Shorter Contributions to General Geology 1968 GEOLOGICAL SURVEY PROFESSIONAL PAPER 614 This volume was published as separate chapters A-F UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director CONTENTS [Letters designate the separately published chapters] (A) Micromineralogy of galena ores, Burgin mine, Hast Tintic district, Utah, by Arthur S. Radtke, Charles M. Taylor, and Hal T. Morris. (B) Isotopic composition of diagenetic carbonates in marine Miocene formations of Cali- fornia and Oregon, by K. J. Murata, Irving Freidman, and Beth M. Madsen. (C) The October 1963 eruption of Kilauea Volcano, Hawaii, by James G. Moore and Robert Y. Koyanagi. (D) Pegmatitic trachyandesite plugs and associated volcanic rocks in the Saline Range, Inyo Mountains region, California, by Donald C. Ross. (E) Distribution of thorium, uranium, and potassium in igneous rocks of the Boulder Batholith region, Montana, and its bearing on radiogenic heat production and heat flow, by Robert I. Tilling and David Gottfried. (F) Studies of celadonite and glauconite, by Margaret D. Foster. A{ :~" . yorary 13/115; fiUCKRam ws \ Nox CaT. e m $575” PC ¢, 614-150 Micromineralogy of SCIENCES LIBRARY Please bin» "~~ Galena Ores, Burgin Mine »" East Tintic District Utah E 2 a | f/k, GEOLOGICAL SURVEY/ PROFESSIONAL PAPER /614-A bonnes, * r | o U 1T | | | I | MAR 20 19863 | (G ACT LIBKAC erm AF CA IEORN Kop L1. OF GAL ann y.5.5.DB. Micromineralogy of Galena Ores, Burgin Mine East Tintic District Utah By ARTHUR S. RADTKE, CHARLES M. TAYLOR, and HAL T. MORRIS SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER. 614-A A study of the distribution of a variety of chemical elements in the major and minor minerals of a silver-rich lead and ginc replacement ore body UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON.; 1969 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 40 cents (paper cover) CONTENTS Page | Mineralogy and chemistry-Continued Page Al Pyrite and ce. A9 0s 1 mias . a9_ io Pes oe o 1 Cemissitec...... 0 lil . e e 9 Purpose and procedure......._............l._l..l.cl.. d Siver ..:... ll _ pally. 11 Mineralogy and 2 Hematit 11 se ell cll l. css tad sas 2 emIa ( ees vam - orto ote fama ll.. ll t 3 Calfnte and . 13 i ici incl Aliso tly 3 Barite and .A ac duce. 13 l=. icc. nn rie y | Element ites, 13 Lead-antimony oxide or carbonate _ __-_________--.- § | Summary and 16 Lk ect un - nous - 8 :| References oy 17 ILLUSTRATIONS [All illustrations are photomicrographs] Page FIGURE 1. Chainlike series of polybasite and tetrahedrite inclusions in A4 2. Large grain of tetrahedrite locked in fine-grained 5 3. Granular intergrowth of jalpaite and galena surrounded and replaced by cerussitec 5 4, Mimetite forming along a microfracture in 6 5. Replacement of remnant grains of sphalerite, quartz, and barite by c.. er nicee es ea oe aaa augen 8 6. Veinlet of late(?) barite containing fragments of pyrite, chalcopyrite, sphalerite, and galena cutting across quartz... 9 7. Mimetite along contact between galena and barite L_ 10 8. Alteration of galena to cerussite localized along cleavage 11 9. Fine-grained secondary silver sulfide dispersed in secondary 12 10. Replacement of remnant quartz and segmented barite by galena, which was subsequently altered to ..... 13 11. Alteration of galena to L enses seul on subs uss 14 TABLES Page TABLE T: Minerals in the galena ores of the Burgin mine. aun be unt ue ans bess ce fe A2 2. Chemical and spectrographic analyses of galena ores, Burgin 3 3-7. Analyses of- 8: Polybasite: ras ans an an a aab an hin mu hil b tiple » e mn a Bd a c abl is in ae ain n am ave - trie ia he ire aie ation s a ~ Telrahedrife. . 2. ...n one ien ok ane an aie aan alee mn mln al al a an an is in an s in a a in a le on ao i in aris ia i arta ie ale ate 3 B ... z. - 12s $o ob aat oes n ae aan an in armin m oie tol a al bm B ge a hen ae ie al in in in ien fei in io in a s in a ain ie n in e a. eee rae 5 6 * Sphalerite.. . . 2 cel co o Lee o con's aln ale mee ale o o piel bk im a n ie me d al foe ae a al deine in in i us ina e la an in i he ne s arin in an a he ae e tale a ala aie 8 ee MImetihe . . 222 20 2 so ss o o oe o al a oe e mg e e ne ae an ae c lge n a h od ta hn an Te a et he a d n o as as e an haes n he ap nne cles it In o ad ne e rate ae e aet 9 IH SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY MICROMINERALOGY OF GALENA ORES, BURGIN MINE, EAST TINTIC DISTRICT, UTAH By Arraur S. Raptr, M. Taymor, and Ham T. Morris ABSTRACT Analyses of argentiferous galena ores from the Burgin mine, Utah, by electron microprobe, emission spectograph, and wet chemical method indicate distinctly different amounts of silver in two general types of galena. Massive coarse-grained galena contains an average 0.22 percent silver by weight (approxi- mately 64 ounces per ton), whereas fine-grained galena has less than 0.04 weight-percent silver (approximately 12 ounces per ton). The primary silver minerals dispersed in the galenas in- clude polybasite, tetrahedrite, and jalpaite. Secondary silver sulfides, including argentite (or acanthite) and jalpaite, are concentrated in cerussite along fractures. INTRODUCTION The Burgin mine, in the East Tintic mining district, Utah, has recently become a major source of lead, zinc, and silver ores in an area that is widely known for rich and extensive replacement deposits (Lovering and Morris, 1960, p. 1116-1147; Bush and Cook, 1960, p. 1507-1540). The principal ore body in the mine, from which the samples described in this report were taken, is localized in sheared and brecciated rocks near the sole of the East Tintic thrust fault. This ore body is reported to contain more than 1,250,000 tons of ore with an average content of 10 ounces of silver per ton, 15 percent lead, and 12 percent zinc (Mining Congress Journal, 1961). Several times this amount of lower grade ore forms a casing around the high-grade ore body, and other ore bodies, estimated to contain an even larger quantity of medium- and low-grade ore, have been discovered nearby. In general, the ores consist of various proportions of argentiferous galena, sphalerite, pyrite, and minor quantities of other metallic minerals in a gangue of rhodochrosite and baritic jasperoid. These minerals replace brecciated masses of Cambrian limestone that have been overturned and thrust over argillaceous limestones of Ordovician age. All the Pale- ozoic rocks in the mine area are concealed beneath al- tered quartz latite lavas of Eocene age that postdate the structural events but predate ore deposition. The two ore types described in this report are repre- sentative of two major varieties of lead ore in the cen- tral part of the main Burgin ore body. Samples of both were collected by H.T. Morris in November 1965 from the 1,200-foot level of the mine shortly after the main ore body was first reached on this level. Type 1 ore is massive coarse-grained galena which forms a lead-rich zone near the foot wall of the deposit. Type 2 ore is some- what finer grained galena that is intergrown with minor sphalerite near the hanging wall of the deposit. The contact between the two types of galena ore is abrupt and apparently indicates either a change in the compo- sition of the ore solutions during deposition or postdepo- sitional leaching and recrystallization of part of the ore body. ACKNOWLEDGMENTS The writers thank Mr. Gale Hansen, former mine su- perintendent, and Mr. William M. Shepard, mine geolo- gist, for permission to sample the ore body of the Bur- gin mine. Dr. Victor Macres, President, Materials Analysis Co., generously permitted the use of Model 400 electron-beam microprobe analyzers and other facili- ties at the company laboratories in Palo Alto, Calif. PURPOSE AND PROCEDURE Examination of the galena ores from the Burgin mine was undertaken to (1) study the distribution of silver, (2) identify all ore and gangue minerals, (3) study tex- tural and physical relationships of the minerals, and (4) study chemistry and element distribution of the minerals. Mineral identifications were made by using the com- bined techniques of electron microprobe analysis, X- ray powder diffraction, and microscopy. The extremely small grain size of many of the phases necessitated ex- tensive use of the electron microprobe analyzer. Sample preparation was done by Radtke and Taylor in the laboratories of the Materials Analysis Co., Palo Alto, Calif., and the U.S. Geological Survey, Menlo Park, Calif. All analytical work was done with Materials Al A2 Analysis Co. Model 400 two-channel and three-channel electron-beam microprobe analyzers. Mineral textures and physical relationships were studied in polished section and, to a lesser extent, in hand specimen and are shown in numerous photomicro- graphs. The polished sections were made by mounting thin wafers of ore in epoxy casting resin set in stainless steel rings. The surfaces selected for study were ground, impregnated, and polished following the method de- scribed by Taylor and Radtke (1965). Use of the stain- less steel mounting ring and careful sample prepara- tion resulted in virtually no loss or plucking of galena and other minerals from the surface, as well as in ex- tremely low relief between galena and quartz. Bulk samples of the galena ores were analyzed by standard wet-chemical and spectrographic techniques. The chemical compositions of all minerals were deter- mined with the electron microprobe analyzer. Certain aspects studied in detail include chemical zoning within minerals and element-concentration gradients across mineral boundaries. Numerous electron-beam scanning (EBS) X-ray images illustrate element distribution between and within phases. Instrument geometry for the Materials Analysis Co. Model 400 electron-beam microprobe includes (1) elec- tron incident angle, phi ($) = 62.5°; (2) X-ray take- off angle, theta (0) = 33.5°; and (3) geometric factor for absorption corrections is ese (0) sin(g)=1.6071. Operating potentials (excitation potentials) used in semiquantitative and quantitative analyses were 30 ky (kilovolts), zinc and copper; 25 kv, iron and manga- nese; 20 kv, indium, sulfur, cadmium, antimony, lead, silver, calcium, and chlorine; 15 ky, arsenic; 7 ky, oxy- gen. All electron-beam X-ray scanning images were made at 20 ky operating potential except oxygen, which was made at 7 ky. The K« characteristic lines were used for oxygen, silicon, sulfur, chlorine, calcium, iron, cop- per, and zinc; L characteristic lines were used for arsenic, strontium, silver, cadmium, antimony, and barium; the Ma characteristic line was used for lead. Pure-element and compound standards used in the quantitative analyses included Fe, Cu, Zn, Ag, Cd, S10», NaCl, GaAs, SrSO,, BaSO, PDCO;, PbS, and apatite. X-ray mass-absorption coefficients were taken from Heinrich (1966) and from unpublished data assembled by Metals Research, Ltd., Melbourn Royston Herts, England. In the reduction of electron microprobe X-ray intensity data, corrections were made for (1) drift in the incident electron-beam current; (2) background from the continuous spectrum; (3) absorption effects; and (4) atomic-number effects. Atomic-number and absorption corrections were taken from tables by Adler and Goldstein (1965), calculated from absorp- SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY tion corrections by Philibert (1963), and the Duncumb and Shields' (1966) overvoltage correction. MINERALOGY AND CHEMISTRY The first of the two types of ore, designated "type 1," is coarsely crystalline galena with thin coatings of secondary minerals. The second, designated "type 2," is massive fine-grained erystalline galena with minor amounts of sphalerite and abundant overgrowths of secondary minerals. All minerals identified in the ore samples are listed with their compositions in table 1. TaBLE 1.-Minerals in the galena ores of the Burgin mine l PbS ZnS Pyribe: . FeS; CuFeS; Tetrahedrite. _.....___. (Cu, Zn, Ag) 1; (Sb, As)4 Sis Polybasite:....... >...; (Ag, Cu) is Sb: Su Jalpaite _______________ Clio,“ Ag] S Silver sulfide*..._..____. Ag,S Lead-antimony oxide-carbonate_. _._. (Pb-Sb-C-O) t Coerussite.c.:......l.._/ PbCO; sa 0. CaCO; Minmetite=.....~..y..}. Pb;(AsO,)s CJ FeO; Quartz. >> SiO; J_}? BaSO, Anglesite(?) __ _...... /.. PbSO, 1tf’xfgcgggictglggggfigggaét determined. Small bulk samples of both type 1 and type 2 ores were crushed and ground for analysis. Chemical anal- yses for total lead and complete semiquantitative spec- trographic analyses of galena ores are given in table 2. GALENA The coarsely crystalline galena of the type 1 ore con- tains small amounts of dispersed and associated gangue minerals, including quartz, barite, and calcite. Small rounded or oval grains of polybasite, tetrahedrite, and jalpaite are also dispersed through the galena ; no acan- thite or argentite was identified as primary inclusions or as exsolution blebs in the galena, and sphalerite is ex- tremely rare. In contrast to type 1, the fine-grained crystalline galena of the type 2 ore contains slightly greater amounts of quartz, barite, and calcite gangue minerals. Minor amounts of pyrite, tetrahedrite, and chalcopy- rite are associated with the gangue minerals, and sphal- erite is locally abundant. Compared with the coarse- grained type 1 galena, the fine-grained variety is rela- tively free of dispersed rounded grains or inclusions that are commonly attributed to exsolution. The most MICROMINERALOGY OF GALENA ORES, BURGIN MINE, EAST TINTIC DISTRICT, UTAH abundant inclusions are tetrahedrite. Jalpaite is minor, and no polybasite was noted. TaBum 2.-Chemical and spectrographic analyses of galena ores, Burgin mine [Spectrographic analyses by Chris Heropoulos. Values in weight percent] Massive Massive Massive Massive coarsely fine-grained coarsely fine-grained Element crystalline _ crystalline Element crystalline _ crystalline galena galena galena galena (type 1) (type 2) (type 1) (type 2) 85.3 75.0 0. 0007 0. 2 . 05 2.0 . 007 . 03 . 005 .02 . 00015 . 0003 005 O1 . 02 . 07 . 0002 . 0007 . 005 005 . O1 . 0005 . 0005 . 007 5 .03 005 Di *Determined by chemical gravimetric technique. Note: Spectrographic results are reported in percent to the nearest number in the series 1, 0.7, 0.5, 0.3, 0.2, 0.15, 0.1 , which represent approximate midpoints oss. dlimfé (oeil in e pram alls ou on n me ha th Ai. ND Pa. on, ho, io An th Po Hb t 9: w, 'Y, ¥b, 2. The two types of galena show distinct differences in silver and antimony contents. The fine-grained galena does not contain detectable amounts of either silver or antimony in the mineral structure. The limit of detec- tion for both elements in PbS by electron-beam micro- probe analyses is 0.04 percent. In contrast, the coarsely crystalline galena has an approximate average content of 0.22+0.02 weight-percent silver and 0.25+0.02 weight-percent antimony. The value for silver is con- siderably higher than the 0.16 weight-percent reported by Taylor (1967) for galena-clausthalite associated with rich silver ores from Republic, Wash. Small increases in both the silver and the antimony content in galena are apparent within 100 microns of polybasite inclusions. Such increases were not found within corresponding distances from the tetrahedrite in- clusions. Other elements-such as copper, zinc, iron, cad- mium, arsenic in tetrahedrite, and copper in polyba- site-show no concentration in galena near these two minerals. POLYBASITE The small oval grains of polybasite dispersed in the massive coarsely crystalline galena of type 1 occur both singly and as a series of grains in a chainlike or linear orientation (fig. 1). The general composition of these grains in the galena host is given in table 3. Polybasite is the antimony-rich end member in the polybasite-pearceite isomorphous series. In polished section under reflected light, it is pale brownish gray. The largest grains observed in the Burgin galenas are approximately 8 microns long and 4 mircons wide. A83 Tasus 3.-Analyses of polybasite Element \ Weight percent in .n nena anes ae oo 12-16 SDLLE sellin tients s an sues 10-14 Al Allele ean sues Cn. s oot in l ic naaa s 3-4 Ag- LA E ocr iriver ae ak tale 65-70 1 Determinations by electron microprobe semiquantitative analysis. 2 Values represent element abundance ranges. * Arsenic tested for but not detected (<0.03). No other elements detected. TETRAHEDRITE Rounded or oval grains of tetrahedrite are dispersed through both types of galena (fig. 1) and also occur closely associated with gangue minerals in the fine- grained type 2 galena. All grains of tetrahedite ana- lyzed contained small and varying amounts of silver, indicating that the variety in the Burgin ores should be designated as "silver-bearing tetrahedrite." Chemical analyses of tetrahedrite dispersed through galena and of tetrahedrite associated with gangue minerals show small variations in composition (table 4). A photo- micrograph of a relatively large grain of tetrahedrite in galena is shown in figure 2. TaBus 4.-Analyses of tetrahedrite [Values, given in weight percent, represent element abundance ranges) Element ! Dispersed in With gangue Element! Dispersed in With gangue galena in galena galena in galena 24-28 24-28 | Fell.... 0.7 0. 4 16-20 20-26 9 32 1.1- 1.3 3.2- 8.5 3. 5- 5 3- 4 35-40 35-40 B- 1.6 .6 6- 6.5 6- 6.5 ' Determination by electron microprobe semiquantitative analysis. No other elements detected. Chemical analyses of tetrahedrite show the ratio of antimony to arsenic, in weight percent, to be about 4-6 : 1. Tetrahedrite dispersed through galena compared with that associated with gangue contains consistently higher contents of arsenic and iron, and lower contents of antimony and silver (table 4). Preliminary studies show that the silver content in individual grains vary randomly up to the limits given in table 4; similar work was not done for other elements in tetrahedrite. Large amounts of zine (6.0-6.5 percent) are present in the tetrahedrite and are accompanied by significant amounts of cadmium. Of the numerous tetrahedrite analyses listed by Palache, Berman, and Frondel (1944) none contain cadmium, which ranges to half a percent or more in the Burgin tetrahedrites. Tetrahedrite in coarse-grained type 1 galena com- monly occurs in relatively large (20-30 micron) grains. In contrast, tetrahedrite grains in fine-grained type 2 galena are much more numerous and much smaller (<1-2 microns in diameter). Under reflected light tetrahedrite in these ores is pale gray. A4 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY «4§FicurE 1.-Chainlike series of polybasite (Po!) and tetrahedrite (Tet) inclusions in galena (Goal). In general, inclusions of these two minerals in galena show no preferred orientation. Magnification X 445. A-D, X- ray scanning images for Sb, Ag, Cu, and Pb in the area outlined in figure 1. The symbols in the lower left-hand corner indicate the element represented by the bright areas of the photograph. Magnification X 650. MICROMINERALOGY OF GALENA ORES, BURGIN MINE, EAST TINTIC DISTRICT, UTAH A5 FrcurE 2.-Large grain of tetrahedrite (Tet) locked in fine-grained galena (Gal). Dark- gray phase is quartz (Qz) gangue, and dark band along top of figure is mounting resin. Magnification X 330. Chemical data on this large tetrahedrite grain are given in table 4. Gal JALPAITE Minor amounts of primary jalpaite were identified in coarse-grained type 1 galena as small clusters of round grains or inclusions (fig. 3). Although not closely as- sociated with tetrahedrite, jalpiate is commonly pres- ent in and near areas of significant amounts of tetrahe- drite. (See fig. 4.) The mineral is also secondary in origin and occurs in small grains (<1-2 microns in diameter) scattered through cerussite. Chemical analy- sis of jalpaite is given in table 5. The formula for jal- piate, as reported by Skinner (1966), is or (0.45Cu,8-1.00Ag,8), having a cation to sulfur ratio FicurE 3.- Granular intergrowth of jalpaite (Jal) and galena (Gol) surrounded and re- placed by cerussite (Cer). Fine-grained fragments in cerussite are remnant galena (Gal). Magnification X 645. 317-359 O - 69 - 2 of 2:1. From the analysis given in table 5 for copper and silver, and by calculating the atomic ratios and nor- malizing the cations to 2.00, the formula for Jalpaite in Burgin ores is Cus,sAgisDicse. Of 1.54Ag,8). 5.-Analysis of jalpaite Element ! Weight percent cul men 15. 5+ 0. 5 cin ll tool ore Tek 1 ciel e eins 12. 5+ 0. 2 100. 0 ! Determinations by electron microprobe quantitative analysis. No other elements detected. A J ¥p aunong AT MICROMINERALOGY OF GALENA ORES, BURGIN MINE, EAST TINTIC DISTRICT, UTAH '00f X uoneogrtusept 'qydeasojoud oy} ;o stort jqS1q oy}; 4q juowsto o[} JJ BOTDPULI Jou.10) pUBU-}J9] JoMOI o[} Uf SLOGuI4s oU.J, *p ut Bot ou} UI Io) put 'qy 'sy 'g 'qgq 'uz 'no 'or 'O "Ig 4oJ soSeut Sutuuros 4e1-x 'y-y 'ogg X uon -egIUuSsE]( 'oJBU0(GIEB) 10 optxO Luowtjus-peol € st ueyq} 4nustts) oseyd 48418 cogjour pus 'optupoye1j9} st ostqd 4818 1914SI oy} St astyd jsoy1ep au} store asoy} uj Jo opts JSL UO UI SYJMOISId]UIOJOIU JO SBoIEB OMj 9J0N 'tuo[e3 Ut poyoo[ ort ure13 (Jof) ews quo put (i91) JO afe OM, 'onSues (zit) ore store YIED 981%7T (JDO) Buore3 ut t Sutui10} (wiw) ornowutpy-'p AS LEAD-ANTIMONY OXIDE OR CARBONATE An unknown secondary mineral containing large amounts of lead, antimony, and oxygen, plus a signifi- cant amount of iron, is closely associated with mimetite in the ore. Although the mineral is present in both types of galena ore, it is much more abundant in the fine- grained ore. In the massive coarse-grained galena ore, it is confined to, and dissemintaed through the cerussite coatings on galena. In the fine-grained galena ore, too, the mineral is confined to, and dispersed in, cerussite coatings on galena but is also common with mimetite and cerussite along microfractures in the galena (fig. 4) and as a thin (<1 micron) selvage between quartz grains and galena. Although the mineral is abundant, it everywhere is present only as particles 24 mircons in diameter. Detailed quantitative electron microprobe study indi- cates that the mineral contains approximately 40 per- cent lead, 22 percent antimony, 2.2 percent iron, and 0.4 percent calcium. Sulfur is not present, and wavelength- shift studies on S;« show no sulfate (SO,) to be present near the mineral. Behavior of the mineral under electron bombardment strongly suggests that it is an- hydrous. Since the antimony, lead, iron, and calcium contents, when converted to oxides, are considerably less than 100 percent, and the mineral is confined to cerussite or a system rich in CO," molecule, it seems likely that the carbonate, or bicarbonate, is present. If the mineral is a carbonate, a general formula close to the following would be reasonable on the basis of known data : 8 (Phb,Fe,Ca) CO;: Sb;,03, or 3 (Pb,Fe,Ca) (HCO;) ; Sb,0; SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY The small particle size and interference from carbon and oxygen in cerussite surrounding the mineral pre- vented accurate carbon and oxygen analyses. SPHALERITE Sphalerite was not observed in the coarse-grained type 1 ore, but it is abundant as small grains interlocked within galena in the fine-grained type 2 ore. Locally, it is concentrated with quartz and barite, commonly rimmed by galena, which clearly replaces the earlier sphalerite crystals (fig. 5). Chemical analyses of sphalerite grains are given in table 6. Numerous analyses were made on sphalerite grains, and the general discussion on composition of sphalerite given here incorporates data obtained from all analyses on the samples studied. TaBus 6.-Analyses of sphalerite Element ! Weight Element ! Weight percent ® percent 2 30 -85 Od- . eS. ser creds ce 0.5 - 1.0 60 -70 Mir:. iol ees .02 - .3 <€.08- : M7 I UUP reine Pinca vue rane s . 09 elélgeellzgzgéficttiggls by electron microprobe semiquantitative analysis. No other 2 Values represent element abundance ranges. Continuous quantitative analyses for iron, cadmium, and indium were made along traverses across nu- merous sphalerite grains. The concentration of iron in most of the sphalerite grains is below the limit of detection (0.02 percent). The maximum amount found was 0.07 percent, but the minor concentrations appar- ently vary widely between grains and even within indi- vidual grains. In all the traverses the cadmium content varied directly with that of iron, and the maximum amount of cadmium found, 1.0 percent, was in the area FicurE 5.-Replacement of remnant grains of sphalerite (Sph), quartz (Qtz), and barite (Bar) by galena (Gol). Note alteration of galena to cerussite (Cer) near center of fig- ure. The mineralogy and chemistry of micro- intergrowth areas in galena, outlined in the figure, are shown in detail in figures 4, 44- K, and 11, 114-K. Mimetite formed along microfractures is present in both outlined areas. Magnification X 80. MICROMINERALOGY OF GALENA ORES, BURGIN MINE, EAST TINTIC DISTRICT, UTAH A9 containing 0.07 percent iron. In contrast, the distribu- tion and abundance of indium in sphalerite does not correspond to that of iron or cadmium. The highest indium content found was 0.3 percent, and the average is estimated to be 0.04 percent. Other elements specifi- cally tested for in sphalerite but not found include silver, manganese, mercury, tin, gallium, and thallium. Of particular interest is the cathodoluminescence of the sphalerite in the Burgin ores, which is apparently dependent on the minor element content. Under high- energy electron bombardment, the cadmium-bearing sphalerite emits radiation in the visible spectrum range that apparently varies with the concentration of cad- mium in the following way : (1) >0.08 percent Cd, faint yellow white; (2) 0.05-0.07 percent Cd, faint greenish yellow; (3) 0.03-0.05 percent Cd, pale blue; (4) <0.03 percent Cd, pale red. Since the iron content varies directly with the cad- mium content, these color variations may also reflect small differences in the abundance of iron. Other ele- ments considered to influence cathodoluminescence in sphalerite, such as manganese, may affect it. However, if present, such elements occur in amounts below the limit of detection in the samples studied. PYRITE AND CHALCOPYRITE Rare grains of both pyrite and chalcopyrite are present in the fine-grained type 2 ore, but are absent in the coarsely crystalline type 1 ore. Both minerals com- monly occur in grains less than 10 microns in length. (See fig. 6.) No complete chemical analyses were made on these phases. MILMETITE - Mimetite in small amounts was also identified in type 2 ore. It occurs with cerussite, minor amounts of angle- Gal FirGurE 6.-Veinlet of late(?) barite (Bar) containing fragments of pyrite (Py), chal- copyrite (Cpy), sphalerite (Sph), and galena (Gal) cutting across quartz (Qtz). Note tiny inclusions of tetrahedrite (Tet) and sphaler- ite in galena fragments, and cerussite (Cer) along margin of veinlet. Magnification X 330. site (?), and calcite as a coating or overgrowth on galena. Commonly, it fills minor voids between the quartz or barite grains and the galena (fig. 7), as well as the mi- crofractures within galena (fig. 4). Other secondary minerals commonly associated elsewhere with mimetite, such as smithsonite, hemimorphite, and wulfenite, were not identified. Their absence may reflect the general low abundance of iron, zinc, vanadium, and copper in the galena ores. The general low abundance of mimetite in the ore also doubtless reflects the low concentration of arsenic in the Burgin galena ores. A complete chemical analysis of mimetite in the Bur- gin ores is given in table T. TaBus 7.-Analysis of mimetite Element ! Weight percent leila navels me ol 69 +2 CB =e sre 4% .1 ASL AAE Ane anaes- nest 15 41 C1. .e. nsf gve e dans ae aect 2. St .2 Onye i- rak annul aan 13 42 Totals. sesli iela ue 100. 2 ! Determinations by electron microprobe quantitative analysis. No other elements detected. CERUSSITE Large amounts of cerussite are present in both types of ore, both as surface coatings and along fractures and cleavage directions in galena. The cerussite commonly contains intergrowths of mimetite, jalpaite, secondary silver sulfide, secondary lead-antimony oxide or carbon- ate, and minor amounts of hematite, calcite and angle- site(?). The typical alteration of galena to cerussite along galena cleavage planes is shown in figure 8. Even in areas where this alteration is massive and virtually complete, numerous small inclusions of remnant galena remain. Although minor amounts of anglesite may be MIO Gal SHORTER CONTRIBUTIONS 4A TO GENERAL GEOLOGY «€FrcurE 7.-Mimetite (Mim) along contact between galena t 3 DP (Gal) and barite (Bar) gangue. Small amounts of cerus- site (Cer) are also present. Small white grains in mime- tite are remnant galena grains. Magnification X 1,290. A-F, X-ray scanning images for Cl, As, O, Ba, Pb, and S in the area outlined in figure 7. The symbols in the lower left-hand corner indicate the element represented by the bright areas of the photograph. Magnification X 1,500. MICROMINERALOGY OF GALENA ORES, BURGIN MINE, EAST TINTIC DISTRICT, UTAH present with cerussite, no typical intermediate zone of lead sulphate was observed between the lead carbonate and the lead sulfide. SILVER SULFIDE Large amounts of argentite or acanthite of secondary or supergene origin containing minor amounts of cop- per (0.5-0.8 weight-percent Cu) are admixed with, and dispersed through, cerussite (fig. 9). The extremely small size of individual silver sulfide grains (<1-2 mi- crons) precluded obtaining either the optical data or the X-ray diffraction data necessary for positive identi- fication. Aggregates of granular grains that formed contemporaneously with cerussite are shown in X-ray scanning images (fig. No primary argentite (or acanthite) was identified with the galena either as an individual intergrown min- eral or in dispersed rounded exsolution-type grains. HEMATITE Minor amounts of hematite are present with cerus- site in areas of extensive alteration of galena to the lead carbonate. The low abudance and erratic distribution of hematite reflects the corresponding occurrence of primary iron-bearing sulfides. X-ray scanning images for oxygen (fig. 112) and for iron (fig. 110) show the close association between hematite, cerussite, and the other alteration products. FicurE 8. -Alteration of galena (Gal) to cerussite (Cer) localized along cleavage planes. Note remnant galena grains in cerus- site. Small black spots are micropits pro- duced in sample preparation. Magnification X 80. A12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY Gal FIGURE 9A = <5 Ce =" n s L HC we * of a" while PE ¥. ¥ "% be + v FicuUrE 9.-(Explanation on next page.) MICROMINERALOGY OF GALENA ORES, BURGIN MINE, EAST TINTIC DISTRICT, UTAH CALCITE AND ANGLESITE(?) Minor amounts of both calcite and anglesite(?) are present with cerussite in both types of ore in areas where alteration of galena to cerussite is relatively com- plete. The calcite is commonly associated with mime- tite and anglesite(?). As previously noted, anglesite is not concentrated along galena-cerussite contacts. BARITE AND QUARTZ Small amounts of barite and quartz form the gangue minerals in fine-grained type 2 ore. Only a few scattered grains of each were recognized in coarse-grained type 1 ore. Both minerals are dispersed through galena, and textural evidence clearly shows replacement of the early gangue minerals by galena (figs. 5, 10). The elongated and segmented appearance of barite crystals shown in figure 10 is common in the fine-grained ore. Barite gen- erally occurs with quartz, although the reverse is not necessarily true. Small veinlets of barite cutting galena and quartz suggest a second generation of late postore barite. These veinlets commonly contain numerous fragmental in- clusions of early sulfide minerals (fig. 6). ELEMENT DISTRIBUTION Electron-beam scanning techniques allow graphic portrayal of the distribution of many key elements in the galena ores. These EBS X-ray scanning images are presented throughout the report. Silver.-In the primary minerals silver is present in polybasite jalpaite, and tetrahedrite, although in tetra- hedrite the silver content is far below the 17-19 percent apparent maximum level for freibergite. The two types of galena contain different amounts of silver. In areas Ficurs 9.-Fine-grained secondary silver sulfide (Arg) dispersed in secondary cerussite (Cer). Note argentite or acanthite concentrated in narrow zone between galena (Gal) and inclusion-free cerussite. Black areas are micropits in cerussite. Magnifica- tion X 160. A-C, X-ray scanning images for Ag, S, and Pb in the area outlined in figure 9, Magnifica- tion x 240. D and H, X-ray scanning images for Ag and S at high magnification, showing typical fine- grained (<2 microns) silver sulfide dispersed through the lead carbonate matrix. Magnification X 1,800. FIGURE 10.-Replacement of remanant quartz (Qz) and segmented barite (Bar) by galena (Goal), which was subsequently altered to cerussite (Cer). Small white grains in cerussite are remnant galena and ~ secondary silver sulfide (Arg). Magnification x 80. , scanning images (figs. 10, 4D, 11D). A13 near the polybasite inclusions, silver occurs as a small but definite concentration. Argentite, commonly re- ported to be intergrown with, or dispersed in, "silver- bearing galena," was not identified in the galena of the samples studied. Jalpaite and argentite (or acanthite) of supergene or secondary origin apparently formed with cerussite. This is particularly evident in figure 9 and in the X-ray scanning images for silver, sulfur, and lead distributions in figure A minimum of 50 percent of the total silver in the samples studied was represented by this latter occurrence. Antimony.-The distribution and concentration of antimony in primary minerals in the ore is similar to that of silver with antimony concentrated in polybasite and tetrahedrite and in coarse-grained galena. The dis- tribution of antimony, silver, copper, and lead between polybasite and tetrahedrite locked in galena are shown in X-ray scanning images (fig. 14-D). During oxidation and alteration of galena, antimony is concentrated in the secondary lead-antimony oxide or carbonate phase in cerussite. No antimony is present in the cerussite itself, with the limit of detection for an- timony in lead carbonate of 0.04 percent. Copper.-The minor amount of copper in primary sulfide ores is closely associated with silver and anti- mony in polybasite and tetrahedrite, and associated with silver in jalpaite. Only trace amounts of chalcopy- rite were observed, and no simple sulfides of copper were recognized. The paucity of secondary or supergene copper minerals reflects the apparent general low abun- dance of copper compared with lead in the ore body. Jalpaite, the silver copper sulfide, was the only second- ary copper mineral identified. The concentration of cop- per in sulfosalt minerals in galena is shown in X-ray h ¥IT awoong A15 MICROMINERALOGY OF GALENA ORES, BURGIN MINE, EAST TINTIC DISTRICT, UTAH 6 *(uoit 'p 'u0s4xo 'g) omewoy pus '(wunrapeo '; 'our (g) oquioreqds '(our0rgo "y 'otuosie ' H 'q 'uoséxo opnpo -ur sogeuwt sutuueos £¥1-x UI uMoys jng TJ ornsy UI UMOYS Bort 0g} Uf popNJOUI J0U S[BJoUIUI [BJoA4dg X 'qydeasojogd oy} 30 stort oun £q pojyuosodot juowidfo 9} @}8ITDPUT JoUI0) JoMO[ 9G} UI Stoqur4s ou.1, 'IT oun@g ut pout[no Bole oy} Ut to pus 'g 'pop 'sy 'ag 'ad 'uz 'no 'o 'O Ig sof soseurt Sutuusos 481-x x¥-¥ < uoneogtuseJy 'tuo|e3 ut poyoo| puUtB UMOIS.JojUT (iO!) JJ0N (103) 01 (Do) tuote3 Jo Uone19}[Y-'TT ZunpLf A16 Arsenic.-In contrast to antimony, the concentration of arsenic is low in primary galena ores (table 2). No arsenic sulfides or arsenic-rich sulfosalt minerals were identified. Mimetite has formed near contacts between gangue and galena (fig. 7) and along microfractures in galena (fig. 4). X-ray scanning images for arsenic in corresponding areas are given in figures 47 and 72. Concentration of arsenic in mimetite associated with other secondary minerals suggests some migration of arsenic in the ore during oxidation and alteration. Lead.-Galena is the dominant primary lead mineral in the ore and accounts for more than 95 percent of the total lead in the samples studied. No primary lead-bear- ing sulfosalt minerals were identified in the galena. 'The low lead content (<0.04 percent) in tetrahedrite sur- rounded by galena suggests a very limited solubility for lead in tetrahedrite. In the sections studied, less than 5 percent of the total lead is in -the alteration mineral, cerussite. Altera- tion of galena to cerussite with remnant inclusions of galena reflects in situ oxidation of the sulfide ore. The apparent tendency for lead sulfide to alter directly to lead carbonate without the formation of lead sulfate suggests that oxidation took place in a slightly acid to alkaline environment with relatively high activity of total carbonate species. (See Eh-pH diagram, Garrels and Christ, 1965, p. 237, 238.) Zinc.-The dominant part of the zinc in the primary sulfide ore is present as sphalerite, although tetra- hedrite, relatively rare in the ore, contains significant amounts of zinc. Zine in tetrahedrite and sphalerite are shown in X-ray scanning images, figures 42 and 112, respectively. Secondary zinc minerals such as smith- sonite and hemimorphite were not identified with cerus- site and mimetite, even in those areas showing intense alteration. The apparent absence of any zinc carbonate or zine sulfate species may be explained by their much greater solubility relative to the corresponding lead species. Cadmium and indium.-Both cadmium and indium are concentrated in the sphalerite of the Burgin ore body. Significant amounts of cadmium, along with large amounts of zinc, are also present in tetrahedrite and reflect the similar geochemical behavior of the two elements. The binary cadmium sulfide, greenockite, was not identified. Indium was identified in sphalerite only. Analysis of secondary minerals associated with both types of galena ore shows no concentration of either cadmium or indium. Iron.-Iron is low in abundance in both types of galena ore (table 2). This is reflected mineralogically by the very limited amount of pyrite in the ore sample studied. Other iron-bearing minerals, including chal- copyrite and tetrahedrite, are minor in abundance, and SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY the sphalerite is virtually iron free. Secondary minerals coating the samples and filling fractures cutting across them are also virtually iron free. No iron carbonate was found, although small amounts of the iron oxide hematite were identified locally. Small amounts of hematite, closely associated with cerussite, lie just out- side the area shown in figure 11. The iron oxide mineral is shown in X-ray scanning images for iron (110) and oxygen (11B). Figure 6 shows a photomicrograph of pyrite and chalcopyrite in barite. SUMMARY AND CONCLUSIONS The mineralogy of massive galena ores of the Burgin mine in general is deceptively simple, but in detail it pre- sents many complexities. Substantial mineralogical dif- ferences exist between the two general types of ore and are reflected in chemical analyses of the bulk ore. The massive coarse-grained type 1 ore is chiefly galena, whereas the massive fine-grained type 2 ore, although dominantly galena, carries significant amounts of sphalerite. The oxidation and alteration of galena to cerussite along fractures and cleavage directions is well developed in both types of ore. In contrast, sphalerite, identified only in the fine-grained ore, is fresh and unaltered. Compared with the apparently limited movement and the lack of zinc enrichment in the ore, silver-as a constituent of the obviously secondary or supergene minerals jalpaite and argentite (or acanthite)-is con- centrated in, and dispersed through, secondary lead carbonate. Although the amount of silver sulfide-rich cerussite is small compared with the primary galena in the samples studies, more than 50 percent of the total silver content is in this form. Much of the silver in the primary sulfide ore is in grains of polybasite and tetrahedrite and in smaller amounts of jalpaite dispersed through galena. In many "argentiferous galena" ores the small disseminated in- clusions commonly assumed to be "exsolution argentite" probably are actually complex sulfosalt minerals. The coarsely crystalline "argentiferous galena" con- tains detectable amounts of silver in the galena, and locally, near the sulfosalt inclusions, there is a notable concentration of the element. In contrast, the fine- grained galena is very low in silver, containing only about one-fifth of the silver found in the coarsely crys- talline type. The solubility of silver in galena varies directly with that of antimony. Only minor amounts of zinc-rich tetrahedrite were found disseminated through galena. That its occurrence is only minor is an advantage, for the presence of larger amounts of the mineral would introduce harmful amounts of zine and antimony into any lead concentrate produced by flotation milling. MICROMINERALOGY OF GALENA ORES, BURGIN MINE, EAST TINTIC DISTRICT, UTAH REFERENCES CITED Adler, I., and Goldstein, J., 1965, Absorption tables for electron probe microanalysis: NASA Tech. Note TND-2084, 276 p. Bush, J. B., and Cook, D. R., 1960, Bear Creek Mining Company studies and exploration, Pt. 2 of The chief oxide-Burgin area discoveries, East Tintic district, Utah; a case his- tory: Econ. Geology, v. 55, no. 7, p. 1507-1540. Duncumb, P., and Shields, P. K., 1966, Effect of critical excitation potential on the absorption correction, in McKinley, T. D., Heinrich, K. F. J., and Wittry, D. B., eds., The electron microprobe, Proceedings of Symposium sponsored by the Electrothermic and Metallurgy Division, The Electrochem- ical Society, Washington, D. C., 1964: New York, John Wiley & Sons, p. 284-295. Garrels, R. M., and Christ, C. L., 1965, Solutions, minerals, and equilibria : New York, Harper & Row, 450 p. Heinrich, K. F. J., 1966, X-ray absorption uncertainty, in McKinley, T. D., Heinrich, K. F. J., and Wittry, D. B., eds., The electron microprobe, Proceedings of the Symposium sponsored by the Electrothermic and Metallurgy Division, Electrochemical Society, Washington, D.C., 1964: New York, John Wiley & Sons, Inc., p. 296-377. Lovering, T. S., and Morris, H. T., 1960, U.S. Geological Survey studies and exploration, Pt. 1 of The chief oxide-Burgin A17 area discoveries, East Tintic district, Utah; a case history : Econ. Geology, v. 55, no. 6, p. 1116-1147. Mining Congress Journal, 1961 [Data on ore content assays] : Mining Cong. Jour., v. 47, no. 4, p. 108. Palache, Charles, Berman, Harry, and Frondel, Clifford, 1944, Elements, sulphides, sulfosalts, oxides, v. 1 of The system of mineralogy of James Dwight Dana and Edward Salis- bury Dana [7th ed.]: New York, John Wiley & Sons, Inc., 834 p. Philibert, J., 1963, A method for calculating the absorption cor- rection in electron probe microanalysis, in Pattee, H. H., Cosslett, V. E., and Enstrom, Arne, eds., X-ray optics and X-ray microanalysis, International Symposium on X-ray optics and X-ray microanalysis, 3d, Stanford, Calif., 1962: New York, Academic Press, Inc., p. 379-392. Skinner, B. J., 1966, The system Cu-Ag-S: Econ. Geology, v. 61, no. 1, p. 1-26. Taylor, C. M., 1967, Mineralogical applications of the electron- beam microprobe in the study of a gold silver deposit, Knob Hill mine, Republic, Washington: Stanford, Calif., Stan- ford Univ., Ph. D. thesis. Taylor, C. M., and Radtke, A. S., 1965, Preparation and polishing of ores and mill products for microscopic examination and electron microprobe analysis: Econ. Geology, v. 60, no. 6, p. 1306-1319. U.S. GOVERNMENT PRINTING OFFICE: 1969 - O-317-359 EEEEE No. 7 DAY b+ -£ DISPLAY J.%S.D. Isotopic Composition of Diagenetic Carbonates in Marine Miocene Formations of California and Oregon GEOLOGICAL SURVEY PROF-iESS'IONAL PAPER 614-B Isotopic Composition of Diagenetic Carbonates in Marine Miocene Formations of California and Oregon By K. J. MURATA, IRVING FRIEDMAN, ard BETH M. MADSEN SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 614-B A discussion of the isotopic composition Of dz'agenetz'c carbonates in terms of chemical processes that operate in deeply buried marine sediments UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969 UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 45 cents (paper cover) CONTENTS Page ADSHACbL cL B1 | Oxygen isotopic composition of dolomite__________-_--.-- IntrOGUCtION.L 1 Nature of formation water and its influence on oxygen Acknowledgments 2 isotopic composition of dolomite__________------- Mode of occurrence of diagenetic carbonate 2 | Dolomite with variable light carbon and independently Factors that control carbon and oxygen isotopic composi- VAriAble OXY§@M.L L L HOM. c c 4 | Dolomite with variable heavy carbon and relatively con- Time and circumstance of diagenesis_________--------- 5 stant ReAVY Methods of preparation and analysis of the samples. 6 | Carbonate with variable heavy carbon and covariable Petrography Of SAMpI@S-__________________o__--------- T OXY§@ML L General aspects of the isotopic 11 Calcium content of dolomite as related to isotopic Normal marine C@AIDON&At@________________c__--------- 11 LLL Diagenetic 12 | ILLUSTRATIONS FigurE 1. Map showing localities of the carbonates that were StUdi@U.L L c 2. Photograph of typical outcrops of Miocene diagenetit CAFDONAb@$L L_ 3. Photograph of dolomite from the Miocene of Berkeley 4. Graph showing covariance of CaCO; content of dolomite and the d-spacing of [222]--_______________--_----- 5. Photomicrographs of thin sections of carbonate 6. Graph showing relationship between isotopic compositions of carbon and oxygen in carbonates._________-_----- 7. Graph showing temperatures indicated by the oxygen isotopes of dolomites from Palos Verdes Hills -____----- 8. Graph showing temperature dependence of O8 content Of CAIGit@L L L 9. Histograms of carbon isotopic composition of coexisting methane and carbon dioxide_________________-__----- 10. Graph showing isotopic temperature and depth of origin Of NAtUTAL 11. Graph showing carbon isotopic composition as related to the CaCO; content of diagenetic dolomite____-_--_.-- TABLE Tari - 1. Carbon and oxygen isotopic composition of diagenetic L2 III Page B13 15 16 17 20 20 21 Page B2 3 4 6 8 12 15 16 18 19 21 Page B9 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY ISOTOPIC COMPOSITION OF DIAGENETIC CARBONATES IN MARINE MIOCENE FORMATIONS OF CALIFORNIA AND OREGON By K. J. Murara, Irvine Frtmomax, and Berra. M. Mapsex ABSTRACT Diagenetic carbonates, mostly protodolomite, occur as thin beds or concretionary zones in marine Miocene shales of Cali- fornia and Oregon. They manifest an unusually wide range of carbon isotopic composition (8C of -25 to {-21 per mil rela- tive to Peedee belemnite standard) and are divisible into a light-carbon group (8C of -2 to -25 per mil) and a heavy- carbon group (8C of +5 to {+21 per mil). The range of their oxygen isotopic composition coincides generally with that of normal marine carbonate (80° of 26 to 34 per mil relative to standard mean ocean water). Calcite of Foraminifera and other fossils (8C of 0O+4 per mil) in consolidated sediments was dissolved and redeposited along certain favorable zones to form diagenetic calcite. The formation water involved must have contained at least as much O* as sea water for the 850° of diagenetic calcite to lie in the range of normal marine carbonate. Dissolved in the water were carbonate methane, naphthenates, and other substances formed through diagenetic alteration of organic matter in the sediments. The 3C" of the deposited calcite differed substantially from that of the original fossils, the carbon being heavier or lighter de- pending on whether or not isotopic equilibrium was attained between the carbonate ion and methane. Isotopic exchange equilibration between carbonate ion and methane concentrates * in the carbonate and is the most likely mechanism for producing heavy-carbon carbonates. Without this equilibration, the carbonate ion will be rich in C", in keeping with its derivation from organic matter, and will yield dia- genetic carbonates of the light-carbon group. The exchange could take place between finely porous carbonate and methane gas without necessarily involving water ; this could lead to variation in the carbon isotopic composition among some heavy-carbon carbonates without a concomitant variation in oxygen composi- tion or in crystalline texture. At most places, the first-stage diagenetic calcite was soon dolomitized by brines that tended to make the oxygen somewhat heavier (maximum 80° of 35.4 per mil) and the carbon, as well, if the carbonate- methane equilibration was operating. Proto- dolomite with a calcium content as high as 58 mol percent formed first, but in time exsolved the excess calcium to become ideal dolomite. Among some heavy-carbon dolomites, these sev- eral adjustments led to an inverse relation between the content of calcium and either the carbon or oxygen isotopic composition. The temperature implications of the carbon isotopic composi- tion can be explored in a preliminary way if it is assumed that the methane involved in the equilibration had the same isotopic composition as the methane of natural gas from oil fields (most prevalent 3C" values of -52 to -38 per mil). Under this as- sumption, the heaviest and the lightest carbonates (8C" of {+21 and -+-5 per mil) within the heavy-carbon group limit equilib- rium temperatures to 34° to 140°C. Existing data on genesis of petroleum indicate that transformation of organic matter into petroleum proceeds at an appreciable rate only at temperatures higher than about 115° C. Most of the diagenetic carbonates seem to have formed at somewhat lower temperatures, but their isotopic and mineralogical natures clearly suggest them to be byproducts of petroleum-generating reactions. An interesting variant of diagenetic carbonate is the calcite cement of the Miocene oil-bearing sands that occur as coastal plain deposits lapping onto the granite of the Sierra Nevada. The formation water in these marine sands is virtually fresh. The presence of fresh water accounts for the fact that the car- bon and oxygen isotopic composition of the calcite cement is similar to that of fresh-water limestone and explains why the cement escaped the almost universal dolomitization that oc- curred elsewhere. INTRODUCTION Diagenetic carbonate in the form of indurated beds of concretionary zones is a minor but conspicuous rock in the Miocene Nye Mudstone of Oregon and the Monterey Shale of California (Snavely and others, 1964; Bram- lette, 1946). Recently, interest in this carbonate has grown because of the realization that it is mostly dolo- stone rather than limestone and that, in many places, it is extraordinarily rich in carbon isotope C" (Murata and others, 1967). In the present paper, we discuss the mineralogy and the carbon and oxygen isotopic composition of 9 sam- ples of this and related carbonates from widely scat- tered localities (fig. 1). The main impetus for the study springs from the possibility of gaining new insight into the diagenesis of organic-rich sedimentary rocks such as the Monterey Shale. For our purpose, we adopt the broad definition of dia- genesis given in the "Dictionary of Geological Terms" of the American Geological Institute (1962) : "Process involving physical and chemical changes in sediment B1 B2 after deposition that converts it to consolidated rock; includes compaction, cementation, recrystallization, and perhaps replacement as in the development of dolomite." Existing data on the formation and diagenesis of lime- stone and dolostone have been well summarized in two recent reviews (Pray and Murray, 1965; Chilingar and others, 1967). Diagenesis of the Monterey Shale has attracted the attention of many investigators because the shale is unusually rich in opaline silica (diatoms) and because it is considered to have been the source bed of much petroleum in California. Besides the carbonates that constitute the subject of our inquiry, diagenesis has produced chert from the opaline silica of diatoms (Bramlette, 1946), zeolites and montmorillonite from Yaquina Bay (1-3) 0 R E G 0 N CALIFORNIA N E V A D A Point Arena (4) Berkeley Hills (7-9) SAN FRANCISCO Santa Cruz Mts (10-11) Ano Nuevo Point (12) .Re||z Canyon (13-26) .Nacnmiento Dam (27-32) ® Round Mtn (41-45) ® Chico Martinez Cr (33-40) ® Temblor Range (46) & Cuyama Valley (50) San Rafael Mts (51-53) Ynez R (54-55) ®Pine Mtn (56-57) Pismo syncline\ ® (47-49) C &, Santa Palos Verdes Hills (61-75) LOS ANGELES ®San Juan San Clemente r Capistrano (76) Island (77) U La Jolla and Coronada Canyons (78-79) 100 0 100 MILES FIGURE 1.-Localities of the carbonates that were studied. Numbers in parentheses are the sample numbers listed in table 1. SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY volcanic tuffs (Bramlette and Posnjak, 1933), and oil from sedimentary organic matter (Trask, 1932; Phi- lippi, 1965). The middle Miocene part of the Monterey Shale is highly phosphatic (Gower and Madsen, 1964; Dickert, 1966). Although no diagenetic effects have been noted so far in the phosphorite, future studies will prob- ably disclose such effects even in this relatively stable material. ACKNOWLEDGMENTS We are indebted to Prof. D. L. Inman of Scripps In- stitution of Oceanography for sample 78 from La Jolla Canyon. Advice regarding stratigraphy, paleontology, 'and isotope geology of west coast Tertiary rocks was generously given by Prof. M. N. Bramlette and Dr. Arthur Jokela of Scripps Institution of Oceanography, by Prof. R. H. Jahns of Stanford University, by Dr. S. R. Silverman of Chevron Research Co., and by our colleagues, W. O. Addicott, Ivan Barnes, R. H. Camp- bell, T. W. Dibblee, Jr., D. L. Durham, G. W. Moore, J. R. O'Neil, R. O. Rye, J. E. Schoellhamer, Patsy J. Smith, P. D. Snavely, Jr., H. F. Tourtelot, and R. E. Zartman. Laboratory investigations were carried through with the able assistance of R. R. Bruegger, Chere N. Barnett, Joy Church, and James Gleason. MODE OF OCCURRENCE OF DIAGENETIC CARBONATE Figure 24 shows the typical mode of occurrence of diagenetic carbonate. It is found in conformable beds or concretionary zones, generally less than 3 feet thick, spaced 5 to 50 feet apart in the shale or mudstone of the west coast Miocene. The associated shale or mudstone may be devoid of any carbonate or may be moderately rich in calcareous Foraminifera. Figure 22 illustrates an occurrence of diagenetic carbonate in a sequence of alternating layers of shale and sandstone ; the thin sand- stone layers are hardened by a calcite cement. A closer view of the lowest of the three carbonate beds of figure 2B is shown in figure 2C. Although very ordinary in appearance, this bed has an exceptionally heterogeneous composition. Its top is dolomitic; its bottom, calcitic; and the middle part, a variable mixture of the two car- bonates. It apparently represents an incompletely dolo- mitized bed of diagenetic limestone; all the dolostone beds may originally have been limestone. Thin sections of the dolomitic and calcitic parts of the bed are shown in figures 5C and 5F. In demonstrating the secondary (diagenetic) origin of carbonate concretions and lenticular beds in the Monterey Shale, Bramlette (1946) called attention to the relation of carbonate concretions to the bedding in the enclosing strata. Beds continue through the concre- tions and are thicker inside of them than outside, as if the concretions had preserved an earlier thickness of DIAGENETIC CARBONATES, MIOCENE FORMATIONS, CALIFORNIA, OREGON FiaurE 2.-Typical outcrops of Miocene diagenetic carbonates. A, Dolomitic beds and lenses in the Nye Mudstone, north side of Yaquina Bay, Lincoln County, Oreg. Sample 2 of table 1 was obtained from the bed near the man. Photograph by P. D. Snavely, Jr. B, Carbonate zones (the three thicker beds) in the Monterey Shale of Pine Mountain, Ventura County, Calif. At the beds against the compressive load of the overlying sediment. He emphasized the general impurity of the carbonate: "All the concretions include more or less sedimentary material similar to that forming the adja- cent beds, and most of them are little more than car- bonate-cemented nodular masses of the sedimentary material forming these adjacent beds." In some places, impregnation by carbonate protected opaline diatoms from being dissolved and glassy pumice from altering to montmorillonite. The dolomite content of the dolostone generally de- pends on the porosity of the host sediment and varies B3 this locality, the shale contains numerous thin layers of calcite-cemented sandstone. The rucksack is 30 inches long. C, Detail of the lowest carbonate bed in view B. The upper part of this bed is dolomitic ; the lower part, calcitic ; the two parts are represented by samples 56 and 57, respectively, in table 1. The hammer is 11 inches long. among our samples from about 40 weight percent in dolomite-cemented sandstone to about 85 percent in dolomitized diatomite. The massive, apparently lami- nated, dolostone shown in figure 3 contains 84 percent dolomite without any calcite; its insoluble residue con- sists almost entirely of tests of diatoms with very few detrital minerals. Pure diatomite may have a porosity as high as 75 percent and upon impregnation with dolomite could yield a rock containing about 80 percent dolomite. Thus, this sample of dolostone probably formed mainly through impregnation rather than re- placement of laminated diatomite, a type of sediment B4 common in the Monterey Shale. A few dolomitized tests of Foraminifera in the dolostone further indicate that any and all calcite originally present in the diatomite would have been converted to dolomite. A thin section of this sample is shown in figure 52. T'wo aspects of the diagenetic carbonate are especially significant with regard to its origin and nature. First, the widely spaced thin beds of the carbonate constitute a minor constituent, making up perhaps a percent or so of the total thickness of a typical section. Second, with a few exceptions, the diagenetic carbonate is dolomite rather than calcite. The frequent presence of dolomite along sharply delimited horizons in moderately cal- careous shale sections indicates that dolomitization was effected only along certain favorable zones by solutions introduced from outside. Later in the report, we shall postulate certain isotopic reactions between methane (generated by burial meta- morphism of organic matter) and the carbonate, in order to account for abnormally heavy carbon in the carbonate of Monterey Shale. The effect of such reac- tions would be greatest in a formation like the Monterey FiGurE 3.-Dolomite (sample 8 of table 1) from the Miocene of the Berkeley Hills, Calif. This hard massive rock originated through impregnation of laminated diatomite with dolomite (84 weight per- cent). Insoluble matter consists largely of opaline tests of diatoms with very few detrital minerals. Vertical cracks contain very late calcite. SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY Shale in which the ratio of organic shale to carbonate is large and in which the reacting fluids moved only along certain zones rather than by diffusing throughout the section. FACTORS THAT CONTROL CARBON AND OXYGEN ISOTOPIC COMPOSITION Because comprehensive reviews of the general geo- chemistry of carbon and oxygen isotopes have been published recently (Craig, 1963; Rankama, 1963; Sil- verman, 1964; and Degens, 1965), only data and con- cepts that are pertinent to the origin of diagenetic carbonates need to be considered here. Carbon isotopes of sedimentary rock can be discussed in terms of two major categories of materials with distinctly different isotopic composition : (1) Marine limestone (3C"*=0+4 per mil relative to the PDB, or Peedee belemnite, stand- ard of University of Chicago), and (2) organic sub- stances (such as coal, oil, and dispersed organic matter of soils or shales), containing much lighter carbon (8C"=-35 to -8 per mil) and carbon dioxide derived through oxidation of these organic substances. The rela- tively constant isotopic composition of marine lime- stone results from precipitation of the limestone under conditions of isotopic equilibrium with the bicarbonate of sea water, which is virtually constant isotopically (8C"=-2.5 to -1.3 per mil according to Craig, 1954). The light carbon that characterizes all organic sub- stances originates in the marked preference shown by plants for isotope C** during photosynthesis. Carbon of plant tissues can be deficient in C* by as much as 30 per mil relative to atmospheric carbon dioxide (Park and Epstein, 1961). The following table gives the isotopic composition of atmospheric carbon dioxide, plant tissue, derivative sedimentary organic matter, and other related substances. The isotopic composition of organic matter of animals is similar to that of the plants on which they feed. Material Atmospheric CO;- ___. Marine bicarbonate. __. 5C" (per mil, PDB) Reference -7A to -6.7 - Keeling (1958). -2.5 to -1.3 Craig (1954). +4 Marine limestone. ___ -4 to Baertschi (1957). Marine plants-________ -30 to -12 Deuser and Degens (1967). Terrestrial plants______ -29 to -23 Craig (1953). Organic matter of -S31 to -14 Craig (1953) and marine sediments. Sackett (1964). Coall L -21 to -22 Craig (1953). Petroleum. ___________ -34 to -22 Silverman (1964). Bicarbonate in fresh -23 to -4 Oana and Deevey water. (1960). Fresh-water limestone.. -18 to -3 Clayton and Degens (1959) and Miinnich and Vogel (1959). In comparison to sea water and marine limestone, terrestrial waters and limestone contain carbonate whose carbon is generally lighter and more variable DIAGENETIC CARBONATES, MIOCENE isotopically, as indicated in the table. Fresh water con- tains, besides the normal bicarbonate based on carbon dioxide of the atmosphere, substantial and variable amounts of much lighter bicarbonate (8C of -30 to -20 per mil), based on the carbon dioxide derived from the decaying organic matter of soils. The variation of isotopic composition of fresh-water bicarbonate and limestone from such light values to those of marine limestone depends on the extent to which circulating fresh water encounters and dissolves marine limestone (Oana and Deevey, 1960). Oxygen isotopic composition of sea water is fairly con- stant at 530® of 0 +1 per mil relative to standard mean ocean water (SMOW), according to Epstein and May- eda (1953) and Craig and Gordon (1965). In the hydro- logic cycle, water is evaporated from the ocean under nonequilibrium conditions to yield vapor of light H0 with 30® of -14 to -10 per mil (Craig and Gordon, 1965). Water-laden air from lower latitudes moves pole- ward and precipitates rain or snow on land and sea, as it encounters progressively lower temperatures. H,0*® tends to condense preferentially from the migrating vapor, giving rise to changes in the isotopic composi- tion of the precipitation depending upon the latitude. As a result, polar ice can be as light as 30® of -50 to -40 per mil (Epstein, 1959). The contrast between sea water and fresh water in the middle latitudes would be 80® of zero for the former and of -15 to -5 per mil for the latter. This contrast in oxygen isotopic composition of marine and terrestrial waters also appears in marine and fresh-water limestones. Besides the original iso- topic composition of the water, temperature of the water also determines the amount of O* in carbonates. Because oxygen of sea water is nearly constant iso- topically, the range of 80® in marine limestones (mostly 26 to 34 per mil, SMOW) is largely attributable to variations in temperature of the water, and indeed such variations form the basis for determining the paleo- temperatures of ancient seas (Urey and others, 1951; Bowen, 1966). Fresh-water carbonates (30® mostly in the range of 9 to 23 per mil) are more complex because they form from solutions that vary widely in both isotopic composition and temperature (Weber, 1964). With both carbon and oxygen tending to be lighter in terrestrial than in marine limestone, isotopic compo- sition would seem to be a useful indicator of the depositional environment. The criterion has proved to be ambiguous for some samples because of moderate overlap of isotopic compositions, but is accurate in some 80 percent of the samples (Clayton and Degens, 1959; Keith and Weber, 1964; Keith and others, 1964). The isotopic composition of most of our diagenetic car- 334-795 0O-69--2 FORMATIONS, CALIFORNIA, OREGON B5 bonates indicates derivation from neither fresh water nor sea water, but rather from a different: category of natural water, probably that which is commonly called formation water or oil-field brine. TIME AND CIRCUMSTANCE OF DIAGENESIS Diagenetic carbonate of the Monterey Shale evi- dently formed before the final compaction of the host sediment (Bramlette, 1946), but neither the circum- stance of its formation nor the kind of chemical solu- tion involved is known. Lithification of modern cal- careous sediments lying exposed on the sea floor has been reported by Fischer and Garrison (1967). However, the hard carbonate layers in the Monterey Shale could not have formed in this way, because their carbon isotopic composition indicates equilibration with solutions very different from sea water and because they commonly involve carbonate-poor materials, such as diatomite and volcanic tuff. Studies of modern marine sediments and their in- terstitial waters (Emery and Rittenberg, 1952; Siever and others, 1965; Berner, 1966; among others) have generally disclosed little diagenetic change in these sediments to a depth below the sea floor of 10 meters or so, representing a timespan of 10° to 10° years. The interstitial water of some modern marine sediments, however, is enriched in phosphate and silica and de- pleted in sulfate as the result of diagenetic reactions (Emery, 1960; Brooks and others, 1968). Even older sediments, such as the Miocene ooze re- covered during experimental mohole drilling of the Pacific sea floor (Riedel and others, 1961; Murata and Erd, 1964), are soft and porous and have no indurated zones of carbonate or chert. The interstitial water in this ooze differs very little from normal sea water (Rit- tenberg and others, 1963; Siever and others, 1965) ; it is, however, somewhat deficient in deuterium (Fried- man, 1965). The calcite and dolomite of the coze have carbon and oxygen isotopic compositions in the range of normal marine carbonate (Degens and Epstein, 1964). Compared to such relatively unaltered sediments, the Monterey Shale and overlying sediments accumulated to much greater thickness in nearshore basins, under- went moderate burial metamorphism, and at many places were uplifted into the zone of weathering during Pliocene and later orogenies. The Los Angeles basin contains over 25,000 feet of Miocene and Pliocene sedi- mentary rocks (Barbat, 1958; Yerkes and others, 1965), and other basins, such as Chico Martinez Creek (Bram- lette, 1946) and Reliz Canyon (Durham, 1963), have more than 7,000 feet of middle and upper Miocene sedi- mentary rocks which, before much of the cover of B6 younger deposits was eroded away, must have lain at depths of 10,000 to 12,000 feet. At such depths, sedi- ments would have been subjected to temperatures higher than 100° C, which would be sufficient to con- vert sedimentary organic matter into petroleum (Philippi, 1965), accelerate the reconstitution of bio- genic opaline silica into chert, and induce many other chemical changes in a geologically short time. Unlike the sea-water-like interstitial solutions found in undisturbed deep-sea oozes of even Tertiary age, for- mation waters (oil-field brines) that occur in the Mon- terey Shale and related formations vary greatly in salinity and composition. They range in salinity from fresh water to brines several times more saline than sea water (Jensen, 1934; White, 1965). Deeply circulating meteoric water becomes converted into saline formation water by passage through semipermeable shales and by undergoing numerous other chemical reactions that alter and reconstitute sedimentary materials (Clayton and others, 1966; Anderson and others, 1966; Ritten- house, 1967). As will be shown later, the isotopic com- position of the diagenetic carbonates indicates that for- mation waters from which most of the carbonates sepa- rated resemble neither fresh water nor sea water, but rather constitute a different category of natural water. METHODS OF PREPARATION AND ANALYSIS OF THE SAMPLES To prepare the samples, one or two pieces of each car- bonate were first sawed perpendicularly to the bedding plane and stained with alizarin red sulfonate (Fried- man, 1959). Samples shown by the staining test to be finely intergrown mixtures of dolomite and calcite were discarded. Many of these consisted of calcareous Foram- nifera imbedded in a fine-grained dolomitic groundmass and probably represented a stage of alteration of cal- careous shale transitional to dolostone in which both the Foraminifera and the groundmass are dolomitic. Some of the rejected samples were fractured dolostones ex- tensively veined by late calcite. X-ray diffractometer patterns of all samples that passed the staining test were made with nickel-filtered copper radiation at a scanning speed of 1° of 20 per minute, in order to verify the purity of the carbonate and to determine the nature of the associated noncar- bonate minerals. Special X-ray patterns were prepared, by using a scanning speed of %, ° of 20 per minute and SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY elemental silicon as the internal standard, to determine accurately the unit-cell dimensions of 14 dolomite sam- ples by means of the computer method of Evans, Apple- man, and Handwerker (1963). The relationship be- tween cell dimension and composition of dolomite (Goldsmith and Graf, 1958) indicated that the calcium content of the 14 samples varied substantially, ranging from 50 to 58 atomic percent. A crude but useful index of the cell dimension and composition was also obtained for all other dolomites from the patterns taken at 1° per minute, by determin- ing the d-spacing of the {222} reflection, using the main quartz peak at 3.3438A (angstroms) as the internal standard. The principal {211} peak of dolomite is so broad in these patterns that its spacing could not be measured as precisely as that of the {222} peak. The working curve that relates atomic percent of calcium (determined on the 14° per min patterns) and the Gs» spacing (determined on the 1° per min patterns) is given in figure 4, and compositions derived from this curve are listed in table 1. Even a crude estimate of the com- position is of much interest from the viewpoint of the thermal history of the samples, because dolomites con- taining more than 50 atomic percent of calcium (protodolomites) are unstable above about 200°C (Goldsmith, 1959). 60 - 55 |- CaCOs IN DOLOMITE, IN MOL PERCENT poul f cat apa aaa f 4 oa 4 aa aa 44 a} 2.670 2.680 2.690 2.700 doz SPACING, IN ANGSTROMS FrGurE 4.-Covariance of CaCO; content of dolomite and the d-spacing of {222}, Equation for the line is as follows : CaCO; (mol percent) =322.6 ( das-2.668) +-49.0. DIAGENETIC CARBONATES, MIOCENE Carbon and oxygen isotopic compositions were de- termined on carbon dioxide, which was obtained by de- composing the samples with 100 percent phosphoric acid (McCrea, 1950) and was analyzed in an isotope ratio mass spectrometer (McKinney and others, 1950). Prior to decomposition by acid, many of the samples were held at 300°C for an hour in a stream of helium in order to remove the more labile organic matter. Carbon isotopic composition is expressed relative to University of Chicago PDB standard (Craig, 1957), and oxygen isotopic composition, relative to SMOW (Epstein and Mayeda, 1953; Craig, 1961). The reproducibility of the determinations is +0.1 per mil. The analytical results are given in table 1. Where two or more samples are listed from a given locality, they are in stratigraphic order, the youngest sample being given first. The sampled beds represent from a quarter to a half of the total available and are usually spaced 30 to 100 feet apart with one or more unsampled beds in between; a few pairs of samples (such as Nos. 10 and 11, and 59 and 60) are from beds over 500 feet apart. Of the 79 samples listed in table 1, 60 represent dolomite beds or concretions; 1, vein dolomite; 1, dolomitic ce- ment of sandstone ; 8, calcareous fossils or Foraminifera- rich shales; 4, limestone lenses; 4, calcareous cement of sandstones ; and 1, vein calcite. PETROGRAPHY OF SAMPLES Photomicrographs of thin sections of a dozen samples of different textural type and isotopic compositions are presented in figure 5 in order to show the microscopic character of the carbonate, whose megascopic appear- ance was illustrated in figures 2 and 3. The individual samples are described in detail in the legends to the figures. The photomicrographs were made at a magnifi- cation of X100, because at this high magnification in- herited textures can be largely ignored and attention focused on the crystalline character of the diagenetic carbonate itself. Figure 5 shows clearly that diagenetic calcite and dolomite, though differing greatly in isotopic composi- tion, are commonly very fine grained with crystal diameters of the order of 5 microns (compare figs. 5A and 57). The lack of correlation between isotopic com- position and grain size of dolostone has already been emphasized by Weber (1964). The texture of dolomite sample 8, the hand specimen of which was illustrated in FORMATIONS, CALIFORNIA, OREGON B7 figure 3, is shown in figure 52 ; its variability is simply inherited from the original varved diatomite. Thin sections of the dolomitic and calcitic parts of a single bed are shown in figures 5C and 57, respectively. The coarser texture of the dolomitic part suggests that a con- siderable enlargement of crystal size sometimes occurs when fine-grained calcite is dolomitized. Figure 57 illustrates a type of dolostone that is made up of euhedral crystals of dolomite each with a nucleus of oily material or a gas, probably carbon dioxide. The carbonate-carbon of this dolomite is so light (8C#®=-11.7 per mil) that it suggests a genetic relation- ship to the light carbon of the nuclear material (Spotts and Silverman, 1966). Simple impregnation of a carbo- nate rock with tar or other organic matter, however, does not automatically introduce light carbon into the car- bonate mineral. Sample 21 of table 1 (+0.03, 28.4) is a tar-soaked coquina of Foraminifera which well illus- trates a lack of isotopic exchange between organic matter and carbonate. Although organic matter clearly has a role in determining the carbon isotopic composi- tion of many samples, no general correlation between concentration of organic matter and isotopic composi- tion could be established; this agrees with the results of Jeffrey, Compston, Greenhalgh, and De Laeter (1955). The state of oxidation of many samples can be judged roughly from the relative abundance of small blebs of authigenic pyrite and blebs of hematitic pseudo- morphs of the pyrite. However, no consistent relation- ship between the state of oxidation and the isotopic composition is apparent. Primary calcite still retaining to a large degree the isotopic composition of normal marine carbonate is exemplified by the barnacle shell in figure 57 and by foraminiferal debris in the shale in figure 5G. A type of secondary calcite, with isotopic characteristics indi- cating equilibration with fresh water, cements the grains of the marine sandstone shown in figure 57. The calcite consists of crystals as coarse as the sand grains and very irregular in shape (almost amoebiform). We have been able to attach genetic significance to only a few of the different petrographic features illus- trated in figure 5. The figure is presented mainly to show typical samples and to indicate concisely the ap- parent lack of correlation between petrographic char- acter and isotopic composition. B8 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY Ficur®E 5.-Photographs of thin sections. (Explanation on facing page.) DIAGENETIC CARBONATES, MIOCENE FORMATIONS, CALIFORNIA, OREGON B9 5.-Photomicrographs of thin sections of carbonate E. Calcitic shell sample 14 (-1.4, 27.7) of a barnacle is seen rocks, all taken with plain light at magnification of X 100. All in cross section along the bottom of the figure. Tiny sections were cut perpendicularly to the bedding. In the legend euhedral dolomite crystals (+7.0, 30.2) and opal make for each sample, the first of two numbers enclosed in paren- up the light-colored matrix; the matrix also contains theses is the carbon isotopic composition relative to the PDB a moderate amount of detrital minerals, such as those standard, and the second is the oxygen isotopic composition lying in the concavity of the shell. relative to SMOW. Sample numbers are the same as those F. Calcite sample 57 (+10.2, 27.3) from the same bed as used in table 1. dolomite sample 56 of C above. Fine-grained calcite with much brown organic matter. Foraminifera is filled with sparry calcite. This rock probably was precursor to the dolomite of C. A. Dolomite sample 33 (+20.8, 33.8). Fine-grained impreg- nation of a diatom-rich sediment; contains the heaviest carbon. . Calcareous shale sample 29 rich in foraminiferal debris B. Dolomite sample 8 (+19.4, 33.1). Vertical variation in (-0.69, 27.6) and disseminated opal. grain size in part of a varve layer was inherited from the H. Dolomite sample 28 (+11.1, 30.8) from a bed in the cal- original varved diatomite; rich in diatoms (elongate ob- careous shale of G. Unusually large dolomite crystals in jects in figure) and formless opal that fills the porous vuggy parts. parts of each varve. A hand specimen of this sample is I. Dolomite sample 66 (-11.7, 31.6). Moderately coarse illustrated in figure 3. grained; each dolomite crystal has a nucleus of an oily . material or of carbon dioxide. This type of dolomite C. Dolomite sample 56 (+12.5, 31.4). From same bed as was described in detail by Spotts and Silverman (1966). calcite sample 57 of F below. Upper part of bed shown in J. Calcite sample 44 (-14.4, 20.8), cement of marine sand- figure 2C. Moderately coarse texture. For'amlmfera‘m stone. Calcite crystals (gray) are as coarse as the sand lower left corner is partly dol.om1t1zed ar}d is filled with grains (white) and very irregular (almost amoebiform). sparry crystals of both dolomite and calcite. K. Dolomite sample 62 (-15.4, 34.3). Uniformly fine grained D. Calcite sample 51 (+ 15.2, 28.8). Fine-grained groundmass. rock moderately rich in remains of diatoms. Other parts Interior of diatoms and Foraminifera are filled with have many dolomitized tests of Foraminifera. sparry calcite, chalcedony, or hematite probably after L. Dolomite sample 78 (-25.3, 34.6). Fine-grained dolomite pyrite. Dolomite of A above may have had calcite like enclosing many detrital minerals along with blebs of this as a precursor. pyrite and organic material; contains the lightest carbon. TaBus 1.-Carbon and oxygen isotopic composition of diagenetic carbonates and calcium carbonate content of diagenetic dolomites in Miocene rocks of Oregon and California and of comparison materials [Samples 1, 2, and 3 are from Oregon; samples 4 through 79, from California. Carbon isotopic results are relative to University of Chicago PDB standard; oxygen isotopic results are relative to SMOW; CaCO; results are in mol percent] Sample - Lab. Locality Latitude and longitude Formation Rock Hed 508 - CaCO%in Reference No. per mil per mil dolomite 1.:...... 3277-4 Yaquinéa Bay, Lincoln - 44°38" N., 124°01' W..... Nye Mudstone.... Dolomite bed 1........... +5. 8 32.7 55 Srisgfly and others, ounty. . AAsivete 8277-5 _.... oc _....dolll... evo -|-7.8 33. 2 53 Do. 8277-6 ___.. OOL GOL 22 +11. 0 34.0 51 Do. |_. ] When ooo 3277-3 - Point Arena, Mendocino 38°52" N., 123°39' W..... Skooner Gulch Dolomite cement of -9. 5 29. 3 57 W. O. Addicott, 1967. County. Formation. sandstone. 3279-1 Daakes tBay, Marin 38°02" N., 122°58' W.. __. Uilfiamed upper - Dolomite bed....._____.__ +11. 3 31.9 51 Galloway, 1962, pl. 26. ounty. ocene. . 3279-7 - Bolinas Point, Marin 37°54" N., 122943" W.. __. Monterey Shale... Dolomitized porcelanite... +10.2 35. 4 50 Jennings and Burnett, County. 1961; Clark Blake, oral commun., June 1967. $e 8277-1 Bigkeéeychi Contra - 37°52" N., 122°14' W..... Claremont Shale.. Dolomite bed 1... ....._._. 417.7 32.9 49 - Lawson, 1914, pl. 7., osta County. Sisssises 3279-30 ..... CO2 2 ........... +19. 4 33.1 51 Do. 9. cam i.. 8277-2 ..... 10. . 22222 seee del do ..do -_ 4 33.1 52 Do. 10.2 -.. 3279-2 - Santa Cruz Mountains, . 23.6 58 Cummmgs and others, Santa Cruz County. 1962. 3279-3 __... O22 do do +9. 6 30. 4 52 Do. 12..¢iv... 3277-31 A531) Igluego Pfimt San 37°07 N., 122°20' W..... Monterey Shale... Dolomite bed ............. +6. 4 30. 4 52 J enrgings and Burnett, ateo County 18...;...: 3277-23 Reliz Canyon, 36°13" N., 121°17" W.__._. Pancho Rico Dolomite cement of +7.0 30. 2 57 Durham, 1963; Dur- Monterey County. Formation coquina. ham and Addxcott (Pliocene). 1965. 14........ 3279-28 _.... ccc ccc QL 22222 ccc do............. Calcitic shell of Balanus -1. 4 2M. T Do. in coquina. Monterey Shale... Dolomite bed 1... +16. 5 30. 3 B1 Do Me - 31. 3 52 Do 30. 0 52 Do 30.9 52 Do 31. 2 53 Do s 6 32.3 52 Do Foraminiferal coq: +0. 03 28. 4 122000000. Do ber of Monterey Shale. 277-15 ..... lll ccc do.._._________ Foraminiferal shale........ 28.0 Do --- .. do... h 31.9 52 Do 8277-18 ._... do.......... _.. do... 31.3 52 Do secs ees 8277-11 ..... do.......... ._. do... 27.1 55 Do 8277-12 ..... cc Q2 ccc ccc lll cc do.._..________ - Vein calcite in bed 9.0.0 19.5 ccccllccl! Do 334-795 0O-69--3 B10 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY TaBu® 1.-Carbon and oxygen isotopic composition of diagenetic carbonates and calcium carbonate content of diagenetic dolomites in Miocene rocks of Oregon and California and of comparison materials-Continued Sample - Lab. Locality Latitude and longitude Formation Rock 50H 508 _ CaCO; in Reference No. per mil per mil - dolomite MF.. 3277-25 Nacimiento Dam, San - 35°45" N., 120°52" W...... Sandholdt Mem- - Dolomite bed 1............ +7.1 28. 9 56 Jennings, 1959. Luis Obispo County. gar 1of Monterey ale. 8277-26 ._... ens ecs cns Go.._.____._..__. 411.1 30. 8 54 Do. do. - Foraminiferal shale -O. 69 TO i:. cscs Do. do. - Dolomite bed 3... 29. 3 55 Do. do. ® §... 31.5 53 Do. ¥ do cca ese .n Foraminiferal shal 2A Do. 89........ . 8279-12 Chico Martinez Creek, - 35°26" N., 119°47 hico Martinez i 33.8 49 * Woodring and others, Kern County. Chert of 1940; McMasters, McMasters 1947, fig. 36. (1947). 34........ 3270-13 ..... do....__....... 32.0 49 Do. 3270-14 do. - 34.0 50 Do. 3279-15 do. 2 32. 4 52 Do. 3279-16 do. a 34. 6 51 Do. 3279-17 do. 2 33.1 50 Do. 3279-18 do. 32.5 51 Do. 40 3279-19 do. 8 32.9 49 §1........ 3279-20 Round Mountain 0 35°28" N , 118055 W._____ ound Mountain - Calcite cemented sand- -13. 0 29.7. L.. Godde, 1928; Addi- field, Kern County. Silt of Diepen- stone. cott, 1965. brock (1933). 42........ 8279-21 ..... 07. 4... sso unl ede a Limestone bed............ -14.7 80.8 ....-.«..- Do. 49.......2 8270-22 .._... o.... 35°20" N., 118950" W...... Jewett Sand of Calcite cemented sand- -15.8 $L1 Do. Godde (1928). stone 1. 3270-23 ..... lll G.. cinc ene nees enne ec ives Caéclte gemented sand- -14. 4 20.8 Do. stone 45.2.0... 3279-20 ___.. 00000000022 even.. ccc ccc Calcitic shell of pecten in +2.9 80. 5 Do. sandstone 2. 46........ 3276-18 Temblor Range, San 35°05" N., 119°35" W...... Monterey Shale... Dolomite bed............. -|-19. 3 31. 6 50 J. G. Vedder, oral Luis Obispo County. commun., Septem- ber 1967. 47........ 3279-26 Pismo syncline area, 35°13" N., 120°42" W..... Obispo Tuff Dolomite-impregnated -17.3 26.5 49 Halland Surdam, San Luis Obispo Member of tuff. 1967. County. Monterey For- mation. 3279-24 ___.. CQ 35°10' N., 120°42" W ._... Monterey Shale, _ Dolomite concretion...... -2.7 27.0 49 Do. immediately underlying Pismo Forma- tion. 49... ___.. 3279-25 ._... do 35°09'° N., 120°41" W..... Monterey Shale... Dolomite bed in tidal -10. 2 27. 4 53 Do. zone. 50....._... 3279-27 Cuyama Valley, Santa | Santa Margarita, _ Limestone lens...._....... -10.1 24.0 122220000. J. G. Vedder, oral Barbara County. Sandstone. commun., Septem- ber 1967. Bl........ 3276-15 San Rafael Mountains, - 34°47 N., 119°36' W..... Monterey Shale... Limestone lens 1...._..... 415. 2 28.8 Do. Santa Barbara County. §2........ 3276-16 ..... do..... 34°47! N., W.. ..___._. do............ Pee -4+-5.3 246 :......... Do. . 8276-17 .._. O0. seee 34°47" N., 119941" W - Dolomite bed............. -12.5 32.6 56 Do. 3279-9 Santa Ynez River area, 34°35 N., 1120023" W.2 22220... Dolomite bed (100 ft -13.1 30.6 55 Dibblee, 1950. Santa Barbara thick). County. B6........ 3279-10 ._... OO 34°35" N., W......_... d0.c.......... Dfilgfiltfifed reef rock (40 _ -14.6 28.0 56 Do. $6........ 3279-32 Pine Mountain, Ven- 34°38" N., 119°23' W.. ... Monterey Shale, _ Dolomitic upper part of +12.5 31. 4 55 Dickinson and Lowe, tura County. muddy phase bed. 1966. 3270-33 ..... G-... 0 Len cas Calcitic lower part. ....... +10. 2 PNB ..... Rel Do. §8........ 3279—6 Point Dume, Los 34°01" N., 118047" W.. ._. Monterey Shale... Dolomite bed 1........... 410.1 34.9 55 Durrell, 1954; R. H. Angeles County. Campbell, oral commun., Septem- ber 1967. 3270-5 ..... Q2 cence neden ecienes e rues UO 222 d0...-..i..... Aer +10. 2 34.2 53 Do. 60........ 3279-4 ..... 0 .es do........... _do 3.. 10 -5.6 30. 2 55 3276-3 _ Palos Verdes Hills, 33°48" N., 118°24" W...... Repetto Siltstone - Foraminiferal sand________ -0. 52 $0.8 Woodrlng and others, Los Angeles County‘ (lower Pliocene). 1946 62........ 3276-5 .._. 00. 2. cers ln nel O2 Malaga Mudstone Dolomite bed_...._....... -15.4 34.3 56 Do. Member of Monterey Shale. 63......_. 3276-10 ___.. 33°48" N., 118°21" W...... Valmonte Diato- _ Dolomitized diatomite -8.5 33.3 54 Do. mite Member of (soft). Monterey Shale. 64.1.1... 3276-2 ___. oc 33°47" N., 118°20' W. ._.. Lomita Marl Foraminiferal sand...__... -3.3 PTB Do. Member of San Pedro Forma- tion (lower Pleistocene). ©5........ 3276-11 ._... Q2 33°45" N., 118022" W _ ___. Valmonte Diato- _ Dolomite bed 1........... -8.3 34.1 55 Do. mite Member of Monterey Shale. 66........ 3276-6 ___.. 02. 222 lieben Q2 22 nels do............ 2 (8 ft -11.7 31.6 52 Do. thick). 8277-1 ___.. Q. . cs peng ean Q9. . c. incense nse. do............ Dolomite bed 3 ___________ -6.5 38. 4 52 Do. 68...._._. 3276-12 _____ A9. . ...... sellin inns doot Q2 Upper part of _ _- 42222222220. -5.2 32.6 55 Do. Altamira Shale Member of Monterey Shale. 60;:....... 3276-7 ___. G. L1. ce ences | een te ie. B. -8.8 32. 4 57 Do. 8276-13 ___.. OOL O2 Middle part of -11.3 30.9 56 Do. Altamira Shale Member of Monterey Shale. Ti........ 8276- ._.. D. 1 eli cic ii Q. . .ll lvn css nene nen ess do......._...- 7 (dark -10.5 28.1 50 Do. 14D core). Te ere 8276-0 0 2000. AB. . . . . ccc lled ine ovens. O2 do. Dolomite bed 7 (bleached _ -10.2 28.5 50 Do. 14L exterior). 18........ 8276-8 ____. O2 QL Len ie eee ene nene Dolomite bed 8........... -6.8 28.9 50 Do. 74...._... 8276-9 ____. 33°45" N., 118°23" W...______.. Ag.. Dolomite cement of glassy . -14.9 28. 4 50 Do. tuff. DIAGENETIC CARBONATES, MIOCENE FORMATIONS, CALIFORNIA, OREGON B11 TABLE 1.-Carbon and ozygen isotopic composition of diagenetic carbonates and calcium carbonate content of diagenetic dolomites in Miocene rocks of Oregon and California and of comparison materials-Continued Sample - Lab. Locality Latitude and longitude Formation Rock 508 50® _ CaCO®% in Reference No. per mil per mil dolomite 8276-1 ___. dou 22222222222222222 33°44" N., 118°21' W ___. Intrusive basalt... Drusy dolomite in frac- -18.1 25.1 54 Do. ures. 76.00.22. 3276-4 - San Juan Capistrano, 33°35 N., 117°40' W. ___. Niguel Formation Dolomite cement of sand- - -15.3 33.5 53 Vedder and others, Orange County. (upp)er Plio- stone. 1957. cene). 3279-8 - San Clemente Island.... 33°00' N., 118°33' W ___.. Monterey Shale... Dolomite bed............. -1. 8 33.0 57 Olmstead, 1958. 3279-11 La Jolla submarine 32°55" N., 117°34' W. ._.. Cored sediment _ ..... oc -25.3 34.6 51 Inman and Goldberg, canyon. 1 +5 per mil). Using the average geothermal gradient of the Los Angeles basin (89.1°C per kilometer according to Philippi, 1965) for illustration, these temperatures would corre- spond to depths of 1,100 to 10,000 feet. As the 8C" of the carbonate becomes more negative, the indicated tem- perature becomes higher; carbonate with 8C" of -4.0 per mil and methane with 3C" of -38 per mil would be at equilibrium at 192°C, corresponding to a depth of about 14,500 feet. Judging from the temperature data on couplets of methane and carbon dioxide from oil field gas (fig. 10), very few, if any, of our carbonates would have been subjected to temperatures higher than about 190°C. The conclusion seems inescapable that carbonates with 3C" more negative than about -5 per mil could not have equilibrated with much methane. They, therefore, con- stitute the separate class of light-carbon carbonate pre- viously discussed, whose carbon was largely derived through nonequilibrium oxidation of organic matter and whose carbon isotopic composition has no relevance to the temperature of formation. An upper limit of about 190°C would also be in harmony with the fact that a great many of the dolomites hold excess calcium, which would exsolve at around 200°C (Goldsmith, 1959). B20 The computed temperatures of 34° to 140°C in the tabulation given above and the correlative depths of about 1,100 to 10,000 feet are only crude indications of the conditions under which these dolomites could have formed. In detail, no systematic variation of 3C" with stratigraphic position is observed at the two localities concerned. At the Berkeley Hills (samples 7 to 9), the sampled interval was only about 100 feet, correspond- ing to a temperature difference of only 1°C along a normal geothermal gradient, whereas the spread of tem- perature indicated isotopically (assuming methane of constant composition) is as high as 26°C. At Chico Martinez Creek (samples 33 to 40) the sampled section was 2,200 feet thick, which would allow a temperature contrast of around 25°C; the difference between mini- mum and maximum carbon-isotopic temperature is 41°C. These apparent temperature discrepancies among heavy-carbhon dolomites are resolved if it is assumed that equilibration between methane and a normal marine carbonate either proceeded at variable rates or involved natural gas with variable content of methane. Such con- ditions could yield carbonates manifesting all composi- tions between some maximum value of 3C for the lo- cality and the value of around zero for the original marine carbonate. Thus, the spread of isotopic values at each place would be a measure of the kinetic or con- centration limitations on isotopic exchange rather than an indication that the exchange process had occurred at widely different temperatures. The heavy carbonates under consideration in this see- tion (points in envelope 6 in fig. 64) contain heavy oxygen, with 30® of 32.0 to 34.6 per mil and an av- erage of 33.2 per mil-values which would limit the maximum temperature to about 45°C (according to fig. 8) if oxygen isotopic equilibrium were also to be con- sidered. In this case, 3C" of methane must lie between -52 and -48 per mil; 30 of formation water, be- tween +7 and +9 per mil ; and the temperature, between 34° and 45°C, as indicated in the tabulation given above. Only 3C" values between +17 and +21 per mil in car- bonates would then represent equilibrium compositions. These conditions are so restrictive that we rejected at- tainment of oxygen isotopic equilibrium as a necessary condition in the final equilibria involving these carbonates. CARBONATE WITH VARIABLE HEAVY CARBON AND COVARIABLE OXYGEN Examples of this isotopic category are grouped to- gether in the upper right side of figure 6B. Group d; consist of samples of a slightly variable dolomite from the Monterey Shale of Reliz Canyon (samples 15 to 20 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY of table 1). Its average 80® value of 31.0 per mil is too light for the group to have been included in the heavy- carbon heavy-oxygen group of figure 64 at b. Group d; consists of samples from a lower member of the Monte- rey Shale in Reliz Canyon and in a section near Naci- miento Dam (samples 23 to 25, 27, 28, 30, and 31 of table 1); these samples are isotopically more variable than those of group d,, carbon varying directly with oxygen. Paleotemperatures of the four samples with the heavi- est oxygens in group d, were discussed on page B11. Other representatives of this isotopic category are three from Oregon (samples 1 to 3) and pairs of samples (Nos. 10 and 11, 51 and 52, 56 and 57, and 59 and 60) from several localities. A simple explanation for the covariation of carbon and oxygen isotopic composition would be to have these carbonates undergo exchange reactions with meteoric water. The presence of late fresh-water calcite (sample 26) in cracks in dolomite sample 25 would corroborate the view that this dolomite was more affected by meteoric water than any others of group G., and that the isotopic trends within the group were due to the variable degree of equilibration of an originally heavy-carbon heavy- oxygen carbonate with meteoric water. The same action of meteoric water was already used on page B11 to explain the discordant paleotemperatures within the dolomite-calcite pair of samples 56 and 57. However, the calcium content of the dolomites under consideration in this section cannot be reconciled with the postulated reaction with meteoric water, as will be shown next. CALCIUM CONTENT OF DOLOMITE AS RELATED TO ISOTOPIC COMPOSITION The calcium content of the dolomites is plotted against the carbon isotopic composition in figure 11, and several significant relationships are apparent. Calcium appre- ciably in excess of the 50 mol percent of ideal dolomite is common among both heavy-carbon and light-carbon dolomites. Graf and Goldsmith (1956) and Goldsmith (1959), who gave the name protodolomite to dolomites of such nonideal composition, have shown that proto- dolomite is a widespread and persistent metastable phase in sedimentary rocks. It can be made to invert to stable ideal dolomite by heating to temperatures greater than about 200° C. Among the samples from Palos Verdes Hills (black circles in fig. 11) only samples 72 to 74 are ideal dolo- mites. Previously, in figure 7, these very samples were seen to have deviant oxygen compositions-deviations which were explained in terms of possible heating effects of nearby igneous sills; the same effects could have accelerated the inversion to normal dolomite. Otherwise, no relationship between isotopic and calcium DIAGENETIC CARBONATES, MIOCENE +22 +20 +16 +12 +8 +4 aC'3 PER MIL (PDB) -20 -24 78 O |ll|l|ll|||l|ll||Ill|l|ll|ll|lll|l|||l|||l||]|ll[‘|_| -28 L 1 I 1 1 1 1 1 1 1 I 49 50 51 52 53 54 55 56 57 58 59 CaCOs3 IN DOLOMITE, MOL PERCENT FIGURE 11.-Carbon isotopic composition as related to the CaCO; content of a diagenetic dolomite. Groups di and d: are the same as those so labeled in figure 6B. Numbered triangles, squares, and open circles that are linked by tie lines denote the same dolomite samples as those linked by the dashed lines in figure 6B. Black circles represent samples from Palos Verdes Hills; open circles without tie lines, samples from all other localities. compositions is apparent among light-carbon dolomites in figure 11. Among the heavy-carbon dolomites in the upper half of figure 11, group d; outlines an inverse trend between 8C® and the calcium content. Other heavy- carbon dolomites (samples 1 to 3, 10 and 11, and 59 and 60) also show the same trend. The increase in calcium content with decrease in 8C" might be interpreted to mean that ideal dolomite alters into protodolomite in the zone of weathering-a direct contradiction of earlier work (Graf and Goldsmith, 1956), which indicated that ideal dolomite, not protodolomite, is the stable phase under near-surface conditions. 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C., 1959, C*-Altersbestimmung von Siisswasser-Kalkablagerungen: Naturwissenschaften, v. 46, pt. 5, p. 168-169. Murata, K. J., and Erd, R. C., 1964, Composition of sediments from the experimental Mohole project (Guadalupe site) : Jour. Sed. Petrology, v. 34, no. 3, p. 633-655. Murata, K. J., Friedman, I. I., and Madsen, B. M., 1967, Carbon- 13-rich diagenetic carbonates in Miocene formations of Cali- fornia and Oregon: Science, v. 156, no. 8781, p. 1484-1486. Nakai, Nobuyuki, 1960, Carbon isotope fractionation of natural gas in Japan: Nagoya Univ., Jour. Earth Sci., v. 8, no. 2, p. 174-180. Natland, M. L., 1957, Paleéoecology of West Coast Tertiary sedi- ments, in Ladd, H. S., ed., Treatise on marine ecology and paleoecology, Vol. 2, Paleoecology : Geol. Soc. America Mem. 67, p. 543-571. Northrup, D. A., and Clayton, R. N., 1966, Oxygen-isotope frac- tionations in systems contaihing dolomite: Jour. Geology, v. 74, no. 2, p. 174-196. Oana, Shinya, and Deevey, E. S., Jr., 1960, Carbon 13 in lake waters, and its possible bearing on paleolimnology: Am. Jour. Sci., v. 258-A (Bradley Volume), p. 253-272. Olmstead, F. H., 1958, Geologic reconnaissance of San Clemente Island, California : U.S. Geol. Survey Bull. 1071-B, p. 55-68. O'Neil, J. R., and Epstein, Samuel, 1966, Oxygen isotope frac- tionation in the system dolomite-calcite-carbon dioxide: Science, v. 152, no. 3719, p. 198-201. Ovsyannikov, V. M., and Lebedev, V. S., 1967, Isotopic carbon compositions of gases of biochemical origin: Geokhimiya, 1967, no. 5, p. 537-542. B24 Park, R., and Epstein, Samuel, 1961, Carbon isotope fractiona- tion during photosynthesis: Geochim. et Cosmochim. Acta, v. 21, nos. 1-2, p. 110-126. Philippi, G. T., 1965, On the depth, time, and mechanism of petroleum generation; Geochim. et Cosmochim. Acta, v. 29, no. 9, p. 1021-1049. Pray, L. C., and Murray, R. C., eds., 1965, Dolomitization and limestone diagenesis, a symposium: Soc. Econ. Paleontol- ogists and Mineralogists Spec. Pub. 13, 180 p. Rankama, Kalervo, 1963, Progress in isotope geology : New York, Interscience Publishers, Inc., 705 p. Riedel, W. R., Ladd, H. S., Tracey, J. L., Jr., and Bramlette, M. N., 1961, Preliminary drilling phase of the Mohole proj- ect-Pt 2, Summary of coring operations (Gaudalupe site) : Am. Assoc. Petroleum Geologists Bull., v. 45, no. 11, p. 1793-1798. Rittenberg, S. C., and others, 1963, Biogeochemistry of sediments in experimental Mohole: Jour. Sed. Petrology, v. 33, no. 1, p. 140-172. Rittenhouse, Gordon, 1967, Bromine, in oilfield waters and its use in determining possibilities of origin of these waters: Am. Assoc. Petroleum Geologists Bull., v. 51, no. 12, p. 2430-2440. Russell, K. L., Deffeyes, K. S., Fowler, G. A., and Lloyd, R. M., 1967, Marine dolomite of unusual isotopic composition : Science, v. 155, no. 3759, p. 189-191. Rye, R. O., 1966, The carbon, hydrogen, and oxygen isotopic composition of the hydrothermal fluids responsible for the lead-zine deposits at Providencia, Zacatecas, Mexico: Econ. Geology, v. 61, no. 8, p. 1399-1427. Sackett, W. M., 1964, The depositional history and isotopic carbon composition of marine sediments: Marine Geology, v. 2, p. 178-185. Schleicher, D. L., 1965, Emplacement mechanism of the Miraleste tuff, Palos Verdes Hills, California: University Park, Pennsylvania State Univ., Ph. D. thesis, 62 p. Siever, Raymond, Beck, K. C., and Berner, R. A., 1965, Com- position of interstitial waters of modern sediments: Jour. Geology, v. 73, no. 1, p. 39-73. Silverman, S. R., 1964, Investigations of petroleum origin and evolution mechanisms by carbon isotope studies, Chap. 8 in Isotopic and cosmic chemistry: Amsterdam, North-Hol- land Publishing Co., p. 92-102. Smith, P. B., 1964, Recent foraminifera off Central America. Ecology of benthonic species : U.S. Geol. Survey Prof. Paper 429-B, p. B1-B55. Snavely, P. D., Jr., Rau, W. W., and Wagner, H. C., 1964, Miocene stratigraphy of the Yaquina Bay area, Newport, Oregon: Ore Bin, v. 26, no. 8, p. 133-151. Spotts, J. H., and Silverman, S. R., 1966, Organic dolomite from Point Fermin, California: Am. Mineralogist, v. 51, no. 7, p. 1144-1155. SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY Thode, H. G., Wanless, R. K., and Wallouch, R., 1954, The origin of native sulphur deposits from isotope fractionation studies: Geochim. et Cosmochim. Acta, v. 5, no. 6, p. 286-298. Trask, P. D., 1932, Origin and environment of source sediments of petroleum: Houston, Tex., Am. Petroleum Inst., Gulf Publishing Co., 323 p. Urey, H. C., Lowenstam, H. A., Epstein, Samuel, and McKinney, C. R., 1951, Measurement of paleotemperatures and tem- peratures of the Upper Cretaceous of England, Denmark, and the southeastern United States: Geol. Soc. America Bull., v. 62, no. 4, p. 399-416. Vedder, J. G., Yerkes, R. F., and Schoellhamer, J.. E., 1957, Geologic map of the San Joaquin Hills-San Juan Capi- strano area, Orange County, California: U.S. Geol. Survey Oil and Gas Inv. Map OM-193, scale 1 : 24,000, Vogel, J. C., 1959, Uber den Isotopengehalt des Kohlenstoffs in Siisswasser-Kalkablagerungen : Geochim. et Cosmochim. Acta, v. 16, no. 4, p. 236-242. Wasserburg, G. J., Mazor, E., and Zartman, R. E., 1963, Isotopic and chemical composition of some terrestrial natural gases, in Earth science and meteoritics: Amsterdam, North-Hol- land Publishing Co., p. 219-240. Weber, J. N., 1964, Carbon isotope ratios in dolostones-Some implications concerning the genesis of secondary and "pri- mary" dolostones: Geochim. et Cosmochim. Acta, v. 28, no. 8, p. 1257-1265. Weber, J. N., Bergenback, R. E., Williams, E. G., and Keith, M. L., 1965, Reconstruction of depositional environments in the Pennsylvanian Vanport basin by carbon isotope ratios : Jour. Sed. Petrology, v. 35, no. 1, p. 36-48. White, D. E., 1965, Saline waters of sedimentary rocks, in Fluids in subsurface environments-A symposium: Am. Assoc. Petroleum Geologists Mem. 4, p. 342-366. White, D. E., Hem, J. D., and Waring, G. A., 1963, Chemical composition of subsurface waters, in Fleischer, Michael, ed., Data of Geochemistry [6th ed.]: U.S. Geol. Survey Prof. Paper 440-F, p. F1-F67. Woodring, W. P., Bramlette, M. N., and Kew, W. S. W., 1946, Geology and paleontology of Palos Verdes Hills, California : U.S. Geol. Survey Prof. Paper 207, 145 p. Woodring, W. P., Stewart, Ralph, and Richards, R. W., 1940, Geology of the Kettleman Hills oil field, California-stratig- raphy, paleontology, and structure: U.S. Geol. Survey Prof. Paper 195, 170 p. Yerkes, R. F., McCulloh, T. H., Schoellhamer, J. E., and Vedder, J. G., 1965, Geology of the Los Angeles basin, California- an introduction: U.S. Geol. Survey Prof. Paper 420-A, p. A1-A57. Zartman, R. E., Wasserburg, G. J., and Reynolds, J. H., 1961, Helium, argon, and carbon in some natural gases: Jour. Geophys. Research, v. 66, no. 1, p. 277-306. U. S. GOVERNMENT PRINTING OFFICE ;: 1969 O - 334-795 s 3C" 7 pay The October 1963 Eruption of Kilauea Volcano Hawan GEOLOGICAL SURVEY PROFESSIONAL PAPER 614-C The October 1963 Eruption of Kilauea Volcano Hawa By JAMES G. MOORE and ROBERT Y. KOYANAGI SHORTER CONTRIBUTIONS FO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPEE 614-=C UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969 UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 € C r c la C] r CONTENTS N I Page k LLL. IC: Lirc. cual ioe rect bana eo- aid ae a ele ble aan o aln aie a nae a an C1 Introduction -_ 220. ital il nl lel cull EUT Ue bane tle's aie bee a ain e an aln a an mie a= o a a inin io male 1 Distribution of volcanic activity on the east rift sone 1 Events of 1963 preceding the 5 L Description of the eruption.. _ l eles tal 5 Lava flows.. :l 0.0L IPL -use een nly I wana none ma wis ae aln leo ao ie alk a o ain ole ail ae 7 - Earthquakes, tremor, and ensue bus sek eee 10 Petrology of the aun ane ss 11 ita.2.... l snare - Metanet ae 13 %o a ILLUSTRATIONS Page Prats 1. Geologic map of the central part of the east rift zone of Kilauea In pocket FiaurE 1. Map showing the summit region of Kilauea Volcano. C2 2. Diagram showing the longitudinal distribution of eruptive vents on the east rift zone of Kilauea Volcano from a 1954 to 10965... n. erence ll id aes our ainia an rhe te a ang Cine a e alte aan mind ak aie aa o alee aio hee a ale onle ale 3} 3. Graph showing relations of Kilauean eruptions and collapses, ground tilting, and earthquakes-___-_--_----- ---. 5 4. Graph showing chronology of events during the October 1963 Kilauea east rift eruption-__------------------- 6 5. Oblique aerial photograph of eruptive vents extending east of Napau Crater 8 6. Oblique aerial photograph of lava flow in Napau Crater 9 R ® TABLES Page CABLE 1. Area and volume of lava erupted in: October 1963....- .._... ___.... oe cae CZ | 2. Lime and iron content of recent lavas erupted on the summit and east rift zone of Kilauea-________-_-__-------- 11 | 3. Chemical analyses, norms, and modal analyses of basalts from October 1963, eruption of Kilauea Volcano and i adjacent pars raun ce bem as abs an aio aln een cul on bake - oo ne 12 III SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY THE OCTOBER 1963 ERUPTION OF KILAUEA VOLCANO, HAWAII By Jaxmrs G. Moor® and RosErt Y. Kovyanact ABSTRACT The eruption of October 5-6, 1963, occurred along an 8-mile section of the central part of the east rift zone of Kilauea Volcano. About 9 million cubic yards of lava was erupted from more than 30 fissures which show a slight right-offset en echelon pattern; the new lava covered an area of 1.3 square miles. A few hours before the actual outbreak, the summit of Kilauea began to subside, and strong harmonic tremor and earthquakes commenced at both the summit and the site of the later activity near Napau Crater. These phenomena were apparently caused by subsurface flow of magma from the sum- mit reservoir through the rift zone conduits to the eruptive vents 8 miles distant. The lava of the eruption is a tholeiitic basalt with an average of 5.6 percent olivine. In general, lavas that erupted toward the eastern end of the eruptive zone are richer in olivine. These lavas, like others that erupted in historic times on the rift zone, show a slight differentiation when compared with lavas that erupted from the summit. The differentiation, apparently caused by cooling and crystallization within the rift zone, can be measured by the ratio Fe,0,), which is greater than 1 for summit lavas and which decreases systematically for lavas that erupted progressively eastward along the rift zone. INTRODUCTION Kilauea Volcano erupted on its upper east rift zone (fig. 1) on October 5-6, 1963, just 43 days after the last eruption, which occurred at Alae Crater 3 miles west of the region affected in this eruption. The eruption oc- curred along an 8-mile section of the central part of the rift zone extending from near Napau Crater east- ward to Kalalua Crater. This activity is the fourth con- secutive small eruption along the east rift zone during the past 2 years, and it marks a curious departure from the usual pattern of one or more summit eruptions fol- lowed by a flank eruption. During the eruption, lava flowed from more than 30 fissures that are arranged in a right-offset en echelon pattern. Some flows are more than half a mile wide, and the total area covered by new lava is 1.3 square miles. The volume of the erupted lava is about 9 million cubic yards (disregarding lava which flowed back down cracks). Although this volume exceeds that of the pre- vious three flank eruptions (September 1961, December 1962, and August 1963), it is small compared with earlier flank eruptions. The eruption occurred in a remote area distant from roads and overgrown with dense tropical vegetation. Most of the observations during the eruptive activity were made from the air, and subsequent fieldwork was limited owing to the inaccessible terrain. Sampling of the flow was accomplished with the aid of a light helicopter. This report includes a summary of observations and data collected by the staff of the Hawaiian Volcano Observatory, whose contributions and help are grate- fully acknowledged. The Hawaii Army National Guard provided air reconnaissance support, and personnel of Hawaii Volcanoes National Park contributed data and observations. DISTRIBUTION OF VOLCANIC ACTIVITY ON THE EAST RIFT ZONE Most of the mapping shown on plate 1 has been done with the aid of aerial photographs, because roads do not extend east of Makaopuhi Crater, trails are widely sep- arated, and the region is overgrown by a tropical forest that thrives on an annual rainfall of 100-200 inches. Fortunately a series of photographs were taken in 1954, and additional photographic coverage of varying qual- ity was made after each successive flank eruption. The older lava flows of the rift zone have been di- vided into three map units which are based on the char- acter of the covering vegetation. The oldest unit, early prehistoric basalt, includes flows whose forest cover is the most mature. Ohia trees average about 30 feet in height, and beneath the trees a canopy of tree ferns from 10 to 15 feet high obscure the ground from the air. There is little surficial material except for ash and spatter near vents, and the lava surface is generally fresh although locally it is covered with a few inches of dead vegetation and a tangle of fallen trees. The early C1 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY EXPLANATION A Seismograph station @-» Tilt station Showing tilt vector, August 8 to October 8, 1963 Fault Dashed where approximately located C» Volcanic cone or crater (hachured) 0332-0544 October 5 1207 October 6 - 2128 October 7 O 1026 October 5 - 0928 October 6 2131 October 7 - 0853 October 9 1001 October 6 - 1156 October 6 Epicenters of earthquakes of magnitude 1.5-3.5 Arranged in time sequence. Earthquakes beneath Kilauea Caldera not included 0 40. 60 0 1 2 3 4 MILES Lae cd _L ouf cae f __ d Tilt vector scale, in microradians per month 155°15' 155°05' I &" a is- ne o, euro =- O rater a + (349: O EAST O 19° {Desert 20'| - "4 # 7% O poo 0 FIGURE 1.-Summit region of Kilauea Volcano. Geology modified from Stearns and Macdonald (1946). THE OCTOBER 1963 ERUPTION OF KILAUEA VOLCANO, HAWAII prehistoric basalt is considered to be younger than ap- proximately 10,000 years because in no place is it cov- ered by the Pahala Ash, a widespread ash unit which has a radiocarbon date of 10,000-17,000 years old (Ru- bin and Berthold, 1961). Moreover, the Pahala Ash has not been identified in any of the pit craters and so is presumed to underlie the several hundred feet of lava flows exposed in the pit crater walls. The next younger unit, late prehistoric basalt, is in most places densely covered by small to medium ohia trees 10-20 feet high. Judging from this vegetation, the late prehistoric basalt is assumed to be older than about 300 years and younger than a few thousand years. The pit craters within the mapped area probably formed during this period because several of them (Napau Crater, the small crater southeast of Napau Crater, and Makaopuhi Crater) cut this unit, and lava of this unit fills the older east pit of Makaopuhi Crater. The next younger unit, very late prehistoric and early historic basalt, is overgrown by small trees that are up to 10 feet high and by low staghorn ferns a few feet high. This unit includes lava flows known to have been erupted in 1840 and others that have similar vegetative cover. Throughout historic time, the east rift zone has been the more active of Kilauea Volcano's two rift zones. Fifteen eruptions have been reported on the east rift zone between about 1750 and 1965, and only three on the southwest rift zone (Stearns and Macdonald, 1946). Information on early eruptions is scanty, but probably more unreported eruptions broke out along the east rift than along the southwest rift because it is more inac- cessible, covered with heavier vegetation, and more likely to have a heavy cloud cover. For convenience, the location of the historic eruptive vents is projected onto a straight line 32 miles long drawn from the center of the principal vent of Kilauea Caldera (Halemaumau) to the east cape of the island (fig. 2). Distances are measured along this line from Halemaumau. The October 1963 eruptive fissures are on the 8- to 14-mile segment of this line. The geologic map (pl. 1) includes the 6- to 19-mile segment of the rift zone. The earliest historic activity along the east rift zone, the eruptions of 1750 (*) and 1790 ( 2), is recorded main- ly in Hawaiian folklore, and details of these eruptions are only assumed. The 1750( ?) lava was erupted from a vent at the 19-mile point of the rift zone just beyond the east end of plate 1. The 1790( ?) vents were located along the 21- to 23-mile segment. Records of the next recorded eruption, in 1840, are incomplete. Lava was erupted along the 5- to 9-mile seg- ment; eruptive vents occurred within Alae Crater, C3 | north and east of Makaopuhi Crater, and in and north | of Napau Crater. The greatest volume of lava, however, | was extruded from vents farther east along the 20- to 24-mile segment; much of this lava poured into the sea. According to old records (Dana, 1849, p. 189) natives reported additional activity in the interior between the 5- to 9-mile and 20- to 24-mile active segments of the rift zone. Aerial photographs of the region show several lava flows of similar age (based on vegetation) in the intermediate zone, and these flows have been tentatively 19°45" 156 10 20 MILES 19°00" 1953 1954 1955 1956 1957 1958 1959 0.92 1960 —= 1961} _o.96 1962 1963 1964 1965 1966 1 1 | | 1 I 10 15 20 25 30 DISTANCE, IN MILES FicurE 2.-Longitudinal distribution of eruptive vents on the east rift zone of Kilauea Volcano from 1954 to 1965. Posi- tion of vents is projected (at right angles) onto line AB. which connects a point in the center of Halemaumau in Kilauea Caldera with a point on the east cape of the island. Distance is measured from the center of Halemaumau. Num- bers are the CaO/(FeO-+0.9Fe:0);) ratio at 50.4 percent NiO. of rift zone and summit lavas as shown in table 3. In- dex map shows southern half of island of Hawaii. C4 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY assigned to the 1840 eruption on the geologic map. They occur in the 9- to 12-mile segment of the rift zone. In 1884 an apparent submarine eruption occurred just east of the east cape of the island in shallow water (Stearns and Macdonald, 1946, p. 111); a column of water, steam, and smoke shot several hundred feet in the air. Small eruptions occurred on the upper part of the rift zone in May 1922 and August 1923 (pl. 1). The 1922 eruption (Stearns and Macdonald, 1946, p. 115) pro- duced a small pond of lava in the west crater of Makao- puhi (6% miles on the rift zone) ; the pond was fed from a fissure which was later active in the March 1965 eruption. A second 1922 vent, at the 9-mile position, built a small spatter cone on the east rim of Napau Crater; this cone is still visible even though lavas of the October 1963 and March 1965 eruptions have swept around its base. A small eruption in 1923 (Stearns and Macdonald, 1946, p. 116; Finch, 1923; Stearns and Clark, 1930, pl. 1) poured out a small amount of lava on the west side of Makaopuhi Crater at the 6-mile position of the rift zone. The cited references do not agree on the exact position of the lava that erupted in 1923. The two flows shown on plate 1 as belonging to the 1923 eruption appear on aerial photographs as young flows overgrown with the same amount of vege- tation. The northeasterly flow corresponds to the map- ping of Stearns and Clark (1930) and the description of Finch (1923) ; the southwesterly flow corresponds to the mapping of Stearns and Macdonald (1946, pl. 1). These small flank eruptions were followed in 1924 by violent phreatic explosions in Halemaumau and by cracking, faulting, and formation of a graben on the east cape of the island along the 30- to 32-mile segment of the rift zone. This sequence of events indicates a rapid draining of the summit reservoir of the volcano and is possibly related to migration of magma out along the east rift zone where lava may have emerged farther east as an unobserved submarine eruption. A new pattern of increased frequency of east rift zone eruptions began in 1955. From 1955 through 1965, eight rift eruptions occurred, whereas from 1750 to 1955 only five east rift eruptions (and two questionable submarine eruptions) were reported. Part of this dif- ference is no doubt due to incomplete records of early activity, but it is unlikely that any but the smallest eruptions were unreported since 1840. The inhabitants of the volcano region, both native and immigrant, have always had a keen awareness of volcanic activity. The position of east rift eruptive vents of the last decade is summarized in figure 2. Almost every part of the 32-mile-long rift zone has been active, though since 1961 activity has been restricted to the 4- to 20-mile segment. Eruptive activity seems to be generally moving up the rift zone despite several notable excep- tions. Clearly, predictions of the site of the next eruption are little better than guesses. Prior to the 1961 eruption, every flank eruption of Kilauea Volcano (and Mauna Loa Volcano as well) was followed by at least one summit eruption. During much of the historic period up to 1924, continuous lava- lake activity occurred in and near Halemaumau. Since 1961, however, the increased east rift zone activity has been accompanied by total absence of any summit activity." The individual eruptive vents on the east rift zone (pl. 1) are the upper ends of magma-filled fissures or feeder dikes that extend from the summit reservoir beneath Kilauea Caldera. Most of the vents are discon- tinuous on the surface, rarely exceeding half a mile in length, though at some shallow depth they must be continuous. Within the mapped area, most of the dikes of the last decade occur in a narrow zone less than a quarter of a mile wide except near the west margin of the map where the right-offset en echelon arrangement of the vents produces the north curve of the rift zone (Moore and Krivoy, 1964, p. 2042). The activity of the last decade has occurred about midway across the 2- mile width of the rift zone. The individual vents of each eruption, as well as the rift zone as a whole, show a characteristic en echelon arrangement. In the map area most of the 1840, 1922, 1961, 1962, October 1963, and March 1965 eruptive fis- sures are slightly offset to the right. This effect is visible on a finer scale when an eruption is viewed from the air, for each mapped fissure is actually made of shorter elements, each slightly offset to the right. On a larger scale, the entire rift zone where it curves northward toward Kilauea Caldera (on the west end of pl. 1) shows a profound right offset en echelon pattern. The 1962 vents, for example, are offset nearly half a mile near Alae Crater west of Makaopuhi Crater (Moore and Krivoy, 1964). Generally wherever the offset of eruptive fissures is greatest (from 0.1 to 0.5 mile), pit craters occur between the offset segments. This relation holds on the upper east rift zone and also on the extreme lower east rift zone where the zone curves slightly north and is offset to the left at the group of pit craters 6 miles southwest of the east cape of the island (see Macdonald and Eaton, 1964, pl. 1). 1 Activity within Halemaumau resumed in November 1967 and was still continuing in June 1968. THE OCTOBER 1963 ERUPTION OF KILAUEA VOLCANO, HAWAII EVENTS OF 1963 PRECEDING THE ERUPTION Following the December 1962 eruption, Kilauea Vol- cano was especially active. Three periods of collapse and associated ground cracking and one small eruption pre- ceded the eruption in October (fig. 3). Each of these four events was preceded by an uplift of the caldera region, presumably caused by magma from depth enter- ing and inflating the reservoir beneath the summit. This uplift is indicated by northwest ground tilting at Uwekahuna, located on the northwest side of the region of uplift. Each of the collapses and eruptions was ac- companied by a dramatic subsidence of the summit caused by underground movement of magma from the reservoir eastward into the rift zone. No lava reached the surface during any of the three collapses (May 9, July 1, and August 3, 1963), but extensive ground crack- ing of the Koae fault zone south of the Caldera oc- curred during the first two (Koyanagi and others, 1964). The daily number of shallow earthquakes originating beneath Kilauea Caldera that are recorded on the North Pit seismometer is tabulated as a general index of local seismicity (fig. 3). The pattern of these earthquakes during 1963 bears a crude relation to the amount of magma in the summit reservoir as recorded by daily ground tilting at Uwekahuna. Generally when the sum- mit reservoir is most empty, as after an eruption, the C5 daily count of earthquakes is less than about 40 per day. As the reservoir fills and apparently stresses its roof, walls, and floor, the count of earthquakes increases to more than 100 per day. This relation, however, is com- plicated by swarms of earthquakes which seem to be related to the rate of filling rather than to the amount of filling. Such a swarm occurred in late December 1962 and early January 1963. Another factor not represented in figure 3 is the movement of the center of inflation, which will affect the north-south and east-west com- ponents of tilt at any one station differently. Any local- ized uplift directly south of the Uwekahuna station, for example, may produce earthquakes, but the uplift will not be recorded by the east-west component of ground tilt at that station. DESCRIPTION OF THE ERUPTION The eruption was immediately preceded by a marked subsidence of the summit of Kilauea which began at 0306 October 5 (fig. 4). This subsidence and the result- ant tilting of the ground was indicated by deflection of the long-period seismographs at Uwekahuna. At 0316, seismographs at the summit (North Pit) and upper east rift zone (Makaopuhi) began recording strong harmonic tremor and local shallow earthquakes. The subsidence and the harmonic tremor presumably resulted from magma moving from the summit reservoir T T T T T T T T T 1" fa- T E C C C= E E iP b s- x a 41 “MA/M fl/Jl/‘f 2 S als West b % € */ €. 1 f E 0 [- East w u W |- 0 E E< 80 IUD 5 0 S £ 40|- 5.6 e c 2 ¢ | w 0 1 | 1 1 | | | 1 1 | | e | | | nov _ DEC JAN _ FEB - MAR - APR MAY JUNE JULY AUG SEPT OCT NOV - DEC JAN 1962 1963 1964 Ficuke 3.-Relations of Kilauean eruptions (E) and collapses (C), ground tilting at Uwekahuna, and daily number of shallow caldera earthquakes recorded at the North Pit seismometer during the period November 1962 to January 1964. 339-928 0O-69--2 C6 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY I I I I I I I I f I I I I I I I I I I I I I M (Faint) Trace(?) » Makaopuhi worth Pit HARMoNIC TREMOR Relative amplitude at Pahos nn are i m mk four seismographs Mauna Loa - .-~---~-----_-__ 20 |- =] Caldera area s To —MN/ EARTHoUAKE swarms #] hea $ © o ie 3 [= 3 A4 '~ B € [=] o A $ E ae 's £ 2 s a A & & and younger than 10,000 years. Gener- % ally covered by large trees and tree ferns 165f10' > Y QUATERNARY Dashed where aerial photographs not available; dotted where concealed. Because of low relief, dense forest, and lack of surface drainage, contacts may be misplaced as much as several hundred feet 1961" aeg Fault Year of formation shown where known. Dashed where covered by younger flows. Bar and ball on downthrown side. Faults bounding pit craters not shown Crack Dashed where covered by younger flows 19°25 .... TRUE NORTH AGNeyr, 1 C NORTH # APPROXIMATE MEAN DECLINATION, 1969 ~19°2230" Base from U.S. Geological Survey: Volcano, 1963; Makaopuhi Crater, 1963; Kalalua, 1966; Kalapana, 1966; Pahoa South, 1966 GEOLOGIC MAP OF THE CENTRAL PART OF THE EAST RIFT ZONE OF KILAUEA VOLCANO, HAWAII zs3p NOTE ADDED IN PRESS As of the beginning of 1969, three more eruptions of Kilauea Volcano have occurred: (1) November 1967 to July 1968 within Halemaumau at the summit; (2) August 22-26, 1968, on the east rift zone from Heake Crater to Kalalua Crater; and (3) October 7-16, 1968, on the east rift zone from Kane Nui o Hamo to Puu Kamoamoa. The second and third eruptions produced new lava flows in the area of plate 1. SCALE 1:24 000 ; n T n T i £ 0 1 KILOMETER G- ~- - O- -f CONTOUR INTERVAL 20 FEET DATUM IS MEAN SEA LEVEL 155°00' 19°25" Geology by J. G. Moore, 1963-65 INTERIOR-GEOLOGICAL SURVEY, WASHINGTON, D.C. -1969-G69025 Pegmatitic Trachyandesite Plugs and Associated Volcanic Rocks in the Saline Range-Inyo Mountains Region, California GEOLOGICAL SURVEY PROFESSIONAL PAPER 614-D Prepared in cooperation with the Calzform’a Department of Conservation Division of Mines and Geo/0g y Pegmatitic Trachyandesite Plugs and Associated Volcanic Rocks in the Saline Range-Inyo Mountains Region, California By DONALD C. ROSS SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 614-D Prepared in cooperation with the Calzform'a Department of Conservation Division of Mines and Geology UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1970 UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Page sL 220 c 2 le een ee ana cl ac an ea ne n ae oes D1 | Trachyandesite caprocks of the Saline Range__________. Dil introduction... s.. _ NRE 1 Field description and relations____________________ 11 General name for the Saline Range caprocks___________ 1 Microscopic __ 14 Intrusive 4 Chemical sol onl cL lol oe lene cue a 15 Field description and relations____________________ 5 | Age of the _ 15 Microscopic L. 5 | Felsic to intermediate volcanic rocks-.________________. 17 Chemical i en uns 9 |-Owens Valley voleanic 18 Comparison of pegmatitic core rocks with similar Chemical characteristics of the Saline Range suite. _._.___ 20 rocks of Other areas... .c ul ect rense. 10 | Regional indications of an alkaline suite. .____________. 27 Other occurrences of intrusive trachyandesite___._._. 11 I References cited MLL LOC L_ 2: 28 ILLUSTRATIONS Page Prate 1. Geologic maps of volcanic rocks in the northern Inyo Mountains region, Inyo County, California.. In pocket FigurEs - 1-3. Photograph of- 1. Tuff beds capped by cinder and ash G I_ set D4 2. Crudely bedded red ash and cinder deposits and sill-like part of north intrusive body of trachyandesite in window in Saline Range. . . cone??? nolo 2 ne cee oll ene nese de anv be aeon ance ea 4 8. Contact between intrusive body and cinder 5 4. Photographs showing relations of pegmatitic core rocks to marginal rocks in the southern trachyandesite Plug: ce- eine etn e- onlera ana a ss o anes aar anns bed ano ue eae anes ates 6 5. Photomicrographs of rocks from intrusive trachyandesite PIU&8- 7. 6. Photographs of selected hand specimens from intrusive trachyandesite PIUgS- _ 8 7. X-ray diffraction patterns of selected minerals from specimen 9 8. Photographs of vesicular and amygdaloidal trachyandesite 12 9-11. Oblique aerial view- 9. Across south end of trachyandesite field of Saline Range.. 13 10. Of locality of specimen neben in an ibo alo bald 14 11. Northward along east side of the Saline Range trachyandesite field ____________________________ 14 12. Graph showing refractive index of fused samples of volcanic rocks of the Saline Range-Inyo Mountains ane anale ane! nle ens tent cole 2s Le 16 13. Plot of total alkalis and silica for volcanic rocks of the Saline Range AMA 19 14. Variation diagram showing classification of Saline Range-Inyo Mountains suite according to alkali-lime index - - 20 15. Graph showing relation of refractive index of fused-glass beads to percentage of oxides in chemically ana- lyzed volcanic cases een t e arenes an sone scenes 21 16-18. Graphs showing- 16. Selected oxide ratios of volcanic rocks. ern L_ OLL MO Lila J all lft 22 17. Variation of common oxides and silica in volcanic 23 18. Relation of oxide percentage to differentiation index of volcanic rOCKS_______________________.__._ 24 19. Ternary plots of selected oxides and normative minerals of volcanic __ 25 20. Graphs showing trace-element trends in volcanic Src. 26 TABLES Page 1. Analyses of volcanic rocks in the northern Inyo Mountains-Saline Range D2 2. Thin-section modes, Saline Range volcanic rocks...__.._..._ .". L..: {j __ n t i us. 15 S.. Potassium-argon ages L_... Os. [O PMc .po Mt {ral 16 4. Normative mineral comparison of Owens Valley ; _: D 19 III SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY PEGMATITIC TRACHYANDESITE PLUGS AND ASSOCIATED VOLCANIC ROCKS IN THE SALINE RANGE-INYO MOUNTAINS REGION, CALIFORNIA By Donaurp C. Ross ABSTRACT Two small late Tertiary volcanic plugs (made up of interme- diate plagioclase, sanidine, clinopyroxene, olivine, and metallic opaque minerals) on the west flank of the Saline Range grade from fine-grained pilotaxitic trachyandesite at their margins to coarse-grained, miarolitic, pegmatitic rocks at their cores. The plugs intrude layers of cinder, ash, and tuff that lie immediately beneath a vast volcanic cap which covers about 125 square miles and whose black olivine-bearing rocks are dominantly trachy- andesites that contain about 2 percent K,0. Chemical analyses and semiquantitative spectrographic anal- yses were determined for 16 volcanic specimens from the area. These chemical data are supplemented by determinations of the index of refraction of fused-glass beads from 54 volcanic spec- imens. Potassium-argon determinations from K-feldspar and clinopyroxene of one of the plugs and from three whole-rock samples of the capping flows give uppermost Pliocene ages. These new chemical data extend the large areca in the Sierra Nevada and western Great Basin where late Cenozoic mafic volcanic rocks seem to be typically alkaline and particularly rich in K,O. INTRODUCTION On the west flank of the Saline Range, a window in a vast field of upper Cenozoic trachyandesitic volcanic rocks exposes two small plugs that have coarse-grained pegmatitic cores that grade marginally into a fine- grained pilotaxitic rock. The purpose of this report is to describe these rather unusual rocks and their volcanic associates and to present the chemical and petrogra- phic data that have been accumulated on these vol- canic rocks during the geologic mapping of a cross section of the Inyo Mountains (Independence and Waucoba Wash 15-minute quadrangles). Location of the volcanic rocks of this region is shown on plate 1. Geologic maps of and reports about the two quad- rangles already published (Ross, 1965, 1967a, b) show their distribution in more detail and also contain brief descriptions of these units. Published chemical data on volcanic rocks of this region are scarce. It therefore seems particularly desir- able to get chemical data on the volcanic rocks of the Inyo Mountain-Saline Valley region into the literature, as field examination and even thin-section determina- tion is insufficient to ascertain the chemical character- istics of these largely aphanitic rocks. As the chemical data slowly accumulate, it is becoming apparent that there is a large alkalic province of Cenozoic volcanic rocks in this region that might go undetected without such data. The great bulk of the volcanic rocks cover most of that part of the Saline Range that is within the Waucoba Wash quadrangle and adjoining parts of the lower east slopes of the Inyo Mountains. These volcanic rocks are dominantly a great caprock flood of trachy- andesite flows and agglomerate. A window on the west flank of the Saline Range exposes the unusual fine to coarse-grained intrusive rocks of the plugs as well as cinder beds, tuff, and tuffaceous sedimentary rocks that underlie the volcanic caprock. Beneath the capping trachyandesite elsewhere in the Saline Range are a few exposures of more felsic volcanic rocks and alluvial deposits with ash and pumice-rich layers. Volcanic rocks are absent at higher altitudes in this part of the Inyo Mountains, but low on the western slopes there are several patches of dark basaltic ash and cinders, in part mixed with alluvial material that originated from a small volcanic center just north of the area of plate 14A at the west base of the Inyo Mountains or that originated from volcanic centers at the east base of the Sierra Nevada to the west. The volcanic rocks shown in Owens Valley are the ends of basaltic flows that originate along faults (marked by alined cinder cones) along the east front of the Sierra Nevada (Moore, 1963). GENERAL NAME FOR THE SALINE RANGE CAPROCKS The flood of black volcanic rocks capping the Saline Range, which commonly contain visible phenocrysts of olivine and (or) pyroxene are, by field classification, basalt. 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T000 "0 carat hn nos many sosireus oryde.Sonpods oanenuenbruog (own 3reseq -£uo®1) uo 3res8q quydor _ oququda; areseq 4s0u(s) ~Au081 roydou . oftfoudou ~£Kuo81} qreseq oqrsopue areseq outsopus offo4yi - oqtsopus - onrouoyd qreseq areseq outsopus qreseq owsopue - oqrsopue . -@uIAIO _ SPOS ©pog n°T ~Auo81pf _ IEC x18C - - -outANIO owsopuy -@ulANOQ -4uornp ~~ oureu y901 HESS—N 9 "B9 9 T 829 LAcl 0 'I 9T © °08 9 OF L '6F 808 0 "IF T 'SF PMC 8:08 9 °Eg 0 "sg Lal 98° F0 ' 20 ° €1° 90° LL SL £8" 98° AL' 9° L8" 8€° F6 8° F0 86 °0 tr 0 98 °0 0€ '0 Fo°0 18 °0 80 °0 810 $0 810 IEO 12 °0 $0 °0 T8 '0 soureu yoou pus uueargryg D4 1962, p. 361-363). The Saline Range volcanic rocks are high in alkalis and low in magnesia, as compared with basalts of similar silica content. Also, coarse-grained intrusive phases of these rocks contain modal K-feld- spar. Normative plagioclase is generally andesine, less commonly oligoclase, and is labradorite in only one of the 11 chemically analyzed specimens. By using the rock-classification system of Rittmann (1952), various names apply to these rocks (table 1), but andesine basalt and trachyandesite are the most common. By using the classification of MacDonald and Katsura (1964, p. 88-89), the Saline Range volcanic rocks are | all part of the alkalic suite, and the less silicic varieties are alkalic basalts or alkalic olivine basalts. Some speci- mens more nearly fit hawaiite, as they contain both normative and modal andesine. Some specimens with more than 5 percent normative, but no modal nepheline, are basanitoid. : No one name from a chemical classification will apply equally well to all the somewhat variable Saline Range volcanic caprocks. It seems, however, that the name "trachyandesite'" best reflects the general chemical na- ture of this suite, which is high in alkalis and has a normative plagioclase of less than Ang,. For the pur- poses of this report, the entire suite of caprocks and their presumably intrusive equivalents will be referred to as the trachyandesite of the Saline Range. The me- dium to coarse-grained cores of the plugs might more precisely be called syenodiorite. The bulk of the intru- sive rocks, however, are not much coarser grained than the trachyandesitic caprocks of the volcanic field. To help focus attention on the fact that these plugs are closely related to the extrusive volcanic rocks, the term "trachyandesite"" will be used to refer to all these intru- sive rocks, regardless of grain size. The terms "peg- matitic' and "diabasic' as here used for these intrusive rocks refer solely to texture and have no compositional connotation. INTRUSIVE TRACHYANDESITE The intrusive plugs and related dikes of these rocks, which crop out over an area of only about one-tenth of a square mile, are shown on plate 1B. These plugs are in part sill-like and intrude a sequence of cinder layers, tuff, and tuffaceous sedimentary rocks (fig. 1). In part, these intrusive rocks are coarse grained and miarolitic; they contain primary K-feldspar along with olivine and clinopyroxene and have a texture that can best be described as pegmatitic. As far as I know, this occurrence of pegmatitic rocks in a mafic volcanic suite in this region is unique. The northernmost pegmatitic plug is isolated from the capping trachyandesite, but the southern mass appears to be overlain by the caprock sequence. The rapid gradation in these plugs from dense SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY volcanic rock to pegmatitic cores within the space of a few tens of feet (figs. 2, 3) suggests that the plugs were very shallow intrusions that were sealed off, and thus the volatile-rich fluid of the core was permitted to crys- tallize to a pegmatitic and miarolitic texture. Even though the plugs are near the surface, however, an erosion interval seems to separate their emplacement from the extrusion of the capping flows, as coarse- grained core rock of the south plug directly underlies caprock (see also p. D17). These plugs and the asso- ciated pile of cinder layers suggest proximity to a vol- canic center. Also, the window that exposes the plugs and cinder layers spans one of several northeast-trend- FrGurE 1.-View to the west from north of the northernmost plug of trachyandesite in dominantly pink to white tuff beds, which are thin bedded in part and may be lacustrine. Red cinder and ash beds cap the tuff. FiraurE 2.-Crudely bedded red ash and cinder deposits and sill-like part of north intrusive body of trachyandesite (out- lined on photograph) exposed in window in Saline Range volcanic field. VOLCANIC ROCKS, SALINE RANGE, CALIFORNIA Coarse central h part Fraur® 3.-Closer view of trachyandesite intrusive of figure 2. Contact of marginal rocks with cinder beds is sharp. Contact between marginal and pegmatic core rocks is gradational, but relatively abrupt. Specimens 3, 4, 5, and 6 were collected approximately where indicated by open circles. ing faults that traverse the caprocks and that may reflect cracks that were a source of feeders for this vol- canic sequence. FIELD DESCRIPTION AND RELATIONS The rocks of the plugs are generally medium gray to medium dark gray, but some of the finer grained marginal facies are reddish gray. In both plugs and in the smaller associated dikes, a range in grain size is notable from margin to core. The marginal rocks look much like the trachyandesite of the flows, though they are lighter in color than the caprocks. The cores of thin dikes that are only a few feet thick are fine to medium grained and have a prominent network of platy euhedral feldspar crystals separated by open pore space. The cores of the plugs are coarse grained, the largest crystals being 5-10 mm in maximum dimension. Coarse reticulate platy feldspar crystals are most conspicuous. The core rocks have much open pore space into which terminated crystals of clinopyroxene and olivine extend. These miarolitic cavities locally contain calcite and other secondary minerals. The north plug is grossly sill-like in its southern part, but markedly cuts across the intruded tuff beds near its northern end. A marginal envelope of fine- grained rock grades abruptly into a core of coarse pegmatitic and miarolitic rock (figs. 2, 3). The south plug markedly cuts across the intruded cinder beds. In contrast to the simple envelope and core of the north plug, the south plug is typified by veins several inches to several feet thick of coarse- grained, miarolitic core rock intruded into the finer- D5 grained marginal rock (fig. 44) and by many patchy areas of coarse-grained, spongy core rock (fig. 4B,0). The south plug shows clearly that the core rocks are coarse spongy equivalents of the marginal rocks and not a separate later intrusive pulse. MICROSCOPIC DESCRIPTION The finer grained marginal rocks of the plugs and dikes have a pilotaxitic (felty) texture, and some are trachytic; some specimens are also weakly porphytritic. These rocks are generally dense, but some are vesic- ular. The plagioclase laths are as much as 0.2 mm long; dark minerals and opaque clots are as large as 0.8 mm in largest dimension, but more commonly are 0.1-0.2 mm. The marginal rocks grade in grain size into a rock with a diabasic or intergranular texture with grains generally from 1 to 3 mm in longest dimen- sion. Further coarsening of grain size produces rocks that approach a pegmatitic texture with crystals 5-10 mm in length. The medium-grained (diabasic) rocks are only in part miarolitic, whereas the coarser varieties are invariably miarolitic. None of these rocks contain micrographic (micropegmatitic) interstitial ma- terial, which seems so widespread and common in mafic pegmatitic differentiates in other areas. The textural gradation from margin to core can be seen in the photo- micrographs of figure 5A-C. Figure 6 shows some of the representative rocks of the trachyandesite plugs in hand specimen. What is probably the most abundant rock type of both plugs is illustrated by photomicro- graphs in figure 5D and E and by hand specimen in figure 6A. Figures 6B and 6C show the general appear- ance of the coarser grained core rocks, figure 6D shows the rapid grain-size change from margin to core. The mineralogy of these rocks is grossly the same from the fine-grained margin to the coarse-grained core. Intermediate plagioclase is dominant, both clinopyroxene and olivine are abundant, and metallic opaques are also common. K-feldspar, probably sani- dine, forms a mantle on plagioclase crystals in the coarsest grained varieties (fig. 5G). Attempts to dis- tinguish the plagioclase from the K-feldspar by staining of slabs and thin sections were generally unsuccessful. Sanidine, unlike the other K-feldspars, does not seem to take a good stain. The one thin section (specimen D) stained well enough to permit a distinction by point counting showed about three times as much plagioclase as K-feldspar. This 3:1 ratio, which is generally com- patible with the normative composition of the intrusive trachyandesite, is at least the correct order of magnitude for the feldspar content. Probably K-feldspar (sanidine) is present as 10-15 percent of these rocks. Small amounts of biotite and amphibole are also found. Table 2 shows the modes of seven specimens of these intrusives; D6 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY C FrGur® 4.-Relations of pegmatitic core rocks to marginal rocks in the southern trachyandesite plug. Finer grained phases are sim- ilar to specimens C and D, shown in photomicrographs 5D, F. Coarser grained phases are similar to specimens 6, 7, and G shown in photomicrographs 5C, F, and G. Knife used for scale in all views. A, Relatively sharp-walled pegmatitic vein. B, Patchy gradational contact. C, Gradational, but abrupt grain-size change. Coarse phase here extremely miarolitic. specimens 5, C, D, and E are from the medium-grained (diabasic) phase; specimens 6, F, and G are from the pegmatitic core rocks. Two of the coarsest grained specimens (6 and G) are notably poorer in olivine and richer in feldspar than the other specimens; they also contain amphibole. The other coarse specimen (F), however, is very similar to the medium-grained rocks. Plagioclase is the most abundant constituent of these rocks; most is calcic andesine to sodic labradorite. There appears to be no noticeable difference between plagioclase of the marginal and core rocks except grain size. K-feldspar is present in some of the medium- and coarse-grained rocks as interstitial grains and as mantles on the plagioclase; it is not conspicuous in the fine-grained marginal rocks. The index of refraction of the K-feldspar is about: n,=1.527, n,=1.535. The 2V is negative and ranges from near 0° to 30°. A pure- mineral separate from specimen 7 has a K.O content of 6.5 percent; its X-ray diffraction pattern is shown in figure 7. This mineral probably is mostly sanidine. Olivine, where fresh, is in pale-yellow crystals that have a 2V of near 90° and a birefringence of about 0.035-0.04-features that suggest magnesian olivine of the variety chrysolite. In most specimens olivine is intensely or completely altered to iddingsite(?) and iron oxides. Some black metallic masses seem to be pseudomorphing olivine. Clinopyroxene, which is deep green in grains, is pale green to colorless in thin section. A pure mineral sepa- rate made from specimen 7 has the following properties: indices of refraction, nx=1.700, m»,=1.708, ng=1.725; r>V distinct; and 2V (+), moderate. The X-ray diffrac- VOLCANIC ROCKS, SALINE RANGE, CALIFORNIA D7 Ficurr 5.-Photomicrographs of rocks from intrusive tra- chyandesite plugs. A, Pilotaxitic texture near contact with wallrock cinder layers of north plug. B, Diabasic texture intermediate between contact and core rocks of north plug. C, Pegmatitic core rock of north plug. D, Diabasic-tex- tured rock, the most common type in south plug. Crossed- nicols view to emphasize plagioclase laths and texture. E, Same view as D under plane light to emphasize clinopyroxene, olivine, and magnetite. F, Pegmatitic core rock of south plug. View dominated by coarse blades of plagioclase and triangular- shaped miarolitic cavities. G, Pegmatitic core rock of south plug showing K-feldspar coating plagioclase and filling large interstitial spaces. Symbols used: ap, apatite; clp, clino- pyroxene; Kf, K-feldspar; mgt, magnetite; ol, olivine; PI, plagioclase. G D8 R SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY Figur® 6.-Photographs of selected hand specimens from intrusive trachyandesite plugs. A, Diabasic marginal rock (similar to rock in figures 5D and E). B, Pegmatitic core rock (sawed surface). C, Bladed feldspar and miarolitic cavities of pegmatitic core rock. D, Gradational, but rapid grain-size change at contact between marginal rock like rock in figure 6A and pegmatitic miaro- litic rock like rock in figure 6C. VOLCANIC ROCKS, SALINE RANGE, CALIFORNIA D9 T I I I I I I | I I i T I I I I T J I I r 3.22 K-feldspar 3.74 7-4 spacing 6.47 3.45 2.16 3.3 | | | | I | | 1 | 1 | | | | | I | | I 1 | 56 52 48 44 40 36 32 28 24 20 16 12 DEGREES 20 I I T I I I I I T T I I I I I I I I I I I I I I I 2.95 Clinopyroxene 1.63 ,d spacing 3.23 ' 64 40 36 12 DEGREES 20 Fraur® 7.-X-ray diffraction patterns of selected minerals from specimen 7. tion pattern for this sample also is shown in figure 7, Locally, pale clinopyroxene is rimmed by moderate- green to dark-yellowish-green clinopyroxene(?), which is pleochroic from light olive brown to moderate green. Trace amounts of light-brown amphibole and red- brown biotite are fairly widespread. Apatite is an abundant colorless accessory mineral. Metallic opaque minerals are abundant and make up some of the largest crystals and crystal aggregates in the coarser grained rocks. Magnetite is common, but some of the "metallic" aggregates are presumably ilmenite, as they are not attracted to a hand magnet. CHEMICAL DATA The five specimens (8, 4, 5, 6, and 7) that were chemically analyzed (table 1) were selected to provide a sample from the margin to the core of these intrusive rocks. Specimens 3, 4, 5, and 6 were collected from the west contact of the north plug as shown in figure 5: at the contact, 20 feet below the contact, 40 feet below, and about 60 feet below, respectively. Specimen 6 is representative of the coarse core of the north plug; specimen 7 is a coarse variety from the south plug, presumably a coarser crystallized phase of rocks like specimen 6. D10 The chemical analyses do not show as much chemical variation as I had expected for rocks with such a radical textural variation. The sampling was neither systematic nor detailed, but the specimens selected looked typical and represented the textural range of these rocks as they were studied in the field. The most consistent trend is in total Fe as FeQ, which shows a consistent increase to the core-a relationship that was guessed from field relations. Somewhat as expected, TiO; follows total Fe and is concentrated in the core relative to the margin. MgO is less common in the core rocks, which seems to correlate with a lower amount of olivine in these rocks. Less directly tied to predictable trends from field observation are the decrease coreward of Al;0; and the increase coreward of P;0,;. COMPARISON OF PEGMATITIC CORE ROCKS WITH SIMILAR ROCKS OF OTHER AREAS The pegmatitic trachyandesite intrusive rocks are a rather startling textural variate of the typical volcanic rocks of this region. Pegmatitic variations in basaltic and diabasic rocks, particularly in intrusive sheets, however, are rather widely referred to in the literature. Lacroix (1928) briefly described three localities from France, Polynesia, and China of what he called "Les pegmatitoides des roches volcaniques & facies basal- tique." He described coarse-grained segregation veins composed of labradorite, titaniferous augite, olivine, and magnetite with groundmass orthoclase and reported chemical analyses of the basaltic rocks and their peg- matitic derivatives. * Shannon (1924) described the mineralogy and petrol- ogy of an intrusive diabase at Goose Creek, Va., that has pegmatitic phases. These pegmatitic rocks are char- acterized by labradorite, titaniferous augite, and large skeletal crystals of "iron ore" (as large as 5 mm). Interstitial micropegmatite with orthoclase is common. These rocks become quite coarse-grained, and although chemically they are much less alkalic than the Saline Range rocks, they have a similar physical appearance (Shannon, 1924, pls. 1-3). Kennedy (1933, p. 244-247) described some general characteristics of late differentiates of basaltic magmas, noting "that many lava flows and minor intrusions of basaltic composition show a remarkable tendency to segregate contemporaneous veins which differ in composition from the parent rock." He divided the differentiates into two types: (1) those from tho- leiitic basalts that have calc-alkaline derivatives, and (2) those from olivine basalts that give alkaline de- rivatives. The latter are characterized by the absence of quartz, abundant K-feldspar associated with basic plagioclase, and abundant iron ores. Kennedy (1933, p. 245) has tabulated eight analyses of these alkaline SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY derivatives, including those reported by Lacroix (1928, p. 324), that have the following average oxide percentages: Si0;, 49.2; A1;,0;, 14.7; 10.7; MgO, 3.6; CaO, 7.9; Na,0, 4.2; K;,0, 2.9; and TiO ;, 3.2. These values are closely comparable to the alkaline pegmatitic rocks of the Saline Range area (table 1). Kennedy made the point (1933, p. 246) that his examples characteristically show a marked decrease in MgO coupled with a strong increase in alkalis in the differentiates and that the parent rocks are not from originally alkaline magma but are "normal olivine basalt." By contrast, the visible parent rocks of the Saline Range rocks are themselves alkaline; of course we cannot be sure that the entire Saline Range suite is not already differentiated from a less alkaline parent, but this seems unlikely for the large volume of volcanic rocks involved. The end-member pegmatitic rocks of the Saline Range volcanic suite are similar chemically to alkaline differentiates from parent rocks of much lower total alkali value in other areas. Walker (1953, p. 41-44) described the field occurrence of "pegmatitic differentiates of basic sheets." He noted that such rocks occur as schlieren, patches, and crosscutting veins or dikes. The examples he cited from many localities are generally much less alkaline than the Saline Range rocks, but their textures and general mineralogy appear grossly similar. Walker noted (1953, p. 43) that most dolerite (diabase)- pegmatite is in sharp unchilled contact with adjacent dolerite (diabase). In contrast, many of the contacts in the small plugs in the Saline Range are gradational (figs. 4B, C), even where grain-size variation is con- siderable. The general appearance suggests that the coarse spongy core rock was a late crystallizing volatile-rich pegmatitic phase relative to the fine- to medium-grained (diabasic) matrix rock. Even some of the more distinctive veins (fig. 44) are gradational and blend with the matrix rock. This probably mainly reflects crystallization in place for the core rocks of the Saline Range plugs and only minor squirting around of vein material. Apparently, in other areas there is more mobility of later juicy pegmatitic phases, which creates more crosscutting, dikelike relations. Thick sheets and transgressive bodies of dolerite (diabase) from Tasmania have been described by McDougall (1962). These rocks are more silicic than the Saline Range rocks, and they all seem to have abundant modal quartz in the coarse-grained dif- ferentiates. Even though they are chemically dissimilar, the appearance of the hand specimens is remarkably similar (McDougall, 1962, following p. 302) to some of the Saline Range differentiates. This brief summary of some reported occurrences of mafic, coarse-grained, and pegmatitic differentiates VOLCANIC ROCKS, SALINE RANGE, CALIFORNIA points out their textural similarity to the alkalic rocks of the Saline Range. But it also points out that alkalic differentiates like the coarse-grained pegmatitic core rocks of the Saline Range plugs can be derived from parent rocks that have a considerable range in composition. OTHER OCCURRENCES OF INTRUSIVE TRACHYANDESITE The rather unique character of the pegmatitic plugs in the window on the west flank of the Saline Range is further pointed out by the presence of dikes, sills, and small plugs elsewhere in this area that have not developed coarse miarolitic cores. Along the east base of the Inyo Mountains and in the large outcrop area of Paleozoic rocks in the Saline Range are dikes and sills, generally only a few feet thick, that at least in part penetrate the capping trachyandsite flows. Similar dikes have been seen within the capping flow sequence. Some of these dike rocks, particularly the long south- ernmost dike in the Inyos and the several dikes in the north part of the large Paleozoic outcrop area in the Saline Range, are so altered and weathered that their composition and relation to the caprock sequence is difficult to determine. Fresher dikes and the sill-like intrusives in the area of specimen 11 and dikes near the locality of specimen 14 in the Inyo Mountains appear to intrude and feed the caprock sequence. All the dikes and sills outlined above are believed to be genetically related to the capping trachyandesite sequence. These intrusive rocks, where fresh, look much like the capping trachyandesite. They are composed of labradorite, clinopyroxene, olivine, and minor apatite and ilmenite-magnetite. Typical of the altered dikes are chloritic pseudomorphs after ferromagnesian min- erals that are chiefly pyroxene, iddingsite pseudo- morphs after olivine, and clouds of sericitic material replacing plagioclase. Some of the dikes are vesicular and amygdaloidal, and one (fig. 8) has a sharply chilled margin against quartz monzonite wallrock. Only one sample of this group of rocks has been chem- ically analyzed (specimen 11, pl. 1), and it is from a pluglike mass that is dark gray, dense, has numerous small iron-stained spots marking altered olivine crystals, and weathers to a distinctive olive-drab color and blocky surface. It has an intersertal texture with labradorite, clinopyroxene, and olivine crystals set in a brown glass matrix. The glass contains crystallites; some of the forms referred to as cumulites and margarites are pres- ent. Alteration of the olivine in this rock is very unusual. Rather than the red iddingsite (?) alteration so common in this area, the olivine is veined and rimmed by dark- D11 greenish-yellow, weakly pleochroic material that is also found as interstitial masses in the rock. Chemically, (table 1) this rock is trachyandesite after the classifica- tion of Rittmann, and despite its somewhat different microscopic appearance, it seems to be chemically com- patible with the caprocks, of which it is probably an intrusive equivalent. TRACHYANDESITE CAPROCKS OF THE SALINE RANGE FIELD DESCRIPTION AND RELATIONS About 125 square miles of the Saline Range in the Waucoba Wash and adjoining Waucoba Spring, Last Chance, and Dry Mountain quadrangles is covered by a veritable flood of trachyandesitic flows and associated agglomeratic and flow breccia layers. Similar rocks, isolated by erosion from the main field, lap up onto the east face of the Inyo Mountains. Other probably correlative rocks are common in the south part of Dry Mountain (fig. 9) and in the Last Chance Range (Ross, 1967B). Numerous small cinder cones appear to be superimposed on the trachyandesite, but there are no major volcanic centers associated with it. A prominent set of northeast-trending faults, which have displace- ments of as much as several hundred feet, cut the volcanic field. The apparent association of large tongues of volcanic rock with this fault system suggests these are fissure flows that were extruded along a set of weak planes that have continued to be active and have now chopped up the surface of the flows by their continued movement. Numerous 1- to 10-foot-thick dikes have been seen in the Paleozoic and Mesozoic basement rock; some can be traced up into the capping flows, but no major feeders have been found. The window of cindery layers and the plugs of partly pegmatitic intrusive trachyandesite athwart one of these north- east-trending faults in the north-central part of the Waucoba Wash quadrangle may represent a partly uncovered volcanic center for part of the caprock. Mainly, however, it appears that the flood of trachy- andesite has buried its feeder sources. The maximum thickness of the sequence may be more than a thousand feet, as sections over 600 feet thick are exposed in fault scarps in the Saline Range. These sections are composed of numerous flows, a few feet to a few tens of feet thick, intercalated with agglo- meratic and flow breccia layers. In addition, bedded tuffaceous material and gravelly alluvium containing volcanic fragments are interbedded in some volcanic sections. Unfortunately, the best exposed and thickest sections of flows in the Waucoba Wash quadrangle are in areas where access is particularly difficult (south of the area of specimen 8 and west of specimen 13, pl.1). The main sequence of flows is underlain by mixtures of D12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY Fraur® 8.-Two-foot-thick, vesicular and amygdaloidal trachyandesite dike chilled against quartz monzonite walls about one mile SSE. of locality of specimen 14. A, Straight-walled segment at wash level in small canyon. Pick used for scale. B, Irregular discontinuously exposed segment higher on canyon wall. tuffaceous and alluvial material at many places. A repeating sequence of volcanism and alluviation is represented in these sections. Where the trachyande- sites lap up on the Inyo Mountains, an initial out- pouring of flows and a showering down of tuffs covered alluvial material similar to the present fanglomerates of the area. This volcanism was followed by deposition of more fanglomerate rich in granitic debris and fault displacement of the flows (fig. 10). Then a second period of volcanism provided the capping trachyande- sites, which presumably are correlative with the main caprocks of the Saline Range. The trachyandesite presents a dark, somber, monot- onous terrane broken into numerous block faults (fig. 11). The rocks are typically medium to dark gray, dense to vesicular, and have scattered olivine pheno- erysts. Much less commonly, the flow rocks are dark shades of brown to red. They are particularly susceptible to desert varnish, which develops to an iridescent metallic sheen on some exposed surfaces. Cinder cones are present and seem to be in part superimposed on top of the flow sequence as though they represent a dying phase of the volcanic period. The largest con- centration of cinder layers, however, is beneath the caprocks near the north margin of the Waucoba Wash quadrangle (fig. 2). The intercalation of ash, lapilli, and cinder layers in the flow sequences indicates that pyroclastic activity was important throughout the volcanic sequence. Megascopically these rocks do not offer any clues to their potassium-rich nature. They have the physical appearance of typical olivine basalt; in fact, they are D13 VOLCANIC ROCKS, SALINE RANGE, CALIFORNIA 'ouonp uoA purjoy 4q yde1s0j0uq 'pjoy otueojoA Suey ourreg oy} YjIm snonumuo9 uoaq oaty {tur syoo1 urejunopy 41g 941 Jo sxoo1 otueojoA podem put pojng; 04 Suey ourreg oy} Jo pjoy oftsopute4uo®1} ;o puo JNOs sSOIOE MoTA [BII9Y 6 omsiq 864-310-10--2 D14 SHORTER Figur® 10. View north to area of locality of specimen 14. Flow of trachyandesite (Tt) beneath intravolcanic alluvium (Tal) is more highly faulted than trachyandesite caprock. Intravolcanic alluvium is rich in granitic debris of same com- position as weathered granitic rock (Mzgr) in foreground. In background, across Waucoba Wash, is volcanic field of the Saline Range. called basalt on the geologic map of the Waucoba Wash quadrangle (Ross, 19672). MICROSCOPIC DESCRIPTION Only 18 thin sections of the trachyandesite were examined-a very small sample of a volcanic field of this size (see Ross, 19672, fig. 1 for index to specimen coverage). These rocks are pilotaxitic with fresh plagio- clase laths mixed with interstitial grains and small phenocrysts of dark minerals; most commonly clino- pyroxene is interstitial and olivine is in small pheno- erysts. The rock is liberally sprinkled with metallic opaque grains that appear to be dominantly magnetite, although the titanium content of the chemical analyses indicates ilmenite is also present. In some specimens the plagioclase crystals are alined to give a trachytic texture. Most specimens are somewhat porphyritic. Phenocrysts are most commonly olivine in crystals as large as 2 mm in maximum dimension; less common are phenocrysts of clinopyroxene and plagioclase of about the same size. Nearly all specimens are vesicular; most vesicles are clean, but some have fillings of calcite and other secondary material. Some specimens have a texture that suggests three generations of crystals: they contain phenocrysts from 1 to 3 mm, a distinctly finer grained generation of plagioclase microlites, and an equally distinctive, much finer grained groundmass rich in magnetite, very fine grained plagioclase needles, and clinopyroxene and olivine. Five thin-section modes of samples selected to give a good geographic range are listed in table 2. These samples show that in mineral composition the rocks CONTRIBUTIONS TO GENERAL GEOLOGY Aerial view looking north along the east side of the trachyandesite field of the Saline Range, here broken by a series of north-trending normal faults, downthrown to the west. Striped rocks in the middle distance are Cambrian sedimentary rocks. White patches along east edge of volcanic field are deposits of active and dormant hot springs. FrGgurE 11. are about % to % plagioclase, that pyroxene exceeds olivine, and that metallic opaques are abundant. In addition to these major constituents, apatite is wide- spread and secondary calcite is found as vesicle lining, as thin veins, and scattered through the rocks. Most hand specimens will effervesce slightly with dilute HC], showing that secondary calcite in small amounts is widespread in these rocks. The plagioclase in these rocks is dominantly inter- mediate andesine to calcic labradorite. Sodic andesine is present in one red anomalous-looking specimen that has had all its ferromagnesian minerals replaced by iron oxide(?). In other specimens the plagioclase is largely clean and unaltered. Locally some plagioclase "phenocrysts" are embayed and spotted and streaked with microcrystalline alteration(?) material that sug- gests they are exotic crystals that have reacted with the trachyandesitic lava. In another specimen, frac- tured grains of quartz are certainly exotic; one is insulated from the rest of the rock by a reaction rim of clinopyroxene. The clinopyroxene is pale greenish gray and is generally unaltered in thin section. On the basis of extinction angle, birefringence, and 2V, it is augite. Olivine, where fresh, is colorless or somewhat yellow. It has a 2V of close to 90° and therefore is considered to be magnesian chrysolite olivine. The olivine is rarely fresh, however, but is veined and replaced along the typically curving fracture of olivine by VOLCANIC ROCKS, SALINE RANGE, CALIFORNIA D15 2.-Thin-section modes, in percent, of Saline Range volcanic rocks Trachyandesite caprocks Sample No. Plagioclase Clinopyroxene Olivine and alteration Metallic opaques products nett ae n'n b uld i on a aln a b als a nel min m nial nd 65 15 13 T Tee e endo aan an an alule an ole bina i al a ane i 51 27 15 T Tiere iii ro caanesnenik a 2h ahl he 59 19 18 4 ane atas. s as aln ad a mine tle bo almln uel an of 55 28 11 6 e ne sell nel au nana ao al aie ail a al h alig 67 17 12 4 Average. ~... 59 21 14 6 Intrusive trachyandesite Sample No Total feldspar (Plagio- Clinopyroxene Olivine and alter- _ Metallic opaques Biotite Amphibole clase+ K-feldspar ation products 65 17 14 3 I }.: Mmmm y f 76 11 4 8 1 63 17 11 8 t* s ece as 1 61 17 13 8 1} aCe 66 17 12 4 1 > Gare Pa 64 18 11 6 1}; 70 17 6 0 ee anes 1 67 16 10 6 1s -_ ! Plagioclase=47, K-feldspar=14 (stained thin section, but inconclusive). reddish-brown alteration material generally referred to as iddingsite. Some olivine is completely pseudomor- phosed by this alteration material. CHEMICAL DATA Six specimens of the trachyandesite were chemically analyzed, and five of them were flow rocks; one (speci- men 11) from the Saline Range is from an intrusive plug, but it is presumed to be related to the trachyande- site caprock sequence also. One of the most interesting features of this admittedly sparse sample is the range of 9 percent in SiO; content in a suite of rocks that were presumed to be rather homogeneous, judging by their field appearance. Even this small chemical sample, however, points out the alkalic nature of these rocks; they are anomalously high in KO compared with many volcanic rocks of comparable SiO,. The generally K,O-rich nature of this suite seems to have exceptions though, for specimen 13 has a KO content closer to that of "normal" calc-alkaline basalt. In order to supplement the chemical data, 26 addi- tional samples were selected for fused-glass-bead index determinations. The results of this work, plotted in figure 12, confirm a suspicion from the fieldwork that the specimens with higher SiO; may be less important quan- titatively than the varieties with lower The glass- bead clustering suggests that the bulk of the trachyan- desite rocks have an SiO); content in the range of 47-49 percent. The present study has revealed that there are sig- nificant variations in composition over the surface of this volcanic field. Thus, detailed sampling in some of the massive, several-hundred-foot-thick sections of those flow rocks exposed in the fault scarps might be worthwhile in determining the range of chemical variations with time in this trachyandesitic suite. AGE OF THE TRACHYANDESITE The trachyandesite caprock and plugs of the Saline Range are late Cenozoic. The caprock flows are under- lain and overlain by and interlayered with alluvial ma- terial, but there have been no data to establish their age precisely. The nearest volcanic rocks to be dated radiometri- cally are from the Deep Spring Valley area, about 30 miles to the northwest. Dalrymple (1963, p. 387) re- ported an age of 10.8 m.y. (million years) from a basalt capping a tuff from which sanidine yielded an age of 10.9 m.y. Other radiometric ages that were reported by Dalrymple in the same publication and that were made on latitic and basaltic rocks from the Sierra Nevada range from 2.6 to 9.6 m.y. and point out a widespread province of Pliocene volcanic rocks northwest of the Saline Range volcanic field. D16 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 75 71 63 57 53 50 47 45 UH EAE AEA p t f f if err f {_ f 3 _--| | | | | | | | | | | 4 | I | I I I | SiO;, in percent (Approximate trend for all volcanic rocks) c 56.5 54 51.5 49 46.5 EXPLANATION | __ t-] o a SiO,, in percent (Trend for caprocks only) <0.004 _ >0.004 e Bead variation A z a Trachyandesite of the A 0 ® _ 50 s* 5 ve ..6. 5 o Saline Range 1 11°. "2 14 9 13 Bead of chemically _ analyzed sample (Saline Range) L L & & (Isolated specimens) e _ R # O lol ® Intrusive trachyandesite oo 0 o 6° 3 7 5° 4 & LU e eo @ O @ } Intermediate to felsic volcanic rocks 10 12 8 I M | | | i | ad 1.48 1.50 1,92 1.54 1.56 1.58 1.60 1.62 Refractive index of fused beads Fraur® 12-Comparison of refractive index of fused samples of volcanic rocks of the Saline Range-Inyo Mountain region. Silica estimates from To help determine how the trachyandesites of the Saline Range fit into this picture, three whole-rock samples from the caprock flows and clinopyroxene and K-feldspar mineral separates from the south trachy- andesitic plug were submitted for potassium-argon age determination. The data and results of these deter- minations are shown in table 3, the location of the samples, on plate 1. Two of the three caprock samples, specimens 14 and F-158, gave essentially the same age. The third sample, W-352, may also be about the same age, con- sidering its larger analytical error. Specimen W-352 also contains small amounts of what looks like devitri- curves in figure 15. Tasum 3.-Potassium-argon ages of trachyandesites [Ages were calculated using the following constants: K« decay constants:\.=0.585% 10-! year-!, AS=4.72X10-! year-; Abundance ratio: K#Y/K=1.10%X10- atom per atom. Potassium analyses on a Baird flame photometer using a lithium internal standard by Lois Schlocker. Argon analyses made by using standard techniques of isotope dilution and age calculations by Jarel Von Essen] Specimen Material Percent _Arirad_ _ Apparent KO (moles pergm) _ Arar age (m. y.) Whole rock...... 1. 54 6. 865X10-12 0. 05 3. 0+1.2 ... Whole rock........ 1.28 5. 367% 10-12 11 2.84 .5 -_. Whole . 620 5. 996X10-12 . 04 6. 443.2 ... K-feldspar........ 6. 50 3. 368X10-11 . 40 3.5% .1 Clinopyroxene.... .126 1. 274%10-12 . 05 6.843. 9 Specimen locations, given in California Grid System coordinates, Zone 4: 14: 2,314,000 E., 559,300 N. F-158: 2,336,900 E., 594,600 N. W-352: 2,328,500 E., 574,600 N. 7: 2,336,800 E., 596,300 N. VOLCANIC ROCKS, SALINE RANGE, CALIFORNIA fied glass, in contrast to the other samples, which are clean and completely crystalline. Thus there are both petrographic and analytical reasons for placing less confidence in the age determination for specimen W-352. Probably an age of 2.5-3.5 m.y is the best esti- mate of the age of the caprock we can make with the present data. The radiometric age determination on the K-feldspar from the south plug is analytically very good with 40 percent radiogenic argon. The determination of the clinopyroxene is much less certain, and analyti- cally we can say it is not distinguishable from the age determination of the K-feldspar. Thus, we can assign, very tentatively, an age of about 3.5 m.y. to the south plug. This means the trachyandesite plugs are probably slightly older than the flow rocks, which fits the most plausible field interpretation, that the plugs were some- what eroded before the flows were emplaced. These radiometric ages place both the plugs and the flows in the later part of the Pliocene, although some current estimates of the length of the Pleistocene might include these rocks in the Pleistocene. FELSIC TO INTERMEDIATE VOLCANIC ROCKS The felsic to intermediate volcanic rocks which underlie the caprocks in a few places are quantitatively unimportant in present-day exposures but would prob- ably be of considerable areal extent if the capping trachyandesitic rocks were stripped away. These expo- sures give hints of a more complex volcanic history for this area than might be supposed from first encount- ers with the overwhelming flood of rather monotonous caprock extrusives. Exposures of these older, more felsic volcanic rocks are limited to the eastern part of the Waucoba Wash quadrangle. Outerops of rhyolitic rocks are found to the north at about the same longitude in the Waucoba Springs quadrangle (C. A. Nelson, written commun., 1965). To the east in the Dry Moun- tain quadrangle, rather extensive areas of more felsic rocks are exposed beneath the caprock (B. C. Burchfiel, written commun., 1964). The two small felsic rock exposures in the Paleozoic rocks of the Saline Range are definitely intrusive. The other exposures could be either intrusive or extrusive. The northernmost exposure of older volcanic rocks, on the north border of the Waucoba Wash quadrangle, is the tip end of a more extensive exposure in the Wau- coba Springs quadrangle. According to C. A. Nelson (written commun., 1966), these rocks are a series of rhyolitic and obsidian flow rocks with associated tuff beds. D17 The exposures in the area of specimen 8 (pl. 1) are the largest exposures of felsic to intermediate volcanic rocks in the area, but they cover an area of only about one-twentieth of a square mile. The rocks are pale reddish brown, generally massive, but with some faint flow banding. They are overlain by tuff and alluvium, which in turn is covered by the trachyandesite caprock. According to the Rittmann classification (1952), this rock is a latite. It has an aphanitic, pilotaxitic ground- mass liberally sprinkled with phenocrysts of andesine. Much less common are small phenocrysts of biotite and amphibole nearly masked by iron oxides and relatively clean pale-green clinopyroxene. One specimen has a few small patches that may be pseudomorphs of olivine. This rather distinctly colored latite is older than the caprock by an interval of erosion of unknown length. To my knowledge it is a unique rock type in this region. Two dikes of rhyolitic rocks are intrusive along faults into the Paleozoic rocks of the Saline Range. The northernmost occurrence, which intrudes the Rest Spring Shale of Mississippian age, is made up of white to light-gray rocks that look felsic but appear to be altered; their original composition is somewhat uncer- tain. In part, this is a breccia mass that looks as though it might have been a feeder for pyroclastic material. The southern exposure (specimen 10, pl. 1) is a dike of light- to dark-gray flow-banded obsidian and rhyolite which was intruded along a fault in Lower Cambrian clastic rocks. The dike rock is dominantly glass studded with small phenocrysts of quartz, sodic plagioclase, and biotite (hyalopilitic). Embayed quartz phenocrysts with square outlines indicate crystallization as high-temper- ature (beta) quartz. Some rounded quartz grains may be xenocrysts, as the quartzitic wallrocks would be an easily available source of quartz grains. Devitrification of the groundmass glass is taking place by the devel- opment of what looks like sericitic material along curved cooling cracks. Also, microcrystalline patches indicate devitrification elsewhere of the groundmass glass. Chem- ically, specimen 10 (table 1) is a soda rhyolite according to Rittmann (1952). Rhyolitic pumice and ash in tuffaceous beds that are present elsewhere beneath the capping trachyandesite flows are also part of what may have been a rather widespread period of rhyolitic volcanism and pyroclastic activity, now largely buried by the trachyandesite. The southernmost exposure of felsic to intermediate volcanic rocks is in the area of specimen 12 near the south end of the Saline Range. A window along a fault searp exposes some 400 feet of nearly horizontally layered volcanic rocks below the capping trachyande- D18 site. The lower part of the section consists of layers of ash and lapilli and dense olive-gray flow rocks that contain olivine, completely pseudomorphed by red iddingsite(?), and clinopyroxene in a pilotaxitic to trachytic matrix of plagioclase laths. The upper part of the section consists of dusky-yellow, light-olive- brown, and moderate-brown dense glassy volcanic rocks overlain by red cinder layers which are in turn capped by the trachyandesite flows. These glassy (byalopilitic) rocks have a very faint felty texture. Rounded to embayed plagioclase crystals are the most common visible crystals, but thin plates of biotite rimmed with iron-rich material are also scattered through the rock. Specimen 12 is one of these glassy rocks; it is a soda rhyolite by the Rittmann (1952) classification. Neither in the field nor from thin- section examination was it suspected that these glassy rocks were so felsic, as no quartz was visible. The upper thyolitic part of this section as well as the thyolitic dikes just discussed and the rhyolitic pyroclastic layers in some older alluvium probably are part of a sequence of rhyolitic volcanism that preceded the caprock trachyandesitic volcanism. This rhyolitic interval was itself preceded by earlier andesitic or basaltic volcanism with accompanying pyroclastic activity. The position of the latitic rocks of the area of specimen 8 in this volcanic history is unknown except that it preceded the caprock trachyandesite volcanism. OWENS VALLEY VOLCANIC ROCKS In the course of mapping the Independence quad- rangle, only cursory observations were made of the volcanic rocks in and adjacent to Owens Valley. After chemical data had been accumulated on the Saline Range volcanic field, it seemed worthwhile to have some comparative chemical data. Consequently one fresh, and hopefully representative, sample was selected from each of the two flows (Specimens 15, 16, pl. 1) and chemically analyzed. To supplement the descriptions of Moore (1963, p. 135-138) of the west- ward extension of these flows, which he refers to as the olivine basalt west of Aberdeen, a few samples were examined in thin section and the index of refraction of several glass beads, prepared from powders of these volcanic rocks, was determined. The volcanic rocks in Owens Valley (pl. 1) in the northwestern part of the Independence quadrangle are the ends of basaltic flows that originate along faults (marked by alined cinder cones) along the east front of the Sierra Nevada (Moore, 1963). The older olivine basalt of Oak Creek (pl. 1), exposed farther south, also originates from the lower eastern slope of the Sierra Nevada. In addition, there are several patches of black volcanic ash and fine-grained SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY lapilli (Qba on pl. 1) plastered along the west side of the Inyo Mountains near the north boundary of the Independence quadrangle. Mixtures of ashy material with granitic debris in some of the alluvial deposits suggests that originally the ash deposits were much more extensive but have largely been washed off the granitic exposures. Some of these ash patches are virtually continuous with a volcano and associated flows to the north in the Waucoba Mountain quad- rangle (Nelson, 1966). The only volcanic rocks that originated within the Independence quadrangle form an areally insignificant, but nevertheless interesting, occurrence in the SEH sec. 30, T. 11 S., R. 35 E. (Ross, 1965, pl. 1). Here, a small ringlike outcrop, only a few hundred feet across, is made up of basalt ash, lapilli, bombs, and cinder chunks as long as 5 feet. Presumably this outcrop, which straddles a shear zone in the underlying granitic rocks, represents a single pulse of volcanic activity. Felsic volcanic rocks are not exposed in the In- dependence quadrangle, but there are two occurrences of rhyolitic material in tuffaceous sedimentary rocks. One, first noted by Knopf (1918, p. 52), consists of several feet of bedded pumice and crystal fragments resting on granitic rock and is exposed in the north- central part of sec. 6, T. 13 S., R. 36 E. Probably these beds are similar to the lower layer of the older alluvium in the SEX sec. 5, T. 13 S., R. 36 E., which is rich in rhyolitic pumice and ash (Ross, 1965, p. O51). The nearest exposed source of rhyolitic material is an extru- sive dome of pumiceous rhyolite about 18 miles to the northwest (Mayo, 1944). The rhyolitic volcanic rocks beneath the capping flows of the Saline Range are about the same distance to the northeast (Ross, 19672; C. A. Nelson, oral commun., 1966), but they are presumably somewhat older. It seems most probable that these pumiceous and ashy layers in the Independ- ence quadrangle represent fallout from airborne material associated with the outpouring of the voluminous Bishop Tuff of Pleistocene age (Bateman, 1965, p. 151). In the field the Owens Valley basaltic volcanic rocks are dark gray to black, dense to vesicular, and some specimens have visible olivine phenocrysts. The sur- faces of both flows are locally ropy; in part, they are extremely vesicular, and even scoriaceous. Vesicles are commonly elongate and show conspicuous but varied foliation in the flows. Cindery, blocky breccia is common along the surface and is particularly abundent near the exposed margin of the north flow. Some bombs have been found on the surface and embedded in the breccia. From field examination the north and south flows look like similar typical olivine basalt. Microscopically there is some difference between the flows. The north flow is mainly equigranular, and the VOLCANIC ROCKS, SALINE RANGE, CALIFORNIA largest crystals of olivine are about 1 mm in maximum dimension. It is holocrystalline pilotaxitic, and almost all grains are coarse enough to be optically identifiable. The south flow, on the other hand, is more porphyritic, although the largest crystals are not much larger than those of the north flow. The grain-size variation is much greater, as the south flow has a very fine grained groundmass, though it seems to be holocrystalline. Olivine is notably fresh in the north flow, but is in part altered to iddingsite(?) in the south flow. Another difference is reflected in the refractive index of glass beads from these flows. Those from the north flow average 1.594, those from the south flow, 1.600. Even though the number of samples is small and there is a slight overlapping of indices between the two flows, there does appear to be a real, if small, difference in their glass-bead index of refraction. This is all the more unusual because they have almost identical SiO, and total iron contents (table 1). It would seem the differ- ence in glass-bead indices must be accounted for by the differences in Al;O;, MgO, and CaO content in these two flows. Although there are striking textural differences between these two flows, they have the same general mineralogy. Both have microlites of calcic labradorite. Both have clear olivine that has a 2V near 90°, and both have pale-yellowish-green clinopyroxene (probably augite). Judging by the chemical variations of the two flows with regard to Al;0;, CaO, and MgO, however, the percentages of these minerals are different, although no modal counts were made on these rocks. The major difference between the two flows in normative minerals (table 4) is that olivine exceeds diopside in the north flow, whereas these two normative constituents are almost equal in the south flow. The total of these two ferromagnesian minerals also is higher in the norm for the south flow. TaBur 4.-Normative mineral comparison, in percent, of Owens Valley flows South flow (sample 16) North flow (sample 15) Normative minerals Orthociase................... 1 4 1 1 00 i- o po 4 Diopside..i-.l.......l._.l.ll.. OVYIANCY EEE seca csa eae alas. Magnetite-ilmenite-apatite.. ... cll ll 1 }29. 6a. 4 }36. F 3 6 4 0 6 2 © 00 Or c 1 to go os go bo ye rlgo . | 0.1 100. 2 o The chemical analysis of these two flows (table 1) bear out the conclusions from microscopic examination that these two flows are different. Moore (1963, p. 135), who had the opportunity to study a much larger DIO area of exposure of these rocks and their source area, noted "the similarity and apparent contemporaneity of these volcanic rocks." He did not, however, have the benefit of chemical analyses. Petrographic and chemical differences in the selected samples from the two flows suggest the need of further study of the Big Pine vol- canic field to see if these variations are significant, or if they represent fortuitous differences due to the small size of the sample of the present study. Both of the analyzed samples have trachybasalt affinities according to the chemical classification of Rittmann (1952). The sample from the north flow is an olivine-andesine trachybasalt, whereas the sample from the south flow is an andesine basalt, but is almost on the border of Rittmann's trachybasalt field. The Owens Valley basaltic volcanic rocks are alkali basalts (fig. 13) that furnish one more set of chemical data on a volcanic province that seems to be char- acterized by high K,0. There can be little doubt that the Owens Valley flows are much younger than the Saline Range flows. Those in the Owens Valley still preserve some original form and original surface features. Moore (1963, p. 135) has noted, however, that these rocks are not as young as casual inspection of their forms might suggest, for as much as 30 feet of valley fill has accumulated since 9 T T - - -I EXPLANATION * O 1 N n o alm drat ns Cian Ns S UDE meters 10m A m Caprocks H o & i {oM # Py Intrusive rocks _ p 6 |- ”E ® at a 6 be 8 6: Owens Valley __ ui 5 |- "a / ll rocks & z : E l? % / 6 < $ 4a |- ¢ + ea $ # f/ s* a -I 3 & ..... % g | S. 21- / <0o xl £- < 0 1 | | 40 45 50 55 60 SiO;, IN PERCENT Ficur® 13.-Plot of total alkalis and silica for volcanic rocks of Saline Range area. Dotted lines show fields of Hawaiian lavas after MacDonald and Katsura (1961, p. 367). D20 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY I ALKALIC- I CALC- I i ALKALIC | CALCIC ( ALKALIC | ! (51) ! (56) ! (61 I o \o. - 10 |- 2 o Ox 0 % C & 0 R E o L 9 o X O 5|- Og ® [9- o 1 1 1 1 | 45 50 - (52.2) 55 60 65 70 75 SiO,, IN PERCENT FicurE 14.-Variation diagram showing classification of Saline Range-Inyo Mountains suite according to alkali-lime index of Peacock (1931). Solid points are Owens Valley volcanic rocks. their eruption. Nevertheless, these flows are definitely Quaternary and may be Holocene. The Saline Range flows, by contrast, are strongly dissected by canyons and are disrupted by faults of several hundreds of feet of vertical displacement. Chemical comparison shows the Owens Valley flows are considerably higher in MgO and somewhat lower in N&a,0 than the Saline Range flows. Other oxide percentages seem to overlap for the two areas. CHEMICAL CHARACTERIS’II‘JIIg%OF THE SALINE RANGE S Rocks of the Saline Range suite, as stated in the "Introduction," are markedly alkalic. In the plot of K°'O-+Na":0 against SiO; of MacDonald and Katsura (1961, p. 367), the Saline Range rocks plot well up into the alkalic basalt field (fig. 13). High total alkalis are probably the outstanding chemical characteristic of these rocks. By using Rittmann's classification (1953), p value, ' all the chemical samples are alkaline. Peacock (1931) has subdivided the calc-alkaline and alkaline fields and has established two intermediate classes, alkali-calcic and calc-alkalic, on the basis of the S10; content where total alkalis equal CaO. Using this clas- sification, the suite is alkali-calcic, but near the alkalic boundary (fig. 14). ! p is derived as follows: p=(Si0;) (An+0.7), where An=(Al-Alk)/(Al+Alk); Al=0.9Al0; and Alk=K>0-+1.5Na:0 in weight percentage. p values higher than 55 are calc-alkaline, 55 or lower are alkaline. Fused-glass beads were prepared from powders from each chemically analyzed specimen according to the method described by Rinehart and Ross (1964, p. 60) and modified somewhat by Huber and Rinehart (1966, p 103). The plot of the indices of these beads against SiO; of the chemical analysis, shown in figure 15, is not as linear as one would hope. There is a suggestion that the capping trachyandesites plot on one roughly linear trend (dashed line in fig. 15), and the intrusive trachy- andesites and the intermediate to felsic volcanic rocks plot on a somewhat different curved trend. When an average curve is fitted for all the points, there are some rather anomalous points. Specimens 6 and 7 can prob- ably be discounted, as they are probably too coarse grained for a powder of a small sample to represent any average composition. Specimen 1 plots as the most anomalous point and has an anomalous composition, being high in SiO;. Yet it appears to be fresh and defi- nitely is part of the caprock sequence. The reason for its anomalous position is unknown. Specimen 11, the fine-grained intrusive of the Saline Range, also appears to be anomalously high in silica and off the general trend. In view of these irregularities, it is probably best to use the average curve as the best approximation for the whole suite-we just do not have enough data to evaluate the possibility of two trends. Although generally only SiO; is plotted against the refractive index of fused-glass beads, and this plot is VOLCANIC ROCKS, SALINE RANGE, CALIFORNIA D21 75 7 T T T T T T 12 T T T T £5 wave T T T T T s 70 |- 3 - 6 |- | a 10 |- - A 35 10 | 12 "a 65 |- < ST & 4 'an? t 8 |- a 2“ Q1 7 13 = 1 l > L- - * 4l- 'o ge ® _" S. & e/ 15+" 55 F n 3}- 36 4 |- 50 |- it 2 1 | 1 | | 1 2 |- 45 | - T T T u T T 0 a' 19 T T T T + 3 f- { 4 12 T T T T T by; L 2 *n (s E s a E s 10 |- Jts - 14 “0015- 5 ‘s 1 13 6 17 - e §* - 2 |- in $1 me* r g ist" o y/ 4u 34 16 |- a} # - SL _ e < < o 2 it a> 15 |- a (* A 1 }- - 4 |- - as Al 7 2 L 14 - a - - =d 10, /A; j 13 'a 1 N | 1 1 1 0 A101 1 | 1 1 1 0 | 1 | I 1 | 1.48: 1.90 - 1.62., 1.54. 1.56 1.58 1.60. '1.62 REFRACTIVE INDEX 12 T T T T f 6 T T T T T T 6 12 t a $ { i EXPLANATION |- A 3] 10 01 5 A* ( SALINE RANGE ® 3 s |- "ta, 5 ¥ 12. a5 5, 2 a A3 Trachyandesite caprocks Ag # (6 L * 11 s & 3 |- : a' s 1 = e° al a = a Intrusive trachyandesite ® o 4} - 2 |- it 15 - +- RU \“.‘9 y Other volcanic rocks y P 2T 12 =I 1 |- ~ Cm A OWENS VALLEY *13 0 & | 1 1 1 i 1 015 0 1 1 1 L 1 1 148 1.50 182 154 156 158 160 162 148 (1.50 ~452 1.54 L596 158 160 1.62 REFRACTIVE INDEX Trachybasalt REFRACTIVE INDEX Numbers refer to table 1 (chemical analyses) Fraur® 15.-Refractive index of fused-glass beads plotted against oxides, in weight percent, for chemically analyzed volcanic rocks, Saline Range-Inyo Mountains area. used to estimate SiO; percentage of fused-glass-bead specimens for which there are no chemical analyses, some authors (for example, Callaghan and Sun, 1956; Shilov and others, 1958) have used this refractive index of fused-glass beads to determine other oxides. To see how valid this was for the Saline Valley suite, all the oxides were plotted against this refractive index (fig. 15). From these plots it seems that the index of a fused-glass bead of this suite is a good clue to the amount of CaO and the amount of total FeO (with the exception of the two coarse-grained intrusives, speci- mens 6 and 7), a fair help in estimating K,0, MgO, and TiO;, but rather hopeless for estimating Al;O; or Na,0. The CaO plot against refractive index is extraordi- narily linear; even the coarse-grained specimens 6 and 7 and the much younger and chemically dissimilar specimens 15 and 16 fall on the trend. Only specimen 13, which is unusually rich in CaO, is off the trend. In current petrochemical discussion, the interrela- tions of total Fe as FeQ, MgO, CaO, and TiO, are considered significant. Various pairs of these oxides have been plotted in figure 16. The plots are generally self explanatory, and show good correlations but with some marked divergences. By far the best correlation is obtained from the plot of total Fe as FeO against TiO». Standard variation diagrams (fig. 17) show that these volcanic rocks follow good normal trends and that the suite appears to be a chemically related one. The best linear trends are shown for total Fe as FeQ, K;,0, and CaO. Somewhat comparable trends were plotted by using the differentiation index of Thornton and Tuttle (1960). These plots (fig. 18) show even more concen- D22 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 20 T T T is 14 ca I I o & 16 |- - 12 |- a' - a 13 ® LU 3 a5 - - = O 0 i g ® < 8 |- a z H £ < E o "4.1 in < # & 5 - 0 -] 0 12 12 T T wy T m a5 $ a° 10 |- o 3 o 4 f ‘ y 369. o 1 2 3 a 5. cleft TiO e U5 6 2 s pal $e, § 7 i Fe 5 a* Eo 4} A EXPLANATION 2 |- 12 a A SALINE RANGE P xi ais o L L 1 | 1 fachvandesi o 7 a # ® To T2 Trachyandesite caprocks CaO Fo 12 T #, T T £ Intrusive trachyandesite a6 AO 9p 58 &° *A Other volcanic rocks e' $ , 15 8 "elt a'* * «* | owEnNns VALLEY & a> 5 C 1 al o Trachybasalt 0 6 |- - u P: (o a o A Numbers refer to table 1 +. A. % (chemical analyses) 2 112 I Note: All figures in weight percent a!° 0 1 1 1 1 1 0 2 4 6 8 10 12 MgO Figur® 16.-Selected oxide ratios of volcanic rocks, Saline Range-Inyo Mountains area. trated linear trends than the standard variation dia- | specimens, including the two younger rocks from Owens grams. CaO, and K,0 have particularly good | Valley, make a relatively compact field. Normative An linear trends. is the most variant constituent and its range in the A ternary diagram was prepared to show the plot of | suite is the major control of the elongate field. It can the normative feldspar molecules (fig. 194). All the | also be stated that the relation between normative An VOLCANIC ROCKS, SPECIMEN NUMBER Al;O3 TOTAL Fe AS FeO 8 $2 = g. 6 |-* ® a ~ U 4 |- a e U 3 ip ' ams n_ a 0 1 1 L 1 1 h A 12 AAL ___, e I- ; { } D* A 10 Fae 5 s* .\I. A + { 3 a £ o 4 |- w F we =I 0 I L. L 1 L 1 i 46 50 54 58 62 66 70 74 78 Si02 SALINE RANGE, CALIFORNIA D23 SPECIMEN NUMBER Us! 15:6 tA t . 10s __. 6 T T T "%"" T T T Pe *e # A____—————-——A A o, 4 [w JIT’T' a - B Z 2 - al 0 | 1 1 1 1 1 1 6 I T T I I I T 4 |- /A o J a ® _/A x 2 "/.0”‘/. < ® o * 1 1 1 1 1 1 1 6g putt y tas f { } 50 _'. LJ a ["e a s U 40 |- \ a & ® U < -I 3 A & = |- 2 § |-* .. A @ a 20 |- & _ f \ a a \A * |? ~ 0 1 1 1 1 1 1 1 46 50 54 58 62 66 70 74 78 Si0 EXPLANATION SALINE RANGE A m Other volcanic Intrusive rocks trachyandesite ® Trachyandesite caprocks OWENS VALLEY O Trachybasalt Numbers refer to table 1 (chemical analyses) Figur® 17.-Variation diagram of common oxides, in weight percent, in volcanic rocks of Saline Range-Inyo Mountains area plotted against SiO;. and Or is the most significant, normative Ab being much less variable. The ternary A-C-F diagram (fig. 19B) shows a good clustering of points except for specimen 10. This diagram tends to show unaltered igneous rocks clustering along the F-An join. The displacement of the field of the Saline Range region suite below this join reflects the related richness in alkalis, which gives a somewhat lower than normal A value for comparable C and F values. The markedly aberrant specimen 10 has an unusually low content of iron oxide, even for so felsic a rock. Total Fe as FeO is only 0.5 percent, in contrast to about 2 percent total Fe as FeQ for the average rhyolites of Nockolds (1954, p. 1012, table 1). This glassy dike rock is incipiently devitrified and iron has possibly been leached out. From thin-section study, however, this rock did not look extensively altered. It is not a normal rhyolite chemically, but the single small dikelike occurrence does not permit any definitive statement as to whether this was originally an unusual rock or was made unusual by subsequent alteration- of course it is possible that the low iron value is an analytical error. The Alk-F-M diagram (fig. 190) is shown for comparison with other chemical plots. This rather standard ternary diagram does not show a good linear- differentiation trend, but if specimens 15 and 16 from Owens Valley and specimens 6 and 7 of coarse-grained D24 SHORTER SPECIMEN NUMBER 3 H1 2 40 a5 ax f oal c dine 80 T f= J = 7 a f al 40 | 1 1 1 1 20 T T T T T T T U A + * re d A a "o - & aqs- - A a "" % % =t " a el A CC r =>" 0 1 1 1 1 1 3t O Ed s -t - T T T T T T € - s < 10 [- b -~ 2 TOTAL Fe AS FeO &n T » a \.\ + 1 LLL _ T I Mgo ® 0 Ack. 1 1 L I | 1 100 90 80 70 60 50 40 30 20 DIFFERENTIATION INDEX (NORMATIVE Q+or +ab) CONTRIBUTIONS TO GENERAL GEOLOGY SPECIMEN NUMBER 3 7 2 1's u Ill s R -o 4 15 Iflil‘fi ‘F' re.. T T Pad m 10 |- /_o. U -I o zt‘ Fo 5 e 5- s A A i- g 1 | | 1 Z lad o fed a. 10 T T T T T fo 3 8 Aad o % & ——AA‘\L e - 2 2° Cot os -_ as m * % < 0 1 1 I #3 1 | 10 ; ; | ; ; lll IIIII l11m ll o # I| Taa m- _n 1 Am- QHA..\ + ae -*- * | | 1 1 1 (*s a_ I 100 90 80 70 60 50 40 30 20 DIFFERENTIATION INDEX (NORMATIVE Q+ or+ab) EXPLANATION SALINE RANGE U a A Trachyandesite Intrusive Other volcanic caprocks trachyandesite rocks OWENS VALLEY * Trachybasalt Numbers refer to table 1 (chemical analyses) FiGur® 18.-Oxide percentage plotted against differentiation index of volcanic rocks, Saline Range-Inyo Mountains area. intrusive are ignored, there is a fairly good clustering of the mafic rocks that ties reasonably well with the more felsic rocks along the plotted trend line. Semiquantitative spectrographic analyses of the 16 volcanic rock specimens are shown in table 1. Some 22 minor elements were detected and 29 others that are listed were looked for but not found. Because there is almost no comparable published data on other volcanic rocks of this region, these data constitute a first tie point of the minor-element concentrations in volcanic rocks of this region. Comparisons have been made with igneous-rock averages of Vinogradov (1956) as compiled by Green (1959, table 2). Vinogradov's averages for mafic and felsic rocks are generally similar to chemically comparable volcanic rock types in the Saline Range area. Notable exceptions are lanthanum (La), strontium (Sr), and neodymium (Nd). Both lanthanum and stron- tium are some two to four times more abundant in the Saline Range rocks, and neodymium in the trachyan- desite rocks is some 15-30 times more abundant than Vinogradov's average for mafic and intermediate igne- ous rocks. Until more trace-element data are available in the western Great Basin and Sierra Nevada region, it will be difficult to evaluate the significance of these figures. Comparison of trace-element concentration in the Saline Range and Owens Valley specimens (table 1) shows comparable values except for nickel and chro- mium, which show about 300-500 ppm (parts per million) and 500-700 ppm, respectively, in Owens Valley rocks as compared with only about 20-100 ppm and 20-300 ppm in the Saline Range rocks. The con- siderably higher MgO and CaO in the Owens Valley analyses may indicate a higher concentration of ferro- magnesian minerals that could carry the nickel and chromium, although Saline Range rocks with nearly VOLCANIC ROCKS, SALINE RANGE, CALIFORNIA A=A1;0;-(Na;0+K;0) C=Ca0-(3P;0,;+CO,;) B +2Fe;0,-TiO; FraurE 19.-Ternary plots of selected oxides and normative comparable MgO and CaO (specimens samples 9 and 13, table 1) do not have comparatively high nickel and chromium concentrations. Possibly, the higher concen- tration of nickel and chromium could be a clue to a deeper source for the Owens Valley lavas. Moore (1963, p. 138) has suggested, from other evidence, that these lavas did originate at great depth, perhaps near the Mohorovici¢ discontinuity. Semiquantitative spectrographic analyses have been published from the latite series of the Bridgeport-Sonora Pass area of the eastern Sierra Nevada (Nockolds and Allen, 1954, p. 280). Most of the 17 trace elements determined from this latite series are present in con- centrations comparable to concentrations in rocks of similar composition from the Saline Range region. Diff- erences are: Gallium (Ga) is two to three times more abundant in the Bridgeport-Sonora Pass area; lantha- num (La) was not detected in the Bridgeport-Sonora Pass andesitic and basaltic rocks, whereas it is present in concentrations of 50-200 ppm in the Saline Range F=FeO4+MnO+MgoO D25 EXPLANATION SALINE RANGE ats Trachyandesite caprocks a3 Intrusive trachyandesite 41" Other volcanic rocks OWENS VALLEY a15 Trachybasalt Numbers refer to table 1 (chemical analyses) F=FeO4+2Fe;0,+MnO Alk=K;04+Na;0 M=Mgo minerals, volcanic rocks, Saline Range-Inyo Mountains area. rocks; scandium (Sc) was generally not detected in the Sierra rocks, but is present in concentrations of 10-50 ppm in the Saline Range rocks; nickel (Ni) is found in comparable concentrations in the two areas except for the Owens Valley rocks, in which it is more abundant (see p. D24). Spectrographic data from the late Cenozoic trachy- volcanic rocks from the Death Valley area (Drewes, 1963, p. 21) show the following comparison with the Saline Range volcanics: Cesium (Ce), lanthanum (La), niobium (Nb), scandium (Se), strontium (Sr), ytterbium (¥b), and zirconium (Zr) are more abundant in the Saline Range rocks, and lead (Pb) is less abundant. All the other trace elements have comparable values. In all the comparisons, lanthanum appears to be more highly concentrated in the Saline Range rocks, both in specific volcanic suites and for general crustal averages of similar rocks. In order to give some idea of the trends of the minor elements for the Saline Range suite, graphs using the D26 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 0.3 [ 02k" n (3 t -l Q 0.1 - 0 C - 0.1 p [ l E ts I C 0.05 |- s 0.05 |- P . 5 [- d o @ & & n a. iz L i: { 9 (- l ui L _ 3 £ z TI A ©..,.001 t- # a w ( o 3 # [ - Z < s = - E 0.005 o - -| - zZ 0.005 |- 7 k eo ~ £ i o < > % c 6 4. 'E 2 a - 2 o E CZ} .- L s 0 fyi 0 j 5 0.001(- - O : <7 $ _] "£ 09.001 |- < |- ; 1 _ © R fs - So - A |- - |- "% "b - L - L - Cos & 0.0005 |- q R p - -| - § 0.0005 |- I- - ’_ - |- Q -I |- - 0 0 sl E a Be O 0.0001 | | | I 0.0001 | | | | | Average 8 10 12 3 4 5 6 7 trachyandesite Increasing silica ------*- A VOLCANIC SUITE Margin -----»~ Core B INTRUSIVE TRACHYANDESITE Figure 20. -Trace-element trends in volcanic rocks, Saline Range. semiquantitative center points as plotting points were constructed for the intrusive trachyandesite analyses and for the entire suite (fig. 204, B). It must be remem- bered that these percentage figures are not absolute values; but even so, the slopes show relative increases and decreases in the minor elements. Minor-element trends within the trachyandesite cap- rock (specimens 1, 2, 9, 13, 14) were noticeable for only a few elements and thus were not plotted. Beryllium increases about threefold with increasing silica. Chromi- um decreases threefold to fourfold with increasing silica. Other minor elements have inconclusive trends or are relatively constant for the caprock range of 9 percent in SiO, content. Trends within the intrusive trachyandesite (speci- mens 3-7) are rather pronounced for several elements from the dense marginal rocks to the pegmatitic core rocks. Increases of two or more times from margin to core are noted for vanadium (V), ytterbium (XB), copper (Cu), niobium (Nb), and yttrium (Y). Cesium (Cs), lanthanum (La), zirconium (Zr), and neodymium (Nd) are also somewhat concentrated in the pegmatitic core rocks. The most evident trends, however, are the decreases coreward, by five to ten times, in nickel (Ni) and chromium (Cr). It should be noted that these trends are based solely on physical position relative to the margin of the intrusive plugs. The trend of VOLCANIC ROCKS, SALINE RANGE, CALIFORNIA Si0;, which has a range of 3.5 percent, does not seem to be consistent, at least from the samples analyzed. By using an approximate average for minor-element content for the trachyandesite caprocks and by using the more felsic rocks as plot points, plots of figure 20A show general trends for the entire Saline Range vol- canic suite. Rather strong decreases are noted for barium (Ba), strontium (Sr), vanadium (V), chromium (Cr), nickel (Ni), copper (Cu), scandium (Sc), and cobalt (Co), with the increase in silica from the tra- chyandesite to the rhyolite specimens. The only minor elements that showed concentration with increasing silica were boron (B) and beryllium (Be). Other minor elements show no noticeable trend over this silica range of about 25 percent. REGIONAL INDICATIONS OF AN ALKALINE SUITE One of the first indications of the presence of alkalic volcanic rocks in this region was an analysis reported by Gilbert (1941, p. 792) showing 2.11 percent K;,0 for a rock described by him as unusual from the center of a thick flow in an olivine basalt sequence in the Benton Range. Unfortunately, we cannot determine from this one high value the range in KQ content of average volcanic rocks of that sequence. Two addi- tional chemical analyses of mafic volcanic rocks mapped as basalt are reported by Ross (1961, p. 40) from southern Mineral County, Nev. One of these rocks, which consists of abundant small olivine phenocrysts set in a pilotaxitic groundmass of plagio- clase and clinopyroxene, contains 3.4 percent K;,O. Another, a dense grayish-black vesicular rock composed of plagioclase, hypersthene, and clinopyroxene pheno- erysts set in a hyalopilitic groundmass, contains 3.8 percent K;,0. These three samples of strongly K;,O-rich volcanic rocks suggest that the widespread "basalt" fields of this area, about 80 miles north of the Saline Range area, are part of a K,O-rich suite. Trachybasalt has been described by Bateman (1965, p. 151) from the Tungsten Hills area of the eastern Sierra foothills, about 45 miles to the northwest. These scattered flow remnants are holocrystalline and are composed of tabular to elongate plagioclase, rounded grains of augite and olivine, magnetite, and minor hypersthene; groundmass K-feldspar consti- tutes about 15 percent of the rock. Somewhat detailed work on the volcanic rocks of the Mammoth Lakes area, about 75 miles to the northwest, has been done by Rinehart and Ross (1964) and Huber and Rinehart (1967). These workers have recorded several chemical analyses of rocks mapped as basalt and andesite which contain about 2 percent K,0. It is interesting to note, however, that one sample of basalt from this area has only 1 percent K;0 in a rock D27 that contains 48.8 percent SiO), and normative sodic labradorite. Although this too exceeds the amount of K,0 in most basalts, it is not far out of line, and dictates caution in assuming that all these mafic volcanic rocks have extremely high K,0. Trachyandesite affinities are also shown by dark-colored hypersthene and olivine- bearing volcanic rocks from the Merced Peak quad- rangle, west of the Mammoth Lakes area, that contain from 2.6 to 3.0 percent K,0 (D. L. Peck, written commun., 1968), and by trachybasalts present in the Shuteye Peak quadrangle, just south of the Merced Peak quadrangle, that contain from 2.1 to 3.6 percent K,0 (N. K. Huber, written commun., 1968). Some 75 miles to the west-northwest, in the Hunting- ton Lake area of the Sierra Nevada, are scattered remnants of intrusive plugs and flows of trachybasalts that average 54 percent SiO, and 3 percent K,0 and that contain modal olivine and K-feldspar (Hamilton and Neuerburg, 1956). These rocks, which are also described as vuggy, seem to be very similar to the intrusive trachyandesites of the Saline Range, except that the Huntington Lake occurrence does not attain coarse pegmatitic texture. About 125 miles northwest of the Saline Range region is a volcanic area in the Sierra Nevada that is character- ized by an episode of latitic volcanism. These rocks, named the Stanislaus Formation by Slemmons (1966, p. 203-205), are the source for 10 chemical analyses listed by Nockolds and Allen (1954, p. 280). Five of the analyses are identified as basalt, and they range in composition from 48.0 to 53.8 percent SiO, and from 0.7 to 2.4 percent K,0; grossly, the lower the SiO);, the lower the K;,0. Other rocks in this suite contain as much as 63:7 percent SiO; and 5.4 percent K;,0, with intermediate rocks of about 57 percent SiO), and 3-4 percent K,0. These rocks of the Bridgeport-Sonora Pass area are thus typified by abundant K,0, but, like the Saline Range volcanic rocks, for rocks called basalt in the field, there is a wide range in K;,0, from less than 1 percent to 3 percent. For the area about 50 miles to the east near Beatty, Nev., Cornwall and Kleinhampl (1964, table 2) have published analyses of basaltic rocks that contain from 1.30 to 1.67 percent K,0. Somewhat farther to the southeast in the Funeral Peak quadrangle in the Death Valley area, Drewes (1963, p. 19-22) has tabulated a number of chemical analyses and spectrographic analy- ses for late Cenozoic olivine-bearing andesitic and basaltic rocks. These rocks range in SiO, content from 47 to 57 percent and have about 1.5 to 2 percent K,0. By the Rittmann (1952) classification, they are trachy- basalt, trachyandesite, or andesine basalt. Considering the rather great distances separating the Sierra Nevada, the Mineral County-Benton Range D28 area, and Death Valley from the Saline Range, there is no intention here of suggesting that these areas are part of the same eruptive suite. It is suggested, how- ever, that regionally the Saline Range volcanic rocks are part of a rather extensive late Cenozoic alkaline volcanic province. More chemical data are needed, par- ticularly of "basaltic'' rocks of late Cenozoic age to the south and east of the Saline Range, to help determine the extent and limits of this province. Moore (1962, p. 101) has synthesized a large amount of data on K,0 and Na,0 in Cenozoic volcanic rocks of the western states, and to compare these data he Las used Niggli's k value (the molecular ratio of K,0 :K,0 plus Na,0). Moore has then plotted k against SiO; for the various areas and obtained k value trend lines. The k values where these trend lines crossed 50 and 60 percent SiO; were used as control points to contour these data. Moore stated: "These maps show that potassium is least abundant relative to total alkalis (when rocks of the same SiO; content are compared) in a zone along the Pacific Coast, becomes more abun- dant eastward, and is highest in the Colorado Plateau and Northern Rocky Mountains." The values of k for the Saline Range volcanic suite, 0.23 for 50 percent SiO; and 0.32 for 60 percent SiO», fit into an area of intermediate k values between a local high in the Sierra Nevada and a much larger high to the east in Nevada and Utah (Moore, 1962, p. 128, 129). REFERENCES CITED Bateman, P. C., 1965, Geology and tungsten mineralization of the Bishop district, California: U.S. Geol. Survey Prof. Paper 470, 208 p. Callaghan, Eugene, and Sun, Ming-Shan, 1956, Correlation of some igneous rocks of New Mexico by the fusion method: Am. Geophys. Union Trans., v. 37, no. 6, p. 761-766. Cornwall, H. R., and Kleinhampl, F. J., 1964, Geology of the Bullfrog quadrangle and ore deposits related to Bullfrog Hills caldera, Nye County, Nevada, and Inyo County, California: U.S. Geol. Survey Prof. Paper 454-J, 25 p. Dalrymple, G. B., 1963, Potassium-argon dates of some Cenozoic volcanic rocks of the Sierra Nevada, California: Geol. Soc. America Bull., v. 74, no. 4, p. 379-890. * Drewes, Harald, 1963, Geology of the Funeral Peak quadrangle, California, on the east flank of Death Valley: U.S. Geol. Survey Prof. Paper 413, 78 p. Gilbert, C. M., 1941, Late Tertiary geology southeast of Mono Lake, California: Geol. Soc. America Bull., v. 52, no. 6, p. 781-815. Green, Jack, 1959, Geochemical table of the elements for 1959: Geol. Soc. America Bull., v. 70, no. 9, p. 1127-1183. Hamilton, W. B., and Neuerburg, G. J., 1956, Olivine-sanidine trachybasalt from the Sierra Nevada, California: Am. Min- eralogist, v. 41, nos. 11-12, p. 851-873. Huber, N. K., and Rinehart, C. D., 1966, Some relationships between the refractive index of fused glass beads and the petrologic affinity of volcanic rock suites: Geol. Soc. Amer- ica Bull., v. 77, no. 1, p. 101-110. SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 1967, Cenozoic volcanic rocks of the Devils Postpile quadrangle, eastern Sierra Nevada, California: U.S. Geol. Survey Prof. Paper 554-D, 21 p. Kennedy, W. Q., 1933, Trends of differentiation in basaltic magmas: Am. Jour. Sci., 5th ser., v. 25, no. 147, p. 239-256. Knopf, Adolph, 1918, A geologic reconnaissance of the Inyo Range and the eastern slope of the southern Sierra Nevada, California, with a section on the stratigraphy of the Inyo Range, by Edwin Kirk: U.S. Geol. Survey Prof. Paper 110, 130 p. Lacroix, Alfred, 1928, Les pegmatitoides des roches volcaniques 4 facies basaltique: Acad. Sci. [Paris] Comptes rendus, v. 197, p. 321-326. Macdonald, G. A., and Katsura, Takashi, 1961, Variations in the lava of the 1959 eruption in Kilauea Iki: Pacific Sci., v. 15, no. 3, p. 358-369. 1964, Chemical composition of Hawaiian lavas: Jour. Petrology, v. 5, no. 1, p. 82-133. Mayo, E. B., 1944, Rhyolite near Big Pine, California: Geol. Soc. America Bull., v. 55, no. 5, p. 599-619. McDougall, Ian, 1962, Differentiation of the Tasmanian doler- ite-Red Hill dolerite-granophyre association: Geol. Soc. America Bull., v. 73, no. 3, p. 279-316. Moore, J. G., 1962, K/Na ratio of Cenozoic igneous rocks of the western United States: Geochim. et Cosmochim. Acta, v. 26, p. 101-130. 1963, Geology of the Mount Pinchot quadrangle, southern Sierra Nevada, California: U.S. Geol. Survey Bull. 1130, 152 p. Nelson, C. A., 1966, Geologic map of the Walkcoba Mountain quadrangle, Inyo County, California: U.S. Geol. Survey Geol. Quad. Map GQ-528. Nockolds, S. R., 1954, Average chemical compositions of some igneous rocks: Geol. Soc. America Bull., v. 65, no. 10, p. 1007-1032. Nockolds, S. R., and Allen, R., 1954, The geochemistry of some igneous rock series, Part 2: Geochim. et Cosmochim. Acta, v. 5, no. 6, p. 245-285. Peacock, M. A., 1931, Classification of igneous rock series: Jour. Geology, v. 39, no. 1, p. 54-67. Rinehart, C. D., and Ross, D. C., 1964, Geology and mineral deposits of the Mount Morrison quadrangle, Sierra Nevada, California: U.S. Geol. Survey Prof. Paper 385, 106 p. Rittmann, Alfred, 1952, Nomenclature of volcanic rocks proposed for the use in the catalogue of volcanoes and key-tables for the determining of volcanic rocks: Bull. volcanol., ser. 2, v. 12, p. 75-102. --- 1953, Magmatic character and tectonic position of the Indonesian volcanoes: Bull. voleanol., ser. 2, v. 14, p. 45-58. Ross, D. C., 1961, Geology and mineral deposits of Mineral County, Nevada: Nevada Bur. Mines Bull. 58, 98 p. --- 1965, Geology of the Independence quadrangle, Inyo County, California: U.S. Geol. Survey Bull. 1181-0, p. 01-064. --- 1967a, Geologic map of the Waucoba Wash quadrangle, Inyo County, California: U.S. Geol. Survey Geol. Quad. Map GQ-612. --- compiler, 1967b, Generalized geologic map of the Inyo Mountains region, California: U.S. Geol. Survey Misc. Geol. Inv. Map 1-506. Shannon, E. V., 1924, The mineralogy and petrology of intrusive diabase at Goose Creek, Loudoun County, Virginia: U.S. Natl. Museum Proc., v. 66, art. 2, 86 p. VOLCANIC ROCKS, SALINE RANGE, CALIFORNIA D29 Shilov, V. N., Belikova, N. N., and Ershova, Z. P., 1958, The rocks-I. Differentiation index: Am. Jour. Sci., v. 258, use of the fusion method for determining the approximate no. 9, p. 664-684. chemical composition of Kainozoic rocks of South Sakhalin: | Vinogradov, A. P., 1956, The regularity of distribution of U.S.S.R. Acad. Sci. Proc., Geochemistry See., v. 118, 119 chemical elements in the earth's crust: Geokhimiya (English ed.). (Geochemistry), no. 1, p. 1-43 (English ed.). Slemmons, D. B., 1966, Cenozoic volcanism of the central | Walker, Frederick, 1953, The pegmatitic differentiates of basic Sierra Nevada, California, in Bailey, E. H., ed., Geology of sheets: Am. Jour. Sci., v. 251, no. 1, p. 41-60. northern California: California Div. Mines and Geology | Yoder, H. S., Jr., and Tilley, C. E., 1962, Origin of basalt Bull. 190, p. 199-208. magmas-an experimental study of natural and synthetic Thornton, C. P., and Tuttle, O. F., 1960, Chemistry of igneous rock systems: Jour. Petrology, v. 3, no. 3, p. 341-532. PREPARED IN COOPERATION WITH THE UNITED STATES DEPARTMENT OF THE INTERIOR CALIFORNIA DEPARTMENT OF CONSERVATION PROFESSIONAL PAPER 614-D GEOLOGICAL SURVEY DIVISION OF MINES AND GEOLOGY PLATE 1 S \" “fiQba c Qal Negr - Qal n O C C M m f cy \ -* 2A 7 p Qal W . f *I* s ‘\,/ < \ \ 4 I "dia 7 > /\Mer 3, “7:1 Fis “ w Y, .\ Qal © pe F Qbo , 12 fi ( rfiz.’ Tiv Ris 7 13 al (0) a Tt 3/Mzgr / Independence f f Qal C , 1 # j INDEPENDENCE - A\>~«-L1 /A/ > LZ, ci iy &) x y- \/@UADRANGLE:!\| ~QUADRANG@LE \/ {- f j=€)" R ~ \Bs 36°45 ' - LAL: /% faxiA \. [36°45 ( 117°45 2 0 2 4 6 MILES {0001-0041 £ 4 40 a I | "3 2 0 2 4 6 KILOMETERS Ba L404 1351 1 | I ] A. GENERALIZED GEOLOGIC MAP EXPLANATION Qal Contact Dashed where approximately located Alluvial deposits Includes pre- and intra-basalt layers ‘Ffi Vertical contact >_ C. | < M | ra Simmie v's bait Trachy basalt 83 Fault Qb, Olin??? bagaltf extruded from vents along Z” Dotted where concealed; bar and ball on ' Sterra Nevada front 2 in si f s w Qba, ash and cinders from volcanic area along 0 fommainpin side /Qa}) Inyo Mountain front 30 sli Ob? Strike and dip of beds Olivine basalt of Oak Creek (Moore, 1963) 30 36°58" woth j Strike and dip of foliation 6 $ Chemically analyzed specimen Trachyandesite and related rocks Tt, flows and agglomerate; pattern indicates cinder © 18 cones E Ttp, coarse-grained trachyandesite, pegmatitic and <4 Chemically analyzed specimen with miarolitic in part. Intrusive ~£ 1:1}; i a Tti, intrusive masses & in-secwon mode Ttc, red cinders and agglomerate E Tal, alluvial deposits, mainly gravel @ x Tt i i i Thin-section mode V? Pyroclastic rocks Felsic intermediate a Tuff, pumice, and ash volcanic rocks Potassium-argon radiometrically a dated specimens al TP | oy: ; | Mer 8 U P 5 LW 8 Outline of map B 0L ; f <2 4} MILE Granitic rocks undivided h 0 5 1 KILOMETER o {poo. 3000. 3000" - 4000..." Sbo0 ECT rfig 9 u L 1 1 1 1 d bits : 3:1 3 3 4 fil? N B. WINDOW IN TRACHYANDESITE VOLCANIC FIELD, SALINE RANGE Sedimentary rocks undivided a. INTERIOR-GEOLOGICAL SURVEY, WASHINGTON, D.C.-1969-G69225 GEOLOGIC MAPS OF VOLCANIC ROCKS OF THE NORTHERN INYO MOUNTAINS REGION INYO COUNTY, CALIFORNIA teas # QE 75 lakes 7 DaY PG No. 614-E Distribution of Thorium Uranium, and Potassium in Igneous Rocks of the Boulder Batholith Region, Montana, and Its Bearing on Radiogenic Heat > Production and Heat Flow GEOLOGICAL SURVEY PROFESSIONAL PAPER 614-E ZA DEC 10 1969 / ¢ A #~ m'kk‘iw, 7 f W 7 a [A/A A B3 *e '? ‘L'fl’ , \_ 2UVICNUE P aa einer a *~ y GC \ U.S.S.D. Distribution of Thorium Uranium, and Potassium in Igneous Rocks of the Boulder Batholith Region, Montana, and Its Bearing on Radiogenic Heat Production and Heat Flow By ROBERT I. TILLING erd DAVID GOTTFRIED SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 614-E A study of the relationship between content of hear-producing elements and host-rock chemistry and heat flow UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969 UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 40 cents (paper cover) CONTENTS Page cee enas E1 | Distribution of thorium-Continued =i. 1 Summary and comparison of thorium, uranium, and 2 potassium abundance datac___________________._ GeoIORIC -_ 2 | Nature of variation in thorium and uranium content.... Distribution of thorium, uranium, and potassium in Comparison of thorium and uranium distribution between igneous rocks of the Boulder batholith region____._... 6 contrasting magma Prebatholith volcanic rocks._._._._._...________._.___._ 6 | Radiogenic heat production and heat flow...... Batholith TOCKS-: -.» cll -cell. 6 | +. -_ _._ _. __» enn swans Postbatholith volcanic rocks.._.._..______.________. 6 ILLUSTRATIONS Figur® 1. Sketch map showing the areal distribution of igneous rocks in the Boulder batholith region_________._._____. 2. Generalized geolgic map of the Boulder batholith and vicinity________L___LLLL__LLL_LLLLLLLLLLLLLLLLLLL 3-5. TABLE ¥. . Summary of thorium, uranium, and potassium values and weighted average values for rocks of the Boulder 13. 14. 15. Graphs showing relations of thorium and uranium contents and SiO,, K,0, and CaO/(N2;,0+K,0) in- 3. Prebatholith volcanic rocks of the Boulder batholith region.__._.________________________________ 4; Rooks 'of the Boulder .L .. 2 «22. nee. cael ne chara na van anns con ons nin oun 5. Postbatholith volcanic rocks of the Boulder batholith region... . Thorium and uranium distribution curves of selected igneous Suites. . Graphs showing a comparison of thorium and uranium distribution patterns of selected granodiorites and .. tent TABLES Interlaboratory comparison of results of analyses for thorium and uranium on four standard rock samples. . Summary of thorium, uranium, and potassium values for prebatholith and postbatholith volcanic rocks com- pared with weighted average values for the Boulder batholith______________________________________ . Comparison of the thorium and uranium distribution between hydrothermally altered Butte Quartz Monzonite samples and their frosh, unaltered ll.. . Lime-alkali indices and median Th/U ratios of selected suites of igneous rOCKS____________________________ . Radiogenic heat production in igneous rocks of the Boulder batholith and in some other crystalline materials. 7-12. Distribution of thorium, uranium, and potassium in- 7. Prebatholith volcanic rocks of the Boulder batholith region._____________________________________ 6. Rooks of the Boulder ..-. 9. Mafic inclusions in some rocks of the Boulder 10. Postbatholith Lowland Creek Volcanics (early Eocene)._________________________________________ 11. Postbatholith igneous rocks of probable early Tertiary age in the Boulder batholith region...... 12. Post-Lowland Creek volcanic rocks (Miocene Or Distribution of thorium and uranium in the Butte Quartz Monzonite from underground workings in the Butte district MORALANIRA-=L. -.....» sans sk ar bw s Distribution of thorium, uranium, and potassium in Butte Quartz Monzonite samples from a drill hole for which heat flow has Deen MEASUICU ~~» .. Distribution of thorium, uranium, and potassium in a single stock of Woodson Mountain Granodiorite, South- ern Californian «2: 22.2 del cel n IH Page E10 11 12 17 19 Page E2 10 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY DISTRIBUTION OF THORIUM, URANIUM, AND POTASSIUM IN IGNEOUS ROCKS OF THE BOULDER BATHOLITH REGION, MONTANA, AND ITS BEARING ON RADIOGENIC HEAT PRODUCTION AND HEAT FLOW By Rosert I. Timumc and Davin Gotrrrimp ABSTRACT Thorium and uranium contents in about 150 samples of igneous rocks from the Boulder batholith region generally increase with increasing SiO; content and decreasing K,0) ratio of their host rocks. Prebatholith volcanic rocks (the Upper Cretaceous Elkhorn Mountains Volcanics and its correlatives) vary in thorium, uranium, and potassium contents from one out- crop area to another: two areas contain rocks with average values of 2.4-2.6 percent K, 6.0-6.6 ppm (parts per million) Th, 2.1-2.9 ppm U, and Th/U ratios between 2.4 and 3.2; a third area contains rocks with average values of 3.5 percent K, 16.6 ppm Th, 3.7 ppm U, and a Th/U ratio of 4.2. The older (and more mafic) rocks of the composite Boulder batholith (also Late Cretaceous) have an average content of 6.2-11.0 ppm Th and 1.5-3.4 ppm U, whereas the younger (and more felsic) rocks have an average content of 15.7-36.3 ppm Th and 4.0-9.2 ppm U. The average radioelement content of the batholith, weighted ac- cording to areal abundance of constituent rocks, is 3.3 percent K, 15.4 ppm Th, and 3.9 ppm U. Average Th/U ratios of the batholith samples range from 4.0 to 5.8 and have no apparent correlation with rock composition or position in the intrusive sequence. Postbatholith volcanic rocks (the Lowland Creek Volcanics of early Eocene age and post-Lowland Creek rocks of Pliocene or Miocene age) have thorium, uranium, and potassium contents comparable with those in some of the younger batholith rocks. Comparison of the Boulder batholith with selected igneous suites representative of diverse magmatic provinces shows that its thorium and uranium distribution is typical of that for calc- alkalic suites. The thorium and uranium distribution pattern of any comagmatic igneous suite can be grossly correlated with its lime-alkali (Peacock) index, which in turn reflects differences in tectonic setting and (or) magma provinces. Calculated from average thorium, uranium, and potassium contents (using Birch's heat-generation values), the total radiogenic production of the calc-alkalic Boulder batholith (weighted according to rock abundance) is 6.8 microcalories per gram per year, as opposed to a similarly computed value of 2.7 microcalories per gram per year for the calcic Southern California batholith. Available heat-flow measurements from a drill hole and under- ground workings in the Butte Quartz Monzonite near Butte are about 2.2 microcalories per square centimeter per second. Calculations demonstrate that the observed heat flow can be fully attributed to radiogenic heat produced in a column of surface rocks 25-35 kilometers thick (depending on the average thorium, uranium, and potassium contents used in the calcula- tion), even though limited seismic data indicate that the crust in the Boulder batholith region is about 45 kilometers thick. The apparent discrepancy between radiogenic heat production and heat flow may be resolved by postulating that the content of heat-producing elements in the crust decreases with depth, which has also been suggested by other investigators as partial explanation for analogous discrepancies between heat flow and radiogenic heat in the Sierra Nevada batholith. INTRODUCTION Data on the distribution of heat-producing elements (thorium, uranium, and potassium) in igneous rocks are fundamental to interpretation of petrologic, isotopic, and geologic evidence pertinent to many problems of magma generation and differentiation, radiogenic heat production and heat flow, and, in the broadest sense, evolution of the earth's crust. It is now generally ac- cepted that the observed continental heat flow can largely be attributed to heat generated by the radio- elements. Thus a knowledge of the distribution of these heat-producing elements in the rocks in which heat flow is measured is essential for meaningful interpretation. The geochemistry of thorium and uranium has been ably summarized by Adams, Osmond, and Rogers (1959), and many investigators have studied the dis- tribution of thorium and uranium in several magmatic differentiation series (for example, Whitfield and others, 1959; Larsen, 3d, and Gottfried, 1960; Gottfried and others, 1962, 1963; Heier and Rogers, 1963; Phair and Gottfried, 1964; and Kolbe and Taylor, 19668, b). The relationship between radicoelement content and radio- genic heat has been examined in detail by Birch (1954, 1965), Verhoogen (1956), MacDonald (1964), Wasser- burg, MacDonald, Hoyle, and Fowler (1964), and Wollenberg and Smith (1964, 1968a, b). Previous studies of radioactivity of rocks in the Boulder batholith region were made under the auspices of the U.S. Atomic Energy Commission in connection with the intensive prospecting for uranium in the northern part of the batholith from 1949 to 1956 (Becraft, 1956; Bieler and Wright, 1960; Wright and Bieler, 1960). In conjunction with this exploration pro- gram for uranium deposits, fundamental studies on the distribution of thorium and uranium in fresh, unaltered batholith and prebatholith and postbatholith volcanic rocks were also initiated to better understand the E1 E2 behavior of these elements during magmatic differentia- tion. In recent years, the geologic mapping of the Boulder batholith and vicinity has been nearly com- pleted, thereby making it possible to evaluate the data on thorium, uranium, and potassium distribution in light of a better understood igneous history of the region. In this report, data on about 150 samples are pre- sented, correlated with their rock chemistry, and compared with data on samples from selected igneous rock series of the United States and elsewhere in the world. In addition, radiogenic heat production, calcu- lated from the average radioelement content of the samples, is compared with preliminary heat-flow data on the Boulder batholith. Analyses for thorium and uranium were obtained by wet chemical methods. During the early stages of the study, thorium was determined by the chemical method of Levine and Grimaldi (1958), which subsequently has been greatly simplified with the use of arsenazo III as the reagent for thorium (May and Jenskins, 1965). Most of the thorium data in this report were obtained by the arsenazo III method, which yields results repro- ducible to +10 percent for thorium contents as low as a few parts per million. Uranium contents were deter- mined by the fluorimetric method of Grimaldi, May, and Fletcher (1952) adapted to concentrations of uranium commonly found in igneous rocks. With the fluorimetric method, uranium contents can be deter- mined with a precision of +10-15 percent for concentra- tions greater than 1 ppm; however, for concentrations from 0.5 to 1 ppm, the analytical uncertainty may be as great as +50 percent. A comparison of thorium and uranium values ob- tained by the methods used in this study with those determined by neutron activation, isotope dilution, and gamma-ray spectrometry for four standard rock samples is given in table 1. The agreement between results from TABLE 1.-Interlaboratory comparison of results of analyses, in parts per million, for thorium and uranium on four standard rock samples [Chemical method: Roosevelt Moore and Esma Campbell (this rept.). Neutron activation and gamma-ray spectrometry: Morgan and Heier (1966). Delayed- neutron method: Hamilton (1966)] Chemical _ Neutron Gamma- Isotope Delayed- method activation _ ray spec- dilution neutron Sample No. trometry method and rock type Thor- Ura- Thor- Ura- Thor- Ura- Thor Ura- - Ura- ium ni- ium ni- ium ni- ium ni- nium um um um um G-2, granite. 21.5 20 2.1 216 25.7 21 12.3 !1.04 1. 64 GSP-1, granodi- Orife:. 100 2A NATL: 106 1.7 2106 22. 4 1.80 AGV-1, andesite... 6.1 20 6.47 2.17 6.4 1.9 $6.27 ©1.9 1.47 BCR-1, basalt... .. 6.2 1.6 6.00 1.81 6.1 lo it fet ear. 1. 44 1 Doe, Tatsumoto, Delavaux, and Peterman (1967). * Peterman, Doe, and Bartel (1967). 3 Mitsunobu Tatsumoto (unpub. data, 1968). SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY these independent methods is highly satisfactory and indicates that the accuracy of the determinations is of the same order as the precision. ACKNOWLEDGMENTS We wish to express our gratitude to David D. Blackwell, Southern Methodist University, Dallas, Tex., and to our colleague Eugene C. Robertson, U.S. Geological Survey, for kindly providing their under- ground samples from the Butte district and their heat- flow data in advance of publication. Robertson, along with Harry W. Smedes, Geological Survey, critically read an earlier version of the manuscript and suggested many ways to improve it. Thanks are also due Randolph Chapman, presently at Trinity College, Hartford, Conn., for collecting many of the samples analyzed in this report and Roosevelt Moore, Esma Campbell, Paul Elmore, and other chemists in the Geological Survey for analytical data. We also bene- fited from discussions of mutual problems with Michael Fleisher and Zell E. Peterman, Geological Survey, who also drew our attention to some of the Russian litera- ture on thorium and uranium. Earlier phases of this study were done on behalf of the Division of Research of the U.S. Atomic Energy Commission. GEOLOGIC SETTING The geologic setting of the composite Boulder batholith and satellitic plutons has been described elsewhere and need only be summarized briefly here (Knopf, 1957, 1963; Smedes, 1966a; Doe and others, 1968; Tilling and others, 1968). The Boulder batholith is exposed over an area of approximately 2,200 square miles. It cuts rocks ranging in age from early Pre- cambrian (pre-Beltian metamorphic rocks) to Late Cretaceous (Elkhorn Mountains Volcanics) and is cut and unconformably overlain by the Lowland Creek Volcanics of early Eocene age and by younger rocks (Smedes and Thomas, 1965). At its type locality, the Elkhorn Mountains Volcanics consists of pyroclastic and epiclastic volcanic strata, lavas, and welded tuffs having a combined strati- graphic thickness of more than 10,000 feet and ranging in composition from basalt to rhyolite (Klepper and others, 1957; Smedes, 19662). Because of their similar lithology and stratigraphic position, Upper Cretaceous volcanic and clastic rocks in the Wolf Creek area to the north of the Elkhorn Mountains area and in the Three Forks and Maudlow areas to the south and east (fig. 1) are considered to be grossly contemporaneous with the Elkhorn Mountains Volcanics (Robinson, 1963; Robert G. Schmidt, oral commun., 1968). Smedes (1966a, p. 21) suggested that the Upper Cretaceous THORIUM, URANIUM, POTASSIUM, HEAT PRODUCTION, BOULDER BATHOLITH, MONT. volcanic pile represented by rocks in all these areas may have covered as much as 10,000 square miles. Samples from all these areas are included in this study. Although the Elkhorn Mountains Volcanics is cut by even the earliest rocks of the Boulder batholith, geologic evidence and K-Ar age data indicate that the 113° 112 | I 111: I MONTANA Area of this report 47°|- Wolf Creek\B Maudlow 46°\- 40 MILES icc Ldn sind EXPLANATION > g § § sz § t X_ § Post-Lowland Creek volcanic rocks T4 l— [1d {4 G tq Lowland Creek Volcanics e \[ U ‘1’lyj 2 $ $ Rocks of Boulder batholith 3 § $ U & § < D < ss E o Elkhorn Mountains Volcanics E and correlatives U Contact - . Dashed where approximate FrGur® 1.-Areal distribution of igneous rocks in the Bouldet batholith region. The outcrop areas of the postvolcanic rocks are only partly delimited and represent the areas sampled for this study. (Modified from Robinson and Marvin, 1967.) E3 prebatholith volcanism and the onset of batholith emplacement were separated by only a very short time interval, not detectable geochronometrically, for both events are dated as ~78 m.y. (million years) B.P. (Robinson and others, 1968; Tilling and others, 1968). The composition of the batholith ranges from syenogabbro to leucogranite; rocks of granodiorite and quartz monzonite composition form about 95 percent of the exposed batholith (fig. 2). In general, the younger batholithic rocks are more felsic than the older ones. Earliest in the intrusive sequence are the mafic rocks (syenogabbro, syenodiorite, and monzonite), which occur as plutons near the margin of the batholith. Next in the intrusive sequence are granodiorite plutons. All these are cut by the Butte Quartz Monzonite, which is the largest single pluton of the batholith and constitutes about 80 percent of the total area of the batholith. A comparison of the three granodiorite plutons included in this study shows that the Unionville Granodiorite and Burton Park plutons are distinctly more mafic than the Rader Creek pluton (fig. 2). Field and petrographic evidence demonstrates that a continuous genetically related series exists between the Butte Quartz Monzonite, its silicic variants, and alaskite. Both sharp and gradational contacts were observed between various members of this series, even within a single outcrop; however, where sharp contacts separate any two members, the more felsic of the two generally is the younger. The few economic uranium prospects (now inactive) in the Boulder batholith region are restricted to hydrothermal veins, which cut the Butte Quartz Monzonite or its related silicic facies, and are known to occur only in the northern part of the batholith. Latest in the intrusive sequence in the batholith are leucogranodiorites and leucoquartz monzonites (grouped simply as "leucocratic rocks" in fig. 2 and subsequent data tables), which invariably are in sharp contact with the Butte Quartz Monzonite and older rocks. The Boulder batholith is fringed by numerous satellitic plutons, only the largest of which are shown in figure 2. Because these satellitic bodies are widely separated and intruded into country rock of diverse ages, they cannot confidently be assigned positions in the intrusive sequence of the batholith. However, K-Ar ages on biotite and hornblende from several of these plutons fall within the range of ages obtained for the batholith proper (Tilling and others, 1968). Several samples from satellitic plutons are included in this paper, as well as some samples of mafic inclusions in the batholith rocks. Approximately 20 m.y. after the emplacement of the leucocratic rocks, the youngest rocks of the bath- olith, the Lowland Creek Volcanics was extruded in E4 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 112°30' 112°00' 46°30' 46°00 MONTANA Area of this report 115 MILES FiGur® 2.-Generalized geologic map of the Boulder batholith and vicinity. (Modified from Tilling and others, 1968, fig. 1.) ze THORIUM, URANIUM, POTASSIUM, HEAT PRODUCTION, EXPLANATION QT Lowland Creek Volcanics, post-Lowland Creek vol- canic rocks, bedded sedimentary rocks, terrace gravels, and alluvium ( ( Nat =v mes [PRL : 2. {B3 ime [Nbg he Lime] 4) £63 f 1yle 4} ary Sip Y- ARES T Leucocratic rocks Age relations among these units not known bg, biotite granite of Knopf (1963) ba, biotite adamellite of Knopf (1963) hc, Hell Canyon pluton d, Donald pluton Butte Quartz Monzonite and related silicic facies a, alaskite, aplite, and pegmatite h, Homestake pluton; locally grades into alaskite and into Butte Quartz Monzonite pr, Pulpit Rock pluton; generally grades into but locally cuts Butte Quartz Monzonite bam, Butte Quartz Monzonite; includes and is coextensive with Clancy Gramodiorite sys Granodiorite Age relations among these units not known ug, Unionville Granodiorite rc, Rader Creek pluton bp, Burton Park pluton (w" | Mafic rocks C Syenogabbro, syenodiorite, monzonite and related rocks Upper Cretaceous Ay - Rocks of the Boulder batholith and satellitic plutons K 5 x" * mf V v k "KW 51. % Elkhorn Mountains Volcanics \ Includes intrusive rocks correlated with the volcamics Prevolcanic rocks Include strata of Mesozoic, Paleozoic, and late Precambrian age and metamorphic rocks of early Precambrian age Dashed where inferred FireurE 2.-Continued. 339-847 O - 69 - 2 BOULDER Satellitic plutons mc, Moose Creek pluton Rocks of diverse composition whose positions c, Climax Gulch pluton in the intrusive sequence are not known BATHOLITH, MONT. / t i > & | 2 32m > E £ < ! W- '< Fi 9 0 w S o] 6 a - LJ r U :a vay 4 g <0 s yo ~> 00 {(s) oni x (47) syoo1 omesocongt=[] {(v) ofrysere=+ (whos) opruozuopy z1ren( oj;ng Jo sorow; orfuis=C) {(whq) opruozuopy !(uogn{d oor dopey 24) oquotpous3 !(oguotpoutn ajftauortun pus uogn|d y1eq uojimg-On 'dg) sopuotpous18 oyem=¥W {(w) =f (2) syoo1 yjog}eq ut suotenpout ogeu= ¥ +0°*SN)/O0¥8p pu® O°y "of pajjoId soptnog 94} Jo sy001 UI s;u0ju0d wintusin put (0®x+0%eN)/0°p LN3OH34 LH9I3M NI'O®X LN3ODM3d LH9I3M NI Tolis 0 So O'I SI 0% 9°C 0°€ gE 0+ I NOITMIIW ¥3d4 SLHVd NI 'WNINYVHN NOITIIW ¥434 SLHVd NI 'WNIMOHL EQ THORIUM, URANIUM, POTASSIUM, HEAT PRODUCTION, BOULDER BATHOLITH, MONT. (¢I-0T #199) yo (07d) s4004 orurojoa yearn pusjmoT-4sod=C) {(L ©) syoor snooudt somof (97) sotusojoA xoorqp pusmoT-=-@ *(0O'X+0O®®°N/O®D pur '0'y "Ol pay;ord uordor oy} Jo syoo1 ojuwojoA yjoOyu}¥qjsod ut s;uoju0o0 wintusin pus «uadig (0®x+0%eND)/0°p LN3JOH34 LH9I3M NI'O°X LNJOW3d LH9I3M NI 'Tols 0C SC o._m 9°C I 0 08 SL OL $9 09 EH 0s St I I T T LO 2 WNINYHN « NOITIIW ¥3d SLMVd NI LO NOITIIW 43d SLYUVI NI 'WNIMOHL St E10 Figure 54, C demonstrates that thorium and uranium increase exponentially with increasing SiO), and de- creasing CaO/(Na,0+K,0). Even though the data are much more scattered, thorium, in general, increases with increasing KO in all the postbatholith samples. However, although uranium for the Lowland Creek Volcanics and other lower Tertiary samples varies but little with variation in K,0, uranium for post-Lowland Creek samples increases with increasing K,0 (fig. 5B). Despite differences in rock composition and in abun- dance of thorium and uranium, the average Th/U ratios of all the postbatholith rocks differ only slightly, ranging from 4.0 to 4.7. SUMMARY AND COMPARISON OF THORIUM, URANI- UM, AND POTASSIUM ABUNDANCE DATA The satellitic plutons have the lowest average con- tents of thorium and uranium observed for the batholith (table 2); however, the number of samples analyzed (six) is probably not sufficient to adequately represent these bodies, for, though generally small, they are abundant and have diverse composition. Among the units of the batholith proper, the rocks early in the intrusive sequence have lower thorium and uranium contents than those later in the sequence, primarily because the early rocks are more mafic. However, com- positional control on thorium and uranium abundance is not the only factor involved as shown by comparison of the granodiorite units. Although granodiorites of the Rader Creek pluton are decidedly more felsic than granodiorites of the Burton Park and Unionville Granodiorite plutons, they contain the lowest amounts of thorium and uranium observed for the granodioritic rocks (table 8). SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY The average radioelement (thorium, uranium, and potassium) content of the batholith, weighted according to areal abundance of constituent rocks, is 3.3 percent K, 15.4 ppm Th, 3.9 ppm U; the Th/U ratio is 4.7 (table 2). The youngest rocks of the batholith do not contain the most thorium and uranium; the lower contents of thorium and uranium in these rocks relative to the Butte Quartz Monzonite and related rocks may be in part be- cause they are not the most felsic rocks of the batholith, even though they are the youngest. In addition, in the youngest rocks Na,0 generally is greater than K;,0, whereas in all other batholith rocks K,0 predominates over Na,0 (table 8). Thus, the youngest rocks ap- parently depart from the typical calc-alkalic differen- tiation trend in which the youngest rocks are highest in K;,0 and SiO; as well as in thorium and uranium. That the Boulder batholith may not represent a simple cale- alkalic differentiation series is suggested also by lead and strontium isotope data, which indicate that the leucocratic rocks (of which only the Hell Canyon and Donald plutons were analyzed isotopically) and the rocks of the Rader Creek pluton differ significantly in isotopic makeup from all other rocks of the batholith and from each other (Doe and others, 1968). Average Th/U ratios of the batholith range from 4.0 to 5.8, with no apparent relation to bulk rock composi- tion or to position in the intrusive sequence. Only for the Butte Quartz Monzonite and its silicic differentiates is any possible systematic variation in Th/U ratio ob- served. The average Th/U ratio is 4.5 for the Butte Quartz Monzonite, 4.7 for its silicic facies, and 4.9 for the alaskites. Thus for the Butte Quartz Monzonite- alaskite series, which constitutes about 80 percent of the TABLE 2.-Summary of thorium, uranium, and potassium values and weighted average values for rocks of the Boulder batholith [Rocks listed according to position in intrusive sequence from oldest to youngest (except for satellitic plutons). Data from table 8] Estimated areal abundance Naber at (percentdot Potassium (percent) Thorium (ppm) Uranium (ppm) Th/U umber 0 expose Map unit (fig. 2) Samples _ batholith) Range - Average _ Range _ Average Range - Average Range Average! Mafic FOOKS (M).. .-. 4 0.5 2.3-3.1 2.7 2.7-10.7 6.2 0.4- 2.3 1.5 2.6- 6.7 4.8 Granodiorite 5 4.5 2. 0-38. 4 2.8 - 8.8-16.9 11.0 1. 7- 5.7 3. 4 2.8- 5.5 4.0 Ere: 7 4.5 2. 1-3. 6 2.5 5. 4-12. 6 7.3 .8- 2.7 1.5 3. 4- 9.2 4.8 Butte Quartz Monzonite (bqm) 3.................. 15 73.0 1.8-3.7 3.4 11.8-19.0 16.2 1.6- 7.1 4.0 _ 2.7-11.2 4.0 Silicic facies of the Butte Quartz Monzonite (pr, h).. 6 4.0 3.8-4.1 3.6 14.9-38.5 22.3 26-121 5.9 - 1.5- 7.9 4.7 AlaSKICS .. o.. recede onne 5 1.0 3.9-4.9 4.5 - 26.0-42.0 36.3 - 4.4-20.0 9.2 - 2.1- 6.9 4.9 Leucocratic rocks (hc, d, C)........................ 12 7.5 2. 4-3. 4 2.9 7. 6-30.7 15.7 1.6- 8.8 4.1 1.9- 6.5 4.4 Sutellitic (8)... .... .... -o:clsrcorscectine. 6 5.0 1. 6-4. 0 26 - 25-111 5.9 . T- 2.3 1.4 _ 1.5- 5.8 4.5 Average (weighted according to areal abund- ance Of COnEUEUENE PIMIONE) . 2. :.. 222020001. Piss Devin ansi ce ens $5 ...ickssce.« 16.4 vi cs. $.9 .: 54.7 1 Computed from individual Th/U ratios, not from average thorium and uranium. 2 Age relations between the granodiorite units (ug, bp, rc) cannot be determined because these rocks are separated spatially (see fig. 2). The Unionville Granodiorite (ug) and Burton Park pluton (bp) are arbitrarily placed earlier in the intrusive sequence because of their more mafic composition. 3 Excluding samples 54C-248 and 52C-10a which are xenolith-rich mafic border facies. 4 Excluding the granite porphyry sample (5T-29a). 5 Calculated from average Th/U ratios of the batholith units and not from weighted average thorium and uranium. THORIUM, URANIUM, POTASSIUM, HEAT PRODUCTION, BOULDER BATHOLITH, MONT. exposed batholith, the Th/U ratio appears to increase slightly with differentiation, although the increase may not be statistically significant because of analytical uncertainties (p. E2). In terms of bulk rock composition and thorium and uranium content, the Elkhorn Mountains Volcanics is grossly similar to the oldest rocks of the batholith, the mafic rocks (table 3). This relationship is consistent with field and geochronometric evidence, which indi- cates that the Elkhorn Mountains Volcanics and the oldest batholith rocks were separated by an extremely short time interval. However, the average Th/U ratio of 3.2 for the Elkhorn Mountains Volcanics contrasts with that of 4.8 for the mafic rocks of the batholith. In fact, average Th/U ratios of all the prebatholith vol- canic rocks studied are lower than average ratios of the batholith rocks, if the single volcanic sample from the Maudlow area and the mafic granodiorites are omitted. Similarly, excluding the three basalt samples, the post- batholith volcanic rocks also have lower average Th/U ratios than the batholith rocks (compare table 2 with table 3). With few exceptions, both the prebatholith and post- batholith volcanic rocks are characterized by lower average Th/U ratios than rocks of the Boulder bath- olith. This difference in Th/U ratio no doubt is partly due to differences in rock composition; however, the fact the the combined range in chemical composition of the prebatholith and postbatholith volcanic rocks is similar to that of the batholith suggests that rock composition cannot be the sole factor causing this difference. Although presently available data are in- conclusive, the possibility exists that the difference in Th/U ratio might stem in part from fundamental dif- ferences in the geochemical behavior of thorium and uranium under conditions of plutonism as opposed to volcanism. A similar explanation has recently been in- E1l voked to partly explain variations in cesium distribution in extrusive basalts versus hypabassal intrusive basalts (Gottfried and others, 1968). NATURE OF VARIATION IN THORIUM AND URANIUM CONTENT Primary magnetic abundances of thorium and uran- ium can be modified by later alteration (Hurley, 1950; Neuerburg and others, 1956; Brown and Silver, 1956; Tilton and others, 1955; Larsen, 3d, 1957; Larsen, Jr., and Gottfried, 1961; Ragland and others, 1967). Therefore it is essential to evaluate whether the ob- served variations in thorium and uranium are pre- dominantly due to primary or secondary processes. To minimize the possibility of obtaining secondary variations in thorium and uranium content, the rocks analyzed in this study were carefully collected to avoid zones of hydrothermal alteration and (or) faulting; the samples analyzed appeared fresh both in hand sample and in thin section. Petrographic study indi- cated that sphene is a common primary accessory mineral in nearly all the batholithic rocks. Allanite of probable deuteric origin was found only in trace amounts in many of the batholith rocks but is absent in the mafic rocks and mafic granodiorites. However, there is no apparent correlation between the absence or presence of these thorium- and uranium-rich minerals and the observed thorium and uranium content of the host rock. With the exception of the glassy welded tuff from Maudlow (table 7), which has an anomalously low content of K,0 relative to the other prebatholith volcanic rocks, the major-element compositions also do not suggest any alteration of the rocks. Therefore, because of the lack of contrary evidence, the distribution of thorium and uranium in igneous rocks of the region was assumed to be predominantly primary. TaBur 3.-Summary of thorium, uranium, and potassium values for prebatholith and postbatholith volcanic rocks compared with weighted average values for the Boulder batholith [Data from tables 7, 10, 11, and 12} Numb? of - Potassium (percent) Thorium (ppm) Uranium (ppm) Th/U samples 2 Range Average Range Average Range Average Range Average Prebatholith volcanic rocks: Elkhorn Mountains volcanic field. 10 1. 7-5. 0 2.6 - 2.5-8.9 6.6 .7-3.0 21 - 1.54.8 3.2 (urge POFKS : :r ab ccen 7 1. 1-2. 8 2. 4 2. 7-14. 4 6.0 .8-6.0 2.9 1. 6-9. 6 2.4 Wolf Creek ArBRLLLL ... . .. .o neces anne een den de eel oe, 4 . 7-6.0 3.5 - 6.8-30.9 16. 6 2. 0-5. 6 3.7 3.0-5.5 4.2 MSUdIOW.STOR.. .. ..:. .\ =.. A 0 T5 cone MO 6.7 Boulder batholith (weighted average from table 2). . ___. 60 1. 6-4. 9 3.3 - 2.542 15. 4 . 4-20. 0 8.9 - 1.5-11.2 4.7 Postbatholith volcanic rocks: Lowland Creek Volcanics (early Eocene)..._.._..._________. 11 1. 9-4. 4 29 _ 4.0-20.2 9.0 .8-4.5 2.5 . 9-13, 4 4.3 Other lower Tertiary igneous rocks....._._..._____.._______. 3 1.8-2.6 22 - 2.1- 7.9 4.7 .4-2.1 1.1 _ 3.8-5.3 4.7 Post-Lowland Creek volcanic rocks (Miocene or Pliocene) . . 10 2.83-4.6 3.5 3.244 21.9 _ 2.1-18.0 6.1 - 1.5-7.2 4.0 E12 A test of the validity of the assumption that varia- tions in thorium and uranium are indeed primary was provided by data on a suite of hydrothermally altered Butte Quartz Monzonite collected from several under- ground workings in the Butte district (table 13). Some of these show only slight argillic and sericitic alteration and contain minor sulfides; others are very intensely altered and contain abundant sulfides. In light of data on the ease of leaching uranium, and to a lesser extent thorium, it was anticipated that the uranium and thorium values for these samples which have clearly interacted with hydrothermal fluids should be erratic, depart from primary values, and yield anomalous Th/U ratios. However, a comparison of data on these altered Butte Quartz Monzonite samples with data on their fresh, unaltered counterparts indicates that the thorium and uranium distribution for both are about the same (table 4). TABLE 4.-Comparison of the thorium and uranium distribution between hydrothermally altered Butte Quartz Monzonite samples and their fresh, unaltered counterparts [Data from tables 8, 13, and 14] Butte Quartz Nun}er Thorium (ppm) Uranium (ppm) Th/U Monzonite Samples samples - Range Average Range Average Range Average Hydrothermally altered.... ...... 14 12.1-25.8 17.1 2.2-8.8 4.4 2.2-6.2 4.3 Fresh, unaltere ace)......... 14 11.8-19.0 16.2 1.6-7.1 4.0 2. 7-112 4.0 Fresh, unaltered (drill hole).... 19 12.8-82.4 22.8 8.5-11.8 6.3 2.1-6.3 3.7 This rather unexpected result can be interpreted as either (1) the alteration had little or no effect on the primary thorium, and uranium distribution or (2) what appear to be unaltered samples are in fact also altered. The second interpretation does not seem reasonable, because it would require that the entire mass of Butte Quartz Monzonite be pervasively, and uniformly, affected by ore fluids associated with the Butte mineralization. Such a large-scale alteration, involving distances as great as 60 miles, can be rejected on the basis of isotopic studies (Doe and others, 1968). Though the apparent lack of difference in thorium and uranium distribution between fresh and altered samples is puzzling, the first interpretation nonetheless cannot be excluded. If the first interpretation is assumed to be valid for all samples, then the observed variations in thorium and uranium are probably predominantly primary variations, disturbed little or not at all by later alteration processes. Data on another suite of Butte Quartz Monzonite samples from a drill hole in the Butte district (table 14) provide a means of testing the possibility that thorium and uranium distribution might vary with depth. The samples from this drill hole, which are also discussed SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY in connection with heat-flow data (see p. E18), are extremely fresh, although a few contain thin veinlets (0.5-1.5 mm) of sulfides. The depth of the samples ranges from approximately 850 to 4,000 feet. Exam- ination of table 14 reveals that there is no systematic variation of thorium and uranium with depth and that the thorium and uranium contents of the drill-hole samples are approximately 50 percent higher than those typically observed for the Butte Quartz Monzonite (compare tables 8 and 14). Although the drill-hole samples have values of SiO», alkalies, CaO/(Na,0+K,0) typical to those of the Butte Quartz Monzonite, their average thorium and uranium content is comparable instead to that of the silicic facies of the Butte Quartz Monzonite (table 8). In summary, available data support the assumption that the thorium, uranium, and potassium distribution in igneous rocks of the Boulder batholith region is predominantly primary and that the distribution observed does not vary with depth, at least in a 4,000- foot interval. COMPARISON OF THORIUM AND URANIUM DISTRIBU- TION BETWEEN CONTRASTING MAGMA SERIES It is beyond the scope of this report to summarize and interpret all the available data on thorium and uranium in igenous rocks for comparison with our data. More- over, pertinent petrochemical and geologic information necessary for meaningful interpretation of much of the existing thorium and uranium data is not available, not known, or obscured by postcrystallization phenomena. Therefore, reference is made only to several igneous rock suites (table 5) which were chosen for comparison because: 1. The petrochemistry and geologic history of these suites are reasonably well known. 2. The suites selected represent a wide range of mag- matic provinces and tectonic settings. 3. The suites selected are Cretaceous or younger, there- by minimizing postcrystallization modification which affects many of the older suites. 4. Most of the suites selected encompass a wide com- positional range, representing most stages of mag- matic differentiation and thus avoiding comparison of thorium and uranium distribution on the basis of random rock types alone without reference to relative stage of differentiation. 5. The suites selected probably bracket all the possible variation patterns of thorium and uranium with differentiation undisturbed by postcrystallization events. For ease of such comparison, the thorium and uranium data of these selected reference suites are plotted against THORIUM, URANIUM, POTASSIUM, HEAT PRODUCTION, BOULDER BATHOLITH, MONT. Tasur 5.-Lime-alkali indices and median Th/U ratios of selected suites of igneous rocks Refer- References ence Locality Lime- Median suite alkali- h/U Thorium and Chemical and in fig. 6 index ratio uranium data geologic data AAt.c.t Kamchatka, 64 1.2 Shavrova (1958, Shavrova (1958, U.S.S.R. 1961). 1961). Mariana Islands.. 65 1.2-2.9 Gottfried, Moore, Schmidt (1957); and Campbell Gilbert Corwin (1963). (oral commun ., 1967); Stark (1963). Siske.}s Mount Garibaldi, 6621.6 |- Mathews (1957). 113ml“ Colum- a. Strawberry Mts., 63 2.6 aoe l-. haver (1987). reg. 5.204. Lassen, Calif...... 64 2.7 Larsen, 3d, and Williams (1932). Gottfried (1960). b.uie.ss Modoc, Calif... .. 60 2.8 Powers (1982); Anderson (1941). Tavist Jemez Mts., N. 58 3.4 Ry L. Smith sand Mex. R. A. Bailey (oral commun., 1967). * Big Bend, Tex... 51 28 Gottfried, Moore, Lonsdale and Max- and Caemmerer _ well (1949); J. T. (1962). Lonsdale (writ- ten commun., 1962). «8 | st Charles Milton (oral commun., 1962). Southern Califor- 64 3.5 Larsen, 3d. and Larsen, Jr. (1948). nia batholith. Gottfried (1960); Larsen, Jr., and Gottfried (1961). SiO ; content (fig. 64) ; the use of a differentiation index other than SiO; would not result in significantly different configuration of the plots. R A comparison of the thorium and uranium variation curves (fig. 64) for a given petrographic province in- dicates a rather strong geochemical association of these elements over a wide span of differentiation; accord- ingly, the Th/U ratio remains fairly constant. However, the Th/U ratio does vary between igneous rock series representing different magma types. The median Th/U ratios of the reference suites (table 5) show a gross correlation with their lime-alkali indices. With exception of the Southern California batholith, the calcic volcanic suites seem to have significantly lower Th/U ratios than those of the alkali-calcic and alkalic series of the continental interior regions. Similar variations of Th/U ratio with geologic setting have been noted by Heier and Carter (1964) for tholeiitic basalts: an average of 1.5 for those of orogenic belts as compared with an average of about 4 for those of the plateau type. Doe (1967) also observed a tendency for higher Th/U ratios to occur in rocks of the continental interior than in rocks of the west coast of the United States. Thorium and uranium in the reference suites (fig. 6) show a strong correlation with the lime-alkali (Peacock, 1931) index of the given suite, which in turn is influenced by tectonic setting. Reference suites 1-8 are of volcanic origin; reference suite 9 represents a dike complex. The thorium and uranium patterns in these reference E13 suites are discussed briefly and then they are compared with rocks of the Boulder batholith region and with several other suites of plutonic rocks. The calcic rock suites (curves 1-5, fig. 64) are fairly rep- resentative of the cireum-Pacific petrographic province and of a tectonically active '"island-are'" setting. In these suites thorium can increase gradually to about 6 ppm (curves 3-5), remain nearly constant (curve 2), or decrease slightly (curve 1) with increasing SiO,. The uranium distribution pattern of these calcic suites generally follows that of thorium (fig. 64). Some of the calcic suites represent marked exceptions to the often- stated generalization that thorium and uranium con- tents increase with increasing SiO; or differentiation index. The Southern California batholith differs from the other calcic suites and is discussed later (p. E15). The calc-alkalic reference suites represent tectonic associations of the continental interior, that is, tectonic settings east of the "quartz diorite line' of Moore (1959). These suites are characterized by the curvilinear increase of thorium and uranium with increasing SiO», the increase being most pronounced in the siliceous end members of the suite (fig. 64). This relationship is well illustrated by the Jemez Mountains volcanic suite, which is predominantly calc-alkalic, but less so by the Modoc lavas, which have a lime-alkali index that nearly falls into the calcic grouping. It is generally accepted that alkalic rocks have tectonic associations typical of regions of relative crustal stability. Curvilinear distribution of thorium and uranium are also characteristic of these suites; however, in the rocks of alkalic affinities, both the thorium and uranium contents are significantly higher than in other igneous rocks (fig. 64). Thorium and uranium distribution curves for rocks of the Boulder batholith region (generalized from data of figs. 3-5 and tables 7-12) are plotted in figure 6B, the patterned fields of which are based on the thorium and uranium distribution curves of the reference suites in figure 64. The Boulder batholith rocks (lime-alkali index, ~58) show the curvilinear trend typical of calce-alkalic rocks with respect to both thorium and uranium contents, although the siliceous part of the curve actually lies within the field designated for more alkalic rock suites. This slight departure of the batholith curve from the arbitrarily delimited "calc-alkalic'" field (as shown in fig. 6B) probably has little or no geologic significance and merely illustrates the imper- fect nature of such a composite plot as well as the impossibility of fitting natural rock suites into artificial pigeonholes without having some minor overlap. Like the batholith rocks, the postbatholith volcanic rocks have thorium and uranium distribution curves which E14 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY 87ppm A 45TH“ B 42ppm I [ 4ppm *- p THORIUM, IN PARTS PER MILLION URANIUM, IN PARTS PER MILLION SiO, IN WEIGHT PERCENT Sig, IN WEIGHT PERCENT EXPLANATION EXPLANATION Alkalic and alkali-calcit - Calcic I] 1. Kamchatka, U.S.S.R.(64) 2. Mariana Islands (65) Alkalic and alkali-calcic Calc-alkalic Calcic f 3. Mount Garibaldi, 4. Strawberry Mountains, British Columbia (64) Oreg. (63) PLC=Post-Lowland Creek 5. Lassen, Calif. (64) 6. Modoc, Calif. (60) volcanic rocks 7. Jemez Mountains, 8. Big Bend, Tex. (51) (Miocene or Pliocene) Postbatholith New Mexico (58) 9. Virginia (48) LC=Lowland Creek Volcanics and other lower Tertiary rocks and correlative rocks EMV=Elkhorn Mountains Volcanics Prebatholith (Upper Cretaceous) Fraur® 6.-A, Thorium and uranium distribution curves of selected reference igneous suites (see text and table 5). The lime- alkali index of suite is given in parentheses after locality name. B, Thorium and uranium distribution curves of igneous rocks of the Boulder batholith region and of the Southern California batholith superimposed on fields defined by the reference igneous suites of figure 64. THORIUM, URANIUM, POTASSIUM, HEAT PRODUCTION, BOULDER BATHOLITH, MONT. are characteristic of calc-alkalic rock suites but which also plot partly into the "calcic'" field as outlined. The thorium curve for the prebatholith volcanic rocks is similar in form to, and intermediate between, the thorium curves for the batholith rocks and the post- batholith volcanic rocks. Similarly, the uranium curve for the prebatholith volcanic rocks lies between the curves for the batholith and for the postbatholith volcanic rocks; however, it does not display the sharp increase in the siliceous end. The uranium distribution curve for the prebatholith volcanic rocks is more typical of calcic suites, but the thorium curve is that expected for calc-alkalic suites. The reason for this apparent divergence of trends is not known. Th/U ratios of the igneous rocks of the Boulder batholith region show no systematic variation with respect to degree of differentiation or to age, but there is a tendency for the volcanic rocks, both prebatholith and postbatholith, to have lower ratios than the batholith rocks. Although the average Th/U ratios (tables 7, 8, and 10-12) of the igneous rocks are generally higher than 3.0 and thus are compatible with their calc-alkalic character (see table 5), mafic inclusions in batholithic rocks generally have Th/U ratios less than 3.0 (table 9). The Southern California batholith (Larsen, Jr., 1948) is a typical representative of a calcic suite (lime-alkali index, ~64), yet its thorium and uranium distribution patterns (fig. 6B) resemble more closely those of calc-alkalic suites (for example, the Boulder batholith) than those of the reference calcic volcanic suites (fig. 64). The reason for this difference is not known, but it may reflect inherent differences in crystallization regimes of a plutonic versus the volcanic environment of the reference calcic suites. Though the distribution curves of thorium and uranium of the Boulder and Southern California batholiths are similar in form, they differ markedly in amounts; figure 6B graphically demonstrates that the Boulder batholith is significantly higher in both thorium and uranium than the Southern California batholith. This dif- ference in radioactive element content, which is as- cribed to difference in petrographic province and, hence, tectonic setting, is also well expressed by the average thorium and uranium contents of these batho- liths weighted according to relative areal abundance of the constituent rocks types: E15 Thorium Uranium Region (ppm) (ppm) Boulder batholith (table 2, this rept.) ___.... 15. 4 3. 9 Batholiths of Western United States (South- ern California, Sierra Nevada, and Idaho batholiths; Phair and Gottfried, 1964, table IB).: =s 11. 4 2. b Southern California batholith (table 6, this 5. 5 1.4 Many differences in thorium and uranium related to bulk composition are believed to reflect primarily dif- ferences of a regional or provincial nature; that is, different segments of the earth's crust differ intrinsically in their thorium and uranium content: For this reason, bulk composition is no universal guide to estimating thorium and uranium content, nor are thorium and uranium guides to bulk composition. To illustrate, two examples have been chosen; the first is of similar thorium and uranium contents from plutons that differ markedly in bulk composition, and the second is of markedly different thorium and uranium contents from plutons of closely similar bulk composition (fig. 7). The Rader Creek granodiorite pluton and the Woodson Mountain Granodiorite from the Southern California batholith (table 15) differ significantly in bulk composition, having large differences in, and no overlap of, the SiO; and CaO/(N&,04+ K0) values but nearly identical average thorium and uranium contents (fig. 7). The Rader Creek pluton and plutons from the Snowy Mountains, Australia, are similar in bulk composition. The granodiorite and adamellite from the Snowy Mountains have distinctly higher thorium and uranium contents than the Rader Creek or Woodson Mountain plutons, yet the differences in bulk rock composition are small (fig. 7). This concept of one region of the crust enriched in thorium and uranium ("uranium and thorium prov- inces") relative to another has been examined in some detail by Phair and Gottfried (1964), who demonstrated that the Colorado Front Range as a whole has about twice as much thorium and uranium as do batholiths of the Western United States and the "continental crust" of Heier and Rogers (1963). In summary, the distribution of thorium and ura- nium in igneous rocks of the Boulder batholith region is typical of that observed for calc-alkalic rock suites. The comparison of the data of this report with thorium and uranium data on other comagmatic rock series attempts to take into account the dependence of the thorium and uranium distribution pattern on the SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY E16 ('gt pus 'g 'g wou; pajgoojes surequnopy Smoug oy} Jo pus sojuorpout13 pus uJoujnog) ojlJoIpousJ;) ut¥junopy UuospoOM 04} JO uogn|d ® sopmog) uogn|d yooarq dopey 94} Jo sy001 (g-g ut yeqq 03; souutu ut pojy4ofd) tuntu®.m pus unL10T} JO SHA DIA segue ajeoipul aienbs ysnouy} 'efjejsny '(Ws) sujejunop Amoug 'soyjjqwepe pue saju0ipouei$ jo = ageiory=@ aBeaory=* uyjouyjeq eiuojijep wayinos '(Lm) aquoipoueis uiejunop uospooy=* yjijoyjeq 1apinog '(01) uojn|d y221q aapey =+ NOILYNYVTIAX3 (0®x+0%en)/0ep LN3OH3d LH9I3M NI'O°X LN3JOH3d4 LHOI3M NI COS & G 0 20 P0 9°0 80 O'T S ¢ 0+ 9°C 0°€ SC 9% - 08 SL OL $9 99 a | T T T | T T T | T T L 4 Ly i | A C < o. ~ > |- / A > o1 A “Ia. ( wim CC 1 i- 6T] (- l+ 'm ws ws ¥ al < [+ nt [~ " F | | | | I C 0 A I I - TC 0 a - L- m |- - OT 91 We. uum ( 3 m bd - - -S1 & a C ws 0 z 1 | LC oz V. THORIUM, URANIUM, POTASSIUM, HEAT PRODUCTION, BOULDER BATHOLITH, MONT. petrographic province, and it is concluded that in any calculation of average crustal abundances of thorium and uranium or in any scheme of the evolution of the radiogenic daughters of these elements, the possibility of wide regional ("provincial") differences should be considered. RADIOGENIC HEAT PRODUCTION AND HEAT FLOW The radiogenic heat production of igneous rocks of the Boulder batholith region has been calculated from their thorium, uranium, and potassium contents (table 6) on the basis of Birch's estimates (1954) of heat generation (1 ppm Th=0.20 microcalories per gram per year; 1 ppm U=0.73 ucal/g yr; 1 percent K=0.27 pcal/g yr). Heat-production values for the prebatholith volcanic rocks range from 3.4 to 7.0 ucal/g yr. These values are comparable to those for the earlier and more mafic rocks of the batholith and for rocks of the E17 satellitic plutons. The younger rocks of the batholith, which are volumetrically predominant, are character- ized by higher heat-production values, 7.0 to 15.2 peal/g yr. The average total heat production of the batholith, weighted according to areal abundance of the constituent plutons, is 6.8 ucal/g yr. Of the post- batholith volcanic rocks, the post-Lowland Creek rocks have an average value of 9.8 ucal/g yr, or slightly more than twice the heat-production capacity (4.4 ucal/g yr) of the Lowland Creek rocks. Insufficient data preclude calculating average heat-production values for the prebatholith and postbatholith volcanic rocks, weighted for rock abundance. For comparison, values for total heat production of rocks from other regions are also shown in table 6. The major rock types of the calcic Southern California batholith (tonalite and granodiorite) have distinctly lower heat-production values than the rocks of the 6.-Radiogenic heat production in igneous rocks of the Boulder batholith and in some other crystalline materials Estimated areal Average values ; Map unit or rock type Number of - abund Total heat samples (percent of - Potassium _ Thorium Uranium (ucal/g yr) exposed (percent) (ppm) (ppm) batholith) Boulder batholith region Prebatholith volcanic rocks: Elkhorn Mountains volcanic field. 2.6 6.6 2.1 3.6 Three Forks area. . 2. 4 6.0 2.9 4.0 Wolf Creek area. ._.... 3.5 16.6 3.7 7.0 Boulder batholith: Mafic rocks.... s r+ 4 0.5 2.7 6.2 1.5 3.1 Granodiorite (ug, bp) .. 5 4.5 2.8 11.0 3. 4 5. 4 Granodiorite ms _____ es 7 4.5 2.5 7.3 1.5 3.2 Butte Quartz Monzonite.... 14 73.0 3. 4 16. 2 4.0 7.1 Silicic Butte Quartz Monzonite..... 6 4.0 3.6 22.8 5.9 9.7 Alaskite . . > 5 1.0 4.5 36.3 9.2 15.2 Leucocratic rocks . .... nere 12 7.5 2.9 15.7 4.1 6.9 Satellitic plutons 2s dess 6 5.0 2.6 5.9 1.4 2.9 Average (weighted according to areal abundance of map units) o 3.3 15. 4 3.9 6.8 Postbatholith volcanic rocks: Lowland Creek Volcanics (early Eocene)...._._.._______________ JJ ye 2.9 9.0 2.5 4. 4 Post-Lowland Creek volcanic rocks (Miocene or Pliocene) . . ___. 10 ... l.. AICC 3.5 21.9 6.1 9.8 Southern California batholith ! Gabbro... ¢. 7 0.3 0.6 0.3 0. 4 Tonalite . . 12 63 1.3 4.1 1.5 2.8 Granodiorite . .. 24 28 2.9 8.1 2.1 3.9 Quarts monsonite and SraRIbG . . . ...... L..... colo loco anol cl. cL 2 2 3.8 19.1 5.2 8.6 Average (weighted according to areal abundance of rock types) 17 5.5 1.7 2.7 Sierra Nevada batholith region ? Prebatholith rocks: Metovoleatilc TOGRE . . .. 22 .... 2.000. 2000. Lol lieu. 0.14 0. 98 0. 45 0.6 Clastic, sedimentary rocks ..... k 2. 40 14.5 3. 31 6.0 Sierra Nevada batholith: Cret pl ies z 2.77 21.3 7.17 10.3 Jurassic or Cretaceous (granodiorite of Dinkey Creek) . 1.88 11.0 3. 56 5.3 ic or Jurassic rocks of eastern Sierra Nevada... 3.78 18. 6 4.29 8.1 Baltholiths, Western United States } (Southern California, Sierra Nevada, and Idaho) Gabbro and diorite . ... 9 12.6 0.5 0.8 0.5 0.7 Tonalite........ s z 17 33.7 1.6 5.5 1.7 2.7 Granodiorite... 38 19.1 3.0 12.1 2.5 5.0 Quarts monsonite, granite .... .... .. . oo, . len lleno oen nner LL. 12 34.6 3.6 18.5 4.0 7.6 Average (weighted according to areal abundance of rock types) 2.4 11.6 2.5 4.7 See footnotes at end of table. E18 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY TaBus 6.-Radiogenic heat production in igneous rocks of the Boulder batholith and in some other crystalline materials-Continued Estimated areal Average values Map unit or rock type Number of - abundance Total heat samples (percentdof Izotasslug T(horiu)m U(ranlu)m (ucal/g yr) expose percent ppm. ppm, batholith) Snowy Mountains, Australia 4 Gneissic granite.... 3. 48 19. 9 4.0 7.8 Leucogranite... sey B 3. 87 17.2 8.0 10. 2 Granodiorite and adamellite.. ... 2. 63 17.0 3.8 6.9 Coarsely porphyritic. .. 4.1 21.6 6.5 10.2 Medium grained... e 4.36 26.9 12.0 17.4 Fine grained .. 4.1 23.5 11.6 13.9 Canadian Precambrian shield $ Composite samples (990 FOOKS) . .. ..... ..-... . RR $2 C- 2. 58 10.3 2, 45 4.6 Continental crust " K6 2.6 10.0 2.8 4.7 Some crustal and meteoritic materials ® -so .r cises cer soe cine 0. 0845 0. 0398 0. 012 0. 04 Achondrites High calcium .... as «=% . 0430 «51 . O81 17 Low calcium . 0009 . 0059 . 0021 . 003 CTBNIHIE TOCKS. ...-. o..... ccc cars aners aden dens os oin 3.79 ©18. 5 4.75 8.2 Basalts . . whes e . 84 2.7 .6 1.2 Eclogites Low uranium. z ATG . 0360 .18 . 048 . 08 BHS UrARIUI . .. . . .... .. ... . coo on- Oo babas aul as ans ne be io a ual a bea nbc ne 10/12/12 . 2600 . 45 . 25 . 34 1 Average potassium, thorium, and uranium values calculated from data of Larsen, Jr. (1948), Larsen, 3d, and Gottfried (1960), and Larsen, Jr., and Gottfried (1961). 2 Data of Wollenberg and Smith (19682, b). 3 Average thorium and uranium values from Larsen, 3d, and Gottfried (1960) as combined in Phair and Gottfried (1964); average K values estimated from data of Larsen, Jr., and Schmidt (1958), Bateman (1965), and of this reFort. 4 Average K, Th, and U values taken from Kolbe and Taylor (19662). 5 Average K, Th, and U values taken from Kolbe and Taylor (1966b). 8 Average K, Th, and U values taken from Shaw (1967). ? Average K, Th, and U values taken from Heier and Rogers (1963 ). 8 lasts] from literature as tabulated in Wasserburg and others (1964, table 1). Numbers of samples separated by slashes indicates the number of K, Th, and U analyses, respectively. ? Calculated from average Th/K ratio of 166 granitic rocks. calcalkalic Boulder batholith; the average values for | Wasserburg, MacDonald, Hoyle, and Fowler (1964), total heat production of these masses, weighted accord- | the value for "granitic rocks" (8.2 ucal/g yr) approxi- ing to areal abundance of the constituent rocks, are | mates that for the Butte Quartz Monzonite (7.1 2.7 and 6.8 ucal/g yr, respectively. The Butte Quartz | ueal/g yr) and its silicic facies (9.7 ucal/g yr). Monzonite and its silicic facies, which compose ap- Data by D. D. Blackwell and E. C. Robertson proximately 80 percent of the Boulder batholith, | (written commun., 1968) indicate that the heat flow have a heat production capacity less than that of the | measured in a drill hole in the Butte Quartz Monzonite Mount Givens Granodiorite but greater than that of | of the Boulder batholith is 2.2 ucal/em' see (micro- the granodiorite at Dinkey Creek of the Sierra Nevada | calories per square centimeter per second) and the batholith. Table 6 also reveals that the weighted heat | heat flow calculated from measurements in the under- production of the Boulder batholith is less than heat- | ground workings at Butte is 2.1 ucal/em"' see (see production values of the Snowy Mountains (Australia) | tables 13 and 14 and p. E12). These values are higher and of the Cape Granites (South Africa) but greater | than the mean heat flow of the earth, 1.5 +10 percent than heat production of the Canadian Precambrian | ucal/em>' see (Lee and Uyeda, 1965), but are compatible shield. It is noteworthy that the average radioelement | with the mean value of 2.3 ucal/em* see for six heat- content (hence, heat production also) of the Canadian | flow measurements from the western interior of the Precambrian shield (Shaw, 1967) is virtually identical | United States (see Von Herzen, 1967, table 1) and with that of the "continental crust" of Heier and | with a preliminary value of 2.25 pcal/em"' see near Rogers (1963). Of the values for radiogenic heat | Wallace, Idaho (Lachenbruch and others, 1967). production calculated from average radioelement con- | Heat flow in the Rocky Mountains and western Great tent of crustal and meteoritic materials tabulated by | Basin generally exceeds 1.6 ucal/em' see (Roy and THORIUM, URANIUM, POTASSIUM, HEAT PRODUCTION, BOULDER BATHOLITH, MONT. E19 Blackwell, 1966; Decker, 1966; Lachenbruch and others, 1967). Calculations using mean specific gravity of 2.70 and heat-production values computed from the average radioelement content (table 14) show that the heat flow of 2.2 ucal/cm" see measured in the drill hole can be attributed entirely to radiogenic heat produced by a column of surface rock approximately 25 kilometers thick, if it is assumed that the radioelement distribution in the rock does not vary with depth. As previously noted (p. E12), the radioelement content of the drill- hole samples is atypical, roughly 50 percent higher than that typically observed for the Butte Quartz Monzonite. Computations based on heat-production values cal- culated from the mean radioelement content of the whole Butte Quartz Monzonite ' indicate that a slab of rock about 35 kilometers thick could produce the ob- served heat flow. Limited seismic data (Meyer and others, 1961; McCamy and Meyer, 1964) show that the crust in the Boulder batholith region is about 45 kilo- meters, which is about 10 kilometers thicker than the maximum thickness of the rock column (35 km) that would provide the necessary radiogenic heat to match the observed heat flow. These calculations suggest that radiogenic heat not only can account entirely for the heat flow observed but also would exceed the heat flow, if the radioelements were uniformly distributed through- out the entire 45-kilometer thickness of the crust. Observed heat flow in the Boulder batholith is thus lower than that expected from the radiogenic heat production of the surface rocks, if uniform distribution of radioelement in the crust is assumed. This situation is analogous to that observed for the Sierra Nevada batholith (Lachenbruch and others, 1966; Bateman and Eaton, 1967). Lachenbruch, Wollenberg, Greene, and Smith (1966) have attempted to reconcile the dif- ference between heat flow and radiogenic heat by pro- posing that the radioactive elements were concentrated in the upper part of the crust and subsequently removed by erosion. Whether or not a similar mechanism was operative in the Boulder batholith cannot be fully evaluated with presently available geologic and analytic data. However, erosion on the order of 7-10 kilometers of batholith since the Late Cretaceous, as suggested for the Sierra Nevada batholith (Bateman and Eaton, 1967), probably did not occur in the Boulder batholith region, for geologic evidence indicates that the present erosion surface is probably nowhere more than 2-3 kilometers from the roof of the batholith (see, for exam- ple, Ruppel, 1963). Nonetheless, the possibility that the radicelement content of the crust in the Boulder + Table 4 shows that the distribution of thorium and uranium in unaltered Butte Quartz Monzonite is comparable with that in altered rocks of the underground work- ings at Butte where the 2.1 ucal /cm? sec heat-flow measurement was made. batholith region decreases with depth must be seriously considered as a possible explanation for apparent dis- crepancy between heat flow and heat production. Data presented in this report suggest that, at least down to a depth of little more than a kilometer, there is no decrease (or increase) in radioelement content. However, seismic data (McCamy and Meyer, 1964) suggest that material in the lower half of the approxi- mately 45 kilometer-thick crust is characterized by higher seismic velocity, 7.4-7.6 km/sec (kilometers per second) as opposed to 6.0 km/sec for material in the upper half. On the basis of gravity data, Biehler and Bonini (1966) and Burfeind (1967) suggested that the Boulder batholith probably does not extend to depths greater than 10-15 kilometers. Thus, geophysical data enhance the possibility that the lower part of the crust is composed of denser (more mafic) material that is lower in heat-producing radioelements. Until detailed information on the subsurface configuration of the batholith and on the structure of the crust becomes available, it is not possible at present to determine the contribution, if any, of heat from lower crustal and (or) upper mantle sources to the overall thermal budget of the crust in the Boulder batholith region. The limited geophysical data available, none- theless, are compatible with the interpretation that the abundance of radioelement may decrease with depth in the crust. In summary, the available data indicate that in the Boulder and Sierra Nevada batholiths radiogenic heat production is probably ample to match observed heat flow. The data also suggest the existence of lateral regional ("provincial'"') variations in the distribution of radioelements, and hence of radiogenic heat production, in selected areas of the crust. Clearly, much more data are required on heat flow, especially in regions where crustal thickness and radioelement distribution are known, and on regional variation in thorium, uranium, and potassium distribution patterns before the inter- play of conductive, convective, and radiative heat processes in the earth's thermal regime can be properly evaluated. In addition, data must be gathered on possible vertical variation in radioelement content in crustal and ultimately in mantle materials collected by deep drilling. 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H., 1952, U.S. Geological Survey fluorimetric methods of uranium analysis: U.S. Geol. Survey Circ. 199, 20 p. Hamilton, E. I., 1966, The uranium content of some interna- tional standards: Earth and Planetary Sci. Letters, v. 1, p. 317-318. Heier, K. S., and Carter, J. L., 1964, Uranium, thorium, and potassium contents in basic rocks and their bearing on the nature of the upper mantle, in Adams, J. A. S., and Lowder, W. M., eds., The natural radiation environment: Chicago, Univ. Chicago Press, p. 63-85. Heier, K. S., and Rogers, J. J. W., 1963, Radiometric determi- nation of thorium, uranium, and potassium in basalts and in two magmatic differentiation series: Geochim. et Cosmo- chim. Acta, v. 27, p. 137-154. Hurley, P. M., 1950, Distribution of radioactivity in granites and possible relation to helium age measurements: Geol. Soc. America Bull., v. 61, p. 1-8. Klepper, M. R., Weeks, R. A., and Ruppel, E. 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S., 3d, 1957, Distribution of uranium in igneous complexes: U.S. Geol. Survey TEI-700, p. 249-253, issued by U.S. Atomic Energy Comm. Tech. Inf. Service, Oak Ridge, Tenn. Larsen, E. S., 3d, and Gottfried, David, 1960, Uranium and thorium in selected suites of igneous rocks: Am. Jour. Sci. (Bradley volume), v. 258-A, p. 151-169. THORIUM, URANIUM, POTASSIUM, HEAT PRODUCTION, BOULDER BATHOLITH, MONT. Lee, W. H. K., and Uyeda, S., 1965, Review of heat-flow data, in Lee, W. H. K., ed., Terrestrial heat flow: Am. Geophys. Union Geophys. Mon. Ser. no. 8, Natl. Acad. Sci.-Natl. Research Council Pub. 1288, p. 87-190. Levine, Harry, and Grimaldi, F. S., 1958, Determination of thorium in the parts per million range in rocks: Geochim. et Cosmochim. Acta, v. 14, p. 93-97. Lonsdale, J. T., and Maxwell, R. A., 1949, Petrology of Big Bend National Park, Texas [abs.]: Geol. Soc. America Bull., v. 60, no. 12, pt. 2, p. 1906. Lyons, J. 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S., 1959, The relatlonship between the petrology and the thorium and uranium contents of some granitic rocks: Geochim. et Cosmochim. Acta, v. 17, p. 248-271. Williams, Howel, 1932, Geology of the Lassen Volcanic National Park, California: California Univ. Dept. Geol. Sci. Bull., v. 21, no. 8, p. 195-385. Wollenberg, H. A., and Smith, A. R., 1964, Radioactivity and radiogenic heat in Sierra Nevada plutons: Jour. Geophys. Research, v. 69, p. 3471-3478. 1968a, Radiogeologic studies in the central part of the Sierra Nevada batholith, California: Jour. Geophys. Re- search, v. 73, p. 1481-1495. 1968b, Radiogenic heat production in pre-batholithic rocks of the central Sierra Nevada [abs.]: Am. Geophys. Union Trans., v. 49, no. 1, p. 332-3833. Wright, H. D., and Bieler, B. H., 1960, Primary mineralization of uranium-bearing "siliceous reef" veins in the Boulder batholith Montana-Part I, The host rocks: Econ. Geology, v. 55, p. 56-72. TABLES 7-15 E24 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY TaBu® 7.-Distribution of thorium, uranium, and potassium in prebatholith volcanic rocks of the Boulder batholith region [Thorium and uranium analyses by Esma Campbell, Roosevelt Moore, and Alice Caemmerer (using methods cited in text), unless otherwise noted. Analysis for major elements using rapid methods described by Shapiro and Brannock (1962): Elkhorn Mountains samples, by F. S. Borris, J. M. Dowd, P. L. D. Elmore, H. F. Phillips, and K. E. (militia; gillxl'eelForks samples, by P. L. D. Elmore, S. D. Botts, and M. D. Mack; Wolf Creek and Maudlow samples, by P. L. D. Elmore, S. D. Botts, Ivan Barlow, and n oe] - A Izotasslutx? T(horiu)m U(ranlu)m a Weight percent CaO a s ample 'percen: m] m emar o £9 S9 SiO: KO Nas0 Ca0 - K:0+Na:0 Elkhorn Mountains volcanic field ! §2C-768............ 5.0 3.8 (25 g: g) 1.5 54.5 6.0 3. 4 6.0 0.49 Andesite flow. 52C-8Ma........... 1.8 4.8 "A8 4.8 51.6 2.2 2.4 8.7 1.89 "Gabbro", intrusive equivalent of Elkhorn (1.0, 1.0) Mountains Volcanics. 2.5 0.71 3.5 49.8 2.0 2.2 9.5 2.26 Basalt flow. (0. 74, 0.68) 8.9 (3.0 3.3) 3.1 64.6 8.7 3.8 1.5 .16 Trachyte flow. 8.8 _ aB $8 . Gs 5.2 3.2 2.4 . 29 Do. (2.3, 2.3) 4.9 1.8 2.6 58. 6 5.0 4.3 2.3 . 25 Do. (1.8, 1.9) 5.4 (1.8 i g) 3.0 87.7 3.2 2.5 6. 4 1.12 Andesite flow. 8.9 a C: 3.9 62.5 2.0 2.0 5.8 1.45 Dacite welded tuff. (2.3, 2.3) 8.8 (3.1 g 0) 2.9 87.2 2. 4 1.7 8.0 1.95 Andesite flow. . . 8 8.8 ' 2.6 3.4 - 62.8 2.4 2.2 4.0 .87 Do. (2.5, 2.6) 6.6 2.1 3.2 58. 2 3.6 2.8 5.5 1.07 Three Forks area * 14.4 T1 2.0 63. 6 3.1 3.9 3.0 0.43 Rhyodacite perlite (70 percent glass). (14.9, 13.8) (7.0, 7.2) 3 16.7 3 4, 590 BB ... I carers Glass fraction from sample 16. 2.7 .9 3.0 590. 4 1.3 3.9 6.2 1.19 Andesite flow. (2.6, 2.8) (1.0, 0.8) 3.1 1.6 1.9 62.1 3. 4 3.5 4.2 .61 Do. 5.8 1.8 3.2 58. 5 2.8 3.3 6.1 1, 00 Do. (5.5, 6.1) 9.3 6.0 1.6 57.9 3.0 3.7 5.2 .78 Latite. 4 3.8 2.4 1.6 70.2 3.2 4.2 1.7 .23 }Daclte laccolith correlative with latite (sample 3.1 .8 3.9 T1 4 3.1 4.0 1.6 .23 12).A (3.0, 3.2) (1.2, 0.5) Average....... 2. 4 6.0 2.9 2. 4 63.3 2.8 3.8 4.0 . 64 Wolf Creek area 5 WC-59-20......... 6.0 12.0 4.0 3.0 59.0 7.2 3.9 3.0 0.27 Li?“ flow; upper part of Two Medicine Forma- on. WC-60-23.......... 4.0 ay 3.2) 2.0 3.4 57.3 4.8 3. 4 4.8 . 59 Datalcite flow; lower part of Two Medicine Forma- :B, 7. on. WC-60-37......__. 16.9 3.3 5.1 65.8 . 80 3.0 4.6 1.21 Quartz latite, ashflow unit; lower part of Two (17.1, 16.7) Medicine Formation. WC-50-14....._.... 3. 4 30.9 5.6 5.5 76.1 4.1 4.1 . 31 .04 Rhyolite dike or sill; assumed to be Elkhorn Mountains Volcanics equivalent. Average........ 3.5 16. 6 3.7 4.2 64.6 4.2 3.6 3.2 . 53 Maudiow area 35-§0:...... siecle. 0.5 17.5 2.8 6.2 64.6 0. 58 1.2 5.6 3.11 Glassy welded tuff, described by Robinson and (2.6, 3.0) Marvin (1967). 1 Geologic information regarding samples from R. W. Chapman (written commun., 1956). 2 Geologic information regarding samgles from Robinson (1963). 3 Isotope dilution determinations by Bruce R. Doe, U.S. Geological Survey. 4 Intrusive equivalents of the Elkhorn Mountains Volcanics. 5 Geologic information regarding samples from R. G. Schmidt (oral commun., 1968). THORIUM, URANIUM, POTASSIUM, HEAT PRODUCTION, BOULDER BATHOLITH, MONT. E25 TaBu® 8.-Distribution of thorium, uranium, and potassium in rocks of the Boulder batholith [Rocks lgrouped accord to map units (fig. 2) from oldest to youngest (except for satellitic plutons); letters in parentheses refer to map units. Thorium and uranium analyses sma Campbell, Roosevelt Moore, and Alice Caemmerer (using methods cited in textzs unless otherwise noted Analysts for major elements 1-8 using1 rapid methods escribed by Shapiro and Brannock (1962) 9, 10, using standard methods: 1, F. S. B J. M. Dowd, P. L. D. Elmore, H £8 L. D. Elmore 8. D. Botts, Gillison Chloe, Lowell Artis and Hezekiah Smith; 3, P. L. D; Elmore 8. D. Botts, 'and M. D. Mack; 4 lmore, S 'D. Botts 3. fL Glenn Gillison Chloe, Lowell Artis, and Hezekiah Smith 5 P. L. D; Elmore, I. H. Barlow, S. D. Botts, and Gillison Chloe; 6, P. L D Elmore and K. E. White, 1. P: L.D;. Elmore, K. E. White, and 8. D. Botts; 8, P. Elmore, S. D. Botts, Lowell Artis, Gillison' Chloe, J. L. Glenn, Hezekiah Smith, and James Kelsey; 9 Elaine L. Manson, 10, J. J. Engel in Knopf (1957)! Sagiple IZotassiutm Tajoriuin U(r:niu)m Thu Weight percent CaO . 3 {Analysts 0. percen: pm] pm, emar for major Bi0:r K:0 NO CaO K:0+Na0 elements Mafic rocks (m) Si41g........... 2.6 5.6 1:7 3.3 54.5 3.2 22 6.8 1.2 Syenogabbro (calcic melamonzonite of Smedes, 5 (5.5, 5.7) 19662, table 2, No. 9). 63K-350...._.... 2.3 5.9 2.3 2.6 56.5 2.8 2.7 6.0 1.00 Syeoréaogabbr (Ringing Rocks stock, Prostka, 4 48-C-1..________ 2.7 10.7 6.7 57.9 3.3 2.6 6.6 1.12 Comrosibe of eight samples (Smedes, 19662, 6 (10.0, 11. 4) (14, 17,18) 2, No. 8). 4T-360.......... 3.1 2.7 0.4 6.7 57.2 3.7 3.9 6. 4 .84 Syenodioriw of Knopf 4 (2.7, 2.7) (0.5, 0.4) Average.... 2.7 6.2 1.5 4.8 56.5 3.2 2.8 6. 4 1.08 Granodiorite (ug, rc, bp) ! 53C-140...._..... 3.3 16.9 5.7 63. 2 4.0 2.5 4.6 0.71 (5.9, 5.5) Mafic granodiorite samples from Unionville 4T-349.. gs 27 9.3 1.7 5.5 6114 3.34 2.78 5. 51 . 90 Granodiorite. 10 $T-6500.......... 3. 10.8 2. 4 4.5 63.0 4.1 2.8 4.4 . 64 4 (10.9, 10.6) l: t_. 3 (3.4, g: g) 8 §*0 *+ ** }Mslflc granodiorite; samples from Burton Park { $T-60........... 8.8 $9 1% GIL Z4 19. 6.0 1 13 | pluton (Smedes, 1966b). 4 (9.3, 8.3) Average 2.8 11.0 3. 4 3.6 621 3. 4 2.7 5.3 . 88 (ug, bp) 53C-163...._.... 2.1 5.4 1.2 4.5 69.6 2.5 3.6 3. 4 . 56 1 (5.7, 5.0) (1.3, 1.2) 53C-166...__..... 2.6 6.6 1.2 5.5 66.0 3.1 3.3 4.6 «72 1 61 7.1 : L1 83C-168...._.... 2.2 6.6 (.t ig) 4.1 62.4 2.6 3.3 5.7 .97 1 2T-1057......... 2.3 l z; (0.8: g“; 9.2 68.6 2.8 3.6 3.8 ® |relsic anodia'iw, samples from Rader Creek 2 7.0, T.4) 1.0) pluton (Pilling, I 2T-1065.....___. 2 2 8.1 1.2 6.8 64.6 2.6 3.6 4.8 «77 2 (8.1, 3.9 (L1, 1.3) 2T-1080......... 2.6 (£1 gig) 2T ° $1 OGG L151" A14". 45 69 2 2T-1098......... 3.6 ' 8.0 1 LT : £3 " %% 13 42 2 (8.1, 7.9) (1.8, 1.6) Agar?” 2.5 7.3 1.5 24.8 65.8 3.0 3.5 4.3 .67 re Butte Quartz Monzonite (bam) 520-108......... 3.5 13. 0 2.9 4.5 50.8 4.2 3.0 5.1 0.71 Strongly foliated xenolith-rich border facies; 1 (12. 6, 13.3) (3. 0, 2.8) hese two samples are not plotted in figures 54C-248..._____. 1.8 7.9 (s g f) 2.5 50.6 2.2 3.1 7.2 1.36 and are excluded from averages. 1 §20-62b........ 3.5 19.0 (72735) 21 618 49> 20% . 44 1 §20-60.......... 3.2 18.8 (5:2'233) 3.1. Mo ~ 19°29". &% 62 1 520-65....._.... 3.3 15.2 '24 6.3 62.5 4.0 2.8 4.6 . 68 1 (2.5, 2.4) Medium-grained equigranular varieties . .._____.. B3C-205...__..... 3.3 16.2 5.1 © 38.2 63.5 4.0 3.0 4. 4 63 1 (16. 3, 16.0) (5.2, 4.9) 53C-200....__... 3. 4 16. 2 ar gs) 4.3 64.3 4.1 2.9 4.1 59 1 §20-114......... 3.5 17.9 g 7’ i: g) 112 60.0 > £3 20 £3 .61 1 58C-202......... 3.3 15.6 "'he $0 Gs L0, $0 ° 13 . 60 1 56K-420 3.6 e $5. 40° 42 58 7T """"" @501k2 | (£250) _" : ay | dane meam -*- 56K-494......... 3.7 15. 4 $2 £8 oh3 £50 z7T ss . 53 beatty 7T (15.3, 15.6) (3.2, 3.2) 6K-306..._...... 3. 4 11.8 3.3 3.6 65.4 4.1 2.9 4.0 . 57 7 (11.2, 12.4) (3.1, 3.5) DDH-B-3..._.. 3. 4 24.9 6,4 3.8 66.0 4.1 2.9 4.0 56 3.2 15.5 4.8 - 64.8 3.9 3.1 4.4 63 1 (3.3, a 2) strongly porphyritic varieties 3.3 16.9 4.5 3.8 65.0 4.0 2.8 4.3 . 63 with K-feldspar megacrysts; 3T-273 is the 1 (3. 9, 5.0, 4.6) "type" Clancy Granodiorite; DDH- 3.3 15.6 3.6 4.3 66.6 4.0 3.0 3.8 54 average of 19 samples from drill hole (table 14) 1 (15. 6, 15. 5) (8. 3, 3.8) 3.1 18. 0 4.6 _ 3.9 65.490 3.66 280 42 66 10 (17.3, 18.7) _____________________________ 316.14 3 4. 32 3.7 Average.... 3. 4 16. 2 4.0 - 64.6 4.1 2.9 4.1 . 50 See footnotes at end of table. E26 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY TaBu® 8.-Distribution of thorium, uranium, and potassium in rocks of the Boulder batholith-Continued Safinple IZotasshgl T(horiu)m U(taniu5m Thu Weight percent CaO T T fngya’sts o. percen m} m emar lor major P9 £9 Bi0r K:0 Nar0 Ca0 K:0+Na0 elements Silicic facies of Butte Quartz Monzonite (pr, h) 520-1188........ 3.5 das 83 (a 113: é) L6 oO 42 21> 0. 70 1 18. 3, 18. 8) es 53C-203......... 4.1 4.3 5.3 67.3 4.9 2.9 2.9 .37 \Shown as part of the Butte Quartz Monzonite 1 (4.3, 4.3) (bqm) in figure 2. §8C-206......... 3.6 19.0 3s gs) L6 10.9 - 44. 34°" 23 .31 1 2T-534.......-«. 3.8 38.5 (Mfg) 79. tos %b 80. 24 .32 2 §20-4§.......... 3.3 14.9 . s, g: g) T. 0.4 40 $1 _ 12 .43 Pulpit Rock pluton (pr) (fig. 2)..__.______._.___..._ 1 IK-241.......... 3.5 20.1 ' 8.3 2.4 71.5 4.2 2.9 2.9 .41 Homestake pluton (h) (fig. 2)............._.....~ 5 Average.... 3.6 22.3 S8 LT WT Lt 11. 2s .42 Alaskites (a) §2I04-........... 4.5 2% et is?) $9 ".% Lt 10 _ 1.% 0:08 . ...o c_ ank 1 §80-1§.......... 4.8 34 a: £7,133) 69 TT LB - 22 .81 ~I » +0021 sue e 1 4.2 «0 .. la 2' 13 LB 7.2 £0 ° £0 . 80 AF BAIGSEIED . \-... .c. ee 1 6K-445a..___..._ 3.9 42 (2; "20> L1 1%0 O47 #2 .85 AT 00.900. arene f $2K00........... 4.9 #7 23.3) 9.0 ° A1 M4 L1. 23 .93 [IL . =_ 2 lanl 2 U Average.... 4.5 36.3 912 L9 TI st" at .75 11 $T-%s.......... 4.4 17.6 $8 - _ s¥ A% - s3" Al" 11 .13 Granite porphyry (not shown in fig. 2) ; its 8 relationship to alaskite is not known, but it is poured with alaskite because of chemical similarity. Leucocratic rocks (d, hc, c) 2.4 12.7 1 2255): K5 10.40 290 $90 23 0. 34 1 2.8 dss i 3; 'car" G1. T04 14. 40. 286 .35 {Donald pluton (d) (fig. 2).....____......-...--.-- 2 $0 -= ' $62 $7: LS T.1l- 10 10 29 . 40 2 3.2 af 3.2) ias g) $2 ILY $9 A8 21 27 1 2.8 40. 4 Ito -b5 H2 $4) L2; 21 .33 |Rocks lithologically similar to those of the 1 (1.7, 1.2) Donald pluton (not shown in fig. 2). 2.8 8. (44 313) £0 $4 42> 21 .38 1 3.0 11.8 (M'g‘zn L9 so $28 :% 41 { 1 "t-" % o, £0, $b oi -1f. 4.7 _ g4 [Hell Canyon pluton (he) (fig. 2)..-.......------- 2 3.0 ' 47% '&s0 ) 120° Tos" "Ss ~As ""a% . 30 Bungle from Tobacco Root batholith (across the 3 (8. 6, 8.9) Jefferson River valley from the Hell Canyon plutong lithologically similar to samples 1K- 633 and 2T-797. 608-C-3.... ;: (re g; i) F- t g) 4.6 6518 3.75 3.00 | 4.14 . 60 9 2.8 ~* 16.2 '%5 £1 02 BA A% As _gz Climax Gulch pluton (c) (fig. 2).........-.-.----- 2 RL:..." 3.4 sos £3) £0 $8 mo 41° #1" 29 . 40 2 , 30. Average.... 2.9 15.7 1 Li OIL A450 L6. 29 .42 Satellitic plutons (s) PIL. 2.3 . 1.3) 08 : $7. B.A 18 %2 62 1.03 3 -l. 2.3 (a 1, g: g) T ~ sT wI as 'g4 %% .94 \Three Forks area (Robinson, 1963) .........----- 3 ar ars 4.0 ' 9.8 17 - L8 O1 ~ as ' {L1 $% .36 3 §20-116..:...... 3.5 11.1 s 2 g) £3 tL: "G2 £25 %% . 49 - Kgd (granodiorite undivided) of Knopf (1963)... 1 MS-w5......... 52.0 $2.5 tis" 1s" ND $4; ND; ND .._.._.:.l.._2. }Marysvflle stock; data from Whitfield, Rogers, M6-208....__._. 51.6 $3.5 $11 -§.4 ND: L9 ND ND and Adams (1959). Average.... 2.6 5.9 Li ib Q1 %% 8850 (47 .70 1 Age relations between the granodiorite units are not known because these rocks do not occur in contact with one another. 2 Excluding the anomalously high value of 9.2 for sample 2T-1057. 3 Isotope dilution determination by Bruce R. Doe, U.S. Geol. Survey. 4 Excluding the anomalously high value of 11.2 for sample 52C-114. 5 Thorium, uranium, and potassium determined by gamma-ray spectrometry; data from Whitfield, Rogers, and Adams (1959). THORIUM, URANIUM, POTASSIUM, HEAT PRODUCTION, BOULDER BATHOLITH, MONT. E27 9.-Distribution of thorium, uranium, and potassium in mafic inclusions in some rocks of the Boulder batholith [Thorium and uranium analyses by Alice Caemmerer and Roosevelt Moore, using methods cited in text. Major element anal by F. S. Borris, J. M. Dowd, P. L. D. Elmore, H. F. Phillips, and K. E. White, using rapid methods described by Shapiro and Brannock (1962). Remarks from R. W. Chapman (written commun., 1956)] Sampl 120th Tan-In? U(raniu)m ThU Weight percent CaO > *> ample 'percen m m] emar % yy sor-" "mo ___ Nub _._ Cab -_ §9C-120b............ 4.6 11.7 (4.4.1?) 2.8 58. 8 5.6 2.7 5.4 0. 65 3.5 10.7 5.9 1.8 56.8 4.2 3.6 5.4 . 69 {Host rock is Butte Quartz Mon- (5.7, 6.1) zonite. 1.9 7.9 3.2 2.5 55. 9 2.38 3.7 6.3 1.05 (3.1, 3.4) «7 2.7 (1.1 H) 2.5 46.0 . 84 1.6 9.6 3. 04 58C-1778............ 1.2 5.1 s" 6.1 a 54. 6 1.5 2.9 7.6 1.73 +Host rock is satellitic pluton of (1.7.1.6) "'dioritic' composition. §3C-177b............ 1.1 1.9 2, 4 51.8 1.3 3.3 8.3 1.81 (2.0, 1.8) (0.8, 0.9) Average......... 2.2 6.7 2.8 2.5 54.0 2.6 3.0 7.1 1. 64 TABLE 10.-Distribution of thorium, uranium, and potdssium in the postbatholith Lowland Creek Volcanics (early Eocene) [Thorium and uranium analyses by Roosevelt Moore, Esma Campbell, and Alice Caemmerer, uslnf methods cited in text. Analysts for major elements, using rapid methods described by Shapiro and Brannock (1962): 1, H. F. Phillips, K. E. White, F. S. Borris, P. L. D. Elmore; 2, P. L. D. Elmore, I. H. Barlow, S. D. Botts; 3, P. L. D. Elms-lo, 8. D. Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith; 4, P. L. D. Elmore, S. D. Botts, and M. D. Mack; 5, H. F. Phillips, K. E. White, and J. M. ow Izotassiutl? T(horiu)m U(raniu)m Th/U Weight percent -Ca0 a S IAnnuals“ ample percen emar or major al PF SiO: K:0 N0 Ca0 K:0+N20 elements 78-110......... 1.9 21 g 3) 20° "1% M1 afr L2 £2 0.65 Quartz latite porphyry.........._____._._.______ 2 78-111......... 4.4 ' 15 125 70.0 5.3 2.8 LT . 21 Rhyolite vitrophyre, very near quartz latite.. 2 (19.2, 18.3) (1.4, 1.6) 7B-112......... 3.6 (mo £0 F Kl : FL6 c}4$ %% _ L6 IBVB. .- 2 Si~417......... 21 (4.25.3: g) 8 L0 Wi jas 43 _ .43 Rhyodacite welded tuff.._____.______.__________ 3 68=802......... 3.2 : g g) L9: 1% ms aso." 822 00 Quarts I8HLO ISVB. .- -... 4 §20-54........ 2.2 *' 8.9 ists 1 g) K0 Ml %6 21 .40 Labradorite rhyodacite welded tuff.........__. 1 §2C-56........ 2.8 yes 22:5, 7? f) $8 AJP 11" 190 23 $7 .-. 5 520-85 L...... 2.0 5.0 20. 25 ALO " 2t 13. 48 .84 Andesite dike cutting Kgd (granodiorite un- 1 (21.19 divided) of Knopf 19635 a §0-78........ 2.9 7.9 is 2, g g) 22 O&SB L5) %8 28 .41 - Quartz latite intrusive......___.___.______._____._ 1 §2C-14....... 3.7 8.8 Ye g" g) $4 0.6 £5 ¢ 46 08 ; QUBIHE I8HtE IBVE. 1 ...-. 1 sc ND 4.0 4.5 .9 ND - ND ND ND ND - Rhyolite dike, cutting alteration halo in Butte (3.7, 4.3) uartz Monzonite, 4,100-foot level, Steward mine, Butte district. Average.... 2.9 9.0 25: i MB 15 L6" 17 .42 * The assignment of this sample to the Lowland Creek Volcanics is arbitrary, based on compositional similarity. TaBuE 11.-Distribution of thorium, uranium, and potassium in the postbatholith igneous rocks of probable early Tertiary age in the Boulder batholith region [Thorium and uranium analyses by Esma Campbell and Roosevelt Moore, using methods cited in text. Analysts for major elements, using rapid methods described by Shapiro saggimnock (1962): Three Forks sample, P. L. D. Elmore, S. D. Botts, and M. D. Mack; Wolf Creek samples, P. L. D. Elmore, S. D. Botts, L. H. Barlow, and G: n Potassium _ Thorium Uranium Weight percent CaO Sample (percent) (ppm) (ppm) Th/U Remarks BiO2 K0 Naso Ca0 - K10+N20 Wolf Creek area WC-60-5........ 2.6 2.1 0. 4 5.3 50. 8 3.1 3.8 7.0 1.10 Syenogabbro or trachybasalt which intrudes (1.9, 2.3) (0. 5, 0.4) Two Medicine Formation (R. G. Schmidt, oral commun. 1968). WC-60-15....... 2.2 4.1 0.8 5.1 52.8 2.7 4.8 5.1 .68 Hornblende monzonite dike intruding Adel (4.0, 4.2) (0.8, 0.8) Mountain Volcanics of Lyons (1944) (R. G. Schmidt, oral commun, 1968). Three Forks area $20 1.8 7.9 2.1 3.8 54.5 2.2 3.0 6.5 1.25 Basalt which intrudes Tertiary rocks (Robinson, (7.9, 7.9) 1963). Average.... 2.2 4.7 1.1 4.7 52.7 2.7 3.7 6.2 2.93 E28 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY TABLE 12.-Distribution of thorium, uranium, and potassium in post-Lowland Creek volcanic rocks (Miocene or Pliocene) [Thorium and uranium by Esma Campbell, Roosevelt Moore, and Alice Caemmerer, using methods cited in text. Analysts for major elements, using rapid methods described by Shapiro and Brannock (1962): 1, F. S. Borris, J. M. Dowd, P. L. D. Elmore, H. F. Phillips, and K. E. White; 2, P. L. D. Elmore, S. D. Botts, Lowell Artis, James elsey, Gillison Chloe, J. L. Glenn, and Hezekiah Smith h] m" f oy yip S0. amie otk Rusa yig Si0:r K:0 N0 Ca0 K:0+NaO0 elements 4T-505.......... 4.6 (7.1, i; i) 4.4 8.9 721 5.6 2.1 0.88 0. 14 RthyOlite SVB... .. e 2 §20-1........... 3.3 44 (1% i} g) 4.2 78.0 4.0 4.0 . 34 . 04 Porlphyritic soda rhyolite from small intrusive 1 52C-20b......... 3.3 (44.1 gt. i) (F8 g. g) 4.0 _ 75.6 4.0 2.4 .4 .07 fungal“: erryocg 650851 xflggr (gin 23115: 11913613; 1 s a 52C-20c......... 4.2 42 (iss g. g) 2.9 . "1.6 5.0 3. 4 . 05 . 01 Bhgollte pigs )stone near margin of dike 1 520-798......... 3.3 10. 4 2.6 4.0 _ 67.6 4.0 3.4 24 .32 Quartz Cer ece ec 1 520-80... 4.2 (20.4, E. g) t> a g) 7.2 78.0 5.0 2.2 . 45 205 Rhyolibe 1899. 0002.10.00. cece ccs 1 §20C-B1.......... 3.0 8.8 12% is) 3.7 68.0 3.6 4.2 26 90 Quarts 1 520-888. ........ 2.3 3.2 (23, g g) 1.5 - 66.2 2.8 £2 $% .46 Quartz latite lava, aphanitic and perlitic...... 1 52C-83b......... 2.8 9.6 (22, g: (13) 4.6 _ 71.6 3. 4 3.6 24 .34 Quartz latite lava, vesicular................... 1 53C-135......... 3.8 31. 3 6.2 5.0 _ 76.0 4.6 4.0 . 30 Soden rhyolite i (30.7, 31.9) Average.... 3.5 21.9 6:1 4.0 _ 72.5 4.2 3. 4 1.3 0.18 13.-Distribution of thorium and uranium in the Butte Quartz Monzonite from under- ground workings in the Butte district, Montana [Samples collected by E. C. Robertson, U.S. Geol. Survey. Thorium and uranium analyses by Esma Campbell and Roosevelt Moore, using methods cited in text] Sample Mine Approximate _ Thorium Uranium Th/U depth (feet) (ppm) (ppm) 3, 400 13.2 4.9 2.7 (13.0, 13.4) 3,800 . 2.2 6.7 (15.8, 13.9) 15.6 4.5 3.5 12.1 3.3 3.7 18.6 4.5 4.1 (18. 9, 18. 3) 25.8 5.6 4.6 10.2 4.6 2.2 23.3 3.8 6.1 18.2 3.9 4.7 19.3 8.8 2.2 18.0 2.9 6.2 (17.8, 18.1) 13.5 2. 4 5.6 22.1 6.5 3. 4 3.7 4.2 15.4 (14.9, 15.8) ma bet fa i> im m co THORIUM, URANIUM, POTASSIUM, HEAT PRODUCTION, BOULDER BATHOLITH, MONT. E29 TaBu® 14. -Distribution of thorium, uranium, and potassium in Butte Quartz Monzonite samples from a drill hole (DDH-B-3) for which heat flow has been measured [Samples provided by D. D. Blackwell, Southern Methodist Univ. and E. C. Robertson, U. S. Geol. Survey. Specific gravity: Determined by 8 c-gravity balance, using solid core segments. This report: Potassium analyses (atomic-absorption determinations) by P. L. D. lmore, Lowell Artis, S. D. Botts, Gillison Chloe, J. L. Glenn, James Kelsey, and Hezekiah Smith; thorium and uranium analyses by Roosevelt Moore. Previous analflsis: D. D. Blackwell (written commun., 1968); analyses (gamma-ray spectrometric determinations on solid core segments) by Gordon McKay, Rice Univ.] This report Previous analysis Sample No. and Specific ; depth (feet) gravity _ Potassium Thorium Uranium Potassium Thorium Uranium (percent) (ppm) (ppm) Th/U (percent) (ppm) (ppm) Th/U 2. 68 3.1 32.4 8. 0 4.0 2.72 3.4 25.9 8.2 8.2 2. 68 3.4 24. 9 4.8 5.2 2. 69 3. 4 24.0 3.8 6.3 2.70 3.6 25.3 4.2 6. 0 2.12 3.5 20.0 6.0 3.3 2.71 3. 4 26. 2 6.8 8.9 2.71 3.7 24.0 7.6 38.2 2.70 3.3 24.0 8.2 2.9 2. 69 2.8 . 24.7 6.2 4.0 (24. 8, 24. 6) 2.72 3.6 A 4.8 3.6 (16. 7,17. 6) 2. 74 3. 4 19.5 5.8 3. 4 2. 67 3.0 16.2 4.6 3.5 (15.8, 16.7) 2.70 3.5 25.2 11.8 (25. 2, 25. 3) 22.67 3.2 20. 6.6 3.1 (20. 9, 20. 1) (6. 4,6. 8) 22.71 3.6 1 8.5 3.7 (124,132) (3.0,3.4,4.0) 22.75 3.6 1 6.0 38.1 (17.8, 19.1) (6. 4, 5.6) 22.71 3.7 20.6 7.1 2.9 (21. 0, 20. 0, (7.0,7.2) 20.7) 22. 68 3.9 21. 6 6.4 3.3 4.4 24.0 5.6 4.3 (21.0, 22.2) (6. 4,6. 2) Average........ 2.70 3. 4 22.8 6.3 O. Ti ack s ao ur beide abe ae 1 Composite samgles of three pieces of core collected at the depth or between the depths indicated by sample number. 2 Average of the three chips in composite. NotE: The potassium determinations by gamma-ray spectrometry are systematically higher than those by atomic absorption, by as much as 30 percent. The reason for this lack of agreement is not clear but may perhaps be the different form of the samples: the gamma-ray deter- minations were on oorlgfosites of actual drill-core segments (that is, sample was not powdered), whereas the atomic-absorption determina- tions were on powdered sample material representing approximately 1 inch of each of the three (1.5-inch diameter) core segments making up the original composite samples. Although the agreement between thorium and uranium is much better, the difference in sample form may also account for some of the discrepancy between our data and the Rice Univ. data. TaBu® 15.-Distribution of thorium, uranium, and potassium £11: a kiting}? stock of Woodson Mountain Granodiorite, Southern California atholit IThorium and uranium analyses by Roosevelt Moore, Esma Osagbell, and L. B. Jenkins; major-element analyses by P. L. D. Elmore, S. D. Botts, Hezekiah Smith, and Gillison Chloe (using methods described by Shapiro and Brannock, 1962), unless otherwise noted] Potassium _ Thorium Uranium Weight percent CaO Sample No. (percent) (ppm) (ppm) Th/U Remarks BiO2 K:0 NarO Ca0 _ Na0+K:0 1.9 8.7 1.8 4.8 71.9 2.3 3.9 2.4 0.39 Foliated dark facies near border. 1.9 8.1 1.5 2.1 72.7 2.8 3.8 2.4 . 89 Do. 2.2 11.3 3.3 8. 4 72. 6 2.7 4.0 2.2 .33 Contaminated algmnudiorlte. 1.9 7.1 1.4 5.0 72.5 2.8 3.9 2.5 .40 Typical central phase. 2.8 5.2 1.4 3.7 72.6 2.8 4.3 1.8 .25 Do. 2.2 8.5 2.2 3.8 73.3 2.7 4.0 1.6 . 24 Do. 2.2 8.0 2.2 3.6 73. 4 2.6 4.0 1.8 .27 Do. 2.2 9.1 1.4 6.5 74.8 2.7 8.9 1.5 . 23 Do. 2.8 9.7 2.7 8.6 74.72 3. 40 3.76 1. 62 . 23 Do. 2.8 5.8 1.4 4.1 75.3 2.79 4.12 1.9 «27 Do. 2.1 5.7 1.0 5.7 75.8 2.6 4.0 1.7 .26 Do. 27 7.5 1.6 4.7 75. 4 3.3 8.8 1.3 .18 Do. 2.7 7.9 2. 4 3.3 76.1 3.3 3.7 . 95 14 Do. 3.2 6.9 1.8 3.8 76. 9 3.9 3.9 . 44 .06 Leucocratic phase. 3.8 14.1 2.8 5.0 77.0 4.6 3.5 . 54 .11 Aplite dike. 2.3 7.5 1.9 4.2 74.1 2.8 3.9 1.7 . 26 1 Chemical data for sample SLR-596 from Larsen, Jr. (1948). 2 Chemical data for sample LTS-4 provided by L. T. Silver. * Excluding aplite sampfe U, S. GOVERNMENT PRINTING OFFICE : 1969 O - 339-847 aE 15 f PAY P G ¥. -F Studies of Celadonite and Glauconite GEOLOGICAL SURVEY PROFESSIONAL PAPER 614-F Studies of Celadonite and Glauconite By MARGARET D. FOSTER SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 6l+i-F A study of the compositional relations between celadonites and glaucom’tes and an interpretation of the composition of glauconites UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON; 1969 UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 40 cents (paper cover) Page ADELTACLE_L_._.. :.. .. mon s F1 - Interpretation of glauconite composition._____-___--_--- introduction:... 3. mn... _ fic 1 Relation between trivalent iron and octahedral 4 y j , MUMINUM-. .... ss countess Selection of analyses and calculation of atomic ratios-_. 2 Theo ration............l.. mg.. Acle Relation between the composition of celadonites and Relation between iron and potassium._________-----.- iti gel s 3 Fixation High potassium celadonites and T Deficlfancy in : coptent “““ oce colt ean. Relation between glauconite composition and geo- Low potassium celadonites and glauconites.___._... 9 tegic loo Relation between Si, R+*2(VI), Al(VI), and R*#(VT) . 0 "References cited.... en Fiaur® 1. TABLE 2. 8. > g ng o a CONTENTS ILLUSTRATIONS Chemical composition of representative celadonites and glauconites having more than 0.65 atom of potassium per half cell. Relation between Si+ R+*2(VI) decrease and Al(IV) + increase in high potassium- high charge celadonites and Relation between decrease in Si and decrease in R+*(VI) in high potassium-high charge celadonites and . Relation between increase in Al(IV) and increase in R+*#(VI) in high potassium-high charge celadonites and Relation between excess of R+2(VI) decrease over Si decrease and layer-charge deficiency.. Relation between Fet and Al(VI) in glauconites-_____._______________________________~ . Relation between total iron and potassium in glauconites-__________________-__-_-__-___---- . Extreme and average compositions of Cretaceous and Tertiary glauconites-___________._-- TABLES Chemical analyses of celadonites, together with their calculated atomic Chemical analyses of glauconites having more than 0.65 potassium atom per half cell, together with their calculated AtOMIG . Chemical analyses of glauconites having less than 0.66 potassium atom per half cell, to- gether with their calculated AtOMIG PAtIOS- . Range and median values of principal constituents in high potassium celadonites and Range and median values of principal constituents in low potassium celadonites and glau- 2 2 hin 2 ee a 2 ae o a o wee ee a al el w i t ie per ln m unos in oe me ne ean h me ae d on an hn i anne fn n e i hs n a a un sta in t Tele e in it Al(IV)R+3(VI) replacement of SiR+2(VI) in high potassium-high charge celadonites and EIMICONIGES.. .e. 2 e es semen en o mo ms amis bike m ake ans o aon nlc m ue be a anne man mle mene ne . Al(IV)R+#(VI) replacement of SiR+*(VT) in high potassium-low charge glauconites....... . Al(IV)R+(VI) replacement of SiR+(VI) in low potassium celadonites and glauconites.... III Page 10 10 10 13 13 14 Page F4 ort 11 12 12 Page F13 13 13 14 14 14 15 17 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY STUDIES OF CELADONITE AND GLAUCONITE By Marcearer D. Fosrer ABSTRACT A study based on 10 analyses of celadonites and 19 analyses of glauconites having high contents of potassium (more than 0.65 ion per half cell) shows that both range widely in content of trivalent iron and octahedral aluminum, which seem to bear a reciprocal relation to each other. The relations among Si, R"(VI), Al(IV), and R®(VI) indicate that these micas belong to an isomorphous replacement series that starts with the gener- alized theoretical tetrasilicic dioctahedral end-member, [(Rt R3) and that is characterized by coupled replacement of Si and R+ (VI) by Al(IV) and R"(VI). In this series, celadonites, with Si ranging from 4.00 to 3.75, stand closest to the end-member, and represent lesser degrees of replacement than the glauconites, in which Si ranges from 3.80 to 3.55. Celadonites and glauconites having fewer than 0.66 potassium ion per half cell belong to the same replacement series, and are similar in layer composition to the high potassium celadonites and glauconites. Because celadonites and glauconites have the same crystal structure, are very similar in chemical composition, and are near members of the same replacement series, optical, X-ray, and thermal data at present contribute little to their differentiation. In many celadonites and glauconites the decrease in R' (VI), compared to the theoretical end-member, is greater than the decrease in Si, and the increase in R"(VI) is greater than the increase in Al(IV). The fact that the excess of R (VI) decrease over Si decrease usually agrees closely with the excess of R* (VI) increase over Al(IV) increase strongly suggests oxidation of bivalent iron to trivalent. The further fact that the excess of R(VI) decrease over Si decrease also agrees closely with the layer charge deficiency, as compared with a charge of -1.00 per half cell (the layer charge of the theoretical end-member) seems to suggest that originally all these celadonites and glau- conites had layer charges close to -1.00, but that oxidation has reduced the layer charge by an amount equivalent to the bi- valent iron oxidized. The lack of correlation between iron and potassium in glau- conites suggests that glauconitization is made up of two separate, unrelated processes, incorporation of iron into the octahedral layer, and fixation of potassium in interlayer positions, with in- corporation of iron preceding complete fixation of potassium. According to this concept, the amount of iron incorporated into the glauconite structure is dependent on the iron concentration of the specific environment, with the iron content of the glauco- nite reflecting the degree of iron richness of the environment. A low potassium content may indicate immaturity, or lack of time for more complete fixation of potassium, if it is accom- panied by high layer charge; or it may indicate degeneration, oxidation with attendant loss of layer charge and interlayer cations, if the layer charge is relatively low. The great range in composition of Cretaceous and Tertiary glauconites suggests that factors such as specific environment of development, opportunity for potassium fixation, and ex- posure to oxidation are of greater importance in determining the composition of a glauconite than the geologic age in which it was formed. INTRODUCTION The relation between celadonite and glauconite is a subject of continuing controversy. A comparative study of 10 analyses of celadonite and 40 analyses of glauconite by Hendricks and Ross (1941) confirmed the close com- positional relationship between the two minerals. How- ever, they recommended (p. 708), retention of both names, as "the well established term 'glauconite' is used for a mineral of characteristic sedimentary origin whereas the term 'celadonite' is used for a mineral of quite different occurrence and paragenesis." Later workers, on the basis of optical, X-ray, and thermal data, as well as chemical analyses, have maintained that these minerals are identical. Schiiller and Wohlmann (1951), on the basis of a single analysis of a celadonite, concluded that the composition of celadonite was near that of glauconite, except that silicon is partially re- placed by trivalent iron in celadonite and by aluminum in glauconite. As their X-ray spacings were similar, they suggested their identity. A study of minerals from Karadagh, Crimea, convinced Savich-Zablotzky (1954) that the names "celadonite" and "glauconite" refer to the same mineral, their only difference being mode of origin. Malkova (1956), after electron-microscope and thermal studies of one sample of celadonite, and com- parison of its chemical composition with that of seven other celadonites and glauconites, concluded that cela- FL F2 donite and glauconite are identical and suggested that the name "celadonite" be reserved for ferruginous varieties, and that aluminous varieties be called skolite, discarding the term "glauconite" entirely. Lazarenko (1956) also considered celadonite and glauconite iden- tical, and, on the basis of electron micrographs, and X-ray, optical, and thermal data, placed glauconite, celadonite, and skolite in the hydromica group. In correlating dioctahedral potassium micas on the basis of their charge relations, Foster (1956) found that the Si, end of the trisilicic-tetrasilicice diocathedral micas was represented by a celadonite from near Reno, Nev. In this celadonite, silicon exactly and completely filled the four cation positions (per half cell) of the tetrahedral layer. In another celadonite, silicon filled 3.88 of the 4.00 tetrahedral cation positions, with alumi- num occupying only 0.12 positions. In two glauconites included in the study, silicon occupied only 3.62 and 3.67 tetrahedral positions, respectively. A similar relation between silicon occupancy in celadonites and glauconites is found in atomic ratios calculated from the analyses compiled by Hendricks and Ross (1941), with the atomic ratios per silicon ranging from 3.78 to 4.00 in the cela- donites and from 3.44 to 3.84 in the glauconites. This difference in silicon occupancy in materials so closely related compositionally suggests the possibility that celadonites and glauconites are members of an isomor- phic series. Some of the confusion as to the relation between cela- donites and glauconites may be attributed to the kind of specimens that have been compared. In previous studies of these minerals the importance of the potas- sium content was not realized, and the chemical com- position of specimens containing 8-10 percent K,0 were compared with others containing only 3 or 4 percent K,0, which are often interlayed, most commonly with montmorillonite, hydrous mica, or chlorite, or contain impurities like quartz, calcite, or apatite. In the present study many of the same analyses are used as were used in previous studies, but they are herein grouped for comparison according to potassium content and inter- layer charge, in order to minimize as far as possible the disturbing effects of interlayering or contaminents. SELECTION OF ANALYSES AND CALCULATION OF ATOMIC RATIOS Most of the analyses used to study the compositional relationship between celadonites and glauconites hav- ing more than 0.65 potassium ion per half cell (that is, more than 7.0 percent K,0) were taken from the com- pilation of Hendricks and Ross (1941), although a few were taken from Smulikowsi's (1954) compilation and SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY a few from more recent literature. These analyses made by classical methods are considered more reliable for a study of this kind than analyses made on very small samples (less than 50 mg) by a combination of micro- chemical, spectrochemical, and colorimetric methods. The inaccuracy of the later analyses is indicated by their totals ranging, for example, from 98.78 to 102.34 in Burst's (1958) eight analyses, and from 98.56 to 102.50 in the nine analyses of Bentor and Kastner (1965). Recalculating such analyses to obtain totals of 100.00, as Bentor and Kastner have done, only compounds the errors. In such a recalculation it is assumed that all the values found for the different constituents are equally in error, and the same correction is applied to all. How- ever, it is more probable that only one or two values are in error, as some of the analytical techniques used are much more accurate than others. A slight error in the value obtained by one of the less reliable techniques is multiplied many times because of the very small sam- ples used (less than 0.50 mg in some instances), and the very large factor required to convert the amount found to percent per gram. Analyses made by classical methods are usually made on much larger samples, thus avoiding such multiplication of error. The samples analyzed by the longer classical methods were and are generally very carefully prepared by mag- netic separation, heavy liquids, and handpicking, and examined under a petrographic microscope for impurities. Under the conditions of occurrence of glauconite, calcite or apatite may be present, and their complete removal may be difficult. Consequently, any CO; or P,0;, reported in an analysis used in this study was presumed to indicate calcite or apatite present as im- purities, and their CaO equivalent was deducted from the CaO reported in the analysis before calculating the atomic ratios of the various constituents. Determina- tion of CO, and P.0; is desirable in all analyses of glauconite, for proper allocation of the CaO reported in an analysis of glauconite. If not determined, CO; and P.0;, would escape detection in the course of the analysis, as CO, would be driven off during the deter- mination of inherent water, and any P;0,; present would be included in the value reported for Al,0;. In calculating formulas for celadonites and glau- conites, Hendricks and Ross (1941) followed the usual custom of assigning all the MgO to the octahedral layer. In their formulas the number of octahedral positions occupied ranged from 1.94 to 2.17 per half cell in the glauconites and from 2.01 to 2.24 in the celadonites. As the two celadonites having the highest octahedral oc- cupancy, 2.22 and 2.24, were unusually high in MgO, STUDIES OF CELADONITE AND GLAUCONITE 8.54 and 9.32 percent, respectively, Hendricks and Ross concluded (p. 706) that these two analyses "must have been made on impure material, as a magnesium silicate (serpentine or saponite?) must have been present to account for the high percentages of MgO." They therefore omitted them in formulating their interpre- tation of the chemical composition of celadonite. In their later study of montmorillonite (Ross and Hend- ricks, 1945), they, as usual, allocated all the MgO to the octahedral layer, and obtained octahedral occupan- cies as high as 2.24 positions. From this, they concluded that octahedral occupancy in some dioctahedral min- erals is not necessarily limited to just two-thirds of the available cationic octahedral positions, as had been pre- sumed, but that as much as a fourth of the "vacant" third positions might actually be occupied. However, a subsequent study by Foster (1951) of the exchangeable cations in some of the same montmorillonite samples showed that all of those examined contained some ex- changeable magnesium. In each sample the amount of exchangeable magnesium found was exactly sufficient, on recalculation of the formulas, to reduce octahedral occupancy to 2.00+0.02 ions per half cell. This study demonstrated that, in such minerals as montmorillonite, magnesium can occupy two different positions in the crystal structure, octahedral and interlayer, just as aluminum can occupy both tetrahedral and octahedral positions. This study also suggests that an octahedral occupancy in excess of 2.02 may be indicative of inter- layer magnesium. Kelley and Liebig (1934) found that cation-exchange clays that had been treated with sea water contained more replaceable magnesium than replaceable sodium, 48.35 and 39.13 milliequivalents, respectively. This re- lationship is due to the fact that, although the concen- tration of sodium in sea water is much greater than that of magnesium, its replacing power is much inferior. Because of the high replacing power of magnesium, exchangeable or interlayer magnesium is to be expected in any layer silicate mineral containing exchangeable cations if it comes from an environment that contains magnesium. Owens and Minard (1960) reported ex- changeable magnesium in the two glauconites from coastal plain formations of New Jersey on which they had had cation-exchange determinations made. The presence of exchangeable magnesium in glauconites is not surprising as glauconites are commonly formed in a marine environment. The high octahedral occupancies calculated for many glauconites are probably due to al- location of all MgO present to the octahedral layer, whereas some of it is actually exchangeable and belongs in the interlayer. This is considered so probable that F3 in this study all octahedral cations in excess of 2.00 were considered to represent exchangeable magnesium and were transferred to the interlayer. The two celadonite analyses that Hendricks and Ross (1941) considered to have been made on impure mate- rial, and that must have contained a magnesium silicate to account for their high percentages of MgO, yielded, on calculation, exceptionally high values for octahedral occupancy, 2.19 and 2.24 cations, respectively. Transfer- ence of the excess cations to the interlayer produced very high layer and interlayer charges, +1.20 and -1.14 and +1.16, respectively. Thus, these analyses produce irrational formulas both when all the MgO is considered octahedral and when the excess above 2.00 is considered interlayer. Similar irrational formulas were produced by several other analyses of celadonite. Following Hendricks and Ross, such analyses were considered to have been made on impure material and were not in- cluded in this study. Analyses whose excess octahedral cations, transferred to the interlayer as magnesium, did not produce irrationally high-layer and interlayer charges, were considered acceptable and were included. Customarily, bivalent interlayer cations like calcium and magnesium are reported in terms of charges, not in terms of cations. However, in this study interlayer calcium and magnesium are recorded as cations, in ac- cordance with the usage followed for all the other ca- tions . in the formulas. Thus, the notation *# indicates a total of 0.82 inter- layer cation that carries a combined positive charge of 0.88. RELATION BETWEEN THE COMPOSITION OF CELADONITES AND GLAUCONITES Analyses of celadonite and glauconite are given in tables 1-3. Analyses 1-9 in table 1 are of celadonites whose calculated atomic ratio for potassium is greater than 0.65 atom per half cell; analyses 10-13 of celado- nites containing less than 0.66 atom of potassium. Table 2 presents analyses of glauconites whose calculated atomic ratio for potassium is greater than 0.65 atom per half cell, and table 3 presents analyses of glauconites having less than 0.66 atom of potassium per half cell. The limiting value of 0.65 potassium atom per half cell for differentiating high and low potassium celadonites and glauconites was chosen arbitrarily. Range and median values for the principal constituents in high po- tassium celadonites and glauconites, in terms of atomic ratios, for percent of octahedral positions occupied by bivalent cations, and for negative octahedral charge, are given in table 4. Similar data for low potassium celado- nites and glauconites are given in table 5. F4 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY TABLE 1.-Chemical analyses, in percent, of celadonites, together with their calculated atomic ratios ANSYS crse .-» C1 C2 C3 C4 C5 C6 CT C8 C9 C10 Cll C12 C13 CHEMICAL ANALYSES Celadonites having more than 0.65 potassium atom per half cell [n.d., not determined] Celadonites having less than 0.66 potassium atom per half cell 56. 20 58. 23 55.30 54. 38 54.73 52. 26 52.53 58. 54 56. 88 56. 47 51.36 54. 49 2.05 2.13 10. 90 5.41 7.56 1. 62 4. 97 12.02 11.06 9.09 1. 69 12. 60 19.18 20. 46 6. 95 14. 22 13.44 21.84 18. 62 8. 21 10. 68 12. 36 28.72 9. 52 . 3. 54 3. 56 5.30 4. 45 4. 58 3. 21 2.45 2.19 1.40 2.95 6. 56 6. 40 5.76 5.25 5.35 6. 99 6. 62 5.98 6. 34 6. 24 .................... v0 \ NORG . 01 Trace Trace (2C .05 47 42 .00 Trace . 58 .49 . 94 13 . 54 . 65 .00 £06 None None .07 .19 86 1.29 .48 9. 38 9. 23 7. 40 10.04 7.98 8.43 3. 54 6.49 6. 62 7.41 n.d. ( acres . 25 .09 .20 13 - Trace Trace 5. 21 4. 80 14.15 14. 31 4. 69 4. 69} 5.87 3. 59 1.30 1.16, 10 1.15 2.35 2, 290 1.11 1. 60 99.61 2100.17 100.59 99.71 $100.31 100.09 99.54 _ 100.14 99. 94 99. 58 CALCULATED ATOMIC RATIOS 4.00 4.00 3. 90 3. 88 3. 88 3. 86 3. 83 3.79 3. 76 3.89 3. 88 3.79 3. 76 00 00 .10 12 12 14 4.17 , 21 . 24 Al 12 8,21 . 24 .07 16 . 08 .78 .34 . 49 .00 .d .76 . 79 . 62 .00 .78 + s 1.18 1.01 .43 «55 . 64 1.25 .49 27 . 28 .18 .14 13 .09 ~47. .57 . 50 . 65 . 52 . 61 . 66 . 56 .............................. 00 . 00 .00 .00 iy Damato cie .00 2.02 2.00 2.02 2.00 2. 01 2.00 2.00 -. 95 -. 99 -1.01 -. 77 -. 84 -. 96 -. 97 .00 .07 08 io croc Alsen .03 .08 .00 .05 .04 .07 .08 .04 .05 00 . 00 .01 .02 12 .19 . 06 . 94 . 73 .76 . 31 . 57 . 62 . 65 . 94 . 85 .89 . 55 «77 + . 84 -+. 94 <.97 - +1.01 4.77 -+. 85 +. 95 <-. 97 1 Ignition loss. 2 Includes 0.15 percent LizO, equivalent to 0.04 atomic ratio of Li. 3 Includes 0.03 percent CraOa. C1. C2. C3. C4. C5. Cé. CT. 4 Includes 0.03 atomic ratio of Fet3, 8 Includes 0.06 atomic ratio of Fe#3, LOCATION OF SAMPLES AND REFERENCE Reno, Nev., 23 miles east of (Hendricks and Ross, 1941, table 4, No. 1). In vesic- ular basalt. Analyst: R. C. Wel Krivoi Rog, U.S.S.R. (Serdyuchenko, 1965, p. 566). Replacing aegerine and riebeckite in fractures in iron-bearing quartzite Analyst: M. M. Stukalova. Brentonico, Monte Baldox, Italy (Hendricks and Ross, 1941, table 4, No. 2). Amygdufite masses in basalt. Analyst: G. Levi. Vesuvius, Italy (Hendricks and Ross, 1941, table 4, No. 7). Occurrence and analfist not given. Wind River quadrangle, see. 3, T. 3 N., R. 73 W., Washington (Wise and Eugster, 1964, p. 1034, No. 15). Amy gdulite fifiing in basalt. Analyst: O. yon Knorring. Vail, Arizona (Hendrlcks Ross, 1941, table 4, No. 5). In basalt and basaltic tuffs. Analyst: G. A. Koenig Kursk Mountain, 80 miles northwest of, U.S.S.R. (Sudovikova, 1956, p. 544). In seams in iron-bearing quartzite of the Kursk magnetic anomaly. Analyst B. S. Kopelovich. C8. Bug region, U.S.S.R. (Malkova 1956, p. 308). Veins and vugs in metamorphic Analyst: K. M. Malkov C9. Dolgoye Pole, West Volynya U S S.R. (Shashkina, 1961, p. 400, No. 2). Weath- ered basalt crust. Analyst: Mineral Chemical Laboratory of A. N U. S .R. C10. Berestovets, West Volynya, U.S.S.R. (Shashkina, 1961, p. 400 No. 4). est hered basalt crust. Analyst: Mineral Chemical Laboratory of A.N., C11. Zon uldak Télr’llx‘ey (ggymmgil and others, 1952, p. 245). Cavities in basaltic tu ys C12. Contessa Entellma Palermo, Sicily (Scherillo, 1935, p. 73). Cavities in basalt. Analyst: A. Scherillo. C13. Dolgoye Pole, West Volynya, U.S.S. R. (Shashkina, 1961, p. 400, No. l) Weath- ered basalt crust. Analyst: Mineral Chemical Laboratory of A. N .S.R. 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" 6P 'A 10 '6 c 2C 'L 08 °8 90 'L TE 8 LL'8 99 'A 96 °8 18 6 v6 4 BLT 08 26" IT' l 94° 00 ° SI 28" £6 9p 26 I Bil.: CHT: 94° 96° OL" OL" 29° 68 ° 18° OF ' 98° £9 o fat f rae n ad ap aint Danna tenon 00 ° 20° 10° 100 > Sesnon * ano o e n o n ran AATEC 98°C 90 °6 98 °C SLCC Of 29 8 08 'P 99 €0 € 98 Vars 26 °C LTE F8 'I 98 °C LP'8 00 € 96 ° 08 °C 91 'E 86 °6 LG°€ 86 'I Prard 96 SL 'P L0 'FG 95 "CG OT '61 €6 8T 08 06 'IT IP TC 08 '8I TE '8T 69 '8I 08 "SG 90 '0G 48 'OT a 28 L 9T 6 CQ L ( F9 "PI 06 °0 9P 8 10 °6 €6°6 Of 9 T8 °C LP'6 60 ZP SL CL '8P 68 'Of 89 '0¢ 99 T9 88 ZF 08 "SF 06 'OF 99 "SP d 00 I9 05 '08 £9 '6f SE Tg 68°0-|- wey} s0mof oS.revo ue Sujazy sojrroont]:) 880+ wey; s0u8pf of.reyo ao4eprojur ue SujAzy SHSATVNY TVDINHHO 6D STD MD 9D STD FID SID ITD OD 6D 8D LD 9D SD FD $D ID to ~~ 2 orer eons sts4reuy soupu armomp pammoms may) yam aad wom wnissnjod uny) auowm burany sopuoonn6 fo 'qusouad ur 'sashzpun noruayy-'7 «Tgy I, 336-373-69--2 F6 3.-Chemical SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY analyses, in percent, of glauconites having less than 0.66 potassium atom per half cell, together with their calculated atomic ratios AMAIYSIS. . evie. { eir G20 Gar G22 G23 G24 G25 G26 gaz G28 G20 G30 G31 G32 CHEMICAL ANALYSES Glauconites having an interlayer charge higher than +0.88 Glauconites having an interlayer charge lower than 49. 4 49.0 48. 5 47.6 49. 20 50. 20 48. 54 49.00 49. 53 48. 10 48.19 49.09 10.2 9.2 9.0 9.9 8.17 7. 80 7. 82 8. 35 13. 32 11.07 10. 83 15. 21 18.0 19.5 20.0 21.9 21. 72 17. 90 17. 50 20. 20 12.19 14. 67 17.80 10. 56 3.1 3.3 3.1 1.5 3.19 1. 80 8.07 1. 80 2.27 8.07 . 97 3.06 3.5 3.6 3.7 3.7 3. 85 3. 23 3. 26 3.31 3.89 3.78 3.19 2. 65 ........................................ TACE cocco co .06 Trace 'Trace..........« .6 «5 4 .8 .74 . 81 . 68 s 1.53 1.92 1.06 . 55 1.4 .9 1.5 1. 4 12 Trace .22 Trace . 89 . 56 . 48 1. 21 5.1 6.3 6.1 5.83 6.02 6.42 5.87 6. 6.02 5.49 6. 37 6.05 """"""""""""""""""""""""""" a ys cay. Ar 4g an Ip . 21 6.00 2 + . 8.3 7.6 7.3 7:7 4 60 11.27 6.7 10.07 {4. 05 4.56 4. 41} 11. 64 99. 6 99. 9 99. 6 99.8 2100.35 99.43 _ $99.86 100.30 £100.48 5100.51 8100.22 100. 02 CALCULATED ATOMIC RATIOS 3.62 3. 60 3. 57 3. 49 3.83 3.79 3. T5 3. 66 3. 66 3. 65 3. 63 3.62 .38 .40 . 43 . 51 17 . 21 . 25 .34 . 34 .35 .37 . 38 . 50 .39 . 85 . 35 12 . 49 . 46 . 40 .82 . 64 . 60 . 94 .99 1.08 1.11 1. 21 1.27 1.02 1.01 1.14 . 67 . 84 1. 01 . 59 19 .20 .19 .09 & 21 11 .20 11 14 .19 . 06 19 . 32 .33 .35 i .40 .36 .33 .37 .37 .33 . 33 .29 ........................................ £00 renee irene cern .00 . 00 400 -: 2.00 2.00 2.00 2.00 2.00 1.98 2.00 2.02 2.00 2.00 2.00 2.01 -. 89 -. 93 -. 97 -. 96 -. 78 -. 74 -. 78 -. 76 -. 85 -. 87 -. 75 -. 83 .06 .06 .06 .06 .05 00 .05 00 .04 .10 .03 00 .05 .04 .03 .06 .03 .07 . 04 .06 . O1 .03 .02 .04 . 20 13 21 .20 .02 00 .03 00 13 .08 .07 +427 . 48 . 59 . 58 . 50 . 60 .62 . 58 . 65 . 57 . 53 . 61 57 .79 .82 . 88 .82 .70 . 69 . 70 Myi. «15 .74 .78 . 78 -+. 90 +. 92 <+. 97 4+.94 +. 78 +. 76 +.79 4.77 +. 80 +. 87 +. 78 -+. 82 + Includes 0.01 percent LizO and 0.26 P;05. CaO equivalent to P;0; deducted before atomic ratio calculation. *Includes 0.32 percent P;0s. Equivalent CaO deducted before atomic ratio calculation. 3 Includes 0.05 percent S and 0.14 P;0s. CaO equivalent to P;0; deducted before atomic ratio calculation. G20. Gal. G22. G23. G24. G25. G26. 4 Includes 0.96 percent CO; and 0.12 P:0;. CaO equivalent to CO; and P0; de- ucted before atomic ratio calculation. 5 Includes 0.67 percent CO; and 0.53 P;0s. CaO equivalent to CO; and P;0; de- ducted before atomic ratio calculation. 8 Includes 0.39 percent CO; and 0.30 P:0s. CaO equivalent to CO; and P0; de- ducted before atomic ratio calculation. LOCATION OF SAMPLES AND REFERENCE Monte Brione, Garda Lake, Italy (Hendricks and Ross, 1941, p. 692, No. 21). ge: not given. Analyst: chwzg Northwest of Norwalk, Wis. (Hen ricks and Ross, 1941, p. 692, No. 14). Cam- brian sandstone. Analyst I: Brighto: Woodstown, N.J. (Hendricks and Ross, 1941 p 692, No. 6). Cretaceous marl. Analyst: T. B. Brighton. Near Norwalk, Wis. (Hendricks and Ross, 1941, p. 692, No. 3) Cambrian dolomite. Analyst T. B. Brighton. San Pedro, Calif (Hendricks and Ross, 1941, p. 692, No. 2). Pleistocene marl. Analyst: T. B. Brighton. Whare Flat Bast Talerl, Otago Land, New Zealand (Smulikowski, 1954, table 3, No. 21). Tertiary sandstone. Analyst: Seelye. Villers-sur-mer, France (Smulikowski, 1954, table 4, No. 39). Cretaceous sand. Analyst: Sabatier. G27. G28. G29. G30. G31. G32. Kahoko Creek Otepopo, Otago Land, New Zealand (Smullkowski 1954, table 3, No. 23) Tertiary sandstone. Analyst: See! Cuise- la—Mothe, France (Smulikowski, 1954, table 3, No. 13). Tertiary sand. Analyst: Sabatier. Monte Bonifato di Alcamo, Italy (Pirani, 1963, p. 37, No. 3). Age: not given. Analyst: Pirani. Monte Barbaro di Sigesta, Italy (Pirani, 1963, p. 37, No. 2). Age: not given. Analyst: Pirani. Monte Bonifato di Alcamo, Italy (Pirani, 1963, p. 37, No. 1). Age: not given. Analyst: Pirani. Ashgrove, Elgin, Scotland (Hendricks and Ross, 1941, p. 692, No. 15). Cre- taceous oolitic limestone. Analyst: not given. TaBu® 4.-Range and median values of principal constituents in high potassium (>O.65 atom per half cell) celadonites and glauconites in atomic ratios Celadonites Glauconites Principal constituents Interlayer charge >-4-0.87 (9 analyses) Interlayer charge >4-0.88 (10 analyses) _ Interlayer charge <+-0.89 (9 analyses) Range Median Range Median Range Median 4. 00- 3. 76 3. 88+ 0.12 3. 73- 3.43 - 3.58+ 0.15 3. T9- 8. 56 3. 67+ 0. 11 AllIV) a. . 00- . 24 12% .,. 12 & 27- .57 . 42% .15 . 21- . 44 . 88% . 11 AVI) - a ~~~ n Calas a bl . 00- _. 78 . 89+ . 39 , 14- . 81 . 48+ . 84 & 14- _. 74 . 44+ _. 80 Fre. . 87- 1. 18 Tig _. 41 . 50- 1. 28 . 89+ .39 . T1- 1. 37 1. 04+ . 33 on . 18- .31 & 25% .07 &12- .37 . 25% .13 . 11- . 22 . 16+ .06 MS- sube .a nls ns be anl . 49- _. 78 . 64+ _. 15 . 23- . 48 «83+ .. 10 . 27- . 48 . 85+ .08 Percent 38. 9 -50.8 44.9 + 6.0 19. 5 -35.5 27.5 + 8.0 91. 5 =32. 0 26. 6 k $. 3 Percent RB(VI)..:.......:....se- 49.2 -61.1 55.2 + 6.0 64, 5 -80. 5 72.5 + 8.0 68. 0 -78. 7 78. 4 + 5.8 Octahedral charge._..._.._.__.... -. Ti--. 94 -.85+-.09 _ -. 89--.71 -.55%-.16 - -. 87--.64 _ _-.50%-.14 STUDIES OF CELADONITE AND GLAUCONITE F7 TaBL® 5.-Range and median values of principal constituents in low potassium (<0.66 atom per half cell) celadomites and glaucomites in atomic ratios Celadonites Glauconites Principal constituents Interlayer charge <+-0.89 (4 analyses) Interlayer charge >-4-0.88 (5 analyses) Interlayer charge <+-0.83 (8 analyses) Range Median Range Median Range Median a> ane 3. 89- 3.76 3. 82+ 0. 07 3. 68- 3.49 3.59+ 0.10 3. 83- 8.62 3. 72+ 0. 10 sss bel . 11- .24 . 184%. .07 . 32- . 51 «42+ - .10 . 17- . 88 . 28+ 10 AL MYT) .e . 0O0- . 79 . 40+ _. 40 . 29- . 50 . 40+ 41 , 12- -. 82 . 88 in aer rails . 49- 1, 25 . Sik . 88 .99- 1. 21 1. 10+ «11 . 59- 1. 27 . 93+. . 34 . . 09- . 17 13% .04 . 00- , 24 17+ . 08 . 06- , 21 . 14+ .08 ® . 52- . 66 . 59+ .07 . 82- . 44 . 38+ . 06 . 29- . 40 ; ob% :06 Percent Rf (VH..... > ~g8.-0 -37. 5 35. 2 + 2. 2 $2 0 -3a3.6. 27. s L 5. 8 17..0 26.5 $1. 7 4 4, 7 Percent R (VI)............_... 62. 5 -67.0 64.5 + 2.5 66.4 -78.0 72.2 + 5.8 78.5 -80.5 76.5 + 8.5 Octahedral charge......_......... -. 66--. 75 -.71+-.04 -.44--. 62 -.538+ -.09 -.38--.61 _-.49+#-.11 HIGH POTASSIUM CELADONITES AND GLAUCONITES The data given in table 4 show distinct differences between high potassium celadonites and glauconites, as well as certain differences between high potassium glau- conites having an interlayer charge greater than +0.87 per half cell, and those having less. Silicon in the high potassium celadonites ranges from 4.00 to 3.76 cations per half cell, as compared with 3.73 to 3.43 cations in high potassium glauconites, both in glauconites with in- terlayer charges above and in those with interlayer charges below +0.88. Magnesium in the celadonites ranges from 0.49 to 0.78 cation per half cell (median value 0.64), as compared with 0.23 to 0.43 cation in the glauconites (median 0.33). The median value for biva- lent iron, 0.25 cation, is the same in the celadonites and the high-charge glauconites, although the range is some- what narrower in the celandonites. Thus, the difference in the percentage of octahedral positions occupied by bivalent cations, 44.9 percent in the celadonites and 27.5 in the high-charge glauconites, is due principally to the higher magnesium content of the celadonites, not to significant differences in bivalent iron content. Con- versely, the median octahedral occupancy by trivalent cations is lower in the celadonites than in the high- charge glauconites, 55.2 percent as compared with 72.5 percent, but both range widely in relative content of octahedral aluminum and trivalent iron. Because of their greater content of bivalent octahedral cations and lesser content of trivalent octahedral cations, there is a greater deficiency of positive charges in the octahedral layer of high potassium-high charge celado- nites than in that of high potassium-high charge glauconites. The positive charge deficiency ranges from 0.85+0.09 in the high potassium high-charge celadonites as compared with 0.55+0.16 in the high potassium- high charge glauconites In a few celadonites, in which the tetrahedral layer is completely occupied by silicon, as for example, those from Reno, Nev., and Krivoi Rog, U.S.S.R., Nos. C1 and C2, table 1, the entire negative layer charge is due to deficiency of positive charges in the octahedral layer. In this sense, therefore, the total negative layer charge in these celadonites may be thought of as originating in the octahedral layer. High potassium glauconites with interlayer charges less than +0.88 are very similar in composition to those with interlayer charges greater than +0.87, except for their respective contents of bivalent and trivalent iron. In the high-charge glauconites the medians and ranges for bivalent and trivalent iron are 0.25+0.13 and 0.89 +0.39, respectively, whereas in the glauconites with lower charges they are 0.16+=0.06 and 1.04=0.83, respectively. With respect to content of other bivalent and trivalent cations, there is little difference, the high- charge glauconites having 0.33>0.10 cation of magne- sium and 0.48+0.34 cation of octahedral aluminum, the lower charge glauconites having cation of magnesium and 0.444+0.30 cation of octahedral alumi- num. The lower interlayer charge in these glauconites is attributable, therefore, to their lesser content of bivalent iron and greater content of trivalent iron. The differences in composition between high potas- sium celadonites and glauconites are shown graphically in figure 1. In these diagrams the left-hand column rep- resents percent occupancy of the octahedral layer by A1", Fe®, Fee*, and Mg, the middle column represents percent occupancy of the tetrahedral layer by Si+* and Al", and the right-hand column indicates the number of potassium and exchangeable cations, Na, Ca, and Mg, present per half cell. The figures at the tops of the columns indicate the positive-charge deficiencies in the octahedral and tetrahedral layers (cols. 1 and 2, respec- tively) and the positive interlayer charge (col. 3). The sum of the octahedral and tetrahedral deficiencies should agree closely with the positive interlayer charge. These diagrams demonstrate graphically the higher magnesium content of the celadonites and their lower content of tetrahedral aluminum as compared FS SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY A,. Celadonites, interlayer charge >+0.88 B. Glauconites interlayer charge > +0.88 Reno, Nev. Wind River, Wash. Vesuvius, Italy Urals , U.S.S.R. St. Joseph, Mo. Tipchin vrah, Bulgaria 'Table 1, No. CL Table 1,No. C5 Table 1, No. C4 Table 2, No. G3 Table 2, No. G7 'Table 2, No. G9 109 1.00 -9.09 4 1.93 __-0.94 -0.12 ~60.85 -0.12 O71 -033 . (os. 0.59 -9 38 -0.50 -0.45 a i to o a O B O 1 LAYER OCCUPANCY, IN PERCENT 20 - 0 C. Glauconites, interlayer charge <+-0.89 ~ z ~ EXPLANATION Puglia, Italy Carpathian Mts. Udrias, E.S.S.R. Table 2, No. G19 Table 2, No. G13 Table 2, No. G14 100 037-944 0.56 -0.28 -o. 31 - E y g e =z 60- 2): Fet3 3 8 40- 2-35 Al 2 Exchangeable cations FIGURE 1.-Chemical composition of representative celadonites and glauconites having more than 0.65 potassium atom per half cell. Figures above the first and second columns of each diagram indicate the positive-charge deficiencies of the octahedral and tetrahedral layers; figures above the third column indicate the positive interlayer charge. STUDIES OF CELADONITE AND GLAUCONITE with glauconites. They also illustrate the greater oc- cupancy of the octahedral layer by bivalent cations, Mg and Fe*, in celadonites. On the other hand, the diagrams show that the celadonites and glauconites are similar in that both range widely in content of octa- hedral aluminum and trivalent iron. In some celadonites and glauconites, iron is the greatly predominant triva- lent octahedral cation ; in others, aluminum is equal to, or even dominant, over iron. These two octahedral ca- tions bear a general reciprocal relation to each other in that decrease in trivalent iron content is usually ac- companied by increase in octahedral aluminum. LOW POTASSIUM CELADONITES AND GLAUCONITES Comparison of tables 4 and 5 shows that the range values for Si, Al (IV), Al (VI), and Mg in low potas- sium celadonites fall within or close to the range values for these constituents in high potassium celadonites, but that the range values for Fe in the low potassium cela- donites is lower than in the high potassium celadonites, 0.09-0.17 cation as compared with 0.18-0.31 cation, re- spectively. This decrease in Fe* content is reflected in the slightly higher range values for Fe® in the low potassium celadonites, in their lower occupancy by bivalent cations, and in their lower range of octahedral charge. The low potassium glauconites, like the high potas- sium glauconites, are divided into two groups, depend- ing on their interlayer charges. The first group, com- posed of five (table 3, G20-G24), have interlayer charges in excess of +0.88, because of their high con- tents of exchangeable cations, notably sodium, which contribute between 0.32 and 0.45 positive charge to the interlayer. These five glauconites are very similar to each other, as indicated by the small ranges in the values of their principal constituents. They are also similar, in composition, to diagram GT, figure 1, except for their low content of potassium and high content of exchange- able cations. The second group of low potassium glauconites, con- sisting of eight (table 3, G25-G82), are characterized by interlayer charges of less than +0.89 per half cell. The ranges of values of the principal constituents in them are very similar to those of the low charge-high potassium glauconites. Like them, and also like the high charge-high potassium glauconites, they differ widely in content of trivalent iron and octahedral alumi- num, but they are quite uniform in content of silica, tetrahedral aluminum, and magnesium. Compared with the high-charge glauconites, these glauconites are some- what lower in bivalent iron content. Most of them are similar in exchangeable cation content, but others, nota- 336-373-69--3 FQ bly G29, G30, and G32, have high contents of exchange- able cations. Thus, this study shows that celadonites, whether of high or low potassium content, are very similar in con- tent of silicon, tetrahedral aluminum, bivalent iron, and magnesium content; but both groups range widely, and reciprocally, in content of trivalent iron and octa- hedral aluminum. Likewise, high and low potassium glauconites are relatively uniform in content of silicon, tetrahedral aluminum, and magnesium, and in per- centage of octahedral positions occupied by bivalent ca- tions; but they range widely, and reciprocally, in con- tent of trivalent iron and octahedral aluminum. As a group, the celadonites are higher in silicon and mag- nesium and in bivalent-ion octahedral occupancy than the glauconites, and are lower in tetrahedral aluminum and trivalent-ion octahedral occupancy. RELATION BETWEEN Si, R*(VI), Al(IV), AND R#(VD) It has been shown (Foster, 1956) that progressive replacement in dioctahedral potassium micas of tri- valent octahedral cations, R*(VI), by bivalent cations, is accompanied by equivalent increase in silicon and decrease in tetrahedral aluminum, Al(IV). Starting with muscovite, [Als.,(Sig.qAli.0)Q1o(OH)sl* K.,, the end result of such replacement is tetrasilicic dioctahedral potassium mica in which half of the octahedral cations are trivalent and half are bivalent, as [(R. .,oR+*.0)Si.oO10(OH);]-Ki.,. Although analyses of natural micas representing all steps of this replace- ment may be found, Foster noted that, whereas the first part of this replacement series is represented by micas in which aluminum is the dominant octahedral cation, the latter part is represented by glauconites and celadonites, minerals in which iron is usually the dominant octahedral cation. Thus, neither aluminum nor iron, as the dominant trivalent octahedral cation, appears to form a complete series from trisilicic to tetrasilicic or vice versa, but each a partial series, one starting at trisilicic (musco- vite) end, the other at the tetrasilicic end, with neither extending, apparently, much beyond the middle of the range as, [A120 (Si3.oA11.o) (OH) 2] *Kig- [(A11.5R+2.5) (Si3.5A1.5) O10 (OH) 2] 'K1.o, and I: (Fe+31.oR+21.o) 814.0010 ( OH) 2] ® K1.0_'> [ (Fe+31.5R+z.5) (‘Sis.5A1.5) Oto (OH) z] 5. In these formulas bivalent-octahedral cations are rep- resented by the general expression, R*, as the bivalent cations in both series are usually made up of both mag- F10 nesium and iron. The replacements taking place in both series can be represented by the expression [Sit R=(VH) | "={ XI(I¥) or by the formula [(RB) (Bii-yAl; )O; (OH)] (K, Na, Ca). If the high potassium-high charge celadonites and glauconites belong to such a series, with a tetrasilicic mica as the starting point, then in each sample the de- crease in the atomic ratios of Si and of R(VI) and the increase in the atomic ratios of A1(IV ) and of R=*(VI), with respect to the theoretical tetrasilicic end-member, should agree closely, and the sum of the decreases in Si and R#(VI) should correspond with the sum of the increases in Al(IV) and R#*(VI). Table 6 shows the atomic ratios for Si, R*(VI), Al(IV), and the amount of decrease or increase with respect to the end-member, the sums of the decreases in Si and R* (VI), and the increases in Al(IV) and Com- parison of the amounts of Si+R***(VI) decreases with Al(IV)+R®(VI) increases for the samples studied shows that these sums correspond to each other within 0.02 atomic ratio, except for those for C2 and G4, which differ by 0.07 and 0.09 atomic ratio, respectively. The close agreement between the decreases in Si+R**(VI) and the increases in A1(IV) +R®*(VI) is shown graph- ically in figure 2, in which these two sets of values are plotted against each other. All the points, except those for C2 and G4 fall on, or very close to, the 1 : 1 diagonal. 1.20 G10 1.00 G19 G18 J ao 3 G 6x13, G17 E 0.80 G7,Gs_J G5,G6 G14 C G13®l G15 2 3 G3 MGZ S G12 < 0.60 G11 a G1 & i cg, + 0.40 a w C6 C3y-®n7 c2l8>"ca EXPLANATION 0.20 e - Celadonite, interlayer charge >+0.88 | x - Glauconite, interlayer charge >+0.88 C1 gat ® Glauconite, interlayer charge <+0.89 0.00 1 1 1 0.20 0.40 0.60 0.80 1.00 1.20 Al(IV)+R+3(VI)- ATOMIC RATIO FicurE 2.-Relation between Si+R*(VI) decrease and Al(IV) -+-R"(VI) increase in high potassium-high charge celadonites and glauconites. SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY However, the decrease in Si and the decrease in R* (VI) are not the same in all the samples, nor is increase in Al (IV) always in agreement with increase in R#*(VI). These relations are shown graphically in figures 3 and 4, 0.60 [ I EXPLANATION l x y G10 e Celadonite, interlayer charge >+0.88 0.50 |- * Glauconite, interlayer charge >+0.88 @ Glauconite, interlayer charge <+0.89 # 9x 619® o GB € x ce 0.40 67 cis x G6 o x G17 = ois, """ & 6 xG3 ® G16 xG4 v L? ast .o @G14 " S fo: ®(313 A c9, ®c12 t < zC8 @@! g; 0.20 0 C7. ud ® o ce f x* c3 0.10 C2 0.00 s 0.10 0.20 0.30 0.40 0.50 0.60 DECREASE IN R+2(VI)-ATOMIé RATIO FrcUurE 3.-Relation between decrease in Si and decrease in R* (VI) in high potassium-high charge celadonites and glauconites. 0.60 | T EXPLANATION 10% e Celadonite, interlayer charge >+0.88 0.50 |- X - Glauconite, interlayer charge >+0.88 ® Glauconite, interlayer charge <+0.89 g XG9 6128 . ,as i 0.40 1% r x" Les @§er, o x ® ¥ G5" ©g16 < G3 G15® jar & oso a2 zz man gels s c3, ®ar2 m Cs all 2 0.20 ® ® ht f & Tor Z C6 c5 C4 0.10 C3 0.00 KCL C 0.10 0.20 0.30 0.40 0.50 0.60 INCREASE IN R+3(VI)-ATOMIC RATIO FrGur® 4.-Relation between increase in Al(IV) and increase in R"(VI) in high potassium-high charge celadonites and glauconites. STUDIES OF CELADONITE AND GLAUCONITE respectively. The agreement between decrease in Si and decrease in R**(VI) and between increase in Al(IV) and increase in R*(VI) is very good in about one-half of the samples, C1, C4, C5, C7, CS8, G1, G3, GT, and G9. In two samples, C9 and G8, the decrease in Si is greater than the decrease in R**(VI), and the increase in Al (IV) is greater than the increase in R®(VI) by more F11 than 0.03 atomic ratio. In the other eight samples, de- crease in R+ (VI) is greater than the decrease in Si, and the increase in R®*(VI) is greater than the increase in Al(IV). The fact that the excess of R**(VI) decrease over Si decrease is almost identical with the excess of R®(VI) increase over Al(IV) increase strongly sug- gests oxidation in these samples. TABLE 6.-Al(IV)R+3(VI) replacement of SiR+2(VI) in high potassium-high charge celadomites and glauconites Decrease Decrease - Decrease in Increase Increase - Increase in - Negative Si in Si R+ (VD) in R+ (VI) AlIV) in AlIV) R# (VI) in R# (VD Al(IV) layer {+RH(VD charge Theoretical end-member.............. 4.00 1.00 0. 00 0. 00 1.00 0. 00 1.00 Celadonites Analysis: (g eneral nie ies 4.00 0.00 1.02 0. 02 (O] 0. 00 0. 00 1.00 0. 00 0. 00 0. 96 ece 4.00 . 00 .76 .24 0. 24 . 00 .00 1.19 .19 .19 . 91 B... 22. 3. 90 .10 79 . 21 . 31 10 10 1.21 21 . 81 . 89 4.. 3. 88 12 . 85 15 27 12 12 1.15 15 .27 . 97 Been. 02. 3. 88 12 . 88 12 24 12 12 1.10 10 .22 1.06 Fee eet icec ec uweds 3. 86 14 .80 20 34 14 14 1.20 20 . 34 . 94 see 3. 83 A47 . 84 16 33 217 17 1.18 18 .35 . 95 Se n. B PELL A . eure dia Phew 3.79 . 21 . 78 43 .21 21 1.22 22 .43 . 99 $e AIT... ..... . conde OY 3.76 . 24 83 17 41 . 24 24 1.19 19 .43 1.01 Glauconites 3. 73 0. 27 0. 71 0. 29 0. 56 0. 27 0. 27 1.20 0. 29 0. 56 0. 98 3. 70 . 30 . 63 .37 . 67 .30 .30 1.30 . 39 . 69 .87 3. 67 . 33 . 60 . 31 64 . 33 33 1. 30 30 . 63 1.04 3. 67 . 33 .48 67 90 . 33 38 1. 48 48 . 81 1. 08 3. 64 .36 . 58 47 83 . 36 36 1.47 47 .83 . 89 3. 63 .37 . 54 46 83 . 37 37 1. 46 46 . 83 . 91 3. 62 . 38 . 59 41 79 . 38 38 1. 41 41 79 . 97 3. 58 42 . 63 37 79 42 42 1. 37 .37 79 1.05 3. 55 45 . 56 44 89 45 45 1.46 46 . 91 . 94 3. 43 «57 . 39 61 1.18 «57 57 1. 61 61 1.18 96 1 Increase 0.02. f 2 Includes 0.03 atomic ratio Fet3. In the high potassium-high charge celadonites and glauconites, therefore, the relations between decrease in Si and R*(VI) and increase in Al(IV) and R=®(VI) indicate that these celadonites and glauconites belong to a replacement series, which starts with tetrasilicie mica, and which continues to about halfway between the tetrasilicie and trisilicic end-members, or to a replacement of about 0.5 Si and 0.5 R+ (VD by 0.5 A1 (IV )and 0.5 R* (VI). The discrep- ancies in some of the samples between R**(VI) decrease and Si decrease, and between R**(VI) increase and Al (IV) increase indicate oxidation of to R#® (VI). In the most divergent samples, C2 and G4, octa- hedral occupancy is low, only 1.95 and 1.91 positions, respectively. This low octahedral occupancy may indi- cate faulty analysis or analysis of impure samples, which may account for the fact that the SiR** (VI) and (VI) data for these analyses do not correlate with such data for the other high potassuim-high charge celadonites and glauconites. The data for SiR**(VI) and Al(IV)R®*(VI) rela- tions in high potassium glauconites having interlayer charges less than +0.89 per half cell are given in table T. In all these glauconites there is good agreement between Si+ R(VI) and Al(IV) +R*(VI). However, in these glauconites, R**(VI) decrease is significantly greater than Si decrease, and increase is greater than Al(IV) increase. The amount that the decrease in R* (VI) exceeds the decrease in Si agrees closely with the amount that the increase in R**(VI) exceeds the in- crease in A1(IV). Thus, there is evidence of oxidation in all these glauconites, and this oxidation of bivalent to trivalent iron may account for the lower interlayer charge of these glauconites as compared with those having comparable contents of potassium but higher interlayer charges. Data on Al(IV) and replacement of Si and R* (VI) in celadonites and glauconites having less than 0.66 potassium ion per half cell, given in table 8, also show close agreement between Si+R**(VI) decrease and Al(IV) +R*(VI) increase. In the low potassium F12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY TaBus 7.-Al(IV)R+3(VI) replacement of SiR+2(VI) in high potassium (>O0.65)-low charge (<-|-0.89) glauconites Decrease Decrease ecrease in Increase Increase Increase in - Negative Si in Si R+ (VI) in RH (VI) AlIV) in Al@IV) R# (VI) in RH (VD AlIV) layer +RH(VD charge Theoretical end-member.............. 4.00 1.00 0. 00 0. 00 1.00 0.00 1.00 Analysis: 11 3.79 0. 21 0. 64 0. 36 0. 57 0. 21 0. 21 1.36 0. 36 0.57 0. 85 3. 74 26 . 60 . 40 66 .26 26 1. 40 40 . 66 . 86 3. 72 28 . 56 .44 72 .28 28 1.44 44 . 72 .84 3. 69 31 . 57 43 74 . 81 31 1.45 45 .76 82 3. 66 34 . 57 43 T7 . 34 34 1. 45 45 .79 85 3. 64 36 . 51 49 85 . 36 36 1. 49 49 . 85 87 3. 63 37 . 49 51 88 . 87 37 1.52 52 . 89 3. 62 38 . 49 51 89 . 38 38 1. 51 51 . 89 87 3.56 44 . 48 57 1. 01 . 44 44 1. 59 59 1.03 81 celadonites and glauconites that have interlayer charges greater than +0.85, the decrease in R+**(VI) agrees fairly well with the decrease in Si, and the increase in R®(VI) with the increase in Al(IV), but in the low potassium-low charge glauconites the discrepancy be- tween these values is considerably greater. Again, the close agreement between the amount that the decrease in R+**(VI) exceeds the decrease in Si and the amount that the increase in R*®*(VI) exceeds the increase in A1(IV) suggests oxidation of bivalent to trivalent iron by the amount of the excess. The data on decrease in Si and R**(VI) and increase in A1(IV) and R*(VI) indicate that the low potassium celadonites and glauconites belong to the same iso- morphous replacement series as the high potassium cela- donites and glauconites. However, the amount by which the decrease in R*(VI) exceeds the decrease in Si, and the amount by which the increase in R**(VI) exceeds the increase in Al(IV) are greater in them, and there- fore indicate greater degrees of oxidation. As shown in figure 5, there is a close agreement be- tween the excess of R**(VI) decrease over Si decrease and layer charge excess or deficiency compared with - 1.00, the layer charge of the theoretical end-member, regardless of the amount of excess or the charge excess or deficiency. As excess of R* (VI) decrease over Si de- crease suggests oxidation, and as oxidation reduces the layer charge by the number of ferrous ions oxidized, this close relationship implies that most of these ce- ladonites and glauconites, whether high or low in po- tassium, originally had layer charges very close to the theoretical 1.00. TaBu® replacement of SiR+2(VI) in low potassium (<0.66 per half cell) celadonmites and glauconites Decrease Decrease Decrease in Increase Increase - Increase in - Negative Si in Si R+*(VI) in RH (VI) Sif-R(VI) AlIV) in AlIV) R# (VI) in R# (VD AlIV) layer +RH(VD charge Theoretical end-member.............. €:00 .. 1:00}. 0. 00 0:00 .%... 1000: 0. 00 1.00 Celadonites Analysis: CIO cl.. R. Beil. 3. 89 0. 11 0. 66 0. 34 0.45 0.11 0.11 1. 34 0. 34 0.45 0.77 11. 3. 88 12 .74 .26 . 38 12 12 1.26 26 . 38 P 12... 8. 79 . 21 . 75 . 25 . 46 1,21 . 21 1.25 25 . 46 95 Ae 3.76 . 24 78 27 . 51 . 24 . 24 1.27 27 . 51 97 High charge glauconites (>--0.88) 3. 68 0. 32 0. 68 0.32 0. 64 0. 32 0.32 1. 34 0. 34 0. 66 0. 94 8. 62 . 38 . 51 . 49 .87 . 38 . 38 1. 49 49 . 87 89 3. 60 . 40 . 53 47 .87 . 40 .40 1.47 47 . 87 93 8. 57 .43 . 54 . 46 80 . 43 .43 1. 46 46 . 89 97 3. 49 . 51 .44 . 56 1.07 . 51 . 51 1. 56 56 1. 07 96 Low charge glauconites 3. 85 0.15 0. 61 0. 39 0. 56 0.15 0.15 1.39 0. 89 0. 56 0. 76 8.79 . 21 47 . 53 . 74 . 21 . 21 1. 51 51 . 12 74 3.75 . 25 .53 47 72 . 25 . 25 1.47 47 172 78 3. 66 34 . 48 52 . 86 .34 . 34 1. 54 54 . 88 76 3. 66 34 . 57 49 .83 . 34 . 34 1. 49 49 .83 85 38. 65 35 . 52 48 . 83 .85 . 85 1.48 48 . 83 87 3. 63 37 . 39 61 . 98 .37 . 87 1. 61 . 61 . 98 75 3. 62 38 . 48 52 . 90 . 38 . 38 1.53 53 . 91 ' Includes 0.06 atomic ratio FeS. STUDIES OF CELADONITE +0.28 G4 C2 r 2 ho a x C > +0.20 +0.16 +0.12 +0.08 +0.04 x o o 6 * x ~ a x e x EXPLANATION ~ | e Celadonite EXCESS OF R*2(VI) DECREASE OVER Si DECREASE -0.04 x Glauconite -0.08 +0.08 +0.04 0,00 -0.04 -0.08 -O0.12 -O0.16 -0.20 -0,24 -0,.28 LAYER CHARGE EXCESS OR DEFICIENCY FiGurE 5.-Relation between excess of R+*(VI) decrease over Si decrease and layer charge excess or deficiency. INTERPRETATION OF GLAUCONITE COMPOSITION RELATION BETWEEN TRIVALENT IRON AND OCTAHEDRAL ALUMINUM In the first part of this study it was shown that except for deficiency in potassium content, potassium-deficient glauconites are very similar in composition to the high potassium glauconites. Aside from potassium content, the most variable constituents in these glauconites are the trivalent octahedral cations, iron and aluminum, which bear a reciprocal relation to each other (fig. 6). These relations, especially when found in high potas- sium-high charge glauconites, suggest that a high con- tent of octahedral aluminum in a glauconite may not necessarily indicate degradation or admixture, but might be indicative of the environment in which the glauconite developed specifically with reference to the concentration of iron. Because many glauconites are high in iron, it has been generally postulated that glauconite is developed in en- vironments rich in iron. Even in such environments, however, iron concentration differs greatly, and these differences are reflected in the iron content of the glauconite. THE Fe®:Fet* RATIO Theoretically, the relation between bivalent and triva- lent iron in a glauconite is determined by the oxidation potential in the environment of its development. With change of environment, however, a glauconite may un- AND GLAUCONITE F13 1.00 t t t | EXPLANATION l ® * More than 0.65 K ion per half cell 0.90 x Fewer than 0.66 K ion per half cell -I ® 0.s0 } ® o 2 Ix a 0.70 x e ® 3 2 oso x < 1 > I 0.50 4 x r B ¢ s * $% E . y+ é has U U 6 x L p 0.30 g x | ® < 6 x o 0.20 y xx ® 0.10 0.00 - 0.50 0.60 0.70 0.80 0.90 1.00 1.10 . .1.20 1.30 - 1.40 TRIVALENT (VI) IRON-ATOMIC RATIO FicurE 6.-Relation between Fe® and Al(VI) in glauconites. dergo oxidation ; this appears to have happened to many of the glauconites under review. As has been pointed out, oxidation is strongly suggested when the decrease in R**(VI) is greater than the decrease in Si, and the increase in R®*(VI) is greater than the increase in A1(IV) compared with the theoretical tetrasilicic end- member, and when the amount by which the decrease in R**(VI) exceeds the decrease in Si agrees closely with the amount by which the increase in R*®*(VI) exceeds the increase in A1(IV). The only glauconites included in this study in which R**(VI) decrease agreed closely with Si decrease, and R*®(VI) increase agreed closely with A1(IV) increase, indicating little or no oxidation, are those high potas- sium glauconites that also have layer charges close to -1.00+0.05, G1, G3, G4, G7, and G9. In all the others, the relations between R**(VI) and Si and between R®(VI) and Al(IV) indicate that some oxidation has taken place, and that Fe*®: Fet ratios calculated from the analytical results do not reflect the true oxidation- potential of the respective environments of development. For glauconites that have undergone only a minor degree of oxidation, the calculated Fe® : Fe** approxi- mates the Fo*®: Fet relation in the original glauconite. If, however, two-thirds or more of the original bivalent iron has been oxidized, as in G26, G28, and G31, such calculated ratios would be very misleading, unless the amount of oxidation, as indicated by the amount by which the decrease in R**(VI) exceeds the decrease F14 in- Si and the amount by which the increase in R®(VI) exceeds the increase in Al(IV), is taken into consideration. RELATION BETWEEN IRON AND POTASSIUM As is obvious from figure 7, there is no correlation between potassium and iron in glauconites. Glauconites high in potassium may be high, medium, or low in iron, and others low in potassium may be high, medium, or low in potassium. This is contrary to Hower's (1961) conclusion that in glauconites, potassium and iron should be directly proportional to each other. However, these constituents are incorporated into the structure by different processes, and any proportionality between them is coincidental. After a study of glauconite formation in modern foraminiferal sediments of the southeastern coast of the United States, Ehlmann, Hulings, and Glover, (1963, p. 95) concluded that "the earliest stage material having an FeO, content of 20 percent may be regarded as a potassium-deficient prototype of glauconite with iron already incorporated into the structure." This con- clusion supports the present hypothesis that glauconiti- zation consists of two separate, unrelated processes, incorporation of iron into the crystal structure and fixa- tion of potassium in interlayer positions, with incorpo- ration of iron and development of negative layer charge 0.88 T T t I EXPLANATION I 9.34 |__. * More than 0.65 K ion per half cell + x - Fewer than 0.66 K ion per half cell * 0.80 0.76 * 9 ~ n o & A POTASSIUM - ATOMIC RATIO & Co € 0.48 0.60 1.30 x 0.90 1.00 1.10 1.20 (Fe+2 +Fe+3)-ATOMIC RATIO 0.70 _ 0.80 1.40 1.50 FrGurE 7.-Relation between total iron and potassium in glauconites. SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY preceding complete fixation of potassium. Thus, there is not necessarily a relation between iron and potassium in a glauconite, as has been postulated by some investigators. - FIXATION OF POTASSIUM Fixation of potassium may begin to take place rela- tively early in the development of a glauconite, espe- cially if the parent material is a stripped and degraded layer silicate, with potassium first being taken into exchange positions before becoming fixed. As the potas- sium gradually settles into fixed positions and thus changes the relative proportions of the exchangeable cations present, continual readjustment is required be- tween the exchangeable cations present and the cations of the surrounding water, usually sea water. Eventually, if the process is not interrupted, most of the negative layer charge is neutralized by potassium. As more and more potassium becomes fixed, the sheets become more and more tightly bound together, and further entry of potassium becomes increasingly difficult. Thus, it may be that fixation never proceeds to completion in a glau- conite, and that most glauconites contain some ex- changeable cations. An occupancy of about 85 percent of the possible interlayer positions, as in G3, G4, G15 and G18, may be the maximum to be expected in glau- conites, even though their negative layer charges are equivalent, as in G3 and G4, to those of true micas. DEFICIENCY IN POTASSIUM CONTENT Deficiency in potassium content may be due to failure to attain maximum fixation because of change of en- vironment, burial, uplift, or dilution, or to oxidation, with accompanying decrease in layer charge and loss of some potassium that had been fixed. Failure to at- tain maximum fixation of potassium is exemplified by glauconites G20-G24. These five glauconites all have high layer charges, ranging from -0.89 to -0.97, per half cell, but they contain only from 0.48 to 0.62 ions of potassium, with exchangeable cations neutralizing the remainder of the layer charge. Although a slight excess of R*(VI) decrease over Si decrease and excess of R* (VI) increase over Al(IV) increase indicate some oxidation in those with layer charges less than -0.95, their very high content of exchangeable cations, averag- ing 0.39 positive charge per half cell, and particularly their content of exchangeable sodium, averaging 0.19 ion per half cell, is very suggestive of immaturity. The fact that the R*(VI) and Si and the R*®*(VI) and Al (IV) relations in these glauconites indicate that they have undergone little oxidation also suggests imma- turity as the cause of the low content of potassium. STUDIES OF CELADONITE AND GLAUCONITE Many of the glauconites that have undergone oxida- tion are characterized by relatively lower contents of bivalent iron, lower layer and interlayer charges, lower exchangeable cation content, and a somewhat lower potassium content. For example, the relations between decrease in R+**(VI) and in Si, and between increase in R@®(VI) and in Al(IV) in seven high potassium glauco- nites having layer charges of -0.95 or more, G1, G3, G4, and GT-G10, indicate that these glauconites have undergone no oxidation; whereas in six other high po- tassium glauconites, G11, G13-G15, G17, and G19, the relations between decrease in R*#*(VI) and in Si and between increase in R®"(VI) and in Al(IV) indicate that from 0.10-0.16 ion of bivalent iron has been oxi- dized to trivalent iron. As a result of this oxidation these glauconites have not only lower layer charges, averaging -0.83 compared with an average of -1.00 in the seven unoxidized glauconites, but an average of 0.13 fewer bivalent iron cations, and only about half as many exchangeable cation charges, averaging +0.10 as compared with 0.21. Although there is also a slight decrease in average potassium content, from 0.'T7- 0.73, most of the loss in negative layer charge has been offset by the release of exchangeable cations, not of fixed potassium. In some of the low potassium glauconites it is diffi- cult to determine whether potassium deficiency is due to oxidation or immaturity. The relations between de- crease in R**(VI) and in Si and between R®"(VI) and A1(IV) in these glauconites indicate that they have lost an average of 0.24 cation of bivalent iron by oxidation. In terms of the individual glauconites this means that one-half (for G25 and G27), two-thirds (for G26 and G28), and three-fourths (for G31) of the bivalent iron originally present has been converted to trivalent iron. It also means that there has been a loss in negative layer charge corresponding to the loss in bivalent iron con- tent, as these glauconites have an average layer charge of -0.76. With an average exchangeable cation charge only +0.05 less than that in the unoxidized high charge- high potassium glauconites, G1, G3, G4, and GT-G10, it is difficult at this stage to resolve the reason for the deficiency in potassium in these glauconites, which averages only 0.61 ion per half cell-whether it is due to release because of oxidation, or to immaturity, failure to attain maximum fixation, as in G20-G24, or to a com- bination of the two. RELATION BETWEEN GLAUCONITE COMPOSITION AND GEOLOGIC AGE Attempts to correlate glauconite composition with geologic age have not been altogether successful; too few analyses of glauconites can be reliably referred to F15 definite geologic periods to furnish adequate informa- tion as to trends in composition over the eons of geo- logic time. Smulikowski (1954) based a correlation on average values of the principal constituents of the glau- conites assigned to various geologic periods. He con- cluded (p. 77) that "the older the geological formation, the smaller in its glauconite the prevalence of ferric iron over aluminum in the octahedral layer and the greater in its glauconite the total amount of interlayer cations." He based these broad conclusions on six anal- yses for the Quaternary Period, 18 for the Tertiary, 13 for the Cretaceous, none for the Triassic, Permian, Carboniferous, and Devonian, two for the Silurian, five for the Ordovician, and three for the Cambrian. Smuli- kowski excluded from consideration five analyses of Jurassic glauconites because all were by the same an- alyst, and represented glauconites from only two locali- ties, both in Russia. However, four of his five Ordo- vician analyses were from limestone in Sweden, and these he did include. All in all, he had no data for the long period of time between the end of the Silurian and the beginning of the Cretaceous, and only scanty data (10 analyses) for the Silurian, Ordovician, and Cam- brian, which he grouped together as early Paleozoic, a period of almost 200 million years. Smulikowski's average values for octahedral alumi- num in Tertiary, Cretaceous, and early Paleozoic glau- conites, 0.399, 0.426, and 0.501 ion per half cell, respec- tively, differ by only 0.102 ion for a length of time of more than 500 millions of years. This is a very small difference in amount of octahedral aluminum and very scant data upon which to base the broad conclusions he came to, particularly in view of the great variations in the compositions of the Tertiary and Cretaceous glau- conites upon which his averages for these periods are based. Thus, his average value of octahedral aluminum in Tertiary glauconites is based on values that range from 0.121 to 0.686 ion per half cell, and in Cretaceous glauconites from 0.135 to 0.942 ion. The variation in content of octahedral aluminum in glauconites referred to each of these periods is more than six and nine times greater than the average difference between octahedral aluminum in the Tertiary and early Paleozoic glauconites. The great variety of composition to be found in glau- conites from the Cretaceous and Tertiary, the periods best represented in the glauconites included in this study, is indicated in figure 8. The range in content of octahedral aluminum, bivalent and trivalent iron, po- tassium and interlayer charge among the eight Cretace- ous glauconites is almost as great as among the entire suite of analyses studied. They range from the highly ferruginous, as illustrated by G11, in figure 8, to the F16 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY CRETACEOUS TERTIARY Table 2, No. G11 Average Table 2, No. G1 Table 3, No. G25 Average Table 2, No. G16 -0.64 -0.21 -0.55 -0.30 -0.71 -0.27 -0.61 -0.17 -0.55 -0.33 . -O0.51 -0.36 LAYER OCCUPANCY, IN PERCENT X BA A N. A TEL Q N Mg Fe *® Al K Exchangeable cations FiaurE 8.-Extreme and average compositions of Cretaceous and Tertiary glauconites. Figures above the first and second Columns of each diagram indicate the positive-charge deficiencies of the octahedral and tetrahedral layers; figures above the third column indicate the positive interlayer charge. highly aluminous, as illustrated by G1, in figure 8. They are equally variable in potassium content and in inter- layer charge, ranging from 0.84 to 0.57 ion of potas- sium per half cell, and +0.97 to +0.76 interlayer charge per half cell. Some of the Tertiary glauconites studied are as ferruginous as any of the Cretaceous glauconites, but none of the Tertiary glauconites were as high in octahedral aluminum as G1, the highest octahedral aluminum content occupying only 30.0 percent of the octahedral layer. The range in potassium content in the Tertiary glauconites, from 0.88 to 0.58 ion per half cell, is very similar to that in Cretaceous glauconites, as is also the range in interlayer charge, from +1.03 to 0.77. The variety of compositions found among both the Cretaceous and Tertiary glauconites suggests that geo- logic age has little to do with the composition of glau- conites. Other factors, such as the specific environment in which the glauconite is formed-particularly its de- gree of iron concentration and its oxidation-reduction- and opportunities for potassium fixation and for oxida- tion, are of far greater importance in determining the composition of a glauconite than the geologic age in which it was formed. STUDIES OF CELADONITE AND GLAUCONITE F17 REFERENCES CITED Atanasov, Georgi, 1962, Glauconites from the Jurassic in Bul- garia: Sofia Univ. Biologo-Geologo-Geografski fakultet Godishnik, v. 55, p. 142-157. [ Russian.] Bayramgil, Orhan, Hiigi, Th., and Nowacki, W., 1952, Uber ein Seladonitvorkommen im Gebiete yon Zonguldak [Turkey] ; Schweizer. Mineralog. u. Petrog. Mitt., v. 32, p. 242-250. Bentor, Y. K., and Kastner, Miriam, 1965, Notes on the min- eralogy and origin of glaucenite: Jour. Sed. Petrology, v. 35, p. 155-166. Burst, J. F., 1958, "Glauconite" pellets-their mineral nature and applications to stratigraphic interpretions: Am. Assoc. Petroleum Geologists Bull., v. 42, p. 310-327. Dell'anna, Luigi, 1964, La glauconite nei calcari cretacei della Pensiola Salentina: Periodico Mineralogia, v. 33, p. 521- 545. Dyadchenko, M. G., and Khatuntzeva, A. Y., 1955, On the prob- lem of the genesis of glauconite: Akad. Nauk SSSR Dok- lady, v. 101, p. 151-153. [Russian.] Ehlmann, A. J., Hulings, N. C., and Glover, E. D., 1963, Stages of glauconite formation in modern foraminiferal sediments : Jour. Sed. Petrology, v. 33, p. 87-96. Foster, M. D., 1951, The importance of exchangeable magnesium and cation-exchange capacity in the study of montmoril- lonitic clays: Am. Mineralogist, v. 86, p. 717-7830. Foster, M. D., 1956, Correlation of dioctahedral potassium micas on the basis of their charge relations: U.S. Geol. Survey Bull. 1036-D, p. 57-64. Hendricks, S. B., and Ross, C. S., 1941, Chemical composition and genesis of glauconite and celadonite: Am. Mineralo- gist, v. 26, p. 683-708. Hoebeke, F., and Dekeyser, W., 1955, La glauconite: Recherches LR.S.I.A. [Institut pour Encouragement de la Recherche Scientifique dans lIndustrie et Agriculture], Brussels, Compte rendu, no. 14, p. 103-121. Hower, John, Jr., 1961, Some factors concerning the nature and origin of glauconite: Am. Minerologist, v. 46, p. 313-334. Kelley, W. P., and Liebig, G. F., Jr., 1934, Base exchange in relation to composition of clay with special reference to effect of sea water: Am. Assoc. Petroleum Geologists Bull, v. 18, p. 358-367. Lazarenko, E. K., 1956, Problems of nomenclature and classifi- cation of glauconite: Voprosy mineralogii osodochnykh obrazovanii (Lvov. Univ.) Books 3 and 4, p. 345-379. [Russian.] Malkova, K. M., 1956, Celadonite from the Bug Region: Lvov. Geol. Obshch. Mineralog. Sbornik, no. 10, p. 305-318. [Russian.] Owens, J. P., and Minard, J. P., 1960, Some characteristics of glauconite from the coastal plain formations of New Jer- sey: U.S. Geol. Survey Prof. Paper 400-B, p. B430-B432. Pirani, R., 1963, Sul fillosilicato die livelli eruptivi di Monte Bonifato di Alcamo e di Monte Barbaro di Sigesta e sulla validita di uso della nomenclatura binomia-glauconite- celadonite: Mineralog. et Petrolog. Acta [Bologna], v. 9, p. 831-75. Ross, C. S., and Hendricks, S. B., 1945, Minerals of the mont- morillonite group: U.S. Geol. Survey Prof. Paper 205-B, p. 23-79. Savich-Zablotzky, K. N., 1954, On the question of the chemical composition and genesis of celadonite from Karadagh : Lvov. Geol. Obshch. Mineralog. Sbornik, no. 18, p. 213-220. [Russian.] Scherillo, Antonio, 1935, I basalti di Giuliana e di Contessa Entellina e la loro alterazione-Studio petrografico: Peri- odico Mineralogia, v. 6, p. 61-84. Schiiller, Arno, and Wohlmann, E., 1951, Uber Seladonit und seine systematische Stellung: Neues Jahrb., Mineralogie, Abh., v. 82, p. 111-120. Serdyuchenko, D. P. 1965, Svitalskite and its position in the series of tetrasilicic micas: Vses. Mineralog. Obshch. Za- piski, v. 94, p. 566-577. [Russian.] Shashkina, V. P., 1961, Mineralogy of weathered basalt crust in W. Volynya: Internat. Geology Rev., v. 3, p. 893-407; translated from Lvov. Geol. Obshch,. Mineralog. Sbornik, 1959, no. 13, p. 190-211. [Russian.] Smulikowski, Kazimierz, 1954, The problem of glauconite: Archiwum Mineralogiczne (Polska, Akad. Nauk), v. 18, pt. 1, p. 21-120. Sudovikova, E. N., 1956, A green mica of the iron ore series of the Kursk magnetic anomaly: Vses. Mineralog. Obshch. Zapinski, v. 85, p. 543-549. [Russian.] Valeton, Ida, 1958, Der Glaukonit und seine begleitminerale aus dem Tertiir yon Walsrode: Hamburg Geol. Staatsinst. Mitt., no. 27, p. 88-131. Wise, W. S., and Eugster, H. P., 1964, Celadonite-synthesis, thermal stability, and occurrence: Am. Mineralogist, v. 49, p. 1031-1083. ¢*. Fev 7 DAY € 75 f °C 2. (Geology of the Grandfather Mountain Window and V icinity, North Carolina and Tennessee GEOLOGICAL SURVEY PROFESSIONAL PAPER 615 (Geology of the Grandfather Mountain Window and Vicinity, North Carolina and Tennessee By BRUCE BRYANT end JOHN C. REED, Jr. CGFEOLOGICECAL: - SURVEY. RROFESSEON AL Structural and metamorphic history of rocks exposed in the largest window in the crystalline belt of the southern Appalachians, and a discussion of the significance of the window in interpretation of the tectonics of the region PAPER _ 6135 UNITED SEATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1970 UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director Library of Congress catalog-card No. 77-607595 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Abstract ado a uo dds te lune l u 1 introduction . se . CLOG il 2 Location and physical geography ________________ 2 Previous geologic investigations _________________ 6 Present investigation T Fieldwork and acknowledgments _____________ T Methods sof study -.. ...l Lcol 7 Geographic locations and locations of typical. outcrops 9 Petrographic nomenclature ____________-- 9 General LIL n deca 10 Mountain City window ___.. {x 10 Rock: Units 2. 22 2.0. c eel H La. e eae a all am oo 10 Chilhowee Group ::..}... l 10 Unicol Formation 11 Hampton Formation 11 Erwin Formation ....-.._-.._-_____CL.ll.. 11 AQE "h., cell roan abl oue ae a 11 Shady. Dolomite. _...... ~... 11 Rome - Formation *: _. 11 Structure and metamorphism --.-L_-__.__..c_.._. 11 Blue Ridge thrust sheet: 12 Rock units } lleno ile caa n 12 Mica schist, mica gneiss, and amphibolite ____ 12 Biotite-muscovite schist and gneiss ______ 12 Amphibolite and hornblende schist _____-_-- 17 Granoféels g_ inara cea aa an 19 ARE ?. ist o. nig a ale nel an 19 Mixed roCks ! --..! __ sen eel nal ins 20 Layered gneiss southeast of the Grandfather Mountain -window. ... 21 Cranberry Gneiss .:.. llc ce. 26 Beech Granite : co 36 Quartz monzonite gneiss -__._._.._-.-:._-.____. 40 Aegirine-augite granite gneiss _______________ 41 41 Quarts porphyry . 2 LL 45 Chilhowee Group. ..:. L >-. R L_.LLeLLELLL._eln. 45 Slices north of the Grandfather Mountain window. . 2 Lin ie ole cba onan aud 45 Slices southeast of the Grandfather Mountain window 47 (Correlation >- L on tn indo e io 48 Uittamaiic 48 Granodiorite and pegmatite _________________ 49 Quartz monzonite and pegmatite in the Deep Gap c ees 58 Page Blue Ridge thrust sheet-Continued Structure and metamorphism not related to the Brevard zone" _____2 Lcc llc _s, 53 FoHAtION .*.. 1 . | . 1 2.2. lll. LAC ce ner nein elle a's ale 58 Lineation..:..= cn 53 Minor. folds - -- CL gee eel Uc cus 55 Metamorphism. _: LZ 55 Intermediate sheet 59 Grandfather. Mountain window _-___._-_.____-______._ 59 Autochthonous: rocks .._... 59 Metagabbro and metadiorite _______________--- 59 Wilson Creek Gneiss _.: 60 Megascopic features 60 PetrograpUys . s. cuse l L_ cE 63 Layered gneiss! cl.. 64 Quartz: monzonite pneige -_-.____..._.-.___. 65 Phyllonite and blastomylonite ___________- 65 Other rocks: -.... _>.. -L lui 66 Origin and age . lec. 67 Blowing Rock Gneligs. tL: 67 Brown Mountain Granite 71 Grandfather Mountain Formation __________-- 12 Basal 010 78 Contacts between arkose and siltstone units _ 75 Sedimentary rocks __ 75 ATkOoSe!* - succinate ute 75 Siltstone __ 81 Volcanic rotks ___.... :s 88 Rocks older than Montezuma Mem- ber of Grandfather Mountain For- imation - T0 eal nol ien dens 88 Felsic volcanic rocks _________---- 88 Mafic volcanic rocks _________--- 89 Montezuma Member ____.____._______ 93 Origin and environment of deposition ____ 94 Age and correlation ; ___u 95 Linville Metadiabase 96 Rocks of the Tablerock thrust sheet ____________-- 98 Chilhowee Group: 98 Lower quartzite unit ____.-L________ulc_l 99 Phyllite unit ..:. un 99 Upper - quartzite unit 99 Correlation and regional relationships ___. 103 shady Dolomite 104 Allochthonous rocks of uncertain correlation ______ 105 Structure ~~... 105 III IV CONTENTS Page Grandfather Mountain window-Continued Structure-Continued Faults . - : 2s 2 De LINA UE alc _ 105 Tablerock fault 22020000000... 105 Other faults... - cose re 2. 106 Cleavage . c=! onle ecu ee leat ee reed a ll anne bak 106 Cineation _ scl cul. l_ 106 Folds c aa een 22 aaa ame aa tulad a o 107 Basement rocks -__. 107 Grandfather Mountain Formation _______- 108 Tablerock thrust sheet __.._/.........._.. 110 Metamorphism ._... c : Olin a_ 114 inner Piedmont belt. cls 115 Rock units nul ae Uda eae o aloe 116 Layered rocks 0 116 Biotite pheiss: .... }{... 116 Mica: schist tL. -l. 122 Hornblende gneiss and amphibolite _______ 122 Siflimanite 128 Other interlayered rocks _______ mga 129 Quartzite and quartz schist _________ 129 Cale-silicate rocks 130 Anthophyllite gneiss 131 Marble .. 131 Origin ___.... _. ul on adit t uel 132 (ABE seee e o oils niet i aie nee n aa erd aie ne. 135 135 Henderson Gneiss aint So ow 135 Granitic rocks and migmatite ____________ _-_ 140 Quartz monzonite gheiss __L_L__L_____________ 147 Uitramafic. rocks _ 149 Structure of rocks of the Inner Piedmont not showing metamorphic effects related to the Bre- vard faultizone ._. -__-... 150 Layering and folistion'..-._._L.._.cec_-l.-.____ _ 150 Folds and Aineatiohn -=. 150 Structural geometry 150 Age relations between folding, formation of min- eral lineation, and emplacement of granitic TOCkSE - SPAA IL o ue reer anne me ae amble lem a ae bin 152 Page Brevard fault: zone ._.. .. L ss 28>. aL Iasi dik 152 Rock LL 2200000 ducts o s aoa a 153 PBlastomylonite ln.} 153 Phyllonitic schist and gneiss _______ 155 Structure of rocks of the Brevard fault zone and flanking polymetamorphic rocks ______________- 156 Relation between structures in the Brevard fault zone and structures in rocks to the northwest: 22222222 LCIE LAGU tanec 159 Relation between structures in polymetamor- phic rocks southeast of the Brevard zone and structures in other rocks of the Inner Piedmont '..... ss co00_ Coco atta ul 160 Piabase "2. 2 on o on e o o p n ara inl ant 161 Major faults bounding tectonic units ________________ 163 Thrust Aaults. co. 222... B 00000 Llc e Luis 163 Linville Falls fault 163 Stone Mountain fault: ..} 165 Extent: of thrusting =.:1.l 167 Pirection of thrusting _... 168 Ave of thrusting 108 Origin of the Grandfather Mountain window __ 169 Brevard : Lil LEL us 169 Tectonics. .: nae oo a pias tag ann ibe ap a £71 Summary of metamorphic and structural history _- 171 Regional synthesis 171 Principal tectonic events L_. 173 Precambrian -.. 2223.00.12 0 00 AIL 173 Early Cambrian to Early Ordovician Middle Ordovician to Early Silurian ____- 174 Late Devonian and Early Mississippian _- 176 Late Mississippian, Pennsylvanian, and Permin ...}... ouran Uk. 5) 176 Stratigraphic - and - geochronologic record: fen oo Jane ue wan 176 Structural events 177 References ~ Ll ellen d oe t oe con en ang o. i on aol nala to ie tae + ccie he mal aes 183 Index .; 2... n EAE EH e ane a aa te rie be tae nie as aad ao ie a aie aie arta a a ale oe a 189 PLATE 1. 2, 8. n o pop FIGURE 1. 73. p pp go Po 10. 11. 12,18. 14. 15. 16-19. 20. 21. 22. 28. 24. 25. 26. 27. 28. 29. 30. CONTENTS V ILLUSTRATIONS [Plates are in pocket] Geologic map of the Grandfather Mountain window and vicinity. Explanation, fence diagram, and generalized tectonic map for plate 1. Geologic and tectonic map of the Blue Ridge and Valley and Ridge belts in parts of North Carolina, Tennessee, and southern Virginia. Geologic section and simple Bouguer gravity anomaly profile of the western part of the southern Appalachians. Contour diagrams showing orientation of structures, Grandfather Mountain window and vicinity. Maps showing metamorphic history and grade of the Grandfather Mountain window and vicinity. Contour diagrams of poles to bedding in the Grandfather Mountain Formation and the Tablerock thrust sheet. Page Generalized geologic map of the southern Appalachian region showing location of the Grandfather Moun- tain avindow _: :.. c oto 1 ue ast oon Lr OLd pO tO da c n (Ooo e Oa 8 index map of the Grandfather Mountain area .. 2222... . = 20 L cL nell el olden oan an aa anne nena nen nk 4 Sketch of the Linville Gorge from summit of Hawksbill Mountain _____________________________________ 5 Triangular diagram showing classification of silicic plutonic rocks used in this paper __________________--- 9 Photograph of biotite-muscovite gneiss in the Blue Ridge thrust sheet __________________________________ 13 Photomicrographs of biotite-muscovite schist and gneiss southeast of the Grandfather Mountain window ____ 13 Triangular diagrams showing proportions of quartz, plagioclase, muscovite, and biotite in biotite-musco- vite schist and gneiss ______ hie oal e aan it oor i te alle mare uel o hace a i ohn m arial in se no al calc t a ala ade th mas a he aie a in ie moore a 16 Graph of weight percent Al.O; against sum of KO, Na:0, and CaO for analyzed biotite-muscovite schists and gneisges in the Blue Ridge thrust sheet n. 00.0.0: I LGPL GCLC 17 Na.0:K.0 variation diagram for analyzed biotite-muscovite schists and gneisses in the Blue Ridge thrust sheet .} _ = - oen. , _s Gen oe o ets Aino oo et o doa d no a ote oa Lal Les So o bul ui eal aisi pinna ant / 17 Photomicrographs of polymetamorphic mixed unit rocks 22 Photomicrograph of biotite-muscovite-plagioclase porphyroclast quartz schist ____________________________ 28 Triangular diagrams showing proportions of quartz, plagioclase, muscovite, and biotite in gneiss in the Blue Ridge thrust sheet: 12. ;Nonmiematitic layered _. _) _.. 2 2 2 2a ae ce ooo o Lona Hes n Te d e ane Eee oue mand aide sana me 24 18, Migmatitic layered gneiss _ -__ OL _c ru medic ouilel Out Slaco aed de 25 Sketch showing tectonic lenses of fine-grained biotite gneiss in biotite schist and biotite amphibole gneiss __ 27 Photograph of conspicuously layered Cranberry Gneiss 28 Photomicrographs of- 16.. Typical Cranberry OGnelgs _. : s s.. 2 L _.... s Bus Bo u 0 Seu adn |L iro LLL il A anew. coe e cau nne ae este 30 17; Mess Cranberry Gnelss ~ 222 . sur 2 olo o n LL o Lol eo pac, H c En le braes a' ae iin o a lal e armen anes 32 18. Blastomylonite and phyllonite in Cranberry in''." ~ oa taas ta oui ~. 33 19. Slightly sheared Cranberry Gneiss recrystallized at medium grade _____________________________ 34 Photograph of -southern contact of Beech Granite. -__ 37 Sketch. showing northern contact of Beech Granite <..__.._.-L ___}. DO _.O OO: 87 Photomicrographs of Beech Granite -and phyllonite: 38 Triangular diagram showing proportions of quartz, plagioclase, and potassic feldspar in Beech Granite ___ 39 Photomicrograph of negirine-augite granite gmelgs __._._L.__..__.__ .. _.L ___ l JL__OL _L 2 41 Triangular diagram showing proportions of quartz, plagioclase, and potassic feldspar in aegirine-augite granite Sones." .. _ _ . 2.22 o a 2 o o 2 s ein eeg a ao sree en a ec H a eee an oe cl aah a an aed aree aed a eae ie aie ae e all a a aia oa an 41 Photomicrographs of " Bakersvilie Gabbro 10 DOM Of CO Ogo Ct PLL ees 48 Photomicrographs of rocks of the Chilhowee Group in slices along faults _____________________________---- 46 Triangular diagrams showing proportions of quartz feldspar, and mica in rocks of the Chilhowee Group in tectonic: slices 'in the -Blue Ridge thrust lieu} "o O0 _ ien D 47 Photographs of sheared pegmatite in the Blue Ridge thrust sheet 50 Photomicrographs of granodiorite and pegmatite in the Blue Ridge thrust sheet ____________________-_---- 51 VI FIGURE 31. 32,33. 84. 35. 36. 37. 38. 39. 40. 41. 42. 48. 44. 45. 46. 47. 48. 49. 50. 51. 52. 58. 54. 55. 56. 57. 58. 59. 60. 61. 62-66. 67. 68. 69. CONTENTS Page Triangular diagram showing proportions of quartz, plagioclase, and potassic feldspar in light-colored eranodiorite and pegmatite -_... 2... 2. 22 22 200. 2 2 LC IA Ls Ecce Le te elie cs ot Tine se ln hulud ad haine To te sii td n an o tin e an taa t as an ad e m fa he a 52 Maps of the Grandfather Mountain area showing- $2. Generalized trends of cleavage and foliation -L 54 88. 'Generalized trends of mineral lineation 56 Photographs of Wilson Creek O@neigs. ... _.... . 2... _ 22 L cL Lull lle d n n n cel He + a uae Hh ie ce sci Ie te eed an an he ui a alati Hean ar ae an ace 61 Photomicrographs of Wilson Creel G@neigs . _... : _ _.. -:- 22. 2 o. 2 2 22 202 20.00 2 22 Le L ce l l te wee lhe no h an Hh ie ae mean mn Be e a i aa 62 Triangular diagrams showing proportions of quartz, plagioclase, and potassic feldspar in Wilson Creek |. 3.0.2 2 ool + o a a a a ree ae ol ae an areal Ha e raed ae ae ws ae ao ne sl cale ne ut n ar ane aioe ere e a h ple He he Ur an i ce e ta te n he te Hee alee tehe th nin ies an be s on in aired 64 Photomicrographs of pegmatite in Wilson Creek Gneiss 65 Photographs of Blowing Rock Gneiss o aie einen ane n nee ule ae uel a are 67 Photomicrographs. of- Blowing Rock GnelSe .. .... _ .. _._ _. .. .._ _ 22.2 LoL ll Lol Ll £ ola e doan b oo no aree tie ia 1p ha mn fe an mch n an Bea gece cs 69 Triangular diagram showing proportions of quartz, plagioclase, and potassic feldspar in Blowing Rock Gneiss, light-colored intrusive rocks, and Inclusions 70 Photomicrograph of Brown Mountain Granite __________ ama ee n a Anan ina ae a eae a nie an aie nees 71 Triangular diagram showing proportions of quartz, plagioclase, and potassic feldspar (including perth- Ste) in- Brown Mountain -...... .._. LLL LZ Cu alesse aes -ma alee nle 72 Photographs of arkose in the Grandfather Mountain Formation ______________________________________ 74 Photomicrographs of arkose in the Grandfather Mountain Formation __ ______________________________- 7T Triangular diagrams showing proportions of quartz, potassic feldspar, plagioclase, and mica in arkose of the Grandfather Mountain Formation __________________- sees apc Agne une nec 79 Na.0 :K.0 variation diagram for sedimentary rocks of the Grandfather Mountain Formation and the Chil- fhowee Group - ._.. 2 2 2 2 ele ee Lone a calc c ae Ta m an ante tn he a han le au he hfa an ma oh e n on nfo e me me an me one te cde ae as ani ad he ae e B Ha n Bi ashe ay a ene an b ce mre ae 82 Plot of Al:0; against CaO +Na:.0 + K.0 in sedimentary rocks of the Grandfather Mountain Formation and the Chilhowes HHrOUD: - -. - >= ence o on o an c 2 e ie uo 2) 2) a se ao c nic hale Ls on u aa an at m e e arin hi hn te me ane e s ue in l es had ae at st st H te a munte fe ar cee an n ae e an 82 Photographs of siltstone in the Grandfather Mountain Formation _____________________________________ 83 Photomicrographs of siltstone in the Grandfather Mountain Formation _______________________________-- 85 Triangular diagrams showing proportions of quartz, plagioclase, potassic feldspar, and combined mica and chlorite in siltstones of the Grandfather Mountain Formation __________________________ccocccc____- 86 Photographs of felsic volcanic rOCk® :- -.. .. - ..... .. _ .. _ _. saree onl L o ollo oll ae on E mel bae bl oo ol ha o ul Garen ce ba T cn wie areas une Gn t in us an ie ie alee 88 Photomicrographs of felsic volcanic rocks ..... . -__... ._... ___.. .~. .oo dul anns ans an as is 91 Photograph of amyodaloidal «greenstone. :..... . 4 « ___... c L. _ ___ L4 _L : Oe Hiren lo a enne ue b mine oh He i calne nr mere tn ha e iy m he os I 92 Photomicrograph of:- mafic. porphyry. ... . . _ . . .... _. _. _.... _o ._ L L_ £2 aG o wheal aed te he a ioe te Bos Hoon in Serie Slee a ae arre oe He ve oe 92 Photomicrograph of amygdaloid in Monteeuma Member of Grandfather Mountain Formation ____-______-_-- 98 Diagrammatic stratigraphic section of the Grandfather Mountain Formation _______________________-_-- 95 Photomicrographs of Linville Mctadiabase c 97 Photomicrographs of rocks of the: Chilhowee Group male. 100 Triangular diagrams showing proportions of quartz, feldspar, and combined mica and chlorite in rocks of the Chilhowee Group in the Tablerock thrust sheet and in the lowest arkose of the Grandfather Moun- Fain. FOFMACION . --> _ .. 2 2 2, 2, 2 2 e SE 2 coe cred ue te e cl e net ea ie mere Ul me te me al r md he ae said a bal Ba on e hr es as in e n ad Alle ne i hind d hu sg wt Be oa e ha e a he c a he cn aa ae 101 Photograph of contact between Shady Dolomite and quartzite of the Chilhowee Group _______________---- 103 Contour diagrams showing orientation of minor folds in basement rocks of the Grandfather Mountain :" 22 eH L Sce one a s aaa a a a Le o ie se hase had inh ine 1 hee o on me h ae ged e a hr wt at on t ae cs ma m n uh Ae t an tn Te i h mae Soon ts u hae te in he mn ie e 107 Diagrammatic skitches of- 62. Typical early folds in the Grandfather Mountain Formation __________________________________- 109 63. Late slip fold in siltstone of the Grandfather Mountain Formation ____________________________- 109 64. Possible mode of development of antiform in Shady Dolomite near Woodlawn. __________________ 111 65. Tightly appressed and isoclinal folds in rocks of the Chilhowee Group in the Tablerock thrust sheet __ 112 66. Open folds in rocks of the Chilhowee Group in the Tablerock thrust sheet _____________________- 113 Photograph showing mineral lineation and axes of open folds in quartzite ____________________________ 114 Photograph of interlayered fine-grained biotite and hornblende-biotite gneiss containing small concordant pods of biotite: pegmatite -.: - -. .... . c- ool o che mono e n aree mle nan an be an cn Be we aan ma oad amc a hs n a ste hn he at . l e me as at and l wi ae un ant he e r he ace 117 Photomicrographs of MERE, -.- 2... a 2. 1. o Kn ll. oll d cl hoe cl o coe m elena een a hae i ce oe an pe ue ae ae tn e cf apnea at anc mag sted bice ie hehe n m 118 FIGURE 70. 714. 72. 78. 74. 75. 76. Ti+ 78. 19. 80. 81. 82. 88. 84. 85. 86. 87. 88. 89. 90. 91. 92. 98. 94. 95. 96. 97. 98. 99. 100. 101. 102. CONTENTS VII Page Triangular diagrams showing proportions of quartz, plagioclase, muscovite, and biotite in biotite gneisses of Inner Piedmont : belt:: -.... 2 s.. .s nek a e e een mae ee L a aad o e te w ae wa e ad mams onine f ehe ce hee cp he te a m mnt r ad bi a te l s nt n e u an t e bet 121 Triangular diagram showing proportions of quartz, plagioclase, and potassic feldspar in biotite gneisses of Inner Piedmont belt =. _._-___.. __ _s. e_ L 20 _L_L MILLI O LLM Caen avec neu auks 122 Photomicrographs of "mica schist} _._ -_- L_ LLA eL _L SO dne, 1283 Triangular diagrams showing proportions of quartz, plagioclase, muscovite, and biotite plus chlorite in mica schist of the Inner. Piedmont Delt © .:- 2.2 .._ L. .... _ _ CL Cu cnl ooo onl n eee e EEL we ale t LL 6 He Hae e oo a mace ot aber nent ue pan 124 Triangular diagrams showing modal composition of hornblendic rocks of Inner Piedmont belt ________--- 127 Photomicrographs of sillimanite sohist |.. _. _ . 2 = c- ss 2002.0 22 cl c. 22 oo o oe oE oot eac n u oc SE Led i an ee u ad he an re he oe as tane ie manet 128 Silica variation diagrams comparing layered gneisses, schists, and amphibolites of the Inner Piedmont belt with volcanic rocks of the Aleutian Islands, rocks of the Carolina slate belt, and graywackes ______ 133 Na.0 :K.0 variation diagram comparing layered gneisses, schists, and amphibolites of the Inner Piedmont belt with volcanic rocks of the Aleutian Islands and rocks of the Carolina slate belt ______________-- 184 Variation diagram showing molecular proportions of Al:0;, CaO, and total alkalies in layered gneisses, schists, and amphibolites of the Inner Piedmont belt compared with subaerial volcanic rocks of the Aleutian Islands, rocks of the Carolina slate belt, and graywackes ________________________________ 134 Schematic ACF diagram for layered rocks of the Inner Piedmont belt containing excess SiO, _________- 137 Photomicrographs of Henderson Gneiss ____________- cle 186 Triangular diagrams showing proportions of quartz, plagioclase, biotite, and muscovite in Henderson Gneiss and biotite gneisses transitional into Henderson Gneiss _____________________________________ 189 Triangular diagram showing proportions of quartz, plagioclase, and potassic feldspar in Henderson Gneiss and biotite gneisses transitional into Henderson Gneiss _____________________________________ 140 Photographs of migmatite -and granitic rocks - cu - c_-.. 2. 220.2 Lu Loc t a LL 0. c. e LLL SJ ie me in inl ne hale Sele cl ne Wne Salih te cre ube oa e hn ae 141 Histograms showing percentage of granitic rocks containing plagioclase of various compositions _________ 142 Triangular diagrams showing proportions of quartz, plagioclase, biotite, and muscovite in migmatitic biotite gneisses and granitic rocks related to the Toluca Quartz Monzonite ________________________- 148 Triangular diagrams showing proportions of quartz, plagioclase, and potassic feldspar in migmatitic biotite gneisses and granitic Tools .. _ 2 . . s s 2 sen ae nen e eae ao eo ee ae a m ae ee ove bo ie ss oe hs he ae an tein oo he make a re nm aran ce i ina aes 145 SiO; variation diagrams comparing granitic rocks and migmatites of the Inner Piedmont belt with layered biotite pneisges : 2. -. .. 222 22. . o 2 oll £200. a un ie al ar ae nein an c hn ta a Toe as Hali ae ue ne g mh ap os ay is he ae e e sl he e e an a he e a ae d an in t edn n he haes eae weer 148 Triangular diagram showing molecular proportions of Al;0O;, CaO, and total alkalies in granitic rocks and Mig MAUI . . 22 £22202 23 2 c SNe on ce hhe ee el aria ien at cad a loins bl ea up ae tn as ot Be athe Be te ae o Pe a> t me al ah Bl on an cain as s pen in ps in ag a ha aa te he a ce ue pace citri teric a 148 Na.0:K.0 variation diagram comparing granitic rocks and migmatites of the Inner Piedmont belt with layered biotite phicleses _. _ -.-: il cn oul A won he Hele cnc cl an e hhe ea a e ae he s hie be oss thee he ai Bl o t ont he hhe in haine ae ta is ae 148 Photomicrograph -of quartz monzonite | blastomylonife 149 Sketches of folds in Inner Piedmont rocks nA mane inin Burien ees ain a Shae ks 151 Sketches showing field relations: of granitic ... n on «i d am 153 Photograph of fold in layered biotite and hornblende-biotite gneiss cut by dike of light-colored granitic rock containing blocky inclusions of layered gneiss ___ _ n colors, 153 Photomicrographs of blastomylonite _._. _... _ _ - _ .._. Lou cel ltl bull L l UL L ceed ciel aie a amare ame ie be- oa arene be i ules ha ie aas 154 typical phyllonitic SCNISE - - - _. . -_ _._ _s c onl c le Ene ob LK LL el oin ulin ih ie are nbn lake ram ank a 155 Photograph showing crinkles with steep axial planes and northeast-trending axes in biotite-muscovite schist and gneiss of the Blue Ridge thrust sheet southeast of the Grandfather Mountain window ____-- 158 Sketches showing folds in polymetamorphic rocks of the Inner Piedmont belt near the Brevard fault zone __ 159 Graphs showing ratios of number of measurements of fold and crenulation axes and mineral lineation to number of measurements of layering and foliation in rocks of the Brevard fault zone and adjacent parts of the Grandfather Mountain window, Blue Ridge thrust sheet, and Inner Piedmont belt ___--_-_ 161 Photograph of dike of unmetamorphosed diabase of Late Triassic(?) age in sheared and retrogressively metamorphosed gneiss of the Inner Piedmont belt in the Brevard fault zone ________________________ 162 Photomicrograph :of diabase % .._ 2 .. 2 - .. o o GL n cnn e e ce eee ao oo a uh ne ne ue an ad me mene ct e l ae h es ae nt cire he o nee hn ed us inet he tr bo nan oe a an sn c arate a 162 Photograph of Linville Falls fault at its type locality: _... -= cL L o_ eeu oon nis inane a al meee 163 Photomicrograph of blastomylonite from Linville Falls fault at Linville Falls ___________________________ 164 VIII 103. 104. 105. 106. TABLE 1. $ fi /p f h o p A 10. 11, 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 28. 24. 25. 26. 27. 28. 29. 80. 81. CONTENTS Page Photograph of Linville Falls fault in roadcut along North Carolina Highway 194 about one-half mile east of: Bowers HGAD 222 2 22 nl o 2 20 22 .. L n L eed 2 ole can a in reid mee an nlra ie in cie tail a he arie men a ae Lone mie and Aare aa ae 164 Maps and sections of the northwest corner of the Grandfather Mountain area ___________________________ 166 Photomicrograph of breccia from Stone Mountain fault zone 2... lloc. c lL. fI. 167 Diagrammatic sketch illustrating possible relation between strike-slip faulting and thrusting _____________ 170 TABLES Page Minor-element analyses of rocks from the Grandfather Mountain area _________________________________-- 8 Classification of cataclastic rocks.... >. 2... _. _L ole L puce out oe neu onic eerie esa 9 Chemical analyses, modes, and norms of biotite-muscovite schist and gneiss in the Blue Ridge thrust sheet 15 Chemical analyses, modes, and norms of amphibolite in the Blue Ridge thrust sheet ____________________-_- 18 Chemical analysis, mode, and norm of granofels from the Blue Ridge thrust sheet _____________________-- 19 Chemical analyses, modes, and norms of layered gneiss in the Blue Ridge thrust sheet southeast of the Grandfather Mountain window _...... . s 2 cout c root Loie PBZ Bute E Pbl + a cnl ea ad ie arada einer aah 26 Average modes of various rock types in the Cranberry Gneiss ________________________________________ 34 Chemical analyses, modes, and norms of- Beech Granite :: 40 Chemical analysis, mode, and norm of aegirine-augite granite gneiss 42 Chemicalsanalysiz-of quartz porphyry... ..__%. -. 2 c L coil _ 1 Loe LL andar er anm nel eam 45 Occurrences of ultramafic rocks in the Blue Ridge thrust sheet ____________s__________________________L 48 Ages 'of minerals from-pegmatites of the Spruce Pine district 52 Chemical analyses of Blowing Rock Gneiss and phyllonite _L__.--L__________L___. _. __. _L MOO 70 Chemical analysis, mode, and norm of Brown Mountain Granite - _ 72 Chemical analyses, modes, and norms of arkose and related rocks of the Grandfather Mountain Formation 80 Chemical analyses and norms of siltstone and related rocks of the Grandfather Mountain Formation ____ 87 Chemical analyses and norms of Linville Metadiabase and volcanic rocks of the Grandfather Mountain Formations culos etl an ae Aten a SUL, Hae a anne bic as aes Lo ao sice is mim o an a doo ad in wie ia L co ai nano a, a ce den annal a ned e a a 90 Chemical analyses, modes, and norms of rocks of the Chilhowee Group in the Tablerock thrust sheet _____ 102 Stable-mineral assemblages in biotite gneiss of high metamorphic grade in the Inner Piedmont belt ________- 119 Chemical analyses, modes, and norms of biotitegneiss of the Inner Piedmont belt ______________________-- 120 Chemical analyses, modes, and norms of schists in the Inner Piedmont belt ____________________________-- 125 Stable-mineral assemblages in hornblende gneisses and amphibolites in the Inner Piedmont belt ______-- --- 126 Chemical analyses, modes, and norms of biotite-hornblende gneiss and amphibolite in the Inner Piedmont belt 126 Chemical analysis 'and -modes of sillimanite schist......__._.._.._ c l LLU 129 Stable-mineral assemblages in quartzite, quartz schist, cale-silicate rocks, and anthophyllite gneiss in the inner. Piedmont belt? 2205.10. 22 t e ono on oe cl oon fee i 2, a a an al a Aile a Ne an he al ee ce on an he ae me l be hae pean in an an an a an ar a le a i tren mae an chace ie 130 Chemical analyses, modes, and norms of Henderson Gneiss and biotite gneiss transitional into Henderson Mess 32> 2. £ Le £ sates, sole weil a ne ce oe ba cd mille Te bo an t wae tee ca en lee ot wet a md in t c t e caa a n ari in n he in he aie ien an ie ie e Atac aran roe eaid ie ta a aie aie alie aed 138 Chemical analyses, modes, and norms of granitic rocks probably related to the Toluca Quartz Monzonite __ 144 Chemical analyses, modes, and norms of migmatitic biotite gneisses _____________________________cc_____- 146 Chemical analysis, mode, and norm of typical phyllonitie mica sehist ________________________________ 156 Structural and metamorphic history of the Grandfather Mountain window and vicinity ______________---- 172 Location of typical exposures : Units: _... ... .. _._. .. __... .. O2 . nll a ol aon a me as o o are ce ie be hae e ie re oe is e an Ba ae 'as 180 GEOLOGY OF THE GRANDFATHER MOUNTAIN WINDOW AND VICINITY, NORTH CAROLINA AND TENNESSEE By BRUCE BRYANT and JOHN C. REED, JR. ABSTRACT The Blue Ridge belt in northwestern North Carolina and northeastern Tennessee is composed chiefly of 1,000-million to 1,100-million-year-old metamorphic and plutonic rocks that have been thrust many miles northwestward across unmeta- morphosed Cambrian(?) and Cambrian sedimentary rocks of the Unaka belt. The Blue Ridge thrust sheet is rooted on the southeast along the Brevard zone, a zone of strike-slip fault- ing along which metamorphic and plutonic rocks of the Inner Piedmont belt are juxtaposed with rocks of the Blue Ridge. Near the southeastern edge of the Blue Ridge belt, the Blue Ridge thrust sheet is breached by erosion, and the rocks beneath are exposed in the Grandfather Mountain window, which is 45 miles long and as much as 20 miles wide; it is the only major window so far recognized in the Blue Ridge belt. The rocks exposed within it include 1,000-million-year to 1,100-million-year-old plutonic basement rocks, sedimentary and volcanic rocks of late Precambrian age, and an alloch- thonous tectonic slice of Lower Cambrian(?) and Cambrian sedimentary rocks identified with the Chilhowee Group and Shady Dolomite in the Unaka belt 20 to 30 miles to the northwest. The Blue Ridge thrust sheet surrounding the Grandfather Mountain window consists largely of schist, gneiss, and am- phibolite derived by metamorphism of sedimentary and vol- canic rocks 1,000 to 1,100 m.y. ago, and of Cranberry Gneiss, a complex of migmatite and granitic rocks which underlies the metasedimentary and metavolcanic rocks and which probably formed during the same metamorphic episode. The Cranberry Gneiss is intruded by the Beech Granite, by aegirine-augite granite, and by quartz monzonite, all of which were emplaced during a late stage of or after the plutonic metamorphism. Stocks and dikes of Bakersville Gabbro _ of late Precambrian(?) age and small bodies of ultramafic rock, granodiorite, and pegmatite of early or middle Paleozoic age intrude the earlier Precambrian rocks. Although all these rocks may have been metamorphosed about 450 m.y. ago, the principal Paleozoic dynamothermal metamorphism occurred about 350 m.y. ago. At that time new medium-grade miner- als, including staurolite, kyanite, monoclinic pyroxene, epi- dote, and calcic plagioclase, crystallized in the schist, gneiss, and amphibolite. During the late Paleozoic, most of the plu- tonic rocks were partly reconstituted to low-grade blastomy- lonitic and phyllonitic gneisses containing new biotite, albite, sericite, chlorite, actinolite, and epidote, whereas the overlying rocks were largely unaffected. The contact between low- and medium-grade rocks may be a fault. Layering and foliation in rocks of the Blue Ridge thrust sheet dip away from the Grandfather Mountain window on all sides, and broad flexures in these structures plunge away from its northwest and northeast corners. Minor folds in both the low- and medium-grade rocks are of two genera- tions: (1) tight and isoclinal folds having axial planes paral- lel to foliation and layering and axes trending in various directions in the plane of the foliation, and (2) later open folds and crinkles having steep axial planes and northeast- trending axes perpendicular to a well-developed northwest- trending mineral lineation. The early folds, which are possi- bly 350 m.y. old, perhaps formed during an early stage of thrusting and were themselves deformed during continued thrust movement. The later folds formed in a late stage of the thrusting. In the northwest corner of the area, an intermediate sheet of partly metamorphosed Precambrian plutonic rocks occurs between the Blue Ridge thrust sheet and rocks of the Unaka belt in the Mountain City window. The Blue Ridge thrust sheet overrides both the intermediate sheet and the Mountain City window, in which rocks of the Chilhowee Group of Cambrian(?) and Early Cambrian age and the Shady Dolo- mite and Rome Formation of Early Cambrian age are exposed. The basement exposed in the Grandfather Mountain win- dow is composed principally of nonlayered granitic gneiss (Wilson Creek Gneiss) and coarse-grained augen gneiss (Blowing Rock Gneiss), both 1,000 to 1,100 m.y. old. The plutonic basement rocks are stratigraphically overlain by the Grandfather Mountain Formation, a sequence at least 20,000 feet thick of arkose, siltstone, shale, and conglomerate of late Precambrian age. The formation also contains tuffaceous rocks; flows of basalt, quartz latite, and rhyolite; and sills of diabase. The sediments were derived mainly from adjacent plutonic rocks but partly from volcanic rocks similar to those found in the formation and were apparently deposited in a rapidly subsiding basin. Diabase and felsic porphyry intru- sives in the basement rocks are probably related to the volcanic rocks in the Grandfather Mountain Formation. The main outcrop belt of the Grandfather Mountain For- mation lies on the southeast limb of a large synclinorium overturned to the northwest. Medium- and small-scale folds are overturned to the northwest or west; they are isoclinal in the southeastern part of the belt and are more open in the northwestern part. In most of the outcrop belt, axes of minor folds are subhorizontal, and their axial planes strike north- east parallel to the trend of lithologic units. In the northern part of the outcrop belt, however, axes of minor folds plunge 1 2 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE northeast and axial planes strike north or northwest at a large angle to the trends of the lithologic units. These folds are evidently younger than, and superimposed on, the earlier major structure. Pervasive cleavage, parallel to the axial planes of the minor folds in the upper Precambrian rocks, is parallel to cata- clastic foliation in the underlying basement rocks. Strongly developed cataclastic lineation plunges southeastward on the cleavage and foliation planes. It is generally normal to the axes of the minor folds and is evidently in the a direction. The rocks in the Grandfather Mountain window are of the same low metamorphic grade as the retrogressively metamor- phosed rocks in adjacent parts of the Blue Ridge thrust sheet. Progressive metamorphism of the upper Precambrian rocks was concurrent with retrogressive metamorphism of the basement rocks. Typical metamorphic minerals in both groups of rocks are albite, microcline, epidote, actinolite, chlorite, and iron-rich muscovite. Basement rocks were con- verted to blastomylonitic and phyllonitic gneiss, phyllonite, and blastomylonite. Metamorphism of the rocks in the Grandfather Mountain window apparently reached a thermal climax about 350 m.y. ago. The large synclinorium in the upper Precambrian rocks formed prior to that time, but most of the minor folds and the cleavage, cataclastic foliation, and lineation formed dur- ing the metamorphic episode 350 m.y. ago. The Tablerock thrust sheet is a tectonic slice between the Blue Ridge thrust sheet and autochthonous rocks in the southwestern part of the Grandfather Mountain window. It is composed of Shady Dolomite of Early Cambrian age and several thousand feet of quartizite, arkosic quartzite, and phyllite of the underlying Chilhowee Group of Early Cambrian(?) and Early Cambrian age. These rocks are met- amorphosed to the same grade as the. rest of the window rocks, but their bedding and cleavage are approximately parallel with the cataclastic foliation in the Blue Ridge thrust sheet and are strongly discordant with structures in the underlying autochthonous rocks. Tight and isoclinal folds in rocks of the Tablerock thrust sheet have axial planes parallel with bedding and cleavage, and diversely oriented axes; superimposed on them are open folds having south- east-dipping axial planes and gently southwest-plunging axes which are approximately perpendicular to a well-developed mineral lineation. The geometry of these structures resembles that of the structures in the overlying Blue Ridge thrust sheet; both groups of structures probably formed and were rotated into their present orientation during thrusting. At the southwest end of the window, the Tablerock thrust sheet is overturned, broken by faults, and overridden by sheets of basement rock. Layered gneiss, mica and sillimanite schist, amphibolite, and other associated metasedimentary and metavolceanic rocks in the Inner Piedmont belt are probably of late Precambrian or early Paleozoic age; they were metamorphosed in the early or middle Paleozoic. Where they have not been affected by later metamorphism related to the Brevard fault zone, they contain the apparently stable mineral pairs sillimanite- muscovite and epidote-diopside. Layering and foliation strike north and dip gently to moderately east. The axial planes of minor folds dip gently or moderately north or northeast, and axes plunge gently east, parallel to mineral lineation. Migmatite and granitic rocks in the Inner Piedmont were apparently formed by recrystallization and partial anatexis of the layered rocks of the Inner Piedmont during the climax of high-grade regional metamorphism. The Brevard fault zone is a narrow zone of strongly sheared and retrogressively metamorphosed rocks, including porphyroclastic blastomylonite gneiss, blastomylonite, and phyllonitic muscovite-paragonite schist. Most rocks in the zone were probably derived from the flanking rocks, but the paragonite-bearing schist is apparently an exotic tectonic slice. Foliation in the zone is steeply dipping or vertical. Rocks of both the Inner Piedmont and Blue Ridge belts near the fault zone show well-developed polymetamorphic textures characterized by porphyroclasts of potassic feldspar, plagioclase, or muscovite in a groundmass of recrystallized biotite, garnet, epidote, and oligoclase-andesine. Locally, adja- cent to the fault zone, the rocks are retrogressively metamor- phosed in the chlorite zone. A belt as much as 5 miles wide along the Brevard fault zone shows pervasive structural and metamorphic effects re- lated to the faulting. The Piedmont rocks are overprinted by a subhorizontal northeast-trending cataclastic mineral linea- tion and a northeast-trending and southeast-dipping cleavage. Minor folds in the Piedmont rocks in this belt are subiso- clinal, and their axes trend northeast, parallel to the mineral lineation. Folds become tighter and axial planes and cleavage become steeper as the Brevard fault zone is approached. Southeast-plunging mineral lineation in the Blue Ridge and Tablerock thrust sheets swings abruptly clockwise adja- cent to the Brevard and becomes parallel to and indistin- guishable from the subhorizontal northeast-trending cataclas- tic lineation along the southeast side of the fault zone. This observation suggests that strike-slip faulting along the Bre- vard was contemporaneous with and mechanically related to northwest movement of the Blue Ridge and Tablerock thrust sheets. A dike of unmetamorphosed Upper Triassic(?) diabase cuts the rocks of the Inner Piedmont, the Brevard fault zone, the Blue Ridge thrust sheet, and the Grandfather Mountain window. The rocks of the Blue Ridge thrust sheet moved northwest- ward at least 35 miles over the Grandfather Mountain win- dow after the close of the metamorphism 350 m.y. ago (Late Devonian) and before Late Triassic(?) time. Left-lateral strike-slip movement along the Brevard was concurrent with, but may have lasted somehat longer than, thrusting. Lateral displacement was greater than 1835 miles. INTRODUCTION LOCATION AND PHYSICAL GEOGRAPHY The Grandfather Mountain window in northwest- ern North Carolina exposes rocks beneath the Blue Ridge thrust sheet near the southeastern edge of the Blue Ridge belt (fig. 1). The window, which is 45 miles long and 20 miles wide, and adjacent areas included in this study comprise about 1,000 square miles in North Carolina and adjacent parts of north- eastern Tennessee. The northwestern part of the area lies mainly in the Blue Ridge upland. The south- eastern part includes the deeply dissected southeast- ern slopes of the Blue Ridge upland and extends into the Morganton basin, a northeastward extension of the Piedmont plateau partly enclosed by the Brushy INTRODUCTION 8 EXPLANATION yam 55 Metamorphosed Paleozoic rocks in the Carolina slate belt and the Kings Mountain belt Tertiary and (fitaceous Coastal Plain deposits Triassic rocks { -: _ Metamorphosed Paleozoic and Precambrian(?) rocks A Precambrian rocks Nonmetamorphosed Paleozoic rocks ___ Contact Fault Thrust fault Sawteeth on upper plate a rr toe r Anticline Showing trace of axial plane + mroe. in ioe is Syncline Showing trace of axial plane. Dashed where approximately located 88° 100 MILES 1 1 \ FIGURE 1.-Generalized geologic map of the southern Appalachian region showing the location of the Grandfather Moun- tain window. Generalized from the tectonic map of the United States (U.S. Geol. Survey and Am. Assoc. Petroleum Geologists, 1961). Mountains on the northeast and by the South and Hickorynut Mountains on the south (fig. 2). Grand- father Mountain, from which the area takes its name, is the highest point (alt 5,939 feet) on the crest of the Blue Ridge. The area includes all the Linville, Linville Falls!, Blowing Rock, and Lenoir 15-minute guadrangles; adjacent parts of the Marion 15-minute quadrangle, the Little Switzerland, and Marion East 714-minute quadrangles; parts of the Elk Mills and Sherwood 7/»-minute quadrangles; and parts of the Maple Spring and Grandin 7/-minute quadrangles. Sev- en-and-a-half-minute topographic maps covering all Called Table Rock quadrangle at an early stage in its preparation and referred to by that name in some earlier reports. Not to be confused with the Linville Falls 7%-minute quadrangle, which is the northwestern quarter of the Linville Falls 15-minute quadrangle and which is not mentioned below. the Linville, Blowing Rock, Linville Falls, and Len- oir 15-minute quadrangles are also available. Several main highways (fig. 2) and a network of secondary roads and unmaintained roads and jeep trails make most of the area readily accessible. Even on the sparsely settled southeast flank of the Blue Ridge, it is difficult to find a point on a map more than 2 miles from a road. Hilly and mountainous terrain of the Blue Ridge upland in the northwestern part of the area ranges from 2,500 to nearly 6,000 feet in altitude. The up- land is drained by the South Fork of the New River, which flows northward into the Ohio River; by the Watauga, Elk, and North Toe Rivers, which flow westward into the Tennessee River; and by the Lin- ville River, which flows southward to the Catawba River. Local relief ranges from a few hundred to 2,000 feet. Slopes are locally gentle, but large cliffs GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE 8215 82°00" 45 81°30 T s I 1 \ ELK MILLS 52A - \\ / <5°% ~ s \\ Elizabethton | \, , gEao G A $ C atauga ACS é C v R > K das ~ sHERWOOD s \ fan -, /1 421 New® /\_/L,’ * S aA ~. A ¥ B5 ~s ) $y SN YJ a mm.. A § A f /x \ / 36°15'|- LINVILLE LeA \ (.% o xs \4‘/,,e) -_ t m /\ > 0 X cP A& / do“ s -# _._. .. watauga co "6 w y4 a8 I Blow ing, CALDWELL co - eN A TEN y j R. 0 fy Cc o 4 s ock i {Ob}, sl LQ NORTH C? \_ Newland g? MIA \\ € \ «© Linvill \ \:\ 3 SML e \\ 7 $4 2 § O I ‘ | \ ¢ " / Q‘ \\1\J % ( s > < 3% / | > 36°00 (- & C d INVIL(I_/E FALL LENOIR LC * fl Pra / A Cr \ 2 E -/ R%))\ Ja / s &. * \ MNQEW» \ € 1 / a < C <0 V\ \ / View)” y. 3% I o y I% \ 13 & § UITILE ! > \ / § s SWITZERLAND 21 i § j / lel - % as 32 ; c- {Laces Ev] Rhodhiss \\ Lake 35°45" X Hickory/ O ha *% NTAI /aP 74 \ K | $0 / | & 5 o 5 10 15 MILES (teste fi \ | 4 FIGURE 2.-Index map of the Grandfather Mountain area showing physiographic subdivisions, principal geographic features, political boundaries, and topographic quadrangles. Area of plate 1 outlined. form the crests of some of the peaks, such as Grand- father Mountain. Outerops of fresh rock are abun- dant, especially along the valleys of major streams. In some areas, however, the rock is deeply weathered and covered with colluvium. The Blue Ridge upland has a moist temperate cli- mate. Recorded annual precipitation ranges from 50 to 55 inches, but precipitation is probably greater locally. Mean July temperature in the valleys is 65° to 70°F; maximum summer temperatures seldom INTRODUCTION 5 exceed 85°, and nights are cool. Minimum winter temperatures are as low as -10°F in the valleys and -20°F on the highest peaks, but periods of extreme cold are generally interspersed with milder weather, and snow cover seldom remains all winter. The mean temperature in January in the valleys is 34° to 36°F (figures from U.S. Weather Bureau, 1962). North- ern hardwood forest covers most of the Blue Ridge upland, but fir and spruce grow locally on the higher peaks, particularly on Grandfather Mountain. Rho- dodendron grows luxuriantly in shady places; on sunnier slopes, laurel commonly forms the under- story. Both laurel and rhododendron grow best on soils derived from siliceous rocks. Laurel is com- monly called ivy by the local inhabitants, and rho- dodendron is referred to as laurel. The principal towns of the Blue Ridge upland in the Grandfather Mountain area are Boone (site of Appalachian State Teachers College and county seat of Watauga County), Blowing Rock, Linville, and Newland (county seat of Avery County). The main sources of income are tourism and farming. Tobacco (mountain burley), cabbage, beans, and beef cattle are some of the chief products. Most farms are small, and many of those in the more remote hollows have been abandoned. There are a few small factories in the towns, but many people commute to work in the larger towns of the nearby Piedmont. The Blue Ridge front constitutes the southeastern slope of the Blue Ridge upland. It is a steep, deeply dissected area about 10 miles wide which ranges in Tablerock Mountain 1 The Chimneys altitude from 3,500 to nearly 6,000 feet along the southeastern edge of the upland to 1,100 to 1,300 feet at the northwestern margin of the Piedmont. Aver- age local relief is 1,000 to 1,500 feet, and streams draining the area have steep gradients. The headwa- ters of the Yadkin River drain the northeastern part of the belt, and tributaries of the Catawba River drain the southwestern part. The Linville River cuts through this belt in a spectacular gorge about 1,500 feet deep flanked by cliffs of quartzite (fig. 3). Out- crops are excellent along the streams, but deeply weathered rock is found on the ridge crests and on some of the steep slopes. Tha Blue Ridge front is a zone of transition be- tween the cooler climate and northern hardwood for- est of 'he Blue Ridge upland and the warmer climate and southern hardwood forest of the Morganton basin. It is protected from the strong northwesterly winter winds that bring snow flurries on the Blue Ridge upland, but it receives the full effect of storms coming from the south and southeast. An understory of laurel is especially well developed on the quartz- ites, and rhododendron is found in shady places. Pine is abundant. The Blue Ridge front was once thickly settled. Many farms in the more remote areas are now aban- doned, but some subsistence farming is still done along bottoms of the major valleys. A large part of the front is in the Pisgah National Forest, and consid- erable logging is done. The Linville Gorge Wild Area, which includes the least accessible and most Lake James ; Pinnacle FigurE 3.-View of the Linville Gorge from the summit of Hawksbill Mountain. Morganton basin in the background and South Mountains on the horizon. Rocks in the foreground are arkose of the Grandfather Mountain Formation. Prominent cliffs on Tablerock Mountain and Shortof Mountain are quartzite of the Chilhowee Group. 6 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE scenic part of the valley of the Linville River, is one of the few designated wild areas in the eastern United States. The surface of the Piedmont plateau in the Mor- ganton basin lies at altitudes of 1,100 to 1,300 feet, more than 2,000 feet lower than most of the Blue Ridge upland. Small hills and ridges stand as monad- nocks a few hundred feet above the plateau surface, and streams are incised as much as 200 feet into it, leaving a series of concordant flat-topped interfluves. In the northeastern part of the Lenoir quadrangle the southwest end of the Brushy Mountains stands as hills as much as 800 feet above the Piedmont sur- face. The Morganton basin is drained by the Catawba River and its tributaries. Parts of two large reservoirs are included in the area: Lake James, which is impounded by dams across the Catawba River and the Linville River west of Mor- ganton, and Rhodhiss Lake, which is impounded by a dam across the Catawba northwest of Hickory (fig. 2). Outcrops of fresh rock are scarce in most of the Morganton basin, but some occur along the sides of stream valleys and on the flanks of some hills and ridges that rise above the general level of the basin floor. Numerous exposures of weathered rock and saprolite are found in roadcuts, gullies, and along the shorelines of the large reservoirs. 'The Morganton basin has hot summers and rather mild winters. Snow occasionally falls and tempera- tures as low as 0°F occur during some winters, but 50°F is also a common midwinter temperature. Mean temperatures for July are 76° to 78°F and for Janu- ary, 41° to 48°F. Annual precipitation is about 50 inches (U.S. Weather Bureau, 1962). Pine and southern hardwood forests cover uncultivated parts of the Piedmont. Although laurel and rhododendron are scarce, many vines, including Virginia-creeper, cudzoo, and blackberry, add to the undergrowth. Morganton and Lenoir, the county seats of Burke and Caldwell Counties, respectively, are the largest towns in the Grandfather Mountain area. Furniture and textile manufacturing are the principal indus- tries. Some of the interfluves and many of the al- luvial flats bordering the streams are farmed, but much of the northwestern part of the area is wooded. PREVIOUS GEOLOGIC INVESTIGATIONS The unique nature of the rocks of the Grandfather Mountain area was recognized during the earliest geological reconnaissance in the southern Appalach- ians. Early maps (Maclure, 1817; Kerr 1875) show rocks of "transition" age in the Grandfather Moun- tain area amidst a terrane of gneisses classified as "Primitive" or "Huronian'" in age. In the last decade of the 19th century and the first decade of the 20th, Arthur Keith of the U.S. Geo- logical Survey did much of the pioneer geologic mapping in the crystalline belt of the southern Ap- palachians. He mapped all the Grandfather Moun- tain area and wide expanses in the Blue Ridge to the north, west, and southwest. The Linville and Blowing Rock quadrangles compose the south half of the Cranberry 30-minute quadrangle (Keith, 1903); the Linville Falls and Lenoir quadrangles constitute the north half of the Morganton 30- minute quadrangle (Keith and Sterrett, 1954). The southwestern part of the Grandfather Mountain area is included in the Mount Mitchell 30-minute quadrangle (Keith, 1905). In the Grandfather Mountain area, Keith mapped an extensive area of sedimentary and volcanic rocks of low metamorphic grade to which he assigned a late Precambrian and early Cambrian age. He be- lieved that these rocks occupied a complex syncline bounded on the north and west by faults along which Precambrian plutonic rocks had overridden the younger rocks from three sides. This structural feature was reinterpreted on the geologic map of the United States (Stose and Ljungstedt, 1932) as a window in a major overthrust sheet, now called the Grandfather Mountain window (Stose and Stose, 1944, p. 383). During reconnaissance for the geologic map of the United States, Jonas (1932) recognized that rocks of low metamorphic grade southeast of the Grandfather Mountain window were polymeta- morphic and that they were continuous with similar rocks along strike to the northeast and southwest. She interpreted this belt of retrogressive rocks as marking the sole of a great overthrust continuous with the Martic overthrust of southeastern Penn- sylvania. George and Anna (Jonas) Stose are reported to have mapped the Grandfather Mountain window (Stose and Stose, 1944; 1950; Miser, 1962, p. 146), but their maps were never published and were not available to us during our work. Geologic mapping in connection with mineral resource investigations in the Spruce Pine district west of the Grandfather Mountain area extends into the western parts of the Linville and Linville Falls quadrangles (Parker, 1946; Brobst, 1962). INTRODUCTION T A report on the geology of northeastern Tennessee by King and Ferguson (1960) covered the north- western corner of the Linville quadrangle, and Ham- ilton (in King and Ferguson, 1960, p. 13-27) dis- cussed the basement rocks in that area. Topical studies by Eckelmann and Kulp (1956) and White (1950) have furnished information on various aspects of the geology of the Grandfather Mountain area. Two small areas in the Grandfather Mountain window have been mapped in connection with theses (Bright, 1956; Goedicke, 1950), but the maps were not published. Deposits of the following minerals and commodi- ties in the Grandfather Mountain area have been discussed briefly in various reports: mica (Olson, 1944; Griffitts, 1953; Sterrett, 1907, 1910, 1923), iron (Nitze, 1893; Bayley, 1923; Kline and Ballard, 1948), gold (Nitze and Hanna, 1896; Bryson, 1936; Pardee and Park, 1948), sillimanite (Hash and Van Horn, 1951; Espenshade and Potter, 1960), lime- stone (Conrad, 1960; Loughlin and others, 1921; Watson and Laney, 1906; Lewis, 1893), and quartz (Mertie, 1959). Various short reports and maps of parts of the Grandfather Mountain area have been published as our work progressed. These include Bryant (1962, 1963, 1965, 1966, 1967), Bryant and Reed (1961, 1962, 19702, b), Reed (1964a, b), Reed and Bryant (1964b), Reed, Johnson, Bryant, Bell, and Overstreet (1961), Reed, Bryant, and Hack (1963), and Reed, Bryant, Leopold, and Weiler (1964). The map of the southwestern extension of the window was re- leased to open file (Reed and Bryant, 19642). Economic geology of the Grandfather Mountain area is discussed in a separate short report (Bryant and Reed, 1966). Surficial deposits are not discussed in the present report as we have little to add to our preliminary reports (Bryant 1962; Reed 1964b) and two short topical papers (Reed and others, 1963, 1964). J. T. Hack has studied some aspects of the geomorphology of the area (1966). The first draft of this report was completed in 1964; some references to important more recent work have been added as recently as 1969. PRESENT INVESTIGATION FIELDWORK AND ACKNOWLEDGMENTS Bryant made a reconnaissance of the entire area in August 1956 and began geologic mapping in the Linville quadrangle in September 1956 ; Reed started fieldwork in the Linville Falls quadrangle in March 1957. Both authors spent about 10 weeks in the field each spring and fall in 1957, 1958, and 1959 and 3 to 4 months each fall and winter from 1960 until completion of fieldwork in February 1962. Each author spent approximately 24 months in the field, excluding time spent in reconnaissance in adjacent areas and in field conferences. Of this time, Bryant had field assistants about 16 months, and Reed, 13 months. We wish to acknowledge their help: C. E. Fritts, fall 1956 and spring 1957; F. G. Lesure and C. W. Spencer, fall 1957; William Van Horn and D. V. Lewis, spring 1958; C. E. Harris, Jr., and C. A. Shelby, fall 1958; R. L. Beck and K. E. Billeau, sprin,r 1959; H. W. Sundelius, fall 1961; and D. B. Andvetta, fall 1961 and winter 1962. We wish to thank the inhabitants of the towns in which we stayed for their interest and assistance, and the landowners who, almost without exception, readily gave permission to do geologic mapping on their lands. Mr. Hugh Morton permitted us to use his toll road on Grandfather Mountain. The investigation has been nurtured by discussion with many of our colleagues, especially J. B. Hadley, Warren Hamilton, W. B. Myers, R. A. Laurence, W. C. Overstreet, D. A. Brobst, F. G. Lesure, P. B. King, and J. T. Hack of the U.S. Geological Survey; John Rodgers of Yale University; S. W. Maher and G. D. Swingle of the Tennessee Division of Geology; S. G. Conrad of the North Carolina Department of Conservation and Development (now State Geologist of North Carolina); and H. S. Johnson, Jr., State Geologist of South Carolina. W. M. Cady and D. W. Rankin made helpful sug- gestions concerning presentation of the data and conclusions given below. METHODS OF STUDY Field mapping was done on 1 :48,000-scale enlarge- ments of the 15-minute topographic maps. In moun- tainous parts of the area, most contacts were traced in the field, but in the Piedmont, most contacts were interpolated between scattered exposures plotted on outcrop maps at a scale of 1:24,000. Contacts of surficial deposits were largely sketched from aerial photographs but were locally checked in the field. Approximately 2,100 rocks were studied petro- graphically, and 103 samples were chemically ana- lyzed. Modes of most of the analyzed rocks were determined by standard point counts and are given in the tables of analyses. Some of the analyzed rocks were so fine grained that the modes determined by 8 point counts could not be reconciled with the anal- yses ; these modes were discarded. Semiquantitative spectrographic determinations of trace elements were made on 27 of the analyzed rocks. The trace- GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE TABLE 1.-Minor-element analyses of rocks from the Grandfather Mountain area [Analyses were determined by semiquantitative spectrographic methods by Paul R. Barnett, U.S. Geol. Survey, in 1959 for 1, 2, 7, 12, 13, 14, 15, and 17 and in 1961 for 4, 9, 10, and 19; and by John C. Hamilton, U.S. Geol. Survey, in 1961 for 3, 5, 6, 8, 11, 16, 23, 24, and 26 and in 1962 for 20, 21, 22, 25, and 27. Results are reported in weight percent to the nearest number in the series 1, 0.7, 0.5, 0.3, 0.2, 0.15, 0.1, and so forth, which represent the approximate midpoints of group data element data are given in table 1 for reference, but they are not referred to further in this report. Estimates of the average bulk composition of large heterogeneous rock units were made by counting 50 on a geometric scale. The assigned group for semiquantitative results will include the quantitative value about 30 percent of the time. D, barely detectable; ___, not determined] 1 2 8 4 5 6 7 8 9 10 11 12 13 Table. » 8 9 10 13 14 15 16 Column in given 1. lr eels eate ee cena naaa 2 4 8 1 4 6 1 2 Field No.......... GML-6 GML-5 GML-9 GMB-4 G GML-8 - GML-T _ J-20-1 GMB-2 GML-13 GML-12 GML-3 GML-1 Lab F-2589 _ F-2588 - H-3424 - H-3264 H-3484 H-3423 F-2590 H-3427 H-3263 H-3262 H-3425 F-24188 \ F-2416 0 0.003 0 0 0 0 0.003 0 0 0 0 0 Ba............ 007 .015 .007 15 .005 A1 .07 .8 +07 .07 07 .08 .015 Bée:.:..l..l... 0007 _ .00015 .001 00015 .0002 .0007 .0003 .0007 .0003 _ .0003 _ .00015 0 0 o ty 2 015 .08 0 . 08 .02 0 0 .08 .08 .07 0 0 0 0 0 0 0007 0 0 0003 _ .002 0015 - .0015 0 .008 007 CP:= ss esen d d 0 0007 0 .001 00083 _ .008 007 .007 .002 007 007 00015 .00015 .0005 .0015 .0003 .0003 .00015 .0002 _ .003 .015 0015 - .007 .007 0015 - .0015 _ .005 0015 - .005 . 008 0015 - .005 .003 .003 .002 .003 .008 007 . 03 .005 .015 .01 .005 - 0 .08 .015 015 0 0 U 0003 0 0 0007 0 0 0015 0 0 0 0 0 .008 015 0 .01 .002 0015 - .002 .0015 - .003 _ 0 0015 - .0015 d #09 28 ec .015 O15 /<. 0 .03 . 083 015" csc 002 0 0 «Ni 0 0 0 0 0 .0083 . 003 0015 0 .003 .007 0015 - .0007 0 .0007 _ .005 .0015 d 0016 ~.0015 .0015 .005 0 0 0008 0 .0015 0 0007 - .0008 _ .005 0015 - .0015 - .0015 - .008 0 0 .002 - 0 «001 0 0 0 0 0 0 0 0 iv isu ae . 008 . 0083 .001 .07 .001 .01 .007 .015 . 08 .07 .07 .03 .07 s 0 0 . 08 0 .008 0015 - .002 - 0 .015 .0083 . 08 .08 (W 007 007 015 .003 .02 .005 . 003 .02 .007 007 .005 . 008 .008 0007 .0007 .0015 - .00083 _ .002 0007 _ .0008 _ .001 0007 _ .0007 _ .0005 0003 _ .0003 to . 08 .08 .07 .015 . 05 . 08 .03 02 .015 .08 .02 015 .007 14 15 16 17 18 19 20 21 22 28 24 25 26 27 T ADIGEL a+e - wv ren 17 20 26 27 28 Column in given 8 6 T 8 10 16 2 7 1 8 4 1 8 1 Field No.........: GML-2 W-84 GMB-6 GMB-1 84(L) GMB-5 BR-71A BR-71B 28-1045 15-1061 21-2331 2583(L) B 2028-L Lab: F-2417 F-2419 H-3426 F-2591 1-4057 H-3265 I-4058 I-4059 I-4061 H-8433 H-3482 I-4060 H-3481 I-4062 0 0 0 0 0 0 0 0 0 0 0 0 0 .008 15 15 I 07 A1 .07 A8 15 .01 .07 .07 07 0 0 0003 .0003 .0003 .0002 .0002 0 0002 .0005 .0003 .0002 .0002 0 015 .015 .02 .to 0 0 0 <.02 «<:02 : 0 «02 0 . 008 005 0007 _ .0005 0 0015 .0005 .0007 .0007 0 .001 _ .0007 .0015 .003 0007 .00015 .0003 .00015 .003 .0005 .001 .0007 0 .002 - .0007 .002 0015 002 0007 .0007 .0008 _ .007 _ .001 .002 .001 .0005 .0005 .0007 .001 .008 005 0007 _ .008 9007. . .008 - .002 - .002 .002 .005 .002 - .002 .002 0 .007 .007 .01 .007 .005 0 905 - ~.01 005 ~0 .005 0 0 001 -- 0 0 0 0 0 0 0 0 0 0 0 0015 002 0015 - .003 .008 - 0 0 002 .002 - .015 <0 .0015 0 0 .015 007 .015 007 _- 0 0 0 .015 0 0 0 0 .0083 .001 _ 0 0 0 .003 0 0007 0 0 0007 .0007 . 001 0 .0015 0 .002 0007 ~.002 - .002 > <.008-: .008 ::002 _ ,.005- - .002 .008 .003 .0015 - .0007 0 .001 - .0005 .0015 .002 0 0015 _ .002 0 0 0 0 0007 0 0 0 0 0 0 0 0 .08 07 .015 .02 .007 07 .05 .05 .02 0015.05 .015 - .08 .08 .01 .008 0015 0 .02 .008 : .O1 .005 0 .01 .005 - .02 .008 .005 .003 007 .015 .003 ©;002 -.008 -.005 ~.03 .003 ~.008 .008 . 008 .0005 .0003 .0007 .0015 .0003 .0002 .0003 .0005 .003 .0003 .0003 .0003 015 . 08 . 08 . 03 . 03 ; O15 -. . O01 . 08 .05 . 08 007 .-, _ .01 INTRODUCTION 9 grains in each of a large number of thin sections. The grains were selected by random movements of the thin section on the microscope stage. For many rock units, modes estimated in this way are plotted in triangular diagrams to illustrate the range in composition and degree of heterogeneity of the unit. On these diagrams, the contours represent the per- centage of points that fall within 1 percent of the area of the diagram. Composition and optical properties of most indi- vidual minerals were not determined. However, pla- gioclase compositions were determined by measure- ment of extinction angles in thin section or of indices of refraction in immersion oils, and composition and optical properties of some of the iron-rich muscovitic micas were studied in some detail. All photomicro- graphs were taken with crossed polarizers except where noted. X-ray techniques were used to assist in some of the mineral identifications. GEOGRAPHIC LOCATIONS AND LOCATIONS OF TYPICAL OUTCROPS In order to facilitate reference to specific geo- graphic locations mentioned in this report, an arbi- trary grid has been placed on the geologic map (pls. 1,2). Blocks in the grid are approximately rectangu- lar areas 3.75 minutes of latitude and longitude in extent. They are lettered eastward from A through K and numbered southward from 1 through 10, be- ginning in the northwest corner of the map area. Whenever reference is made to a particular geo- graphic feature, the grid area in which the feature lies is mentioned by letter and number. In order to conserve space, locations of typical out- crops of the various map units are not described in the text, but are given in table 31 at the end of the report. PETROGRAPHIC NOMENCLATURE In this report, mineral modifiers of petrographic names are listed in order of increasing abundance. For example, biotite-muscovite-quartz schist denotes a schist containing more quartz than muscovite and more muscovite than biotite. Many rocks in the Grandfather Mountain area have strongly developed cataclastic textures. The nomenclature used in de- scribing these rocks is outlined in table 2. In most of the cataclastic rocks described herein, recrystalli- zation and cataclasis were virtually synchronous. The term "porphyroclast" is used to describe large mineral fragments or grains set in a finely granu- lated matrix. Porphyroclasts may be fragments or grains that are from an originally coarser grained, TABLE 2.-Classification of cataclastic rocks Degree of cataclasis Degree of recrystallization Rock Rock consists of Rock contains of matrix granulated 90 to 10 percent less than around grain porphyroclasts in 10 percent boundaries fine-grained matrix porphyroclasts Unrecrystallized_. Mortar gneiss... Mylonitic gneiss_____.__ Mylonite. Recrystallized_._. Recrystallized Phyllonitic gneiss Phyllonite. (where recrystallized material is largely sericite). Dos. dies dCors.siig.s Blastomylonitic gneiss (where recrystallized material is largely quartz and feldspar). mortar gneiss. Blastomylonite. even-textured rock and that have escaped granula- tion, or they may have been phenocrysts or prophy- roblasts in the original rock. In many rocks, the ori- gin of the larger grains cannot be determined, but it is clear that they did not recrystallize during the latest cataclastic metamorphism. The term "plutonic rock" refers to rocks that have or can reasonably be inferred to have had a medium- and coarse-grained allotriomorphic or hypidi- omorphic granular texture. Figure 4 shows the clas- sification of silicic plutonic rocks used in this paper. A plutonic metamorphism is defined as a meta- morphic event in which large masses of plutonic rocks were emplaced, no matter what the mechanism of emplacement may have been. Sedimentary rock names are used for arenaceous rocks even where they are of low metamorphic grade. Metamorphic rock names are generally ap- plied to argillaceous of equivalent metamorphic grade. Quartz Quartz f Granite monzonite 10 67 33 Potassic feldspar including perthite p 90 Plagioclase FIGURE 4.-Classfication of silicic plutonic rocks used in this paper. 10 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE Chlorites are classified using the system suggested by Albee (1962, p. 868). GENERAL GEOLOGY The Blue Ridge in western North Carolina and eastern Tennessee is composed largely of gneiss, schist, migmatite, and granitic rock that are the products of metamorphism and plutonism that occurred at about the time of the metamorphism of the Grenville Series in the western Adirondacks a billion years ago. Stratigraphically overlying these rocks are thick sequences of sedimentary and vol- canic rocks of later Precambrian age, deposited after the 1.000-m.y.-old event. These comprise the Ocoee Series, the Grandfather Mountain Formation, and the Mount Rogers Formation (pl. 3). All the Pre- cambrian rocks were subjected to one or more epi- sodes of thermal and dynamic metamorphism during Paleozoic time. During the later part of the Paleozoic and perhaps early Mesozoic, Precambrian crystalline rocks were thrust northwestward at least 35 miles over unmetamorphosed upper Precambrian and lower Paleozoic rocks in the Unaka belt (pls. 3, 4). The Grandfather Mountain window exposes meta- morphosed Cambrian, upper Precambrian, and lower Precambrian rocks below the Blue Ridge thrust sheet many miles southeast of any other exposures of rocks of this tectonic level yet recognized in this part of the Blue Ridge. During and after thrusting, the Blue Ridge thrust sheet was sundered by the Brevard fault, a large- scale strike-slip fault southeast of the Grandfather Mountain window. This movement juxtaposed an en- tirely different terrane of crystalline rocks of the Inner Piedmont belt with the Blue Ridge thrust sheet. The Inner Piedmont belt is composed of gneiss, schist, and granitic rocks of early Paleozoic or older ages, which underwent dynamothermal and plutonic metamorphism one or more times during the early and middle Paleozoic and which were retro- gressively metamorphosed during movement along the Brevard fault zone. A diabase dike probably of Late Triassic age extends undisturbed across both the Brevard fault zone and the thrust fault bounding the Grandfather Mountain window. The area described in this report includes all 470 square miles of the Grandfather Mountain window and adjacent parts of the Blue Ridge thrust sheet. It also includes nearby parts of the Mountain City win- dow in the Unaka belt northwest of the Blue Ridge and parts of the Brevard fault zone and Inner Pied- mont belt southeast of the Blue Ridge. Because of their fundamental differences, the rocks of each of these major tectonic units are de- scribed and their structural and metamorphic histo- ries are discussed separately before possible rela- tions between tectonic units are considered. Discus- sions of the various tectonic units are arranged geo- graphically from northwest to southeast. MOUNTAIN CITY WINDOW Sedimentary rocks of Early Cambrian and Cam- brian(?) age beneath the Blue Ridge thrust sheet are exposed in the Mountain City window in the northwestern corner of the Grandfather Mountain area (A-1, pl. 1). The tectonically lowest rocks in the Mountain City window in the Grandfather Mountain area are in the Doe River inner window (pl. 3) according to Rodgers (1953a). Tectonic slices of basement rocks and sedimentary rocks of Early Cambrian and Cambrian (?) age southeast of the Doe River inner window in the Grandfather Moun- tain area lie beneath the Blue Ridge thrust sheet and occupy an intermediate tectonic position between it and autochthonous rocks in the Doe River inner win- dow. The following descriptions apply to the sedimen- tary rocks exposed in the Doe River inner window in the Linville quadrangle. Basement rocks in the inter- mediate tectonic slices to the southeast are described with the rocks of the Blue Ridge thrust sheet. Sedi- mentary rocks in other parts of the Doe River inner window and in the intermediate tectonic slices were not examined petrographically during this study. ROCK UNITS CHILHOWEE GROUP Arenaceous rocks of the Chilhowee Group have well-developed detrital textures; the more argilla- ceous rocks are cleaved. Quartz is the dominant clastic mineral and is generally strongly strained. Microcline, biotite, and muscovite also occur as clastic grains. Biotite and muscovite clasts are lo- cally altered to sericite. Clastic green tourmaline and zircon are widespread. Sphene, ilmenite or magne- tite, epidote, and rutile are other accessory minerals. The rocks contain various amounts of matrix con- sisting of very fine grained sericite and chlorite. A few samples studied microscopically have a wide range of mica and chlorite content and contain less than 20 percent feldspar. These rocks contrast with the upper Precambrian rocks of the Grandfather MOUNTAIN CITY WINDOW 11 Mountain window but resemble the rocks of the Ta- blerock thrust sheet in their rather low feldspar con- tent, lack of clastic plagioclase, and presence of clastic tourmaline. King and Ferguson (1960, p. 119) recorded 1,975 feet of beds of the Chilhowee Group in an incomplete section on Nowhere Ridge (area C-2, pl. 1). UNICOI FORMATION The Unicoi Formation (Keith, 1903) is composed of fairly coarse to fine-grained, light-gray, tan, and green arkosic quartzite, conglomeratic arkosic quartzite, vitreous quartzite, and darker gray more argillaceous beds. Thin dark-gray to black beds, a millimeter or less thick, are rich in heavy minerals. The lower part of the formation is cut out by a thrust fault. Some gray quartzite locally has many fractures containing iron oxide and pyrite. King and Ferguson (1960, p. 119) measured 1,010 feet of Unicoi on Nowhere Ridge. HAMPTON FORMATION The Hampton Formation (Keith, 1903) consists of thin-bedded gray shale, siltstone, and feldspathic quartzite. Sandy beds 4 to 2 inches thick are lenti- cular in some places. X-ray diffractometer study by Paul D. Blackmon, U.S. Geological Survey, of a sam- ple of shale from the Hampton Formation from area C-1 (pl. 1) indicates that it is composed predomi- nantly of mica, quartz, and chlorite. ERWIN FORMATION The Erwin Formation (Keith, 1903) consists of light-gray, tan, and greenish-gray vitreous quartzite and feldspathic quartzite, locally containing Scoli- thus and shaly partings and beds. A few quartz peb- bles 1 cm (centimeter) in diameter are present. At the top of the Erwin Formation is an interval of light-green shale containing 1- to 3-foot-thick quartz- ite beds. This unit may correspond to the Helenmode Member as used by King and Ferguson (1960), but it is too thin to distinguish on the 1 :62,500-scale geo- logic map (pl. 1). King and Ferguson (1960, p. 119) measured 605 feet of Erwin on Nowhere Ridge. AGE The Chilhowee Group generally lacks diagnostic fossils in northeast Tennessee (King and Ferguson, 1960, p. 36), although Keith (1903) reported some from the top of the Erwin Formation. West of the Great Smoky Mountains, Early Cambrian fossils have been found in the Murray Shale (Laurence and Palmer, 1963), which is equivalent to about the mid- dle of the Erwin Formation in the Mountain City window. The rocks between the lowest diagnostic fossils and the base of the Chilhowee Group are con- sidered to be Cambrian (?) in age by the U.S. Geo- logical Survey. SHADY DOLOMITE 'The Shady Dolomite (Keith, 1903) is locally well exposed in the Elk River valley (C-1, pl 1). It is a fine- to coarse-grained, thin- to thick-bedded dark- to light-gray, blue-gray, and white dolomitic limestone. Some of the gray dolomite has spots of coarse- grained white dolomite. The Shady Dolomite is about 1,200 feet thick. No fossils are known in the Shady Dolomite in northeastern Tennessee (King and Fer- guson, 1960, p. 52), but fossils from the Shady in southwestern Virginia fix the age of the formation as Early Cambrian (Resser, 1938, p. 24-25). ROME FORMATION The Rome Formation (Hayes, 1891, p. 143) is composed predominantly of dull-green to red siltstone and shale and contains few interbeds of white quartzite. Beds of dolomite and shaly dolomite are widespread according to King and Ferguson (1960, p. 53), but none were noted in the Linville quardran- gle. X-ray diffraction study of a red shale from the Rome Formation just north of the Linville quadran- gle by Paul D. Blackmon, U.S. Geological Survey, indicates that it is composed predominantly of mica having a slight amount of mixed layering, and quartz, hematite, and feldspar. No fossils have been found in the Rome Formation in northeastern Tennessee (King and Ferguson, 1960, p. 53), but elsewhere it contains Early Cambrian fossils (Resser, 1938, p. 23-24). STRUCTURE AND METAMORPHISM The sedimentary rocks in the Doe River inner win- dow in the Grandfather Mountain area lie on the eastern limb of a syncline overturned to the west. They dip generally eastward beneath intermediate thrust slices near the sole of the Blue Ridge thrust sheet. Incompetent rocks of the Rome Formation are in subisoclinal folds overturned to the west. Cleavage parallel with the axial planes of minor folds is devel- oped locally in argillaceous rocks. In hand specimen the rocks seem to be unmeta- morphosed, the shales contain mica and chlorite rather than clay minerals, and some of the mica has a slightly mixed layering. It is uncertain whether these 12 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE rocks belong in the quartz-albite-muscovite-chlorite subfacies of the greenschist facies or the zeolite facies of regional metamorphism. BLUE RIDGE THRUST SHEET The Blue Ridge thrust sheet surrounding the Grandfather Mountain window is a complex crystal- line terrane composed of biotite-muscovite schist and gneiss, amphibolite, hornblende gneiss, and migma- tite intruded by plutons of Precambrian granite and quartz monzonite, by stocks and dikes of upper Pre- cambrian metagabbro and metadiabase, and by stocks, sills, and dikes of ultramafic rock, granodior- ite, and pegmatite of early or middle Paleozoic (?) age. Both northwest and southeast of the window, thin slices of Cambrian(?) sedimentary rock are intercalated in the gneisses along subsidiary thrust faults. West of the Grandfather Mountain window, bio- tite-muscovite schist and gneiss, amphibolite, and hornblende gneiss grade northward into granitic rocks. The transition occurs in a broad zone; layers and lenses of granitic material become increasingly abundant, and the rocks pass into layered migmatitic gneiss in which granitic layers predominate and fin- ally into rudely layered and nonlayered granitic gneiss. A similar but narrower transition zone is found north of the Grandfather Mountain window, but in some areas (J-2, J-3, K-3, C-6, C-7, and C-8, pl. 1) mica and hornblende gneiss are in sharp contact with layered migmatitic gneiss. Layering and foliation in rocks of the Blue Ridge thrust sheet generally dip away from the Grand- father Mountain window. West and north of the win- dow is a belt of rocks of dominantly granitic aspect retrogressively metamorphosed to a low grade before or during the thrusting. Tectonically overlying rocks were metamorphosed to medium grade in the middle Paleozoic. Southeast of the window, especially in a narrow belt between the window and the Brevard fault, medium-grade rocks have been partly retro- graded. ROCK UNITS MICA SCHIST, MICA GNEISS, AND AMPHIBOLITE Muscovite-biotite schist and gneiss and amphibol- ite make up the bulk of the Blue Ridge thrust sheet west and northeast of the Grandfather Mountain window (Keith, 1903, 1905; Brobst, 1962). The mi- caceous rocks were called Carolina Gneiss by Keith, a name which he applied to lithologically similar rocks in wide areas in the Blue Ridge and Piedmont (Keith and Darton, 1901 ; Keith, 1907b). He was un- certain whether they were derived from sedimentary or plutonic rocks. Keith called the associated horn- blende-rich rocks Roan Gneiss and believed them to be metamorphosed diorites intrusive into the Caro- lina Gneiss. On Roan Mountain, about 6 miles west of the Grandfather Mountain area, many metamor- phosed dikes of Bakersville Gabbro cut layered gneiss. These dikes were apparently miscorrelated by Keith with the amphibolite and hornblende schist interlayered with the mica schist and gneiss; this interpretation led him to the conclusion that the Roan Gneiss was intrusive. Because the terms "Carolina Gneiss" and "Roan Gneiss" were used by Keith primarily as lithologic terms, they have been abandoned as stratigraphic names (Brobst, 1962). Hornblendic and micaceous rocks are interlayered on all seales from fractions of an inch to a hundred feet, and all gradations between the two types are found. In the Grandfather Mountain area, large units of hornblendic rocks structurally underlie the large mica schist and gneiss units, but in the Spruce Pine area (Brobst, 1962) mappable amphibolite units are intimately interlayered with the micaceous rocks. The map pattern and smaller scale field rela- tions in both areas indicate that the hornblendic rocks and the mica schist and gneiss are everywhere conformable. Where amphibolite is interlayered with mica schist and gneiss, it crops out much more com- monly than the enclosing micaceous rocks, and the soil is reddish brown and is composed principally of small pieces of weathered amphibolite. The mapping of contacts between predominantly amphibolitic units and predominantly micaceous units is therefore extremely subjective in many areas. BIOTITE-MUSCOVITE SCHIST AND GNEISS The mica schist and gneiss are gray to light-gray and fine- to coarse-grained rocks containing muscov- ite flakes ranging from 0.5 to 8mm (millimeters) in diameter. The large muscovite flakes commonly give the rocks a glittery aspect. The rocks commonly con- tain light-pink to red garnet. Where the schist and gneiss are not interlayered with hornblendic rocks, they form light-colored soil containing flakes of hy- dromica. Schist and gneiss are gradational and inti- mately intercalated in layers that are fractions of an inch to a few tens of feet thick. Layers and lenses of amphibolite and a few thin layers of granofels and micaceous quartzite are intercalated with the mica schist and gneiss. The amphibolitic intercalations are BLUE RIDGE THRUST SHEET 13 most numerous adjacent to mapped bodies of amphi- bolite. Parallel arrangement of the micas forms a well- developed foliation which is generally parallel with the compositional layering and wraps around fold noses, although in a few places it cuts through the layering on the noses of isoclinal folds (fig. 5). FigurE 5.-Biotite-muscovite gneiss in the Blue Ridge thrust sheet. Foliation cuts layering in the noses of small folds. Roadcut on Blue Ridge Parkway south of Table Rock No. 2 triangulation station (area C-7, pl. 1). Segregation knots, lenses, and stringers of quartz are especially numerous in areas J-2 and I-2 (pl. 1) and muscovite-bearing pegmatites are locally abun- dant in areas C-5 and C-6. Quartz stringers parallel with the foliation are as much as 1.5 feet thick and several tens of feet long. The quartz has a sugary texture that suggests that it has been granulated and recrystallized. Kyanite occurs in the schist in a few places west of the Grandfather Mountain window. There the trail crosses Elk Ridge in area C-5 (pl. 1), float from a quartz-kyanite segregation contains blades as much as 6 inches long and 3 inch wide. Kyanite was also found north of the window in areas G-2 and H-2 (pl. 1), on the north side of Rich Mountain, on Doe Ridge, and on Howard Creek a mile west of the New River. FIGURE 6.-Photomicrographs of biotite-muscovite schist and gneiss southeast of the Grandfather Mountain window. A, Garnet- and chlorite-bearing muscovite-quartz-plagioclase schist from southeast of Giles Knob (area J-4, pl. 1). Plagioclase is oligoclase. Microfolds have the same orien- tation and style as larger folds that are the result of deformation during movement along the Brevard fault zone. Section cut perpendicularly to mineral lineation. B, Chlorite- and garnet-bearing biotite-muscovite schist from 1.25 miles northwest of the mouth of White Creek (area E-9, pl. 1). Porphyroclasts of muscovite partly granulated and recrystallized in matrix of muscovite, biotite, plagio- clase, and quartz. Plagioclase is partly altered and ranges from An,, to Ang,. Southeast of the Grandfather Mountain window, especially near faults, the larger muscovite crystals are ovoid porphyroclasts resembling fish scales, which are in part converted to finer grained white 14 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE mica (fig. 6B). In such areas, especially in the Lin- ville Falls quadrangle, biotite and garnet are partly converted to chlorite. Northeast of the window, the schist is finer grained where it has been sheared along a few local fault zones. The mica schist and gneiss generally have grano- blastic and lepidoblastic textures, but near faults and shear zones, cataclastic and prophyroclastic textures predominate locally. Equant grains of quartz average 0.1 to 0.5 mm in diameter but locally reach 4 mm. Muscovite forms well-alined grains ranging from 0.1 to 5mm long and averaging about 2 mm. Where the rock has been crinkled musovite flakes form polygonal ares around the folds (fig. 6A). Locally, crystallization lasted longer than movement, and some flakes have grown transverse to the foliation. In other places, deforma- tion continued or took place after crystallization, and the larger muscovite grains are bent. Adjacent to faults southeast of the window and near the contact with the Cranberry Gneiss west and north of the window, the coarse-grained muscovite has been converted to ovoid porphyroclasts, some of which are bent. Southeast of the window, muscovite por- phyroclasts have commonly recrystallized to aggre- gates of finer grained muscovite. Biotite flakes are generally smaller and have ap- parently crystallized later than muscovite. They av- erage about 0.5 mm in diameter, but some are as much as 2.5 mm in diameter. Biotite is synkinematic in some rocks but partly postkinematic in others. It is generally brown or greenish brown ; in a few rocks it is brownish green or dark green. In the rocks con- taining porphyroclastic muscovite, the biotite is all fine grained and seems to have completely recrystal- lized. Plagioclase grains average about 0.5 mm in diame- ter; locally, they range from 0.1 to 3 mm. Some of the larger grains contain inclusions of quartz, mica, and other minerals. Composition ranges from An,; to Ans, except where the plagioclase has been saus- suritized. Normal zoning, with as much as 10-per- cent range in anorthite content, is locally found. Al- tered plagioclase is found in shear zones and espe- cially adjacent to the contact with the Cranberry Gneiss ; locally, it is found southeast of the Grand- father Mountain window. Garnet occurs as subhedral or euhedral grains and locally as anhedral grains or skeletal crystals as much as 1 cm in diameter. Sieve texture is wide- spread; a zonal or spiral (snowball) distribution of inclusions of quartz, plagioclase, muscovite, biotite, and opaque minerals is common. Some of the garnet has a light-red absorption; in a few rocks its core has a somewhat stronger absorption. Anhedral to subhedral sieve-textured kyanite grains, locally as much as 1 em in diameter, contain inclusions of garnet, biotite, quartz, staurolite, pla- gioclase, and rutile. Sieve-textured staurolite forms anhedral crystals 0.5 to 8 mm long that commonly contain inclusions of quartz, plagioclase, garnet, and rutile. The kyanite is partly replaced by muscovite in some rocks, and southeast of the window, relict grains of staurolite and kyanite occur in aggregates of sericite. Sillimanite included in a porphyroblast of muscovite was found in one specimen from a roadcut souheast of Mount Perion school (area J-2, pl. 1). Light-green to green FeMg and MgFe chlorite occurs in small amounts after biotite and garnet. Epidote occurs as anhedral to subhedral grains 0.05 to 0.5 mm in diameter, that commonly contain cores of allanite or metamict allanite and less com- monly cores of zoisite or clinozoisite. Rarely, the epi- dote is iron rich. Opaque minerals are magnetite, ilmenite, pyrrho- tite, and pyrite and range from 0.02 to 2 mm in grain size. Apatite and rounded zircon grains are wide- spread. Sphene, bluish-green to brownish-green tour- maline, rutile, and calcite are less abundant. COMPOSITION AND ORIGIN The biotite-muscovite schist and gneiss were de- rived from interbedded graywackes and argillites. They generally contain more quartz than plagioclase and subequal amounts of biotite and muscovite (fig. 7). The total mica content ranges from 4 to 66 per- cent. The occurrence of kyanite and staurolite is not related to a high mica content in all cases. It may be partly controlled by the ratio of alkalis to alumina (see position of analysis 2, table 3, in fig. 7). The only volumetrically important minerals not plotted on the diagram are garnet and epidote. Garnet occurs in more than half of the thin sections exam- ined, but in most of them it constitutes only a few percent of the rock. One specimen, however, contains 16 percent garnet. Epidote constitutes 2 to 16 per- cent of the rock in about a quarter of the samples. Some of the chemical analyses of schist (table 3, analyses 1, 2, and 7) resemble those of some Pre- cambrian lutites (Nanz, 1953, analyses 8, 10, and 13) and differ from the average shale (Clarke, 1924) in having higher Al,O; and Na,0 and lower CaO con- tents. Except for its higher iron content, analysis 1 BLUE RIDGE THRUST SHEET 15 TABLE 3.-Chemical analyses, modes, and norms of biotite-muscovite schist and gneiss in the Blue Ridge thrust sheet [Analyses 1-6, by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey, 1962. Modes, by point counts; P, present but not intersected in counting. Analyses 7-10 are standard rock analyses by Ruth H. Stokes, U.S. Geol. Survey, 1952, furnished through courtesy of Donald A. Brobst. Major oxides and CIPW norms given in weight percent; modes, in volume percent] 1 2 8 4 5 6 T 8 9 10 Ficd ZD-60-1 X-67-6 ZB-54-2 4-586 RE-58-2 ZB-49-2-a SP1 SP9 SP2 SP8 Laboratory No...... 160178 160181 160183 160186 160179 160184 51-1758CD 51-1766CD 51-1759CD 51-1765CD S1O;- 2-2 cnn 56.0 58.8 60.3 70.6 71.8 78.5 63.14 71.13 12.11 82.46 ic t osa oli ok 17.6 22.2 16.1 13.9 13.3 11.8 16.30 13.14 13.50 8.60 6.0 2.1 1.6 T1 2.6 . 50 2.86 .98 1:77 "19 6.3 4.2 7.2 8.7 3.1 4.1 4.26 3.34 2.52 1.76 2.4 2.1 3.0 1:6 1.4 1.6 2.62 1.51 1.76 1.51 1.4 1.0 8.1 2.2 .98 1.6 1.91 1.95 . 99 74 Nad: z.: 2.0 2.2 8.8 8.8 2.0 3.0 2.30 4.32 1.87 2.64 gO Fcc: 3.2 8.1 3.0 2.8 2.6 2.0 3.42 1.52 3.10 1.16 ...s, 2.2 2.0 1.1 .92 1.0 .91 1.74 .61 1.26 42 ( LL.... 12 . 24 .08 .04 .05 . 06 .30 .04 .10 .06 1.8 .87 1.9 49 .86 T6 1.06 .86 .70 . 88 ( Ofe wer - lel .86 .39 .85 A7 AT 28 A1 .22 .18 .06 .28 .12 'At .10 . 08 . 08 .15 .07 07 .08 .us <.05 <.05 <.05 <.05 <.05 <.05 .08 .02 .01 .01 es ce ames 1 o noen o Nore o aun un aaa be ull 2s an aa aie boe gina ale a ales ae aan ale ane ae i 199. . - 00 cL resin en. ans suas na rebs ss ase mene ss Sol sms 100;:10 -.- Less 10 fOr B... 2 2 .s o 2 CATER L c a aln a nae ule aie a a ale a a - tale ane nl o a a bic a dn a ie le ha i he ania alle il mn an s la ae s. Total. 100 99 100 100 100 100 100.20 99.20 99.94 100.02 Modes Quartz..>......_l.. 13 13 28 38 58 61}: > CXL connie et an congeal ue cues an a ole pul a a ake Plagioclase.__-____. 10 17 31 36 10 T5 «sear rack sb enses Muscovite...___._.. 37 dt? s 9 Los. 4 28 BH LORE ALLE Pe cher base a's aires =t sk ..:. 12 ciel... 39 20 9 Ra C El ous =s real sass Eb ce ene s Garnet............ 17 8 t d 1.0 2 110 1 ong e aaa ne a nb aie ale aaa alel a alle take Epidote..'... :l... tf Leo aed rare na eee a 2.0 PD Po e ae a aa a i in i ne ad t il ina o t ae ehe lan s ie i s in Ae e B ae ie Chlorite...<.~>.._:.. o aa r cubre an ae sins hie nid a an v4 BHEL rea ina adil -a s nae eben e., 6 . o U I ne reek nikkers ant Like aoe ais s ale tees ans be o he'd anne ee eek 0 sn ns tb icp Lue - naw as a abbe a ao rable as eel o dail 9 aa a's a's aleina a aie a's a - in a ele nell alate Opaque minerals. _ . 5 .5 .8 2 1.2 B o Perro an au aioe aie wo U n ad » aa a aerials P rt Lu. a Ner ho naan a ae hauls a a tels ac = an ane -in bin aie cb alain tle fle aie P t cic >in cares P P. PA*} {(lg P 0s o Sot ete bee, £1 os p oen cede .5 4. .8 Nae er esata So l ELS H an a a's 24 a baar bi a a slake oie urea ane Apatite. .:.... A ¥ .8 .8 .2 i EUA a a+ oak ae an a aie e an one eae o UL aul a ah a sie a ate P _ Nene rr ioc iners cans artner stuns ce cee e Calcite... >. 202. EC natl alc nanan Ma eal =a nak neon ner onle P 2 Els Pon nene ey ove ca be - an ain ain ain ane ile a e are Points counted.... 805 1,011 606 600 600 608 > nolan toil oen o aa bel uue eae CIPW norms ( eas a a ne lo wee 24.12 27.31 14.95 33.80 45.49 40 . 48 21.27 32.62 44.20 57.90 (ark erin inkl ens 9.06 14.34 2.63 2.39 5.82 2.32 5.69 1.42 6.12 1.82 Ore ss l c e ny ol 18.91 18.32 17.712 13.59 15.36 11.82 20.21 8.98 18.32 6.85 ADs. 16.91 18.61 27.91 27.91 16.91 25.37 19.45 36.54 15.82 22.38 AMES seein in 4.59 2.41 13.09 9.80 3.175 6.50 8.56 8.11 3.67 3.22 5.97 5.28 7.47 3.73 3.48 3.98 6.52 3.16 4.38 3.16 4.67 4.76 10.04 5.15 2.27 6.01 8.99 4.03 2.14 2.50 Mt:..:.....N..-._. 8.70 3.04 2.18 1.03 3.77 12 4.15 1.42 2.67 .27 Milks s ec s 3.42 1.65 2.47 1.42 1.63 1.44 2.01 1.63 1.33 T2 .85 .92 .88 . 40 . 40 . 52 .26 . 52 . 48 14 CCH lle c l.. nau sted t ob k seein c Lb eC a mals ace a kd .07 04 . .ll. c 1. Medium-gray schist containing garnets as much as 1 cm in diameter long. Plagioclase (Anzo-») in sieve-textured porphyroblasts, as much and muscovite as much as 4 mm long. A few white quartz-feldspar segregation lenses, less than 5 mm thick. Muscovite, partly in poly- gonal ares and partly deformed. Olive-green biotite, as much as 3 mm long. Quartz and plagioclase (An,,), about 1 mm in diameter. Garnet includes quartz and opaque mineral and is altered along margins and cracks to sericite and chlorite. FeMg and MgFe chlorite from garnet and biotite. Mode, probably not representative of whole rock because of coarse grain size and irregular distribution of minerals. Probably contains more plagioclase and quartz and less mica. From 1,900-foot altitude on the east side of the 2,100-foot-high knob east of Carr Mountain (area J-5, pl. 1). 2. Light-gray schist containing kyanite and mica as much as 3 mm in di- ameter. Synkinematic muscovite and brown biotite, as much as 2 mm as 3 mm in diameter, that contain quartz and garnet. Quartz 0.1 to 0.3 mm in diameter. Subhedral garnet 0.15 to 0.3 mm in diameter. Bent and broken sieve-textured porphyroblasts of kyanite as much as 6 mm long are locally altered to sericite; they contain inclusions of quartz, garnet, biotite, and staurolite. Anhedral grains of staurolite, 0.5 to 3 mm in diameter, include quartz and garnet. From roadcut along North Carolina Highway 194, 0.9 mile N. 84° E. of Howard Creek Church (area H-2, pl 1). 3. Dark-gray gneiss containing garnets as much as 5 mm in diameter and biotite as much as 2 mm in diameter. Quartz and plagioclase (Anss) grains, 0.2 to 0.7 mm in diameter; reddish-brown biotite 0.2 to 1 mm long. Garnet includes quartz and biotite. From roadcut along Blue 16 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE TABLE 3.-Chemical analyses, modes, and norms of biotite-muscovite schist and gneiss in the Blue Ridge thrust sheet- Continued. Ridge Parkway at 3,450-foot altitude, 1,000 feet S. 80° E. of 3,670- thrust sheet in the Grandfather Mountain area, but satisfactory modes foot-high knob southwest of Deep Gap (area J-2, pl. 1). of these specimens are not available. 4. Fine-grained gneiss having granoblastic texture with an average grain 7. Garnet-muscovite-quartz-biotite-plagioclase schist containing subordinate size of 0.5 mm and nonoriented micas, 0.2 to 1 mm long. Dark-red- opaque minerals, epidote, chlorite, and accessory apatite and zircon. dish-brown biotite; plagioclase is Ans. Subhedral granet, 0.3 mm in From roadcut on U.S. Highway 19E north of Spruce Pine on the diameter. From roadcut along Blue Ridge Parkway 0.37 mile N. 74° south side of the bridge across the North Toe River in the Spruce E. of Table Rock No. 2 triangulation station (area C-7, pl. 1). Pine 7%-minute quadrangle, 0.43 mile S. 50° W. from the point where 5. Medium-gray schist containing muscovite aggregates as much as 1 cm the Brushy Creek road leaves the west edge of area C-6 (pl 1). long. Muscovite aggregates, made up of individual grains 0.5 to 1 mm 8. Muscovite-biotite-quartz-plagioclase gneiss containing accessory zircon long, which form polygonal arcs; earlier foliation has been isoclinally and sphene. From small creek 1,500 feet N. 5° W. from 3,755-foot al- folded with axial planes parallel with megascopic foliation of rock. Late titude on south ridge of Doublehead Mountain in the Carvers Gap kinematic and postkinematie brown biotite, less than 0.5 mm long. 7%-minute quadrangle, about 1.5 miles west of the west edge of area Granoblastic-textured quartz and plagioclase (An,,), 0.1 to 0.3 mm in C-5 (pl. 1). grain size. MgFe chlorite. (From roadcut along U.S. Highway 821, 0.2 rnet-bearing muscovite-biotite-quartz-plagioclase schist, from roadcut mile north of Stratton Creek (area I-5, pl. 1). *: Gilm North Circling; Highway 80 at bridge across Rebels Creek, Bakers- 6. Light-gray gneiss containing micas as much as 3 mm long and quartz ville 7%-minute quadrangle, about 10 miles west of Linville Falls segregation stringers. Quartz, 0.1 to 0.5 mm in diameter; plagioclase quadrangle. (Ano), as much as 1.2 mm in diameter; and synkinematic brown biotite and muscovite, as much as 2 mm long. From roadcut alolng 10. guise glgggggetilifizg {fgepxllfnlvtihfut of road duncfon at:spof altl- junction at 2,867-foot altitude nortzl of Plumtree, C'arver?l Gui) l71/2- The following descriptions of samples 7 through 10 were furnished by Don- minute quadrangle, 2,300 feet S. 63° W. from the point where Plum- ald A. Brobst. The rocks are rather typical of much of the Blue Ridge tree Creek road leaves area C-5 (pl. 1). Biotite-quartz-plagioclase gneiss containing accessory opaque minerals, chlorite, apatite, and zircon. From roadcut 300 feet north of road Biotite Quartz 3 Biotite Muscovite Plagioclase EXPLANATION 2 % Analyzed specimen © Average Biotite FIGURE 7.-Proportions of quartz, plagioclase, muscovite, and biotite in biotite-muscovite schist and gneiss in the Blue Ridge thrust sheet. Based on counts of 50 random grains in each of 70 thin sections. Contours 1.5, 3, 6, and 9 percent. Number of analyzed specimen refer to analyses in table 3. BLUE RIDGE 1 . *J mes P IIlite / muscovite Als 20 |- U © A *1 0 15 +A % & 9 x4 f 5**+B x8 & & § . sof ed G 10- f 10 > x l < 5»— 4 1 Js o 5 10 15 K,0+Na,0+CaO, IN WEIGHT PERCENT EXPLANATION Albite Exogeosynclinal Pal Analyzed specimen Graywacke © A Biotite Orthoclase A 0 Eugeosynclinal Pelite FiqurE 8.-Plot of weight percent Al,O, against sum of K,0, Na2a,0, and CaO for analyzed biotite-muscovite schists and gneisses of the Blue Ridge thrust sheet. Num- bers refer to analyses in table 3; unnumbered point, from table 5. Selected mineral compositions and averages of sedimentary rock types are shown. A, average analysis of graywacke, from Pettijohn (1957); B, average analysis of graywacke, from Pettijohn (1963); average analysis of pelite, from Shaw (1956); average analyses of eugeosyn- clinal and exogeosynclinal sandstone, from Middleton (1960). resembles the average pelite of Shaw (1956). In figure 8, analysis 2 falls to the left of the muscovite line despite the plagioclase content of the rock. This position is compatible with the presence of kyanite and staurolite in the rock. The schists generally have more K;0 than Na,0 (fig. 9), whereas the gneisses, which contain less mica and more plagioclase, have less K,0 than Na,0. Some of the gneisses (analyses 4 THRUST SHEET 17 4 pee > 3. . %*. 8 9 961 3 % o 3|- a x a. T x" R x* > 6 4. A O 8 a % 3 C x10 J -: bus | | | | 0 1 2 3 4 Na,0, IN WEIGHT PERCENT EXPLANATION xl Analyzed specimen Graywacke A 0 Eugeosynclinal Pelite o] Exogeosynclinal FIGURE 9.-Na,0:K,0 variation diagram for analyzed bio- tite-muscovite schists and gneisses in the Blue Ridge thrust sheet. Numbers refer to analyses in table 3; unnumbered point, from table 5. Selected averages of sedimentary rock types shown: pelite (Shaw, 1956), graywackes (Pettijohn, 1957, A; 1963, B), exogeosynclinal and eugeosynclinal sandstones (Middleton, 1960). and 6, table 3) have chemical compositions typical of graywackes, but most of the gneisses of the Blue Ridge thrust sheet are richer in SiO; and K0 and poorer in CaO than average graywackes (Pettijohn, 1957, 1963; Middleton, 1960). The gneisses are not likely to have been derived from siliceous igneous rocks because they have lower alkali contents in rela- tion to their silica contents than do most igneous rocks. This is reflected in the predominance of quartz over feldspar in their modes. AMPHIBOLITE AND HORNBLENDE SCHIST Amphibolitic rocks interlayered with the mica schist and gneiss in the Blue Ridge thrust sheet are black, greenish black, or mottled black and white and are fine grained to coarse grained. Hornblende crys- tals generally lie along foliation planes parallel to the 18 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE layering; they are commonly 1 to 3 mm long and may be as much as 5 mm long. In many rocks their long dimensions are parallel, forming a conspicuous lineation. Pinkish-red garnets, 0.5 to 2 mm in diam- eter, occur in many layers. Lenses and stringers of granoblastic quartz or plagioclase or both are gener- ally less than an inch thick. The amphibolites contain muscovite-bearing plagioclase-rich pegmatites. The amphibole is hornblende that has Z A c rang- ing from 14° to 22° and an absorption parallel to Z which is green to olive green in most rocks but ranges from pale green and bluish green to brownish green and light brown. In places, the hornblende grains contain inclusions of quartz, sphene, plagio- clase, rutile, and opaque minerals. Plagioclase is anhedral and has a grain size of 0.05 to 0.2 mm, except in a few coarse-grained quartz dioritic layers, where it is as much as 5 mm. It is generally calcic oligoclase and sodic andesine and locally shows normal zoning. In some places, espe- cially near the contact with Cranberry Gneiss in areas C-5 and C-6 (pl. 1), it has been saussuritized. However, in these retrograded rocks the mafic min- erals are very little altered. Locally, the plagioclase contains inclusions of quartz and hornblende. Quartz ranges in grain size from 0.05 to 2 mm, and in some places is concentrated in thin segrega- tion stringers. It locally includes hornblende and epi- dote. Epidote and clinozoisite are 0.05 to 0.5 mm in grain size and range from anhedral to subhedral. Most of the epidote is rather iron poor ; none is pista- cite. Some crystals have allanite or zoisite cores. Most of the epidote belongs with the main assem- blage, but some is derived by retrogressive alteration of plagioclase and, rarely, hornblende. Light-pinkish-tan garnet occurs in subhedral to euhedral crystals 0.1 to 5 mm in diameter and lo- cally shows sieve texture with quartz, plagioclase, and opaque minerals. Diopsidic pyroxene is found in a few amphibolites. Sphene is the most widespread accessory mineral and occurs as round ellipsoidal grains 0.03 to 0.5 mm long. Apatite is found as ellipsoidal to euhedral crys- tals 0.03 mm to 1.5 mm long. Rutile locally occurs as cores of sphene grains. Magnetite, zircon, pyrite, brown biotite, chlorite, and sericite are other acces- sory minerals. COMPOSITION AND ORIGIN Although rocks of this unit vary rather widely in composition, most of them are amphibolites that are composed principally of hornblende and plagioclase and that contain small amounts of quartz and epi- dote. Some layers are hornblende schists that lack plagioclase and are richer in quartz than the amphi- bolites. Chemical analyses of typical amphibolite (table 4, analysis 1) and hornblende schist (table 4, analysis 2) show that the schist contains less CaO, Al;O;, K,0, MgO, and more SiO, and total iron than the amphibolite. The amphibolite has a composition simi- lar to basalt, whereas the hornblende schist contains less Na,0 and K0 than do most igneous rocks of its silica content, such as the average tholeitic andesite (Nockolds, 1954). Field relations indicate that nei- ther rock can be a flow. The amphibolite was from a TABLE 4.-Chemical analyses, modes, and norms of amphi- bolite in the Blue Ridge thrust sheet Analyses 1 and 2, by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey, 1962. Analyses 3 and 4 by standard methods by Ruth H. Stokes, U.S. Geol. Survey, 1952 (Wilcox and Poldervaart, 1 8, table 5). Modes, by point count of 600 grains in each thin section; P, present but not intersected in counting. Major oxides and CIPW norms given in weight percent; modes, in volume percent] 1 2 8 4 4-550A FA-34-87 160185 160182 Field SP7 SP6 Laboratory No...... 51-1764CD 51-1763CD Major oxides SIO: uide sels + 48.5 51.8 49.58 50.28 . enews 15.3 12.6 16.11 16.11 2.5 4.7 1.33 1.46 8.8 11.3 8.42 8.65 6.5 5.2 7.42 7.76 11.4 8.9 11.47 9.99 2.7 2.0 2.84 2.94 .65 .22 42 45 1.0 1.8 > . 19 1.48 .06 .24 .03 .09 1.9 1.5 .99 .54 .28 .24 19 .18 .20 22 +17 18 10 <.05 .28 .00 _______________________________________________ Toll. Subtotal: Pool pic meer ncs , 100:10 :-: seee... Less O for 8. ... 3. [1 nucs none anu ewan nen be (O0 er Totals iss. 100 100 100.04 100.06 Modes Hornblende.........._... 63 T5 QUBIEE: .L .L 1.5 15 Plagioclase . x 27 .8 Epidote . s 8 5 Biotite: -o-. on.. cused eens econ o Garnet... . 1.7 T Sphene........ Pee 2.8 ig Opaque mineral._..___.~. .€ 8 Apatite. ~.. 5 2 BLUE RIDGE THRUST SHEET 19 TABLE 4.-Chemical analyses, modes, and norms of amphib- olite in the Blue Ridge thrust sheet-Continued 1. Amphibolite, in layers 0.5 to 1.5 feet thick in biotite gneiss. Well-alined drab-green hornblende is as much as 2 mm long; quartz and plagioclase (An,,) are as much as 0.5 mm in diameter. Plagioclase is locally saussuritized. Anhedral garnet as much as 1 mm in diameter includes hornblende, quartz, and plagioclase. From roadcut along Blue Ridge Parkway on east side of Humpback Mountain at contact of Cranberry Gneiss (area C-6, pl. 1). 2. Dark-gray-green schist containing a few thin quartz segregation string- ers. Green hornblende, 0.1 to 2 mm long; zoned grains of epidote 0.05 to 0.5 mm in diameter have iron-rich cores; quart grain 0.01 to 0.2 mm diameter are concentrated in segregation stringers. Rock grades to biotite gneiss in outcrop. From roadcut along county road 0.7 mile S. 20° E. of road intersection at spot altitude of 3,123 feet on U.S. High- way 421 (area I-2, pl. 1). Petrographic descriptions and locations of samples 3 and 4 furnished by Donald A. Brobst. No satisfactory modes of these specimens are availa- ble. 8. Garnet-hornblende-plagioclase gneiss containing accessory sphene and biotite. From outcrop 0.6 mile N. 13° E. of Slippery Hill Church (southeast corner of area C-4, pl. 1). 4. Plagioclase amphibolite containing subordinate quartz and sphene. From 200 feet S. 70° W. of summit of Copperas Bald in the Carvers Gap quadrangle. Copperas Bald is 0.4 mile N. 73° W. of Slip- pery Hill Church (area C-4, pl. 1). layer less than 114 feet thick intercalated with bio- tite gneiss, and the hornblende schist grades into bio- tite-muscovite gneiss; the amphibolite may have been a mafic tuff or a voleanic-derived sediment. The hornblende schist is too low in K.,0 and Na.0 to have been derived from a normal mafic igneous rock; it may have been a dolomitic shale or a mixture of sedimentary and mafic igneous detritus. The interlayering of the hornblende-bearing rocks with mica schist and gneiss and the intergradation between the rock types suggest that many of the hornblendic rocks must be derived from sedimentary or tuffaceous rocks. However, some of the thicker, more uniform amphibolite layers may have been mafic flows or sills. GRANOFELS Bluish-gray granofels layers superficially resem- bling quartzite are locally intercalated with schist, gneiss, and amphibolite in the Blue Ridge thrust sheet. The granofelses range from light-yellowish- brown rocks containing as much as 90 percent epi- dote and 10 percent quartz to gray rocks rich in plagioclase and pyroxene. Pyroxene occurs in anhedral to subhedral grains 0.02 to 2 mm in diameter. It is light-green salite with 2V about 60°, n, 1.693, and Z A c =42°. Amphiboles in the granofelses tend to be lighter green than in the amphibolites and are probably less aluminous. Epidote is generally rather iron poor. Plagioclase is generally more calcic than in the enclosing rocks; in one specimen it is labradorite. The one analysis (table 5) can hardly be representa- tive of the wide variety of granofelses, but in general they probably have a higher CaO content in relation to K0 and Na,0 than do the schist, gneiss, and am- phibolite. They were probably derived from calcar- eous siltstones or sandstones. TABLE 5.-Chemical analysis, mode, and norm of gramofels from the Blue Ridge thrust sheet [Determined by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey, 1962. Mode, by point count of 600 grains; P, present but not intersected in counting. Major oxides and CIPW norm given in weight percent; mode, in volume percent] 1 Fida Noell te dle lean ek asr on sews sep N-62-2 Laboratory (No. AT COLE LAL ah one n o she s 160180 Major oxides NOK ATA AILEEN Ain i rere alkene an a 69.0 MsO;.: u n i belo i nein rece oul a oh an - ena ue 14.7 F8203 ___________________________________________ 1: 2 LII cE? cL a 14 MeO. AII C Te aoe hye ne ne oo aetna arate aat t.:1 Cad.. on d Loie are reset alec es 8.0 crack sabe 1.4 ssi s co lns a oe alee oke Hain mele a ie aon am ale la alg t ae wll ia 28 M;:0o4 .!: ISI ale on ae bose - 2 aes pik cae aad ole 1.4 (RSLI ERE EL 4 += ank eek eee tid mace aoe .90 TO;. m ie tule c fen be co-ed eee aba . 88 SON ed nl ls ak oll ne clan cole an an wie o ale a aie aes a .26 MnO... 2: Lenee IH : -- -an esc bee ls - oracle een tn. .01 CO; ..S scant = dente cate cenkes sees aan 05 E, 2G C Euan s a male c c nanan 100 Mode Quart 20220 2 sin nnis a oh Sule sabe n alain dh + be paid t 49 Plagioclase... re.. cll 22 ll cnn nines a - airman a a al al o anale 32 Epidate ss s(n c A Uit a 15 futile.. };. y.. 328 ray 1 one d ee uy Alara ceo Aai Arar P s2 t cos oo e e UTL Lar cee ae a ae aia ne 7B Sphene ss.. irs: ee crl iL Icl UDA e 1.0 ADEUINC:L : k_ ina o ape ak ch a emind - /a Amphibole. : ~.. ... .s oss o eto lene eae ea i cle aan m 5 CIPW norm ea oa A NEAL E as an ae r e s ree eine en 483.08 Or ee sel lee L Hae nail a an cece recte 1.36 AD.. . ne da uke oe Me EBE dan ae os a a ale nle rele aud 11.84 An L sore AM VLE ETI aT - Le saws P ber 33.15 u. il ec ein a aas 2. in diva ena ee s 1.82 Enes osis s ce aan an eee eae d o ne ale aan o ain ain s 2.74 Hm se. ec fico edi EL LE nae cole wed 1.20 Mrs cece t ..i MTI - Los se 1.58 Tse US oes so o Le u e nau aa dle a nom baal bln a a alle eae ie 11 don ens 62 OC 2 ro Ve chin main a o oun anale Riau be un ai aa en sagen 11 1. Light-gray medium-grained amphibole-epidote-plagioclase-quartz granofels. Mosaic of quartz and plagioclase (Anss), 0.5 to 0.8 mm in grain size, epidote, 0.2 to 0.5 mm, and light-green amphibole, 0.4 to 0.5 mm. From roadcut on south side of Little Plumtree Creek, 0.85 mile S. 5° E. of Mount Pleasant Church, Linville quadrangle (area C-5, pl. 1). AGE The mica schist, mica gneiss, and amphibolite of the Blue Ridge thrust sheet were considered to be of early Precambrian age by Keith (1903) because of their degree of metamorphism and their strati- graphic position below sedimentary rocks of pre- sumed late Precambrian age. Stose and Stose (1949, p. 315) stated that these rocks are directly traceable into rocks that they mapped as Lynchburg Gneiss of late Precambrian age in the Gossan Lead district, 20 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE Virginia, 40 miles northeast of the Grandfather Mountain area. However, Dietrich (1959, p. 58) ex- pressed doubt that the rocks in the Gossan Lead dis- trict could be correlated with the Lynchburg Gneiss. He found that similar schist and gneiss in Floyd County, Va. are gradational with plutonic rocks of the Blue Ridge complex, which have an age of 1,000 to 1,100 m.y. in northern Virginia (Tilton and oth- ers, 1959, table 13). Hornblende at the Ore Knob copper deposit in the Blue Ridge thrust sheet about 10 miles northeast of the Grandfather Mountain area has a potassium-argon age of 1,130 m.y. (Thomas, 1963). About 60 miles west of the Grandfather Mountain area, schist, gneiss, amphibolite, and migmatite and plutonic rocks are overlain by upper Precambrian rocks of the Ocoee Series (Hadley and Goldsmith, 1963). Isotopic ages of zircon in layered gneiss at Deyton Bend on the northwest side of the Spruce Pine dis- trict are 950 and 1,270 m.y. The layered gneiss at Deyton Bend grades southward into typical schist and amphibolite of the Blue Ridge thrust sheet. Sim- ilar ages are found for zircon from layered gneiss from the intermediate sheet in the Mountain City window at Pardee Point (Davis and others, 1962, table 3). This gneiss is directly overlain by the Unicoi Formation of Cambrian (?) age. These facts favor assignment of the mica schist, mica gneiss, and amphibolite of the Blue thrust sheet to middle Precambrian age in terms of the informal, local usage of the U.S. Geological Survey. The rocks are older than their metamorphism, which was 1,000 to 1,100 m.y. ago, and possibly younger than 1,270 my., which is the age of detrital-ap- pearing zircon in the gneiss at Deyton Bend. In terms of present knowledge of the absolute time scale of the Precambrian of North America these rocks would be middle late Precambrian or Neo- helikian (Stockwell, 1964, table 2). Since the above was written, recent work by Rankin (1967, 1970), Hadley. (1970), _ and Hadley and Nelson (1970) has reopened the question of the age of the metamorphic rocks of the Blue Ridge thrust sheet. Near the North Carolina-Virginia State line, Rankin has found good evidence of an unconformable relation between gran- itic basement rock and rock called the Ashe Forma- tion (Rankin, 1969), which can be traced into the schist and amphibolite of the Grandfather Mountain area. In the Grandfather Mountain area, however, this contact does not appear to be an unconformity, for reasons stated below. Nevertheless, some of the metamorphic rocks in the Blue Ridge thrust sheet in the Grandfather Mountain area may be of late Pre- cambrian age. MIXED ROCKS A diverse group of rocks transitional between pre- dominantly amphibolitic rocks and predominantly granitic Cranberry Gneiss crop out in a broad area west of the Grandfather Mountain window (area C-4, pl. 1) and in a narrow belt north of the window areas (G-2, H-2, I-2, J-2, pl. 1). Elsewhere these rocks are missing. The mixed rocks consist of inter- layered and intergrading amphibolite, cale-silicate granofels, biotite-hornblende gneiss, hornblende-epi- dote-biotite gneiss, biotite-hornblende-plagioclase schist and gneiss, epidote-biotite-plagioclase schist and gneiss, and granitic gneiss ranging from quartz diorite to quartz monzonite. Plagioclase prophyro- clasts as much as 2 cm long are widespread in the biotite-hornblende-plagioclase and epidote-biotite schist, and hornblende porphyroclasts as much as 1 ecm long occur in some rocks. These rocks are mapped an a unit, the contacts of which are drawn at the first occurrence of layers of granitic rock in the amphibolite on one side, and at the place where granitic layers become dominant on the other side. Many bodies of metamorphosed Bakersville Gabbro occur among the mixed rocks, especially west of Newland in Area C-4 (pl. 1), but only the larger ones are mapped separately. Some may not have been recognized because of the difficulty of distin- guishing the amphibolite from younger sheared and recrystallized metagabbro. Typical exposures of the mixed rocks show well- developed compositional layering and a variety of rock types. In such outcrops, darker layers are am- phibolite or biotite-hornblende gneiss, and lighter colored layers are quartz-feldspar gneiss. Locally the layering is sharp, but in other places it becomes dif- fuse and gradational. The gradation of dark-colored amphibolite to light-colored gneiss commonly takes place through biotite amphibolite, epidote-plagio- clase-biotite schist, and epidote-biotite-plagioclase porphyroclast schist and gneiss. In many outcrops the more feldspathic layers contain larger feldspar and hornblende grains than do the amphibolitic lay- ers. Other outcrops show a patchy and irregular dis- tribution of light-colored rock rich in quartz and feld- spar in darker rock more rich in biotite and horn- blende. In some of the more granitic rocks the mafic minerals occur in irregular clots. Feldspar-rich rock BLUE RIDGE THRUST SHEET 21 also occurs in lenses, stringers, and crosscutting veinlets. The unit contains bodies of nonlayered granitic rocks as much as several tens of feet thick. Contacts of these bodies are seldom exposed, but many are apparently gradational with the less granitic country rock. Light-greenish-gray granofels forms layers rang- ing from less than an inch to several feet in thick- ness. The granofels is composed of various propor- tions of epidote, quartz, and, locally, hornblende. Rare layers of biotite-muscovite schist and gneiss with or without garnet were found in the unit. Gran- ofels layers are well exposed in area C-4 (pl. 1) near Gooseneck Branch above U.S. Highway 19E. Considerable vein quartz and pegmatite float is found in the outcrop area of the mixed rock, and a few pegmatites crop out. Several of those pegmatites on Bellvue Mountain (area C-4, pl. 1) have been prospected and mined for mica and feldspar. All the mixed rocks are of medium metamorphic grade and are composed principally of quartz, calcic oligoclase, biotite, and hornblende. The biotite is generally synkinematic, but crystal- lization locally outlasted deformation. However, the larger feldspar or hornblende crystals seem to be porphyroclastic rather than porphyroblastic (fig. 10A and B). Some finer grained, strongly sheared rocks are apparently mesozonal blastomylonites (fig. 10C and E). An unusual but interesting rock type con- tains round feldspar and hornblende grains as much as 2 ecm in diameter in a fine-grained matrix of bio- tite, quartz, and plagioclase (fig. 10C). It resembles a clastic rock in outcrop and hand specimen, but none of the "clastic" grains are quartz. The most plausible interpretation of the rock is that it is a porphyro- clastic blastomylonite. Other rocks have mortar texture, bent feldspar and biotite, and strained quartz (fig. 10D). In some, oligoclase is altered to albite and epidote. These ef- fects are found locally throughout the map unit. Green and slightly bluish-green hornblende and dark-brown biotite are characteristic of the unit. Garnet has a light-red tinge. Epidote is commonly zoned; cores are allanitic or calcic, and rims are somewhat more iron rich. In most of the more grani- tic rocks, the potassic feldspar is fine-grained bleb- by-textured perthite. In a few granitic rocks, the po- tassic feldspar is nonperthitic microcline. Potassic feldspar-bearing rocks contain small amounts of myrmekite. One quartz monzonite contains pale- green diopsidic augite. Sphene, apatite, and magne- tite are widespread accessory minerals. Rutile, zircon, pyrite, and ilmenite also occur. The mixed rocks probably originated through in- cipient and local feldspathization of rocks similar to the adjacent amphibolite. Subsequent medium-grade metamorphism has destroyed textures attributable to replacement and converted porphyroblasts to por- phyroclasts. Strongly developed layering may repre- sent either sheared-out migmatitic layering or modi- fied bedding. The most strikingly layered rocks are the most sheared. Less sheared rocks are generally more granitic and have a migmatitic aspect. This indicates that most, if not all, of the layering has been produced by shearing of migmatitic layering and is not relict bedding. The more sheared and lay- ered parts resemble the more strongly layered parts of the adjacent Cranberry Gneiss, and the more granitic and less strongly layered parts resemble the more granitic parts of the Cranberry. The mixed rocks seem to be a migmatitic gradation zone be- tween Cranberry Gneiss and schist, gneiss, and am- phibolite, all of which were subsequently metamor- phosed. LAYERED GNEISS SOUTHEAST OF THE GRANDFATHER MOUNTAIN WINDOW The Blue Ridge thrust sheet southeast of the Grandfather Mountain window is largely composed of interlayered mica gneiss, mica schist, plagioclase porphyroclast gneiss, and granitic rock. These rocks resemble the rocks of the mixed unit west and north of the window, but the unit southeast of the window contains only minor amounts of amphibolite and hornblende gneiss. On its southeastern side the layered gneiss is in sharp contact with blastomylonite and phyllonitic schist and gneiss in the Brevard fault zone. This contact is exposed in a cut along the Clinchfield Railroad near Hankins (area C-10, pl. 1). Contacts between the layered gneiss and bodies of mica schist and gneiss in the Blue Ridge thrust sheet southeast of the Grandfather Mountain window are gradational. An impression of the relationships can be obtained in the gullies on the north side of the valley of Licklog Branch (area J-5, pl. 1). There, large amounts of pegmatitic and granitic material in both units occur near the contact. The rocks of the layered gneiss unit are musco- vite-biotite-plagioclase-quartz gneiss and schist, pla- gioclase porphyroclast gneiss, blastomylonitic quartz dioritic gneiss, garnet-biotite schist, amphibolite, and pegmatite. Layers ranging from fractions of an inch to several inches in thickness are well developed. GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE 22 BLUE RIDGE THRUST SHEET 23 FIGURE 10.-Photomicrographs of polymetamorphic mixed unit rocks. A, Hornblende porphyroclast gneiss from roadcut along North Toe River road on sharp curve west of Row Branch, Linville quadrangle (area C-4, pl. 1). Porphyro- clasts of hornblende and quartz in a coarsely recrystallized groundmass of oligoclase, biotite, quartz, and epidote. Field relations suggest that this was a migmatitized amphibolite. Present texture is probably due to remetamorphism during the Paleozoic. B, Blastomylonitic biotite-hornblende gneiss from 4,000-foot altitude on north side of Bellvue Moun- tain, Linville quadrangle (area C-4, pl. 1). Porphyroclasts or hornblende and plagioclase in a matrix of recrystallized oligoclase, biotite, quartz, epidote, and hornblende. Out- crop contains gradations between this rock and amphibolite. C, Porphyroclastic blastomylonite of medium grade from east side of North Toe River south of Minneapolis, Linville quadrangle (area C-4, pl. 1). Porphyroclasts of hornblende and plagioclase in a groundmass of recrystallized oligoclase, quartz, biotite, and epidote. D. Biotite-feldspar-quartz gneiss from east side of the top of Spanish Oak Mountain, Linville quadrangle (area C-4, pl. 1). Grains of strained quartz, perthite with very fine grained blebby texture, and oligoclase surrounded by mortar and recrystallized mortar consisting of quartz, biotite, and feldspar. In the outcrop, layers and lenses of felsic and mafic composition have a patchy and irregular pattern. Section cut perpendicularly to mineral lineation. E, Blastomylonite from south ridge of Little Haw Mountain, Linville quadrangle (area C-4, pl. 1). Plagioclase is sodic andesine. Granitic and pegmatitic material occurs in lenses and pods and locally in layers as much as 15 feet thick. All gradations between granitic rocks and peg- matites occur. Thin granitic layers grade into feld- spar porphyroclast gneiss or schist. The coarser grained pegmatites have a grain size of 1 to 2 inches; they commonly contain deformed muscovite books. Accessory garnet is found in the granitic rocks. Pods and lenses of amphibolite a few feet in diameter and layers 1 inch to 10 feet thick occur in most outcrops, but nowhere does amphibolite com- FIGURE 11.-Photomicrograph of biotite-muscovite-plagioclase porphyroclast quartz schist interlayered with gneiss south- east of the Grandfather Mountain window. Specimen from 1,540-foot altitude in Licklog Branch, Blowing Rock quad- rangle (area J-5, pl. 1). Mica gneiss, amphibolite, and pegmatite are interlayered in outcrop. Porphyroclasts of sodic oligoclase and muscovite in matrix of biotite, musco- vite, sodic oligoclase, and quartz. Some zones of mortar. pose an important part of the unit. One amphibolite pod was found inside a granite-pegmatite layer. Most of the ricks have microtectures indicative of cataclasis and recrystallization (fig. 11). Porphyro- clasts of feldspar and muscovite occur in a matrix of recrystallized quartz, muscovite, brown biotite, and plagioclase; the matrix generally has a grain size of 0.05 to 0.2 mm but locally is as coarse as 0.5 mm. A few specimens contain scattered porphyroclasts of biotite that average about 3 mm but that locally are as much as 7 mm in diameter. The amphibolites gen- erally do not have polymetamorphic textures. Most of the feldspar porphyroclasts are plagio- clase, which is locally bent and in some rocks is partly or wholly saussuritized. Locally, the porphyroclasts are saussuritized, whereas in the groundmass, pla- gioclase is clear. Twinning in the plagioclase is poorly developed in some rocks, but albite and peri- cline twinning are found in others. A few rocks con- tain a few porphyroclasts of microcline or perthitic microcline. Occasional layers of quartz monzonitic composition contain numerous porphyroclasts of mi- crocline with coarse-textured string perthite. Muscovite porphyroclasts typically are wedge or lens shaped. They are commonly bent and in- many places partly or wholly granulated and recrystallized into aggregates of finer grained muscovite. The matrix consists of mosaic-textured quartz and feldspar and lepidoblastic muscovite and brown bio- tite. Quartz is locally strained. The new micas are synkinematic, and in some rocks crystallization lasted longer than movement. Epidote makes up as much as 12 percent of the martix in some layers. Generally it is the same size as the other matrix minerals but locally is as much as 1.5 mm long. Garnet occurs as anhedral to subhedral grains, in 24 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE places partly altered to biotite, sericite, and chlorite, and as irregular sieve-textured grains containing in- clusions of quartz, plagioclase, biotite, and opaque minerals. It is not clear whether the garnets are of the same age as the large mica and feldspar porphy- roclasts or whether they are porphyroblasts. The fact that they are about the same size in the sheared rocks, as in rocks lacking cataclastic features, sug- gests that they might be inherited from the earlier stage in the evolution of the rock. If so, they must have been nearly in equilibrium during the later shearing and recrystallization. Many of the rocks contain minor amounts of chlor- Biotite ite largely derived from garnet and biotite. It gener- ally is a green FeMg variety, although some MgFe chlorite also occurs. Relict kyanite was found in a sericite aggregate in one rock, and similar sericite aggregates lacking relicts were found in a few other rocks. Other common accessory minerals are magne- tite, ilmenite, zircon, sphene, and apatite. Tourma- line and allanite occur rarely. A pod of cale-silicate rock sampled contains equal amounts of epidote and quartz and minor amounts of sphene, actinolite, and garnet. The nonmigmatitic types (fig. 12) of layered gneiss contain less muscovite than most rocks of the Biotite Plagioclase Muscovite EXPLANATION *3 Analyzed specimen Biotite FIGURE 12.-Proportions of quartz, plagioclase, muscovite, and biotite in nonmigmatitic layered gneiss in the Blue Ridge thrust sheet southeast of the Granfather Mountain window. Based on counts of 50 random grains in each of 18 thin sections. Contours 6 and 11 percent. Number of analyzed specimen refers to analysis in table 6. BLUE RIDGE THRUST SHEET 25 mica schist and gneiss unit (fig. 7), and their total mica content is somewhat less. However, the relative proportions of quartz and feldspar are similar. Granitic parts of the layered gneiss (fig. 13) contain less mica and more plagioclase than the ordinary gneiss. Most of them are leucocratic quartz diorite, but a few layers and pods of granodiorite and quartz monzonite occur. Chemical analyses of nonmigmatitiec samples (table 6, analyses 1 and 3) suggest that the original rocks ranged from impure quartzites to graywackes. A typical migmatitic rock (table 6, analysis 2) con- tains more Na,0 and less Fe,0;, than nonmigmatitic Biotite Quartz gneiss with comparable SiO; content (table 6, analy- sis 1). The layered gneiss southeast of the Grandfather Mountain window was apparently derived from less argillaceous rocks than the mica schist and gneiss to the west, northwest, and north. In addition, it has been subjected to plutonic metamorphism. Feldspar porphyroblasts (mostly plagioclase) are developed in the gneiss, and this porphyroblast gneiss grades into layers and lenses of leucocratic diorite, granodiorite, or quartz monzonite. These granitic rocks are locally coarse grained enough to be considered pegmatites. The layered gneiss resembles some parts of the Biotite Plagioclase Muscovite EXPLANATION *2 Analyzed specimen Biotite FiqgUrE 13.-Proportions of quartz, plagioclase, muscovite, and biotite in migmatitic layered gneiss in the Blue Ridge thrust sheet southeast of the Grandfather Mountain window. Based on counts of 50 random grains in each of 21 thin sections. Contours 5, 10, and 20 percent. Number of analyzed specimen refers to analysis in table 6. 26 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE TABLE 6.-Chemical analyses, modes, and norms of layered gneiss in the Blue Ridge thrust sheet southeast of the Grandfather Mountain window [Determined by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey, 1962. Modes, by point counts of 600 grains in each thin section; P, present but not intersected in counting. Major oxides and CIPW norms given in weight percent; modes, in volume percent] 1 2 3 Field NO% : 222m 124 keene ee's o a ale 19-1854 19-1855 23-2590 Laboratory 160166 160167 160168 Major oxides S10; -e .cn ae. 64.5 65.0 80.5 AsO; sete seed 15.3 15.2 8.T eee 8.5 1.5 1.83 | y O o area S eve poot 2.5 8.6 1.8 MgO evel ine ec s 1.6 1:8 1.1 of {re ost rio y 8.1 8.2 . 16 Ee Pe ara eu lec caes 3.4 5.2 1.5 OP n ea ie ne an uue onle a o wie wie 8.0 1.2 1.3 OYE ener eca ells 1.7 1.6 1:6 HsO ese eee PrP ae ia nene n oe 26 14 . 22 Ojai r. ei ec es an .92 .96 55 P Operan. ns ere e ae wal ee ene n en .87 .25 06 llc .06 .06 02 COFS e ede a <.05 .20 <.05 100 100 99 Modes Q 222 s. c eee ee dea. 29 28 70 31 52 12 Potassic feldspar............. $ tao ear eel Pun a . Biotite. re eca ls 21 T 12 Muscovite 2:2. 4 .8 1.1 9 5 2.8 Chlorites ._ cL L000 OLL BUL ire 5 1.2 1.2 sc. <4. AD sil een cns CAlCILEE : zee c e, EEV, Opaque mineral........_._.... ; maen e ot ta 1.2 se ner nellie etl 22 P P P Garnet 2 ccc. .l n i LEL e c. P CIPW norms (} s s se her eee aa ee eee oan 25.15 20.12 63.09 Ces Ra n ie wann n e eee 1.71 . 60 3.59 OF. 2 2 ooo tk nen oa cee whe neces 17.12 7.09 7.68 ADs slo A rani licen. 28.15 43.98 12.69 AM ee EE Pe ee ee c auc ana ake 12.96 12.97 3.38 En e sere eli. cern evs 3.98 4.48 2.74 sr e sou led nt 29 3.90 1.36 \ (rara ina o s 's 5.08 2.18 1.88 T ee n Rie ninae 1.75 1.82 1.04 AD. reel .88 .59 14 Op- pee dr ece c (40 /... 1. Strongly foliated and thinly layered fine-grained biotite gneiss. I‘orphyro- clasts of plagioclase (altered to albite), as much as 1.5 mm; perthitic microcline, 1 mm; and muscovite, 0.5 mm in grain size in a ground- mass of recrystallized quartz, albite, microcline, brown biotite, musco- vite, and epidote. Groundmass has a grain size of 0.05 to 0.3 mm. From outerop on south side of Shooks Creek 0.3 mile west of its junction with the Linville River (area D-9, pl. 1). 2. Fine-grained very finely layered porphyroclastic biotite-muscovite gneiss. Porphyroclasts of plagioclase (altered to albite) as much as 3 mm in diameter and lenses of very strongly strained and incipiently brecciated quartz in a matrix of quartz, albite, brown biotite, epidote, and FeMg chlorite derived from alteration of biotite. Grains in matrix range from 0.05 to 0.3 mm in diameter. Zones of mortar are abundant. From east side of Linville River 0.4 mile N. 25° W. of bridge on North Carolina Highway 126 (area D-9, pl. 1). 8. Medium- to fine-grained porphyroclastic biotite gneiss. Scattered porphy- roclasts of partly saussuritized plagioclase (about Anz) and muscovite as much as 3 mm in diameter in a groundmass of quartz, albite, brown biotite, epidote, and chlorite derived from alteration of garnet and bio- tite. From roadcut on west side of county road 0.75 miles S. 51° W. of road junction by South Mountain Institute (area D-9, pl. 1). Cranberry Gneiss described below but contains fewer granitic layers than typical Cranberry. The layered gneiss grades into biotite-muscovite schist and gneiss and is therefore essentially the same age. The plutonic features probably date from the first metamorphism 1,000 to 1,100 m.y. ago. Later metamorphism destroyed or modified plutonic textures and converted porphyroblasts to porphyro- clasts. CRANBERRY GNEISS The Cranberry Gneiss (Keith, 1903; Bryant, 1962) is a metamorphosed plutonic complex that composes a large part of the Blue Ridge thrust sheet in the Grandfather Mountain area and in areas to the northeast and southwest. Although the Cran- berry is homogeneous in gross aspect, it is heteroge- neous in detail. Keith's (1903) description of the for- mation in the Cranberry quadrangle is quite accu- rate, except that he termed it granite rather than gneiss. He recognized the cataclastic textures and structures, the presence of nongranitic layers, and the general increase in the degree of shearing and recrystallization from northwest to southeast. Eckel- mann and Kulp (1956) showed that the rocks that Keith (1905) mapped as Henderson Gneiss north- west of the Brevard fault in the Mount Mitchell quardrangle are traceable into the Cranberry Gneiss. Our work confirms their conclusion (Reed, 1964b) but shows that the rocks mapped as Henderson northwest of the Brevard fault are not similar to Henderson Gneiss mapped by Keith in its type area southeast of the fault (Reed and others, 1961). North of the Grandfather Mountain area, Hamilton (1960) attempted to subdivide the Cranberry into informal map units, but we were unable to distin- guish most of the lithologic varieties on the map ei- ther because of the small size of the individual rock bodies or because of their indistinct boundaries. CONTACT RELATIONS The Cranberry Gneiss lies above the rocks of the Grandfather Mountain window along the Linville Falls fault on the west and north sides of the win- dow. The contact with tectonically overlying amphi- bolite is gradational through the unit of mixed rocks already described and is nowhere completely ex- posed. The sharp contact between Cranberry Gneiss and the overlying biotite-muscovite schist is well exposed along the Blue Ridge Parkway on the east side of Humpback Mountain (area C-6, pl. 1). There, the BLUE RIDGE THRUST SHEET 27 Cranberry Gneiss consists of interlayered fine- grained dark-gray layered biotite schist, light-gray quartz diorite gneiss, and a few layers of amphibol- ite and cale-silicate granofels. Garnet amphibolite and biotite gneiss of the Cranberry are interlayered in 0.5-to 1.5-foot-thick layers through a zone 50 feet thick below the contact. The biotite-muscovite schist contains many sheared and foliated muscovite-bear- ing, plagioclase-rich pegmatites, some of which have been granulated and drawn out into trains of porphy- roclasts. Discordance between the amphibolite and the underlying Cranberry Gneiss indicates at least local movement along the contact. Amphibolite layers a few tens of feet thick and pods and stringers of pegmatites are widespread along the contact between the Cranberry Gneiss and biotite-muscovite schist in the Linville Falls quad- rangle. The pegmatites are locally as much as a hundred feet thick. Near this contact the Cranberry Gneiss locally contains pods of amphibolite and bio- tite gneiss in which layering is at a large angle to foliation and layering in the surrounding gneiss (fig. 14). Amphibolite layers are also found along the contact east of the belt of mixed rocks in area J-2, plate 1. MEGASCOPIC DESCRIPTION The Cranberry Gneiss consists predominantly of gray to light-gray and locally light-pink or pinkish- green layered and nonlayered granitic gneiss, but contains many dark-green to black nongranitic lay- FIGURE 14.-Tectonic lenses of fine-grained biotite gneiss (a) in biotite schist (b) and biotite amphibole gneiss (c). Porphyroclasts of feldspar and small knots of pegmatite are scattered in the enclosing schist. Cranberry Gneiss, just below contact with coarse-grained mica schist and gneiss, 0.9 mile N. 55° W. of Linville Caverns, (area C-6, pI.:1). ers and lenses and scattered bodies of nonlayered hornblendic rock. The granitic parts range in compo- sition from diorite to granite and have an average composition of quartz monzonite or granodiorite. Nongranitic layers are amphibolite, hornblende gneiss, epidote-biotite schist, and epidote-biotite-pla- gioclase schist and gneiss. Thin layers of cale-silicate granofels and quartzite occur locally. In many places, layering consists of white or pink gneissic quartz monzonite or granite intercalated with gray or green-gray, more biotitic layers of granodiorite or quartz diorite gneiss. Layering is most conspicuous near the contact with the tectonically overlying amphibolite and mica schist. It is least conspicuous northwest of the Grandfather Mountain window in areas C-2 and D-2 (pl. 1). The layering is on all scales from a fraction of an inch to tens of feet (fig. 15). In some exceptionally good exposures, individual layers can be traced continuously for several hundred feet. Lay- ered gneiss locally grades into nonlayered gneiss, and some outcrops have a patchy pattern of granitic and nongranitic material, the latter in rod-shaped bodies elongated parallel to the lineation. Cataclastic foliation formed by planar orientation of micaceous minerals is generally well developed in the Cranberry Gneiss. Lineation is also conspicuous ; it is formed by strung-out aggregates of biotite or grains of feldspar or quartz and alined amphibole needles and biotite flakes. Foliation is parallel with layering in most places, but it cuts the layering on the noses of folds. Obvious cataclastic foliation is lacking only in the vicinity of Little Hump Mountain (area C-3, pl. 1), where the Cranberry Gneiss is of medium metamorphic grade. The effects of the shearing that produced the folia- tion are variable; the rocks range from phyllonite and blastomylonite to slightly sheared mortar gneiss. Rocks containing abundant potassic feldspar or hornblende are generally less sheared than plagio- clase-rich rocks. Much of the Cranberry is blastomy- lonitic gneiss containing conspicuous porphyroclasts of white or pink potassic feldspar in a greenish-gray matrix of quartz, feldspar, and mica. Quartz, plagio- clase, or biotite also form porphyroclasts in some of the blastomylonitic gneisses. Some less sheared but retrogressively altered granodioritic or quartz mon- zonitic rocks have a distinctive mottled appearance caused by pink potassic feldspar and green plagio- clase. These rocks, which resemble unakite, are most widespread north of Dark Ridge (areas C-2 and D-2, pl. 1). 28 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE FIGURE 15.-Conspicuously layered Cranberry Gneiss. Saprolite exposure in roadcut near Hines Gap about 0.5 mile west of Boone City limit, Blowing Rock quadrangle (area G-2, pl. 1). Overlain by typical colluvium. Blastomylonites are fine grained, generally light- gray rocks and resemble felsic volcanic rocks, espe- cially where they contain scattered feldspar porphy- roclasts. Some blastomylonites have been enriched in magnetite but not as much as the phyllonites. Blasto- mylonites locally grade into unrecrystallized mylon- ites and breccias. Silvery-gray, light-green, dark-green, dark-gray, or black phyllonites form distinct zones a few inches to 20 feet thick in many places. In other places, phyl- lonite grades into the country rock along or across strike. In the phyllonites, porphyroclasts of quartz 1 to 10 mm in diameter are common, and potassic feld- spar porphyroclasts are scarce. Segregation lenses and stringers of quartz occur. In many places, the phyllonites have been enriched in magnetite or hem- atite and have been prospected for iron; a few zones north of Dark Ridge have been prospected for graphite. A phyllonite zone on Big Ridge (area D-2, pl. 1) contains fragments of granitic gneiss as much as 1.5 feet long in a matrix of phyllonite and could be called a pseudoconglomerate. Dioritic and quartz dioritic rocks in the Cranberry Gneiss commonly have a distinctive gray to green- ish-gray color, and some contain amphibole rather than biotite. They contain bodies of pegmatite as much as 3 feet thick. The pegmatite is bluish gray to white and consists of biotite, plagioclase, and quartz or quartz and potassic feldspar. The Cranberry Gneiss contains white to pink peg- matite lenses, stringers, pods, and a very few dikes ranging from an inch to several feet thick. The peg- miatites are sheared, but they are much more weakly foliated than the country rock because of their coarse grain size and the abundance of potassic feld- spar and quartz. In the more mafic rocks, the pegma- tites contain more plagioclase. Quartz veinlets are widespread; locally, fluorite and chlorite accompany the quartz. Some veinlets formed before shearing ceased, but others appar- ently postdate shearing. Epidote forms segregation lenses and veinlets in some of the plagioclase-rich rocks. Seams of magnetite and hematite are found locally. Coarse-grained granitic gneiss occurs in layers and lenses a few inches to at least a hundred feet thick. Some gneiss contains potassic feldspar por- phyroclasts as much as 1 em in diameter. These po- tassic varieties resemble the Beech Granite. A few bodies of nonlayered granitic gneiss were distin- guished on the maps and described below ; other sim- ilar bodies may be present, but they are too small or indefinitely defined to be mapped on the seale of pl. 1. Some of the bodies mapped separately in the Cran- berry Gneiss are alkalic granites. Smaller bodies of similar rock are widespread but are not distin- guished on the map. The field relations of many of the alkalic rocks are obscure. Most of the alkalic BLUE RIDGE THRUST SHEET 29 rocks occur in a zone within a mile of the contact between the Cranberry Gneiss and the mixed rocks. 'The most widespread lithology among the non- granitic layers is fine-grained, strongly lineated bio- tite schist in which the micas are about 0.1 mm in diameter. The amphibolite layers in the Cranberry Gneiss generally are sheared, and most of them con- tain biotite, although biotite is lacking in amphibol- ite layers where the Cranberry Gneiss is of medium metamorphic grade, as on Little Hump Mountain (area C-8, pl. 1). In a few places, the Cranberry contains small bod- ies of nonlayered hornblende-rich rock, some of which were mapped as Linville Metadiabase by Kieth (1903). The best exposure is at 2,900-foot altitude on the Dark Ridge Creek road (near the boundary of areas C-2 and D-2, pl. 1). There the rock consists of various proportions of stubby randomly oriented prisms of hornblende as much as 15 mm long sur- rounded and cut by quartz-plagioclase veinlets hav- ing both sharp and gradational contacts. Contacts of the rock body are gradational and difficult to locate in many places, although locally they are sharp and sheared. The body of hornblendic rock is about 2,000 feet long and 500 feet wide. Smaller bodies of similar hornblende-rich rock were found along a fault zone on the west of the north fork of Left Prong (area C-4, pl. 1) and between the Linville Falls fault and the Beech Granite (area E-3, pl. 1). A body of somewhat less mafic rock is exposed along the Clingman Mine Branch road on Ward's Mountain (area D-2, pl. 1). It also has indistinct contacts. A thin mafic layer in the Cranberry Gneiss is mapped near Blevens Creek (area D-3, pl. 1). Some metamorphosed mafic intru- sive rocks are younger than the plutonic metamorph- ism, but most are of the plutonic generation or older. On Little Hump Mountain (area C-3, pl. 1) the Cranberry Gneiss is migmatitic. Hornblende, biotite, and garnet occur in clots, stringers, layers, and ir- regular lenses in granitic rock. Locally, layers of granitic pegmatite, with a grain size as much as 1 inch, grade to more mafic rock types. Gradations from amphibolite to granitic rock through biotite- plagioclase porphyroblast gneiss occur. PETROGRAPHY Typically, the Cranberry Gneiss is an inequigran- ular blastomylonitic gneiss composed of grains of po- tassic feldspar, plagioclase, quartz, biotite, and am- phibole that are a few millimeters in diameter in a matrix of recrystallized quartz, plagioclase, sericite, biotite, potassic feldspar, and amphibole, the matrix having a grain size of about 0.1 mm (fig. 16). Blastomylonites have a mosaic texture and a grain size of about 0.1 mm. Phyllonites are very fine grained, and those lacking porphyroclasts cannot be distinguished from phyllites except by their grada- tional relations with recognizable cataclastic rocks. All gradations between slightly sheared and altered granitic rock through recrystallized mortar gneiss, blastomylonitic gneiss, and phyllonitic gneiss to blas- tomylonite and phyllonite are found in the Cran- berry Gneiss (figs. 16, 17, 18). In a few places, the cataclastic rocks have not been much recrystallized and are mylonitic gneisses and mylonites. The non- sheared rocks generally have a granoblastic texture (fig. 17A and B). Potassic feldspar is the most conspicuous porphy- roclastic mineral (fig. 16A, B, C). It is generally microcline, but in the more potassic rocks it is patch or vein perthite (fig. 17D). The porphyroclasts are commonly bent and broken and healed by recrystal- lized quartz and albite (figs. 16D, 17D). Fragments commonly are strung out into pod-shaped aggregates parallel with the northwest-trending lineation. The potassic feldspar porphyroclasts contain inclusions of quartz, plagioclase, and biotite. Potassic feldspar in the recrystallized matrix is microcline. Plagioclase is practically everywhere altered (fig. 17C) and in many places is completely converted to fine-grained albite and epidote. Porphyroclasts are generally saussuritized and show strain shadows and bent twin lamellae. Most of the altered and recrystal- lized plagioclase is An, to An,. Unaltered plagio- clase, which is rarely found, is oligoclase. Locally, especially near the contacts of the Cranberry Gneiss, the plagioclase is only partly altered and has a com- position of about An,. Quartz typically occurs as mosaic-textured aggre- gates in the groundmass of the gneisses and in places forms porphyroclasts that are very strongly strained and partly brecciated (figs. 17C ; 18C, D). In the least sheared rocks, the quartz shows more cataclas- tic effects than the accompanying feldspar. However, the predominant porphyroclastic mineral in phylloni- tic gneisses and phyllonites is quartz (fig. 18C) ; thus, quartz appears to be more resistant when sub- jected to phyllonite-forming conditions than to when subjected to cataclasis not accompanied by chemical breakdown of feldspars, as in the formation of the blastomylonitic gneisses. Biotite is found as 0.1-mm grains in the matrix and less commonly as party altered porphyroclasts. It is generally brown or greenish brown and tends to be concentrated in layers and lenses. Locally, the new GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE 30 BLUE RIDGE THRUST SHEET FIGURE 16.-Photomicrographs of typical Cranberry Gneiss. A, Blastomylonitic sericite quartz monzonite gneiss from roadcut along Elk Creek just east of Triplett (area J-2, pl. 1). Porphyroclasts of microcline and saussuritized plagioclase in a matrix of recrystallized quartz, iron-rich muscovitic mica, albite, and microcline. Section cut parallel to mineral lineation. B, Blastomylonitic granite gneiss from ridge southwest of Smoky Gap (area C-4, pl. 1). Broken porphyroclasts of perthite in a matrix of recrys- tallized quartz. C, Blastomylonitic epidote-muscovite-biotite quartz monzonite gneiss from 3,360-foot altitude in valley south of Cranberry (area C-3, pl. 1). Rock fragments, rich in somewhat perthitic microcline, and porphyroclasts of microcline, and saussuritized plagioclase in a matrix of 81 recrystallized quartz, albite-oligoclase, muscovite, biotite, and epidote. Section cut perpendicular to lineation. D, Blastomylonitic granite gneiss from hill south of Cranberry High School (area C-3, pl. 1). Porphyroclasts of microcline, a few small porphyroclasts of muscovite, and saussuritized plagioclase in a matrix of recrystallized quartz, sericite, albite, and epidote. Section cut parallel to mineral linea- tion. E, Biotite-chlorite-epidote-hornblende porphyroclast amphibolite from north ridge of mountain southeast of Cranberry (area C-3, pl. 1). From a layer in more felsic rock. Lenticular porphyroclasts of hornblende in a matrix of partly recrystallized hornblende, sodic oligoclase, clino- zoisite, biotite, and chlorite. Section cut parallel to min- eral lineation. biotite reaches 1.5 mm in length. It is generally syn- kinematic, but locally crystallization lasted longer than deformation. Porphyroclasts are mostly reddish brown and sagenitic. They are commonly bent, and in some rocks they are elliptical in sections perpen- dicular to the foliation. In places they are partly al- tered to a green or pale-brown biotite or to chlorite. Porphyroclasts of biotite are most widespread in the quartz dioritic gneisses. Sericite forms stringers and lenses, and bent por- phyroclasts of muscovite are found in a few rocks. Some of the sericite is light green and probably iron rich. Sericite in two samples of phyllonite derived from rocks containing considerable plagioclase was examined by E. J. Young of the U.S. Geological Sur- vey by X-ray diffractometer for the presence of par- agonite, but none was detected. Most of the sericite is apparently muscovite. Amphibole porphyroclasts (fig. 166) have green to brown cores and blue-green or very light green rims that have the same extinction as the cores. The porphyroclasts are commonly lens shaped, and in the more amphibolite-rich rock they are locally sur- rounded by needles of recrystallized amphibole. Some of the alkalic granitic gneisses contain dark- green hornblende in the recrystallized matrix. In some rocks its extinction angle is as much as 34°, indicating that it may be kataphorite. Crossite was found in one rock which may be a small intrusive. Chlorite is predominantly the FeMg type, although the MgFe type also occurs. It generally forms pseu- domorphs after biotite. In a few of the more mafic rocks, chlorite apparently formed contempora- neously with biotite in the martix. Some rocks lack biotite, probably because of a deficiency of potas- sium; in these rocks, chlorite is the principal mafic mineral and is locally accompanied by recrystallized amphibole. Garnet occurs both as euhedral crystals and as trains of fragments partly altered to chlorite, seri- cite, and biotite. It contains inclusions of quartz, epi- dote, opaque minerals, and sphene. Some garnet is tannish pink, but some blastomylonites contain a light-yellowish-brown garnet which shows anoma- lous birefringence. Acmite occurs in equilbrium with the groundmass of recrystallized quartz, feldspar, and biotite in a layer of alkalic granite gneiss in more plagioclase- rich gneiss on the ridge west of Mast Gap and north of the Watauga River (area F-2, pl. 1). In another somewhat similar alkalic rock from near the head of the southwest fork of Cooper Brach (area C-3, pl. 1), partly altered pyroxene is aegirine-bearing au- gite with Z Ac of 58°. Relict grains of light-green monoclinic pyroxene were found in one rock. Light tan monoclinic pyroxene occurs in the hornblende- rich rock on the Dark Ridge Creek road. Fluorite occurs in the intergranular area and in veinlets cutting feldspar. It tends to be most abun- dant in potassic rocks and near zones of phyllonite. Green tourmaline is found in some phyllonites and granite gneisses. Carbonate, rutile, and stilpnome- lane are other accessory minerals. Opaque minerals are widespread and are espe- cially a bundant in phyllonite and blastomylonite. Magnetite, commonly in euhedral or subhedral grains, predominates. Other minerals in decreasing order of frequency are ilmenite, pyrrhotite, pyrite, hematite, graphite, and chalcopyrite. Sphene occurs as round to euhedral grains and commonly rims opaque minerals. Zircon is generally in rounded grains, but is occasionally subhedral to euhedral. Some prismatic grains have rounded ter- minations. A few crystals are zoned and have over- growths. Apatite ranges from round to euhedral. Ep- idote forms equant subhedral grains or very tiny 82 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE FIGURE 17.-Photomicrographs of less sheared Cranberry Gneiss. A, Biotite quartz diorite from boudin 2 feet thick in porphyroclastic biotite and amphibole gneiss. From road- cut on Blue Ridge Parkway on northeast side of Humpback Mountain (area C-6, pl. 1). Plagioclase, altered to albite. Quartz and plagioclase, somewhat bent. Coarse-grained granoblastic texture. B, Garnet-bearing quartz monzonite from roadcut in Dark Ridge Creek road near Tennessee- North Carolina State line (area C-2, pl. 1). Strained quartz and microcline. Oligoclase, locally altered to albite. grains still included in plagioclase. The iron content and zoning are variable. Allanite is rimmed by epi- dote in some rocks and is locally metamict. The Cranberry Gneiss on Little Hump Mountain (area C-3, pl. 1) ranges in texture from granoblastic (fig. 19) to partly cataclastic and has apparently Large garnet in upper right corner. Granoblastic-textured rock has been squeezed and slightly altered. C, Cataclastic biotite quartz diorite gneiss from ridge south of Double Knobs (area D-3, pl. 1). Strained quartz, saussuritized plagioclase, and bent biotite. D, Partly recrystallized mor- tar granite gneiss from just east of fault on hill north of White Oak Creek (area C-3, pl. 1). Vein perthite is broken and healed by fine-grained mosaic-textured quartz, epidote, magnetite, and sphene. Matrix also contains perthite frag- ments and some recrystallized albite. been recrystallized under medium-grade conditions. Perthitic potassic feldspar in this rock contains very thin elongate blebs of albite and is similar to the potassic feldspar in rocks of the mixed unit. Potassic feldspar replaces plagioclase, and myrmekite is found in adjacent plagioclase grains. Plagioclase BLUE RIDGE THRUST SHEET 383 FIGURE 18.-Photomicrographs of blastomylonite and phyllon- ite in Cranberry Gneiss. A, Porphyroclastic blastomylonite from 3,390-foot altitude on south fork of Cooper Branch (area C-3, pl. 1). Scattered small porphyroclasts of micro- cline and biotite in a matrix of recrystallized quartz, sericite, albite, epidote, and biotite. Section cut nearly perpendicularly to mineral lineation. B, Porphyroclastic blastomylonite from roadcut on Blue Ridge Parkway west side of bridge across Linville River (area C-6, pl. 1). Large porphyroclast of saussuritized plagioclase (broken and healed by quartz), smaller porphyroclasts of saussuri- tized plagioclase and sagenitic biotite in a matrix of recrys- ranges from An,, to An;,. One rock (fig. 19) con- tains hypersthene and a small amount of light-green monoclinic pyroxene that is partly converted to green hornblende. The occurrence of hypersthene tallized quartz, albite, biotite, sericite, and epidote. Section cut parallel to mineral lineation. C, Chlorite-magnetite-seri- cite phyllonite containing quartz porphyroclasts from ridge northeast of head of Cooper Branch (area C-3, pl. 1). Probably derived from quartz diorite. D, Mylonitic gneiss from along the Stone Mountain fault at 3,740-foot altitude on south side of gully at head of Dark Ridge Creek (area D-2, pl. 1). Very elongate porphyroclasts of quartz and smaller porphyroclasts of biotite and hornblende in a mylonitic matrix of quartz, feldspar, sericite, and epidote. Section cut parallel to mineral lineation. suggests that at one stage in its history the Cran- berry Gneiss may have had charnocitic affinities. In the pyroxene-bearing rock, aggregates of biotite and hornblende surround garnet and magnetite grains, 34 FIGURE 19.-Photomicrograph of slightly sheared Cranberry Gneiss recrystallized at medium grade. Hypersthene and potassic feldspar-bearing biotite quartz diorite in amphi- bolitic migmatite from 4,740-foot altitude on east ridge of Hump Mountain (area C-3, pl. 1). Granoblastic texture. Clots of fine-grained secondary biotite and amphibole in many places surround larger grains of garnet or magne- tite. Amphibole rims pyroxene. Plagioclase is calcic oligoclase. and fine-grained late biotite is found in other rocks in the Hump Mountain area. One rock contains some skeletal porphyroblasts of sceapolite. COMPOSITION Table 7 shows the average composition of some rock types in the Cranberry Gneiss. The rather small GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE number of samples of quartz monzonite and grano- diorite is due to sampling bias. Nongranitic layers and granites were oversampled, and typical granitic gneiss was undersampled. Within each type of grani- tic rocks, the content of mafic minerals is quite vari- able, although the averages show the usual tendency for diorites to be more mafic than granites. Amphibolite and biotite schist were selected as typical end-member types of nongranitic layers in the Cranberry Gneiss. Most of the 56 samples of non- granitic layers that were petrographically studied fall between the two rock types. They generally con- tain more plagioclase than quartz, but gradations to more quartzitic layers do occur. Four samples con- tain more than 50 percent quartz accompained by various amounts of garnet, calcite, amphibole, pla- gioclase, and epidote. Rocks classified as blastomylonite were derived from the various types of granitic rock. Some may have been silicified. They locally contain as much as 10 percent magnetite. Part of the iron for the mag- netite may have been derived by destruction of bio- tite, which is lacking in these rocks, some iron may have been introduced. Similarly the phyllonites were derived from var- ious original rock types. In these rocks the feldspar has been hydrolized, and sericite and quartz have formed (Bryant, 1966). A significant increase in ser- icite is apparent in the average for this rock type in relation to the granitic rocks from which it was de- rived. The average does not show an increase in quartz content in relation to the granitic gneisses, although some samples contain as much as 50 per- cent quartz. The extra quartz generated by the reac- TABLE 7.-Average modes, in volume percent, of various rock types in the Cranberry Gneiss [Average of the number of 50-grain counts indicated in parentheses at head of column. P, present in half or more of samples. Difference between sum and 100 made up by accessory minerals] Rock type Mineral Granite Quartz Gran- Quartz Diorite Blasto- Phyllonite - Amphibolite Biotite monzonite odiorite diorite mylonite schist (27) (16) (19) (51) (8) (19) (27) (14) (8) QUSTIZ _-. ananas - 31 29 27 26 4 34 28 3 8 Potagsic feldspar__........___. 47 24 18 a E 21 T ANIL areas s 10 29 41 49 64 12 1 22 15 Didtite.2 22 anl ane uel 8 5 6 10 10 6 1 9 51 Muscovite and sericite_ ___.... 5 5 5 5 1 13 57 lD .cn 8 4 8 8 9 5 P 6 16 P 2 1 2 6 r - 52 6 Opaque minerals. __________._. P P r P 2 6 1 1 =e in ian P P PAs sili- P P ~s ier e ccae ries P} 0: £ P 1 P 3 2 APMLCEE L: Lele rere ck ags aah mage ain m P P P Pin via . .. pased . Chlorites. conn lIe eL oo nar ene alpaca nnn nle as es at s a 1 1 1 "...... cl dais cases s a a [ LLA s cans an seeder 10+ an onle ge Quartz and feldspar undifferentiated an mes 5 C." } FiOfite _L. LLE Lepere c arre ananas «Beware aln s a blepa bas sa o a aisle o L ain mer on ik a an 1 BLUE RIDGE THRUST SHEET 35 tion was in part deposited in quartz segregation len- ses, which were avoided during sampling of the phyl- lonites. Some quartz from the reaction may have con- tributed to silicification of the blastomylonite zones. Epidote minerals, which might be expected from al- teration of plagioclase, are rare. Opaque mineral content of the specimens studied reaches a maximum of 32 percent. Much of it is magnetite, but hematite, ilmenite, sphalerite, and pyrite also occur. Redistri- bution of elements apparently was widespread dur- ing the formation of the phyllonites. Mafic silicates, principally biotite, were destroyed and could have furnished a source for iron, which was locally con- centrated in the phyllonite zones. In some zones, the mica is green muscovite rich in ferric iron, which could have developed by a coupled reaction between oxidizing aqueous solutions and hydrolytic decompo- sition of feldspar. In relation to the granitic rock from which it was derived, the typical quartz-sericite phyllonite has lost calcium and sodium and gained potassium (Bryant, 1966). ORIGIN The Cranberry Gneiss is a plutonic complex of the catazone as described by Buddington (1959). He es- pecially pointed out the uncertainty of the origin and mode of emplacement of catazonal plutons (p. 714-715). All compositional gradations between amphibolite, epidote-biotite schist, and granite are found in the Cranberry Gneiss. In the main body of the Cran- berry Gneiss in the Grandfather Mountain area, shearing and partial recrystallization of low meta- morphic grade have obscured original textural rela- tions and have even modified some of the small-scale field relations indicative of origin. The widespread compositional layering, the occurrence of nongrani- tic layers in the gneiss, the erratic distribution of rock types, and the lack of well-defined contacts sug- gest that the Cranberry Gneiss is a complex of mig- matite possibly containing melted or homogenized parts and small intrusive bodies. Keith (1903) thought that some of the layers in the Cranberry Gneiss were inclusions in an intrusive mass and that some were the result of metamorphic differentiation, but stated that "the prevalent meta- morphism * * * and the heavy forest cover make it difficult to obtain precise evidence of eruptive con- tacts with adjoining formations." The possibility that some of the layers were pro- duced by metamorphic differentiation during the low-grade cataclastic metamorphism cannot be dis- missed. Indeed, more and less phyllonitic gneisses were produced during shearing, and some material was mobile at that time. However, the porphyro- clasts of different minerals occur in differing propor- tions in the various layers, which indicates that most of the layering is inherited form the original plutonic complex. Eckelmann and Kulp (1956) believed that the Cranberry Gneiss was a conformable sequence below the mica schist and amphibolite of the Spruce Pine district and that this underlying sequence had under- gone metamorphism and varying degrees of feld- spathization. They suggested that all the layering was inherited from sedimentary bedding. They ob- tained rounded zircons from the layered granitic rocks and euhedral ones from some of the massive coarse-grained nonlayered rocks. Their interpreta- tion of the origin of the Cranberry Gneiss differs from ours in that we would emphasize plutonic proc- esses, such as feldspathization, palingenesis, and ana- texis and deemphasize the stratigraphic relations. We believe that the coarse-grained quartz-feldspar layers were produced by lit-par-lit granitization of schist, gneiss, and amphibolite, perhaps similar to the overlying rocks, rather than formed from beds of coarse-grained rocks of granitic composition. Am- phibolite layers tended to be resistant and survived where granitization was rather weak. They were successively converted to epidote-biotite schist, epi- dote-biotite-plagioclase porphyroclast schist and gneiss, and dioritic and quartz dioritic layers with increasing feldspathization. At the same time that schist and gneiss layers were converted to rocks of granodioritic and quartz monzonitic composition, the layers were emphasized by shearing in many places. Locally, especially where granitic layers make up a small proportion of the rock or where the shearing was less intense, the pattern of granitic and non- granitic parts is more irregular and patchy. The bodies of nonlayered hornblende-rich rock with indistinct boundaries transected by quartz-feld- spar veinlets may have been derived from Pre- cambrian ultramafic bodies to which silica and alka- lies were added during the plutonic metamorphism. Amphibolite may form by reaction of serpentine with igneous gabbro and hornblende diorite (Cater and Wells, 1953, p. 102). Hornblendites, gabbro, and diorite in many respects similar to older mafic bodies in the Cranberry Gneiss have formed by granitiza- tion of ultramafic rocks in the Northern Cascade Mountains of Washington State (Crowder, 1959, p. 864, pl. 4). Some of the more uniform granitic rock may have been mobilized and intruded during a late stage of 36 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE the plutonic metamorphism. Certainly, the zircon data of Eckelmann and Kulp (1956, p. 314-315) sup- port this. Subsequent shearing and metamorphism during Paleozoic time has obscured many of the orig- inal relations. The contact between the Cranberry Gneiss and the overlying mica schist is discordant with amphibolite bodies and structural trends in the schist and gneiss both north and west of the window (pl. 1 and fig. 32). This discordance shows that there must be at least an unconformity between the Cranberry Gneiss and the overlying rocks. If the overlying rocks are younger, they must have been deposited against a basin margin having about 2 miles of relief in a dis- tance of 6 miles to account for the apparent discord- ance, but the absence of conglomerates in the mica schist and gneiss seems to rule out this possibility. It seems unlikely that all the layers and bodies of coarse-grained feldspar-rich rocks that had granitic textures before the Paleozoic metamorphism were sedimentary rocks, for we find no gneisses of simi- lar composition with relict sedimentary textures. The development of the granitic texture and compo- sition seem related. Consequently, we believe that the Cranberry Gneiss formed by metasomatic and mag- matic processes and is younger than the nonplutonic rocks it invades. The discordance between the Cranberry Gneiss and adjacent rocks indicates that either the Cran- berry Gneiss is younger than the mica schist, as we have previously interpreted (Bryant, 1962; Bryant and Reed, 1962, p. 164), or the gneiss is in tectonic contact with the schist. The former conclusion cer- tainly applies to the Cranberry at the west margin of the area that was not affected by the low-grade meta- morphism. Where the contact between the Cranberry and the mica schist is sharp, the rocks along the contact are strongly sheared but no more sheared than the main body of Cranberry Gneiss beneath the contact. Am- phibolite layers 1 to 50 feet thick commonly occur along the contact. These layers and all or part of the mixed rocks north of the window might be in tec- tonic slices along a fault zone. On the other hand, it is difficult to place such a fault in some areas, particu- larly in areas C-4 and C-5 where digitations of am- phibolite, mixed rocks, and mica schist and gneiss project well into the Cranberry Gneiss. Thus, the local evidence is inconclusive whether or not a major fault separates the Cranberry Gneiss from the over- lying rocks. If the data on the age of the schist, gneiss, and amphibolite of the Blue Ridge thrust sheet (Rankin, 1967, 1970; Hadley, 1970) that have been obtained since the above was written can be extended to the Grandfather Mountain area, we think that they sup- port the hypothesis of a major thrust fault between Cranberry Gneiss in the basement and the Ashe For- mation overlying it (Rankin, 1970), for units in the younger rocks rather than in the older rocks appear truncated along the contact. Such a relationship sug- gests that there may be Precambrian metamorphic rocks of two ages in the Blue Ridge thrust sheet in the Grandfather Mountain area : upper Precambrian rocks only metamorphosed during the Paleozoic occurring in a thrust sheet above a basement com- plex of schist, gneiss, amphibolite, migmatite, and granitic - rock, which - was metamorphosed 1,000-1,100 m.y. ago. AGE Geologic relations and mineral ages indicate that the Cranberry Gneiss is of Precambrian age. In the eastern Great Smoky Mountains, 50 miles to the west, and in the Gossan Lead district, 30 miles to the northeast, similar plutonic rocks are overlain by rocks of late Precambrian age (Hadley and Gold- smith, 1963 ; Stose and Stose, 1957). About 12 miles northwest of the Grandfather Mountain area at Pardee Point on the Doe River, rocks of the Unicoi Formation of Early Cambrian (?) age rest unconformably on plutonic rock (King and Ferguson, 1960, fig. 7) similar to but less sheared than the Cranberry Gneiss in the northwestern part of the Grandfather Mountain area. However, a major thrust fault separates the plutonic rocks at Pardee Point, which is in the intermediate sheet, from those in the Blue Ridge thrust sheet. Isotopic ages of zircon, micas, and hornblende con- firm the Precambrian age of the Cranberry Gneiss and indicate that it formed 1,000 to 1,100 m.y. ago. BEECH GRANITE The Beech Granite (Keith, 1903) crops out in an east-trending belt northwest of the Grandfather Mountain window (pl. 1) and extends westward into Tennessee beyond the limits of the area mapped. The granite forms the summit and north slopes of Beech Mountain, the second highest peak in the Grand- father Mountain area. Hamilton (1960) included this rock among the rocks he described as "the complex of Lunsford Branch area." The Beech Granite is a coarse-grained inequigran- ular white to light-pink cataclastic granite or quartz monzonite gneiss containing potassic feldspar crys- BLUE RIDGE THRUST SHEET 37 tals that are mostly 5 to 7 mm but are locally 25 mm in diameter. It typically forms large slabby outcrops and cliffs. Cataclastic foliation is obscure in many places, but lineation, which is conspicuous, is formed by aggregates of biotite and epidote and, in places, by drawn-out quartz and feldspar grains. Quartz len- ses and veinlets along joints and irregular knots of quartz are widespread. Aplite veins are rare, and no pegmatite has been found. In a few places, the gran- ite contains ellipsoidal inclusions, a few inches long, of a gneiss which is finer grained and more mafic than the granite. Inclusions of this type are exposed on the north side of Mill Creek at about 3,100-foot altitude (area C-2, pl. 1). Near Skalley Branch (area C-3, pl. 1) are some outcrops resembling Cranberry Gneiss, but whether they are inclusions or roof pendants could not be determined. The granite is cut by zones of black to green and gray phyllitic rocks. Some of these rocks are plainly phyllonites because they contain porphy- roclasts of minerals from the Beech Granite. Others contain a higher proportion of chlorite and (or) opaque minerals and are probably greenstones de- rived from mafic dike rocks. CONTACT RELATIONS Contacts of the Beech Granite in the Grandfather Mountain area are sharp. In many places, especially along the southern contact, shearing and an un- known amount of movement have taken place. Such a sheared contact is exposed on the road northwest of Heaton (area C-3, pl. 1), where coarse- to medi- um-grained nonlayered gneissic cataclastic Beech Granite is in sharp and slightly discordant contact with layered Cranberry Gneiss. The Cranberry Gneiss near the contact consists of dark fine-grained layers of dioritic composition (now albite-actinolite- epidote-biotite schist) and gradations to coarse- grained light-colored blastomylonitic quartz dioritic gneiss containing white feldspar-rich pegmatite stringers and pods (fig. 20). The strongly sheared character of the underlying Cranberry Gneiss could be due to thrust faulting of the Beech Granite over the Cranberry or to the difference in mechanical properties between the homogeneous granite and the layered Cranberry Gneiss. Along the southern contact of the Beech Granite, cataclastic foliation is concordant or semiconcordant with the contact, the attitude of which may have been modified by move- ment parallel with the foliation. Commonly, the Beech Granite is slightly finer grained within a few to a few tens of feet of the contact. In some places along the northern con- FIGURE 20.-Southern contact of Beech Granite. Contact between Beech Granite and Cranberry Gneiss on road northwest of Heaton (area C-3, pl. 1). Massive rock on left is Beech Granite, a homogeneous blastomylonitic gran- ite gneiss; rock on right is Cranberry Gneiss, a heterog- enous blastomylonitic layered gneiss. Amount of move- ment along contact is unknown. tact, the Beech Granite is poor in mafic minerals and contains considerable amounts of green iron-rich sericite. It appears crushed rather than strongly fol- iated. The best exposure of the northern contact of the Beech Granite (fig. 21) is in Beech Creek (area D-2, pl. 1). There, greenish-gray porphyroclastic quartz monzonite to granodiorite gneiss containing aplite dikes and sills is in sharp contact with white to Cranberry Gneiss 1 foot FIGURE 21.-Northern contact of Beech Granite exposed on a slab in Beech Creek (area D-2, fig. 4). Cranberry Gneiss is well foliated and contains potassic feldspar porphy- roclasts as much as an inch long. Beech Granite is poorly foliated and medium grained next to the contact. Between A and B later movement along the contact has produced 4 -inch-thick well-foliated rock parallel to the contact in the Cranberry Gneiss, between B and C foliation has wrapped around irregularity in contact, and between C and D foliation has cut the contact and is obscure over a 1- to 2-inch interval. 38 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE FIGURE 22.-Photomicrographs of Beech Granite and phyl- lonite. A, Coarse-grained recystallized mortar gneiss from Jones Falls (area C-2, pl. 1). Perthite broken and healed by quartz. Smaller grains of plagioclase altered to albite. Recrystallized mortar is mostly quartz but contains some albite, sericite, and biotite. Section cut subparallel to poorly developed lineation. B, Coarse-grained blastomy- lonitic gneiss from north side of Mill Creek valley (area C-2, pl. 1). Large broken porphyroclast of vein perthite containing patches of albite is healed by recrystallized quartz. Small plagioclase porphyroclasts altered to albite. Large plagioclase grain included in darker Carlsbad twin of potassic feldspar bent near quartz-filled fracture. light-gray Beech Granite. The contact dips 65° to 75° SW. The outcrop pattern of the contact in this area also indicates that it dips more steeply than the cataclastic foliation. A thin section from the contact in an adjacent roadcut shows that the Beech Granite Matrix of recrystallized quartz, albite, epidote, and biotite. Section cut at about 45° to mineral lineation. C, Coarse- grained blastomylonitic gneiss. From east side of Laurel Creek at 3,030-foot altitude (area C-2, pl. 1). Porphyrociast of vein perthite in a matrix of recrystallized quartz, albite, sericite, and biotite. Porphyroclasts of quartz are bent, broken, and strung out to form mineral lineation. Section cut parallel to lineation. D, Porphyroclastic phyllonite from about 3,300-foot altitude on north side of Big Pine Mountain (area C-2, pl. 1). Porphyroclasts of quartz, plagioclase (altered to albite), and potassic feldspar, in a matrix of sericite, quartz, and biotite. Section cut par- allel to lineation. is fine grained at its contact and that the foliation cuts the contact. The Stone Mountain fault forms the contact of the Beech Granite along the northwestern margin of the Grandfather Mountain area. BLUE RIDGE THRUST SHEET 39 PETROGRAPHY The Beech Granite consists of numerous porphyro- clasts of microperthite, a few porphyroclasts of quartz and albitized plagioclase, and rare porphyr- oclasts of biotite, in a fine-grained matrix of recrys- tallized albite, quartz, biotite, epidote, and sericite, generally having a grain size of 0.05 to 0.2 mm in diameter (fig. 22). The proportion of groundmass to porphyroclasts is highly variable. Some rocks have undergone only incipient crushing accompanied by formation of rims of mortar around quartz grains and by bending of biotite, whereas at the other ex- treme are blastomylonites and phyllonites in which no porphyroclasts remain. The potassic feldspar is everywhere perthitic (fig. 22 A, B, C) and both string and patch varieties occur. Carlsbad twinning is widespread, and grid twinning is quite common but rather poorly devel- oped. Plagioclase grains are locally included in the perthite. Many of the large crystals have been bro- ken and healed by quartz (fig. 22A, B). The orange, pink color of much of the perthite seems to be due to numerous minute inclusions, which may be exsolved iron oxide. Plagioclase occurs in grains as much as 5 mm in diameter and in groundmass size. Locally, plagio- clase porphyroclasts are bent. Quartz porphyroclasts are as much as 5 mm in diameter, and in some strongly sheared rocks, they have been stretched into rods as much as 2 em long (fig 22C). In one specimen of slightly sheared gran- ite from the western edge of the Linville quadrangle, quartz is interstitial to the feldspar. Quartz was gen- erally the first mineral to break down during cata- clasis; it recrystallized to mosaic-textured aggre- gates with a grain size of 0.05 to 0.2 mm. However, it was a resistant mineral under conditions leading to formation of phyllonite, and it occurs as porphy- roclasts in that rock type (fig. 22D). Greenish-brown, dark-green, and brown biotite is generally in aggregates accompanied in many places by epidote. Porphyroclasts as much as 2 mm in diameter are found in some of the less-sheared rocks. i Dull-green to brownish-green amphibole is found as a relict primary mineral in some of the Beech Granite south of Skalley Branch (area C-3, pl. 1) ; it is partly rimmed by a bluish-green or green amphi- bole having a greater extinction angle. Zircon is the most common accessory mineral. Most zircon grains are prismatic, but others range from euhedral to round. Sphene and opaque minerals occur, and sphene rims the opaques in many places. Purple fluorite is widespread. Generally it is inter- granular, but in places it is also found on fracture planes and in quartz segregations. It is especially noticeable near the margins of the granite along the Stone Mountain fault on the road down Dark Ridge and in the Long Ridge area. Metamict allanite is in places rimmed by epidote. Stilpnomelane in radiat- ing aggregates, apatite, carbonate, and sericite are other accessories. COMPOSITION AND AGE Approximate modal compositions of the Beech Granite (fig. 23) fall rather uniformly in the granite field. One aplite veinlet (point 3, fig. 23) contains more plagioclase, but the potassic feldspar in that rock is less perthitic than in the wallrock and most of the sp :cimens of Beech Granite examined. The speci- men from the contact on Beech Creek (point 4, fig. 23) also contains only slightly perthitic microcline and plots near points calculated from analyses. Chemical compositions of typical Beech Granite (table 8) are very similar to the average alkali gran- ite of Nockolds (1954). Recalculation of the chemical analysis into ideal orthoclase and albite places that sample quite close to the eutectic for the system NaAISi,0,-KAISi,0,-Si0,-H,0 at pressure of 500 kg/ cm' (kilograms per square centimeter) (Tuttle and Bowen, 1958, p. 75). The Beech Granite probably intruded the Cran- berry Gneiss after or during a late stage of plutonic metamorphism. No detailed intrusive relationships QUARTZ * , Granite field PLAGIOCLASE POTASSIC FELDSPAR FIGURE 23.-Proportions of quartz, plagioclase, and potassic feldspar in Beech Granite. Perthite is plotted as potassic feldspar. Points 1 and 2 are calculated modes of analyzed samples (table 8) in terms of pure orthoclase and albite. Line connects calculated mode with measured mode; 3, aplite dike cutting granite; 4, Beech Granite at contact on Beech Creek. All points except 1 and 2 based on count of 50 random grains in each thin section. Granite field is outlined. 40 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE were found, but the gross map pattern, the sharp contacts, lack of layering, uniformity in composition, and perthitic character of the potassic feldspar sup- port this interpretation. The original intrusive con- tacts have been modified by shearing during the Pa- leozoic metamorphism, which produced the present foliation, lineation, and texture of the Beech Gran- ite. TABLE 8.-Chemical analyses, modes, and norms of Beech Granite [Analysis of sample 1 determined by standard rock analysis by Dorothy F. Powers, .S. Geol. Survey, 1959. Minor-element analysis of sample 1 given in table 1. Sample 2 analyzed by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey, 1963; F and Cl determined by Vertie C. Smith, U.S. Geol. Survey. Modes, by point counts; P, present but not intersected in counting. Major oxides and CIPW norms given in weight percent; modes, in volume percent] 1 2 Ficla No.... ill: f X;... ACLA Ai Ln. reece GML-6 O-59-1 Laboratory NOL: ceo ous nes #k a alul c oue F-2589 161255 Major oxides S1O;r. coleus eep i eet dawn salsa aod s 74.61 74.2 AMO nne . on n o on a wae 12.82 12.4 { . 49 1:2 sXe UL . eled n eanath. 1.44 1.0 cC EU 20. olden need. . 08 .19 Cadre lier cu rele ria aio aie T9 1.2 (NA.. RGNL cie ei cane raw 3.68 3.2 OL ND LOC nw 2 5.12 5.2 M;O4 1.2.4 ie tL o coll al .66 fOr sane aii Lesso .04 .04 2.0002 ._ Oc Pi ros itn. .14 21 P 20g eee PLH Lee ue al wo .01 .04 MAO. c 22 ee L eee bl od Pe t LE ake .04 .04 CO mes aa read ias o hace eee n a a niall . 08 'At I o see Gee o aie Au mial b ab aw aie cle we aan ean ieee nnn 12 Cts on oe eee t oe ee ae n ne ca a ae aide wie o a 01 99.55 100 Modes Q UATLE L222 c o PA Sos s rons Se Tet an Pere cule 32 36 Potassic feldspar and perthite__________. 39 40 Cuft} _ 0. 20 11 eac 4 6 Diotite. L scsi ti el 4 2.8 Calcite... s. s eon a P 2.4 Sphene.. :i elle sane ans .6 AMHManite.s2...22 200002 eed erase as P . 2 .X r erg e te Opaque ? .6 sesso _n {lo Pon dial AIC P P. ADALINE - 22 - : 222 22 o a 2h oe ne ale ae anne ake P P oo ed ass P: sss ite P " Points 706 672 CIPW norms ()+ eee a e nea ik e enon ena naren saat 31.12 34.25 | Ce talin set earing niet ine, - anni . 0.00 + 71 OF s eel 2 o eaten e hol eee ona ina ae aora oo 30.25 30.72 ADEA Eee PTE e t ar ean ah uk be ances ae 31.12 26.99 Ans ee o pn nibo carne cee 3.35 2.84 HIs sc 02s} coco ats r ANAL T .L 02 Ms 22% 2 e o Lice od ee e ie aide eave ee a alel .20 47 se ee ro A CUC - a une nea ne e eee 2.08 57 MCLS OOO mL NOL LOANS Ner VIE T1 1:14 H:: 9s. ee in olo L Pen oc POETS His 40 Ap. Ae r ri non oe cre de nib iad 02 10 P Te met ea ae abe no ae Pil c g LURE wie a n aln aa bien » ae 24 Opes 2h ee rC Io cui aaa naaa a en s .18 93 1. Medium-grained gray cataclastic biotite granite gneiss with conspicuous lineation formed by aggregates of fine-grained biotite. Bent and broken porphyroclasts of microcline, microcline microperthite, and saussuri- tized plagioclase as much as 2 mm in diameter in a matrix of recrys- TABLE 8.-Chemical analyses, modes, and norms, Beech Gra- nite-Continued tallized quartz, albite, potassic feldspar, and subordinate green biotite and sericite having a grain size of 0.05 to 0.1 mm in diameter. Acces- sory calcite, allanite, stilpnomelane, epidote, fluorite, zircon, and opaque mineral. From quarry in area C-2 (pl 1) at 3,950-foot altitude on south side of Timbered Ridge just east of road between Heaton (area C-3) and Whaley (area D-2). 2. Coarse-grained cataclastic granite gneiss with light-pink potassic feld- spar porphyroclasts as much as 1 cm in diameter. Porphyroclasts of string perthite, as much as 5 mm; quartz, 4 mm; and bent plagioclase (altered to albite) 1 mm in diameter in a matrix of recrystalized quartz, sericite, albite, potassic feldspar, subordinate brown and green- ish-brown biotite and calcite, and accessory allanite, sphene, opaque mineral, zircon, and apatite. From roadcut on east shoulder of Beech Mountain at 4,150-foot altitude near south contact of Beech Granite south of Oliver Hollow (area E-2, pl. 1). QUARTZ MONZONITE GNEISS Several elongate bodies of nonlayered crushed and gneissic quartz monzonite crop out north of the Grandfather Mountain window within a mile of the Linville Falls fault. They generally form better out- crops than the adjacent more mafic and plagioclase- rich Cranberry Gneiss. The contacts of the quartz monzonite bodies are strongly sheared in most places, and in places phyllonite is found along them. The bodies of quartz monzonite gneiss may be either tectonic lenses along faults in the Blue Ridge thrust sheet or intrusive bodies in the surrounding layered Cranberry Gneiss. They do not closely resemble rocks exposed along the southeast side of the Grand- father Mountain window, as might be expected if they were tectonic slices. The quartz monzonite gneiss is a light-pink to gray coarse-grained rock which ranges from well fol- iated to massive. Pink potassic feldspar crystals form porphyroclasts as much as 2 ecm long which have been localled fractured and the fractures filled with quartz. Various amounts of greenish-gray pla- gioclase are seen in some outcrops. Biotite occurs sparingly, both as porphyroclasts and as recrystal- lized flakes. Purple fluorite occurs on shear planes and as disseminated blobs as much as 2 cm in diame- ter. Pyrite and rarely chalcopyrite are visible. Epi- dote and quartz segregation veinlets occur. 'The rocks are mainly recrystallized mortar gneisses but range to blastomylonite gneisses. The recrystallized mortar consists of quartz, albite, seri- cite, and microcline 0.05 to 0.2 mm in grain size. Quartz, plagioclase, microcline, and microcline mi- croperthite grains are bent, broken, and healed by quartz. In the more strongly sheared rock, only the potassic feldspar remains as porphyroclasts. Propor- tions of plagioclase and potassic feldspar are varia- ble, and the rocks mapped as quartz monzonite gneiss include some granodiorite and granite. Large plagioclase grains have been albitized. All the biotite in many of the rocks has recrystallized, and it is generally green or light brown. Some rocks contain subhedral grains of sphene as much as 1.5 mm long. BLUE RIDGE THRUST SHEET 41 Opaque minerals are magnetite, ilmenite, and py- rite. Fluorite, zircon, apatite, epidote, allanite, chlorite, carbonate, and stilpnomelane are other ac- cessory minerals. AEGIRINE-AUGITE GRANITE GNEISS Small bodies of coarse-grained gray nonlayered granite gneiss were mapped in the Cranberry Gneiss near Crossnore (area C-5, fig. 4) and east of Boone (area H-2, pl. 1). These bodies are poorly exposed, and their contacts with the surrounding layered gneiss are covered, but judging from float the con- tacts are sharp. The granite gneiss is moderately well foliated. Perthitic feldspars as much as 1 em in diameter have finer grained mafic minerals between them. The large feldspars are porphyroclasts of perthite and antiperthite in a groundmass of recrystallized quartz, albite, microcline, and mafic minerals (fig. 24). Quartz, albite, and stilpnomelane are found in fractures in the feldspar porphyroclasts. Mafic min- erals are dark-brown biotite, pyroxene, and amphi- bole. The pyroxene is aegirine or aegirine-augite. It occurs as porphyroclasts as much as 1.5 mm in diam- eter and as small grains in the groundmass, but whether these grains are fragments or have recrys- tallized is uncertain. The amphibole is derived from proxene. It has 2V=85°, ZAc=-18°, Z=dark green, Y=bright green, and X-light brownish green and is probably hastingsite. Other accessories FigurE 24.-Photomicrograph of aegirine-augite granite gneiss from quarry 0.7 mile northwest of Crossnore (area C-5, pl. 1). Porphyroclasts of vein perthite and antiperthite in groundmass of recrystallized quartz, microcline, albite, and subordinate aegirine-augite, biotite, hastingsite, and stilpnomelane. Analyzed specimen, table 9. are zircon, sphene, allanite, magnetitie, ilmenite, tourmaline, and carbonate. Approximate modes for rocks of this unit (fig. 25) fall into the granite field if perthite is counted as potassic feldspar. However, the perthite is coarse grained enough to count the two phases separately. When this is done, the modes fall near the composi- tion calculated for the analyzed specimen. Chemical analysis (table 9) of rock from the Crossnore quarry most nearly resembles average fer- rohastingsite granite (Nockolds, 1954). Compared with the Beech Granite, the granite is poorer in SiO; and richer in Na;,0, FeQ, and combined Na,0 and K20. Eckelmann and Kulp (1956) pointed out that the body near Crossnore lacks layering and contains eu- hedral zircon ; they therefore suggested that it is in- trusive into the Cranberry Gneiss. BAKERSVILLE GABBRO The term "Bakersville Gabbro" was first used by Keith (1903) in referring to the rocks on Hump Mountain (areas C-3 and C-4, pl. 1) which he corre- lated with similar rocks near Bakersville, about 13 miles to the southwest. He believed that these rocks were unmetamorphosed and regarded them as being of Triassic age. Subsequently, however, he did not map Bakersville Gabbro in the type locality west of the Grandfather Mountain area because he thought that it occurred only as small irregular bodies (Keith, 1907a). He probably confused Bakersville Gabbro with Roan Gneiss, the name applied to the QUARTZ EXPLANATION fnd Perthite counted as potassic feldspar } Components in perthite counted separately % Calculated from analysis PLAGIOCLASE POTASSIC FELDSPAR FiGurE 25.-Proportions of quartz, plagioclase, and potassic feldspar in aegirine-augite granite gneiss. Granite field outlined. Leaders connect composition calculated on basis of analysis (table 9) with modes determined by point count. Other points determined by count of 50 random grains in each thin section. 42 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE TABLE 9.-Chemical analysis, mode, and norm of aegirine-augite granite gneiss [Standard rock analysis by Dorothy F. Powers, U.S. Geol. Survey, 1959. Mode, by Rgint count, 709 points counted; P, present but not intersected in counting. inor-element analysis of this rock is given in table 1. Major oxides and CIPW norm given in weight percent; mode, in volume percent] GML-5. Field Field Laboratory No........~.... F-2588 - Laboratory No..________- F-2588. Major oxides 71:24 / K30-. 22. owe oak cns 5.51 u.. 19.68. __. 18 .64 (O o Pinan aln anld ais .05 ns 2.78 . 28 MeO._--: 04 __.. 04 CaO: ELE A.C. 1.14 09 NasQ-_L_Li c- 4.12 08 Total.: .:...." 99.77 Mode H 21 Hastingsite:......_.. 1.3 Microcline, perthite, Stilpnomelane. _._. 1.6 and antiperthite___ 54 Opaque mineral. ___. P 18 P irecon s m P Aegirine-augite_____- 2.9 | 7 . Allanite...='"-.._-.... P CIPW norm C) Lebel e int. 22.62 (Fe. cscs els cut i 4.28 OF :. 2 ice in ms $2.55 Mt.... .98 AD: ee aac an . S4.84 IM.. __ . 58 An.:2:c sal sauces P49 .10 WOL... aa ec ua aeg s 1,02} e>. 18 x0 NortE.-Description of analyzed specimen follows: Gray cataclastic granite gneiss. Porphyroclasts of perthite and antiperthite as much as 8 mm in diameter in a groundmass of recrystallized grains 0.06 to 0.6 mm in diameter of quartz, microcline, and albite. Subordinate aegirine-augite, brown biotite, hastingsite, and stilpnomelane and accessory zircon, sphene, allanite, and (zpaqule) riggneral. From quarry in area C-5 (pl. 1) 0.7 mile N. 88° W. of Crossnore area D-5). amphibolites of the region, because he considered the Roan Gneiss to be composed of diorite or gabbro in addition to amphibolite, hornblende, schist, and hornblende gneiss. He also believed that the Roan Gneiss cut the Cranberry Gneiss, but feldspathic lay- ers associated with the Cranberry are found in the hornblende gneiss, and the Bakersville cuts both Cranberry and hornblende gneiss. Bayley (1923) recognized that the Bakersville Gabbro in the vicinity of Cranberry has been meta- morphosed, but he regarded it as a part of the Roan Gneiss because, following Keith, he considered the Roan to be metamorphosed gabbro and diorite. Kulp and Poldervaart (1956) recognized that the Bakersville Gabbro cuts the Cranberry Gneiss but has been metamorphosed, thus furnishing excellent evidence for the polymetamorphic history of the rocks of the Blue Ridge thrust sheet. Wilcox and Poldervaart (1958) made a compre- hensive study of the Bakersville Gabbro near its type locality ; they believed that it occurred solely as dikes and sheets and that the dike swarm did not extend east of Roan Mountain. However, fieldwork by D. A. Brobst (oral commun., 1960) indicates that dikes of Bakersville Gabbro are found between Roan Moun- tain and the Linville quadrangle. o In the Grandfather Mountain area, the Bakersville Gabbro consists of metamorphosed gabbro, diabase, and basalt which occur in dikes and stocks in a belt extending from Hump Mountain (areas C-3 and C-4, pl. 1) to Spanish Oak Mountain (area C-4, pl. 1). Many of the bodies contain inclusions or septa of granitic gneiss and amphibolite. The best exposures are on the steep sides of Little Hump Mountain where many dikes and larger bodies cut Cranberry Gneiss and rocks of the mixed unit. The dikes trend N. 30°%-60° W. and are generally vertical. Some have chilled margins; others are foliated parallel with their margins. The contact of the metagabbro body on Hump Mountain is drawn where dikes apparently make up more than half the rock. Thus, there is much dike material outside the contact and much country rock inside it. In the interior of the body little feldspathic gneiss or amphibolite is found. This metagabbro body is similar to that mapped by Brobst (oral commun., 1960) west of Bakersville. Ir- regular bodies of metagabbro southeast of Hump Mountain do not have many dikes associated with them. A few small bodies of metagabbro occur in the Cranberry Gneiss north of the Grandfather Moun- tain window. Dikes and sills a few inches to several tens of feet thick of fine-grained met:morphosed mafic rocks in the Cranberry Gneiss and Beech Granite may be related to the Bakersville Gabbro. Some of these bodies are exposed in roadcuts along U.S. Highway 19E near the North Carolina-Tennes- see State line (area C-3, pl. 1). The metagabbro is a dark-gray to black rock con- taining primary labradorite, monoclinic pyroxene, and opaque minerals. The rock has been recrystal- lized to varying degrees. It is massive to schistose and is locally porphyritic. In the completely recrys- tallized rocks, hornblende, garnet, calcic oligoclase- sodic andesine, monoclinic pyroxene, biotite, and opaque minerals are the principal constituents. The Bakersville Gabbro has porphyritic, diabasic, and ophitic textures (fig. 26). All the igneous tex- tures have been modified to some extent by recrystal- lization. Most of the rocks have relict porphyritic textures that have been partly converted to grano- BLUE RIDGE THRUST SHEET FIGURE 26.-Photomicrographs of Bakersville Gabbro. A, Ophitic texture in slightly altered gabbro from interior of dike exposed in roadcut on Little Horse Creek Road where road crosses creek (area C-4, pl. 1). Laths of sodic labra- dorite, partly enclosed in large monoclinic pyroxene grains. Pyroxene, rimmed by light-green actinolitic hornblende; some pyroxene recrystallized. Garnet locally replaces plagioclase; plagioclase, locally altered. Small amount of hypersthene. B, Amphibolite from sheared part of same dike as A. Nematoblastic hornblende and sodic andesine. blastic textures during recrystallization. Most of the mafic minerals have been converted to hornblende, garnet, and biotite, and much of the plagioclase has been reconstituted, but some phenocrysts of plagio- clase as much as 5 ecm long are still recognizable. Where the porphyritic rocks have been strongly metamorphosed, the phenocrysts have been con- verted into aggregates of plagioclase and garnet. A few of the rocks have relict basaltic textures. One C, Porphyritic basalt from 4,950-foot altitude on west ridge of Hump Mountain (area C-3, pl. 1). Phenocrysts of sodic labradorite, in matrix of sodic andesine, hornblende, and pyroxene. Garnet porphyroblasts replace groundmass. D, Biotite-garnet-pyroxene metagabbro from 4,200-foot altitude on ridge south of road opposite Bellvue Church (area C-3, pl. 1). Plagioclase, recrystallized to oligoclase, although some lath-shaped crystals remain. Monoclinic pyroxene, partly altered. Garnet porphyroblasts and ir- regular blobs of magnetite. body northeast of Chestnut Ridge (area D-3, pl. 1) contains feldspar laths as much as 2 ecm long and has diabasic texture. Wilcox and Poldervaart (1958) described similar rocks in the Bakersville Gabbro in the Bakersville- Roan Mountain area. Some of the orthoamphibolites they described are identical to gneissose or schistose Bakersville Gabbro from the western part of the Linville quadrangle. Other rocks described by them 44 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE may be orthoamphibolites derived from older rocks. They attributed the textural variations of the Bak- ersville Gabbro to variations in the amount of water available during the recrystallization. In the Grand- father Mountain area, more completely recrystallized gabbro has gneissose structure; therefore differences in intensity of shearing may also have been a factor in the textural variations. In one exposure on the road up Little Horse Creek (area C-4, pl. 1), all gradations from coarse-grained nonfoliated metagabbro to garnet amphibolite occur (fig. 26 A and B). The amphibolite contains lenses of feldspar representing the former phenocrysts. In ex- posures on the steep slope west of Tucker Hollow (area C-3, pl. 1), gradations from coarse-grained metagabbro in the center of dikes to amphibolite at the margins can be seen. Fine-grained hard skarnlike rocks with pink and green mottled appearance form a distinctive type of metamorphosed Bakersville Gabbro. They consist of plagioclase and interstitial pyroxene. The pyroxene is partly altered to hornblende, and anhedral skeletal porphyroblasts or aggregates of garnet irregularly replace the earlier minerals and produce the mottled aspect. Plagioclase forms 12 to 52 percent of most of the rocks. It is mostly calcic and best retains its shape in fine-grained diabasic-textured rocks in which it forms laths as much as 0.5 mm long. Plagioclase as calcic as An,; is found in some rocks. Well-devel- oped normal zoning with as much as 830-percent variation in An content from core to rim is found in some of the less altered grains. Plagioclase pheno- crysts have been partly decalcified without destruc- tion of their identity. Epidote minerals locally occur in the phenocrysts. In other places, the phenocrysts have altered to garnet and mosaic-textured aggre- gates of calcic oligoclase or sodic andesine grains 0.05 to 0.2 mm in diameter (fig. 26D). Where shearing has been strong, these aggregates have been made into lenses and strung out, until, with further shear- ing and recrystallization, the new plagioclase be- comes evenly distributed. The lath-shaped igneous plagioclase is more complexly twinned than the gra- noblastic plagioclase. Pyroxene forms 0 to 35 percent of the rocks ; it is mostly monoclinic, although a minor amount of hy- persthene was found in one specimen. The least met- amorphosed rocks contain more pyroxene than the more altered ones (fig. 264). Pyroxene forms pheno- crysts as much as 6 mm in diameter and equant grains 0.01 to 0.1 mm in diameter. The phenocrysts have Z Acranging from 40° to 44° and are probably augite and diopsidic augite. Generally, they are largely altered to hornblende and biotite and are filled with many tiny opaque inclusions. Much of the pyroxene occurs only as relicts in aggregates of hornblende grains. The smaller equant grains of py- roxene lack inclusions and have a light-green tinge. In many rocks they seem to be metamorphic, al- though in some, such as the skarnlike rocks, evidence of their origin is inconclusive. Amphiboles in the Bakersville Gabbro range from green to olive green, brownish green, dull green, and rarely to bluish green. Amphibole forms 3 to 74 per cent of the rocks. It seems to have been derived mainly from pyroxene. Rocks with granoblastic tex- ture contain amphibole and no pyroxene. The amphi- bole generally occurs as equant grains 0.1 to 0.2 mm in diameter, but in the more metamorphosed gabbros it is found as alined needles as much as 2 mm long (fig. 26B). It surrounds many of the pyroxene grains and in places has a porphyroblastic habit and in- cludes grains of quartz, plagioclase, opaque minerals, and rutile. Z A cranges from 14° to 18° 'but is gener- ally 15° to 16°. Garnet occurs in skeletal crystals and aggregates as much as 5 mm in diameter (fig. 26C), which form a trace to as much as 32 percent of some-rocks. It has a light-red hue and is commonly sieve textured and contains inclusions of plagioclase, pyroxene, horn- blende, and opaque minerals. As the content of horn- blende increases, the content of garnet decreases. Magnetite, lesser amounts of ilmenite, and minor amounts of pyrite, occur as irregular grains as much as 1 mm long and in amounts ranging from a trace to 14 percent. Brown to orange-brown biotite in flakes 0.2 to 1 mm long forms as much as 20 percent of the rocks. It is derived from garnet and hornblende. Epidote and chlorite tend to be in and adjacent to late shear zones. Quartz, sphene, and apatite are common accessory minerals, and rutile is a less common one. Three small bodies of rock entirely surrounded by Cranberry Gneiss are mapped as Bakersville Gabbro, but they differ from the gabbro bodies elsewhere and from each other. The one southwest of the Cranberry High School (area C-3, pl. 1) probably was origi- nally porphyritic and contains pigeonite (now al- tered to green and colorless amphibole), a few small albite porphyroclasts (relict phenocrysts?), and much very fine groundmass composed of epidote, al- bite, and sericite. The body east of Heaton (areas C-3 and D-3, pl. 1) has more porphyroclasts of pla- BLUE RIDGE THRUST SHEET & 45 gioclase and partly altered pyroxene and has a fine- grained blastomylonitic groundmass. The body east of the east fork of Curtis Creek (area D-3, fig. 4) is relatively unsheared and has a very coarse diabasic texture in which the saussuritized plagioclase laths are as much as 15 mm long, and the interstitial titan- augite as much as 5 mm in diameter is partly altered to amphibole. Small dikes of mafic rock possibly related to the Bakersville Gabbro cut the Cranberry Gneiss and Beech Granite. Most of them are very fine grained and consist of biotite, albite, sphene, actinolite, epi- dote, chlorite, and opaque minerals. In some dikes, the plagioclase forms laths as much as 2 mm long. Chemical analyses of Bakersville Gabbro from the Bakersville-Roan Mountain area indicate that the rocks represent a differentiated suite of olivine ba- salts (Wilcox and Poldervaart, 1958, p. 1351 and table 5). Older analyses from the Cranberry-Hump Moun- tain vicinity (Bayley, 1923, p. 44; Clarke, 1900) are quite similar, but rock and locality descriptions are inadequate to determine beyond doubt that they came from Bakersville Gabbro rather than older am- phibolites. As Wilcox and Poldervaart were unable to establish any chemical differences between Bak- ersville Gabbro, orthoamphibolite, and para-amphi- bolite north of the Spruce Pine district, it is not surprising that completely metamorphosed Bakers- ville resembles some of the older amphibolites. QUARTZ PORPHYRY Thin dikelike bodies of aphanitic light-green to white quartz porphyry too small to map are exposed in the Blue Ridge thrust sheet and the Grandfather Mountain window. They are lenses or tabular masses 10 inches to 50 feet thick, in places parallel to and in places cut by the cataclastic foliation. Some seem to be associated with faults, such as the ones north of Dark Ridge Creek at the southwest end of Horse Ridge (area C-2, pl. 1) and south of Liberty Hill school (area G-2, pl. 1). Others seem to be dikes, such as the ones east of Whaley (area D-2, pl. 1) and north of Aho (area H-3, pl. 1). The quartz porphyry consists of euhedral to sub- hedral embayed phenocrysts of quartz as much as 3 mm in diameter in a groundmass of quartz and seri- cite having a grain size ranging from 0.01 to 0.2 mm. In many rocks the sericite is alined forming folia- tion. No feldspar was identified with certainty in most of the specimens, but one contains lath-shaped potassic feldspar with Carlsbad twinning. Accessory minerals are biotite, sphene, magnetite (ilmenite?), apatite, and zircon. The rocks contain various proportions of quartz and sericite, and in several, sericite is dominant. Analysis of a typical quartz porphyry having vol- canic texture (table 10) shows that it has lower CaO, Na,0, and K,0 contents than volcanic rocks of equivalent SiO, contents. Evidently the rock has been partly silicified. Some of the rocks megascopically resembling quartz porphyry lack any well-defined relict texture and could be silicified blastomylonites. The quartz porphyry bodies may be related to the felsic volcanic rocks of the Grandfather Mountain Formation. CHILHOWEE GROUP SLICES NORTH OF THE GRANDFATHER MOUNTAIN WINDOW Quartzite, feldspathic quartzite, and quartz pebble conglomerate occur as tectonic inclusions along sev- eral faults within the Cranberry Gneiss within a mile of the Linville Falls fault and locally along the Linville Falls fault itself. They also are found along faults of the Stone Mountain family near the Moun- tain City window. The slices range from a few feet to as much as 1,000 feet thick; many of the thicker lenses are well exposed and form prominent ridges. The ridges sup- port thick growths of rhododendron and are mantled with quartzite float which makes it difficult to deter- mine the exact width of the quartzite lenses. In some places, phyllonite derived from the granitic gneisses is found adjacent to the quartzite slices, but in other places, the degree of shearing in the gneisses adja- cent to the quartzites does not significantly differ from that of the surrounding rock. The width of many of the slices has been exaggerated somewhat TABLE 10.-Chemical analysis of quartz porphyry [Standard rock analysis by C. L. Parker, U.S. Geol. Survey, 1961. Results given as major oxides in weight percent. Minor-element analysis given in table 1] Field No. segs GML-D. «Field GML-9. Laboratory No............. H-8424 - Laboratory No.._______.. H-3424. Te-18 - :c 1.64 MOF n cns ene awa 12.94 10 F9203 _______________ 1.24 TiOz ________________ , 12 80 incus 01 15 MnQ:.::....:....._s .01 Cade l 00 ~ . 00 00.00.00... AQ gic ae . 00 erika sess ense, Lad v2 ie unas ian AT 99.63 NotE.-Description of analyzed specimen follows: \ Light-greenish-gray rock in dike(?) 30 feet thick in Cranberry Gneiss. Nearly eubedral quartz grains as much as 1.2 mm in diameter. Groundmass of quartz 0.05 to 0.2 mm and sericite 0.01 to 0.03 mm in grain size. Groundmass quartz is partly in spheroidal growths filled with sericite inclusions. Approximate composi- tion: quartz, 64 percent; sericite, 36 percent. From roadcut at 8,270-foot altitude along road east of Whaley (area D-2, pl. 1). Norm, not computable. 6 46 a on the geologic map (pl. 1) ; some of the thinner ones are not shown. The slices typically pinch and swell, and some are reduced to a string of separated lenses connected by zones of phyllonite. Bits of quartzite only a few inches thick occur in the phyllonite. Where the quartzite slices or the phyllonite zones are no longer traceable, the faults are dropped on the map, although they may extend farther. The quartzites are thin bedded to thick bedded, light greenish gray, light green, white, gray, and, less commonly, bluish gray and purplish gray. They contain beds of green sericite phyllite and dark-gray beds rich in heavy minerals. Clastic feldspar grains are light pink. Conglomerates contain quartz pebbles as much as 5 inches long. Feldspar and fine-grained gray to red volcanic(?) rock fragments occur spar- ingly in the conglomerates. Clasts of muscovite are locally visible. The rocks commonly have numerous veinlets of segregation quartz. Most of the veinlets seem to be younger than the main shearing, but some are older. Cleavage is generally parallel to bedding and subparallel or parallel to the attitude of the slices themselves. Clastic grains and pebbles are elongated in a northwesterly direction. In one speci- men, quartz pebbles are drawn into flattened rods 1 ecm thick, 2 em wide, and 10 ecm long. Very locally, the quartzite is enriched in iron and contains abundant disseminated magnetite octa- hedra or hematite. On the ridge west of Whitehead Creek (area D-3, pl. 1), quartzite and adjacent phyl- lonite have been enriched in iron. Pyrite also occurs there and locally elsewhere in the quartzites. Most of the rocks have well-preserved clastic tex- tures. They consist of round or subround clastic grains of quartz, microcline, and rarely muscovite and plagioclase, in a matrix of recrystallized quartz and sericite (fig. 27). Where the rocks are exten- sively sheared or mineralized, they are difficult to dis- tinguish from blastomylonite or phyllonite derived from the Cranberry Gneiss. Quartz clasts are commonly strongly strained, and the larger ones are partly granulated and recrystal- lized into mosaic-textured aggregates (fig. 27A). In some rocks they are extremely flattened and elon- gated. Microcline clasts are fractured and healed by quartz. A minor amount of the potassic feldspar is a finely textured perthite resembling that in the felsic volcanic rocks of the Grandfather Mountain Forma- tion. Clastic grains of muscovite and altered biotite(?) as much as 1 mm long are found locally. Clastic grains of tourmaline and zircon average about 0.2 mm in length. IImenite also occurs as GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE clastic grains. The matrix of recrystallized quartz and well-alined muscovite has a grain size ranging from 0.01 to 0.2 mm. Some of the muscovite is light green in thin section and is similar to the iron-rich muscovite of the Grandfather Mountain Formation (see below). That mineral accounts for the green FIGURE 27.-Photomicrographs of rocks of the Chilhbwee Group in slices along faults. A, Sheared quartz-pebble conglomerate from slice about 300 feet thick along a sub- sidiary of the Linville Falls fault, 3,515-foot altitude on old road up north fork of Cooper Branch (area C-3, pl. 1). Quartz pebbles in various stages of granulation and re- crystallization into mosaic-textured aggregates. Matrix of magnetite, iron-rich muscovite, and zircon. B, Biotite- bearing sericitic arkosic quartzite from roadcut on North Carolina Highway 80 near Lake Tahoma dam (area A-10, pl. 1). From slice about 250 feet thick along Linville Falls fault. BLUE RIDGE THRUST SHEET 47 color of many of the rocks. Recrystallized ilmenite, magnetite, and hematite are also found. Other acces- sory minerals are sphene, epidote, and apatite. Figure 28A shows the compositions of sedimen- tary rocks from the tectonic slices. Most of these rocks are shaly quartzites and quartzites. A few are feldspathic quartzites, and even fewer contain sub- stantial amounts of green sericite and would be con- sidered graywacke in some classifications. However, even the mica-rich rocks are rather light colored. Dark-colored rocks generally contain more opaque minerals, which make up as much as 25 percent of a few rocks. SLICES SOUTHEAST OF THE GRANDFATHER MOUNTAIN WINDOW Thin slices of quartzite and feldspathic quartzite are also found on the southeast side of the Grand- father Mountain window from near Collettsville (area H-6, pl. 1) southwest to the end of the win- dow. They occur along the Linville Falls fault and are intercalated along subsidiary faults in the adja- cent gneisses. Slices are also found along the Linville Falls fault on the west side of the window southwest of Lake Tahoma (area A-10, pl. 1). The slices range from a few inches to 40 feet thick, and some can be traced for half a mile along strike. All the slices within the Blue Ridge thrust sheet lie within a few hundred feet of the trace of the Linville Falls fault. Those in the Wilson Creek Gneiss are generally near mapped subsidiary faults, but a few probably mark faults that were not mapped. MICA & > FELDSPAR QUARTZ A The quartzite is a fine-grained white, gray, or light-green sugary rock containing dark partings of heavy minerals and dark-green parting of sericite. Bedding and cleavage are parallel and are conforma- ble with the foliation in the enclosing rocks. Small clastic grains of feldspar are visible in some out- crops, and in a few places small quartz pebbles are present. The quartzite consists of a mosaic of fine-grained quartz 0.05 to 0.5 mm in grain size, fine-grained green iron-rich muscovite, and minor amounts of feldspar. Angular to subrounded clasts of microcline (fig. 27B) and less common microperthite generally form 5 to 10 percent of the rock. In a few specimens from the southwestern extension of the window, small flakes of brown biotite are associated with the sericite. Accessory minerals are magnetite, ilmenite, tourmaline, zircon, and sphene. The quartzite in most of the slices closely resem- bles the quartzite of the Chilhowee Group of the Ta- blerock thrust sheet in mineral composition (com- pare fig. 28B with 59A and B), including even the presence of clastic tourmaline. The slices are inter- preted as having been carried up from the buried part of the Tablerock thrust sheet along the Linville Falls fault and subsidiary faults. Quartzite in a few of the slices in the Lenoir quad- rangle lacks the distinctive clastic texture and tour- maline of most of the rocks elsewhere; it resembles sericite quartzite of uncertain origin found at scat- tered localities in the Wilson Creek Gneiss. MICA FELDSPAR QUARTZ B FIGURE 28.-Proportions of quartz, feldspar, and mica in rocks of the Chilhowee Group in tectonic slices in the Blue Ridge thrust sheet. A, In the Blue Ridge thrust sheet between the Grandfather Mountain and Moun- tain City windows. Based on a count of 50 random grains on each of 23 thin sections. Contours 4, 9, 13, and 17 percent. B, Along Linville Falls fault on the southeast side of the window. Based on count of 50 random grains in each thin section. 48 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE CORRELATION Rocks in the tectonic slices resemble those of the Chilhowee Group in the Mountain City window and the Tablerock thrust sheet in the Grandfather Moun- tain window in their lack of clastic plagioclase and content of clastic tourmaline. In these respects they differ from sandstones in the Granfather Mountain Formation. Conglomerate is more abundant in the slices north of the Grandfather Mountain window than in the upper part of the Unicoi and younger formations of the Mountain City window and in the incomplete(?) section of the Chilhowee Group in the Tablerock thrust sheet. Rocks in the tectonic slices are more micaceous than the Chilhowee rocks in the Tablerock thrust sheet (compare fig. 28 with fig. 59). Some of the slices may have been derived from lower units or different facies of the Chilhowee than are present in the exposed part of the Tablerock thrust sheet. ULTRAMAFIC ROCKS Small bodies of variously metamorphosed ultra- mafic rock are found in the Blue Ridge thrust sheet west and north of the Grandfather Mountain win- dow. These are part of a belt of similar rocks extend- ing along the western part of the Appalachian belt from Alabama to Newfoundland (Pratt and Lewis, 1905). All the ultramafic bodies are less than 400 feet long (table 11), and some may have been over- looked. Most of these bodies have been prospected for anthophyllite or soapstone, and the prospects are shown on the geologic maps. The ultramafic rocks are massive, crudely foliated, or schistose. The rocks are light green to very dark green, greenish gray, or black and contain various proportions of olivine, enstatite, antigorite, antho- phyllite, tale, tremolite, Mg-rich chlorite and minor amounts of chrysotile, chromite, pyrite, and carbon- ate. The olivine-rich rocks weather tan and serpen- tine-rich rocks, a milky white. Keith (1903) assigned the ultramafic rocks an Ar- chean age because of their metamorphism. On the basis of evidence from the northern and central Ap- palachians, Pratt and Lewis (1905, p. 159) suggested that the age of the ultramafic rocks might be Paleo- zoic, but they pointed out that such an age was not established for the bodies in North Carolina. In a later publication, Lewis (1921, p. 111) concluded that there may be two ages of ultramafic rocks but that both were Paleozoic. Hadley and Goldsmith (1963) found no ultramafic bodies in the upper Pre- cambrian rocks of the Ocoee Series in the eastern Great Smoky Mountains, although they found nu- merous ones in the underlying rocks, some adjacent to the basal contact of the Ocoee Series. They sug- gested that the ultramafic rocks are of early Paleo- zoic age. Kulp and Brobst (1954) noted that pegema- tites of the Spruce Pine district cut one ultramafic TABLE 11.-Occurrences of ultramafic rocks in the Blue Ridge thrust sheet Agaproximate Country Location inzfnsgons Trend rock Mineralogy Structure eet, Bellvue Mountain (area C-4). One of 400 by $50... N. 25°%W...... Amphibolite . . .. . . . Olivine, enstatite, antigorite with minor _ Crude foliation two bodies mapped together on chrysotile, tale, Mg-rich chlorite, parallels that in late 1. magnetite, chromite, carbonate. country rock. 0.35 mile N. 27° E. of Mount Pleasant 820 by 60..... N. 60% W..: ... sae reskin Enstatite, tale, antigorite, olivine, Schistose at margin. Church (area C-5) at asbestos mine anthopixyllite and accessory carbonate shown on plate 1. and pyrite; relatively talc-rich at margin. Roadcut along road southwest of Golden Unknown.... Unknown.... Cranberry Gneiss... Antigorite in core; tale and asbestiform Do. Creek in area C-5, 1.5 miles S. 44° W. amphibole at margin. of center of village of Crossnore (area D-5). Roadcut along road south of Squirrel 20 wide...... Unknown.... Amphibolite. . .. . .. Mg-rich chlorite, tale, anthophyllite. ... Schistose. Creek 0.2 mile N. 59° W. of Mount Pleasant Church (area C-5). South slope of Hawshore Mtn. (area C-5) 8300 by 50... .. N: OS® W...... ~ 1.204 Tremolite, tale, enstatite, antigorite, Foliated and lineated at asbestos prospect shown on plate 1. olivine, Mg-rich chlorite. where rich in tale. 0.8 mile N. 77° W130! Mount Pleasant 160 by 50... .. N. 10°ER.....: -se isch a wes Talc, asbestiform amphibole. .___._.... Unknown. Church (area C-5) at asbestos prospect shown on plate 1. 0.8 mile N. 81° E. of R‘urkey Knob Unknown.... . Unknown.... i Tale, tremolite, Mg-rich chlorite, opaque Do. (area I-2) at soapstone prospect minerals. shown on plate 1. ' 0.6 mile N. 89° E. of Turkey Knob 170 by 80... .. N.80°B...... vn eo ie Tale, amphibole, antigorite, chlorite. .. . Do. (area I-2) at soapstone prospect shown on plate 1. o : % 1.1 mile N. 78° W. of Trivett Gap Unknown.... Unknown.... i esse Asbestiform amphibole..........__.__. Do. (area G-2) at asbestos prospect shown on plate 1. s 0.5 mile N. 77° W. of summit of Rich Unknown.... . Unknown.... neater Anthophyllite, tale. Do. Mtn. (area G-2) at asbestos prospect shown on plate 1. R f 1.0 mile S. 19° E. of Green Valley Unknown.... Unknown.... Mica schist? ._.... Tremolite, chlorite....._...__.___.._.. Schistose. Church (area C-6). § p A ; _ Along Camp Branch 0.7 mile S. 26° W. N.: 85° w...... Cranberry Gneiss... Tremolite, Mg-rich chlorite, asbestiform _ Unknown. 150 by 250 .... of school in village of Triplett f (area J-2). amphibole, opaque minerals. BLUE RIDGE THRUST SHEET 49 body on the west margin of the district; and there- fore, that body is older than 350 m.y., the minimum age of the pegmatite (see below). In the Grandfather Mountain area we have no direct evidence of the age of the ultramafic rocks. None are known in the basement rocks of the Grand- father Mountain window. Therefore, their absence in the upper Precambrian rocks in the window shows nothing about their age. They are older than the lat- est medium-grade regional metamorphism of the rocks in the Blue Ridge thrust sheet. GRANODIORITE AND PEGMATITE Irregular stocks, sills, and pods of medium- to coarse-grained white granodiorite (Spruce Pine Alaskite of Hunter and Mattocks, 1986) and pegma- tite are found in the schist, gneiss, and amphibolite of the Blue Ridge thrust sheet west, north, and southeast of the Grandfather Mountain window. This area is the east margin of the Spruce Pine peg- matite district. These rocks are best exposed in the valley of Brushy Creek (areas C-5 and C-6, pl. 1) and in the valley of Plumtree Creek (area C-5, pl. 1). They are not found in the adjacent Cranberry Gneiss or in the rocks of the Grandfather Mountain window. They are also absent from the Blue Ridge thrust sheet rocks south of White Oak Branch (areas C-7 and C-8, pl. 1), except near the contact between the Cranberry Gneiss and the mica schist and gneiss. Very few of these rock bodies occur in the mica shist and gneiss north of the window, except in the Deep Gap-Stony Fork area (area J-2, pl. 1), where they occur with biotite quartz monzonite. Because of their economic importance the pegma- tites and "alaskites" of the Spruce Pine district have been studied by many geologists (Sterrett, 1901, 1910, 1923; Maurice, 1940; Olson, 1944; Parker, 1946, 1953; Brobst, 1962; Lesure, 1968). Most of the pegmatite bodies are too small to delineate at a scale of 1:62,500. Concordant bodies range from 1 inch to about 100 feet in thickness; most are less than 30 feet thick. The larger bodies are generally granodior- ite, but they also contain pegmatite and all grada- tions between the two rocks. Larger pegmatite bod- ies are commonly flanked by small satellitic stringers and lenses. In the larger and more mica-poor bodies, the granodiorite and pegmatite are weakly foliated, but cataclastic foliation is conspicuous in smaller bodies and along the margins and in restricted zones within larger bodies (fig. 29). Zoning is not well developed in most pegmatites in the Grandfather Mountain area. The smaller pegmatite bodies are generally more foliated and have lineation formed by alined micas. This lineation is parallel to lineation in the wall- rocks. In some larger pegmatites, lineation is con- spicuous only near or at the margins, or in restricted shear zones. Lesure (1959) noted foliation and linea- tion in pegmatite throughout the Spruce Pine dis- trict but found them to be best developed near the margins of the district. Inclusions of schist are found in the granodiorite and locally in the pegmatite. The pegmatite and granodiorite bodies generally have sharp contacts; where they are in contact with amphibolite, a thin selvage of biotite schist is commonly developed. The granodiorite and pegmatite contain plagio- clase, quartz, perthitic microcline, muscovite, and biotite. In the more strongly foliated bodies, porphy- roclasts of plagioclase, microcline, quartz, and mus- covite are set in a matrix of coarsely recrystallized quartz, plagioclase, microcline, and muscovite. Size of the porphyroclasts is variable. Many of the larger quartz and feldspar grains in the granodiorite are 0.5 to 1 inch in diameter. Perthite crystals in pegma- tite are as much as 4 feet long. Muscovite forms books as much as 2 feet in diameter, but most flakes are only 1 to 2 inches in diameter. Muscovite books are commonly bent and ruled. Muscovite flakes 1 to 3 mm long, the same size as those in the adjacent mica schist, occur on foliation planes; fine-grained light- green sericite occurs on widely spaced shear planes. In many places, the last two types of muscovite form a northwest-trending lineation that is parallel to that in the country rock. Biotite forms books and flakes as much as 3 inches in diameter. Red garnet as much as 3 inches in diameter is a common accessory min- eral. Apatite, epidote, and allanite are minor acces- sory minerals, and a host of rarer minerals has been reported from the pegmatites of the Spruce Pine dis- trict (Maurice, 1940, p. 59). Thin sections show large porphyroclasts of plagio- clase, microcline, quartz, and muscovite surrounded by mosaics of coarsely recrystallized plagioclase, quartz, microcline, and flakes of muscovite (fig. 30). The quartz and feldspar grains are as much as 2 mm in diameter. Plagioclase porphyroclasts are bent and locally broken. Some grains have a weak normal zon- ing. Plagioclase in different pegmatite bodies ranges from An, to Ang; Quartz rarely forms porphyro- clasts but commonly forms coarse-grained mosaics, especially in some quartz-muscovite parts of the peg- matites. 50 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE FIGURE 29.-Sheared pegmatite in the Blue Ridge thrust sheet. A, Boudin of foliated pegmatite in biotite-muscovite schist on Blue Ridge Parkway near contact between schist and Cranberry Gneiss on west side of Humpback Mountain (area C-7, pl. 1). Note feldspar porphyroclasts in schist adjacent to pegmatite. B, Sheared pegmatite and feldspar porphyroclasts in schist. Same locality as A. C and D, Foliated pegmatite from gneissose zone exposed in the Slippery Elm Mine, north side of Plumtree Creek west of Fall Branch (area C-5, pl. 1). Thin-section study shows that porphyroclasts of plagioclase and muscovite occur in a groundmass of recrystallized quartz and plagioclase which resembles the surrounding schist and gneiss in grain size. BLUE RIDGE THRUST SHEET 51 FIGURE 30.-Photomicrographs of granodiorite and pegmatite in the Blue Ridge thrust sheet. A, Muscovitequartz peg- matite from ridge of Rich Mountain 0.4 mile east of summit (area G-2, pl. 1). Elliptical porphyroclasts of muscovite in a mosaic of recrystallized quartz. (Some separation in section, especially around porphyroclasts.) B, Foliated pegmatite from Bellvue Mountain (area C-4, pl. 1); Porphyroclasts of oligoclase in a matrix of recrystallized quartz and oligoclase (feldspar, slightly weathered). Acces- Muscovite porphyroclasts are elliptical (fig. 304) and commonly are bent. Many are partly recrystal- lized into aggregates of muscovite 0.5 to 2 mm long. Microcline is perthitic an dseems to replace pla- gioclase in less-sheared rocks. COMPOSITION AND AGE Most specimens of nonpegmatitic rocks studied in sory microcline.). C, Foliated pegmatite from a layer 4 feet thick on south side of Doe Hill Mountain (area C-6, pl. 1). Porphyroclasts of oligoclase in a matrix of recrystallized quartz, oligoclase, and muscovite. Section perpendicular to mineral lineation. D, Quartz diorite from Mill Race mine near Brushy Creek (area C-6, pl. 1). Oligoclase porphyro- clasts in a coarse-grained mosaic-textured matrix of quartz and oligoclase. Porphyroclasts are bent and broken. thin section are oligoclase quartz diorites containing few or no mafic minerals, but some are granodiorites and quartz monzonites (fig. 31). Most of the rocks of this unit are too coarse grained for thin sections to be representative, and field estimates indicate that most are granodiorite. Brobst (1962) estimated that the average mineral composition of "alaskite" and pegmatite in the Spruce Pine district is oligoclase, 52 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE QUARTZ PLAGIOCLASE POTASSIC FELDSPAR FIGURE 31.-Proportions of quartz, plagioclase, and potassic feldspar in light-colored granodiorite and pegmatite. Based on count of 50 random grains in each thin section. 40 percent; quartz, 25 percent; perthitic microcline 20 percent) and muscovite, 15 percent. Chemical an- alyses of rocks from near Sprune Pine cited by Hunter (1940) and Olson (1944) show that the "alaskite" there is higher in SiO; and K0 and lower in CaO than most granodiorites but has more CaO, Al;0;, and Na,0 and less K.,0 than most alas- kites. No information on the mineralogy of the ana- lyzed samples is available, however. The mineralogy of rocks of this unit in the Grandfather Mountain area indicates that they may contain more Na;0 and less K;O0 than the analyzed specimens. The granodiorite and pegmatite have been par- tially sheared and recrystallized under the same con- ditions as the wallrocks during the latest meta- morphism. Keith (1903) recognized the polymeta- morphic character of the wallrocks and concluded that the pegmatites had been intruded after one me- tamorphism but before the latest metamorphism. He mentioned that they "were thoroughly crushed and drawn out by the second deformation and retain in many places only a fraction of their original coarse- ness." He believed they were of Archean age. Maurice (1940) recognized three sets of textures in the pegmatities: magmatic, late magmatic, and cataclastic. He stated that "in the majority of the pegmatites the magmatic crystallization was brought to a close by stress resulting in secondary texture * * **" (compare Maurice's figs. 1, 2, 6, and 7 with our fig. 30). On the basis of a chemical age determination on uraninite, he suggested that the pegmatites were of Paleozoic age. Kulp and Poldervaart (1956, p. 339) stated that "the pegmatites and alaskite are believed to have been placed during the later phases of the second regional metamorphism * * * since there is no evi- dence of deformation or metamorphism of these rocks" but added "near the contacts, however, expo- sures may show faint to distinct foliation which par- allels the foliation of the country rocks." Eckelmann and Kulp (1957, p. 1123) stated that "the formation of the pegmatites appears to have concluded the meta- morphic history of the region." Many isotopic age determinations have been made on minerals from the pegmatites of the Spruce Pine district; all samples have come from west of the Grandfather Mountain area. Table 12 summarizes these results. All methods give ages in fair agree- ment at about 350 m.y. It has been widely assumed that this is the date of emplacement of the pegma- tites. However, because micas from the pegmatites and the country rocks give the same results, Long, Kulp, and Eckelmann (1959) recognized that the date of 350 m.y. "probably represents the date of the height of the last regional metamorphism of the area." Whether the age determinations on the miner- als from the pegmatites indicate the date of the last regional metamorphism of the rocks of the Blue Ridge thrust sheet or the date of emplacement of pegmatites is unknown, for the effects of meta- morphism on the contents of the various isotopes of lead and uranium in the uraninite are uncertain. The granodiorite and pegmatite may be synorogenic, vir- tually the same age as the determinations indicate, or they may be older. Pegmatites in the Blue Ridge thrust sheet north and southeast of the window have not been dated, but their mineralogy and structural habit is similar to pegmatites of the Spruce Pine district, and they are probably correlative. Lesure (1968) postulated that the pegmatites in the Blue Ridge belt formed during Paleozoic regional TABLE 12.-Ages of minerals from pegmatites of the Spruce Pine district Age Type of Number of Mineral (Million determina- determina- Source years) tion tions Uraninite_._.... 810-365 Chemical... 8 Eckelmann and Kulp, 1957, table 6 (summary of published deter- minations). Uraninite.__.... 822-875 Isotopic... 8 Eckelmann and Kulp, 1957, table 5. Samarskite . . . __ 300-405 Pb*/Pb* 4 Do. Muscovite. ..... 334-848 K:Arc_____ 8 Long, Kulp, and Eckel- mann, 1959, table 2. Uraninite_._.... 370-420 Isotopic... 2 Davis, Tilton, and Wetherill, 1962, table 4. Muscovite. ..... 335 1 Do. Muscovite. .... 875 1 Do. Microcline... ... 385 f Do. Muscovite. .... 328-348 3 Deuser and Herzog, 1962, table 2. Biotite: =...}... 311 1 Do. BLUE RIDGE THRUST SHEET 53 metamorphism of at least kyanite grade rather than by differentiation from a large underlying batholith because of their simple mineralogy, lack of rare ele- ments that might be expected in late differentiates of a large granitic batholith, and their localization in rocks of at least kyanite grade. QUARTZ MONZONITE AND PEGMATITE IN THE DEEP GAP AREA A swarm of small intrusive bodies of granitic rock and pegmatite of variable composition cuts mica schist and gneiss in the Blue Ridge thrust sheet in northeastern part of the Blowing Rock quadrangle. Dikes, sills, and lenses 1 to 60 feet thick locally make up about 50 percent of the bedrock. The rocks are generally crudely foliated, especially those which are mica poor. One leucocratic granitic sill 8 feet thick contains concordant stringers of peg- matite % to 2 inches thick and has 4 inches of peg- matite at its margins. Xenoliths of wallrock were seen in one pegmatite dike. Some bodies contain partings, indistinct lenses of schist, and numerous feldspathic lenses. Gradations between granitic rock and pegmatite occur. Some quartz and feldspar grains in the coarsest pegmatite are as much as 6 inches in diameter. Muscovite books as much as 3 inches in diameter occur; they are generally bent or ruled. Biotite books as much as 1 inch in diameter occur in a few places. The rocks have a white to salt-and-pepper appear- ance and are fairly fine grained to pegmatitic. They contain various proportions of microcline. Textures are granoblastic to lepidoblastic. 'The larger musco- vite crystals are porphyroclasts. In finer grained rocks having granoblastic textures, microcline re- places oligoclase. Quartz in grains 0.2 to 2 mm in diameter occurs in mosaic-textured aggregates be- tween the larger plagioclase grains. Locally, equant quartz aggregates suggest outlines of a former large grain. These rocks were emplaced before or during the latest metamorphism. They could be the same age as the light-colored granodiorite and pegmatite, or they could be older. They differ from that unit by having a higher biotite and potassic feldspar content and a somewhat more crosscutting structural habit. STRUCTURE AND METAMORPHISM NOT RELATED TO THE BREVARD FAULT ZONE The gross structure of the Blue Ridge thrust sheet in the Grandfather Mountain area is that of an ir- regular dome with foliation and layering dipping away from the Grandfather Mountain window (pls. 1, 2 and fig. 32). Foliation and layering in the Cran- berry Gneiss near the window are generally subpar- allel to the Linville Falls fault ; dips are gentle in the western part of the Linville Falls quadrangle and steep near the southwestern extension of the win- dow. They steepen somewhat near the subsidiary faults northwest of the window. These faults dip more steeply than the Linville Falls fault and are probably cut by it at depth. Abrupt large-scale flex- ures in the foliation and layering trend N. 50° W. from the northwest corner of the window and N. 30° E. from the northeast corner. Large gentle folds are found in the Cranberry Gneiss and Beech Granite in the structural saddle between the Grandfather Mountain and Mountain City windows. In the rocks tectonically overlying the Cranberry Gneiss, dips are gentle and strikes are erratic. The structural and statigraphic relations of these rocks to the Cranberry are complex and are not well un- derstood. In much of the Grandfather Mountain area they are separated from the Cranberry by a meta- morphic and a structural discontinuity which may mark a major fault, a possibility that will be dis- cussed in more detail below. The pattern of the am- phibolite layers in the mica schist and gneiss (pl. 1) does not indicate structural complexity, but minor folds suggest that the gross structure may consist of recumbent sheared-out isoclines. The structure and metamorphism of rocks of the Blue Ridge thrust sheet southeast of the Grand- father Mountain window are intimately related to the Brevard fault zone and are discussed in connec- tion with it. FOLIATION Foliation marked by alined micas, tabular quartz- feldspar laminae, and planar arrangement of amphi- boles is well developed in almost all the rocks in the Blue Ridge thrust sheet. In most of the mica schist, gneiss, and amphibolite, cataclastic effects are lack- ing, and foliation apparently formed during synkine- matic recerystallization. In most of the plutonic rocks and in some of the pegmatites in the mica schist and amphibolite, on the other hand, the foliation is a cataclastic structure formed by partial or complete granulation and recrystallization ; this structure we refer to as cataclastic foliation. Where compositional layering is present, it is parallel with foliation except in the noses of tight minor folds. LINEATION Lineation formed by alined minerals and mineral aggregates and by elongated porphyroclasts and bou- dins is ubiquitous. It is best developed in the Cran- 54 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE 4 45\45 3\35 59 [so iso 45 J as so os fl 3s! pes 4 10 \/5 EX PL ANA TIO N 3+ 25 40 so k ik m__ ___ RoN en etal ain, c Thrust fault Unclassified fault Generalized strike and dip of h Dashed where approximately located . cleavage or foliation Sawteeth on upper plate: K. klipe FiGUrRE 32.-Map of the Grandfather Mountain area showing generalized trends of cleavage and foliation. Grid indicated by letters and numbers along margin corresponds to grid on plate 1. BLUE RIDGE THRUST SHEET 55 berry Gneiss and Beech Granite, where it is a cata- clastic structure formed by drawn-out and incom- pletely granulated and recrystallized minerals. North and west of the window, the lineation in these rocks trends consistently northwest, but the trend ranges from about N. 60° W. by the northwest corner of the Linville quadrangle to N. 20° W. in the mica schist and gneiss in the west-central Linville Falls quad- rangle (fig. 33). In the mica schist and gneiss tecton- ically above the Cranberry Gneiss, the lineation, like the foliation, formed during synkinematic recrystal- lization. Lineation in these rocks generally trends northwest, but in a few places both west and north of the window, northeast-trending lineations are found. In one outcrop near the junction of Pine Run with the New River (area I-2, pl. 1), where both lineations were present, the northwest-trending one seems to be the younger. Regional geologic relations which require north- west transport of the Blue Ridge sheet over the Grandfather Mountain window and Unaka belt indi- cate that the northwest-trending mineral lineation in the thrust sheet is in the a direction. Local evidence in the rocks of the Grandfather Mountain window (discussed below) confirms this interpretation. The northeast-trending lineation may be relict from a de- formation older than the thrusting, but until the structures in adjacent areas are more thoroughly studied, these relations will remain undetermined. MINOR FOLDS Two principal sets of minor folds are found in layered rocks of the Blue Ridge thrust sheet north and west of the Grandfather Mountain window: tight and isoclinal folds having axial planes parallel or nearly parallel with the foliation and layering and axes trending in various directions, and open folds and crinkles having steeply dipping axial planes and northeast-trending axes. The tight and isoclinal folds are most conspicuous in the markedly layered parts of the Cranberry Gneiss near the contact with the overlying mica schist and gneiss and amphibolite. They are well ex- posed in roadcuts along Golden Creek (area C-5, pl. 1) and along the Elk River (area J-8, pl. 1). Large folds of this type are exposed just north of the Grandfather Mountain area (Hamilton, in King and Ferguson, 1960, pl. 44). The rare outcrops in which foliation and layering are not parallel are in the noses of these folds (fig. 5). The folds have an ampli- tude ranging from a few inches to at least tens of feet. Figure 14 on plate 5 shows the orientation of the tight and isoclinal folds west of the Grandfather Mountain window and their geometric relations to the foliation, layering, and mineral lineation. Axial planes of the folds are parallel with the foliation and layering, and their axes form a girdle in the foliation which has a maximum parallel with the mineral lin- eation. Figure 1B on plate 5 shows that a somewhat simi- lar relationship exists north of the window in the Blowing Rock quadrangle, but there the axes are more evenly distributed along the girdle and do not form a maximum near the mineral lineation. The open folds and crinkles are most conspicuous in phyllonite and phyllonitic gneiss in the area of outcrop of Cranberry Gneiss. These folds are most numerous in the saddle between the Mountain City and Grandfather Mountain windows. In places, slip cleavage has developed parallel with their axial planes. Very few folds of this type were identified west or northeast of the window. A photograph (Hamilton in King and Ferguson, 1960, pl. 7) shows typical crinkles of this type in a phyllonite zone just north of the Grandfather Mountain area. The axial planes of the open folds are generally steeply dipping and perpendicular to the mineral lin- eation; their axes are subhorizontal or northeast plunging and are perpendicular to the mineral linea- tion (compare pl. 5, fig. 1, A-2 and B-2 with pl. 5, fig. 1, C-1). Plotting the attitudes of axes and axial planes of open, transitional, and tight folds north of the win- dow in the Blowing Rock quadrangle shows a transi- tion between them (Bryant and Reed, 1970, fig. 6). These relations suggest that open folds may have developed during thrusting and then may have been progressively rotated, tightened, and flattened dur- ing northwestward transport of the Blue Ridge thrust sheet. ‘ METAMORPHISM The rocks of the Blue Ridge thrust sheet have been metamorphosed at least twice and perhaps as many as four times. The present distribution of rock of different metamorphic grades is largely due to the most recent metamorphism, which may be of a dif- ferent age in different areas. Biotite-bearing low- grade rocks form a rim almost surrounding the Grandfather Mountain window and occupying the saddle between the Grandfather Mountain and Mountain City windows (pl. 6D). This rim almost coincides with the distribution of the plutonic rocks. 56 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE o G | H | | | J 81°30" & N by * 10 x #s . vG so May ASL Osy 36°15 30 /o\ Pike is «"o 1s & \w>ys\\“\°\§\ s. a 35 Hecht A 5 MILES l EX~PL ANAT IQ N ~ --t %--> wooo thew hous, ae me ime a Gira i i 5 Plunging Horizontal 11 Thrust fault Unclassified fault Generalized bearing and plunge Dashed where approximately located. of mineral lineation Sawteeth on upper plate; K, klippe FicurE 33.-Map of the Grandfather Mountain area showing generalized trends of mineral lineation. Grid indicated by letters and numbers in margin corresponds to grid on plate 1. Mineral lineation throughout the area lies on cleavage or foliation planes whose attitudes are shown in figure 32, but because of the generalized nature of the maps, plunges of lineation shown in this figure cannot everywhere be reconciled in detail with attitudes of the planar structures. BLUE RIDGE THRUST SHEET 57T West, north, and northeast of the window, outside the rim of low-grade rocks, are medium-grade rocks approximately coinciding with the outcrop area of schist, gneiss, and amphibolite. Southeast of the window and in the intermediate sheet between the Mountain City and Grandfather Mountain windows, the latest metamorphism is of chlorite grade. The metamorphic histories of these rock units and var- ious regional geologic relations and mineral ages dis- cussed below allow us to decipher a history of the rocks and to point out some problems and uncertain- ties. The earliest metamorphic event recorded in the rocks of the Blue Ridge thrust sheet is the plutonic metamorphism during which the Cranberry Gneiss formed and sedimentary and volcanic rocks were converted to schist, gneiss, and amphibolite (pl. 6B). The hypersthene and monoclinic pyroxene in the Cranberry Gneiss on Little Hump Mountain may be relicts from this episode of metamorphism. If so, the metamorphism probably took place under conditions of the granulite facies, and the Cranberry Gneiss may have once been a charnockite. Jonas (1935) rec- ognized similar rocks in northern and central Vir- ginia, and she believed that the granitic rocks in southwestern Virginia and northwestern North Car- olina were the metamorphosed equivalents of the py- roxene-bearing plutonic rocks farther north. The southernmost occurrence of hypersthene in the Blue Ridge belt so far mentioned in the literature is in northern Floyd County, Va. (Dietrich, 1959). Uranium-lead age determinations on zircon from the Cranberry Gneiss and related rocks in the region indicate that the Cranberry Gneiss formed 1,000 to 1,100 m.y. ago, at about the same time as basement rocks in the Grandfather Mountain window (Davis and others, 1962) ; one potassium-argon age determi- nation of 1,130 m.y. on hornblende (Thomas, 1963) indicates that the date of metamorphism of the schists, gneisses, and amphibolitee was approxi- mately the same. Whether the granitic rocks intru- sive into the Cranberry Gneiss were emplaced at that time or somewhat later is uncertain. Isotopic ages of zircon from the Beech Granite in northeastern Ten- nessee and from the granite just west of the window are all discordant and range from 360 to 820 m.y. (Davis and others, 1962). Unfortunately, no isotopic age determinations are available for zircon from Cranberry Gneiss near those localities. The granites might have been emplaced 800 to 900 m.y. ago, rather than immediately after the metamorphism 1,000 to 1,100 m.y. ago. If some of the metamorphic rocks of the Blue Ridge thrust sheet are part of the Ashe Formation, as recently suggested by Rankin (1970) and Hadley (1970), these rocks would have been subjected only to the Paleozoic events described below. Field rela- tions described above suggest that at least some of the metamorphic rocks are older, as they are all con- sidered to be in our interpretation. In Canada, many potassium-argon dates on micas from the Grenville province have a frequency peak between 930 and 940 m.y. but have considerable scat- ter. On the basis of these ages, a "Grenville Oro- geny'" is placed in the age range from 880 to 1,000 m.y. (Stockwell, 1964, fig. 2). A compilation of iso- topic ages of uraninite, zircon, and cleveite from southern Canada and northern New York State shows a peak at about 1,030 m.y. (Tilton and others, 1960, table 3, samples 1-7). Potassium-argon ages on mica are lower than the ages on the zircon from the same rock (Tilton and others, 1960, table 4, sample 5), and rubidium-strontium ages are generally greater than the potassium-argon ages. These data might be interpreted to show that the "Grenville Or- ogeny" lasted 150 m.y. or that the orogenic belt was not uplifted and cooled until 150 m.y. after the be- ginning of the orogeny. We prefer to consider that the "Grenville Orogeny" in the southern Appalachi- ans occurred 1,000 to 1,100 m.y. ago, and we suggest that the apparent younger age of that event in the Grenville province of the Canadian Shield is due to the use of only potassium-argon ages of micas in determining the age of the orogeny. The Bakersville Gabbro and related mafic rocks intruded the rocks of the Blue Ridge thrust sheet after this metamorphism and before the next, as Kulp and Poldervaart (1956) recognized. It is tempt- ing to correlate the Bakersville with the Linville Me- tadiabase in the Grandfather Mountain window be- cause of general similarity in lithology and age, but there is no direct evidence for such a correlation. Potassium-argon ages of micas from several places in the Blue Ridge thrust sheet suggest that there may have been an episode of metamorphism between 450 and 550 m.y. ago (Kulp and Eckelmann, 1961). One of these samples is from about 10 miles west of the Grandfather Mountain area; another is at Ore Knob 10 miles northeast of the area (Thomas, 1963). Medium-grade dynamothermal - metamorphism during the middle Paleozoic (pl. 6C) has produced kyanite, staurolite, biotite, muscovite, garnet, and calceic oligoclase or sodic andesine in schist and gneiss; epidote, monoclinic pyroxene, and andesine- 58 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE labradorite in cale-silicate rocks; and diopside, epi- dote, hornblende, and calcic oligoclase or sodic ande- sine in amphibolites. An attempt to construct an ACF diagram for these medium-grade rocks was un- successful, probably because of their polymeta- morphic character. Potassium-argon ratios in micas from the Spruce Pine district indicate that this me- tamorphism occurred about 350 m.y. ago. The peg- matite and granodiorite may have been emplaced prior to or during an early stage of this metamor- phism. Whether the Cranberry Gneiss underwent the me- dium-grade metamorphism during the middle Paleo- zoic is unknown, but if it did not, the existence of a major fault between it and the structurally overlying rocks would be proved, as the overlying rocks are of medium grade (pl. 6D). Ages of micas from base- ment rocks in the intermediate sheet between the Blue Ridge thrust sheet and the Mountain City win- dow are inconsistent but range from 890 to 660 m.y., showing that those rocks were never subjected to medium-grade metamorphism during the Paleozoic (Long and others, 1959; Davis and others, 1962). Biotite from rocks mapped in the Blue Ridge thrust sheet in northeastern Tennessee by Rodgers (1953a) gave potassium-argon ages in the range from 380 to 695 m.y., and one sample gave a rubidium-strontium age of 715 m.y., showing that the biotite in those rocks was not completely recrystallized during the Paleozoic. No dates are available from the Cranberry Gneiss of the Blue Ridge thrust sheet in the Grand- father Mountain area ; the nearest date in Tennessee is from a locality about 5 miles away. Biotite from medium-grade Cranberry Gneiss on Roan Mountain, also about 5 miles west of the Grandfather Mountain area, has a potassium-argon age of 357 m.y., indicat- ing a Paleozoic history for the rock similar to that of the main part of the Spruce Pine district. If a major fault exists between the rocks of low metamorphic grade and those of medium grade, it may connect with the Gossan Lead and Fries faults of southwestern Virginia (Stose and Stose, 1957, map), as was shown on the 1944 tectonic map of the United States (King and others, 1944). The same fault should lie between Roan Mountain and the belt of iron prospects in northeastern Tennessee (Bayley, 1923). No information is available on the possible position of this fault west of the Spruce Pine district, but the geologic map of the Roan Mountain quadran- gle (Keith, 19072) indicates that it might be found near Hughes Gap on the North Carolina-Tennessee State line and somewhere between Red Hill and Re- lief in North Carolina. If the Ashe Formation extends into the northern part of the Grandfather Mountain area, as suggested by Rankin (1970), we think that the map and struc- tural relations along the Cranberry contact necessi- tate a major fault. A considerable thickness of the Ashe is cut out in a southeasterly direction along the contact in much too sudden a manner to represent a basin margin. Coarse-grained clastic rocks are ab- sent in the metamorphic rocks along this contact. Most of the plutonic rocks in the Blue Ridge thrust sheet in the Grandfather Mountain area have been incompletely metamorphosed at low grade (pl. 64, B). New albite, microcline, biotite, and sericite occur in rocks containing sufficient potassium, whereas chlorite, actinolite, epidote, and albite occur in rocks deficient in that element. Some of the sericite is of the green iron-rich variety. Granitic rocks have been incompletely metamorphosed in most places and con- tain numerous porphyroclasts of earlier minerals in a matrix of later finer grained minerals produced by cataclastic breakdown and subsequent recrystalli- zation of minerals of the plutonic generation. Most of the rocks are blastomylonitic or phyllonitic gneisses. Locally, all traces of the plutonic minerals have been destroyed to produce blastomylonite and, under certain conditions, phyllonite. Mafic igneous rocks contain rare relict pyroxene, and the arena- ceous rocks in the fault slices have reliet clastic tex- ture. The low-grade mineral assemblages and textures are the same as those of the Grandfather Mountain window. 'The boundary between the incompletely metamor- phosed rocks of low grade and the rocks of medium grade is approximately at the contact between the Cranberry Gneiss and the tectonically overlying mica schist and amphibolite, except at the west mar- gin of the Linville quadrangle, where the boundary is in the Cranberry Gneiss. Near this contact, the medium-grade rocks are partly crushed and contain retrogressive mineral assemblages. No date for the low-grade metamorphism has been established. The well-developed northwest-trending cataclastic mineral lineationand the cataclastic fol- iation subparallel with the Egnville Falls fault sug- gest that the metamorphism may have occurred dur- ing some phase of thrusting. Thus, it probably occurred later than the metamorphism 350 m.y. ago and earlier than the end of thrusting in the late Pa- BLUE RIDGE THRUST SHEET 59 leozoic. (See below for discussion of the age of thrusting.) The tight and isoclinal folds in the rocks of the Blue Ridge thrust sheet are inferred to have formed during an early stage of thrusting because of the similarity in pattern between them and similar folds in the Table Rock thrust sheet (discussed below). They seem to have formed by tightening, flattening, and passive rotation of earlier more open folds origi- nally formed perpendicular to the direction of trans- port of the thrust sheet (Bryant and Reed, 1970, b) The crinkles and open folds may have formed at a late stage of thrusting or immediately afterwards. Their orientation in relation to the mineral lineation indicates a response to similarly oriented stress. Adjacent to the Linville Falls fault southeast of the window in the Blowing Rock quadrangle, a nar- row zone of biotite-muscovite schist and gneiss has polymetamorphic textures and has been recrystal- lized to medium grade. These rocks contain porphy- roclasts of muscovite in a fairly fine grained matrix of recrystallized quartz, oligoclase, muscovite, and biotite. Cataclasis and recrystallization probably took place during thrusting. Most of the rocks southeast of the Grandfather Mountain window in the Blue Ridge thrust sheet have been overprinted by struc- tural and metamorphic effects associated with the Brevard fault and described below. Table 30 (p. 172) summarizes the structural and metamorphic events and their established or inferred dates and sequence. INTERMEDIATE SHEET In the intermediate sheet, which is mapped as Cranberry Gneiss in the northwestern part of the Linville quadrangle (pl. 1) and which lies tectoni- cally below the Blue Ridge thrust sheet and above the Mountain City window, layering of the plutonic generation is cut by southeast-dipping phyllonite zones. We do not have enough information to deduce the pattern formed by the layering. Much of the rock has not undergone shearing and retrogressive meta- morphism, and the pervasive northwest-trending mineral lineation characteristic of the adjacent rocks of the Blue Ridge thrust sheet is poorly developed. Some northeast-trending crinkles are found in the phyllonite zones. Presumably, the plutonic rocks in the sheet were formed 1,000 to 1,100 m.y. ago, at the same time as those in the Blue Ridge thrust sheet and the Grand- father Mountain window. Paleozoic metamorphism was local and incomplete, and the partial recrystalli- zation was in the chlorite zone. Biotite from a partly altered and somewhat sheared rock on Dark Ridge Creek gave a rubidium-strontium age of 810 m.y. and a potassium-argon age of 660 m.y. (Davis and others, 1962). GRANDFATHER MOUNTAIN WINDOW The oldest rocks in the Grandfather Mountain window are layered migmatite gneiss and metamor- phosed diorite and gabbro that grade into and are cut by granitic rocks of Precambrian age. In the northwestern part of the window, these plutonic rocks are unconformably overlain by the Grand- father Mountain Formation, a thick sequence of in- terlensing arkose, siltstone, shale, and volcanic rocks of late Precambrian age. The bedded rocks occupy the southeastern limb of a complex synelinorium in which at least two generations of structures are su- perimposed. The northwestern limb is concealed be- neath the Blue Ridge thrust sheet northwest of the window. The basement rocks are intruded by bodies of metadiabase and felsic porphyry which probably correlate with extrusive rocks in the bedded sequence. The granitic rocks were pervasively sheared, retro- gressively metamorphosed, and locally converted to phyllonite and blastomylonite at the same time that the overlying sedimentary and volcanic sequence was progressively metamorphosed. Both the granitic rocks and the overlying sequence are apparently au- tochthonous. Also exposed in the window are Cambrian and Cambrian (?) quartzite and phyllite of the Chilhowee Group and the overlying Lower Cambrian Shady Do- lomite. These rocks form the Tablerock thrust sheet, which has ridden at least 15 miles northwestward over the autochthonous rocks beneath. The Tablerock thrust sheet occupies much of the southwestern part of the window and contains minor structures geo- metrically related to those in the overlying Blue Ridge thrust sheet. Rocks in the Tablerock thrust sheet have been metamorphosed at the same grade as the underlying autochthonous rocks. AUTOCHTHONOUS ROCKS METAGABBRO AND METADIORITE Small bodies of greenish-gray gneissose to phyl- lonitic metagabbro and metadiorite crop out in a few places among the plutonic basement rocks in the Grandfather Mountain window. Along Brown Creek and near Upton (areas H-3, G-4, and G-5 pl. 1) several bodies are large enough to map. These bodies 60 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE are not well exposed, and the relations of the Brown Creek body are obscure enough to make its correla- tion with the other bodies uncertain. Interior parts of some of these bodies resemble the Linville Meta- diabase, but they have gradational boundaries with adjacent granitic rocks and are cut by quartz-feldspar pegmatites. The metagabbro and metadiorite are older than the adjacent granitic rocks, but their age relations to nongranitic layers in layered Wilson Creek Gneiss are not known. The rocks lack obvious relict igneous textures. They are composed of saussuritized plagioclase, hornblende, quartz, actinolite, and chlorite in propor- tions that depend on the original composition of the rock and the degree of shearing and recrystallization during the latest metamorphism. Sphene, opaque minerals, and apatite are accessory minerals. A few grains of relict pyroxene occur in hornblende in the Brown Creek body. WILSON CREEK GNEISS The Wilson Creek Gneiss (Bryant, 1962) is a meta- morphosed plutonic complex of Precambrian age occupying much of the eastern part of the Grand father Mountain window. Various parts of the Wil- son Creek Gneiss were mapped by Keith (1903) as Cranberry Granite, Carolina Gneiss, Beech Granite, metarhyolite, and Unicoi Formation. The Wilson Creek Gneiss consists principally of nonlayered plu- tonic rock having compositions ranging from diorite to granite and averaging quartz monzonite. Parts of the gneiss are layered, and in areas I-5, J-4, and K-4, the predominantly layered rocks are distin- guished on the geologic map (pl. 1). Areas where the gneiss is predominantly dioritic are also separated on the map but have very indefinite contacts (areas I-4, I-83, J-8, J-4). Phyllonite, blastomylonite, and mylonite, mostly derived from Wilson Creek Gneiss are mapped separately in areas I-3 and I-4, and a small area of more uniform quartz monzonite in the Wilson Creek Gneiss near Rose Mountain is deline- ated in areas E-7, E-8, F-7, and F-8. The Wilson Creek Gneiss forms large outcrops along Wilson Creek and many other major streams on the Blue Ridge front. Locally, the gneiss is diffi- cult to distinguish from coarse-grained nonbedded arkose of the Grandfather Mountain Formation be- cause both rock types are pervasively sheared and the arkose lacks bedding. However, the gneiss gener- ally contains white quartz-plagioclase-potassic feld- spar pegmatites, whereas the arkose contains only segregations of quartz or quartz and pink microcline. Much of the Wilson Creek Gneiss contains biotite, which the arkose lacks. MEGASCOPIC FEATURES The main body of Wilson Creek Gneiss has a vari- ety of aspects because of shearing and recrystalli- zation. It ranges from almost massive, slightly crushed granitic rock to phyllonite and blastomylon- ite that locally resemble phyllite and volcanic rock. Typically, it is light-gray to greenish-gray medium- to coarse-grained biotite-quartz-feldspar gneiss (fig. 34). The gneiss has a conspicuous cataclastic foliation defined by folia of fine-grained sericite and biotite and a well-developed cataclastic lineation formed by streaks of mica, quartz, or feldspar trains of alined mica, and grooving. Plagioclase-rich gneiss generally his stronger foliation than gneiss rich in potassic feldspar because of decomposition of calcic plagioclase into albite, sericite, and epidote during Paleozoic retrograde metamorphism. The gneiss lo- cally contains layers and pods of biotite schist and biotite amphibolite. Dikes and pods and irregular bodies of mica-poor quartz-feldspar pegmatite are widespread in the Wil- son Creek Gneiss. They range from a few inches to more than 100 feet in thickness and contain feldspar crystals as much as 4 inches long. Some have sharp contacts with the surrounding granitic rock, but other contacts are gradational. The pegmatite con- sists of quartz, perthite, and plagioclase. It locally contains books of muscovite as much as 1 inch in diameter, but they are rare. Allanite and garnet lo- cally occur as accessory minerals. Generally, the peg- matite seems to be little foliated, especially in expo- sures perpendicular to the cataclastic mineral linea- tion, but in many places it has crude foliation despite its lack of mica. Locally, it is an augen gneiss. In some places, the gneiss is cut by thin dikes of fine- to medium-grained light- to medium-gray quartz mon- zonite in which foliation is parallel to that in the surrounding coarser grained gneiss. These dikes cut some of the pegmatites and are in turn cut by others. In addition to the pegmatites, the gneiss contains segregation lenses, stringers, and veins of quartz, quartz and chlorite, and rarely, quartz and pink mi- crocline or quartz, calcite, pink microcline, and chlor- ite. Quartz veinlets are found along joints in some rocks. Veinlets of epidote are found in some of the more plagioclase-rich rocks. Angular blocks of finely layered amphibole gneiss occur as xenoliths in a coarse pegmatitic phase of the GRANDFATHER FIGURE 34.-Wilson Creek Gneiss. A, Typical cataclastic granitic gneiss. From roadcut on east side of North Caro- lina Highway 181 about 0.5 mile south of bench mark 2868 on Ripshin Ridge (area E-6, pl. 1). B, Cataclastic granitic gneiss and pegmatite. From road-metal quarry on east side of Wilson Creek about 0.5 mile northwest of Mortimer (area F-6, pl. 1). This is the location of the sample from MOUNTAIN WINDOW 61 which zircon was dated by Davis, Tilton, and Wetherill (1962). C, Blastomylonite zone in granitic gneiss and pegmatite. From outcrop along Wilson Creek at 1,760-foot altitude (area F-5, pl. 1). View subperpendicular to cata- clastic lineation. Note gradations to blastomylonite con- taining feldspar porphyroclasts. 62 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE GRANDFATHER MOUNTAIN WINDOW 63 FiGuUrE 35.-Photomicrographs of Wilson Creek Gneiss. A, Recrystallized quartz monzonite mortar gneiss from south fork of Harper Creek 1 mile downstream from Kawana (area F-6, pl. 1). Granoblastic texture with zones of re- crystallized quartz mortar. Poikilitic inclusions of quartz and plagioclase in microcline. Microcline bent, broken, and healed by quartz. Larger quartz grains bent and partly broken down into mosaic-textured areas. Plagioclase saus- suritized. B, Blastomylonitic quartz monzonite gneiss from Harper Creek at the Avery-Caldwell County line (area F-6, pl. 1). Porphyroclasts of plagioclase (pseudomorphosed by albite), microcline, and quartz surrounded by mosaic- textured quartz and recrystallized biotite, sericite, and albite. Section perpendicular to mineral lineation. C, Blastomylonitic biotite-epidote-sericite quartz monzonite gneiss from quarry on Bark Camp Ridge (area F-5, pl. 1). Bent and broken porphyroclasts of plagioclase (pseudo- morphosed by albite) and microcline in a groundmass of recrystallized quartz, sericite, albite, biotite, and epidote. Section parallel with mineral lineation. D, Porphyroclastic mylonite from east end of hill 3715 about 1 mile south- east of Dry Pond Gap (area I-3, pl. 1). Crushed and strung-out porphyroclasts of plagioclase (pseudomorphosed by albite), potassic feldspar, and quartz in a matrix of quartz, feldspar, and recrystallized iron-rich muscovite. Section parallel to mineral lineation. E, Porphyroclastic blastomylonite from outcrop along Wilson Creek at an altitude of 1,760 feet (area F-5, pl. 1). From specimen of fine-grained rock shown in figure 34C. Small porphyro- clasts of saussuritized plagioclase and biotite in a matrix of recrystallized quartz, sericite, biotite, and epidote. Sec- tion perpendicular to lineation. F. Blastomylonite from an altitude of about 2,600 feet in tributary valley east of Yadkin River just east of Blowing Rock Gneiss contact and north of Watauga-Caldwell County line (area H-4, pl. 1). Quartz, albite, sericite, biotite, and subordinate microcline. Wilson Creek Gneiss on Upper Creek 0.5 mile above the mouth of Burnthouse Branch (area E-6, pl. 1). PETROGRAPHY The typical Wilson Creek Gneiss (fig. 35B and C) is composed of porphyroclasts of microcline, plagio- clase, biotite, and quartz set in a matrix of fine- grained quartz, plagioclase, sericite, microcline, bio- tite, chlorite, and epidote. The porphyroclasts are a few millimeters to as much as 1 cm in diameter; matrix grains range from 0.01 to 0.5 mm in diame- ter, but most are about 0.1 mm. More mafic varieties of gneiss contain porphyroclasts of amphibole. Mus- covite porphyroclasts are rare. The potassic feldspar forms conspicuous porphy- roclasts, some as much as 5 cm in diameter. Rock containing such large porphyroclasts resembles the Blowing Rock Gneiss (see below). The common grain size for the potassic feldspar porphyroclasts is 5 mm. They contain inclusions of plagioclase, quartz, and mica. The porphyroclasts are bent and broken, and the fractures are healed by quartz, sericite, al- bite, biotite, carbonate, or a combination of two or three of these minerals (fig. 35B and D). Most of the potassic feldspar is microcline, but some is fine-tex- tured string perthite. Quartz granulates and recrystallizes more easily than the feldspar in many rocks. It forms mosaic- textured areas between feldspar grains even in rela- tively unsheared rocks (fig. 354 and B). In places the quartz is concentrated in lenses and stringers which may represent strung-out and recrystallized larger grains. Porphyroclasts, which are as much as 8 mm in diameter, are strongly strained. Many are brecciated or partly granulated and recrystallized into mosaic-textured aggregates or recrystallized grains or mortar. Plagioclase forms porphyroclasts as much as 8 mm in diameter. They are commonly bent and locally are broken and healed by the minerals of the recrystal- lized matrix. Albite twinning is widespread, peric- line twinning, common, and Carlsbad twinning rare. Almost all the plagioclase grains have been saussuri- tized, except those in a few rocks in the tectonic slice of Wilson Creek Gneiss above the Tablerock thrust sheet in North Cove (area A-10, pl. 1). There, relict andesine and oligoclase occur. In a few places the altered porphyroclasts have a rim of clear albite. In more sheared rocks the albite, sericite, and epidote derived from breakdown of the plagioclase become separated in varying degrees into clear recrystallized albite, lepidoblastic aggregates of sericite, and dis- crete grains of epidote. Brown or reddish-brown biotite forms porphyro- clasts as much as 2 mm in diameter. Biotite porphy- roclasts are commonly bent and have sagenitic struc- ture. They are generally partly altered to chlorite or lighter colored biotite. Biotite in the matrix is brown, greenish brown, or brownish green and occurs in undeformed flakes about 0.2 mm long. A few flakes are as much as 0.5 mm long. Most muscovite is fine grained and is derived from breakdown of feldspars, especially plagioclase, but a few bent and partly altered porphyroclasts occur. Limited X-ray data indicate that paragonite is not present in detectable amounts. The new muscovite is mostly synkinematic, but some is postkinematic. It forms flakes generally 0.1 mm long but as much as 64 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE 0.3 mm long. The new mica in some rocks is green and has a high index of refraction ; these properties indicate that the mica is similar to the green iron- rich muscovite so widespread in the Grandfather Mountain Formation. Both the new biotite and mus- covite are concentrated in segregations along cleav- age planes. Anhedral grains and aggregates of epidote are widespread in the groundmass of the gneiss. Grain size averages 0.05 to 0.1 mm but reaches as much as 0.5 mm, especially in segregation veinlets. Epidote seems to have been derived mainly from plagioclase. Locally, it has a light-brown absorption, indicating some rare-earth content. Mafic rocks contain porphyroclasts of hornblende or actinolite as much as 2 mm long. Hornblende com- monly has a rim of actinolite. Recerystallized amphi- bole is actinolite. Garnet is found locally as an accessory mineral in the Wilson Creek Gneiss. It occurs in the ground- mass as subhedral to euhedral crystals that seem to be contemporaneous with the rest of the groundmass minerals, and it also occurs as earlier inclusions in plagioclase porphyroclasts. Some of the garnet has a light yellow absorption. Chlorite is a major constitutent of a few of the mafic rocks. Most of it is drived from biotite. It is generally green and is mostly the FeMg variety, al- though MgFe chlorite also occurs. Other accessory minerals are sphene, ilmenite, leu- coxene, magnetite, apatite, zircon, stilpnomelane, QUARTZ PLAGIOCLASE POTASSIC FELDSPAR carbonate, allanite, and fluorite. Zircon ranges from round to euhedral. Normal Wilson Creek Gneiss ranges in composi- tion from diorite to granite (fig. 36A). Pegmatites range from somewhat altered rocks containing only partly chloritized biotite or saussuritized plagioclase to slightly cataclastic rocks containing a small amount of recrystallized mortar to blastomylonitic gneiss (fig. 37). They range in composition from granite to quartz diorite (fig. 36B), but they lack biotite, whereas the Wilson Creek Gneiss typically has a few percent of that mineral. The fine-grained discordant dikes are similar in composition and tex- ture to the pegmatites. Most compositional varieties of the Wilson Creek Gneiss are so irregularly dis- tributed that they cannot be mapped separately. LAYERED GNEISS A conspicuously layered phase of the Wilson Creek Gneiss crops out over an extensive area in the north- eastern part of the window (pl. 1). Contacts with typical nonlayered Wilson Creek Gneiss are almost everywhere gradational, and layers are found in the Wilson Creek Gneiss at scattered outcrops as much as 2 miles away from mapped units of layered gneiss. The lens of nonlayered Wilson Creek Gneiss in Chestnut Mountain (area I-5, pl. 1) however, has sharp contacts with the enclosing layered gneiss. The two bodies of layered gneiss mapped in the central Blowing Rock quadrangle (areas I-3 and H-4, pl. 1) are not well exposed. They are phyllonitic and more mafic than the adjacent rock. QUARTZ PLAGIOCLASE POTASSIC FELDSPAR FIGURE 36.-Proportions of quartz, plagioclase, and potassic feldspar in Wilson Creek Gneiss. Epidote and sericite counted as plagioclase. A. Typical granitic gneiss. Based on count of 50 random grains in 116 thin sections of rock retaining well-defined plutonic aspect. Contours 1, 2, 4, 6, and 8 percent. B, Pegmatite and dikes of granitic rock cutting typical gneiss. Based on count of 50 random grains in each of 19 thin sections. Contours 5, 10, and 15 percent. GRANDFATHER MOUNTAIN WINDOW 65 FIGURE 37.-Photomicrographs of pegmatite in Wilson Creek Gneiss. A, Sheared pegmatite from discordant dike 8 inches thick in roadcut along North Carolina Highway 181 at the head of Ripshin Branch (area E-6, pl. 1). Large crystals of perthitic microcline, plagioclase (pseudo- morphosed by albite), and quartz are bent, broken, and healed by quartz, albite, epidote, sericite, and calcite. Section at 45° to mineral lineation. B, Sheared pegmatite from railroad cut east of Woodlawn (area B-10, pl. 1). Porphyroclasts of quartz partly broken down and re- crystallized into mosaic-textured aggregates. Porphyro- clasts of perthitic microcline bent and broken. Porphyro- clasts of plagioclase saussuritized and smeared out. Section perpendicular to mineral lineation. The layered gneiss consists of interlayered and in- tergrading plagioclase porphyroclast gneiss, blasto- mylonitic granitic gneiss, biotite-chlorite schist, mus- covite-biotite schist, biotite gneiss, epidote amphibol- ite, biotite amphibolite, hornblende gneiss, and peg- matite. Layers are one-half of an inch to tens of feet thick. Late segregation lenses are composed of quartz, quartz and pink microcline, quartz and chlor- ite, and quartz, calcite, biotite, and chlorite. The layered Wilson Creek Gneiss is similar to much of the Cranberry Gneiss of the Blue Ridge thrust sheet, but the irregular and gradational contact between the layered and the typical Wilson Creek Gneiss indicates that the layered gneiss is in- side the window. Cranberry Gneiss may have been adjacent to and southeast of presently exposed lay- ered Wilson Creek Gneiss before thrusting. The granitic layers are similar to the bulk of the nonlayered Wilson Creek Gneiss, except that directly adjacent to the edge of the window they are more coarsely recrystallized. Most of the biotite schist lay- ers have been completely recrystallized, but in horn- blende gneiss and amphibolite, much of the amphi- bole is porphyroclastic and unaltered or only partly altered to actinolite. Plagioclase is mostly altered to albite even where preserved as porphyroclasts. QUARTZ MONZONITE GNEISS One area of little-foliated coarse-grained light-col- ored quartz monzonite in the Wilson Creek Gneiss is mapped near Rose Mountain (areas E-7, E-8, F-7, F-8, pl. 1). Other unmapped bodies of similar aspect may occur in the Wilson Creek Gneiss, but if so, they are probably less than a few hundred yards in diameter. The rock is a blastomylonitic gneiss or recrystallized mortar gneiss containing porphyro- clasts of microcline, - microcline-microperthite, quartz, and saussuritized plagioclase several millime- ters in diameter in a recrystallized groundmass of mosaic-textured quartz and minor amounts of brownish-green biotite and sericite. The scarcity of mica accounts for the poor development of foliation. Other accessory minerals are epidote, zircon, fluorite, chlorite, allanite, and opaque minerals. 0 PHYLLONITE AND BLASTOMYLONITE Phyllonite and blastomylonite are found in zones a few inches to thousands of feet wide throughout the Wilson Creek Gneiss, but they are most abundant in the southeastern part of its outcrop area. In the cen- tral part of the Grandfather Mountain window west of the belt of Blowing Rock Gneiss, phyllonite gener- ally occurs in well-defined zones concordant with the foliation in the enclosing gneiss, but the zones are discontinuous both laterally and vertically. 66 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE East of the outcrop belt of the Blowing Rock Gneiss, the degree of shearing and recrystallization is somewhat greater, and blastomylonites and phyl- lonites are so widely distributed and intimately mixed with recognizable plutonic rocks that they generally cannot be mapped separately. Near their margins, the phyllonite zones com- monly contain lenses, layers, and pods of less- sheared gneiss and relatively unsheared pegmatite and porphyroclasts of quartz, feldspar, and mica. In many places, the phyllonites lack these relicts and cannot be distinguished from phyllites. The phyllonite is generally medium gray but may be silvery gray, black, or greenish gray. It commonly weathers brown or red. Locally, phyllonite forms the matrix of a brec- cia composed of gneiss and pegmatite fragments. The phyllonite zones locally contain disseminated graph- ite or pyrite. The graphitic zones are a few inches to a few feet thick, and individual flakes of graphite are as much as 1 mm in diameter. Some phyllonite zones only a few inches thick are lens shaped when viewed perpendicularly to the lineation. Some zones only 1 to 3 mm thick are as much as 1 foot long parallel with the lineation. The larger discontinuous phyllonite zones may also be elongate parallel with the linea- tion. The phyllonites in many places have two lineations at approximately right angles, one formed by elon- gated and alined mineral grains and the other formed by small crinkles in the foliation. The phyllonites contain quartz segregation string- ers and knots. Epidote knots containing pyrite and sphalerite(?) occur in a few places. Locally, the phyllonites have been enriched by iron in the form of hematite, magnetite, or pyrite, by titanium in the form of ilmenite, and by uranium in the form of torbernite (perhaps originally uraninite). The phyllonites and blastomylonites contain 30 to 80 percent sericite, 4 to 60 percent quartz, 0 to 30 percent biotite, 0 to 30 percent chlorite, and 0 to 25 percent epidote, depending on the composition of the parent rock. All gradations between gneiss and phyl- lonite are found. A typical phyllonite contains about 30 percent quartz, 60 percent sericite, and the re- maining 10 percent opaque minerals, biotite, chlor- ite, epidote, and sphene. In some phyllonites, the only mafic mineral is magnetite or ilmenite, but it is finely distributed and gives the rock a dark-gray color. Other accessory minerals are pyrite, calcite, stilpno- melane, zircon, apatite, tourmaline, allanite, pyrrho- tite, and porphyroclastic garnet. Quartz, potassic feldspar, plagioclase, biotite, muscovite, or amphi- bole form porphyroclasts in some phyllonites. The blastomylonite mapped in areas I-3 and I-4 (pl. 1) was in part mapped by Keith (1903) as me- tarhyolite. Some of the rocks closely resemble felsic volcanic rocks both in outcrop and thin section. However, along Bailey Camp Creek, gradations in texture between blastomylonitic gneiss and rocks re- sembling felsic volcanics can be seen. In other places, the blastomylonite contains relict lenses of phylloni- tic and blastomylonitic gneiss and pegmatite. The blastomylonite is a gray to dark-gray and green aphanitic rock locally containing carbonate or quartz-chlorite segregations. It contains porphyro- clasts of quartz and feldspar in a matrix of sericite, quartz, and feldspar too fine grained to be accurately identified under the microscope (fig. 35D). Some of the rocks contain considerable amounts of chlorite and epidote and were apparently derived from dio- rite or quartz diorite. A few of the mafic rocks con- tain lath-shaped plagioclase grains and may there- fore have been derived from mafic igneous rock. Lo- cally, the quartz porphyroclasts are subhedral and give the blastomylonite the appearance of a volcanic rock. Some of the rocks mapped as blastomylonite may be volcanic rocks, but if so, they constitute only a minor and unmappable part of the unit. Blastomy- lonite is found locally in the Wilson Creek Gneiss outside the area mapped as blastomylonite. OTHER ROCKS Light-colored gneiss and rocks somewhat resem- bling metamorphosed sedimentary rocks occur lo- cally in the Wilson Creek Gneiss. These rocks are light green to white and are intimately associated with typical Wilson Creek Gneiss and phyllonite. Lo- cally, they contain pegmatite and lenses of material resembling plutonic rock. They lack sedimentary structures and seem to grade into typical plutonic gneiss. The origin of these rocks is obscure, but they probably belong to the plutonic complex. Whether they are somewhat silicified blastomylonitic gneisses or some older sedimentary rock is unknown. These rocks contain 40 to 75 percent quartz and 15 to 50 percent sericite. Some lack feldspar, but others contain as much as 25 percent microcline and 15 per- cent plagioclase. Rocks of similar aspect but contain- ing more mica and less quartz were called phyllon- ites. All gradations are found between light-colored quartzitic gneisses and green to gray micaceous phyllonites. GRANDFATHER MOUNTAIN WINDOW An interesting but rare rock type is a pure granu- lar white quartzite. It forms part of a prominent ledge on the east side of upper Wolfden Branch (area I-3, pl. 1) The outcrop can be seen from Thun- der Hill on the Blue Ridge Parkway about 2 miles to the west. The quartzite caps the ledge and is as much as 30 feet thick; it extends about 500 feet along strike. Much of the quartzite has a coarse texture which is partly obscured by shearing. The quartzite locally grades to gneiss. The bulk of the outcrop below the quartzite is light-colored plutonic gneiss containing numerous stringers and pods of similarly sheared quartzite. A layer of pure quartzite about 30 feet thick crops out on the east side of the valley of Buffalo creek west of Bradshaw school (area I-4, pl 1). The quartzite contains segregation veinlets of quartz along joints. The same layer is 10 feet thick where it crosses Spanish Oak Creek south of the school. These rocks consist almost entirely of mosaic-tex- tured quartz grains averaging about 0.5 mm in diam- eter. Rocks in somewhat similar bodies in the Lin- ville Falls quadrangle contain some sericite. These quartzites are probably sheared and recrystallized quartz veins. ORIGIN AND AGE Intense cataclastic metamorphism obscures the or- igin of the Wilson Creek Gneiss. The oldest textures preserved are granoblastic, but they are rare. The granitic gneiss has broadly gradational contacts with layered migmatitic schist, gneiss, and amphibolite, but locally the contacts are sharp. In a few places, blocky inclusions of hornblende gneiss occur in the granitic gneiss. Large areas of granitic gneiss lack layers or inclusions yet have various proportions of plagioclase and microcline. Apparently, the Wilson Creek Gneiss is a typical plutonic complex formed by a combination of metasomatic and igneous processes, but the relative importance of these processes cannot be evaluated. Isotopic ages of zircon from the Wilson Creek Gneiss from the quarry along Wilson Creek in area F-6 (pl. 1) range from 640 to 1,000 m.y., indicating that the Wilson Creek Gneiss is at least 1,000 m.y. old (Davis and others, 1962). BLOWING ROCK GNEISS The Blowing Rock Gneiss (Keith, 1903) occupies a belt 3 to 5 miles wide and 15 miles long in the central part of the Grandfather Mountain window (pl. 1). The rock is very coarse grained strongly foliated 67 blastomylonitic or phyllonitic augen gneiss contain- ing white porphyroclasts of potassic feldspar in a black biotitic matrix (fig. 38). The gneiss is gener- ally quartz monzonite in composition, but granitic and granodioritic varieties are also present. It con- tains inclusions of finer grained, generally more B FIGURE 38.-Blowing Rock Gneiss. A, Typical Blowing Rock Gneiss. From roadcut on U.S. Highway 321 on northwest side of south fork of the New River about 0.3 mile west of Brown Creek (area H-3, pl. 1). Note inclusion of fine- grained mafic rock. B, Tectonic lenses of pegmatite in phyllonite derived from Blowing Rock Gneiss. Lenses, 1 to 2 feet thick. From railroad cut through ridge north of Green Branch in Tweetsie Amusement Park about 3.2 miles northeast of village of Blowing Rock (area H-3, pi. 1). 68 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE mafic rock and dikes and small bodies of light-col- ored granitic rock and pegmatite. In places it has been converted to phyllonite or blastomylonite. The contact between the Wilson Creek Gneiss and the Blowing Rock Gneiss is generally gradational over a few tens to a few hundred feet, except where it is extensively modified by shearing. In the north- eastern part of the Linville Falls quadrangle (area F-6, pl. 1), phyllonite occurs almost everywhere along the contact, which is steep and parallel with the regional cataclastic foliation. The contact seems to be a fault, but if so, the fault leaves the contact and is lost in the southwestern part of the Blowing Rock quadrangle (area G-5). Near the head of Jack- son Camp Creek (area H-4) the contact is also highly sheared, but local irregularities and the lack of offset in the metadiabase to the north indicate that there is no major fault along it. Gradational zones along contacts generally consist of Wilson Creek Gneiss and Blowing Rock Gneiss in various proportions. In some places, large potassic feldspar crystals characteristic of the Blowing Rock Gneiss appear gradually in the Wilson Creek Gneiss ; in other places, fully developed Blowing Rock Gneiss occurs as patches and zones in the Wilson Creek Gneiss. Continuous exposures through gradational contact zones are lacking, but it is generally possible to locate contacts within a few hundred feet. Relative ages of the Blowing Rock Gneiss and Wil- son Creek Gneiss cannot be determined from contact relationships. Isolated pods of Blowing Rock Gneiss in Wilson Creek Gneiss may be either inclusions of older material or zones in the Wilson Creek Gneiss partly converted to Blowing Rock Gneiss. Both gneisses are cut by dikes of light-colored granitic rocks and pegmatite and are believed to be of the same general age. The Blowing Rock Gneiss is less variable in com- position than the Wilson Creek Gneiss. Most notice- able variations are in the content of potassic feldspar and biotite. The rock ranges from dark biotitic types containing scattered white porphyroclasts of potas- sic feldspar to light-colored augen gneiss containing a rather small amount of biotite. Foliation in the Blowing Rock Gneiss is defined by folia of fine-grained mica that wrap around the large feldspar porphyroclasts. Lineation is formed by alinement of fine-grained mica streaks, elongate quartz and feldspar (especially plagioclase) grains, and crude alinement of potassic feldspar porphyro- clasts. Layering is rare in the Blowing Rock Gneiss, but it is found locally in area G-6 (pl. 1) and near the contact with the thin unit of layered Wilson Creek Gneiss in area I-3. Some layers consist of granitic gneiss like typical Wilson Creek Gneiss; others are fine-grained biotite gneiss. Small inclusions of mafic rocks occur locally in the Blowing Rock Gneiss. The mafic rocks are biotite quartz diorite gneiss, epidote-plagioclase-biotite schist, hornblende-biotite schist, amphibolite, and biotite schist. The largest inclusions are at least sev- eral tens of feet long. One large inclusion of layered amphibolite is exposed south of the large roadcut on U.S. Highway 321 north of the Blue Ridge Parkway underpass (area H-3, pl. 1); smaller inclusions a few inches to a few feet long are also visible in the same exposure. The inclusions are generally tabular, and some contain scattered porphyroclasts of potas- sic feldspar. In places, inclusions of mafic gneiss grade to typical Blowing Rock Gneiss, and the ma- trix of the Blowing Rock Gneiss resembles the in- cluded mafic gneiss. Some of the areas mapped as Flattop Schist by Keith (1903) contain these earlier mafic rocks. Pods, dikes, and lenses of pegmatite in the Blow- ing Rock Gneiss are a few inches to several feet thick and are crudely foliated. Some have straight contacts and are fracture controlled ; others are irregular and discontinuous. They consist principally of quartz, perthite, and plagioclase. The average size of the larger grains is 2 inches, but the maximum is at least 8 inches. Muscovite is less than an inch in diameter and occurs only locally in the pegmatites. Light-colored equigranular granitic rocks form bodies that range from a few inches in thickness to at least 150 feet in diameter and that cut the Blow- ing Rock Gneiss. The grain size of the granitic rocks ranges from 1 to 12 mm, and some grade to pegma- tite. Quartz segregation pods, lenses, and veinlets a few millimeters to a foot thick are widespread in the Blowing Rock Gneiss. Calcite, chlorite, and locally occurring pyrite accompany quartz in many of the segregation veinlets. Veinlets of epidote are less widespread. Locally, quartz veins as much as 10 feet thick are found ; they have a granular texture, suggesting that they have been sheared and recrystallized. Typical phyllonitic augen gneiss grades into gray or dark-blue-gray phyllonite which occurs in zones 1 to 20 feet thick parallel to the foliation. Zones of GRANDFATHER MOUNTAIN WINDOW 69 FIGURE 39.-Photomicrographs of Blowing Rock Gneiss. A, Porphyroclastic quartz monzonite augen gneiss from road- cut along U.S. Highway 321 just south of contact of Grandfather Mountain formation near the mouth of Gold- mine Branch (area H-3, pl. 1). Porphyroclast of potassic feldspar containing patches of plagioclase in a matrix of saussuritized plagioclase, biotite, sericite, calcite, and quartz. A few small porphyroclasts of quartz. B, Typical porphyroclastic quartz monzonite augen gneiss from road- metal quarry along U.S. Highway 321 south of Bailey Camp School (area H-4, pl. 1). Porphyroclasts of potassic feldspar contain patches of plagioclase. Plagioclase por- phyroclasts represented by patches of strung-out saus- surite. Small biotite porphyroclasts. Matrix composed of recrystallized quartz, sericite, biotite, epidote, and chlorite. Section cut parallel to mineral lineation. Analyzed speci- mer 1, table 13. C, Recrystallized quartz monzonite mortar gneiss from dike 6 feet thick in Blowing Rock Gneiss from roadcut along U.S. Highway 321 south of Brown Creek (area H-3, pl. 1). Perthitic microcline, saussuritized plagioclase, and partly broken down quartz grains in a matrix of recrystallized quartz, albite, and minor amounts of mica. light-gray or green phyllonite may have been derived from the light-colored granitic rocks. The phyllonites weather red brown. Numerous phyllonite zones are exposed in roadcuts along U.S. Highway 321 north of the village of Blowing Rock (area H-3, pl. 1). In the valley of Matney Branch (northeast part of area H-3, PI. 1), phyllonite derived from the Blowing Rock Gneiss is in contact with metamorphosed sedi- mentary and volcanic rocks from which it can be distinguished only with difficulty. The Blowing Rock Gneiss is studded with porphy- roclasts of potassic feldspar averaging 3, to 1 inch in diameter and gray porphyroclasts of quartz aver- aging 3 to 5 mm in diameter ; locally it contains por- phyroclasts of biotite 2 to 3 mm in diameter. The porphyroclasts are set in a black to gray matrix of fine-grained recrystallized biotite, sericite, quartz, al- bite, and epidote. The grain size of the matrix is about 1 mm, but ranges from 0.01 to 0.5 mm (fig. 39A and B). Locally, the potassic feldspar porphyro- clasts are as much as 3 inches long. The potassic feldspar is perthitic microcline. It contains inclusions of plagioclase, quartz, and bio- tite. The microcline porphyroclasts are bent, broken, and healed by quartz, albite, epidote, biotite, and calcite (fig. 39B). Plagioclase occurs as bent, broken, and saussuri- tized porphyroclasts averaging 0.5 to 3 mm in diame- ter but locally is as much as 1 em long. Plagioclase grains included in microcline commonly have saus- suritized cores but clear rims. The old altered grains and new recrystallized groundmass grains are both albite. 70 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE Quartz is found as strongly strained and drawn- out porphyroclasts and as mosaic-textured aggre- gates of recrystallized grains. Biotite occurs in amounts ranging from a trace to 16 percent of the rock. It occurs as bent and partly altered brown porphyroclasts having sagenitic struc- ture and as green or brown new synkinematic flakes 0.1 to 0.2 mm long in the groundmass. Green FeMg chlorite derived from biotite occurs in some rocks. Locally, it composes as much as 12 percent of the rocks. Sphene rims opaque minerals, probably ilmenite. It occurs in amounts ranging from 0 to 4 percent. Epidote constitutes 0 to 6 percent of the rocks. Al- lanite occurs in prisms as much as 1 mm long and as cores of epidote grains. Zircon is in euhedral prisms as much as 0.1 mm long and also as subhedral and anhedral grains. Other accessory minerals are apa- tite, ilmenite, pyrite, stilpnomelane, and calcite. The very coarse and uneven grain size of the Blowing Rock Gneiss makes it difficult to obtain accurate modes. Figure 40 shows the proportions of quartz, plagioclase, and potassic feldspar in the Blowing Rock Gneiss and in the inclusions and the dikes cutting it. Because of intense alteration of pla- gioclase and redistribution of the alteration products in some rocks, the estimated proportion of plagio- clase may be too low. (Note especially the mode of the chemically analyzed specimen 1, table 18.) Most rocks are in the quartz monzonite and granite ranges. The Blowing Rock Gneiss was not sampled extensively because petrographic classification based QUARTZ EXPLANATION N Light-colored intrusive rock U Inclusion a 2- by 3-inch thin section of Blowing Rock Gneiss isl 1- by 1-inch thin section of Blowing Rock Gneiss PLAGIOCLASE POTASSIC FELDSPAR FiGurE 40.-Proportions of quartz, plagioclase, and potassic feldspar in Blowing Rock Gneiss, light-colored intrusive rocks, and inclusions. Based on-counts of 50 random grains in thin sections. Numbers refer to anlyzed specimens, table 13. TABLE 13.-Chemical analyses of Blowing Rock Gneiss and phyllonite [Analysis of sample 2, standard rock analysis by C. L. Parker, U.S. Geol. Survey, 1961. Other analyses, by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, Lowell Artis and Hezekiah Smith, U.S. Geol. Survey, 1963. For analysis of sample 1, S determined by induction furnace by I. C. Frost, U.S. Geol. Survey. F and Cl determined by V. C. Smith, U.S. Geol. Survey. Major oxides and CIPW norms are given in weight percent] Gneiss Phyllonite 1 2 3 4 Field No.% baul ios GMB-3 GMB-4 P1 P3 Laboratory Nolrt .. cC. 161252 H-3264 161253 161254 Major oxides S10; -: 1.28 ota arent iol. 61.2 63.30 73.6 63.1 M;Os- ... soul 14.2 14.87" 11.8 16.8 Eul c.. 1.8 1;15 1:1 ge A {unl oB 4.6 3.74 2.1 3.1 Meo: -:. t" 1.6 a T9 1:1 Cad... rat ieri a iio nou 4.7. 3.14 2 1.8 ign 3.4 3.18 1.0 19 (Oso d NEY Ir 3.1 4.70 4.0 6.5 1:2 1.20 1.4 2.2 10 .. . Sv inch e aon .05 . 04 .04 . 06 se U- ere ns. 1:6 1.12 12 t ity. . 80 40 26 T4 cma ad 14 . 09 04 04 a aree thes .82 1.09 94 <.05 ________________________ 22 h ( olen ua a C1 20 ret l N+ . 01 OU recibir eal. DaQ:.%..,.. e ananas nos 16922... Pause cl ________________________ . 03 06 . esta ese aol. 99 99.61 99 99 CIPW norms C ice in av esu dann au uli 20,20 20.95 . 50.18 33.13 Cnr as ees hel nn we 1.03 2. T2 5.34 1.95 a) ale nein a apap aik 18:92 25.08 38.40 AD: A io aoa i a oo 28.68 - 26.82 8.46 1.61 "n os sors rau onus ool ou 11.55 5.03 79 4.10 M.:? ss le oot oe s cen nani. 02 eerie AML Eo aa ale Fh. ss refed tr e ciao 3.98 3.16 1:97 2.74 Ps..:ln clans ie. 4.52 4.11 1.83 1.31 Mt.. :l cat sa ae ue . cue 2.61 1.67 1.60 3.19 Misi sea soak et ov ude 3.04 2.18 1.37 3.04 Apia s. -u Le =o us ie ank 1.90 .95 .62 1.175 Pre ess cL ¥ occas .38 iPr ille clone eee PT: .ll lose tea nice irae. .06 M er ec eae anl CC. 2 or nnis save uae 1.86 2.48 I4 ic. NotE.-Minor-element analysis for sample 2 given in table 1. 1. Gray and white cataclastic augen gneiss containing porphyroclasts of white potassic feldspar as much as 3 cm long in a green-gray matrix of fine-grained mica. Porphyroclasts are microcline and perthitic mi- crocline that have been broken and healed by recrystallized quartz and calcite, somewhat bent brown biotite as much as 1 mm long, and saus- suritized plagioclase as much as 3 mm in diameter. Matrix is composed of quartz, green biotite, sericite, subordinate FeMg chlorite (replacing biotite), and epidote. Accessory minerals are ilmenite, sphene, apatite, pyrite, allanite, stilpnomelane, and zircon. From small quarry along U.S. Highway 221-321 near spot altitude of 3485, south of Chetola Lake (area H-3, pl. 1). 2. Dark-gray and white cataclastic augen gneiss containing porphyroclasts of white potassic feldspar as much as 2 cm long in a green-gray ma- trix of fine-grained mica. Porphyroclasts of microcline and perthitic microcline as much as 2 em long in a matrix of recrystallized albite, sericite, quartz, and green biotite; accessory minerals are sphene, calcite, allanite, zircon, and epidote. Porphyroclasts are filled with in- clusions of quartz and plagioclase. From roadcut along U.S. Highway 3821 through meander of the South Fork of New River 1.75 miles N. 50° E. of junction between U.S. Highways 321 and 221 in the village of Blowing Rock (area H-3, pl. 1). ? 3. Porphyroclastic phyllonite derived from Blowing Rock Gneiss. Gray seri- citic rock containing numerous quartz and feldspar porphyroclast§. Porphyroclasts of quartz are as much as 7 mm in diameter; perthitic microcline porphyroclasts, 5 mm; and plagioclase porphyroclasts, 2 mm. Matrix is chiefly recrystallized sericite and quartz but contains acces- sory calcite, pyrite, sphene, zircon, and apatite. From same locality as specimen 2. aie 4. Phyllonite derived from Blowing Rock Gneiss. Dark-gray phyllitic rock containing a few quartz porphyroclasts. Quartz porphyroclasts as much as 2 mm in diameter in a groundmass of recrystallied sericiye, FeMg chlorite, quartz, and accessory sphene, ilmenite, pyrite, apatite, allan- ite, and zircon. From roadcut along U.S. Highway 221-321, near Mid- dle Fork Church (area H-3, pl 1). GRANDFATHER MOUNTAIN WINDOW T1 on thin-section study is probably no more accurate than classification based on field estimates. Chemical analyses (table 13) of two samples show that the Blowing Rock Gneiss is quite similar to the average quartz monzonite (Nockolds, 1954) but that it con- tains less SiO,, more FeQ, and more total iron. The light-colored intrusive rocks contain little mica and therefore do not have well-developed folia- tion. In thin section, however, the effects of partial cataclasis and recrystallization are evident. The rocks contain zones of recrystallized mortar and saussuritized plagioclase and are blastomylonitic gneisses or recrystallized mortar gneisses (fig. 39C). They are quartz monzonites and granites (fig. 40) that differ from the country rock principally in their lack of mafic minerals and in their smaller and more even grain size. The inclusions are predominantly fine-grained quartz-plagioclase-biotite gneisses, but range from granodiorite to amphibolite (fig. 40). Phyllonites in the Blowing Rock Gneiss contain 40 to 80 percent sericite and 0 to 20 percent biotite or chlorite. Quartz porphyroclasts are widespread and feldspar porphyroclasts, less common ; some phyllon- ites lack porphyroclasts. Chemical analyses (table 13) suggest losses in CaO, Na.0, and MgO and gains in K,0 and H,O during phyllonite formation. The petrographic data and field evidence do not conclusively establish the origin of the Blowing Rock Gneiss. Less sheared contacts are generally grada- tional over a few to a few tens of feet, and none are demonstrably intrusive. In some places the contact with Wilson Creek Gneiss is gradational through greater distances. Inclusions of nongranitic rocks are mostly small and resemble inclusions in plutonic igneous rocks; layering is rare. Isotopic ages of zircon from the Blowing Rock Gneiss north of the village of Blowing Rock are nearly concordant and range from 990 to 1,055 m.y., suggesting that the rock formed about 1,055 m.y. ago in the same gen- eral metamorphic-plutonic episode as the Wilson Creek Gneiss (Davis and others, 1962). BROWN MOUNTAIN GRANITE The Brown Mountain Granite (Reed, 1964b) is medium to coarse grained, light colored, and homoge- neous. It is generally weakly foliated and nonlayered but commonly has a distinct cataclastic lineation. It characteristically crops out in flat exfoliation slabs which make measurement of foliation and lineation difficult. The contacts of the Brown Mountain Granite are poorly exposed. On the southeast side, the main body is overridden by the Linville Falls and associated faults, and on the west side, the granite apparently overrides volcanic and sedimentary rocks of the Grandfather Mountain Formation. Rocks of the Grandfather Mountain Formation unconformably overlie the granite east of bench mark 1195 on North Carolina Highway 181 area F-7, pl. 1.) and north- west of Adams Mountain (area G-6). Conglomerates in the Grandfather Mountain Formation locally con- tain pebbles of Brown Mountain Granite. Dikes of granite cut the Blowing Rock and Wilson Creek Gneisses northeast of Adams Mountain (areas G-6 and G-7). Local zones of silvery medium-gray phyllonite, small bodies of quartz-perthite pegmatite, and small quartz segregation lenses and veinlets occur in the granite. Fluorite is found locally as thin coatings on joint surfaces. The granite is a recrystallized mortar gneiss or blastomylonitic gneiss composed of porphyroclasts of potassic feldspar as much as 1 cm long and smaller and less abundant porphyroclasts of plagioclase and quartz in a matrix of recrystallized albite, quartz, biotite, sericite, and microcline having grain sizes ranging from 0.05 to 0.5 mm (fig. 41). 5 mm FIGURE 41.-Photomicrograph of typical Brown Mountain Granite from roadcut on east side of Wilson Creek 0.7 mile northeast of Brown Mountain Beach (area G-7, pl. 1). Porphyroclasts of string perthite in groundmass of re- crystallized quartz, albite, potassic feldspar, and biotite. Section approximately perpendicular to mineral lineation. Analyzed specimen, table 14. 72 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE The potassic feldspar is string and patch perthite and antiperthite. The large porphyroclasts are bent, broken, and healed by quartz, albite, and sericite. Plagioclase porphyroclasts are as much as 6 mm long. They are generally saussuritized and altered to albite. Albite is a major constituent of the ground- mass and locally forms rims about 0.05 mm thick on the potassic feldspar grains. Biotite is green or brown and forms irregular clots of recrystallized grains; a few biotite porphyroclasts occur locally. Colorless or light-green sericite forms small flakes in the albite-quartz mosaic. Fluorite occurs as small ir- regular grains in the mosaic. Accessory minerals are metamict allanite(?), epidote rimming allanite, sphene locally jacketing opaque minerals, zircon, il- menite, magnetite, and stilpnomelane. The Brown Mountain Granite is quite uniform in composition (fig. 42). One analyzed specimen (table 14) is siliceous alkali granite low in K,0 and Al;O;. Recalculation of the chemical analysis into ideal or- thoclase and albite places the sample close to the eu- tectic for the system NaAISizO0&;-KalSi;0;-H0 at about 1,000 kg per cm' water-vapor pressure (Tut- tle and Bowen, 1958, p. 75). The Brown Mountain Granite probably intruded the Blowing Rock and Wilson Creek Gneisses during a late stage of, or after, the Precambrian meta- morphic-plutonic episode. Except for the dikes, no intrusive relations were found, but the map pattern, sharp contacts, lack of layering, uniformity in com- QUARTZ Granite field PLAGIOCLASE POTASSIC FELDSPAR FIGURE 42.-Proportions of quartz, plagioclase, and potassic feldspar (including perthite) in Brown Mountain Granite. Line connects point count on analyzed specimen (table 14) with calculated volume proportion of albite, orthoclase, and quartz. Granite compositional field outlined. Based on count of 50 random grains in each thin section. TABLE 14.-Chemical analysis, mode, and norm of Brown Mountain Granite [Standard rock analysis by C. L. Parker, Denver, Colo., 1961. Mineral composition determined by point count; P, present but not intersected in counting. Major oxides and CIPW norms given in weight percent; mode, in volume percent] Field Ci-Fidd Mo....:........_.... G Laboratory No._.._....... H-3434 - Laboratory No__________. H-3434 Major oxides SiO: se To.8b: KO: 4.66 MiOQ;. .:.. ie.. 12:06 .18 F 9203 ________________ .85 1-120 | c inane nel Ahi tai " thie tol OL FeQ::_: y-: sc ¥.08 @ .s 10 08 : 00 Cad: sacri news on's "at 02 02 Total........... 99.67 Mode 48 4 9 -B Quartz=...2...0.00.3 40 P 2.7 - Muscovite. P Points counted.... 600 CIPW norm $41.94 .07 OP! sus inne cae ae reek 1.08. teensy 1.57 Ab. 1 tet ites 82.170 MC -:"°.l..c_.ccct0s. .51 Anis 1:78 19 M- Cel .04 NoTE.-Minor-element analysis of specimen is given in table 1. Description of analyzed specimen follows: 2 } ; Coarse-grained, light-colored granite with conspicuous lineation and faint foliation. Crystals of string perthite as much as 7 mm in diameter in a ground- mass of quartz, albite, potassic feldspar, and biotite 0.1 to 1 mm in grain size. Most of the groundmass apparently recrystallized during metamorphism. A porphyroclast of biotite 1.5 mm in diameter. From roadcut on east side of Wilson Creek 0.7 mile northeast of Brown Mountain Beach, Lenoir quadrangle (area G-7, pl. 1). position, and highly perthitic character of the potas- sic feldspar support this interpretation. The original contacts have been modified during the Paleozoic by shearing and faulting, which produced the present texture and structure of the Brown Mountain Gran- ite. GRANDFATHER MOUNTAIN FORMATION The Grandfather Mountain Formation of late Pre- cambrian age (Bryant, 1962) unconformably over- lies the Wilson Creek Gneiss, the Blowing Rock Gneiss, and the Brown Mountain Granite and is tec- tonically overlain by the Blue Ridge thrust sheet and the Tablerock thrust sheet. The formation is a thick interlayered and intertonguing sequence of meta- morphosed arkose, siltstone, and volcanic rocks. It crops out in the northwestern third of the Grand- father Mountain window, where the more resistant rock units control the topographic form of many of GRANDFATHER MOUNTAIN WINDOW 78 the higher mountains on the Blue Ridge, and in sev- eral smaller isolated areas on the Blue Ridge front. The first specific mention of the rocks of the Grandfather Mountain Formation was by Elisha Mitchell (1905, p. 52) in his account of an ascent of Grandfather Mountain in 1828. He observed that the top of the mountain was made of "grau wacke" and the lower part of "clay slate," to both of which he assigned a "transition" age. Kerr (1875, p. 135-136) assigned the rocks of the Grandfather Mountain Formation to the "Huronian'"' and described them as follows : But the most remarkable part of this belt, both for breadth and peculiar lithology, is found in the region of the Grand- father and Yellow Mounsins [sic], about the headwaters of Toe River, Linville, Elk and Watauga. On upper Linville, and towards the base of the Grandfather and head waters of the Watauga, there are limited beds of argillaceous and hydra- mica slates and shales; but the prevalent rocks are feldspathic and quartzose slates and grits, sometimes gneisslike, and chloritic and epidotic schists, with epidosites, the latter some- times enclosing bright red rounded grains of jasper. Along the high spurs of the Grandfather, to the northwest, the Yellow Mountains, are large bodies of greenish epidotic sand stone, feldspathic and quartzose; and along the northward spur of Hanging Rock, hard and gnarled dark gray quartzo- argillaceous slates prevail; but these are also much veined with irregular masses and reticulations of epidote. Passing eastward to the Watauga, from Valley Crucis up the river, the the prevalence of epidotic and chloritic, massive or obscurely bedded rocks, is most striking. Some of the masses are much veined with fine seams of white quartz, while others are amygdaloidal, sprinkled with grains of gypsum and quartz and epidote; while still further east across the Rich Moun- tains, occur chloritic amygdaloids in which the grains are feldspar, which are much weathered so as to leave the surface of the rock deeply honey-combed. Alternating with these conspicuous and dominant masses along the river, are the slate and gneiss-like grits of Linville, and occasionally silvery, gray greenish and spotted argillaceous and feldspathic slates and shales. On the Elk River occurs a greenish quartzofeldspathic, thick bedded, compact to friable slate and grit, whigh grad- ually passes into a nacreous light-colored, coarse slate-conglom- erate-a fine-grained argillaceous quartzite, filled with rounded and flattened pebbles of white and reddish quartz and of hard quartzo-argillaceous slates. Keith (1903) correlated the arkose units in the Grandfather Mountain Formation with the Unicoi Formation and the siltstone units with the Hampton Formation of the Chilhowee Group of the Valley and Ridge province. He also believed that all the volcanic rocks underlay the arkoses. This naturally led to structural interpretations which differ from ours. Stose and Stose (1944) correlated the rocks of the Grandfather Mountain Formation with the Chil- howee Group, whereas Rodgers (1953a, p. 22) corre- lated the volcancic rocks of the formation with those of the Mount Rogers area in southern Virginia, ex- treme northeastern Tennessee, and northwestern North Carolina. Variations in thickness and facies occur along strike in the Grandfather Mountain Formation. Gen- erally speaking, a variety of volcanic rocks occurs near the base of the formation in the easternmost areas of outcrop, whereas arkose forms the basal deposit farther west. A body of metabasalt, the Mon- tezuma Member, is found in the upper part of the formation. In area D-5 (pl. 1}, the upper arkoses and the Montezuma Member pinch out. The thick basal arkose unit partly interfingers with siltstone to the northeast (area G-2), and the second arkose unit above the base pinches out completely. Stratigraphic sections of the Grandfather Mountain Formation cannot be measured because of incomplete exposures, complex structure, and lack of distinctive marker horizons, but the formation is probably 10,000 to 30,000 feet thick. No major stratigraphic unit is entirely repeated by folding. All the rocks of the Grandfather Mountain Forma- tion have been metamorphosed, but as original sedi- mentary and volcanic structures and textures are abundant in the arenaceous sedimentary rocks and the felsic volcanic rocks, sedimentary and igneous rock names are used to describe them. Metamorphic names are generally used for the pelitic rocks and the mafic volcanic rocks which more readily show the effects of metamorphism. BASAL CONTACT The contact between the Grandfather Mountain Formation and the underlying plutonic rocks is ex- posed in only a few places. In some places its exact position is difficult to determine because of the close resemblance between coarse-grained arkose and phases of the Wilson Creek Gneiss. In many places, however, the basement rocks can readily be distin- guished because they contain pegmatite and biotite and are coarser grained than the arkose. Bedding is generally poorly developed in the arkose near the contact. 7A GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE GRANDFATHER MOUNTAIN WINDOW 75 FIGURE 43.-Arkose of the Grandfather Mountain Forma- tion. A, Crossbedding in arkose bed in middle part of exposed Grandfather Mountain Formation. Roadcut along North Carolina Highway 184 northwest of Linville Gap (area E-4, pl. 1). Overturned beds dip east. B, Bedding in coarse-grained arkose and conglomerate. Upper part of the lowest continuous arkose unit. Altitude of 3,850 feet on ridge northeast of Shulls Mills (area G-3, pl. 1). C, Conglomeratic arkose near top of lowest continuous arkose unit. Lithic fragments flattened along cleavage, which cuts bedding. At 3.850-foot altitude on ridge noth- east of Shulls Mills (area G-3, pl. 1). D, Conglomerate in highest arkose unit. Quartz pebbles and lithic fragments of sedimentary and volcanic(?) rock elongated parallel to cleagave. Bedding visible. Outcrop has slumped to the left; cleavage originally dipped 80° to the right (east). Near the head of Pigeonroost Creek (northeast part of area E-3, pl. 1). E, Conglomerate containing cobbles of plutonic and felsic volcanic rocks in upper part of lowest continuous arkose unit. Roadcut about 0.5 mile southeast of Shulls Mills (area G-3, pl. 1). An overturned contact between Wilson Creek Gneiss and basal arkose of the Grandfather Moun- tain Formation is exposed along a Forest Service road (not shown on the base map) northeast of Sas- safras Knob (area E-5, pl. 1). Light-green sericite arkose containing scattered clastic grains of quartz and feldspar structurally underlies green to gray cataclastic granitic gneiss that contains porphyro- clasts of potassic feldspar and fine-grained sericite and biotite. The gneiss contains small lenses and pods of pegmatite that are lacking in the arkose. Well-developed cleavage dips 25° SE., slightly less than the contact. The contact between arkose and basement rock is also exposed in an area on Lost Cove Cliffs (area E-5, pl. 1). Arkose on the crest of the cliffs contains beds of quartz and feldspar pebbles that are as much as 2 inches in diameter. Cleavage dips 15° SE. The map pattern indicates that the contact dips gently westward at the top of the cliff, but on the part of the cliffs where it is best exposed, it dips about 35° W. Near the base of the cliffs, it dips 35° SE. to form a fold that is nearly recumbent, although not iso- clinal. The contact between siltstone of the Grandfather Mountain Formation and the Blowing Rock Gneiss is exposed along U.S. Highway 321 near Goldmine Branch (area H-3), where dark-blue-gray biotitic siltstone containing calcareous beds overlies coarse- grained augen gneiss. It is difficult to locate the contact precisely, and several inches of sediment at the base probably contain much material derived from the gneiss. CONTACTS BETWEEN ARKOSE AND SILTSTONE UNITS Contacts between arkose and siltstone units are generally well defined, except on and northeast of Flattop Mountain in the eastern part of the main outcrop area of the Grandfather Mountain Forma- tion (areas G-2, G-3, and H-3, pl. 1). There, units mapped as siltstone contain some interbeds of arkose and mafic volcanic rock, and arkose and siltstone are interbedded for a few tens of feet to perhaps 100 feet near their contacts. Interbedding is also found on the north side of Hodges Mountain (area G-2) and north of the Pack Hill School (area G-4), where arkose and siltstone units interfinger. Gradational contacts between the two lithologies are exposed on the hill east of Newland (area D-4), on Peak Moun- tain (area E-3), and on Rich Mountain (area G-3). A sharp contact between arkose and siltstone is well exposed in roadcuts along North Carolina High- way 181 north of Jonas Ridge (area D-6). Coarse- grained, massive, white to light-green conglomeratic arkose grades upward into green, fine-grained, slightly calcareous arkose containing scattered feld- spar grains as much as 6 mm in diameter. The ar- kose is overlain by highly crenulated light-green, dark green, and black phyllite containing light-gray siltstone beds 1 to 2 feet thick. Soils and float derived from the arkose and silt- stone are easily distinguished. The siltstone units form chippy soil containing abundant small rock fragments, whereas the arkose forms granular sandy soil made up of individual grains of quartz and feld- spar. SEDIMENTARY ROCKS ARKOSE Mrcoascoric FEATURES Arkose is the predominant rock type of the Grand- father Mountain Formation ; it forms large cliffs and slabby outcrops on Grandfather Mountain, Peak Mountain, Sugar Mountain, Hawksbill Mountain, and other peaks and ridges along the Blue Ridge 76 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE front. Excellent exposures in roadcuts are found at almost any point at which roads cross arkose units. The arkose units consist of fine- to coarse-grained, thin-bedded to massive, green, light-green, tan, and gray sericitic arkose and feldspathic quartzite con- taining minor beds of conglomerate, green to gray sericite phyllite, and siltstone. Much of the arkose is poorly bedded, especially in the lowest arkose unit southeast of Grandfather Mountain. Bedding is marked by concentrations of heavy minerals and so- lution of calcareous zones. Crossbedding (fig. 484) is found locally, but graded bedding is rare. Some of the crossbedding is of the torrential variety. Oscilla- tory ripple marks are found in a few places. They are well displayed along the road between U.S. High- way 221 and Gragg, where it crosses the west branch of Wilson Creek (area F-4). Sericite phyllite beds one-half of an inch to 6 feet thick occur sparingly in the lower arkose unit but are much more abundant in the upper part of the exposed section. Differences in composition, grain size, and color are more wide- spread in the higher arkose units. The color of the highest arkose unit is especially variable; siltstones and phyllite beds are gray, green, purple, and ma- roon. Careful examination of roadcuts along any main highway reveals local bedding in all the arkose units. Beds and discontinuous lenses of conglomerate (fig. 43B, C, D, E) containing pebbles and cobbles of quartz, feldspar, siltstone, phyllite, felsic volcanic rock, arkose, light-colored granoblastic plagioclase- quartz rock, granitic rock, and pegmatite are found throughout the formation. The conglomerate bodies are thickest and most numerous along the crest and on the northwest and north sides of Grandfather Mountain (areas E-4 and F-4, pl. 1) and on Hang- ing Rock Ridge north of Foscoe (area F-3). Most of the conglomerate lenses are less than 10 feet thick, and the thinnest are pebbly beds only a few inches thick. The thickest and most spectacular conglomer- ate outcrop is at an altitude of 4,200 feet in Falls Hol- low north of the highest peak of Grandfather Moun- tain (northwest corner of area F-4). There the con- glomerate is more than 100 feet thick and contains pebbles, cobbles, and boulders of felsic volcanic rock and muscovite-plagioclase quartzite as much as a foot long. About half a mile to the northeast, boulders of felsic volcanic rock are as much as 2 feet long. Excellent exposures of conglomerate in the upper part of the lowest arkose unit may be seen along the Blue Ridge Parkway about 0.3 mile southwest of Flat Rock (area E-5). There the conglomerate con- tains pebbles of quartz and feldspar and lithic frag- ments of felsic volcanic rock, black or dark-gray ar- gillite or siltstone, gray quartzite, light-green and gray sericite phyllite, light-colored granitic rock, pegmatite, and red jasper. Cleavage is well devel- oped, and pebbles of less competent rocks have been deformed into blade-shaped fragments as much as 10 inches long and only 1 to 2 inches thick, whereas the more competent cobbles are a maximum of 4 inches in diameter. Bedding can be seen in the southern part of the outcrop. Typically, the conglomerates in the highest ex- posed arkose unit contain quartz pebbles and frag- menss of gray, maroon, and purple rock and resem- ble the finer grained interbeds in that unit. Other rock types occur but are much less abundant. The dark rock fragments are flattened in the plane of the cleavage and locally elongated in a northwest-south- east direction parallel to the lineation. These con- glomerate beds are generally a few inches to a few feet thick. Rarely do the rock fragments exceed 4 inches in length. Outcerops of such conglomerates (fig. 43D) may be seen along North Carolina High- way 184 in the village of Banner Elk and at the head of Pigeonroost Creek (area E-3, pl. 1). An unusually thick, coarse-grained, and diverse conglomerate in the uppermost arkose is exposed along old North Carolina Highway 194 on Mill Timber Creek (area D-5, pl. 1). Several conglomerate beds form a se- quence about 40 feet thick. Coarse-grained conglom- erate at the bottom contains cobbles as much as 8 inches long. It grades up into interbedded pebble conglomerate and coarse-grained arkose. In addition to the usual quartz and dark-colored argillite peb- bles, the conglomerate contains fragments of felsic volcanic rock and quartzite. An exposure in the lower part of the second arkose unit west of Linville Gap (area E-4, pl. 1) on North Carolina Highway 184 has unusually well-developed sedimentary structures and textures. The arkose (fig. 43A) contains quartz and feldspar clasts and lithic fragments of argillaceous rock as much as 2 ecm in diameter. Dark beds are rich in heavy minerals. Crossbedding and graded bedding are conspicuous, and the cleavage is poorly developed. In many places the arkose contains seattered large grains or pebbles of feldspar (especially in the low- est arkose), quartz, and fine-grained dark-colored rock fragments. The green color of the arkose is due to evenly dis- tributed green mica. Locally, especially in the lower arkose unit, the green mica is concentrated in segre- GRANDFATHER MOUNTAIN WINDOW T7 FIGURE 44.-Photomicrographs of arkose in the Grandfather Mountain Formation. A, Fine-grained arkose from quarry on east side of Wilson Creek at 3,550-foot altitude (area F-4, pl. 1). Clastic grains of quartz, microcline, and plagio- clase (pseudomorphosed by albite) in a matrix of iron-rich muscovite, quartz microcline, and albite. Analyzed speci- men 2, table 15. B, Fairly coarse grained, poorly sorted feldspathic quartzite from roadcut at 3,250-foot altitude on Rough Ridge (area F-4, pl. 1). Clastic grains of quartz, microcline, and subordinate plagioclase (pseudomorphosed by albite) in a matrix of recrystallized quartz, iron-rich muscovite, and microcline. Analyzed specimen 5, table 15. C, Coarse-grained arkose from roadcut on North Carolina Highway 184 northwest of Linville Gap (area E-4, pl. 1). Clastic grains of quartz, microcline, and plagioclase (pseu- domorphosed by albite) and fragments of very fine grained quartz-feldspar rock in a small amount of matrix com- posed of recrystallized quartz and sericite. Some of the larger microcline grains are factured and healed by quartz. From outcrop shown in figure 434. gation lenses and laminae less than a millimeter thick that are parallel with the cleavage. The mica- ceous folia are 5 to 10 mm apart and separated by white feldspar-quartz rock. Caleareous beds and lenses weather brown and in many outcrops are represented only by voids left by solution. On North Carolina Highway 181, 1.2 miles south of the Jonas Ridge School (area D-6, pl. 1), dark-blue-gray arkose contains thin calcareous beds and ellipsoidal light-gray calcareous knots as much as 3 inches long. The knots are surrounded by a halo of bleached arkose. TexturE anp MinErALocY Sedimentary textures are well preserved in much of the arkose (fig. 44). Clastic grains more than 0.2 mm in diameter of the major minerals are easily recognizable, and some smaller clastic grains are identifiable. Some grains of accessory minerals as small as 0.05 mm have the appearance of clasts. Most of the arkose is poorly sorted. Much of it has well-de- veloped cleavage which is generally parallel with bedding southeast of Grandfather Mountain but which cuts across bedding in many places in the northwestern part of the outcrop area. Typical arkose contains angular to subrounded clastic grains of quartz, potassic feldspar, and pla- gioclase in a mosaic-textured groundmass of recrys- tallized quartz and feldspar containing various amounts of well-alined light-green mica. Some of the clasts of quartz and feldspar are several inches in diameter and must have been derived from quartz veins and pegmatite. The larger clasts in typical coarse-grained arkose are generally potassic feldspar about 5 mm in diameter, quartz clasts 2 mm in diam- eter, and plagioclase 0.8 mm in diameter (fig. 44B 78 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE and C). Recrystallized groundmass averages about 0.1 mm in grain size. Rarely, the groundmass is as coarse as 0.2 mm, and in some rocks, especially those in the northwestern part of the window, it is less than 0.05 mm in grain size. 'Clastic zircon (locally metamict) is ubiquitous. Clastic grains of apatite are fairly widespread. Sphene is as abundant as zircon, but some of it has recrystallized. Magnetite and ilmenite are at least partly clastic, but some may have recrystallized. Py- rite, which occurs rarely, has recrystallized. Both clastic and metamorphic epidote are widespread. Metamorphic epidote is especially abundant in a few places next to the Linville Metadiabase or the Mon- tezuma Member of Grandfather Mountain Forma- tion, where it has been introduced either during the emplacement of the igneous rock or during meta- mophism. Calcite is metamorphic. Clastic allanite occurs, and clastic tourmaline is rare. The clastic grains of potassic feldspar are gener- ally microcline. Some of the larger ones contain in- clusions of quartz and plagioclase. These clasts re- semble potassic feldspar in the basement rocks in the Grandfather Mountain window. Perthite is much rarer. Some was derived from the plutonic rocks un- derlying the Grandfather Mountain Formation. Other perthite grains that have a very fine texture and much finely divided opaque material resemble phenocrysts in the felsic volcanic rocks of the Grand- father Mountain Formation. These grains are found in arkose adjacent to felsic flows and in conglomer- ates containing pebbles of felsic volcanic rock in the Linville and Blowing Rock quadrangles and on the east margin of the southern area of arkose in the Linville Falls quadrangle, about 3 miles west of the largest area of outcrop of the felsic volcanic rocks. Many of the larger potassic feldspar grains are bent and broken and have been healed by quartz and sericite and rarely by albite and calcite. In places, quartz clasts are strongly strained and brecciated. Some of the broken grains are healed by mosaics of recrystallized quartz. All plagioclase clasts have been altered to albite, and plagioclase in the groundmass is also albite. Some of the clasts are bent and broken. A much larger proportion of plagioclase than potassic feld- spar is found in the groundmass, and the finer grained arkose generally contains more plagioclase. In eight rocks from the lowest arkose unit, which have a maximum grain size of 0.5 mm, the plagio- clase makes up 70 percent of the feldspar; in 17 rocks, which have a maximum grain size of 0.5 to 1 mm, 35 percent of the feldspar is plagioclase. The green mica is an iron-rich muscovite (Foster and others, 1960). One analyzed sample of this mica has an index of refraction for 8 of and the pleochroism is «, colorless; 8, light green; and y, light green. Index of refraction for 8 meas- ured on numerous other samples ranges from 1.598 to that of the analyzed sample. Generally, the micas with the higher indices of refraction have the stronger absorption. Colorless muscovite was not studied, and the mica in some rocks may be normal iron-poor muscovite. The range in the index of re- fraction and absorption suggests that there is proba- bly a range in the iron content of the muscovite in the arkoses. The mica is generally well alined and, especially in some of the more phyllitic rocks, is bent into small crinkles one to a few millimeters in wavelength. The larger flakes are generally 0.1 mm long; rarely are they as much as 0.5 mm long. Radiating aggregates of stilpnomelane are found in the arkose in a few places. In one place the stilp- nomelane is associated with a heavy-mineral seam. Green and greenish-brown biotite occurs rarely as a metamorphic mineral of the same grain size as the iron-rich muscovite. Recognizable clastic grains of biotite are even less abundant than the metamorphic biotite. The biotite is locally altered to chlorite. Phyllite interbeds contain sericite, opaque miner- als, and accessory zircon, apatite, epidote, tourma- line, and biotite. Some have a fragmental texture. Fragments of light-green sericite phyllite several millimeters to 2 em long and lacking opaque minerals occur in a matrix of dark-gray sericite phyllite con- taining opaque minerals. This rock type is found ad- jacent to the greenschists and greenstones of the Montezuma Member and also as beds a few to sev- eral inches thick in the arkose. Heavy-mineral beds in the arkose are composed of opaque minerals, sphene, zircon, quartz, feldspar, and mica. The brown-weathering carbonate beds are com- posed of sericite, calcite, albite, and quartz. Calcite makes up 10 to 25 percent of the beds. In one place, where calcite occurs in beds and ellipsoidal bodies in dark-blue-gray arkose, carbonates have replaced most of the plagioclase and some of the quartz and make up 50 percent or more of the rock. Epidote is abundant. A halo of bleached rock surrounding the calcareous bodies contains epidote and calcite and GRANDFATHER MOUNTAIN WINDOW 79 lacks the small proportion of chlorite and biotite that gives the color to the country rock. Calcite also occurs as irregular grains and veinlets in the country rock. ComposITION AND CLASSIFICATION In figure 45, estimated modal compositions of ar- kose are plotted in terms of four major components : quartz, potassic feldspar, plagioclase, and combined mica and chlorite. The content of rock fragments, a component which usually enters any classification of sandstones, is not shown. Rock fragments do not, however, make up a large proportion of most of the samples, and these rocks would not fall into the lithic graywacke or subgraywacke fields of Pettijohn (1957, p. 292) or the lithic wacke or lithic arenite fields of Gilbert (in Williams and others, 1954, p. 292-2983). Most of the arkoses are poorly sorted and contain a few to 50 percent iron-rich muscovite in their ma- trix. The mica seems to have been derived from clay minerals and iron oxide rather than through break- down of feldspars, because many small grains of feldspar have not been converted to mica. Most of the arkoses are light colored and sandy looking. Nev- ertheless, in the classifications of Pettijohn (1957) and Gilbert (in Williams and others, 1954) many of these rocks would be called graywacke. In discussing Mica Quartz Potassic feldspar Plagioclase rocks of similar mica content, although of generally darker color, Hadley and Goldsmith (1963, fig. 14) used the terms muddy arkose and muddy subarkose. We have chosen the classification of Packham (1954) in which sedimentary features are first considered before a name is attached. In his classification, these rocks would fall into the arkose and feldspathic sandstone division of the arkose-quartzose sandstone suite. The mica-rich arkoses probably were derived from muddy arkoses. These light-colored sandstones, which we call arkoses, contrast with beds of dark- gray sandstone found in the siltstone units, which we call gray The highest exposed arkose (fig. 45B) contains less plagioclase and less total feldspar than the low- est arkose (fig. 454). The upper arkose unit contains more numerous siltstone and phyllite interbeds, and the mica content of the unit as a whole is therefore higher than in the lower units. Rocks in the lowest arkose unit (fig. 454) resem- ble those of the Newark Series in Connecticut (Kry- nine, 1950), if it is assumed that muscovite in the arkoses represents original clastic mica and clay. The sandstones of the Newark Series, however, con- tain considerably less clay and mica than would be indicated by metamorphic mica in arkoses of the Grandfather Mountain Formation. Thus, the Grand- Micag Potassic feldspar 8 Mica FIGURE 45.-Proportions of quartz, potassic feldspar, plagioclase, and mica in arkose of the Grandfather Mountain Formation. Numbers of analyzed specimens refer to analyses in table 15; *, average. A, Lowest arkose unit. @, Weighted average of arkose in the Newark Series as used by Krynine (1950). Based on point counts of 24 thin sections and counts of 50 random grains in each of 55 other thin sections. Contours 1.3, 4, 8, and 12 per- cent. B, Highest exposed arkose unit. Based on counts of 50 random grains in each of 24 thin sections. Con- tours 4, 8, 17, and 25 percent. 80 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE father Mountain arkose is less sorted than arokose of Chemical analyses (table 15, Nos. 1-7) of repre- the Newark Series. . sentative samples of arkose and feldspathic quartzite TABLE 15.-Chemical analyses, modes, and norms of arkose and related rocks of the Grandfather Mountain Formation [Samples 2 and 8, standard rock analyses by C. L. Parker, U.S. Geol. Survey, 1961; sample 4, standard rock analysis by D. F. Powers, U.S. Geol. Survey, 1959; other samples analyzed by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey, 1962. Modes, by point fount; P,l prgsenfi [31m not intersected in counting. Major oxides and CIPW norms given in weight percent; modes, in volume percent. CIPW norms not computable or samples 8 an. Upper arkose unit Lower arkose unit Arkose beds Phyllite beds 1 2 3 4 5 6 T 8 9 FIGO ilo .v. seco J-8-4a GML-8 RE-102-3 GML-7 M-54-3 P-69-1 BA-644 3-20-1 P-54-3-d Laboratory 160190 H-3423 160191 F-2590 160189 160193 160192 H-3427 160194 Major oxides :.:... anew sas 67.0 74.05 74.6 76.39 78.9 75.8 84.9 46.46 46.1 AOpen e dene ca end 17.0 13.04 10.6 11.18 9.1 11.83 T.B 19.77 28.8 CNY e Saran P aoe s ple malo n ana 2,56 2.31 2.8 _. 2.16 3.5 1.8 .63 13.25 13.2 Pe. ene ao SAL cle an .64 .16 1.0 .63 .30 .28 s vg 1.15 1.0 MG O- 2a 0+ ae n eee ee o al he ie in a Galore .88 .59 . 58 . 46 . 46 1.3 42 2.17 1.0 Cad: 2 ln une. . 58 . 64 1.9 . 40 1.8 .85 .06 00 .26 NBM EARL anat + fnk bi mln 2.4 2,42 .87 1.25 .84 TT 1.6 21 19 Ol r ee REIV. a o eni ans 6.6 4.70 3.8 5.29 3.3 5.0 3.0 8.97 9.5 es IIL. cin tas 1:2 1.12 1.24 1.00 15 1.2 TT 3.89 2.7 $0 ecu us .07 .04 04 .04 .04 .07 05 21 12 T 22. Ln iene dele liaise. 66 48 33 . 52 . 62 37 09 2.67 1.6 P3;O; 2. Ple ere eee neue cea 26 16 07 (12 .16 10 03 29 . 39 MnQ. s..: c C RC LG:. 03 03 04 .02 .08 03 Ol 02 06 COAL IL: En E L ike ien enn 05 OL 1:8 . 01 . 52 43 <.05 (Di <.05 Penco B RDL Ivel ev oy 04% .-. 2:2. ..o ane c nn ens. 06" yee l ere ie erea alanna eanke's kel as Bk (00 : . <. anld ane ae pauls ao a da ain han oa 00. }.: Subtotal: . dds As 99 NOL i. oleate able nels dn a te nate . o bee an baie aa a $9.14 ' Less - 00 lll ll LUE 02 es souls h oe t g tice a s at io a ane mic os apne cae ce on ie an an t Loa rove 02 +S cr s eus s Total: 100 99.77 99 99.47 100 99 100 99.72 100 Modes Quartz. c. esl ites: cl... 25 41 48 50 62 49 65 5 4 Potassic feldspar.___________._ 16 8 8 16 11 13 9. c 8 Plagioclase 16 13 .5 8 2.1 .8 9). . evo nn aloe notice 41 35 4T 25 23 38 15 79 65 o l 2.2 2.0 .8 1.3 1.9 .6 .4 5 2 Opaduie Minerals =~... aas elses as ene abo nn nants ams s ba | an eat OU pe ll Hoc 1 (n t wae 11 28 Calcite err n Ia al aa eae as a (I Pon 1.6 (DH. \ Aerie » ences Epidobe. 222 e cll e ll e rae uue caer a wane talk P %: .9 (AP Lie aE she wank t Bn wine ae M _ eee ane. Lol nk as P P P P r iI e P P ADRLINEL 2 rene ellen P Mai e ol P r P= S ccr. ho ore uce ate al P *} ¢: ae anne santas ose nes Lans aln ald nt bn nn ae an bl _c lnc.. akane oe ans al T ;s? seee. s edo ie bee ane edes o Points 500 1,118 600 1,005 1,000 706 742 600 600 CIPW norms (J Eee Leen e vena s 26.12 40.33 53.89 4.11 59.79 49.89 HAD : oen eel lan nines (Oe ask es aud 5.60 3.30 4.78 2.98 3.37 4.91 (B8 OL Palin e is | o Suen epee te s " n me car Ge aire s 1 38.99 Bt. 11 22.45 81.25 19.50 29 . 54 1T io il cece a av e. s.. um aot atacy thee.," 20.30 20.47 7.36 10.57 7.10 6.51 19.58" res OLL LLCO Ci mead a it s - .86 1.83 75 1.14 2.12 . 84 IQ.: Ae Pia a be cea was sc ii ille 2.19 1.47 1.32 1.14 1.14 3.24 1:05 coe ly.. Mt. . __. oc ered stl. ise. RP tie ah ean aln 2.40 59 : ele ea et 82 oso ecto etc Hm. 2.88 2.81 .65 1.75 3.50 1.30 41 © coe orice o ik Lc LIL: 1.25 . 40 .63 99 67 . 66 17 c {ece COLL... sbo Up p LOR t} fale ce snl l uel 25 102 +4 s Sot u Pe oreo cul iL Ap. EE Y ITE sinall » a ain 62 38 A7 28 38 24 07. sical Lali re no aA ia LIL En our nae ho ave 07 EH NEIL . Ab eac -s alain ce ber tn c when a's o ae n s eee au ae t al a Ceed ae ee aca in ahi an a 11 02 2.96 02 1.18 98s LIU ne c areal da ben ne as b aas NoTtE.-Minor-element analyses for samples 2, 4, and 8 given in table 1. CRANDFATHER MOUNTAIN WINDOW 81 TABLE 15.-Chemical analyses, modes, and norms of above and related rocks of the Grandfather Mountain Formation- Continued 1. Light-greenish-gray well-sorted sericite arkose. Clastic grains of quartz, microcline, and plagioclase (altered to albite) in groundmass of recrys- tallized iron-rich muscovite, quartz, microcline, and albite with a grain silzeltif 0.1 mm. From 3,080-foot altitude on Lost Cove Creek (area E-5, pl. 1). 2. Light-green sericite arkose. Few clastic grains of quartz and microcline as much as 0.5 mm in diameter in groundmass of recrystallied iron- rich muscovite, quartz, microcline, and albite. From quarry on the east side of Wilson Creek at 3,550-foot altitude (area F-4, pl. 1). 3. Green phyllitic quartzite containing lenses of calcite. Clastic grains of quartz and microcline as much as 2 mm in diameter in a matrix of re- crystallized quartz 0.02 mm in diameter and light-green iron-rich mus- covite 0.01 to 0.2 mm long. A few small quartz-microcline calcite segre- gation lenses. Mica probably overcounted in mode because of fine grain size. From roadcut 0.8 mile S. 59° E. of Shulls Mills (area G-3, pl. 1). 4. Green arkosic quartzite. Clastic grains of quartz as much as 2 mm in di- ameter, microcline 1 mm in diameter, and plagioclase (altered to al- bite) 0.5 mm in diameter in a matrix of recrystallied iron-rich mus- covite, quartz, microcline, and albite with a grain size averaging 0.1 mm. From same outcrop as mica analyzed by Foster (Foster and oth- ers, 1960). From roadcut on U.S. Highway 221, 1.9 miles N. 62° E. of summit of Grandfather Mountain (area E-4, pl. 1). 5. Light-green feldspathic sericite quartzite containing heavy-mineral seams and a few small rock fragments. Clastic grains of quartz and microc- line as much as 2.5 mm in diameter and plagioclase (altered to albite) as much as 1 mm in diameter in a matrix with a grain size of 0.05 to 0.1 mm of recrystallized quartz, iron-rich muscovite, and microcline. A of the Grandfather Mountain Formation resemble those of similar rocks elsewhere. They have a high FeO; :FeOQ ratio, which is characteristic of unmeta- morphosed red arkoses. In the arkoses of the Grand- father Mountain Formation, the ferric iron is mainly in the green muscovite. Na,0 content of the arkoses is a function of the amount of plagioclase; A1l;0; contents are related to the total feldspar and mica contents. Analyses of typical arkoses defined strictly on a mineralogical basis (Pettijohn, 1957, 1963) have a somewhat higher proportion of alkalis to alu- mina, probably because they contain a higher pro- portion of feldspar in relation to mica than do most Grandfather Mountain Formation arkoses. Figure 46 compares alkali proportions and con- tents of arkoses of the Grandfather Mountain For- mation with averages of sandstone clans according to Middleton (1960) and Pettijohn (1963). Arkoses of the Grandfather Mountain Formation contain more alkalis than the average arkoses, but the alkalis are in about the same proportions. The K,0:Na»0 ratio is substantially greater than 1, a characteristic of - taphrogeosynelinal - sandstones - (Middleton, 1960), most of which are arkosic. Most arkoses of the Grandfather Mountain Formation have about the same ratios of Al.0O, to CaO-+-Na,0-+ K.0 as aver- age arkoses (fig. 47). Analysis 1, which has a greater content of alumina and alkalis, is a muddy arkose that differs from graywacke in having a high K;,0 :Na,0 ratio. Analysis 3, which is of a rock poor in feldspar, falls near typical arkoses in this diagram because of its high mica content. Calculated mineral composition suggests that this rock contains about 10 percent feldspar and 30 percent mica and is a muddy feldspathic sandstone. few calcite grains as much as 0.5 mm in diameter occur in the matrix. From roadcut at 3,250-foot altitude on Rough Ridge (area F-4, pl. 1). 6. Light-green feldspathic sericite quartzite. Clastic grains of quartz as much as 1.5 mm in diameter and microcline and plagioclase (altered to albite) as much as 0.5 mm in diameter in a matrix of recrystallized sericite and quartz 0.01 to 0.06 mm in grain size. From roadcut in North Carolina Highway 184 south of Banner Elk near spot altitude of 3645 feet (area E-3, pl. 1). 7. Light-gray well-sorted sericite arkosic quartzite. Clastic grains of quartz and microcline as much as 0.5 mm in diameter and plagioclase (altered to albite) as much as 0.2 mm in diameter with a small proportion of matrix consisting of recrystallized quartz 0.01 to 0.05 mm in diameter and sericite 0.01 to 0.1 mm long. From 4,500-foot altitude on the north ridge of Bald Mountain (area D-3, pl. 1). 8. Light-gray sericite phyllite containing flattened fragments of light-green sericite phyllite as much as 2 em long. Scattered clastic grains of quartz as much as 1 mm long and phyllite fragments in a matrix of very fine grained iron-rich muscovite and sphene and containing grains of hematite as much as 0.5 mm in diameter. Bed about 4 feet thick in arkose. From a roadcut along North Carolina Highway 181 just east of Montezuma (area D-4, pl. 1). 9. Mottled green, maroon, and gray sericite phyllite. Scattered clastic grains of quartz and microcline as much as 0.35 mm in diameter in a matrix of light-green sericite 0.01 to 0.2 mm long and hematite 0.005 to 0.5 mm in grain size. Hematite overcounted in mode. From roadcut along North Carolina Highway 194 just north of bridge over Blevens Creek crossing (area C-3, pl. 1). The mineral composition diagram (fig. 45) indi- cates that analyses 4 and 5 are most nearly repre- sentative of rocks of the lower arkose unit. These anlyses plot quite near the arkose averages on figure 47. Analysis 6 is typical of the rocks in the highest exposed arkose unit; it does not differ in many re- spects from analysis 4 of the lowest arkose, except for lower contents of iron oxides and Na,0 and higher CaO content. Analyses 8 and 9 are of sericite phyllite in in- terbeds in the highest arkose unit. These rocks are chiefly mixtures of iron-rich muscovite, hematite, and quartz. Their origin is problematic. The frag- mental texture of sample 8 suggests that it is a tuf-. faceous rock, but the rock contains clastic grains of quartz. The K.0 and total iron oxide contents are higher than those of most shales. Chemical analyses of rocks believed to be derived from a volcanic-de- rived saprolite (Reed, 1955) show that they contain less K0 and Al;O; and more total iron oxide and CaO. Perhaps the material in these phyllite beds in the Grandfather Mountain Formation was derived from a residuum containing clay and iron oxide, which was enriched by K0 during diagenesis or me- tamorphism, or perhaps it was derived from a mix- ture of volcanic glass fragments with a minor amount of normal detritus and underwent marked compositional changes during diagenesis and (or) metamorphism. SILTSTONE Mrcascoric FEATURES Siltstone units are interbedded and interfinger with arkose units in the Grandfather Mountain For- mation. The siltstone commonly underlies valleys be- 82 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE 768T9 oss /r 6" X To / | 57/D5\ // l- [ | 24 / A1\ Cad -. f iss 4 \D6/ /6 s / \ I Ao & 7 A E Toof 20/A3 X 9 | J A ra | / 2 «8 .. I \ 24> + / \ é \\ of / £ | Lg D7 \\\ 05 // A5 / 214 lox 7]. // O; 1 [I Z\\/09 // 5 "8 | (\\A8 2" /// | re e> "s! Teg D8: G ile | D9? /. > 1 1 1 | 0 1 2 3 4 Na,0, IN WEIGHT PERCENT EXPLANATION 03 Arkose o Arkose and subarkose [ts Average of 5 typical arkoses g6 Chilhowee Group A Eugeosynclinal a Exogeosynclinal FIGURE 46.-Na,0:K,0 variation diagram for sedimentary rocks of the Grandfather Mountain Formation and the Chilhowee Group. Numbers of analyzed specimens refer to analyses in tables: arkose (table 15); siltstone (table 16) ; Chilhowee Group (table 18); mean compositions of gray- wacke, orthoquartzite, quartzite, and arkose and subarkose (Pettijohn, 1963); averages of taphrogeosynclinal, exogeo- synclinal, and eugeosynclinal sandstones (Middleton, 1960) ; Graywacke ® Orthoquartzite 0 P Quartzite A Siltstone A Taphrogeosynclinal average of five typical arkoses (Pettijohn, 1957). tween ridges of arkose, such as the one between Lin- ville (area E-4, pl. 1) and Shulls Mills (area G-3). The siltstone units consist of dark-blue-gray, gray, green-gray, and light-gray, fine-grained, thin-bedded chlorite- and biotite-bearing silstone, phyllite, and 25 20 |- 10 Al,O3, IN WEIGHT PERCENT &n 19(- |_ Middleton, 1960 / [ 6% e # 4A = mel: / lllite / Ideal . / 09 muscovite rep Chilhowee ¥ D57 s phyllites< J /0g \\ ¢ 7 he ly R l A F 1Ca I D6 / / 1 \ A o / i £ \ Shaw, 1956 Siltstones | Clarke, 1924! X45 ASCa/ Pettijohn, 1957 CP A Pettijohn, 1963+ 2Ca//_7 iddleton, 1960(A o NKE] 0/x\ 7Ca / 2/ AGCa -£ AL. /06 x // / 0 A 30 4 OPeguohn, 1957 F * Arkose d ye (/70A80 PP/ettijohn, 1963 XE R> 431 D7\ / /s D2 _LChilhowee quartzites 0 / n3 /. 4 ©middlefon, 1960 / ¥, ®Pettijohn, 1963 I | | 5 10 15 CaO +Na,0+K,0, IN WEIGHT PERCENT EXPLANATION A Albite Orthoclase O1 ® Arkose Orthoquartzite o 2 O Chilhowee Group Pelite A 0 Eugeosynclinal Quartzite 0 bis Exogeosynclinal Shale 4 a8 Graywacke Siltstone ® A Iron-rich muscovite Taphrogeosynclinal FIGURE 47.-Plot of Al,O, against CaO +Na,0+K,0 in sedi- mentary rocks of the Grandfather Mountain Formation and the Chilhowee Group. Positions of selected minerals and sedimentary rocks shown. Numbers of analyzed specimens refer to analyses in tables: arkoses of the Grandfather Mountain Formation (table 15); siltstones of the Grand- father Mountain Formation (table 16); rocks of the Chil- howee Group (table 18). Ca indicates calcite content greater than 1 percent. graywacke. They locally contain calcareous or dolo- mitic sandstone layers and lenses and a few lenses of GRANDFATHER MOUNTAIN WINDOW FIGURE 48.-Siltstone in the Grandfather Mountain Formation. A, Calcareous phyllite containing numerous quartz-calcite segregations. Roadcut along North Carolina Highway 105, about 1 mile northeast of Foscoe (area F-3, pl. 1). B, Laminated argillite and siltstone exposed on a cleavage plane. Roadcut along old North Carolina Highway 195 north of Blevens Creek about 0.1 mile from the Linville Falls Fault (area D-3, pl. 1). 83 84 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE sandy marble. Light-gray to white fine-grained sand- stone and phyllite or siltstone in many places alter- nate in beds 1 mm to 2 inches thick (fig. 484). Very thinly laminated argillite and siltstone beds occur in the uppermost siltstone unit (fig. 48B). Arkose beds as much as 20 feet thick are found locally. The sand- stones in many places are calcareous and in a few places grade to beds of sandy marble. The marble beds average a foot or two thick but in places are as much as 5 feet thick. All the carbonate-bearing rocks become brown and vuggy upon weathering. Thin sandy beds are lenticular in places, although their shape in some outcrops is plainly due to having been broken and drawn out by movement along cleavage planes that cut across the bedding. Graded bedding is found locally. Graywacke and pebbly graywacke are poorly bedded. They contain clastic grains of pla- gioclase and quartz and pebbles of mafic volcanic rock, fine-grained quartz-plagioclase rock, and dark sericite phyllite. Crossbedding, which is very wide- spread in the light-colored arkose, is absent in the gray biotitic and chloritic graywackes. Many of the calcareous phyllites (fig. 48A) con- tain segregations of quartz and calcite which locally also contain chlorite and rarely, purple fluorite. The calcite is pink in many places. The lowest siltstone unit in the Blowing Rock quadrangle (areas G-3 and H-3, pl. 1) is siltier than the others and contains interbeds of volcanic mate- rial and numerous sandstone beds and lenses. On Flattop Mountain and east of the New River (area H-3), sandstone and volcanic beds are particularly abundant, and the map units, which are quite valid for the rest of the outcrop area of the Grandfather Mountain Formation, become less useful. The middle siltstone, which is continuous from Long Arm Moun- tain (area D-6) to Hodges Gap (area G-2), is the most calcareous of the siltstone units. Pebbly gray- wacke is abundant in the uppermost siltstone unit next to the Linville Falls fault in the northwestern part of the Grandfather Mountain window. Texture anp MinErALocYy The rocks of the siltstone units are generally com- pletely recrystallized because of their original small grain size (fig. 494 and B). The coarser grained graywackes exhibit relict sedimentary textures (fig. 49C) like those in the arkose units; grains larger than 0.1-0.2 mm retain their clastic outlines, and rock fragments, their original textures (fig. 49D). Some of the siltstone, especially in the uppermost unit in the northwestern part of the window, seems to have recrystallized less completely; rather fine grained quartz and plagioclase has a fragmental rather than a mosaic texture (fig. 490). Quartz and plagioclase generally occur in recrys- tallized grains 0.01 to 0.15 mm in diameter. Clastic grains as much as 3 mm in diameter are found in the coarser or more poorly sorted rocks. Both clastic and recrystallized plagioclase are now albite. Sericite is the most abundant micaceous mineral. It is 0.01 to 0.2 mm long and well alined. In places it has been deformed by later slip cleavage. A few 0.3- to 0.4-mm grains of muscovite seem to be clastic. No paragonite was detected in several samples of seri- cite phyllite which were examined by X-ray diffrac- tometer by E. J. Young, U.S. Geological Survey. Probably most of the sericite is muscovite. Biotite is mostly green and greenish brown, but in the lowest siltstone unit, brown biotite is dominant. Brown biotite is rare in the other siltstone units. Biotite is 0.01 to 0.4 mm long and not as well alined as sericite. Light-green FeMg chlorite is intergrown with ser- icite and biotite. Calcite is a major constituent in some rocks, espe- cially in the lowerst stilstone unit in the Linville quadrangle. It is in grains 0.02 to 0.8 mm in diame- ter. Abundant accessory minerals are zircon, sphene, tourmaline, apatite, epidote, ilmenite, magnetite, and pyrite. Opaque minerals are the major constituent of some rocks of the siltstone units, especially in the Blowing Rock quadrangle. Less abundant minerals are allanite and stilpnomelane. Clastic grains of microcline, hornblende, and epi- dote are found rarely. Composition anD CrassIFICATION Estimated modal compositions of rocks from the three major siltstone units in the Grandfather Moun- tain Formation are plotted in figure 50. Chemical analyses (table 16) indicate that modes of these fine- grained rocks are unreliable, but the diagram can be used to compare the units and place the analyzed specimens in relation to the compositional range within each unit. Little information is available from the literature on chemical and mineral compositions of siltstone compared with sandstone. In the Grand- father Mountain Formation, the siltstones lack po- tassic feldspar and are much richer in micaceous minerals than the arkose. Biotite and chlorite are GRANDFATHER MOUNTAIN WINDOW 85 FIGURE 49.-Photomicrographs of siltstone in the Grand- father Mountain Formation. A, Chloritee and biotite- bearing sericite phyllite from roadcut along North Caro- lina Highway 183 about 0.9 mile west of Cranberry Knob (area D-6, pl. 1). Thin laminae rich in quartz and mica probably represent original silty and shaly beds. Analyzed specimen 5, table 16. B, Chlorite-sericite phyllite and silt- stone from uppermost exposed siltstone unit at about 3,000-foot altitude west of first sharp bend in Watauga River (area F-2, pl. 1). Silty and shaly laminae tightly folded; axial planes are parallel with north-south cleavage present in the siltstone but are almost absent in the arkoses. The lowest siltstone unit (fig. 504) generally con- tains more mica than the others. The middle siltstone (fig. 50B) contains more plagioclase. About half the samples from each of the siltstone units contain bio- tite. The lowest siltstone unit contains more chlorite of that area. C, Graded bed cut by cleavage in thinly laminated siltstone and argillite from uppermost exposed siltstone unit. North side of hill 3945 north of Newland (area D-4, pl. 1). Outcrop resembles rock shown in fig- ure 48B. D, Chloritee and biotite-bearing sericite sub- graywacke from roadcut along North Carolina Highway 194, 0.7 mile south of Miller Gap (area D-4, pl. 1). Clastic grains of quartz and plagioclase (pseudomorphosed by albite) and fragments of fine-grained plagioclase-rich felty- textured igneous rock in a matrix of sericite, quartz, chlor- ite, and biotite. and opaque minerals than the others, probably be- cause of admixtures of sedimentary or pyroclastic materials derived from mafic volcanic rock. Calcite occurs in about half the specimens studied from the middle siltstone, and it constitutes more than 10 per- cent of the rock in about a sixth of the specimens examined. 86 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE 4 A Mica and 1 chlorite V. Potassic feldspar Mica and chlorite Plagioclase Mica and B chlorite Mica and chlorite Plagioclase Potassic feldspar Mica and Mica and chlorite chlorite Mica and C Mica and chlorite chlorite Quartz < uw Plagioclase Potassic feldspar Mica and chlorite FIGURE 50.-Proportions of quartz, plagioclase, potassic feldspar, and combined mica and chlorite in the siltstones of the Grandfather Mountain Formation. Numbers of analyzed specimens refer to analyses in table 16. *, average. A, Lowest siltstone unit. Based on counts of 50 random grains in each of 24 thin sections. Contours 4, 8, 17 and 33 percent. B, Middle siltstone unit. Based on counts of 50 random grains in each of 30 thin sections. Contours 3, 7, 13, and 27 percent. C, Highest exposed siltstone unit. Based on counts of 50 random grains in each of 19 thin sections. Contours 5, 10, 21, and 42 percent. The mineral composition diagrams indicate that analysis 1 (table 16) is most typical of the lowest siltstone unit; analysis 5, of the middle unit; and analysis 7, of the uppermost unit. The analyses indi- cate that the stratigraphically higher siltstone units are generally poorer in AlsO; and alkalis than those lower in the section. This is in accord with the min- eral composition data, even though X-ray studies and chemical analyses indicate that the amount of albite in most rocks has been undsrsstimated. The rocks of the siltstone units have a lower K0 :Na,0 ratio than the arkoses (fig. 46); some of the values are less than one. Sandstones intercalated with the silt- stones (analyses 6 and 7) would lie in the field of eugeosynelinal rocks of Middleton (1960), most of which are graywackes. The rocks of the siltstone units generally have a higher ratio of alkalis and CaO to alumina than most shales and argillites (fig. 47). This difference is due to their higher plagioclase content. GRANDFATHER MOUNTAIN WINDOW 87 TABLE 16.-Chemical analyses and norms of siltstone and related rocks of the Grandfather Mountain Formation [Samples 1, 4, and 6, standard rock analyses by C. L. Parker, U.S. Geol. Survey, 1961; other samples analyzed by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey, 1962; CO: of sample 3 determined by I. C. Frost, Denver, Colo., 1964. Major oxides and CIPW norms given in weight percent] Middle siltstone unit Upper siltstone unit Lower siltstone unit 1 2 3 4 5 6 T 8 Fig -=- alas s seeds no neue GMB-2 X-18-2-a O-58-2 GML-13 2-268 GML-12 R-57-2-a P-37-1-b Laboratory No. : . . {SOL. cL 22 cule ie nake a H-3263 160188 160187 H-3262 160197 H-8425 160195 160196 Major oxides Ellen tras -on sk 55.65 66.7 60.5 60.65 64.0 64.91 68.3 82.6 MO; REL C... Ln rn Palen aloes bulk aula bae bln b 18.61 14.0 15.4 14.35 15.6 12.45 11:9 #¥i le ede eer eous - tep a ube 6.62 2.5 5.2 2.34 3.3 . 22 8.71 1.4 Pedi ire Ge LL onle a alee cl 2.14 2.0 1.6 3.69 3.0 1.81 2.0 .69 MOORE: clueless r- ct on ukmaks 1.94 1.5 2:5 4.51 2.4 . 50 1.8 ~19 Caddis LCL I-Ie ..- e anes a ae 1.32 2.5 3.0 2.47 1.1 6.25 3.3 .28 Nad et: rec TPLO 3.26 2.6 2.6 3.87 3.5 4.51 2.9 1:8 (OUS: il. nul 5.74 4.1 4.8 4.26 8.4 3.69 2.4 2.5 MOR LY: DNI UIG tane ab es 1.98 1.4 2.0 1.34 1.9 .41 1.4 . 92 .in ick -at en ule .06 .07 11 .04 .06 .07 . 08 . 08 .% . 68 ve . 64 . 92 .34 .98 .36 P0 rr s eir ee ill dik ane a ee ack ae eme on .20 22 49 12 .30 . 28 .28 . 08 LAr esi vt .04 . 06 . 08 12 12 08 . 08 .03 CO AE YYY e ee LL Hh Bids a ak a an ale ania nich ons .84 1.7 1.74 . 63 .07 4.60 1.4 .13 Pad r sn o e L aU o eee ne se haw e aas 0T" ...le lee cee unt s a (0b Esc oa ain Is Ubs n an apenas _____________________________________ ccd cleo. usa .} sOL oce sel a ele cies 3 Eire aln eaid a oe nil alu mine cie ae 200 ~> E- inu ens aa 00 :: eins cie be o - a+ od ue al Osi ss a oy Aae n oo aoa lane aan o e bina F002: cul. ice anit a 200 200" } Ros}. Subtotal. -__- _ ccna n 99.T8 } 99.09 i........ $9.64 . .no ol cee Less ll.}: . EI Eu Sues o oes Tel N Large wares sr ues 08 222 ece ov TOMA reer Een ail inin a cen danes eines 99.52 100 100 99 . 483 100 99.61 100 99 CIPW norms C anand a e el PLL bad ald ah ane a ='a ae ae be 12.60 33.30 22.16 11.95 25.08 23.01 37.13 61.38 CPi LLIN cr cece nee nen 7.04 5.21 4.96 2.22 5.05 1.04 2.33 2.02 Of. snel eos o aot beeen e a eom ua 33.91 24.22 28.36 25.17 20.09 21.80 14.18 14.77 ADLZ 10. d 27.57 21.99 21.99 32.18 29.60 38.14 24.583 15.22 LLY o = anale cae oo ak a ae .22 2.64 8.15 8.05 6.02 .04 Enis: el nitrites aaa sa 3.58 3.78 6.22 11.23 5.98 1.24 3.24 1.97 Petree .l .l cue ick es on ean nig » naan oad Lo Sd a ae a 160° 22202. 4.01 1.49 1.8L. sc... cll ene EEL Ll PLL L ca rants 4.80 3.62 3.36 3.39 4.78 .82 3.87 1.28 ss. .s sos oo IRL I aa ores anl as s Gee GOLA. Sm mee eu ee nue SU ue oute e ale 1.03 . 52 Nerses oo ane AEL cites e ranna toe 1.46 1.30 1.35 1.22 1.75 .65 1.86 . 68 ADES QA ila sar- 47 . 52 45 .28 TT . 52 . 54 19 Pes L eran pub ks nea -a ar naan nk ans 99: Ale oss sad oa aand 1.294. 2.0.0.0. (AC ee ALEC err nee MQ EL EEL nel.. a whan ale s 105; 2. leanne cke ac atl C r eae s nuns o SOT ; .} E2 -s aw (Cee ss t ao L hae ino i a hea a Wae as .6 3.87 3.96 1.43 .16 10.45 3.18 .30 NotE.-Minor-element analyses for samples 1, 4, and 6 given in table 1. clastic plagioclase (altered to albite) as much as 1 mm in diameter. Gray calcareous metasiltstone containing light-green sericite, quartz, and albite with a grain size of 0.02 to 0.06 mm, subordinate magnetite and calcite, and accessory sphene and apatite. From roadcut at sharp out- wiarld) curve 0.4 mile N. 80° E. from the top of Martin Knob (area G-3, pl. 1). Medium greenish-gray, poorly sorted calcareous sandy biotite-sericite phyllite. Clastic grains of quartz and plagioclase (altered to albite) as much as 1 mm in diameter in a groundmass of recrystallized sericite, quartz, and albite with a grain size of 0.06 mm and subordinate brown biotite and calcite. Accessory sphene, opaque minerals, chlorite, Zircon, and apatite. From roadcut along U.S. Highway 221, 150 feet southeast of spot altitude of 3,803 feet east of Raven Rocks (area G-3, pl. 1). Dark blue-gray fine-grained biotite and calcite-bearing sericite phyllite. Sericite, quartz, and albite 0.05 mm in grain size and subordinate greenish-brown biotite, calcite, and opaque minerals. Accessory sphene and apatite. Roadcut along U.S. Highway 221 in area E-5 (pl. 1) at first curve south of the village of Linville (area E-4). Dark-gray calcareous biotite-sericite phyllite. Segregations of calcite, quartz, and chlorite contain some purple fluorite. Sericite, green biotite, quartz, and albite 0.03 to 0.16 mm in grain size with subordinate epi- dote and calcite and accessory sphene. Roadcut along North Carolina Highway 105, 0.4 mile S. 73° W., spot altitude of 3,007 feet in village of Foscoe (area F-3, pl. 1). . Dark-gray phyllite, Sericite, quartz, and albite with a grain size of 0.05 to 0.1 mm and subordinate epidote, FeMg chlorite, sphene, opaque min- erals, green biotite, and accessory calcite and apatite. A few grains of Roadcut along North Carolina Highway 183 about 0.7 mile due west of Cranberry Knob (area D-6, pl. 1). Light-gray calcareous biotite arkose. Albite, microcline, and quartz grains are 0.05 to 0.5 mm in diameter; larger grains retain clastic out- lines. Subordinate green biotite and calcite. Accessory minerals are sphene, apatite, epidote, allanite, opaque mineral, and zircon. From a 2-foot bed in siltstone in roadcut on North Carolina Highway 105, 0.5 mile southwest of the settlement of Grandfather (area F-3, pl. 1). 7. Dark-greenish-gray pebbly graywacke containing fragments of mafic vol- canic rock, sericite phyllite, and granoblastic quartz-plagioclase rock. Contains clastic grains of quartz and plagioclase (altered to albite) as much as 1.5 mm in diameter in a groundmass of quartz, albite, and sericite and subordinate epidote, calcite, sphene, and opaque minerals. Grains in matrix are 0.005 to 0.3 mm in diameter. From roadcut on the north side of a sharp bend of Watauga River at spot altitude of 2,718 feet (area F-2, pl. 1). 8. Gray sandy biotite- and chlorite-bearing metasiltstone. Contains a few fragments of mafic volcanic rock. Clastic grains of quartz as much as 2 mm in diameter and microcline and plagioclase (altered to albite) as much as 1 mm in diameter in a matrix of quartz and sericite 0.01 to 0.05 mm in grain size and accessory greenish-brown biotite, FeMg chlorite, sphene, opaque minerals, calcite, apatite, and tourmaline. From roadcut along North Carolina Highway 194, 0.1 mile northeast of Smoky Gap (southwest corner of area D-8, pl. 1). 88 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE Analysis 6 is of a rock from an atypical bed ; it has a high ratio of CaO and alkalis to Al;O; and con- tains more calcite and feldspar than the other sam- ples. VOLCANIC ROCKS ROCKS OLDER THAN THE MONTEZUMA MEMBER OF GRANDFATHER MOUNTAIN FORMATION FeErsic Vorcanic Rocks Felsic flows, tuffs, and tuffaceous sedimentary rocks occur in the basal part of the Grandfather Mountain Formation, especially in the isolated out- crop areas southeast of the main outcrop belt. It is often difficult to distinguish felsic flows from thick units of tuff or welded tuff even in unmetamorphosed rocks; as all rocks in the Grandfather Mountain For- mation are metamorphosed, it is impossible to tell what proportion of the felsic volcanic rocks are of pyroclastic origin. A few dikes and small plugs of felsite cut basement rocks in the Linville Falls and Lenoir quadrangles, but most of them are too small to map. A Some bodies of felsic rocks are homogeneous, but others are heterogeneous, especially near their mar- gins. The body underlying the Montezuma Member of the Grandfather Mountain Formation in the northern part of the window contains interbedded arkose, siltstone, and graywacke adjacent to the Lin- ville Falls fault. A small basalt flow is intercalated with felsic rocks in this body on the lower part of Hodges Mountain (area G-2, pl. 1). Where it is ex- posed along North Carolina Highway 105 farther to the southwest, the body is homogeneous and coarsely porphyritic. Along Wilson Creek in the southeast part of area F-6, basalt, arkose, and tuffaceous arkose are in- terbedded with the felsic volcanic rocks. Faint color banding and differences in grain size may represent bedding, but in some outcrops such differences are obviously flow banding. Adjacent to some of the ba- salt flows, intricate convolution of color banding (fig. 51A) and pseudointrusive relations between the felsic rocks and the basalt may indicate that the un- derlying felsic material was unconsolidated at the time of extrusion of the basalt. In a few exposures, FIGURE 51.-Felsic volcanic rocks. A, Contorted flow banding in felsic volcanic rocks of Grandfather Mountain Formation. East side of Wilson Creek about 0.1 mile upstream from bench mark 1410 (area F-6, pl. 1). B, Fragments of base- ment rock in a matrix of coarse nonbedded volcanic sandstone or tuff. North side of Wilson Creek, 0.15 mile from west edge of the Lenoir quadrangle (area G-6, pl.1). GRANDFATHER MOUNTAIN WINDOW 89 faint suggestions of crossbedding or local concentra- tions of lithic fragments suggest that the rocks are water-laid crystal tuffs or tuffaceous sedimentary rocks. Cobbles and boulders of Brown Mountain Granite (fig. 51B) are found in nonbedded felsic vol- canic rocks at the base of the unit north of Brown Mountain (area G-6, pl. 1). A few thin felsic flows or beds of tuff occur in the mafic volcanic units at the base of the Grandfather Mountain Formation near Phillips Creek and Gragg Fork (area G-4, pl. 1). A 6-inch bed of felsic vol- canic rock at the base of the arkose northeast of Tablerock Mountain (area D-7, pl. 1) is identical with that in the larger extrusive and intrusive bodies and must be a tuff. The felsic volcanic rocks are light to medium gray and superficially resemble vitreous quartzite, arkose, or mylonitic gneiss. In most outcrops, however, euhedral or partly resorbed phenocrysts of gray to blue-gray quartz 1 to 2 mm in diameter are found. In some exposures, euhedral tan or light pink phenocrysts of potassic feldspar as much as 1 cm long are conspicuous. Some rocks, especially those underlying the Montezuma Member, contain plagio- clase phenocrysts. Quartz segregation veinlets are abundant, and felsic volcanic rocks in the south- eastern part of the window contain widespread quartz-microcline segregations. Foliation is defined by sericite flakes and elongate lenses and laminae of somewhat coarser grained quartz and feldspar in the fine-grained matrix. It is most conspicuous in the rocks near the Linville Falls fault east of the Brown Mountain Granite. In thin section the felsic volcanic rocks contain euhedral to anhedral embayed quartz phenocrysts and euhedral phenocrysts of microperthite and mi- croantiperthite in a fine-grained groundmass com- posed of quartz, feldspar, sericite, and opaque min- erals (fig. 52). Perthite in the phenocrysts has a fine-grained patchy pattern that differs from the coarser and more veinlike pattern of perthite in the plutonic rocks. Both the felsic volcanic rocks immediately below the Montezuma Member and those southwest of Brown Mountain contain plagioclase phenocrysts which have been altered to albite. Some of these plagioclase grains have more complex twinning than that found in plagioclase from the underlying plu- tonic rocks (fig. 52C). Some large feldspar grains seem to be crystal fragments. In some rocks, the fine-grained muscovite is light green and is prob- ably iron rich. The groundmass is so fine grained (0.005 to 0.05 mm) in many specimens (fig. 524) that it is difficult to estimate its composition. The groundmass is generally coarser grained in the rocks north and east of Brown Mountain in the Linville Falls and Lenoir quadrangles than elsewhere (fig. 52C and D). Quartz veinlets and lenses consist of recrystallized mosaic-textured grains 0.05 to 0.2 mm in diameter. Pyrite is a common accessory min- eral. A few rocks contain scattered radiating ag- gregates of stilpnomelane as much as 0.25 mm in diameter. Zircon occurs as subhedral to euhedral prisms as much as 0.25 mm long in the volcanic rocks; in tuffaceous sedimentary rocks both euhe- dral and round zircon grains are found. Both green and brown biotite are found, and biotite constitutes as much as 25 percent of the rock. Other accessories are epidote, sphene, allanite, calcite, chlorite, apatite, magnetite, dark-blue-green amphibole, and fluorite. Chemical analyses (table 17, Nos. 8-16) show that the felsic volcanic flows are rhyolite and quartz la- tite. Rocks believed to be tuffs or tuffaceous sedimen- tary rocks are chemically similar to the flow rocks. Maric Vorcantc Rocks Mafic flows, flow breccias, or tuff breccias occur locally in the lower part of the Grandfather Moun- tain Formation below the Montezuma Member. All the larger bodies of mafic volcanic rocks contain all these rock types and interfinger with sedimentary rocks at their margins. Beds of tuff and tuffaceous sedimentary rocks, and possibly even very thin flows, are widely distributed in the siltstone units in the Blowing Rock quadrangle and are more numerous near the mapped bodies of mafic volcanic rocks. Some mafic dikes found in the basement rocks may be correlative with the extrusive rocks. Mafic flows and associated pyroclastic rocks on the southwest slopes of Brown Mountain (area F-7, pl. 1) and near the southeastern contact of the main body of the Grandfather Mountain Formation near Ripshin Ridge (areas E-6 and E-7) are interbedded with lustrous blue phyllite. The mafic volcanic rocks are dark to light blue gray, gray, green gray to green, and generally con- tain phenocrysts or crystal fragments of plagioclase that are 5 mm but locally as much as 2 ecm long. The large plagioclase grains are commonly partly re- placed by calcite. Brown weathered calcite and quartz-calcite segregations are common; chlorite segregations are rare. A few rocks contain segrega- 90 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE TABLE 17.-Chemical analyses and norms of Linville Metadiabase and volcanic rocks of the Grandfather Mountain Formation [Samples 1, 2, 3, 6, 8, and 10, standard rock analyses by D. F. Powers, U.S. Geol. Survey, 1959; samples 7 and 16, by C. L. Parker, U.S. Geol. Survey, 1961; other samples analyzed by rapid methods by Paul Elmore, I. H. Barlow, Samuel Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey, 1962. Sample 4, by W. F. Hillebrand (Clarke, 1900). Major oxides and CIPW norms given in weight percent] Linville Map unit (pl. 1) ..... Meta- _ Mafic volcanic rocks of - Other mafic volcanic rocks Felsic volcanic rocks diabase the Montezuma Member 1 2 8 4 1 5 6 T 8 9 10 11 12 13 14 15 16 Fisld No:.:.:...0.:.. GML-3 GML-1 GML-2 ________ 10-5274 W-84 GMB-6 GMB-1 24-27 84(L) U-29 10-5783 8V-13 8W-85B RE-94-4 GMB-5 Laboratory No...... F-2418 F-2416 F-2417 ._______ 160149 F-2419 H-3426 F-2591 160151 14057 154236 160150 160153 160152 160154 H-3265 Major oxides 47.85 - 45.6 46.15 66.68 67.7 68.87 69.0 72.8 78.1 75.2 76.2 76.79 16.51 17.3 16.36 18.76 14.1 18.57 14.3 12.5 13.0 12.4 12.1 11.10 4.16 6.3 6.35 5.11 3.6 2.45 3.5 1.0 1.7 .8 £ F 7.43 6.0 7.79 .80 8.1 8.17 .94 2.8 1.8 6.24 6.5 2.61 .67 .59 .39 .56 4 4 7.00 6.3 5.80 1.60 1.6 1.78 1.0 .8 3.20 4.0 4.79 2.47 3.2 8.54 8.5 .2 .82 .09 C 4.7 5.08 5.5 4 4.00 4.0 8. .89 54 .56 4 .21 z R .05 .05 2.28 8. 2. .62 .50 4 . 1: .22 12 # 10 .08 1: <.05 .61 s ia s oto .01 TCH ______________________________ 100.17 onto ABLE O L2 ou (Udon L 2. bis es Ooi eel LLC ie "als an unl io wh wih anl 1, a bale als £09 reena vr 0D ays eee reals aoe oue 99.80 100.29 100 100.03 100.08 99.63 100 99.77 100 100 100 CIPW norms 1.18 1.45 1.283 26.54 26.58 26.37 1.76 .46 2.81 1.72 1.37 1.10 .58 .83 9.93 85.68 27.77 80.01 88.83 41.44 40.44 20.89 27.06 29.86 24.19 18.52 10.55 3.97 6.50 3.18 ____________ ( Pury - Lan . 02 (16.18 co 1.64 1.28 ~~9.13 ~~ 8.55 ~ 5.92 T 98 1.18 .28 ______ o .. poe aon strait "'1.36 Timo Tilso O ' 1 Other elements given: ZrO», 0.03; 0.01; V20s, 0.05; CoO, NiO, 0.03; SrO, trace?; LizO, trace. Complete total 100.41. NOTE.-Minor-element analyses for samples 1, 2, 3, 6, 7, 8, 10, and 16 given in table 1. 1. Fine-grained gray-green metadiabase containing porphyroclasts of tassic feldspar, albite, sericite, and sphene. Accessory minerals are ep- actinolite as much as 4 mm long and ilmenite as much as 2 mm in di- ldoteh biotite, opaque mineral, and carbonate. Roadcut along North ameter in a fine-grained matrix of FeMg chlorite, epidote, actinolite, Cgrolma Highway 105 about 0.2 mile south of bridge across Watauga albite, sphene, sericite, ilmenite, and accessory brown biotite and apa- River (southwest corner of area G-2, pl. 1). tite. Roadcut along North Carolina Highway 105 at Linville Gap Medium-grained greenish-gray volcanic sedimentary rock or tuff. Con- (area E-4, pl. 1). tains perthite grains as much as 6 mm in diameter and plagioclase 2. Greenish-gray greenschist containing actinolite and epidote as much as (pseudomorphosed by albite) as much as 3 mm in diameter in mosa- 0.5 mm in diameter in a matrix composed of FeMg chlorite, albite, ic-textured groundmass of recrystallized quartz, albite, and potassic sphene, ilmenite, and accessory carbonate and quartz, and relict pi- feldspar 0.005 to 0.25 mm in grain size. Matrix contains irregular geonite. From abandoned quarry on north side of Bald Mountain clots and folia of biotite and epidote. Accessories include opaque min- (area D-3, pl. 1). erals, carbonate, sericite, and chlorite. Outcrop in area G-6 (pl. 1) on 3. Fine-grained gray-green greenschist containing hematite and maghemite east side of Wilson Creek, 1.3 miles N. 64° W. of summit of Adams grains as much as 5 mm in diameter in a matrix composed of epidote, Mountain (area G-7, pl. 1). actinolite, FeMg chlorite, albite, and accessory sericite, apatite, and 10. Flute-grained gray felsic volcanic rock containing phenocrysts of per- quartz. Outcrop contains abundant epidote-quartz segregations. Road- thite as much as 5 mm in diameter and plagioclase (altered to albite) cut along North Carolina Highway 184, 0.1 mile south of spot altitude as much as 2 mm in diameter in a matrix composed of fine-grained of 3,645 feet, south of Banner Elk (area E-3, pl. 1). microcline, albite, quartz, sericite, biotite, and magnetite. Accessory 4. Analysis from Clarke (1900, p. 53). "Epidote-chlorite schist one-half minerals are sphene, epidote, calcite, and zircon. Rock is strongly mile northeast of Montezuma-contains epidote and feldspar with less sheared and contains quartz-calcite segregations. Cut along road on chlorite, hornblende, and magnetite." Locality in area D-4 (pl. 1). south side of Johns River 0.17 mile northwest of mouth of Crawley 5. Fine-grained dark-green faintly schistose greenstone from body 50 to Branch (on boundary between areas G-6 and G-7, pl. 1). 100 feet thick interlayered with lustrous gray and blue phyllite. Felted 11. Light-gray to white felsite containing phenocrysts of white feldspar 1 mass of MgFe chlorite, albite, epidote, sphene, quartz, and accessory mm long and gray quartz blebs 5 mm in diameter. Rock is finely lami- opaque mineral, carbonate, and apatite containing a few plagioclase nated and strongly foliated. Phenocrysts are plagioclase and perthite; phenocrysts (altered to albite) as much as 0.8 mm long. From tribu- groundmass is a mosaic of quartz and feldspar 0.01 to 0.05 mm in tary to Steels Creek 0.4 mile northwest of the confluence of Buck grain size containing discontinuous skeins and streaks of sericite. Creek and Steels Creek (area E-7, pl. 1). Accessory minerals are magnetite, sphene, calcite, biotite, and epidote. 6. Fine-grained light-greenish-gray schist having relict pilotaxitic texture Roadcut on east side of North Carolina Highway 181, 0.18 mile south and containing albite (mainly pseudomorphous after plagioclase laths of bench mark 1195 (north edge of area F-8, pl. 1). 0.05 to 0.15 mm long), sericite, epidote, sphene, magnetite, MgFe 12. Fine-grained dark-blue-gray faintly schistose felsite containing a few chlorite (almost isotropic), and carbonate. Rock locally contains feldspar phenocrysts, some as much as 5 mm in diameter. From a amygdules filled with epidote, quartz, and chlorite. From northeast dike 15 to 20 feet thick cutting Wilson Creek Gneiss. Phenocrysts of side of Wilson Creek about 0.1 mile north of bridge over Wilson Creek quartz and perthite in a groundmass of mosaic-textured quartz and near mouth of Craig Creek (area F-6, pl. 1). feldspar 0.01 to 0.05 mm in grain size. Groundmass contains scattered 7. Greenish-gray porphyry containing laths and stubby prisms of plagio- flakes of sericite and biotite and grains of epidote. Accessory minerals clase as much as 1 cm long. Plagioclase phenocrysts (altered to al- are allanite, carbonate, and fluorite. From outcrop in area E-8 (pl. 1) bite) in fine-grained groundmass composed of sericite, FeMg chlorite east of Simpson Creek, 1.4 miles west of bench mark 1195 on North (almost isotropic), magnetite, sphene, epidote, and accessory biotite Carolina Highway 181 (north edge of area F-8, pl. 1). and apatite. From roadcut along the Blue Ridge Parkway 0.95 mile N. 13. Medium-grained dark-blue-gray felsite or tuffaceous sedimentary rock 52° E. of Raven Rocks (area G-8, pl. 1). Greenish-gray metavoleanic rock containing phenocrysts of potassic feldspar and plagioclase as much as 2.5 em long and quartz as much as 2 mm in diameter. Phenocrysts of perthite, plagioclase (altered to albite), and quartz in a very fine grained groundmass of quartz, po- containing blue quartz grains (resorbed phenocrysts?) as much as 3 mm in diameter and a few feldspar grains (phenocrysts?) as much as 5 mm in diameter. The large quartz and perthite grains are set in a fine-grained matrix composed of quartz, feldspar, and sericite with a grain size of 0.02 to 0.01 mm. Accessory minerals are allanite, epidote, GRANDFATHER MOUNTAIN WINDOW 91 TABLE 17.-Chemical analyses, and norms of Linville Metadiab ase and volcanic rocks of the Grandfather Mountain Formation -Continued biotite, zircon, carbonate, and opaque minerals. From cut along Wil- son Creek road (boundary between areas F-6 and G-6, pl. 1). 14. Medium-grained gray-green rock, probably a lithic crystal tuff or tuffa- ceous sedimentary rock containing light-pink feldspar grains as much as 2 mm long. Quartz, potassic feldspar, plagioclase (altered to al- bite), and fragments of fine-grained felsic volcanic(?) rock as much as 1 mm in diameter in a matrix of light-green sericite, quartz, feld- spar, and sphene. East end of bridge across Wilson Creek near mouth of Craig Creek (area F-6, pl. 1). 15. Dark-greenish-gray schistose rock, probably lithic crystal tuff or tuffa- ceous sedimentary .rock containing aphanitic clastic fragments as much as 5 mm in diameter. Grains of quartz and potassic feldspar as FiguUrE 52.-Photomicrographs of felsic volcanic rocks. A, Felsic volcanic rock from roadcut along Blue Ridge Park- way about 0.8 mile north of Cone Lake (area H-3, pl. 1). Embayed phenocrysts of quartz and patch perthite in groundmass of very fine grained quartz, feldspar, sericite, and opaque minerals. Analyzed specimen 16, table 17. B, Felsic volcanic rock from roadcut along Wilson Creek at east edge of Linville Falls quadrangle (area F-6, pl. 1). Phenocrysts of quartz, fine-textured patch perthite and, rarely, plagioclase (altered to albite) in groundmass of sericite quartz, and feldspar. Analyzed sepcimen 13, table 17. C, Volcanic sandstone or tuff from outcrop on much as 1.5 mm long and very fine grained fragments of felsic vol- canic rock in a fine-grained matrix of recrystallized yuartz, feldspar, light-green sericite, and accessory opaque minerals, fluorite, carbonate, and sphene. Same locality as 14. Blue-gray felsite containing quartz and perthite phenocrysts 0.5 to 2.5 mm in diameter in a very fine grained matrix of quartz, sericite, feld- spar, and opaque minerals. Accessory minerals are pyrite, allanite, sphene, zircon, and carbonate. From roadcut along Blue Ridge Park- way opposite overlook, 1.25 miles N. 15° W. from junction of U.S. Highways 321 and 221 in the village of Blowing Rock (area H-3, pl. 1): north side of Wilson Creek 0.15 mile from west edge of Lenoir quadrangle (area G-6, pl. 1). Phenocrysts of fine- textured perthite and complexly twinned plagioclase (pseu- domorphosed by albite) in a groundmass of quartz, albite, potassic feldspar, biotite, and epidote. See figure 51B for photograph of outcrop. Analyzed specimen 9, table 17. D, Felsic volcanic rock from south side of Johns River in area G-6, pl. 1, 1.2 miles northwest of Collettsville (area H-7, pl. 1). Grains of fine-textured perthite and plagio- clase (pseudomorphosed by albite) in a matrix of micro- cline, albite, quartz, sericite, biotite, and magnetite. Ana- lyzed specimen 10, table 17. 92 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE tions of epidote, but they are neither as large nor as ubiquitous as those in the Montezsuma Member. Amygdules are as much as 5 ecm in diameter and commonly contain quartz and epidote and rarely calcite, albite, or chlorite (fig. 53). Some rocks con- tain enough magnetite to deflect a compass needle. Some of the rocks have relict felty texture formed by laths of plagioclase (now albite) generally 0.02 to 0.5 mm long, and in some rocks, gradations in size between laths in the groundmass and lathshaped phenocrysts are found. Other rocks contain stubby euhedral phenocrysts (fig. 54) or anhedral crystal fragments of plagioclase which are bent, broken, and healed by metamorphic minerals and have been al- tered to calcite and albite. A few have rounded out- lines suggesting partial resorption. All gradations exist between rocks having a very marked relict ig- neous texture and those lacking it. Where plagioclase has recrystallized, it has a mosaic texture and a grain size of 0.01 to 0.2 mm. Flakes of nearly isotropic FeMg and MgFe chlor- ite are 0.01 to 0.3 mm long. Sericite is of similar size. FiGUurE 53.-Amygdaloidal greenstone. dules in mafic flow intercalated in felsic volcanic rocks of the Grandfather Mountain Formation on east side of Wilson Creek about 0.1 mile north of bench mark 1410 (area F-6, pl. 1). Note epidote segregation in upper left part of photograph. Epidote-filled amyg- FIGURE 54.-Photomicrograph of mafic porphyry from out- crop along Blue Ridge Parkway in area G-3, plate 1 about 0.6 mile northwest of Cone Lake (area H-3, pl. 1). Laths of plagioclase (altered to albite) in matrix of seri- cite, albite, chlorite, magnetite, and sphene. Analyzed spe- cimen 7, table 17. Sphene is found in tiny grains 0.001 to 0.1 mm in diameter, which commonly form aggregates. IImen- ite and magnetite grains are 0.002 to 0.3 mm in di- ameter. Brown biotite flakes 0.05 to 0.2 mm long are intergrown with chlorite. Epidote, in part pistacite, is most abundant in amygdaloidal zones, where it is found in the groundmass as well as in amygdules. Quartz occurs with epidote in amygdules and amyg- daloidal zones but is most abundant in tuffaceous se- dimentary rocks, where it occurs as clastic grains as much as 2 mm in diameter and in mosaics of recrystallized grains 0.05 to 0.2 mm in diameter. Some phyllites mapped with the mafic volcanic rocks lack definite volcanic texture and contain little albite or chlorite. They are composed of chlorite, sphene, ilmenite, sericite, and opaque minerals. They were probably fine-grained tuffs or tuffaceous sedi- mentary rocks. In the belt of mafic volcanic rocks that extends across Ripshin Ridge from Buck Creek to Upper Creek (areas E-6 and E-5, fig. 4), phyllonites de- rived from the adjacent Wilson Creek Gneiss and phyllites intercalated with sedimentary rocks and mafic volcanic rocks are intimately associated. Lo- cally, the arkose overlying the volcanic rocks con- tains phyllite fragments. In some outcrops in this area it is impossible to distinguish phyllites from phyllonites, but commonly the phyllonites can be GRANDFATHER MOUNTAIN WINDOW 98 identified because they grade into gneiss and contain lenses of less-sheared gneiss and pegmatite. Both the phyllites and phyllonites are rich in sericite and con- tain various quantities of quartz, magnetite, sphene, chlorite, epidote, and plagioclase. The mafic volcanic rocks in the lower part of the Grandfather Mountain Formation differ from the Montezuma Member in their porphyritic character, partly tuffaceous nature, lack of amphibole, and greater content of albite. Chemical analyses (table 17, Nos. 5-7) show that these rocks have SiO, contents typical of basalts, but their rather high Na,O and low CaO contents sug- gest that they have been albitized. Concentration of CaO and some SiO; in segregations and amygdules may have modified the chemistry of the rest of the rock. Their Na,0 contents are greater than those of the Montezuma Member, which indicates that at some stage in their development they were more in- tensely albitized. MgO contents are lower than those of the Montezuma Member, which probably accounts for the lack of amphibole, and the total iron oxide contents are also slightly lower. TiO; contents are similar to those of the Montezuma. MONTEZUMA MEMBER The Montezuma Member of the Grandfather Mountain Formation is composed of metabasalt and is found at a higher stratigraphic horizon than the other volcanic rocks of the Grandfather Mountain Formation. Keith (1903) named the unit the Monte- zuma Schist from exposures near the village of Mon- tezuma in the Linville quadrangle (south edge of area D-4, pl. 1). He believed that it was older than the associated sedimentary rocks. Bryant (1962) found that the metabasalt is interbedded with the surrounding arkoses and renamed it the Montezuma Member of the Grandfather Mountain Formation. It crops out in a continuous belt 15 miles long, mainly in the Linville quadrangle, and is well exposed along the Watauga River and on most hillsides along the outcrop belt. It is cut off by the Linville Falls fault on the northeast and pinches out to the south. Meta- basalt also crops out in small areas west of the main outcrop belt; whether these areas are part of the main body brought up in anticlines or separate flows at a stratigraphically higher horizon is generally un- certain. The Montezuma Member is commonly composed of greenstone and locally, of greenschist. Much of the rock is intermediate between greenstone and greenschist and has a moderately strong cleavage and only an indistinct igneous texture, except for prominent amygdaloidal zones. For convenience, it will be called greenstone. The greenstones are green, blue-green, or blue-gray fine-grained rocks contain- ing epidote and quartz-epidote knots, lenses, and veinlets. Amygdules containing epidote, albite, and quartz occur in zones of massive dark-blue-gray rock. Locally, chlorite and calcite occur in amyg- dules. Many outcrops contain enough magnetite to deflect a compass needle. Neither pillow structure nor columnar jointing was identified. Amygdules average 2 to 5 mm in diameter and reach a maximum length of 1.5 em. They generally stand out on weathered surfaces. Epidote is the most abundant filling, followed, in order of abundance, by albite, quartz, chlorite, and calcite. Chlorite-filled amygdules are commonly flattened and elongated. Some amygdules have epidote rims and quartz cores (fig. 55) ; others have three zones : albite on the rim, epidote, and quartz in the core. Still others have chlorite cores and epidote rims. Epidote and albite predominate in different amygdules in the same rock. Yellowish-green to apple-green epidote and quartz-epidote segregations range from irregular stringers and veinlets to lenses as much as 3 feet long. Locally, they contain calcite and fibrous actinol- ite. The segregations make up about 10 percent of FIGURE 55.-Photomicrograph of amygdaloid in Montezuma Member of Grandfather Mountain Formation exposed in roadcut along Watauga River about 0.1 mile west of east edge of the Linville quadrangle (area F-2, pl. 1). Amyg- dules have epidote rims and quartz cores. Matrix consists of plagioclase (partly in tiny lath-shaped grains altered to albite), actinolite, sphene, opaque mineral, and chlorite. 94 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE the rock in most outcrops ; in some they constitute as much as 20 percent. A few greenstone outcrops lack segregations. Quartz or quartz and chlorite locally occur in veinlets within epidote segregations. Rare veinlets of asbestiform actinolite as much as 1 inch thick are found. Boundaries between flows are commonly marked by a few inches to a few feet of fragmental phyllite. The phyllite is rich in sphene and opaque minerals and may be a tuff or tuffaceous sedimentary rock. It consists of flattened fragments of light-green sericite phyllite 5 mm to 3 ecm long in a matrix of dark-gray sericite phyllite. In a few places, similar rocks occur as thin beds in arkose within a few hundred feet stratigraphically of the greenstone. Arkose or silt- stone overlying greenstone have been locally epido- tized. In places, tops of flows contain fragments of dark-green-gray rock 1 to 6 inches long. The rock in the fragments is rich in opaque minerals, sphene, and chlorite. It lacks albite and locally contains amygdules. The fragments are in a matrix of lighter gray rock that contains albite and lesser amounts of epidote, actionolite, chlorite, and opaque minerals. Albite in the greenstone generally forms small an- hedral grains 0.01 to 0.05 mm in diameter, but in some rocks it occurs as relict subhedral laths, gener- ally less than 0.5 mm long but locally as much as 1.5 mm long; relicts of more calcic plagioclase were not found. Light-green actinolite is generally well alined and is locally somewhat bent. It ranges from 0.02 to 1.5 mm long and averages about 0.1 mm long. In places, the larger crystals are randomly oriented postkine- matic porphyroblasts. Hornblende rimmed by actin- olite occurs in one rock. Green FeMg and MgFe chlorite forms flakes 0.01 to 0.06 mm long. Epidote is pistacite which occurs in anhedral to subhedral grains 0.008 to 0.4 mm in diameter. In a few rocks, epidote grains are red in hand specimens and have a light pink color in thin section, perhaps because of admixtures of piedmontite. Opaque minerals are magetite, maghemite, il- menite, and hematite 0.005 to 0.3 mm in diame- ter. Long aggregates of opaque minerals are gener- ally nonmagnetic and probably ilmenite. Sphene and leucoxene occur as anhedral grains and aggregates and as rims on opaque minerals. Anhedral relicts of pigeonite were found in only one sample. Although the proprotions of albite and mafic min- erals in single thin sections of the Montezuma Mem- ber vary widely, the unit is rather homogeneous overall. Local variations are probably due to the widespread segregations and amygdule fillings. Chemical analyses (table 17, Nos. 2, 3, 4) of green- stones of the Montezuma Member show that they are of basaltic composition. Sample 2, which has low Na,0 and high CaO contents, was free of epidote segregations and may be the most representative analysis of the metabasalt. The other analyses sug- gest slightly albitized basalt. 'The Montezuma Member does not easily fit into any of the categories of basalt. It has the high iron and titanium contents characteristic of tholeiite, but its alumina and (except for analysis 2) soda contents are higher than those of tholeiite. Its alumina con- tent lies between that of tholeiite and high-alumina basalts. Compared with rocks of quite similar aspect from the Catoctin Formation of central Virginia, which may have been derived from tholeiitic basalt (Reed, 1964c), the Montezuma Member contains more alumina and less soda but is quite similar in other respects. ORIGIN AND ENVIRONMENT OF DEPOSITION Abrupt changes in thickness and lithology along strike indicate that the Grandfather Mountain For- mation was deposited in a rapidly subsiding basin. Differences between rocks exposed in the southeast part of the window near Brown Mountain and those in the main outcrop belt to the northwest suggest that similar stratigraphic variations occur across strike. Bedding and fold attitudes, many of which are not shown on the maps, but which are summa- rized below, suggest that the exposed thickness of the formation is about 7,000 feet along the northern edge of the window in the Blowing Rock quadrangle, at least 20,000 feet in the center of the Linville quad- rangle, and 9,000 feet in the southern part of the Linville quadrangle (fig. 56). There is no informa- tion on thickness and facies variations in a north- west-southeast direction in the main outcrop area. The conglomerate-filled channels, crossbedding, and ripple marks in the arkose units and the poor sorting of the arenaceous rocks indicate that they were deposited in alluvial fans or deltas adjacent to a region of high relief. Whether the water in the basin was marine or fresh is not known. The miner- alogy of the arkoses indicates that they could have been derived in a large part from the underlying basement rocks. Volcanic material in the conglomer- ates might have come from part of the Grandfather Mountain Formation itself. The laminated siltstone was probably deposited in shallow water farther from the source of detritus. GRANDFATHER MOUNTAIN WINDOW 95 SW Tablerock thrust sheet 25,000" - 20,000 15,000" 10,000' 5000' VERTICAL EXAGGERATION x 2 LINVILLE FALLS FAULT iy “1&4“ A“.--;“¥’h \~\\\\\\\\\\\\\— * ta gr v halk le op sp M n «6 Nev 4 v 068 v a 4 < 4 Y az a 4 Arkose Felsic volcanic rocks FIGURE 56. -Diagrammatic stratigraphic section of the Grandfather Mountain Formation constructed on the as- sumption that rock units cropping out can be projected to a vertical plane parallel with the trend of the units. Sec- Remoteness from source of clastic material allowed local deposition of admixed carbonate. Compositional differences between siltstones and arkoses may be due to mechanical sorting ; the coarse-grained quartz and potassic feldspar remained on the alluvial fans or deltas, while clays or micas and the finer grained quartz and plagioclase were washed farther into the basin. Potassic feldspar in the basement rocks is in large grains, whereas much of the plagioclase and some of the quartz is finer grained. Occasionally, coarser grained unsorted material was transported beyond the delta and was deposited as graywacke and conglomeratic graywacke. The up- permost siltstone unit, which contains more gray- wacke-type rocks than the other silstone units, may have been deposited in deeper water. The positions of the shorelines and deltas and thus the sites of deposition of the two principal lithologies shifted from time to time. Except for extrusion of the basalts of the Montezuma Member, volcanism occurred principally in the area east of the main outcrop belt of the Grandfather Mountain Formation and at the beginning of the filling of the part of the basin presently exposed. Montezuma Member and other mafic volcanic rocks + +04 ula ca Metadiabase Siltstone tion extends from the north-central part of the Linville Falls quadrangle to north-central part of the Blowing Rock quadrangle. Circles indicate occurrences of conglomerate. AGE AND CORRELATION Stratigraphic relations between the Grandfather Mountain Formation and rocks of Cambrian(?) age cannot be determined in the Grandfather Mountain window because the two sequences are in thrust contact. The Grandfather Mountain Formation, how- ever, differs from the Chilhowee Group in the lenti- cular stratigraphy, greater and more variable total thickness, greater content of volcanic rock, lack of distinctive marker horizons, absence of orthoquartz- ite beds, abundance of feldspar, and presence of con- siderable clastic plagioclase. The differences in min- eralogy produce the marked contrast in the chemis- try illustrated by figures 46 and 47. Heavy-mineral suites from sandstones of the Grandfather Mountain Formation contain abundant zircon and very little tourmaline, whereas those from Chilhowee sand- stones in the Tablerock thrust sheet and in the Unaka belt contain subequal amounts of tourmaline and zircon. Thus, it is unlikely that the Grandfather Mountain Formation is correlative with the Chil- howee Group. Nor does the Grandfather Mountain Formation resemble any known post-Chilhowee rocks in the southern or central Appalachians, and nowhere in the region are post-Chilhowee rocks in stratigraphic contact with the Precambrian meta- 96 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE morphic and plutonic rocks. The Grandfather Moun- tain Formation is therefore probably older than the Chilhowee Group and younger than the rocks (1,000-1,100 m.y. old) on which it rests; it is be- lieved to be of late Precambrian age. The Grandfather Mountain Formation contains the southernmost volcanic rocks of late Precambrian age known in the Blue Ridge. The Mount Rogers Formation, 25 miles north of the Grandfather Moun- tain window (pl. 3) contains the nearest similar rocks. They rest on basement rocks and at least part of them are in the Shady Valley thrust sheet. Availa- ble descriptions of the Mount Rogers Formation (Jonas and Stose, 1939; Stose and Stose, 1944, 1949, 1957; King and Ferguson, 1960) indicate that it is composed predominantly of felsic volcanic rock, shale, conglomerate, and arkose. Mafic volcanic rock is a minor constituent. Some of the sedimentary rocks of the Mount Rogers Formation bear a re- markable resemblance to those in the Grandfather Mountain Formation. Green arkoses, conglomerates, and some gray finely laminated slaty rocks in the Mount Rogers Formation resemble some rocks of the Grandfather Mountain Formation (D. W. Rankin, written commun., 1965). The Grandfather Mountain Formation, however, lacks the agglomerates, ex- tremely coarse conglomerates, and red slates found in the Mount Rogers Formation. The site of deposi- tion of the part of the Mount Rogers Formation in the Shady Valley thrust sheet must have been much closer to the site of deposition of the Grandfather Mountain Formation before the northwestward movement of the thrust sheet. In view of the abrupt stratigraphic variations in the Grandfather Moun- tain Formation, it would be surprising if there were any close correlation between its stratigraphy and that of the Mount Rogers Formation. Rocks of late Precambrian age are found in the Blue Ridge thrust sheet near Mount Mitchell, 30 miles southwest of the Grandfather Mountain For- mation (fig. 12). They have been neither mapped nor studied, but they resemble rocks of the Great Smoky Group in the Blue Ridge thrust sheet 80 miles to the west-southwest (J. B. Hadley, oral commun., 1959). The sedimentary rocks of the Grandfather Mountain Formation more nearly resemble rocks of the Snow- bird Group of the Ocoee Series, which are character- ized by current bedding, than they do those of the Great Smoky Group, which are characterized by graded bedding (King and others, 1958; J. B. Had- ley, oral commun., 1959). Rocks of the Snowbird Group (Hadley and Gold- smith, 1963, table 7; Hamilton, 1961, table 2), how- ever, differ somewhat from those of the Grandfather Mountain Formation in mineralogy and chemical composition. The sandstones in the Snowbird Group are richer in clastic plagioclase and total feldspar and poorer in mica than the Grandfather Mountain arkoses and therefore have lower K;,0O:Na,0 ratios. The Snowbird siltstones are very rich in mica and consequently in K0 and Al;0O;; they chemically re- semble shale. Thus, the proportions of CaO, KO, and Na,0 are reversed between the sandstones and siltstones of the Grandfather Mountain Formation and those of the Snowbird Group. Relationship between the rocks of the Grandfather Mountain Formation and those of the Ocoee Series cannot be definitely established. Whether the Grand- father Mountain Formation and perhaps the Mount Rogers Formation are a different facies of the Snow- bird Group deposited in a different part of the Ocoee basin or whether they were deposited in separate basins and perhaps at somewhat different times is unknown. Hadley (1970) summarizes the characteristics of the Snowbird Group, Mount Rogers Formation, and Grandfather Mountain Formation and reaches simi- lar conclusions. Recent isotopic dates on zircon from felsic volcanic rock of the Grandfather Mountain Formation, the Mount Rogers Formation and the Catoctin Forma- tion in southern Pennsylvania indicate an original age of 850 m.y. and prove the approximate syn- chroneity and Precambrian age of these units (Ran- kin and others, 1969). These authors note that pub- lished zircon ages of the Beech Granite and aegi- rine-augite granite in the Blue Ridge thrust sheet fall close to the discordia curve for these rhyolites, which suggests that they are correlative. LINVILLE METADIABASE The Linville Metadiabase intrudes basement rocks and rocks of the Grandfather Mountain Formation stratigraphically below the Montezuma Member. Keith (1903) named the Linville Metadiabase and recognized its relation to the Montezuma Member of the Grandfather Mountain Formation, although he thought that part of it constituted the lower part of a flow. The map relations shown by him were compli- cated by his belief that the arkoses unconformably overlay the mafic igneous rocks. GRANMDFATHER MOUNTAIN WINDOW | - 97 FIGURE 57.-Photomicrographs of Linville Metadiabase. A, Altered and slightly sheared coarse-grained diabase from north side of Boone Fork about 0.25 mile east of west edge of Blowing Rock quadrangle (area G-3, pl. 1). Plagioclase laths (altered to albite) in a matrix of re- crystallized albite, actinolite, sphene, chlorite, quartz, epidote, and magnetite. B, Foliated medium-grained metadiabase from roadcut along North Carolina High- way 183 on south side of Camp Creek 1.3 miles N. 75° W. of summit of Cranberry Knob (area D-4, pl. 1). Laths of _ plagioclase (altered to albite) in a matrix of chlorite, epidote, actinolite, sphene, stilpnomelane, and opaque min- erals. C, Schistose metadiabase from roadcut along North Carolina Highway 105 at Linville Gap (area E-4, pl. 1). Broken amphibole porphyroclast (now actinolite) in a fine- grained matrix of chlorite, epidote, actinolite, albite, sphene, sericite, and ilmenite. Analyzed specimen 1, table 17. Most of the mappable intrusives in the basement rocks are within a mile of the contact with the Grand- father Mountain Formation. Thin dikes of metadi- abase or metadiorite are somewhat more widespread in the basement rock than in the Grandfather Moun- tain Formation. Intrusive bodies in the Grandfather Mountain Formation range from concordant to cross- cutting and are as much as 4 miles long. Concordant bodies are generally found in the siltstone units or in thin phyllite beds in the arkose units. Arkose adja- cent to metadiabase has been epidotized in a few places. At the margins of the largest metadiabase intrusive are small bodies of porphyroclastic granite to granodiorite; only one was large enough to map. The Linville Metadiabase is generally poorly ex- posed. It forms a reddish-brown clayey soil, and small bodies of metadiabase in the arkose and base- ment rocks in the steep country southeast of the crest of the Blue Ridge were invariably cleared for farms because the soil is so much less stony and sandy than on the adjacent rock. Natural outcrops are abundant in some of the large bodies on Grand- father Mountain, especially the one at the head of Anthony Creek. Metadiabase in the larger bodies typically weathers into rounded boulders. The metadiabase is dark green, green gray, or blue gray. It contains abundant amphibole megacrysts that range from 0.5 to 10 mm and average about 5 mm in diameter and less abundant laths of plagio- clase 2 to 15 mm long, averaging 2 to 3 mm. Segrega- tion knots and lenses of epidote and quartz are much less widespread than in the Montezuma Member. They are found in about 5 percent of the metadi- abase outcrops as compared with being found in more than 50 percent of metabasalt outcrops. Rarely, the rock contains veinlets and fractures filled with quartz or albite. Metadiabase ranges from com- 98 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE pletely altered but unsheared rock with ophitic or diabasic texture to blastomylonitic greenschist (fig. 57). Much of the metadiabase is a blastomylonitic gneiss is which the large crystals are porphyroclasts. Amphibole is generally light-green actinolite. It occurs as porphyroclasts (fig. 57C) and as needles alined on the foliation planes. Relicts of dark-green to reddish-brown hornblende are found in the cores of some of the porphyroclasts. Both the dark core and the lighter colored rims have the same extinction angle. Relicts of pinkish-tan pyroxene are less common. Tiny sphene inclusions are abundant in the hornblende porphyroclasts. Plagioclase has been completely altered to albite. In less sheared rocks it retains its lath shape (fig. 57A and B) but is generally filled with inclusions of epidote and sphene; in more sheared rocks it has been recrystallized to clear grains 0.05 to 0.2 mm in diameter. Green FeMg chlorite is 0.05 to 0.2 mm long and locally is intergrown with biotite or amphibole. Else- where it is interstitial to the other minerals. Epidote generally occurs in aggregates, grain size ranging from 0.01 to 0.8 mm. In some specimens, the epidote is pistacite. Sphene mantles opaque minerals and occurs in stringlike aggregates as much as 0.7 mm long. Opaque minerals are magnetite, maghemite, and ilmenite and are as much as 0.3 mm in diameter. Brown stilpnomelane is found both as alined flakes and in rosettes. Individual grains reach a length of 0.5 mm. Brown to dark-green biotite is intergrown with chlorite and locally replaces the amphibole por- phyroclasts. Granite associated with the metadiabase near Dave Coffee Branch (area G-4, pl. 1) is gray and contains perthite porphyroclasts as much as 1 em in diameter, mosaic-textured quartz, greenish-brown biotite, saussuritized plagioclase, and granophyric intergrowths. Rock in some small bodies is a grano- diorite that contains plagioclase porphyroclasts. Chemical analysis of the Linville Metadiabase shows that its composition is similar to metabasalts of the Montezuma Member of the Grandfather Mountain Formation. It contains less silica and more iron and titanium than mafic intrusive rocks in the Ocoee Series in the Great Smoky Mountains (Hadley and Goldsmith, 1963, table 12). ROCKS OF THE TABLEROCK THRUST SHEET Rocks of the Chilhowee Group of Cambrian and Cambrian (?) age and the Shady Dolomite of early Cambrian age compose the Tablerock thrust sheet, a thin but extensive thrust sheet that lies between au- tochthonous rocks of the Grandfather Mountain win- dow and the overriding Blue Ridge thrust sheet. The Tablerock thrust sheet occupies much of the south- western part of the Grandfather Mountain window (pl. 1), a prominent klippe caps Tablerock Mountain (area D-7 pl. 1) for which the thrust sheet and the fault at its base are named. The rocks in what is now recognized as the Table- rock thrust sheet were described by Maclure (1817, p. 43), who said, after describing rocks of low meta- morphic grade in the James River, Virginia area: - a similar formation about 15 miles long and 2 to 3 miles wide occurs on the north fork of the Catawba River running along Linnville [sic] and John's Mountain near to the Blue Ridge * * * Kerr (1875, p. 135), after mentioning quartzites on Linville Mountain, said : The dip is very irregular and confused but seems to be pre- dominantly westward. Several beds of compact light-colored and gray limestone crop out along the western base of the mountain in the valley of the North Fork, and almost to the head of it, the upper beds being on the west side. CHILHOWEE GROUP Quartzite, feldspathic quartzite, and phyllite, which make up most of the Tablerock thrust sheet, are part of the Chilhowee Group of Cambrian and Cambrian(?) age. The correlation is based upon lith- olige similarities and similarity of stratigraphic se- quence with rocks of the Chilhowee Group of north- eastern Tennessee (King and Ferguson, 1960). The rocks of the Chilhowee Group in the Grandfather Mountain window are unfossiliferous, except for a few occurrences of the worm tube, Scolithus. Fossils are very scarce in the Chilhowee Group elsewhere (Keith, 1903; King, 1949; King and Ferguson, 1960). Complexity of folding and discontinuity of expo- sures preclude an accurate estimate of the strati- graphic thickness of the Chilhowee Group in the Ta- blerock thrust sheet, but estimates based on struc- tural sections indicate that at least 4,000 feet of Chil- howee beds is present. Rocks of the Chilhowee Group have been subdi- vided into two quartzite units separated by a persist- ent blue phyllite unit, which constitutes the only mappable marker horizon in the sequence. Because of the isolated tectonic position of the rocks in the Tablerock thrust sheet, correlations with specific for- mations in the Chilhowee Group of the Unaka belt have not been attempted. GRANDFATHER MOUNTAIN WINDOW 99 LOWER QUARTZITE UNIT The lower unit of the Chilhowee Group in the Ta- blerock thrust sheet is a sequence of quartzite and feldspathic quartzite containing interbedded green sericite phyllite. The thickness of the unit in the Lin- ville Falls quadrangle ranges from about 800 feet along the Linville River south of Shortoff Mountain (area D-9, pl. 1) to at least 2,200 feet on the east side of Linville Mountain opposite the Chimneys (area D-6). The cliffs on the west side of the Linville Gorge and on Shortoff Mountain, Dobson Knob, the Chimneys, and Tablerock Mountain are composed of the lower few hundred feet of this unit (fig. 3). The higher part of the sequence generally forms rather rounded slopes without prominent cliffs. The unit consists predominantly of medium- and fine-grained white, gray, or light green quartzite and feldspathic quartzite but contains numerous thin in- terbeds of green sericite phyllite. The quartzite is generally thin bedded, but massive beds as much as 30 feet thick of medium- and coarse-grained quartz- ite are common. Much of the quartzite is crossbedded and has dark-blue heavy-mineral streaks parallel to bedding and crossbedding. Angular clasts of pink feldspar as much as 5 mm in diameter are wide- spread in some beds. Beds of quartz-pebble conglom- erate 6 inches to 5 feet thick occur in a few places near the base of the sequence. A few 20-foot beds of vitreous white or gray quartzite occur near the top of the unit. The quartzite (fig. 58) consists of recrystallized quartz grains 0.05 to 0.2 mm in diameter in a mosaic that encloses detrital grains of strained quartz and microcline and, very rarely, microperthite. The finer grained beds are entirely mosaic textured and lack detrital grains, except for heavy minerals. Microc- line content ranges from 0 to 25 percent, and plagio- clase is rare. Scattered flakes and discontinuous folia of fine-grained iron-rich muscovite with weak green absorption define a cleavage parallel with bedding. Accessory minerals, chiefly tourmaline, zircon, sphene, ilmenite, and magnetite, occur as scattered grains or are concentrated in thin laminae parallel with bedding or crossbedding. Opaque minerals gen- erally predominate in the heavy-mineral seams. The phyllites consist mainly of fine-grained iron-rich muscovite, but they contain recrystallized quartz and small amounts of FeMg chlorite and brown biotite. Most of the rocks of the unit are quartzites or muddy quartzites (fig. 594) and have a high content of SiO, and rather low alkali and Al,O;,; contents {table 18). No analyses of the green phyllite are available, but it probably has a higher ratio than the blue phyllite described below. PHYLLITE UNIT The lower quartzite unit is overlain by a thin unit composed of dark phyllite containing interbedded fine-grained gray or white quartzite. The phyllite unit ranges in thickness from a few feet to as much as 400 feet but is generally less than 150 feet thick. The phyllite is a lustrous, finely laminated, dark- blue blue-gray, or blue rock consisting of folia of fine- grained sericite and thin lenses and laminae of gra- noblastic quartz, parallel to a strong bedding folia- tion (fig. 58D). The foliation is commonly cut by slip cleavage which produces minor crenulations on the foliation surfaces. The rock contains minor amounts of FeMg chlorite, biotite, magnetite, and ilmenite and scattered grains of zircon and green tourmaline. Interbeds 0.5 to 6 inches thick of fine-grained light-gray or blue-gray sugary quartzite are common in the phyllite; local layers or blue or white vitreous quartzite are 2 to 20 feet thick, especially where the phyllite is unusually thick. Thin beds of similar phyllite are interlayered with quartzites in the upper part of the lower quartzite unit and throughout the overlying quartzite unit. The phyllites (table 18, analysis 5 and 6) have high contents of K0, AlsO;, and iron oxides and low contents of Na,0 and CaO ; thus they are proba- bly derived from typical clay shales. UPPER QUARTZITE UNIT The upper unit of the Chilhowee Group is a se- quence of thin- to thick-bedded medium- to fine- grained white, greenish-gray, or bluish-gray quartz- ite and felspathic quartzite, ranging in thickness from 1,300 to perhaps 2,500 feet. The unit underlies most of the dip slope on the west side of Linville Mountain. Massive beds of fine-grained vitreous quartzite are more common than in the lower unit, and phyllite is less common. Phyllites of the upper unit resemble the blue phyllite of the middle unit rather than the green phyllites of the lower quartz- ite unit. Small-scale crossbedding is common in the quartz- ites. Conglomerates are absent. Near the summit of Bald Knob and in a few places on the slopes of Lin- ville Mountain, some quartzite beds contain slightly deformed Scolithus tubes similar to those common in rocks of the Chilhowee Group elsewhere (King, 1949; King and Ferguson, 1960). The contact of the upper quartzite unit with the overlying Shady Dolomite is exposed along the North 100 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE FiGurE 58.-Photomicrographs of rocks of the Chilhowee Group. A, Quartz-pebble conglomerate from lower quartz- ite unit near base of Tablerock thrust sheet east of road intersection at altitude of 2,880 feet on Linville Mountain (area D-8, pl. 1). Quartz pebbles, partly broken; mortar along fractures, recrystallized. Quartz and microcline clasts in matrix of recrystallized quartz and sericite. B, Poorly sorted feldspathic quartzite from lower quartzite unit at bottom of Linville Gorge in area D-7, 1.4 miles S. 30° E. of Linville Falls (area D-6, pl. 1). Clastic grains of quartz and microcline in a groundmass of quartz, iron-rich mus- Fork of the Catawba River about 1 mile north of Linville Caverns (area C-7, pl. 1). Massive white quartzite passes up into 15 to 20 feet of thin-bedded quartzite and green sericite phyllite which is over- lain by a 6-inch to 1-foot bed containing 2- to 5-mm well-rounded granules of quartz in a calcareous ma- trix. This bed is directly overlain by the Shady Do- covite, and subordinate microcline. Analyzed specimen 2, table 18. C, Fine-grained well-sorted quartzite from upper quartzite unit at altitude of 1,760 feet in Stillhouse Branch (area C-8, pl. 1). Mosaic of recrystallized quartz and minor amount of sericite. D, Crenulated phyllite from phyllite unit exposed in roadcut on North Carolina High- way 183 about 0.8 mile northeast of bridge across Linville River (area D-6, pl. 1). Sericite and chlorite and fine- grained mosaic-textured lenses and laminae of quartz, crinkled and cut by slip cleavage. Analyzed specimen 6, table 18. lomite (fig. 60). These upper beds probably corre- spond to the Helenmode Member of the Erwin For- mation (John Rodgers, oral commun, 1959). The quartzites and feldspathic quartzites typically consist of mosaic-textured quartz (fig. 58C) and lo- cally occurring microcline 0.05 to 0.1 mm in diame- ter, scattered flakes and partings of sericite or green GRANDFATHER MOUNTAIN WINDOW MICA AND CHLORITE FELDSPAR QUARTZ FELDSPAR QUARTZ FIGURE 59.-Proportions of quartz, feldspar, and combined mica and chlorite in rocks of the Chilhowee Group in the Tablerock thrust sheet and the lowest arkose of the Gran- father Mountain Formation. A, Lower quartzite unit of the Chilhowee Group. Numbers of analyzed specimens refer to analyses in table 18. Based on point counts of analyzed specimens and counts of 50 random grains in each of 44 other thin sections. Contours 2, 4, 12, and 25 percent. B, Upper quartzite and phyllite units of the Chilhowee Group. iron-rich muscovite mica, and some locally occurring brown biotite. Widespread detrital grains of micro- cline and quartz and seattered detrital grains of per- thite are 0.5 to 2 mm in diameter. A few beds contain feldspar clasts as large as 1 ecm and quartz granules as large as 3 mm in diameter. Feldspar content is 0 to 25 percent; plagioclase is almost totally absent. Magnetite, ilmenite, sphene, zircon, and tourmaline are the chief accessory detrital minerals and are commonly concentrated in streaks parallel to bed- ding and crossbedding. Clastic rutile and meta- 101 MICA AND CHLORITE Field of phyllite FI LDSPAR QUARTZ EXPLANATION o4 Arkose © Average D8 Chilhowee Group rocks Number indicates analyzed specimen in table 18. Based on point counts of analyzed specimens and counts of 50 random grains in each of 40 other thin sections. Contours 2, 5, 14 and 28 percent. C, Lowest arkose unit of the Grandfather Mountain Formation. Number indicates analyzed specimens in table 15. Based on point counts of 24 thin sections and counts of 50 random grains in each of 55 other thin sections. Contours 1, 3, 6, and 13 percent. morphic chlorite and stilpnomelane are present lo- cally. The overall composition of the upper quartzite (fig. 59B) is quite similar to that of the lower quartzite (fig. 594) ; both are somewhat muddy and feldspathic orthoquartzites. The main difference between the upper and lower quartzite units is that the upper unit is somewhat better sorted, lacks pebble beds, and has gray rather than green phyllites. Individual outcrops of the units are generally similar, and if the phyllite between 102 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE TABLE 18.-Chemical analyses, modes, and norms of rocks of the Chilhowee Group in the Tablerock thrust sheet [Samples analyzed by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey, 1962. Modes determined by point counts of 600 grains; P, present but not intersected in counting. Major oxides and CIPW norms given in weight percent; modes, in volume percent. CIPW norms for samples 1, 5, 6, and 7 were not computable] Lower quartzite unit Phyllite unit Upper quartzite unit 1 2 3 4 5 6 T 8 9 Field Mo" .z. ccs eid e caval ee Paga RE-20-1b 2-249 RE-90-1 _ RE-93-38a 1-93 2-197 RE-91-1 2-195 2-196A Laboratory OL.... 160173 160176 160174 160177 160172 160175 160171 160170 160169 Major oxides iOS ner a Peri te a our 86.7 90.1 90.7 94.0 61.9 66.4 87.3 90.3 94.4 e ee r o ay ae aus 5.6 4. 8.71 p 20.3 17.5 5.4 4.1 1.4 scenes dele fiers 1.8 .46 .92 45 2.0 1.6 .64 44 2.5 a ile seo . .81 .39 .69 . 88 8.5 3.9 . 60 . 60 Saf. tC. C.. .16 "al 15 .00 1.0 1.1 . 08 27 .07 Cad eles irae dein ge ie .05 A2 .05 .05 .04 14 .06 .05 .07 Nad.. il ssi .09 18 12 . 09 . 88 .26 , 18 . 08 97 sO sss .nl. ul 2.6 2.5 1.8 1.0 6.0 5.1 8.6 1.6 .T0 OF re AR AEL T8 .70 . 56 . 58 8.2 2.9 66 .89 .58 (O= uss line, A2 .07 . 06 . 06 .28 .18 . 09 11 A1 crete Peo R 1.2 .06 1.0 .67 . 88 .81 .90 .66 .30 F ;O;: eo ee ece nena a n't cows .04 .02 .02 .01 .09 Pe d .05 .01 .01 as .00 .00 .00 .00 .02 +08 .02 .00 .00 COs: rari iit. ee cel <.05 <.05 <.05 <.05 <.05 <.05 <.05 <.05 <.05 ll 99 99 100 100 100 100 99 99 100 Modes Quartz. ...l. .e . 0... s. CL T7 80 82 87 26 29 75 T7 92 10 11 5 (Bl M Pree eens -z s 15 1:1 2.6 11 9 12 11 69 60 5 20 cass 2.2 ¥ 1.0 1.7 4 2.8 4 1.3 4A Chlofite: 5. co cer LE ede el ae awh denes ano obs aaa nen so 2.3 9 tp ae... ne deena no teu kas P P. P.}: _ PM {luc 0 P P P co.: ecs i222. crc. P P P ta.! eet Ca ads cg nas 2 P ~A g ve. o aan s ao a asan oa aran aa a o ace a Tn aula a ae aie eae Biotite cll .. ats i s . P.;} } } Plagioclase .s. ee .c ool o ool Lol ia ar e ae ea ae aa hamara aa ana w's o a aa ania n a noen ban a ane eae wenno e P P B CIPW norms (s. on e N iene Bene aie nle o neden e 79.16 82.84 £9.57 . atta ti aaa ece as 83.23 91.09 Cps ia s se a o aon s ea o ile a 1.41 1.51 1:90: . [cl ine i L s . 2.17 42 Or. see f ee t n L Nests ue ailes 14.77 10.64 5.01 +:. 1.20 leased ia bean c 9.45 4.14 AD 4s see eis s ine ail as Poe e anl en a ae ole a s 1.10 1.01 (TD see oe ine n aaa c aed 2 . 68 .59 (AT. 32 ee ea ced de an e aoe anil a a ileal a ania s . 46 A2 AB: so n o apie ios arpa aa e naaa o.. "18 28 hns 2s serene r rece ec denen c anes . 52 NOT sl one Lp nets om ae n ea o ac a .67 h d Fe. r am sr t LLA .IP d sn c VL o cin ood Conia i- ae aa ne aet a c ald ao me aaa tiie dea a an s MESSE ere aia n dne eae (OT isc cul ch aan su oe aan emale . an we eu s ceu .02 +.00 (im- ss ess aa rn cao ao oce alld a en e dae aid ia ae 02 (AD \o." ae eae ae alk slg 18 2.50 (ser rea eae a Ae cole n nae a dew a 11 1.46 10 2122} 2 us ae ae ean ond ola nied o o wine 1.25 «57 UL. lee t o oe o e t auc . 28 s ) | oie e clan feo Sol spe cpl ion be O int enc ja to nage ain res 11 on Ap- .se nec cain onc uo cule ont _ .05 .05 02 Mee cible. oun shia s .02 .02 1. Poorly sorted light-greenish-gray feldspathic quartzite containing numer- ous heavy-mineral partings. Very elongate clastic grains of quartz as much as 7 mm long and clastic grains of microcline as much as 5 mm long and 2 mm in diameter in a matrix composed of grains of recrys- tallized quartz, iron-rich muscovite, and microcline 0.05 to 0.1 mm in diameter. From roadcut on Blue Ridge Parkway 1.2 miles N. 70° E. of bridge over Linville River (area D-6, pl. 1). Poorly sorted light-greenish-gray feldspathic quartzite. Clastic grains of quartz, microcline, and perthitic microcline as much as 1.5 mm in di- ameter in a groundmass composed of grains of recrystallized quartz, iron-rich muscovite, and subordinate microcline 0.05 to 0.1 mm in di- ameter. From outcrop along Linville River (area D-7, pl. 1) 1.4 miles S. 30° E. of Linville Falls (area D-6). Light-gray finely laminated feldspathic quartzite with heavy mineral seams and crossbedding. Clastic grains of quartz and microcline as much as 3 mm in diameter in a matrix composed of grains of recrys- tallized quartz, iron-rich muscovite, and microcline 0.05 to 0.1 mm in diameter. From Wisemans View (area D-7, pl. 1). Fine-grained sugary white to light-green quartzite with thin heavy-min- eral seams parallel to bedding. Scattered clastic grains of quartz as much as 1 mm in diameter in a mosaic-textured matrix of quartz (av- erage grain size 0.1 mm) and iron-rich muscovite. From the summit of Tablerock (area D-7, pl. 1). Blue-green phyllite with conspicuous lineation formed by intersection of slip cleavage and bedding foliation. Lepidoblastic sericite and FeMg chlorite as much as 0.3 mm long and laminae of mosaic-textured quartz (average grain size 0.05 to 0.1 mm). From roadcut in Kistler Memorial Highway S. 80° W. from south end of 4,000-foot contour on Laurel Knob (area D-7, pl. 1). Blue-black highly crenulated phyllite with quartz stringers (included in analysis). Iron-rich muscovite and FeMg chlorite layers 1 to 3 mm thick and lenses and laminae of mosaic-textured quartz (grain size 0.05 to 0.1 mm) are crenulated and offset by slip cleavage. Roadcut along North Carolina Highway 183, 0.85 mile N. 25° E. of bridge across Lin- ville River (area D-6, pl. 1). 7. Blue-gray very fine grained, finely laminated feldspathic quartzite in- terbedded with dark-gray sericite phyllite. Composed of mosaic-textured quartz and microcline 0.05 to 0.1 mm in grain size and lepidoblastic sericite. Finely disseminated opaque material produces dark color. Roadcut along Kistler Memorial Highway at summit of Dogback Moun- tain (area D-8, pl. 1). 8. Fine-grained white to light-green crossbedded quartzite with conspicuous heavy-mineral laminae at 1- to 2-cm intervals. Clastic grains of quartz as much as 3 mm in length and of microcline as much as 1 mm in di- ameter in a matrix of recrystallized quartz and iron-rich muscovite 0.02 to 0.1 mm in grain size. Cut along North Carolina highway 183, 0.55 mile north of bridge across Linville River (area D-6, pl. 1). 9. Fine-grained white finely laminated quartzite with heavy-mineral part- ings. A few clastic grains of quartz as much as 1.5 mm in diameter and of microcline as much as 0.5 mm in diameter in a matrix com- posed of mosaic-textured quartz 0.05 to 0.1 mm in grain size and sub- ordinate iron-rich muscovite and microcline. From cut along North Carolina Highway 183, 0.65 mile north of bridge across Linville River (area D-6, pl. 1). GRANDFATHER MOUNTAIN WINDOW FIGURE 60.-Contact between Shady Dolomite and quartzite of the Chilhowee Group in the Tablerock thrust sheet 1 mile north of Linville Caverns on the North Fork of the Catawba River (area C-7, pl. 1). River flows about at contact of massive quartzite and thin-bedded quartzite of the Helen- mode Member (mapped with upper quartzite unit.) Lower contact of Shady Dolomite by man's right foot. them were absent, they could not be mapped sepa- rately. A few blue-gray beds in each unit contain as much as 10 percent recrystallized opaque minerals and may be equivalent to the beds of ferruginous quartzites described by King and Ferguson (1960). CORRELATION AND REGIONAL RELATIONSHIPS The Chilhowee Group is a sequence of clastic rocks, chiefly quartzite, arkosic quartzite, shale, and argillite, which constitute the earliest Paleozoic de- posits in the Appalachian miogeosyneline. These rocks and their equivalents crop out in a nearly con- tinuous belt along the northwest flank of the Blue Ridge from central Alabama to Pennsylvania. Lo- cally, the Chilhowee Group rests nonconformably on plutonic basement rocks of early Precambrian age, 103 but in many places the basal beds of the Chilhowee lie on sedimentary and volcanic rocks of late Pre- cambrian age with little or no evidence of an impor- tant stratigraphic break. The group is conformably overlain by carbonate rocks of well-established Early Cambrian age. In a few places, the uppermost beds of the Chilhowee contain diagnostic Early Cambrian fossils (Butts, 1940b; Laurence and Palmer, 1963) ; the remainder of the sequence is unfossiliferous, except for the worm tube, Scolithus, and is classed as Lower Cambrian (?) because of its stratigraphic po- sition between upper Precambrian and Lower Cambrian rocks. In the Unaka belt in northeastern Tennessee, northwest of the Grandfather Mountain window, the Chilhowee Group is divided into three formations: the Unicoi (at the base), the Hampton, and the Erwin (at the top). As originally used (Keith, 1903, 1907a, b), these names were applied to units thought to have rather uniform lithology, but later more de- tailed work has shown that no such uniformity ex- ists. The formations were therefore redefined by King and others (1944, p. 28), and their boundaries were placed at widely traceable marker beds without regard to the lithologic character of the intervening strata. King and Ferguson (1960, p. 33) noted that: The units as thus defined are differentiated by tracing indi- vidual beds or groups of beds, and by comparing and correlat- ing measured sections from one area to another. In northeastern Tennessee, the Chilhowee Group occurs in a number of northwestward-traveled thrust sheets. Conspicuous changes in thickness and lithology of formations take place within distances of a few miles along strike in some of the thrust sheets, but by far the most abrupt changes occur between adjoining sheets because of telescoping of sequences that were originally deposited many miles apart. In general, thinner and sandier Chilhowes sec- tions are found in the Doe River inner window of the Mountain City window (pl. 3), whereas thicker and more shaly sections occur in the Shady Valley and Buffalo Mountain thrust sheets which originally lay farther southeast. Basalt flows are widespread in the middle part of the Unicoi Formation in the higher and presumably farther traveled thrust sheets and are found in a few places in the Doe River inner window. The lower part of the Unicoi Formation at- tains its maximum measured thickness in northeast Tennessee along the southeast side of the Shady Val- ley thrust sheet (King and Ferguson, 1960, p. 35), where it may include rocks of late Precambrian age, 104 perhaps correlative with the upper part of the Grandfather Mountain Formation. The comformable contact between the Shady Do- lomite and the Chilhowee Group in the Tablerock thrust sheet shows that at least part of the Erwin Formation is present in the thrust sheet, but whether or not the lower strata are correlative with older formations in northeast Tennessee cannot defi- nitely be established. Lack of volcanic rocks and of appreciable thicknesses of shale and the presence of clean quartzites throughout the section suggest that all Chilhowee rocks in the Tablerock thrust sheet may belong to the Erwin Formation. King and Fer- guson (1960) reported no conglomerate or feld- spathic quartzite in the Erwin in northeasternmost Tennessee, but scattered quartz pebbles occur in the formation in southwestern Virginia (Stose and Stose, 1957) and on Embreeville Mountain in the Buffalo Mountain thrust sheet (Rodgers, 1948). D. W. Rankin (written commun., 1965) found quartz pebble conglomerate and feldspathic quartzite in the Erwin near the Virginia-Tennessee State line, and Oriel (1950) reported that some specimens of quartzite from the Hot Springs window contain as much as 20 percent microcline. Thus, the presence of arkosic quartzite and quartz pebble conglomerate in the Chilhowee section in the Tablerock thrust sheet does not preclude correlation with the Erwin. The lack of any appreciable compositional differences be- tween the upper and lower quartzite units is also in accord with their assignment to a single formation. The Chilhowee section in the Tablerock thrust sheet most nearly resembles the thick sections of Erwin described by King and Ferguson (1960, pl. 9) in the Doe River inner window of the Mountain City window. The minimum thickness of Chilhowee beds in the thrust sheet, however, is nearly twice as great as the thickest section of Erwin they describe. Our synthesis of the structure (see p. 178-179 and pl. 4) suggests that the Tablerock thrust sheet probably originated southeast of the Buffalo Mountain and Shady Valley thrust sheets, both of which must have come from southeast of the Mountain City window. This structural interpretation, however, is not in agreement with the orderly southeastward change to thicker and more shaly sections in the Chilhowee Group inferred by King and Ferguson (1960, p. 81-82), if all the Chilhowee Group in the Tablerocl- thrust sheet is assigned to the Erwin Formation. SHADY DOLOMITE Shady Dolomite, which overlies sandstones of the Chilhowee Group, is exposed beneath the Linville GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE Falls fault in several small areas along the west edge of the Grandfather Mountain window. It may also be present locally beneath the alluvial deposits along the North Fork of the Catawba River. The formation consists of fine-grained white, light-gray, or buff-gray crystalline dolomite, gener- ally massive or vaguely mottled, but locally thin bed- ded or ribboned. Partings and thin beds of light- gray and greenish-gray phyllite are widespread. In some areas the dolomite is silicified to a fine-grained white, sugary or porcelaneous rock resembling quartzite. Near Linville Caverns, partially silicified dolomite contains small veinlets and irregular re- placements of honey-yellow to black sphalerite, accompanied by some chalcopyrite and pyrite. The rock is composed of a granular mosaic of equi- dimensional and somewhat sutured grains of carbon- ate generally 0.02 ot 0.2 mm in diameter, although locally as much as 0.5 mm in diameter. Quartz grains participate in the mosaic texture and constitute a trace to 15 percent in the rocks examined. Bedding is generally not visible in small natural outcrops, but it is conspicuous in large exposures, especially in quarries. The small outcrop area of the formation and the complexity of the structure make it impossible to estimate accurately the stratigraphic thickness of the formation exposed, but it seems that at least 800 feet of dolomite must be present in the hills near the Woodlawn quarrry (area B-9, pl. 1). There, according to Watson and Laney (1906, p. 208), * * * several holes were put down [by the State Geological Survey] in such a manner as to include a thickness of nearly a thousand feet of the stone. Partial analyses of Shady Dolomite from the Ta- blerock thrust sheet (Hunter and Gildersleeve, 1946, p. 27-28) show a MgO content ranging from 10 to 21 percent; Ca) content, 27 to 31 percent ; ignition loss, 41 to 46 percent; Fe, 0.25 to 0.6 percent; and acid insoluble residue, 0.7 to 8.6 percent. Light-colored dolomite has less Fe and MgO than dark-colored do- lomite. Complete analyses show the following ranges: SiO», 0.60 to 5.96 percent; Al;0O;;, 0.60 to 1.76 percent; FeO, 0.49 to 0.73 percent; CaO, 29.13 to 30.98 percent; MgO, 19.56 to 21.22 percent; K0, 0.26 to 0.41 percent; P;0;, 0.01 to 0.02 percent; CO;, 40.07 to 47.10 percent (Loughlin and others, 1921). No description of the analyzed rocks is given. No fossils have been discovered in the dolomite in the Grandfather Mountain window. The Shady is also unfossiliferous in northeastern Tennessee (King GRANDFA'fHER MOUNTAIN WINDOW and Ferguson, 1960), but a few Early Cambrian fos- sils have been collected from the formation in south- western Virginia (Resser, 1938, p. 24-25; Butts, 1940b, p. 54-56). The correlation of the dolomite in the Tablerock thrust sheet with the Shady Dolomite is based on its close lithologic similarities to the Shady in Tennessee (Keith, 1905; John Rodgers, oral commun., 1959; R. A. Laurence, oral commun., 1959) and its stratigraphic position overlying Scoli- thus-bearing quartzite similar to that of the Erwin Formation, with transition beds resembling the Hel- enmode Member of the Erwin Formation. ALLOCHTHONOUS ROCKS OF UNCERTAIN CORRELATION The small body of quartzite and greenschist along the Linville Falls fault west of Hodges Gap (area G-2, pl. 1) is probably a fault slice emplaced early in the structural history of the area, but its structural relations are not entirely clear. Rocks in the slice have east-dipping cleavage parallel to that in adjacent rocks in the window, but the contact between the quartzite and the adjacent Grandfather Mountain Formation is not parallel to contacts within the Grandfather Mountain Formation. The quartzite makes up the north side of a steep knob and is well exposed in a roadcut along North Carolina Highway 105 and in a nearby road-metal quarry. The gross pattern of the hill suggests that the quartzite strikes east and dips north, in contrast to the northeast strike of the adjacent Grandfather Mountain Formation. The rock is greenish gray and fairly coarse grained. It contains a few purple-hued beds and lenses, but in most places bedding is ob- secure. Quartz veinlets are abundant. East of the steep knob the rock has more feldspar and is not quite as resistant. The quartzite contains strongly strained to brec- ciated clasts of quartz as much as 5 mm in diameter in a matrix of recrystallized quartz and iron-rich muscovite 0.02 to 0.08 mm in diameter. Zircon, sphene, opaque minerals, and allanite are accessory minerals. A few fragments of volcanic rock are pres- ent. The more feldspathic rocks contain clastic mi- crocline. The greenschist mapped in the quartzite east of the steep knob seems to be exposed in a gentle anticline in the quartzite; quartzite underlying the green- schist is exposed in a roadcut on North Carolina Highway 105. The quartz-rich composition and lack of plagio- clase in the quartzite suggests correlation with rocks 105 of the Chilhowee Group. The scarcity of bedding and lack of clastic tourmaline, however, argue against that correlation. The presence of the greenschist sug- gests that rocks in the slice may be correlative with either the Unicoi Formation of the Chilhowee Group or the Grandfather Mountain Formation. STRUCTURE FAULTS TABLEROCK FAULT The Tablerock fault, named for its exposures in 'the klippe on Tablerock Mountain (area D-8, pl. 1}, separates Lower Cambrian(?) and Lower Cambrian rocks of the Tablerock thrust sheet from authoch- thonous Precambrian rocks of the Grandfather Mountain window. The fault is parallel with bedding and cleavage in the overriding block, and in most places it truncates cleavage and bedding trends in the underblock. Where Wilson Creek Gneiss is adja- cent to the fault, the fault is marked by a zone of lustrous phyllonite and cataclastic gneiss ranging from a few inches to more than 50 feet thick. Cleav- age in the phyllonite adjacent to the fault is parallel with the fault plane and concordant with the struc- tures in the overriding block; farther beneath the fault the cleavage curves to become parallel with the regional cataclastic foliation in the underblock. These relationships may be seen on the west side of the Linville River valley opposite Tablerock Moun- tain. In a few places, thin slices of quartzite derived from the overblock and of arkose derived from the 'underblock are intercalated with phyllonite and sheared gneiss. Between the Linville River and Long- arm Mountain (area D-6, pl.. 1), arkose of the Grandfather Mountain Formation forms the under- block, and the fault is marked by finely laminated siltstone and calcareous phyllite, probably a slice de- Tived from the siltstone member of the Grandfather Mountain Formation. Near Crossnore (area D-5, pl. 1) a slice of basement rock is found along the fault. There the fault is folded into a syncline and tran- cated by the Linville Falls fault. In the southern part of the Grandfather Mountain 'window, the Tablerock fault is warped into a gentle anticline, the Bald Knob anticline. The southeastern limb dips 25° to 35° near the Linville Falls fault and the Brevard fault zone. The Tablerock fault is cut by subsidiary faults of the Linville Falls fault east of Shortoff Mountain (area E-8, pl. 1) and near Bald Knob (area C-9, pl. 1). In the southwestern part of the window, the Tablerock thrust sheet is completely 106 overridden by higher slices of Wilson Creek Gneiss and Chilhowee quartzite. The Tablerock fault carried younger rocks over older. The presence of intercalated slices from the overriding and overridden blocks in the phyllonites along the fault and the different structural patterns in the two blocks indicate a fault of considerable magnitude rather than an unconformity along which there has been minor movement. The difference in structural patterns indicates that the Tablerock thrust sheet has traveled at least 12 miles northwest- ward over the autochthonous rocks of the window. The Tablerock thrust sheet may be considered a siza- ble subsidiary beneath the larger and thicker Blue Ridge sheet. It will be shown below that the minor structures in these two thrust sheets are similar. OTHER FAULTS Subsidiaries of the Linville Falls fault are concen- trated along the southeastern edge of the window. Small lenses of felsic volcanic rocks in the Grand- father Mountain Formation occur along these sub- sidiary faults in the northwestern part of the Lenoir quadrangle. These faults are parallel with cataclastic foliation in the basement rocks and cleavage in the Grand- father Mountain Formation. Most of them are poorly exposed and are drawn largely on the basis of map relationships, but they are locally marked by zones of phyllonite. In many other areas, the basement rocks are highly sheared, but many of the zones of sheared rocks are not continuous, and they are, therefore, not mapped as faults, although many unrecognized faults may be present. Apparently, these faults formed prior to emplace- ment of the Tablerock thrust sheet, for they are par- allel to cleavage and cataclastic foliation that are truncated by the Tablerock fault. In the southwestern extension of the window (pl. 1), where thrust sheets of Chilhowee Group rocks and of Wilson Creek Gneiss have overridden the Ta- blerock thrust sheet, an exceedingly complex map pattern has been produced. These thrusts are probably of the same general age as the Linville Falls and Tablerock faults and are closely related to them. The allochthonous rocks west of Hodges Gap (area G-2, pl. 1) apparently occur in a slice along an early low-angle thrust fault, older than the Linville Falls but, possibly related to it. GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE CLEAVAGE Cleavage and cataclastic foliation are conspicuous in the autochthonous rocks in the Grandfather Mcuntain window. They are formed by planar orien- tation of mica and chlorite flakes and folia and by quartz and feldspar aggregates. Cleavage in the bed- ded rocks is generally parallel with the axial planes of minor folds and with cataclastic foliation in plu- tonic basement rocks. Both structures generally strike north or northeast and dip east or southeast (fig. 32). They are truncated by the Linville Falls fault along the north and west sides of the window and are discordant with the cataclastic foliation in the tectonically overlying Blue Ridge thrust sheet; along the southeast side of the window, discordance is less obvious. Cleavage in the Grandfather Moun- tain Formation and foliation of phyllonite is locally cut by later slip cleavage (not shown in fig. 32). Locally, cataclastic foliation on the basement rocks has been warped. The most noticeable fold is in the central part of the Blowing Rock quadrangle (areas I-3 and I-4, fig. 32) where the blastomylonite seems to be folded in a sharp but rather local crinkle. The rocks of the Tablerock thrust sheet do not have the pervasive cleavage of the autochthonous rocks. Metamorphic foliation is generally parallel to bedding. Fracture cleavage in quartzite and slip cleavage in phyllite is locally developed in the noses of minor folds. Along the southeastern edge of the Tablerock thrust sheet, adjacent to the Linville Falls fault, cleavage marked by alinement of micaceous folia is locally developed. LINEATION Mineral lineation consisting of alined micaceous streaks, elongated clastic grains, porphyrociasts, mineral aggregates, pebbles, and amygdules is found on most cleavage and foliation planes in the autoch- thonous rocks but is absent on the late slip cleav- age which locally cuts older cleavage. The lineation trends rather uniformly northwest and plunges southeast in the autochthonous rock (fig. 33). In the Tablerock thrust sheet, mineral lineation lies on bedding planes or on foliation planes parallel to bedding. It trends northeast or southwest, parallel to lineation in the autochthonous rocks. Northwest of the Bald Mountain axis, it plunges gently north- west, and southeast of the axis, it plunges gently to moderately southeast or south. Offset of individual beds in the sedimentary rocks and of pegmatite dikes in the plutonic basement GRANDFATHER MOUNTAIN WINDOW rocks shows that movement in the cleavage planes has been parallel with the mineral lineation and dem- onstrates that this lineation is in the a direction. FOLDS BASEMENT ROCKS Small tight to isoclinal folds are well developed in the layered part of the Wilson Creek Gneiss in the Blowing Rock quadrangle. Good examples can be seen in roadcuts along U.S. Highway 321 in area I-5, plate 1, and along the Buffalo Creek road in area J-4, plate 1. The axial planes and axes of about 70 percent of the isoclinal folds and about 40 percent of the nonisoclinal folds parallel the foliation and min- eral lineation (fig. 614). The axial planes at an angle to the foliation nevertheless strike and dip in the same quadrant ; fold axes at an angle with the linea- tion trend more north and east than the lineation. Fold axes (solid contours) n=34 Poles of axial planes (dashed contours) %s 35 Contours 3, 5, 10, 15, and 20 percent A 107 All gradations between isoclinal and nonisoclinal folds occur, and there is no apparent difference in age between the two types. In most outcrops of layered Wilson Creek Gneiss, layering and foliation are parallel. The gross struc- ture of the layered rocks probably consists of tight or isoclinal folds sheared out during the formation of the cataclastic foliation in the window rocks. The blastomylonite and phyllonite in the plutonic basement rocks locally have small-scale crinkles with gently plunging axes trending N. 70° E. and axial planes of various dips trending. about N. 60° E. (fig. 61B). Locally, slip cleavage parallel to these axial planes cuts the main cataclastic foliation. Axes of crinkles and intersections of slip cleavage with foliation (fig. 61B) generally trend about N. 70° E., roughly perpendicular to the mineral linea- tion; they plunge gently northeast. The small maxi- Axes of crinkles or intersection of axial planes of crinkles and foliation h=57 Axial planes of crinkles n=19 Contours 2, 5, and 10 percent B E X P L A N A TI O N Ld Average mineral lineation Pole of mean foliation and layering x Axial planes of crinkles FIGURE 61.-Orientation of minor folds in basement rocks of the Grandfather Mountain window. All diagrams are equal-area projections in the lower hemisphere with planes of projection horizontal and north at top. Contours show percentage of points falling within 1 percent of the area of the diagram. A, Tight and isoclinal folds in layered Wilson Creek Gneiss in the Blowing Rock quadrangle. B, Axes of crinkles or intersection of axial planes of crinkles and foliation. 108 mum of southwest-plunging crinkle axes striking N. 50° E. is from measurements in the Linville Falls quadrangle where the mineral lineation averages about N. 40° W. The crinkles may be structures in the b direction formed under the same stress condi- tions but somewhat later than the mineral lineation. GRANDFATHER MOUNTAIN FORMATION MAJOR FOLDS The main outcrop belt of the Grandfather Moun- tain Formation lies on the southeast limb of a com- plex syncelinorium overturned to the northwest. This structure is cut by both the Linville Falls and Ta- blerock faults. In the east and central parts of the main outcrop area of the formation, beds are gener- ally overturned ; in the northwest part, right-side-up beds are more abundant. The outcrop pattern of rock units is largely controlled by the major synclinorium and its attendant medium- and small-scale folds. No major repetition of rock units is evident, although some minor repetition occurs in the western part of the Linville quadrangle. The northwest limb of the synclinorium is presumably concealed beneath the Blue Ridge thrust sheet northwest of the Grand- father Mountain window. The nearly horizontal northeast trending axis of the major synclinorium is warped around a north- west-trending axis that passes through the north- west corner of the window (area D-3, pl. 1). Other large- and medium-scale folds are locally superim- posed on the earlier syncelinorium. The open syncline plunging gently south under the north end of the Tablerock thrust sheet (area D-5, pl. 1) seems to be a later structure superimposed on the major synclinorium; the two south-plunging folds in area D-4 are moderately sharp and seem to have the same pattern as early minor folds in that area. Other medium-scale folds may influence the outcrop pattern of the arkose unit above the Monte- zuma Member in the northwest part of the window, but scarcity of bedding and 'of criteria for tops of beds makes it uncertain whether this outcrop pattern is primarily due to intertonguing of rock units or to folding. At least one body of greenstone seems to be involved in northeast-plunging folds. The long tongue of arkose projecting southward into base- ment rocks in areas G-4 and G-5 lies in a tight north-plunging syncline apparently superimposed on the earlier northeast-trending folds. GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE MINOR FOLDS Minor folds are ubiquitous and cause an unknown but probably appreciable amount of small-scale rep- etition of rock units. The oldest and most wide- spread folds are asymmetric or overturned to the north or northwest. They are tight or isoclinal in the southeastern part of the main outcrop area of the formation (fig. 624 and B) but are more open in the northwest part of the area (fig. 62C). Siltstone and phyllite, especially where confined between arkose beds, are generally more tightly and complexly folded than the more competent arkose. Such dishar- monic folding precludes accurate estimates of strati- graphic thickness of the incompetent rocks. In the northeastern part of the main outcrop area of the Grandfather Mountain Formation, small-scale open folds are superimposed on the older structures. The open folds deform the cleavage that parallels the axial planes of the earlier folds and have steep to vertical axial planes that strike northeast. Slip cleav- age parallel with the axial planes of the open folds off sets the older pervasive cleavage (fig. 63). AGE RELATIONS BETWEEN FOLDS AND OTHER STRUCTURES Attitudes of bedding in various parts of the main outcrop belt of the Grandfather Mountain Formation are shown by statistical diagrams on plate 7. The orientations of bedding, cleavage, lineation, minor folds, and other structures in the northern and southern parts of the main outcrop belt are summa- rized in figure 2, plate 5. The bedding diagrams for the individual sectors (pl. 7) clearly reflect the dif- ference in style of the early small-scale folds between the southeastern and northwestern parts of the area. In the southeastern part of the area, where the folds are tight or isoclinal, bedding poles cluster around diffuse maxima or rudely developed partial girdles; in the northwestern part of the outcrop area, folds are more open, and the bedding girdles are better defined and more complete. The axes of most of the early minor folds (pl. 5, fig. 2, diagrams A-4 and B-4) are coincident with axes of the bedding girdles. The fold and girdle axes, however, are not everywhere consistent with the gross outcrop pattern of the major lithologic units. In section IV (pl. 7) for example, the girdle axis and fold axes plunge 45° NE., whereas the contacts trend uniformly northeast. In this sector, at least, the minor folds that are reflected in the bedding girdles are apparently superimposed on earlier large-scale folds that control the outcrop pattern. Only locally are the small folds reflected by minor irregularities in the map pattern. Similar relations are found in GRANDFATHER MOUNTAIN WINDOW Ficure 62.-Typical early folds in the Grandfather Moun- tain Formation. For location of sectors see plate 7. A, Isoclinal fold in arkose in sector VII. B, Overturned asymmetric fold in arkose and phyllite on boundary between sectors IV and VII. Fold axis trends N. 45° E. and plunges 5° NE. C, Asymmetric folds in siltstone and arkose in the northwest part of sector XIII. Fold axes trend N. 5° E. and plunge 5° N. 109 NW SE FIGURE 63.-Late slip fold in siltstone of the Grandfather Mountain Formation in sector IV (pl. 7). Slip cleavage parallel to axial plane of this fold cuts earlier cleavage parallel to bedding in the limbs of earlier isoclinal folds. Fold axis trends N. 45° E. and plunges 60° NE. sectors I, III, and V, but elsewhere the discordance between gross map pattern and girdle and fold axes is not evident, and it is therefore uncertain whether the early minor folds are contemporaneous with or superimposed on the large early folds. The conspicuous regional cleavage, S; (pl. 5, fig. 2, diagrams A-2 and B-2), is parallel to the axial planes of the minor folds (pl. 5, diagrams A-4 and B-4) and was presumably developed at the same time. Where the trend of the minor folds diverges from that of the large early folds, the cleavage is clearly superimposed on the older structures. No- where have the two cleavages been found in the same outcrop, but the abrupt change (fig. 32) from north- east-striking cleavage parallel to the trend of lithol- ogic units in sectors V, VI, and VII to cleavage striking north or northwest at high angles to the contacts in sectors II, III, and IV strongly suggests that the cleavages and minor folds are of two dis- tinct generations. Where the minor folds parallel the larger folds, it is not possible to distinguish the minor folds and the cleavage that date from the earliest episode of folding from the later structures. The pervasive mineral lineation (L,) lies on the S, cleavage planes and maintains virtually the same orientation throughout the Grandfather Mountain Formation (fig. 33). In the northern part of the out- crop area the lineation is approximately normal to the maximum concentration of axes of the early minor folds (pl. 5, fig. 2, diagram A-3), but in the 110 southern part of the area it is oblique to the axes of the minor folds. These relations suggest that it was formed concurrently with the second generation of cleavage and minor folds in the northern part of the area, whereas in the southern part it was formed by renewed movement along cleavage planes of the first generation. The striking parallelism in trend be- tween the mineral lineation in the Grandfather Mountain Formation and the cataclastic mineral lin- eation in nearby parts of the Blue Ridge thrust sheet indicates that both structures were formed during northwestward transport of the thrust sheet. The small open folds (pl. 5, fig. 2, diagram A-5) are superimposed on folds of the second generation in the northern part of the area. Nowhere do they affect the outcrop pattern of mappable lithologic units, nor are they reflected to any recognizable de- gree in the bedding girdles. The late slip cleavage parallel to their axial planes (pl. 5, fig. 2, diagram A-6) is also parallel to a strongly developed regional joint system (pl. 5, fig. 2, diagram A-7) which is also recognizable in the southern part of the area (pl. 5, fig. 2, diagram B-5) where the open folds and slip cleavage are absent. The occurrence of similar open folds and joints in adjacent parts of the Blue Ridge thrust sheet (pl. 5, fig. 1, diagram C-1) sug- gests that these structures formed subsequent to or in the closing phases of thrusting. The following sequence of structural events is in- ferred from the relations between folds and other structures in the Grandfather Mountain Formation : 1. Initial folding-formation of medium- and large-scale folds that control the outcrop pat- tern of lithologic units and of associated minor folds and cleavage of the first generation. 2. Second folding-formation of minor folds and cleavage of the second generation ; obliteration of earlier minor folds and cleavage in the northern part of the main outcrop area. Con- current with or closely followed by 3. 3. Reactivation of all preexisting cleavages and for- mation of mineral lineation during northwest- ward movement of the Blue Ridge thrust sheet. 4. Formation of small open folds, crenulations, slip cleavage, and predominant regional joint sys- tem. TABLEROCK THRUST SHEET MAJOR FOLDS Most irregularities in outcrop pattern of the stra- tigraphic units in the Tablerock thrust sheet (pl. 1) GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE are due to intersection of gently dipping units with the rugged topography. Except for the Bald Knob anticline, the shallow syncline in the northern end of thrust sheet, and the folds in the southwestern part of the thrust sheet (pl. 1), no folds large enough to affect the gross outcrop pattern of the lithologic units have been detected. Locally, however, folds are mappable at a 1:48,000 scale. The overturned rocks in the southwestern extension of the window are in- terpreted to be a part of an overturned limb of an isoclinal fold in the rocks of the Tablerock thrust sheet that has subsequently been cut by thrust faults and overridden by basement rock (fig. 64). In the fence diagram (pl. 2), the overturn has been ex- tended well to the north for purposes of illustration ; on the surface, the northernmost extensive area of overturned rocks is on the west flank of Dobson Knob (area C-9, pl. 1). MINOR FOLDS Smaller scale folds, visible in single outcrops or inferred from closely spaced groups of outcrops, are common. The minor folds are of two types and are apparently of at least two generations. Minor folds of the first type (fig. 65) are tightly appressed or isoclinal and have horizontal or gently plunging axes with erratic trends. Many of these folds have flat or gently dipping axial planes, but a few axial planes dip as much as 50°. Most of the folds are overturned to the west, northwest, or north; a few are overturned to the northeast. Up- right limbs are many times longer than steeply dip- ping or overturned limbs. Beds are thickened in fold noses and thinned on the limbs; locally incompetent phyllite layers are pinched out. Thin quartzite layers in phyllite are bou- dinaged and locally remain only as discontinuous len- ses in fold noses. Minor folds in phyllitic layers are commonly disharmonic with respect to more compe- tent quartzite layers. Locally, slip cleavage in phyl- lite and rude fracture cleavage in quartzite are de- veloped parallel to the axial planes of these folds. Minor folds of the second type are open symmetri- cal or slightly asymmetrical folds in quartzite and associated crenulations in phyllite (fig. 66). Most of these folds have nearly horizontal northeast-trending axes, and where the folds are asymmetric, axial planes dip southeast. The wavelength of the open folds ranges from a few inches to several tens of feet but is generally less than 5 feet. Crenulations in phyllite range from about 5 mm to less than 1 mm in amplitude and are GRANDFATHER MOUNTAIN WINDOW 111 NW ridge thrust Sheet gue L\§V\LLE F FAULT Do‘omfl’e (s C ~~ SE 2 h S\AeAQ 7\/§\‘“’ 3 : $ EAT Ia -e s 7 TVs (ye $l‘t/O\I'\LIC\\‘>'J\~ ~ l\\’/\TOC-k§\/\_ ‘\/\,\\\’\/|,|7\/:l\/ 4 ‘ Ne \ utonie v {1/1 2 NRZ Nass £17 2 m de t pe c fr ono ao o enn olo hu anis animae ihc e Ans ofc term bac Ak in de le A% ail melee mens MEPC m 8 Foi Ar DANTAINepMi a) ,\(,\|>z\/\/\f,\f,,|/J,r T (Ya HD % Nik ~' & \1~ _ FALLS __ FauLt nico T s tar ~T ‘(—\,\,:/\ efoek SooirAlL ann A> UIS ANE EAA SL NE Be qincvo e LANEY ICE A el I\>\\/I/1/\—-l—~/\\‘/,-/l‘_\—,:,\\’\\‘—/\/\/ A Roisin s* ANTH fer ke $v 4 LINVILLE T° L—-—§ u‘171f\:LlT§‘3C(//\‘>,n ~__ TA N~/ 7 « RA RVH: isapi teir R rar ant AP 13:1”.45930S’\<‘/_‘/\‘l(.\lll\7)i‘/\’/,p\\\\/}I\'/lTA5H5R\QC5|\7gt't/rff‘yxlj I% mts tm i= o egt io -G AB ~ a aay I Als ALO 12 -_ oo Saa y o totes s Cn a rere be e er oT t 1 t Sa SAT Cn Ae do m anes eyy a Lig aa "‘\"—'\/\’;,\_\|—\q£§,/\>VHS/H"' A ~ —\\/'\'*//\\.\l<|>/\,/\‘,<7‘2//\ CRA I MEV /\ - */ _- - \/ 7 1- 1C GAL 2p V SLN shy p ROR TARA 7 ~ M ~ CRS FIGURE 64.-Possible mode of development of antiform in Shady Dolomite near Woodlawn (area B-9, pl. 1). 1, Table- rock thrust sheet moves as tectonic slice during concurrent movement along Linville Falls and Tablerock faults. 2, Development of isoclinal fold in Tablerock thrust sheet as movement along Tablerock fault ceases. 3, Further overturning of fold during continued movement of Blue Ridge thrust sheet along Linville Falls fault. 4, Table- rock thrust sheet overridden by numerous other fault slices during latest stages of movement along Linville Falls fault. 112 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE 10 feet 25 feet C FigurE 65.-Tightly appressed and isoclinal folds in rocks of the Chilhowee Group in the Tablerock thrust sheet. For location of sectors, see plate 7. A, Re- cumbent folds in quartzite and phyllite in northeast- ern part of sector E. Fold axes trend N. 5° W. and plunge 5° N. B, Overturned asymmetric folds in quartzite and phyllite in southeastern part of sector F. Fold axes are horizontal and trend N. 15° W. Axes of crenulations on the limbs of these folds trend N. 40° E. and plunge 10° NE. C, Nose of recumbent anticline in quartzite and phyllite in the northeastern part of sector E. Fold axis is horizontal and trends N. 25° E. parellel to axes of larger folds of this type in adja- cent quartzite beds. Individual layers maintain rather uniform thick- nesses in the open folds, and boudinage effects are less conspicuous than in the earlier folds. Axial-plane cleavage commonly is found in noses of asymmetric folds but is generally absent in the more open sym- metrical folds. Locally, open folds and small crenulations of the second type appear on the limbs of isoclinal folds of the first type and have axes transverse to axes of the earlier folds. Where folds of the second set are asym- metric or overturned, they become indistinguishable from folds of the first type, and some observed folds may have been assigned to the wrong set. Southeast of the axis of the Bald Knob anticline (pl. 7), open folds and crenulations of the second set are less common. Cleavage dips moderately south- east, and field relations suggest that much of the cleavage in this area is related to shearing along the Linville Falls fault rather than to minor folds. Most of the observed minor crenulation axes are parallel to the intersection of cleavage and bedding. The most strongly developed joints in the rocks of the Tablerock thrust sheet are nearly vertical and strike northeast. These joints are not mineralized and cut all other structures. Joints in the more mas- sive quartzite beds are spaced tens of feet apart, but in phyllite and sericite quartzite beds they are more closely spaced and locally resemble fracture cleavage. No lineations have been observed on the joint sur- faces. STRUCTURAL GEOMETRY The geometry of the various structures in the Tablerock thrust sheet in the Linville Falls and Lin- ville quadrangles is summarized in the contour dia- grams (pl. 5, fig. 3). Structures in the part of the thrust sheet southwest of the Linville Falls quad- rangle were not studied in detail. Separate sets of GRANDFATHER MOUNTAIN WINDOW 5 feet rc _ blest _ ______ B FIGURE 66.-Open folds in rocks of the Chilhowee Group in the Tablerock thrust sheet. For location of sectors, see plate 7. A, Slightly asymmetric open fold in quartzite in southwestern part of sector B. Note development of cleav- age parallel to axial plane. Fold axis trends N. 50° E. and plunges 10° SW. B, Open folds in quartzite near Linville Falls in southwestern part of sector A. Fold axes are horizontal and trend N. 45° E. diagrams have been prepared for the parts of the thrust sheet northwest and southeast of the axis 113 of the Bald Knob anticline. Diagrams of bedding poles for small subdivisions of the area are shown on plate 7. The bedding diagrams (pl. 7 and pl. 5, fig. 3, diagrams A-1 and B-1) show that most of the bedding attitudes are consistent with the generally simple structure of the thrust sheet as inferred from mapping. In most of the diagrams, bedding poles cluster around single high maxima and indicate gentle dips away from the axis of the Bald Knob anticline. Only locally are indistinct bedding girdles evident, and even these largely disappear on the summary diagrams (pl. 5, fig. 3, diagrams A-1 and B-1), showing that the minor folds have little in- fluence on the statistical distribution of bedding poles. Axes of small tightly appressed or isoclinal folds (pl. 5, fig. 3, diagrams A-2 and B-2) form a nearly complete horizontal girdle having low maxima in the east-west, north-south, and northeast-southwest po- sitions. Most axial planes are nearly horizontal or have low dips to the south, southeast, or northeast. Orientations of axes of open folds and crenula- tions and of poles of cleavage planes are summarized in diagrams A-3 and B-3, pl. 5, fig. 3. Fold and crenulation axes northwest of the Bald Mountain an- ticline have a remarkably constant orientation and cluster around a single high maximum at S. 40° W., plunging 10° SW. Cleavage poles are normal to the fold-axis maximum. Most cleavage planes dip gently or moderately southeast and are apparently related to folds of the second type, but some of the gently dipping or horizontal cleavage planes are probably related to the earlier folds. Mineral lineations have very consistent orienta- tion patterns (pl. 5, fig. 3, diagrams A-4 and B-4) : Northwest of the Bald Mountain anticline, the aver- age trend is about N. 40° W. and horizontal; south- east of the axis, the most frequent trend is S. 25° E., plunging 30° SE., but the diagram also shows a small submaximum with a more southerly trend. Diagram C (pl. 5, fig. 3) shows the orientation of joint poles. No differences were noted between the distribution of poles on the two sides of the Bald Mountain anticline; therefore, the data are summa- rized on a single diagram. Most joints are nearly vertical and strike about N. 70° E.; a small propor- tion strike northwest. 114 RELATION BETWEEN STRUCTURES IN THE TABLEROCK THRUST SHEET AND THRUSTING The character of the mineral lineation, the close Tablerock thrust sheet and the similar lineation in correspondence in trend between the lineation in the the overridden rocks (compare diagrams A-4 and B-4, pl. 5, fig. 8, with diagrams A-3 and B-3, pl. 5, fig. 2), and the occurrence of similar linea- tions in blastomylonite along the Linville Falls fault and in retrogressively metamorphosed rocks of the Blue Ridge thrust sheet above the fault all suggest that the lineation is a cataclastic a lineation formed during movement along the major thrust faults. The consistent orientation of the axes of minor open folds and crenulations normal to the lineation (fig. 67) suggests that these structures were formed during the same episode of movement. The direction of asymmetry of these folds is consistent with north- westward movement of the overriding Blue Ridge thrust sheet in the direction of the mineral lineation. The folds may have been a somewhat later response to movement of the thrust sheet. The distribution of the earlier fold axes (pl. 5, fig. 3, diagram A-2) has nearly perfect orthorhombic symmetry, the mineral lineation and the axes of the later folds being along the symmetry planes. The FIGURE 67.-Mineral lineation and axes of open folds in quartzite of the Chilhowee Group in the Tablerock thrust sheet at overlook along National Park Service trail at head of Linville Falls (area D-6, pl. 1). Fold axes trend N. 45° E.; mineral lineation trends N. 45° W. GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE reason for this interesting distribution is not clear, but the pattern suggests that tight folds were not formed by regional folding prior to and independent of the thrusting. They may have formed in diverse orientations as a result of local stress variations dur- ing an early stage of thrusting, but the present pat- tern is probably largely due to rotation of the folds by differential forward movement of the thrust sheet in the direction of the a lineation. The direction of overturning of the earlier folds is consistent with this interpretation. The earlier folds do not involve the Linville Falls or Tablerock faults, but small open folds and crenu- lations of the second type are found in phyllonitic and blastomylonitic rocks along the faults. Where the fault planes are exposed, they are not folded. The younger folds therefore probably formed during a late stage of thrusting but before movement had ceased. The distribution of joint poles (pl. 5, fig. 3, dia- gram C) does not fit in with the symmetry of the other structures in the Tablerock thrust sheet but is identical with the distribution patterns of joint and fracture cleavage poles in the Grandfather Mountain Formation (pl. 5, fig. 2, diagrams A-6, A-7, B-5). It is therefore inferred that the major joint sets formed after the Tablerock thrust sheet reached its present position. Formation of the joints may be re- lated to the gentle arching of the thrust sheet along the Bald Knob anticline. The structural geometry of the Tablerock thrust sheet more nearly resembles that of the adjacent part of the overlying Blue Ridge thrust sheet than it does that of the autochthonous rocks of the Grand- father Mountain window (compare pl. 5, fig. 1 and pl. 5, fig. 2). This seems to confirm that formation of the minor structures in the Tablerock thrust sheet was closely related to the thrusting process. MET AMORPHISM The Precambrian basement rocks in the Grand- father Mountain window (pl. 6) underwent plutonic metamorphism which is dated by uranium-lead iso- topic ratios in zircon from the Wilson Creek Gneiss and Blowing Rock Gneiss as 1,000 to 1,100 m.y. ago, approximately equivalent to the time of metamorph- ism of the Grenville Series in the Adirondacks (Davis and others, 1962). The age and previous his- tory of the pregranitic rocks is unknown. It is also uncertain whether the Brown Mountain Granite was emplaced during this plutonic event or at some later INNER PIEDMONT BELT time before the deposition of the Grandfather Moun- tain Formation. After deposition of the upper Precambrian rocks, all the rocks of the Grandfather Mountain window were sheared and metamorphosed to a low and rather uniform grade. Progressive metamorphism of the upper Precambrian rocks and concurrent retro- gressive metamorphism of the plutonic basement produced mineral assemblages characteristic of the quartz-albite-epidote-biotite - subfacies - of - the greenschist facies of Fyfe, Turner, and Verhoogen (1958). The more mafic Precambrian rocks contain porphyroclastic hornblende and recrystallized actin- olite and chlorite. Relict oligoclase or andesine is found in a few of the rocks. Some basement rocks were only incipiently sheared and recrystallized, whereas others were en- tirely converted to blastomylonite or phyllonite. Original textures in sedimentary and volcanic rock were not completely destroyed, except in some of the finer grained rocks. However, all plagioclase was al- tered to albite, and almost all pyroxene, to chlorite and actinolite. The fine-grained mafic volcanic rocks, such as those of the Montezuma Member, were al- most completely reconstituted to albite-epidote- chlorite-actinolite greenschists. Coarser grained vol- canic rocks contain relict phenocrysts of potassic feldspar, plagioclase altered to albite, quartz, and, rarely, pyroxene. The Linville Metadiabase was met- amorphosed to rocks ranging from strongly sheared and completely reconstituted greenschist to altered but relatively unsheared diabase. The recrystallized minerals are locally coarser grained near the southeast side of the window. In the Blowing Rock quadrangle, recrystallized Wilson Creek Gneiss directly adjacent to the Linville Falls fault contains recrystallized oligoclase (Anj;). This metamorphism was apparently contempora- neous with formation of cataclastic foliation in the basement rocks, the two older cleavages in the Grandfather Mountain Formation, and the cleavage parallel with bedding in the Tablerock thrust sheet. A single rubidium-strontium date on biotite from the Wilson Creek Gneiss (Davis and others, 1962) sug- gests that this metamorphism occurred about 350 m.y. ago, near the end of the middle Paleozoic. Rankin, Stern, Reed and Newell (1969) inter- preted discordant zircon ages from felsic volcanic rocks of the Grandfather Mountain Formation as being due to episodic lead loss at 240 m.y. during late Paleozoic thrusting. The exact time of this lead loss 115 in relation to the sequence of minor structures in the rocks of the window is not known. The inferred ages and relationships of geologic events in the Grandfather Mountain window are summarized in table 29. Green iron-rich muscovite occurs throughout the Grandfather Mountain window in metamorphosed basement rock, sandstone, and felsic volcanic rock. Its occurrence in the arkoses of the Grandfather Mountain Formation could be due to an originally high content of ferric iron, and the arkoses could have been red beds whose color was changed during metamorphism. The environment of deposition is probably not as critical as metamorphic conditions in controlling oxidation or reduction of iron during re- gional metamorphism, however, as the mica is also found in metamorphosed quartzite of the Chilhowee Group, felsic volcanic rock, and basement rock as well as in the arkoses. The widespread occurrence of the iron-rich mus- covite is probably related to bulk composition of the rocks and to metamorphic conditions. Ernst (1963) suggested that a high ratio of fluid pressure to total pressure and low temperature favors the formation of phengite in glaucophane schist terranes of Japan, California, and the Alps. Lambert (1959) analyzed muscovites from psammatic and pelitic rocks of var- ious metamorphic grades in the Moine Schist and found that rocks of low metamorphic grade contain muscovite richer in FeO; than those of higher grade. He attributes the high ferric iron content of the muscovite to metamorphism under unusually strong oxidizing conditions. The presence of the iron-rich muscovitic mica in the rocks of the Grand- father Mountain area may indicate that similar con- ditions prevailed. Bryant (1967) discussed the occurrence of the iron-rich muscovite in more detail and concluded that rocks of the Grandfather Mountain window were open to oxygen during metamorphism, which took place under conditions of high shearing stress, low temperature, and high P.,. INNER PIEDMONT BELT The rocks southeast of the Brevard fault zone form a structurally complex metamorphic terrane consisting principally of layered biotite and biotite- amphibole gneiss, mica and sillimanite schist, amphi- bolite, and large concordant bodies of cataclastic augen gneiss. These rocks are invaded by concordant or semiconcordant plutons of granitic rocks, princi- 116 pally quartz diorite and granodiorite, commonly flanked by zones of migmatites apparently derived from the enclosing rocks. The gneisses and schists in the southeastern part of the area are of uniform high metamorphic grade, but in a belt 4 to 5 miles wide (pl. 6D) immediately southeast of the Brevard fault zone, rocks of the Inner Piedmont belt have been intensely sheared and retrogressively metamor- phosed under medium- and low-grade conditions and have a structural pattern differing from that of the high-grade rocks. Deep colluvial cover, scarcity of float, and lack of continuous exposures seriously handicap detailed geologic mapping in the Inner Piedmont belt. These handicaps, plus the structural complexity and lack of distinctive marker horizons, have obscured the stra- tigraphic sequence of the layered rocks. These rocks are therefore grouped into lithologic units for the purposes of mapping and description ; no implication is made as to their original stratigraphic relation- ships. The intrusive rocks and migmatites are de- scribed in order of their inferred ages. Contacts be- tween rock units in the Inner Piedmont are com- monly gradational and are generally poorly exposed. Many of the contacts shown on the geologic miap are therefore diagrammatic at best. ROCK UNITS LAYERED ROCKS BIOTITE GNEISS Biotite gneiss interleaved with biotite and biotite- muscovite schist and biotite-hornblende gneiss and containing pods and layers of amphibolite, quartzite, quartz schist, and cale-silicate rocks constitutes the bulk of the layered rocks in the Inner Piedmont belt. The typical gneiss is a fine-grained well-layered light-, medium-, or dark-gray rock consisting of var- ious proportions of quartz, plagioclase, and biotite and subordinate amounts of muscovite, epidote, po- tassic feldspar, garnet, and chlorite. Common acces- sory minerals are zircon, apatite, allanite, magnetite, ilmenite, and pyrite. Limonite, carbonate minerals, and clay minerals are common products of incipient weathering. Layers range from fractions of an inch to several feet in thickness. Differences in color and texture between layers are due chiefly to variations in the proportions of quartz, feldspar, and biotite. Contacts between layers are sharp (fig. 68). In exceptionally good outcrops, some individual layers and groups of layers can be traced for several hundred feet along GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE strike without significant variations in thickness, but more competent layers are commonly boudinaged and many layers lens out, apparently because of shearing parallel to the layering planes. Foliation is defined by parallel orientation of mica- ceous minerals and by quartz-feldspar folia. It is generally parallel to the layering but locally tran- sects layering in fold noses, where it lies parallel to axial planes. With the possible exception of the lay- ering, no sedimentary textures or structures have been observed in the gneisses. Where they have not been affected by shearing and retrogressive metamorphism related to the Brevard fault zone, the biotite gneisses are of high meta- morphic grade and are granoblastic or rudely fol- iated (fig. 69A). Quartz and plagioclase form an in- equigranular mosaic of sutured grains generally ranging from 0.1 to 1.0 mm in diameter and ar- ranged with their long dimensions parallel to the fol- iation. Plagioclase also occurs as scattered, faintly zoned porphyroblasts as much as 1 em in diameter. Untwinned potassic feldspar forms intergranular films and small irregular grains interstitial to quartz and plagioclase in the mosaic, and microcline occurs as porphyroblasts as much as 1 ecm in diameter, hav- ing reaction rims of myrmekite at contacts with pla- gioclase. Biotite occurs in stubby subhedral to anhed- ral flakes 0.25 to 2.0 mm long. The smaller flakes are commonly randomly oriented, but the larger flakes are rudely alined with the foliation, suggesting that the mineral is synkinematic or postkinematic. The biotite is most commonly pleochroic in shades from light yellow to deap red brown, but in some speci- mens it is olive green or deep smoky brown. Musco- vite occurs as rudely alined synkinematic flakes com- parable in size with the largest biotite flakes and as a secondary mineral forming fringes on the ends of biotite books, skeletal aggregates replacing feldspar, and sericite flakes in feldspar. Colorless or light-yel- low nonpleochroic epidote forms subhedral prismatic grains, small granules, and anhedral skeletal aggre- gates, generally less than 0.5 mm in diameter. Many epidote grains and aggregates enclose cores of or- ange-red or reddish-brown metamict allanite. Garnet forms irregular skeletal grains and subhedral to eu- hedral porphyroblasts as much as 1.5 mm in diame- ter, which show no evidence of rotation. The garnet is colorless in thin section and deep wine red in hand specimen. Zircon, apatite, sphene, and magnetite are whiquitous minor accessory minerals. Sillimanite occurs as needles and fibrous aggregates in biotite in a few specimens from the area of sillimanite-grade rocks. INNER PIEDMONT BELT FIGURE 68.-Interlayered fine-grained biotite and hornblende-biotite gneiss containing small concordant pods of biotite pegma- tite. Several layers contain scattered porphyroclasts of potassic feldspar. Saprolite exposure in roadcut on northeast side of county road 0.3 mile northwest of Abingdon (area I-7, pl. 1). Scale is about 7 inches long. Plagioclase in the gneiss is generally about Ang,, but ranges from Ans; to An,;. The smaller grains have about the same composition as the porphyro- blasts. In some specimens, plagioclase grains have thin albitic rims. Plagioclase compositions in adja- cent layers differ by as much as 10 percent An, show- ing that the variations in the composition of plagio- clase are controlled more by original bulk composi- tion of the rocks than by the metamorphic grade. Stable mineral assemblages in the biotite gneisses that have not been affected by shearing and retro- gressive metamorphism associated with the Brevard fault zone are summarized in table 19. In addition to the minerals listed in the table, zircon, apatite, sphene, and magnetite are stable accessory minerals in all assemblages. In the belt of shearing and retrogressive meta- morphism associated with the Brevard fault zone, the gneisses have cataclastic and polymetamorphic textures which become more conspicuous as the fault zone is approached (fig. 69, B-F'). Porphyroclasts of plagioclase, potassic feldspar, muscovite, and biotite are set in a fine-grained recrystallized matrix (fig. 69 D-F). Where recrystallization has taken place under medium-grade conditions, the matrix consists princi- pally of quartz, plagioclase, and potassic feldspar in an inequigranular mosaic of sutured grains 0.05 to 0.3 mm in diameter and stubby flakes and anhedral grains of synkinematic and postkinematic biotite 0.1 to 0.5 mm long randomly oriented or rudely alined parallel to the foliation. Biotite in the recrystallized matrix is generally pleochroic in shades of gray, green, or dark brown, although some is reddish brown, similar to that in the high-grade gneiss. Syn- kinematic muscovite forms thin flakes alined with the foliation, and epidote forms skeletal prismatic grains and irregular aggregates as much as 0.5 mm in diameter. Most of the epidote grains seem to be postkinematic, but some contain cores of orange-red or reddish-brown metamict allanite similar to those in epidote grains in the unrecrystallized high-grade gneiss. Scattered small skeletal grains of garnet also seem to be postkinematic. Plagioclase porphyroclasts resemble the plagio- clase porphyroblasts in the unrecrystallized gneiss, but they are generally small and have bent and 118 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE INNER PIEDMONT BELT FIGURE 69.-Photomicrographs of biotite gneiss. A, Plagio- clase-biotite-quartz gneiss from quarry beside U.S. High- way 321, 0.3 mile southeast of Caldwell County courthouse in Lenoir (area J-7, pl. 1). Dark layer in fine-grained biotite gneiss. Polymetamorphic features lacking. Plagio- clase is sodic andesine. B, Biotite-plagioclase-quartz gneiss from roadcut 0.6 mile N. 41° W. of Abingdon (area I-7, pl. 1). Incipient development of mortar, new well re- crystallized. Plagioclase is sodic andesine. Analyzed speci- ment 6, table 20. C, Biotite-muscovite-plagioclase-quartz gneiss from roadcut 0.25 mile east-northeast of Fleming Chapel (area H-7, pl. 1). Recrystallized polymetamorphic texture. Some larger muscovite flakes are porphyroclasts. Plagioclase is calcic oligoclase. D, Porphyroclastic musco- 119 vite-plagioclase-quartz-biotite gneiss from roadcut on east side of road 0.9 mile N. 50° E. of Arneys Store (area G-8, pl. 1). Analyzed specimen 3, table 20. E, Quartz- plagioclase-biotite gneiss 1.5 miles S. 57° E. of village of Happy Valley (area J-6, pl. 1). Porphyroclasts of plagio- clase in a mosaic of anhedral recrystallized quartz, sodic andesine, and well-alined biotite. Analyzed specimen 2, table 20. F, Blastomylonitic microcline-muscovite-quartz- biotite-plagioclase gneiss from roadcut 0.4 mile east of Adako (area G-7, pl. 1). Porphyroclasts of perthitic microcline, calcic oligoclase, muscovite, biotite, and quartz in a matrix of recrystallized quartz, albite, biotite, mus- covite, and epidote. TABLE 19.-Stable-mineral assemblages in biotite gneiss of high metamorphic grade in the Inner Piedmont belt [Stable accessory minerals omitted; X, present] Assemblage Quartz Plagioclase Biotite Potassic Muscovite Epidote Garnet Sillimanite (An content) (color parallel to Z) Feldspar 1s onit X 25 X X ye levied. ond X 27 Olive green-....~....... XK € K s I Ate %K 30 Reddish brown.-.____-... X X $ c oll r dee cece aa ala X 30 Grayish brown.-_________ 4 X Mis Pee e sa Y oe v aln d ae J 27 Grayish brown._....__... Jos bolo oce K XT cot clr wy K 25 Greenish gray._________- yue n pel eee ae X N oc eect anns e rae cle leew ab o ole l es X 33 Reddish brown.-________. JCW el nienke s K . X 24 Dark brown............ JC) _ ea K Me vis Trece X 35 Reddish: brown........_.....s..... XK X KEC TC Ell Ea + aa wale a ale mn ae X 35 Black. :i 22 dei ides. a-ak e XK K l X 29 Reddish X .n. Cen antl o een e a at > 4 $0: rc = aeon g eu ons M:. e JK K Mse l gali a + whew a a X 80°: ._... AO ree Ae acr ca ln. ak ann y XP :| ee X S1 cert.. ser a s sims banns YC n eee aP XL .= X 35 Dark brown -s KC. ade ass KL: ? sim Bef ya 85 Reddisinbrowh X ME .. bens XK id Datk brown r= 208. ;. L_ ances v4 XT! uas Ds ELLY... o c- nases X 35 Olive STEM aer ruen lL lll Lr cea Lene cies aam K 1 Un inoo eun a reales n sie ! Typical mineral assemblage of mica schist, but also found in biotite gneiss. broken twin lamellae and ovid outlines (fig. 696 and F). They are calcic oligoclase or sodic andesine simi- lar in composition to the reerystallized plagioclase in the mosaic, but some have thin rims of albite. Por- phyroclasts of microcline or microperthite and ag- gregates of angular grains apparently derived from crushed porphyroclasts are common in some of the gneiss (fig. 69F). The potassic feldspar porphyro- clasts are generally less than 1 mm long and are commonly surrounded by a jacket of quartz and pla- gioclase formed by recrystallization of myrmekite. Muscovite forms conspicuous porphyroclasts as much as 2.5 mm long alined with the foliation. Some of these have warped or folded cleavages and many have cross sections shaped like a much flattened par- allelogram, similar to the head of a double-bitted axe viewed parallel with the handle (fig. 69D). Porphy- roclasts of biotite are smaller and less common than those of muscovite. They are generally dark brown and have warped cleavages and sagenitic webs of opaque minerals. Adjacent to the Brevard fault zone, and in local zones throughout the belt of shearing and retrogres- sion associated with it, the biotite gneiss has been recrystallized under low-grade conditions. Biotite in the recrystallized matrix has been partly or entirely altered to chlorite, plagioclase is partly altered to albite, and garnet is partly replaced by chlorite. Seri- cite is abundant, and tourmaline is a common acces- sory mineral. Chemical analyses, modes, norms, and descriptions of selected specimens of biotite gneiss are given in table 20, together with average modes of rocks mapped as biotite gneiss in the zone of high-grade regional metamorphism and in the belt of shearing and retrogressive metamorphism associated with the Brevard fault zone. The principal modal variations in all rocks mapped as biotite gneiss are shown in figures 70 and 71. 120 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE TABLE 20.-Chemical analyses, modes, and norms of biotite gneiss of the Inner Piedmont belt [Analyses of samples 2 and 7 determined by standard chemical methods by D. F. Powers, U.S. Geol. Survey. Other analyses, Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey. Modes of analysed samples, by rapid methods by Paul Elmore, Samuel by point counts of 600 grains; P, present but not inter- sected in counting; Tr, trace; Nd, not determined. Major oxides and CIPW norms given in weight percent; modes, in volume percent] 1 2 3 4 5 6 T 8 9 10 ! 11 12 ® Field 80-184502 BR-7IA - A-85B 18-1680 88-2196B 32-2009 BR-71B 80-1895D1 20-1148 | ________ ________ ________ Laboratory 160168 'T-4068" 160156 ~ '160i57? - 160165- 1601646 I-4os90 - A60i6e. déofé® - O Major oxides SIO: eZ iA ican esen 56.9 60.81 _ 66.1 69.0 69.3 69.9 11.12 .* 48.8 10:9 8 2 eee Ure o ae abbas a oan 'is ABO; c 17.6 17.68. 15.8 15.2 14.0 14.4 15.24 - 15.0 (1A eee sa ics. c_ _c nc: __ . 68 1.62 1.5 1.2 3.0 1:1 .97 .10 Fik ose rice cc e.. PreQ:... .. 7.5 4.22 4.5 8.5 2.2 3.0 1.42 1:2 Parmi cronica ln. 2.2 2.42 1:7 1.4 1.5 1.4 .69 . 50 T Hea r aes cea aa aia k 5.0 4.22 1.1 1.6 2.8 2.8 2.85 2.8 (NC Revere en are- a 3.4 3.84 2.7 2.6 3.6 3.6 3.99 £4.17 IB Seen t onne oal. SOLA AI ZI 2.2 2.63 3.6 3.0 1.6 1.8 2.63 TB Bence edi gine. .85 .97 1.9 1.4 .86 .91 . 55 .56 BB" Loe? oui eee nicee O= -__ _icll .04 .07 .06 14 A2 .10 .09 . 03 06 cules eol 2.1 .88 .93 «48 . 68 .56 .34 .20 NY peel io rena ea ne s .on .94 .84 715 .18 .25 .28 A2 .05 MT set oA oc ios ie. . 22 .10 11 .05 . 08 07. .05 . 00 $00 c CO;... ana alles <.05 02. ~ <.05 -'«:05 . 01 15: "i105 .com nl Celie tn. e Nd .02 Nd Nd Nd Nd .01 Nd Nd:": ob {tc reloads. I u oo pea seamed Nd .09 Nd Nd Nd Nd .08 Nd Nd .._... simulant g_) Total............. 100 99.78 100 100 100 100 99.61 99 99 eo t ror cr cta lorn: Modes Quarts-.....:.l..._l.:_. 23 25 82 38 42 38 82 37 51 35.2. 81.6 32.8 Plagioclase._.....:..__ 45 39 15 26 41 41 50 54 32 82.5 29.6 30.6 Potassic feldspar....-.......... .2 «5 1.7 1.0 .8 5 1.5 T 7.2 6.3 6.6 Bidvite._ ._..l....s.... 25 28 36 20 12 18 8 5 1.0 14.3 17.3 16.3 5 15 13 1.0 1:2 4 1.8 9 4.0 9.7 7.8 .8 2.8 .8 «8 1:2 .8 *d Y as evens, 2.9 2.8 2.8 Apatite......."....... 1.2 .2 2 5 A .8 P 4 a . Tr Tr Tr Garnets2.-.cc-........l. 9 A Rid ale avs awl P" ne uaa e e Pal o als ae .8 .8 .4 Chlorite:. -_... -L nie eae ie oo auld a ae raat ane ae g- FT lo canvases Tr Tr Tr vA Y. coa P 2 P .. Prs e +2 Tr Tr Tr Carbonate 22sec ell ...le. cern eases duel balks asl a onl a bn than nents ie o Ir Tr Tr Limonite? -.2.t so}; . e Lebec eda alan's Bo oue sants 1.0 Tr Tr Tr Opaque minerals. __ ___ 2.8 .5 .8 fold 1.0 2 Poss cen. Tr Tr Tr CIPW norms Q-... clue soa uit 12:16 ° 14:19 20.08 s5.47 'g4.84 88.06 S1.69 86.00 - 48.00 Cfl ean aa een 2.79 1.79 5.82 5.20 1.97 2.11 1.95 1.84 rod ~ cnc Or.. Celle a os 18.00 _ 15.54 21.27 17.72 8.86 10.64 - 15.54 4:8 t 18.59 :. onces oon . Ab 29.10 _ 92.90 22.89 21.00 . a0.456 80.45 88.07 89:75 S2.14 10 aaa o l ot 18.66 - 18.04 4.48 6:76 12.25 12.06 . 10.68 12.61 PBS | sega wel on su e ae we fils... 0 cel I OS. NOON ean teres elan enses (U2 ire nee one u Enlists 5.48 6.02 4.283 3.48 3.73 3.48 1.12 1.24 M2) cito nn ity __ T Sere nee we rae eca t bey 10.15 5.14 5.69 4.24 . 59 3.80 1.34 1:19 elmas annem al male an a sak ML: :.: _ }l._ >_ .99 2.85 2.18 1.74 4.35 1.60 1.41 A4 fay cl coli {eir il J__- snel legen aar beare ashe L bee's a- ating bees abe rels Or ary eon aet en LL OLL Messe cr stot oso: 3.99 1.67 1.77 1.48 1.29 1.06 .65 . 88 ted o eron ? ae as Apri clue.. oue cl, 2.23 .80 .36 As'. . 59 . 66 .28 A12 Q2 SoLo 2a 2. Pel le cba el. aa bans M en c al cause o alas o aaa an s Obe oa A Ie. o epi eate Det eee eines a. CLs Pea thas an ar nh iaa n ea IL A. . -o... aand .02 BH EEL lise ae andi an aun, ' Total of 50 random grains counted in each of 44 thin sections of biotite gneisses and related rocks not showing conspicuous polymetamorphic textures. * Total of 50 random grains counted in each of 91 thin sections of biotite gneiss and related rocks showing conspicuous polymetamorphic textures. 3 Weighted average of columns 10 and 11. 2. Quartz-biotite-plagioclase gneiss. 3. Muscovite-plagioclase-quartz-biotite gneiss. NoTE.-Minor-element analyses for samples 2 and 7 given in table 1. . Quartz-biotite-plagioclase gneiss. Fine-grained dark-gray biotite gneiss in 2-em layer interleaved with biotite-amphibole gneiss and quartz-plagio- clase gneiss. Rock is composed of irregular interlocking grains of quartz and plagiocvlase (Anss) 0.1 to 0.5 mm across and ra«ced unde- formed flakes of biotite as much as 0.5 mm long. Biotite is strongly alined parallel to layering. Garnet occurs in skeletal porphyroblasts as much as 5 mm in diameter. North wall of Causby quarry, on east side of Hunting Creek, 0.9 mile S. 10° W. of confluence of the Catawba River and Johns River (area I-9, pl. 1). Fine-grained dark-gray gneiss interlay- ered with quartz-feldspar gneiss and amphibolite. Rock has polymeta- morphic texture and consists of ragged porphyroclasts of plagioclase as much as 6 mm in diameter, in a mosaic of anhedral recrystallized quartz and plagioclase (Anss) 0.01 to 0.5 mm in diameter and well- alined synkinematic or postkinematic biotite 0.05 to 5 mm long. Epidote occurs in irregular prismatic grains 0.1 to 1 mm long, many with al- lanite cores. 1.5 miles S. 57° E. of village of Happy Valley (area J-6, Lil). i Layered fine-grained dark- gray gneiss containing thin layers of white quartz-feldspar gneiss. Light-colored layers not included in analysis. Gneiss contains scattered porphyroclasts of potassic feldspar as much as 1 cm long; flakes of muscovite as much as 3 mm across lie on the foliation planes. Texture is polymetamorphic. The rock consists of a mosaic of anhedral recrys- tallized quartz and plagioclase (Ans) 0.02 to 0.2 mm in diameter and irregular to tabular synkinematic and postkinematic biotite 0.05 to 0.25 mm long. Porphyroclasts of untwinned plagioclase 0.25 to 0.5 mm across are set in the mosaic. Muscovite occurs as small synkinematic flakes and as rhomboid porphyroclasts 0.5 to 3 mm long. Potassic feld- spar forms small irregular grains and intergranular films in the re- crystallized mosaic; no porphyoclasts of potassic feldspar occur in the analyzed specimen. Epidote occurs as small skeletal crystals, some con- taining clinozoisite cores. Roadcut on east side of road east of Johns River, 0.9 mile N. 50° E. of Arneys Store (area G-8, pl. 1). 4. Muscovite-biotite-plagioclase-quartz gneiss. Fine-grained dark-gray schis- tose gneiss containing plagioclase and potassic feldspar porphyroclasts 0.5 to 2.5 em leng. Texture is polymetamorphic. Rock consists of mosaic of granoblastic quartz and plagioclase (Anso) 0.05 to 0.1 mm in diameter and rudely alined flakes of synkinematic and postkinematic biotite 0.05 to 0.2 mm long. Epidote forms irregular poikolitic grains, some containing allanite cores. Small outcrop in tributary of Canoe Creek 0.4 mile S. 10° W. of Tablerock Church on North Carolina High- way 181 (area F-9, pl. 1). INNER PIEDMONT BELT 121 TABLE 20.-Chemical analyses, modes, and norms of biotite gneiss of Inner Piedmont belt-Continued. 5. Biotite-plagioclase-quartz gneiss. Dark-gray fine-grained layered gneiss. to 0.25 mm long. Abundant porphyroclasts of plagioclase (Anzso) 0.25 Texture is polymetamorphic. Rock is composed of mosaic of anhedral recrystallized quartz and plegioclase (Anso) 0.1 to 0.5 mm in diameter and irregular to tabular flakes of synkinematic and postkinematic bio- tite 0.1 to 1 mm long. A few porphyroclasts of twinned plagioclase as much as 1 mm long are set in the mosaic. Epidote occurs as irregular to prismatic grains as much as 0.25 mm long, many containing cores of allanite. From outcrop 250 feet northeast of summit of Peaked Top (area I-7, pl. 1). 6. . Biotite-quartz-plagioclase gneiss. Medium-grained dark-gray gneiss con- taining thin layers of biotite schist and small pods of pegmatite (only the gneiss is included in the analyzed specimen). Texture not polyme- tamorphic. Rock consists of mosaic of anhedral inequigranular quartz 0.05 to 0.5 mm in diameter, zoned plagioclase (Ansos:) as much as 1 mm in diameter, and flakes of synkinematic biotite 0.01 to 1 mm long. Muscovite is in scattered synkinematic and postkinematic flakes, and epidote forms subhedral prisms about 0.1 mm long, commonly contain- ing allanite cores. Roadcut on road between Abingdon and Collettsville, 0.6 mile N. 41° W. of Abingdon (area I-7, pl. 1). 7. Potassic feldspar-biotite-quartz-plagioclase gneiss. Light-gray medium- grained gneiss interlayered with dark-gray biotite gneiss (sample 2). Texture is polymetamorphic. Rock consists of mosaic of irregular grains of recrystallized quartz and plagioclase 0.02 to 0.5 mm in diame- ter and irregular flakes of synkinematic and postkinematic biotite 0.05 Biotite to 1 mm in diameter, some sericitized and partly albitized. Muscovite occurs as scattered synkinematic flakes. Potassic feldspar forms small irregular grains and intergranular films in the quartz-plagioclase mosaic. Same locality as sample 2. 8. Biotite-quartz-plagioclase gneiss. Fine-grained light-gray gneiss in 3-cm layer jnterlayeyed with dark-gray biotite gneiss (sample 1) and biotite amphibole gneiss. Texture is not polymetamorphic. Rock consists of an- hedral grains of plagioclase (Anz) averaging 0.5 mm in diameter, lenltlcular grains of quartz as much as 1 mm long elongated parallel to foliation, and scattered rudely alined stubby flakes of biotite 0.1 to 0.25 mm long. Muscovite occurs as sericite flakes and skeletal grains replac- ing plagioclase. Potassic feldspar forms intergranular films and perthi- tic blebs in plagioclase. Some plagioclase slightly albitized. Same local- ity as sample 1. 9. Potassic feldspar-muscovite-plagioclase gneiss. Light-gray medium- to fine-grained gneiss, well foliated but nonlayered. Texture is not con- spicuously polymetamorphic. Rock consists of mosaic of recrystallized granoblastic quartz 0.02 to 0.5 mm in diameter, clear twinned plagio- clase (Anss) 0.5 to 1.0 mm in diameter, and strongly alined flakes of synkinematic muscovite as much as 0.7 mm long. Potassic feldspar occurs in small irregular grains interstitial to quartz and plagioclase. Outcrop 0.6 mile S. 55° E. of Arneys Store (area G-8, pl. 1). Biotite Plagioclase Muscovite Biotite EXPLANATION *2 Chemically analyzed specimen © Average FIGURE 70.-Proportions of quartz, plagioclase, muscovite, and biotite in biotite gneisses of the Inner Piedmont belt. Based on counts of 50 random grains in each of 135 thin sections. Contours 0.75, 3, 9, and 12 percent. Numbers of analyzed specimens refer to analyses in table 20. 122 QUARTZ EXPLANATION *1 Chemically analyzed specimen © Average ¥ PLAGIOCLASE POTASSIC FELDSPAR FIGURE 71.-Proportions of quartz, plagioclase, and potassic feldspar in biotite gneisses of the Inner Piedmont belt. Based on counts of 50 random grains in each of 135 thin sections. Contours 1.6, 3.2, and 4.7 percent. Numbers of analyzed specimens refer to analyses in table 20. MICA SCHIST Mica schist is interlayered with biotite gneiss in all proportions and locally predominates over gneiss in large enough areas to be distinguished on the geo- logic maps. The schist forms layers ranging from a few inches to several tens of feet in thickness and is conformable with the layering in the enclosing gneiss. The mica schist is a strongly foliated me- dium- to coarse-grained light-gray or greenish-gray rock composed of various proportions of biotite, muscovite, quartz, and plagioclase, and commonly containing porphyroblasts or porphyroclasts of gar- net and plagioclase. Where the schist has not been subjected to shear- ing and retrogression along the Brevard fault zone, it consists of folia of strongly alined synkinematic flakes of muscovite and biotite 1 to 5 mm long alter- nating with folia composed of irregular granoblastic grains of quartz and clear-twinned plagioclase (sodic andesine) 0.25 to 5 mm in diameter (fig. 724). Gar- net forms subhedral to euhedral poikilitic grains, generally less than 5 mm in diameter but locally as much as 3 inches in diameter (fig. 724). Zircon, apa- tite, and magnetite are the most common minor accessory minerals, and rutile occurs in a few speci- mens. Some of the schist contains porphyroblasts of plagioclase as much as 1 em in diameter. The only stable mineral assemblage found in the mica schist unaffected by shearing along the Brevard zone is quartz-plagioclase-biotite-muscovite-garnet. GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE In the belt of shearing and recrystallization along the Brevard fault zone, the schist has conspicuously polymetamorphic textures (fig. 72B-D). Warped and bent books of muscovite and biotite as much as 5 mm long and grains of plagioclase as much as 5 mm in diameter having warped and broken twin lamellae form porphyroclasts set in a finely recrystallized granoblastic matrix of recrystallized quartz and pla- gioclase grains 0.01 to 0.1 mm in diameter. Coarser grained recrystallized quartz also forms segregation folia parallel to the foliation. Garnets contain cores having helicitic structures, commonly jacketed by clear synkinematic or postkinematic overgrowths. Scattered grains of kyanite and staurolite as much as 5 mm in diameter are fairly widespread in the poly- metamorphic schist. Locally, staurolite is associated with an isotropic mineral with very light green ab- sorption and high relief, which might be a spinel. Whether the staurolite and kyanite grains are por- phyroblastic or porphryoclastic near the southeast margin of the zone of shearing and recrystallization (pl. 6D) is not certain, but to the northwest they are clearly porphyroclastic. Plagioclase in the polymeta- morphic schist is generally more sodic than that in the schist to the southeast, and colorless epidote is present as scattered irregular grains having moder- ate to low birefringence. Adjacent to the Brevard fault zone, and in irregularly distributed local areas throughout the belt of shearing and metamorphism associated with it, the latest recrystallization occurred at low metamorphic grade. Plagioclase is partially or completely replaced by albite or sericite, biotite and garnet are altered to chlorite, and stau- rolite and kyanite are jacketed by sericite. In these areas, undeformed needles of green tourmaline are very common in the schist. The variation in proportions of the principal min- erals in all rocks mapped as mica schist is shown in the modal composition diagrams (fig. 73). Chemical analyses, modes and norms of two speci- mens of schist and the average mode of all the mica schists examined are given in table 21. HORNBLENDE GNEISS AND AMPHIBOLITE Fine- and medium-grained medium- to dark-gray biotite-hornblende gneiss, hornblende gneiss, and amphibolite form layers and lenses intercalated with biotite gneiss. These rocks are ubiquitous, but no- where do they predominate over biotite gneiss in a sufficiently large area to be distinguished on the map, and they are therefore mapped with the biotite gneiss. Areas where amphibolite is particularly INNER PIEDMONT BELT - 123 FIGURE 72.-Photomicrographs of mica schist. A, Coarse- grained garnet-bearing muscovite-biotite-plagioclase-quartz schist from outcrop along Lower Creek 1.6 miles southeast of Chesterfield (area H-9, pl. 1). Large grains of sodic andesine, quartz, and garnet in a matrix of quartz, sodic andesine, and synkinematic micas. B, Garnet-bearing plagioclase-quartz-muscovite-biotite schist from roadcut on new road from Abingdon to Collettsville, 1.0 mile N. 39° W. of Abingdon (area I-7, pl. 1). Porphyroclasts of calcic oligoclase and of bent muscovite in a matrix of recrystallized quartz, biotite, muscovite, and calcic oligo- clase. Garnets with sieve texture. Quartz segregation stringer. Analyzed specimen 2, table 20. C, Staurolite- bearing kyanite-chlorite-biotite-garnet-quartz-muscovite abundant are indicated by an overprint on plate 1. Layers of hornblende-bearing rocks range from a few inches to several feet in thickness, and contacts with the biotite gneiss are sharp. Where the biotite gneiss has been sheared, hornblende gneiss and am- schist from roadcut along North Carolina Highway 126, 0.9 mile east of bridge over Linville River (area E-9, pl. 1). Porphyroclasts of sieve-textured garnet, staurolite, kyanite, and bent muscovite in a matrix of recrystallized quartz, muscovite, and biotite. Kyanite partly altered to sericite, and biotite partly altered to chlorite. D, Kyanite- and staurolite-bearing garnet-chlorite-quartz-plagioclase- biotite-muscovite schist from roadcut on North Carolina Highway 181, 1.1 miles southeast of Smyrna Church (area F-8, pl. 1). Porphyroclasts of staurolite and sieve-textured garnet in a matrix of recrystallized quartz, calcic oligo- clase, biotite, and muscovite. Local alteration of staurolite to sericite and of garnet to chlorite. phibolite layers are commonly pulled apart into dis- connected pods and lenses, some of which have been rotated, so that layering within them stands at high angles to layering in the enclosing rocks. 124 Biotite +chlorite Quartz GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE Biotite + chlorite Plagioclase Muscovite EXPLANATION 2 % Chemically analyzed specimen 0 Average Biotite + chlorite FIGURE 73.-Proportions of quartz, plagioclase, muscovite, and biotite plus chlorite in mica schist of the Inner Piedmont belt. Based on counts of 50 random grains in each of 35 thin sections. Contours 3, 6, 12, and 17 percent. Numbers of ana- lyzed specimens refer to analyses in table 21. The hornblende rocks are granoblastic or rudely foliated or lineated, depending on the relative pro- portions of biotite and hornblende. Commonly the hornblende-bearing gneisses and amphibolites do not have recognizable polymetamorphic textures, even in the belt of shearing and retrogression along the Bre- vard fault zone where polymetamorphic textures are conspicuous in the enclosing rocks. Apparently horn- blende-rich rocks were resistant to physical and chemical breakdown under the metamorphic condi- tions prevailing in that belt. The biotite-hornblende gneisses consist of various proportions of quartz, plagioclase, bornblende, and biotite. Anhedral grains of quartz and plagioclase 0.25 to 2.0 mm in diameter form an inequigranular mosaic interstitial to randomly arrayed or rudely alined stubby flakes of biotite as much as 2.0 mm long and irregular to subhedral poikilitic grains of hornblende 0.25 to 3.0 mm in diameter. Much of the plagioclase is faintly zoned ; it is generally calcic oli- goclase or andesine. In some specimens, plagioclase forms porphyroblasts as much as 3.0 mm in diame- ter. Epidote occurs as prismatic grains and irregular skeletal aggregates. It is generally colorless, but some is light yellow and fainly pleochroic. Some grains contain cores of allanite; others contain cores of clinozoisite. Apatite, sphene, and magnetite are INNER PIEDMONT BELT TABLE 21.-Chemical analyses, modes, and norms of schists in the Inner Piedmont belt [Chemical analyses determined by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey. Modes, by point counts of 600 grains; P, present but not intersected in counting; Tr, trace. Brackets indicate chlorite replacing biotite and garnet. Major oxides and CIPW norms given in weight percent; modes, in volume percent.] 1 2 3 ! 4 2 Field No. tec. 16-1008 S2-20124 Laboratory 160141 160142! ili Major oxides : uM avs 58.9 59.0 n. tes OTL. sy. i- a saul , a bi eed e 13.0 1149 . clr o A 4.9 19 | css ius ns -le PeQZ t...: luc... 4.4 bad: .+ cela aas ao a a ae a l... lls aige an 6:7. ioe I puch tat neben . rez :l. Ll nel. a eis a aw . 40 a T osa suis awake asic . 58 aB eal uaa ele Suk ELS cus 3.6 Riese ssi H;04 22... e Vd 1.9 Tro ro teats ane tall .09 (04 2 ce YY i. a ue iO; esr ece s eae ae 1.2 : in an Ls oke oi ssc el 25 SMB Uinta sens 40 MTT cee en rat cease .05 MB Lue le oo ines cen s Aqua regia soluble sulfur as ND (Lens renale Sh - os a ee 99 99 ~- C Modes 36 23 26.9 69.8 f iagioclase............_... 7T 21 13.9 2.8 Potassic feldspar. (CLs. 46 20 20.9 4.2 Muscovite. 2.8 26 29.1 18.2 Aluminum 4A 1.1 Ne mec rani a's dss isan 1.0 (0 ntl Carnet. _~_......:_-s.lz...; 2.1 4 8.3 1.2 Ohnlorite..;./z..... .8 1.8 15.2:1. ..: T llc cy P P Tr Tr Apatite. . len. stol... A d 1.0 Tr Tr P Tr) .} 2 .. io: Tr _ [ccs Opaque minerals.... 2.1 2.8 3.0 1.7 CIPW norms Q ies sii. seine sl oir bak ous S1.76 20,84 NY IALLIERE CIL 8.14 T ics. Or J cect seru. 2L.21 - 1T:.T2 ewes ADF ias es sain 4,90 ~ 23.08 L ous Et. L At cus. .04 61:06: .. ls 02482 asa s 16.68 5.98 sz... i L t cae casi ak I. 9.4902 :.. .cue ME:. ss.. csi el crease des 3.70 2:10 scn cels tly eins HIM rol n enne a en ne ao rs a ae (s eases cie coi dna aaa 2.28 209 2.2.2.2 sl ADRE san . 59 lL PPe eee Yie nee oo one arai 4.80 l le si + as b aii a - PH e A Hernan inay 1 Average of 50 random grains counted in each of 35 thin sections of mica schists. l:_At:;'era.ge of 50 random grains counted in each of 13 thin sections of quartz schists. 1. Quartz-biotite schist. Medium-grained dark-gray biotite schist. Rock con- sists of well-alined flakes of reddish-brown synkinematic biotite 1 to 2 mm long, interleaved with folia of granoblastic quartz 0.05 to 1 mm in diameter. Muscovite occurs as scattered flakes alined with biotite, and plagioclase (Anso), as small clear grains in the quartz mosaic. Garnet and staurolite form large synkinematic or postkinematic poilkilitic grains. Opaque grains are chiefly pyrite. Outcrop 0.8 mile N. 47° W. of school at Oak Hill (area F-9, pl. 1). 125 TABLE 21.-Chemical analyses, modes, and norms of schists in the Inner Piedmont belt-Continued 2. Biotite-plagioclase-quartz-muscovite schist. Lustrous fine-grained biotite- muscovite schist containing porphyroclasts of bent muscovite interlay- ered with dark-gray biotite and biotite-amphibole gneiss. Texture is poly- metamorphic. The rock consists of a mosaic of recrystallized grano- plastic quartz and plagioclase 0.05 to 0.2 mm in diameter and of irregu- lar alined synkinematic and postkinematic flakes of reddish-brown bio- tite 0.05 to 0.5 mm long; the rock also contains porphyroclasts of pla- gioclase (An»-»s) 0.25 to 2 mm in diameter and of muscovite as much as 5 mm long. Garnet occurs in subhedral porphyroblasts as much as 5 mm in diameter, containing poikilitic inclusions of quartz and plagioclase. Roadcut on new road from Abingdon to Collettsville 1.0 mile N. 89° W. of Abingdon (area I-7, pl. 1). common stable accessory minerals, and zircon occurs in some specimens. Chlorite, sericite, and carbonate are products of incipient alteration in many speci- mens. Hornblende gneiss and amphibolite are composed principally of various proportions of plagioclase and hornblende and commonly contain smaller amounts of quartz and epidote. We refer to rocks containing less than 50 percent hornblende as hornblende gneisses and to those containing more than 50 per- cent as amphibolites. These rocks consist of mosaics of rudely alined irregular poikilitic grains of hornblende 0.2 to 2.0 mm long, anhedral grains of plagioclase 0.2 to 0.5 mm in diameter, and smaller irregular grains of quartz that are interstitial to plagioclase and that also occur as poikilitic inclusions in hornblende. Plagio- clase is generally andesine and is well twinned and commonly faintly zoned. Hornblende is pleochroic in shades of green and blue green. Some amphibolites contain scattered anhedral grains of colorless or light-green diopside, and many of them contain scat- tered small flakes of reddish-brown biotite. Colorless or pale-yellow epidote forms skeletal grains and ir- regular aggregates, generally less than 0.2 mm in diameter. A few specimens contain euhedral to sub- hedral grains of garnet 0.05 to 0.2 mm in diameter, concentrated along foliation planes. Sphene is a ubiq- uitous accessory mineral ; it forms anhedral to sub- hedral grains as much as 0.5 mm in diameter and aggregates of smaller grains, generally concentrated along foliation planes. Apatite, zircon, and magnetite are other common accessories. Although polymetamorphic textures are not con- spicuous, hornblende gneiss and amphibolite in the zone of shearing and recrystallization associated with the Brevard fault zone are generally finer grained and more schistose and contain more epidote and a more sodic plagioclase than similar rocks to the southeast. Hornblende is partly altered to chlor- ite or to actinolite. Stable mineral assemblages found in hornblende- bearing rocks are summarized in table 22. Composi- tions, modes, and norms of selected specimens of am- 126 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE TABLE 22.-Stable-mineral assemblages in hornblende gneisses and amphibolites in the Inner Piedmont belt [Stable accessory minerals omitted; X, present. Assemblages 1 and 2 are in biotite-hornblende gneiss; assemblages 3 through 9, in hornblende gneiss and amphibolite] Assemblage Quartz Plagioclase Hornblende Biotite Epidote Garnet Diopside (An content) (color parallel to Z) (color parallel to Z) X $0 to §5 Blue green...._.......... Dark X "es tan ad ge s d 45 Light gray green_______. Reddish brown-.______. Kxx oce a +2 a sos % 38 Gray green......._.__.. Reddish -L - Ll? nen eL econ lee ea Le. e en anak X 45 io. s a asan O- Reddish brown 2.0002. sv le. _ weno it. xX 30 Dark gray green-________ Dark chestnut DrOWR _L 1: l clos cece esl ect X 90 Light gray......_...._.. Red DOWN: L eee ea eal ic en I cr ce beat liens bakk nak bn a pines $5 to 45 . Blue green....--........ Medium browns.. :; 2320522... ol ui ceara =s ss cu 50 to 60 _- Light gray green-______. Red brown. sine es cae abrs s- Ien ee eed o xX 40 Bitic ... cer anes nis tole: JC l u nie ien - amer eas o sss asnlsss 40 to 45" Light gray green.. ...-_cocll. ccs K c. C Arai es ogre Tee aria a a X 85 Gray greon :L ___. arket en i. dha area a T X 40 -to 45. ; _.... 0 Hee. L uunet arr eae seein cen castes dole af y X C ue c rans ain ua a murk a X 45 to 55 - Light gray green. aust. I H ia aoe base X 4 80 to 85 -..... dol}. :l :c cl e AICA bea a a e- ale as ae ane Xs e X 9: are lr l X 90 Light Red brown. . .u s sor A alue va een ans Pal ean ana TABLE 23.-Chemical analyses, modes, and norms of biotite-hornblende gneiss and amphibolite in the Inner Piedmont belt [Chemical analyses determined by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, samples determined by point count of 600 points; P, present but not intersected in counting; Tr, modes, in volume percent] Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey. Modes of analyzed trace. Major oxides and CIPW norms given in weight percent; Amphibolites Biotite-hornblende gneiss Averages F 2 3 4 5 6 T 8 9 1 10 : 113 Field 2.2 28-1111D 29-1343C 29-1218 28-1126 80-1379A 30-1669A 80-1395A 38-2146A | _______L LSbOrAtOrYy 160198" " 160140 : 160139 - 160150. 161250 _ 161051 - 160161. 160186) - sl.... Major oxides -. - 44.2 52.6 58.1 38.1 52.6 52.9 57.0 60.9... NLE ICIT awe bows on en ABOJ-. ens ense sin ane 17:2 15.1 12.5 15.4 18.1 16.4 16.6 15.08! 2.8 1.2 2.6 6.1 .90 1.8 1.0 210 > ce: ece enemee een cruel ee el clik bi 11.2 8.0 5.8 10.6 740 8.4 5.9 4.0 . crue rels on -i use 7.4 7.8 10.8 9.8 5.1 3.6 5.3 $0); celle ln rue ae ne s oin aera e Cade nal een el 12.0 11.0 11.9 10.5 8.83 7.6 7.2 coy ai eed e noe sell -n 1,7 * vf .98 .94 3.3 3.0 3.0 IP EL L ia mes an ae ROE Lie o nian eon on an .89 . 90 .37 1.9 1.6 2:2 1.6 $0 lessee udate e i 1 1 o} e tet pat oen aah on wale: T1 15 .65 1.4 1.0 1:2 .87 PQ elie tiao ena ua awan ee ...f LL ee 04 .04 "af . 22 .07 .10 .05 04+. sry carens l ls TiO; .r ise seen auks 1.0 A71 .24 2.2 1.0 2.5 . 84 GO - : eco eaten at e nig. P Oi. ENI Y.» aan ases .19 18 . 04 2.8 .46 .37 .18 (ao EPP ie ea ernie neo s : -k. .28 19 16 14 16 .18 11 10. red nen CO;. :e: eu han vane na enas .05 05 05 .05 05 .05 12 105) .:: e dri ase an Total. i c 99 99 99 99 100 100 100 1004 ; ..is s ravi at Modes QUATHL ESAS c alee nel wale 10 10° : we lnc.. 4 14 13 19 8.1 5.8 19.1 Plaginclase.:............-..-.«. 20 25 13 21 35 36 42 40 28.8 12.2 38.5 BIiOtite. .5 bt cal.. 18 20 25 21 14 7T. 1.0 15.3 ren on Nas a LY slain Hena ta bias a oen mlcle e Sie 2 4 & 1.2 Tr Hornblende......1i:..........~. 17 60 78 36 34 22 22 18 64.9 55.8 28.2 PyrOXENnE.. . 2 Ll .to ssn .8 18) Lvo l Erste ._ -e rakes s 5 Tr - _. D ia 2.2 1D ee o PL ieee iaa aileen wie 2.5 2.2 19.8 1.1 Apatite: 1 .8 A He He ee ae bas a adie a .8 .8 1T. erea ae alana ne as ser I AEE ia ann - an 1 less le .8 1.5 1.5 (BY Lircon :l LU i ici eure dein P P/: s. aman s new Be Bess Tr Tr Tr 2.0.2 200 ULT cach tha ah baninaman nen ann lenis nle ss sido tin aln his a nln Rd ro tei TF > cease: l-. sinc... nll e ss Pr rire co unseren 8.3(2) 4 1.1 .5 .9 1.0 1.2 Opaque minetals..._._....._:_. 1.5 yO: 4 l ele oan n 2 9 Tr Tr CIPW norms _____________________________________ 7.61 Al". 18 4.16 T.T4% AT.G8 aes UFP e ie: casa e rear a a a a abo 5.26 5.82 2,19 11,22 9.45 13.00 9.45 TGB ... . 2. ewes anak ADL Zbl ct 11.54 7.36 8.29 7.92. 2T.O1. 25.07 . 25.15. - 21,00 An: c nees s6.07T - 94.64 28.02 g2.10 209.806 24.179 27.24 28.55 NCX eee aaa ene en ken Seve esl cas SOF :. eee in on be cea n eine an aes anale ais ae cine am a eich e wie a n ae a W 8. Sune 8.89 (7.80. 12.45 1.90 3.34 4.25 2.90 SHBI is SOL: LC ILL.. re n> dresser occ [luli cee? 4.46 19.42 26.89 1.20 12.70 8.96 14.30 S I0 2 l a =s cana o ucc urea sh este aL Re ake 4.28, :: 12.88 8.48 .58 10.16 - 10.14 8.85 BIBS cles erie ae rak= trem co l cns a ia aus es rane 9 Anus 16:20 .... Ll san o -n ahiele iene oid nee an ena Pa. -se use weld at ae ale one be ue 10.20 s- ec C NZ . oo on fie an dna enh ud J+ a ale nb ale nh nk an- ab mate n an cae a ab Mt. :e ce Armi 3.34 1.74 3.62 7.40 1.30 2.61 1.39 ETT (& o dQ sa ain aik fh ain in male b ud a nie M: ese e iy a rescan t 1.90 1.35 46 4.18 1.90 4.75 1.60 ©4400 ail rea ae nne AD LE L s viv ecteon aa laws 45 .81 10 5.45 1.09 .88 .34 (52 [.is o :A alis lanl tt tek 4 11 11 11 11 Al 30 AT INNER PIEDMONT BELT Continued 127 TABLE 23.-Chemical analyses, modes, and norms of biotite-hornblende gneiss and amphibolite in the Inner Piedmont belt- ! Averages of 50 random grains counted in each of 16 thin sections of amphibolites not affected by shearing along the Brevard fault zone. ? Average of 50 random grains counted in each of nine thin sections of amphibolites from zone of rocks affected by shearing along the Brevard fault zone. 3 Average of 50 random grains counted in each of 23 thin sections of biotite-hornblende gneisses. Plagioclase amphibolite. Medium-grained dark-gray amphibolite in folded layer in medium-grained migmatitic biotite gneiss containing granitic layers and streaks. Amphibolite is weakly foliated but has conspicuous lineation of alined hornblende prisms. Rock is unsheared; it consists of subhedral to anhedral grains of green hornblende 0.5 to 1.5 mm long and anhedral grains of twinned and zoned plagioclase (Anso-so) 0.25 to 1 mm long, some with warped twin lamellae. Biotite is reddish brown and occurs in irregular flakes as much as 1 mm long. North wall of abandoned quarry on east side of Hunting Creek 0.9 mile S. 12° W. of confluence of Johns River and the Catawba River (area H-9, pl. 1). 2. Quartz-plagioclase amphibolite. Medium-grained dark-gray amphibolite in pod 3 feet long in streaky biotite quartz monzonite containing thin lay- ers of biotite schist and gneiss. Moderately strong foliation and linea- tion in amphibolite is due to alinement of hornblende; 1- to 2-mm flakes of biotite are conspicuous on foliation surfaces. Rock consists of rudely alined irregular poikilitic grains of green hornblende 0.25 to 1 mm long and irregular well-twinned and faintly zoned plagioclase (about Anso), both containing small blebs of quartz. Quartz also occurs as small irregular interstitial grains less than 0.25 mm in diam- eter. Thin undeformed flakes of reddish-brown biotite are alined with the foliation and seem to have crystallized with the amphibole and pla- gioclase. Roadcut on north side of U.S. Highway 70 (business route), 100 feet east of the junction of the U.S. Highway 64-70 bypass around Morganton (area H-9, pl. 1). 3. Quartz-plagioclase amphibolite. Medium-grained amphibolite in 2-foot- long pod in saprolitized layered biotite gneiss containing abundant pods and layers of medium- or coarse-grained biotite quartz diorite. Amphi- bolite is well foliated and weakly lineated because of alinement of horn- blende. Rock is unsheared. It consists of well-alined poikilitic prismatic grains of green hornblende as much as 3 mm long, irregular grains of strained quartz, and twinned and zoned plagioclase (An;o-s) as much as 2 mm in diameter that is interstitial to hornblende and occurs as in- clusions in hornblende. A few irregular grains of a colorless pyroxene, probably diopside, are intergrown with hornblende and seem to have crystallized simultaneously with it. Roadcut in area H-8 (pl. 1), 1.4 miles N. 40° W. of the school at Chesterfield (area H-9, pl. 4). Pyroxene-biotite-hornblende gneiss. Dark-gray medium-grained nonlay- ered rudely foliated gneiss from isolated homogeneous outcrop. Proba- bly a layer in layered biotite gneiss. Rock is unsheared and is composed of clear irregular grains of twinned plagioclase (Anss) 0.25 to 1.5 mm in diameter, irregular poikilitic grains of green hornblende 0.5 to 1.5 mm in diameter, stubby flakes of reddish-brown biotite 0.25 to 0.75 mm long, and irregular equidimensional grains of diopside 0.25 to 2 mm in diameter. Diopside is jacketed and partially replaced by hornblende. Magnetite occurs as small irregular grains, chiefly in hornblende; apa- tite forms subhedral prisms and round grains included in amphibole and biotite. Small outcrop north of county road, 0.3 mile N. 64° W. of A Epidote Quartz Epidote Plagioclase ornblende 8. Biotite-quartz-homblende-plagioclase gneiss. Biotite bench mark 1013 at north end of bridge on which North Carolina Highway 18 crosses Johns River (area H-9, pl. 1). 5. Plagioclase-biotite-hornblende gneiss. Medium-grained dark-gray biotite- amphibole gneiss in 10-foot layer in migmatitic biotite gneiss. Rock is unsheared and consists of interlocking irregular grains of twinned and faintly zoned plagioclase (Anss-so) and rudely alined poikilitic gray- ish-green hornblende, both 0.25 to 1 mm in diameter. Biotite is reddish brown and occurs in irregular undeformed flakes 0.25 to 2.5 mm long, alined with the foliation. Sphene occurs in round or wedge-shaped grains 0.05 to 0.2 mm across, forming clusters as much as 1 mm across. More sphene is recorded in the modal analysis than is indicated by the TiO» in the chemical analysis. Roadcut on west side of road north of Huffman Bridge over Rhodiss Lake, 0.4 mile N. 20° E. of south end of bridge (west edge of area I-9, pl. 1). 6. Quartz-hornblende-biotite-plagioclase gneiss. Medium-grained dark-gray well-foliated gneiss containing abundant pods as much as 5 feet long of light-colored granitic rock. Rock is unsheared. It consists of well-alined irregular stubby grains of undeformed brown biotite and anhedral to subhedral green hornblende 0.25 to 1.0 mm long, anhedral grains of quartz 0.25 to 1.0 mm in diameter, and twinned and faintly zoned pla- gioclase (Anss) 0.25 to 1.5 mm in diameter. Sphene occurs in anhedral or wedge-shaped grains, some in clusters as much as 0.5 mm in diame- ter. Roadcut on north side of road, just north of Smoky Creek, 1.5 miles N. 52° E. of south end of Huffman Bridge (area I-9, pl. 1). 7. Quartz-biotite-hornblende-plagioclase gneiss. Dark-greenish-gray coarse- grained well-foliated nonlayered gneiss. Rock is unsheared. It consists of well-alined flakes of brown biotite as much as 2 mm long, irregular small prisms and sieve-textured grains of gray-green hornblende as much as 1 mm long, and folia of irregular granoblastic quartz and twinned and faintly zoned plagioclase (Ans), averaging 0.25 to 0.5 mm in diameter. South end of east wall of Causby quarry, 0.9 mile S. 10° W. of confluence of Catawba River and Johns River (area H-9, pl. 1 f Well-foliated medium-. to coarse-grained gneiss forming streak in migmatite biotite quartz dior- ite (samples 4 and 7, table 28). Rock is unsheared. It consists of large, very irregular grains of faintly zoned plagioclase (Anss) as much as 6 mm in diameter, many having albite fringes and some being partly sericitized and saussuritized. Quartz forms irregular grains 0.1 to 1 mm in diameter interstitial to plagioclase. Brown biotite occurs as irre'gplar poorly alined flakes as much as 3 mm long. S9me partly. chlqutlzed green hornblende forms irregular poikilitic grains 3 mm in diameter containing blebs of quartz and plagioclase. Small irregular grains of colorless epidote are included in biotite and partially replace plagio- clase. Roadcut on east side of North Carolina Highway 18 bypass west of Lenoir, 1.1 miles S. 68° W. of Caldwell County courthouse in Lenoir (area J-7, pl 1). Biotite Plagioclase Hornblende EXPLANATION v Average epidote amphibolite @ Average amphibolite 03 Chemically analyzed specimen *% Average Epidote Biotite FIGURE 74.-Model composition of hornblendic rocks of the Inner Piedmont belt. Numbers of analyzed specimens refer to analyses in table 23. A, Proportions of quartz, plagioclase, hornblende, and epidote in amphibolites. Based on counts of 50 random grains in each of 25 thin sections. Contours 4, 8, 17, and 33 percent. B, Proportions of quartz, plagioclase, hornblende and biotite in biotite-hornblende gneisses. Based on point counts of analyzed specimens and counts of 50 random grains in each of 22 other thin sections. Contours 4, 7, and 15 percent. 128 phibolite and biotite-hornblende gneiss and average modal compositions of these rocks are given in table 23. Modal composition diagrams showing range in proportions of the principal minerals in the biotite- hornblende gneiss and in amphibolite are given in figure 74. SILLIMANITE SCHIST Sillimanite schist interlayered with sillimanite- bearing quartzite and containing a few pods and lay- ers of cale-silicate rocks forms an extensive map unit in the southeastern part of the Lenoir quadrangle. Similar sillimanite schist occurs in layers and lenses interleaved with biotite gneiss northwest of the main body. The sillimanite schist is a strongly foliated lus- trous rock composed principally of sillimanite, mus- covite, biotite, and quartz. It is light to medium gray and commonly has a faint purple cast on slightly weathered surfaces. Aggregates of fine sillimanite needles give the foliation surfaces a silky luster. Por- phyroblasts of muscovite, biotite, and garnet are con- spicuous in most hand specimens, and lenses, knots, and thin folia of quartz and feldspar are ubiquitous. In many outcrops, foliation in the schist is corru- gated by chevron folds, having amplitudes of as much as an inch. Locally, sillimanite needles are rudely alined with the axes of these folds, but else- where sillimanite is randomly arrayed on the folia- tion surfaces and is wrapped around the fold axes. Much of the sillimanite schist is partially or com- pletely altered to lustrous silvery chlorite-sericite schist or phyllite in which sericite pseudomorphs of sillimanite are recognizable. In these rocks, prisms of . black tourmaline as much as 1 ecm long are abundant. The unaltered sillimanite schist consists of a de- formed mosaic of intergrown dark-brown to black biotite flakes as much as 1.0 mm long and sillimanite needles and fibrolite aggregates, some of which par- tially replace biotite. The biotite and sillimanite are bent around noses of small chevron folds visible in hand specimen. Undeformed flakes of reddish-brown biotite as much as 1.0 mm in diameter and skeletal crystals of muscovite as much as 5 mm long form scattered porphyroblasts in the older deformed bio- tite-sillimanite mosaic (fig. 754). Skeletal grains and aggregates of garnet, some as much as 3 ecm in diameter, and a few large undeformed prisms of sil- limanite also seem to be postkinetic. Both musco- vite and garnet contain unrotated helicitic trains of fine sillimanite needles which have the pattern of the chevron folds. Grains of anhedral quartz 0.25 to 1.0 mm in diameter form segregation folia interleaved with the old biotite and sillimanite. These folia are GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE FIGURE 75.-Photomicrographs of sillimanite schist. A, Quartz-plagioclase-muscovite-sillimantie-biotite schist from south side of Rhodiss Lake about 1.6 miles west of Castle Bridge (area J-9, pl. 1). Sillimanite needles and fibrolite bundles partly parallel plane of section and bent around folds and are partly perpendicular to plane of section and parallel fold axes. Biotite in polygonal ares. Muscovite in polygonal ares and as postkinematic porphyroblasts. Quartz and plagioclase (An,,) concentrated in segregation. Polar- izers at 45°. B, Altered sillimanite schist. Garnet-quartz- chlorite-sericite schist from roadcut 0.6 mile northwest of center of Drexel (area I-9, pl. 1). Sericite pseudomorphous after sillimanite, and chlorite, after biotite. Some silliman- ite and chlorite included in garnet rims and in quartz. Polarizers at 45°. wrapped around the noses of the chevron folds, but it is difficult to tell whether they formed during or after the folding. The quartz folia commonly contain scattered flakes of postkinematic biotite and a few INNER PIEDMONT BELT anhedral grains of plagioclase (An..). The larger quartz-feldspar pods and knots in the schist contain as much as 25 percent plagioclase of the same compo- sition. Principal stable accessory minerals in the sil- limanite schist are zircon and magnetite. Rutile occurs in a few specimens. Leucoxene forms fringes on biotite; garnet is commonly partly altered to li- monite, and plagioclase, to clay minerals. Where the sillimanite schist has been altered, silli- manite is replaced by pseudomorphs or structureless felted aggregates of sericite, biotite is replaced by chlorite, and garnet is partly or completely chlori- tized (fig. 75B). In spite of the alteration, the micro- scopic structure and texture of the original silliman- ite schist is generally well preserved. A few needles of sillimanite are preserved as inclusions in musco- vite porphyoblasts and in the cores of garnets. Sub- hedral to euhedral prisms of porphyroblastic tour- maline are abundant in some of the altered rocks. Tourmaline is black in hand specimen and colorless to yellowish brown in thin section. Many grains have light-green rims. A chemical analysis of one specimen of altered sil- limanite schist and average modal compositions of altered and unaltered sillimanite schist are given in table 24. OTHER INTERLAYERED ROCKS Layers and pods of impure quartzite, quartz schist, cale-silicate rocks, anthophyllite gneiss, and marble are locally intercalated with the layered gneisses and mica schist. These rocks constitute a very minor proportion of the layered rocks, but they are important as indicators of the origin of the rocks enclosing them. Stable mineral assemblages in these rocks are listed in table 25. QUARTZITE AND QUARTZ SCHIST Feldspathic quartzite (Used here as a general term for quartz-rich gneiss or quartz granofels.) and quartz schist occur as layers and pods interleaved with biotite gneiss, mica schist, sillimanite schist, hornblende gneiss, and amphibolite. The quartzite occurs as isolated pods and as continuous layers 1 inch 2 feet thick and is most commonly interlayered with sillimanite schist or biotite gneiss. Quartz schist forms layers as much as 20 feet thick and is gener- ally interlayered with mica schist. The quartzite is a medium- to dark-gray sugary to vitreous fine- to medium-grained rock, commonly thinly layered, but only rudely foliated. It consists of 60 to 80 percent anhedral quartz grains 0.5 to 1.0 mm in diameter, 0 to 20 percent plagioclase, and sub- 129 TABLE 24.-Chemical analysis and modes of sillimanite schist [Chemical analyses determined by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey. Mode of analyzed sample determined by count of 600 points; P, present but not inter- sected in counting; Tr, trace. Major oxides given in weight percent; modes, in volume percent] 1 2: 3 : Fida Mo. LAT. ele orl eich nn ens so-1480- -_ L...... Laboratory No-... ei ech v cee un 160148 : .. Major oxides vee cire dl - ari tne abides 40.0 |_ L_ cs .A LZ e dva wank S2. 1... cC 90s. Am:: / ct e v de Fe.. .. S. ce- bes aan er ine aet oil MgO:->:z. s: ccna cke canes s 20 Cad.s. sell l mera el elect 2 cis wed :l etl ere lle el ...n. kates ens . . poot eca s ee moans " TsO. el ee o n Laan Hay noe cale ae awn Bii. SES gis t.2 -Lisa ut sn _s . (Ad - . 1103: s. c ein c aan 1.7 col Nels adage T se eN Un aaa a de aan a ial CALE LGI... MnOF: :).: rd caeues abs a h L dike Monee esen s Souths w c coll ree it Pe den celle a clan ceil a O5: cP. n anc Total-: :=} 99 ~: c Modes Q 8 29.0 2.4 Plagioclase... cet 74 R: 2.0 Biotite: sc sls c rt L Me taa beer ie Ee 31.1 Muscovite. :L ee 61 52.0 16.2 2.2 #: 40 . 4 Garnet. 2220.22. A ental en ae 4 2.5 2.0 Chlorites: ! Perla einai 18 $T c eee Limonite and leucoxene_____________ 1:7 2.5 4.6 T TCONn 22 l oval ca aad o a oen P Tr "Ir Tourmaline. ... rel. .5 Ir Z- clin eer P Tr."! - RAutile .s. _ sun" to nod o LO eeu L L recs Tr Sphene: c: cls: lsc nner IELO -D- th nes a e e a ela Opaque minerals.......-....._..si.. 6 2.8 1.2 ! Average of 50 random grains counted in each of eight thin sections of hydro- thermally altered sillimanite schists. 2 Average of 50 random grains counted in each of five thin sections of unaltered sillimanite schists. 1. Sillimanite-bearing chloride-muscovite schist. Lustrous light-gray fine- grained schist containing abundant small pegmatite pods and quartz segregations. Sillimanite needles and sericite pseudomorphs after sili- manite are randomly oriented on foliation planes. Rock consists of alined felted aggregates of sericite probably replacing sillimanite bun- dles, scattered porphyroblasts of garnet partly replaced by chlorite, and aggregates of anhedral quartz in segregation folia 1 to 2 mm in diame- ter. Small plates and irregular grains of opaque minerals are dissemi- nated in sericite aggregates. Outcrop on south side of road 0.25 mile east of Hoyle Creek, 1.7 mile S. 46° W. of south end of Castle Bridge (area J-9, pl. 1). ordinate amounts of epidote, garnet, hornblende, and biotite. Sphene, zircon, rutile, and opaque minerals are common accessories. The quartz is commonly deeply sutured and has a rude dimensional orienta- tion parallel to the foliation, which is commonly de- fined by folia of coarser and finer grained quartz mosaic. Plagioclase occurs in seattered untwinned grains in the quartz mosaic. It is generally sodic an- desine. Colorless epidote having low to moderate bi- 130 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE TABLE 25.-Stable-mineral assemblages in quartzite, quartz schist, cale-silicate rocks, and anthophyllite gneiss in the Inner Piedmont belt not affected by shearing along the Brevard fault zone [Stable accessory minerals omitted; X, present] Assemblage Quartz - Plagioclase Hornblende Biotite Musco- Epi- Garnet Diop- Sillim- Antho- (An content) (color parallel to Z) (color parallel to Z) vite dote side anite phyllite Quartzite...:....-... Blue green. -__._._._ }.. lir lout ~ X feine moras oo 2. Pun sk. X ea ee benet ne cans tie oo nad be enne anc folle Ln ign ende c aes ble as O:... X 26. Blue green...:...... Yellow brown........_; RD Ce- rea be anita mee as oie whale a - X 55 Light gray green.... Red fe reino ceri ua t 5.22. AC sa nenas s so po clean blaa ale do. e s on. a aao rang :a s dle . 6...:.2. X SS et en iio conect in Re aM m e iain e an! o oes a dale + aa Quartz schist... ..__.. N nae ae ne ae Pence ween aln bee in ae ba ae tu ap s are H toil iano allel oul l AMIE ee de aas cle as ane - o + oink eel anl Light brown...... X erals. . vee a ee se ain a o aam a ale a 9... P a ie rare SLE o - 2a wale a Wale aa n oin X ee AM ried ica ons 10... .. e r cae ne d ua a ued anal o Red brown... -.... INEC C stern: Cale-silicate rocks____ 11.___. X $0 Greenish doles c lenses X o aes cns uaa ons tas... X $0 ..... do.. slc ct null olds 10. eled ias % eal an an hie we cnt 19... :s. X BT.. Dark green.... lol u X s onar dell: 14. fuse X Dark l_. ca. Kaa ) Ale bowen n vae 19.. cs err eee einen ben Stents vie ona cca rE co tines cue acla e $9 22.2 innu l oe s hak alie bcd o aa aaa ie ite o ae maas ae n are oen onn 2h X Anthophyllite gneiss__ 16_____ X refringence occurs in scattered irregular skeletal grains 0.05 to 0.5 mm in diameter, commonly concen- trated in thin layers. Garnet forms anhedral to sub- hedral poikilitic grains 0.1 to 0.5 mm in diameter, commonly forming aggregates parallel to layering. Anhedral grains of hornblende and biotite interstiti- al to quartz and plagioclase are irregularly distrib- uted and randomly oriented. Quartzite layers in silli- manite schist commonly contain fibrolite aggregates alined with the foliation and scattered sillimanite needles in quartz. The quartz schist is a medium- to coarse-grained light-gray to light-pink strongly foliated lustrous rock; it is most commonly intercalated with mica schist, but it is also interlayered in biotite and bio- tite-hornblende gneiss. The layers range from a few inches to as much as 30 feet in thickness; locally, quartz schist makes up a considerable proportion of the rocks mapped as mica schist. The rock consists of 60 to 80 percent quartz, 5 to 25 percent muscovite, 0 to 10 percent biotite, and generally contains small amounts of garnet and plagioclase. Sillimanite and staurolite occur in some specimens, and zircon, apa- tite, pyrite, and magnetite are common accessory minerals. An average modal composition of quartz schist is given in column 4, table 21. As muscovite content decreases, the quartz schist is composition- ally gradational into quartzite; as biotite and plagio- clase content increases, the quartz schist is grada- tional into mica schist. Quartz forms an inequigranular mosaic of sutured elongate anhedral grains 0.1 to 3.0 mm long and seg- regation laminae of larger grains alined with the foliation. A few anhedral grains of calcic oligoclase or sodic andesine are scattered in the quartz mosaic. Muscovite occurs in synkinematic and postkinematic flakes 0.25 to 5 mm long, forming redundant folia or scattered through the quartz mosaic. Some muscovite books contain abundant poikilitic inclusions of quartz. Biotite forms scattered synkinematic flakes, as much as 3 mm long, generally smaller than mus- covite and interleaved with it. Garnet forms subhed- ral to anhedral porphyroblasts, as much as 3 mm in diameter, many of them containing abundant poikili- tic inclusions of quartz and magnetite. Sillimanite forms fibrolite aggregates in muscovite and biotite and scattered rudely alined needles in quartz. In polymetamorphic quartz schist in the belt of shearing and metamorphism along the Brevard fault zone, muscovite forms wedge-shaped porphyroclasts, biotite is largely recrystallized to small anhedral grains interstitial to quartz, and quartz is intensely strained and partially recrystallized. Garnets com- monly have conspicuous snowball streture. In a few places in the belt of polymetamorphic rocks, the quartz schist contains scattered porphyroblasts or porphyroclasts of poikilitic staurolite. CALC-SILICATE ROCKS Fine- to medium-grained medium-gray, greenish- gray, or yellowish-green cale-silicate granofelses form layers or pods 1 inch to 2 feet thick in the gneisses and schists. They are most commonly inter- leaved with hornblende gneiss and amphibolite but also occur in biotite gneiss and sillimanite schist. They consist of various proportions of quartz, pla- gioclase, epidote, hornblende, pyroxene, and garnet. Some cale-silicate rocks contain as much as 50 per- cent quartz and are compositionally gradational with quartzites. Others lack quartz and are composed en- INNER PIEDMONT BELT tirely of plagioclase, pyroxene and epidote, or plagio- clase and epidote. Contacts of the layers are sharp, and their composition, internal layering, and conti- nuity suggest that they were originally sedimentary beds, rather than metamorphic segregations. Textures are generally granular. Quartz and pla- gioclase form mosaics of anhedral sutured grains 0.05 to 0.5 mm in diameter or occur as scattered grains interstitial to epidote, pyroxene, and garnet. Plagioclase ranges from sodic andesine in some rocks to bytownite in others. Epidote forms irregular to subhedral skeletal grains as much as 1 mm in diame- ter. It is generally colorless and has moderate to low birefringence and anomalous interference colors. In some of the cale-silicate rocks, however, it is pleo- chroic in shades of yellow and has high birefring- ence. Pyroxene is colorless to light-apple-green diop- side that forms anhedral poikilitic grains as much as 2 mm in diameter. Hornblende occurs as poikilitic grains generally less than 0.5 mm in diameter. In some rocks it seems to be contemporaneous with diopside, but in others it forms partial jackets around diopside. Garnet forms subhedral to anhedral skele- tal grains generally less than 1 mm in diameter. Sphene, apatite, and magnetite are common acces- sory minerals, and zircon occurs in a few specimens. Near the Brevard fault zone, plagioclase in some rocks is entirely altered to albite and clinozoisite, and hornblende, garnet, and pyroxene are partly al- tered to chlorite. ANTHOPHYLLITE GNEISS In outcrops on the east side of Canoe Creek, 1.2 miles S. 54°W. of Oak Hill School (area F-9, pl. 1), anthophyllite gneiss is interlayered with biotite gneiss, hornblende gneiss, and amphibolite. The an- thophyllite gneiss is a medium-grained dark-gray strongly foliated rock having sheaves of gray antho- phyllite randomly arrayed on the foliation planes. The layers are a few inches to several feet thick. The rock consists of 5 to 10 percent quartz, 40 to 50 per- cent andesine, and 20 to 30 percent anthophyllite and contains a few flakes of biotite and scattered grains of magnetite. Quartz and plagioclase occur in anhed- ral grains 0.1 to 0.5 mm in diameter. Anthophyllite prisms as much as 1 ecm long lie in the plane of foliation but randomly oriented within it. Antho- phyllite is partly altered to a pale-green monoclinic amphibole and locally, to chlorite. Apatite is a minor accessory mineral. MARBLE Layers and pods of impure calcite marble are in- tercalated with sheared and retrogressively meta- 181 morphosed gneiss, schist, and amphibolite in a few places adjacent to the Brevard fault zone. These lo- calities are in: (1) roadcuts on the south side of the county road south of the Catawba River, 2.1 to 2.3 miles S. 52°W. of the junction of U.S. Highways 70 and 221 northwest of Marion (area A-10, pl. 1), (2) roaducts on the east side of the county road east of the Catawba River, 1.0 mile S. 39°W. of the same highway junction (area A-10), (3) a roadcut on the north side of the county road north of the head of Lake James, 0.3 mile N. 58° W. of the center of the Clinchfield Railroad trestle over the head of the lake (area C-10), (4) outcrops on the northwest shore of Lake James, 2.1 miles N. 39° E. of the same trestle (area C-10), (5) roadcuts in area D-9 on the north side of the county road north of Lake James, 1.1 miles S. 84° E. of the U.S. Geological Survey gag- ing station at the mouth of the Linville River (area E-9, pl. 1), and (6) outcrops southeast of the Johns River, 0.5 mile S. 83° E. of Collettsville (area H-7, pl. 1). The first three of these occurrences of marble have been described by Conrad (1960), who de- scribed a similar occurrence about 5 miles southwest of Marion. The marble is a fine- to medium-grained medium- gray rudely foliated rock, commonly having a light- blue or purple cast on fresh surfaces. Porphyroclasts of muscovite and flakes of red-brown biotite as much as 5 mm in diameter are conspicuous in hand speci- mens, and the marble contains thin micaceous part- ings and quartz-feldspar folia, many of them highly contorted. The marlbe occurs in pods and layers con- formable with the layering in the enclosing schist and gneiss. Contacts with the enclosing rocks are commonly gradational but are locally sharp. The ex- posed marble layers range from a few inches to at least 30 feet thick, and Conrad (1960) reported that a diamond-drill hole near the outcrop 1.0 mile south- west of the junction of U.S. Highways 70 and 221 penetrated nearly 130 feet of marble. The marble is interlayered with biotite gneiss, quartz-feldspar gneiss, biotite-muscovite-chlorite schist, pegmatite, and amphibolite, some of which contains diopside. The enclosing rocks have well-developed cataclastic textures and structures related to shearing and re- trogressive metamorphism along the Brevard fault zone, but these features are not everywhere conspic- uous in the marble itself. The marble has a granular to rudely foliated tex- ture. Inequigranular grains of twinned calcite 0.25 to 1.0 mm in diameter form 25 to 90 percent of the rock. Quartz and plagioclase occur in scattered an- 132 hedral grains interstitial to calcite and in thin layers and folia parallel to the foliation. Muscovite occurs as wedge-shaped deformed porphyroclasts, as much as 1 mm long, and as small synkinematic or post- kinematic flakes 0.1 to 0.2 mm long. Reddish-brown biotite forms synkinetimatic and postkinematic flakes and anhedral grains, as much as 0.25 mm long, generally concentrated in the quartz-plagioclase layers. Scattered granules of clinozoisite are common in some specimens, and apatite, sphene, and magne- tite are common accessory minerals. The marble near Collettsville contains porphyroclasts of micro- cline as much as 2 mm in diameter, fringed with myrmekite, and abundant chlorite derived from al- teration of biotite. Plagioclase is clear and well twinned, and some is faintly zoned. Its composition is about Ans;. The carbonate has the optical properties of calcite, but a partial chemical analysis of the marble near Marion quoted by Conrad (1960) indicates that 11 percent of weight of the total carbonate in the rock is MgCO;s. If no disseminated dolomite has been overlooked in the grain mounts, the carbonate must be a highly magnesian calcite. In all the known outcrops, the marble is associated with polymetamorphic rocks within a few hundred yards of the Brevard fault zone, and it is not clear which minerals crystallized during metamorphism prior to faulting and which erystallized during shearing and metamorphism during movement along the fault zone. Mineral assemblages in the marble are therefore not listed in table 25. ORIGIN Shearing and metamorphism have completely obli- terated any textures and structures that might have indicated the nature of the original materials from which the layered rocks of the Inner Piedmont were derived. The layered character of the sequence sug- gests that the rocks were derived from a stratified sequence, but deformation has obscured or obliter- ated original stratigraphic relationships, and even the contacts between individual layers are probably largely tectonic. There is no evidence of any impor- tant modification of the bulk composition of the lay- ered rocks during metamorphism, and the chemistry of the rocks, therefore, furnished the only clues, however tenuous, to the protoliths from which they were derived. Some of the rocks in the Inner Piedmont are clearly of sedimentary origin. These include the quartzite, quartz schist, marble, sillimanite schist, and some of the amphibolites and the lime-silicate GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE rocks containing diopside and bytownite. The origin of the remainder of the gneisses, schists, and amphi- bolites that make up the bulk of the layered rocks of the Inner Piedmont is less obvious. Terranes similar to the layered rocks of the Inner Piedmont are commonly believed to have been pro- duced by metamorphism of thick eugeosyneclinal accumulations of graywackes and volcanic rocks. To test this hypothesis, chemical analyses of gneisses and schists from the Inner Piedmont are compared with analyses of rocks from the Aleutian Islands (Hamilton, 1963, p. 70-71), the Carolina slate belt in the Albermarle-Denton area, North Carolina (A. A. Stromquist, written comm., 1960), and with analyses of graywackes quoted by Pettijohn (1963, table 6). Hamilton (1963, p. 70-71) has briefly described the geology of the Aleutian Islands and has summa- rized a large number of recent rock analyses. The Aleutians are a chain of volcanic islands formed of lavas and tuffs, of sedimentary rocks derived directly from them, and of dikes, sills, and plutons intrusive into them. Two chief groups of rocks are exposed : older eugeosynelinal submarine lavas and tuffs, gen- erally altered and partly reworked by water, and younger unaltered subaerial flows and tuffs uncon- formably overlying the eugeosynclinal rocks and forming great stratovolcanoes, some of which are still active. Where exposed on the islands, the sub- marine volcanic rocks have yielded fossils of various ages from Pennsylvanian to Tertiary. The subaerial volcanic rocks range in age from later Tertiary to Recent and are predominantly andesite with lesser dacite and basalt and rare rhyodacite and quartz la- tite. Compositionally, the submarine volcanic rocks are chiefly andesite, basalt, dacite, and their albitized equivalents, keratophyre, spilite, and quartz kerato- phyre. Rhyodacite and quartz latite are present but are uncommon. The Carolina slate belt is a group of volcanic and associated sedimentary rocks of low metamorphic grade that extends from southern Virginia south- ward across the Carolinas into northern Georgia (King, 1955, p. 343). The rocks of the slate belt form a sequence of alternating rhyolitic to basaltic vol- canic rocks, interbedded with argillite, siltstone, sandstone, and their tuffaceous equivalents, and in- truded by dikes, stocks, sills, and batholiths of grani- tic and gabbroic rocks. Some of the volcanic and sed- imentary rocks are probably submarine, but others are clearly subaerial (Stromquist and Conley, 1959; Sundelius, 1963; Conley and Bain, 1965). Some of the rocks in the Albemarle-Denton area have yielded zircon that has been dated as Ordovician (White and INNER PIEDMONT BELT 133 FIGURE 76.-Silica variation diagrams comparing layered gneisses, schists, and amphibolites of the Inner Piedmont belt with volcanic rocks of the Aleutian Islands, rocks of T I I I T A f the Carolina slate belt, and graywackes. Data for the Aleutian rocks from Hamilton (1963); for the slate-belt Siltstone and ler, 1964), but many are of doubtful reliability and the specimens are inadequately described. A group of 20 |- msg 7/\ argillite f rocks, from A. A. Stromquist, U.S. Geological Survey; - a {8532325 f and for graywackes, from Pettijohn (1963). Amphibolite “20315: ® n and hornblende gneiss analyses from table 23; biotite - SIX Z gneiss, table 20; mica schist, table 21. BSC A others, 1963). Several distinct depositional sequences 6 i have been recognized, and rocks of widely different 10} 7 ages may be present. Many chemical analyses of Z E2085 F 7 rocks from the slate belt have been published (But- W WEIGHT PERCENT 0 unpubiished modern analyses of well-described rocks from the Albemarle-Denton area has kindly been made available by A. A. Stromquist, U.S. Geological Survey, and have been selected for comparison. Analyses of 10 graywackes, many of them asso- ciated with tuffs and submarine volcanic rocks in eugeosynclinal environments, are quoted by Petti- john (1963, table 6) and are used for comparison. These rocks are from a variety of geologic provinces and range in age from Precambrian to Eocene. The silica variation diagrams (fig. 76) show that most of the biotite and hornblende gneisses of the Inner Piedmont are chemically comparable to the ofm > > CaO o pdc 1 P114 1314 Jullullllul Hamm -mm 3 volcanic rocks of the Aleutian Islands and to some of -~ __ _._ j- | the rocks of the Carolina slate belt, as well as to the fran «ilng- § graywackes. Their compositions most closely ap- proach those of the subaerial volcanic rocks of the Na,0 5 EHT T OI ‘f A *% I< - Aleutians, but many of them tend to be slightly a richer in Al,0O; and slightly lower in Na,0. They o mresmea® --- fp J/ 4 ~C] tend to show a complete range of SiO; content, hav- = ing unimodal distribution, from hornblende gneisses resembling andesites in composition to biotite gneisses resembling rhyodacites and having a broad ”X 0 TTT 1 IN WEIGHT PERCENT EXPLANATION frequency maximum in the range of 65 to 70 percent. ROCKS OF THE INNER In the Aleutian suite, the SiO, content also has a PIEDMONT BELT s "as P P P : a unimodal distribution, but it has a frequency maxi- Amph‘etmme mum in the range of 55 to 60 percent, whereas the Biotite gneiss gneisses of the Inner Piedmont more commonly have sees: SiO, contents in the range of 65 to 70 percent. aimless Many of the more silicic gneisses closely resemble + the felsic volcanic rocks of the slate belt, but in the Mica schist slate belt, at least in the Albemarle-Denton area, the fins eres r e dare SiO, content has a strong bimodal distribution, and pep orl 0s volcanic rocks having SiO, contents in the range of 55 to 65 percent are absent. Siltstones and argillites Trend line for subaerial volcanic rocks in the slate belt commonly have SiO; contents in the from the Aleutian Islands latter range, but the gneisses of the Inner Piedmont © having comparable SiO, contents are distinctly Range of submarine volcanic rocks F k f d from the Aleutian Islands richer in CaO and somewhat richer in Na,0. 134 Most of the graywackes for which analyses are quoted by Pettijohn (1963, table 6) have SiO, con- tents in the range 65 to 75 percent. Inner Piedmont gneisses having SiO, contents in this range are ap- preciably richer in Al;O;, somewhat richer in CaO, and poorer in MgO and total iron. Most of the layered gneisses have an excess of Na,0 over K0, a feature that is very characteristic of graywackes (Middleton, 1960; Pettijohn, 1963). Figure 77 shows, however, that both the subaerial volcanic rocks of the Aleutian Islands and the rocks of the Carolina slate belt also have Na,0 : K,0 ratios are greater than 1, so that this property cannot be used to distinguish volcanic rocks from graywackes. The submarine volcanic rocks of the Aleutians have Na,0 :K,0 ratios greater than 1, but the ratios are quite variable because of extensive albitization, and they are therefore not shown on the diagram. Figure 78 is a plot of the molecular proportions of alumina, lime, and total alkalies in the various rocks. The gneisses of the Inner Piedmont fall in a trend very similar to the trend line for the Aleutian sub- aerial volcanic rocks, and many plot in positions intermediate between the felsic and mafic volcanic rocks of the slate belt. Some fall in the same general field as the graywackes, but as a group the gneisses seem to plot along a volcanic trend. 6- $- Z LJ £ 4- / a s Trend line for subaerial f ha volcanic rocks of the C Aleutian Islands G a 3- Ase Tax ® 4 PLN = ert x /0 ! < ( Siltstone and j ra \ argillite he." I® 9. a)= \\ yet @ Felsic ~ 3 fe xe af flows and *y tuffs 1} / \\\_//) % A as 4 ig A /Greenstone/ ces let & 1 | 0 1 2 3 4 5 6 Na,0, IN WEIGHT PERCENT FIGURE 77.-Na,0:K,0 variation diagram comparing layered gneisses, schists, and amphibolites of the Inner Piedmont belt with volcanic rocks of the Aleution Islands and rocks of the Carolina slate belt. Symbols and sources of data are the same as in figure 76. GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE A|203 / Trend line for Siltstone subaerial vol- and/Y $ canic rocks of argillite f thle Aleutian Islands |+¢0 e 2 I+ Felsic flows and tuffs Na,0O + Kzo Cao FiguUrE 78.-Molecular proportions of Al,O,, CaO, and total alkalies in layered gneisses, schists, and amphibolites of the Inner Piedmont belt compared with subaerial volcanic rocks of the Aleutian Islands, rocks of the Carolina slate belt, and graywackes. Symbols and sources of data same as in figure 76. The two analyses of mica schist and the three anal- yses of amphibolite do not fit trends of the biotite and hornblende gneisses in figures 76, 77, and 78. The schists have higher total Fe and KO and lower Na,0 and CaO contents than gneisses of comparable SiO, content, and one of them has low Al;O; and high MgO contents. One of the amphibolites has a lower SiO; content than any of the volcanic rocks of the Aleutians or the slate belt. The other two amphib- olites have higher MgO and CaO and lower Na,0 contents than any of the volcanic rocks of similar SiO, content. One of them also contains considerably less AlsO;. The chemical anlayses suggest that many of the biotite and hornblende gneisses and perhaps some of the amphibolite could have been produced by meta- morphism of volcanic rocks similar to those of the Aleutian volcanic are but that some of the biotite gneisses may also have been derived from gray- wackes. The mica schists were probably interbedded shales or argillites, and at least some of the amphib- olites were shaly dolomites. Also interlayered with the volcanic-graywacke sequence were calcareous and argillaceous sandstone (now metamorphosed to quartzite and quartz schist) and impure dolomitic limestone (now metamorphosed to marble, cale-sili- cate rocks, or anthophyllite gneiss). The sillimanite schist must originally have been an aluminous shale, INNER PIEDMONT BELT perhaps derived from clays produced during chemi- cal weathering of volcanic rocks. AGE The age of the layered rocks of the Inner Pied- mont belt in the Grandfather Mountain area is not obvious. The rocks are, however, older than the mid- dle or lower Paleozoic granitic rocks that invade them. Keith (1905) mapped the biotite gneisses and mica schists in the adjacent part of the Mount Mitchell quadrangle as Carolina Gneiss, and the hornblende gneiss and amphibolite as Roan Gneiss, both of which he believed to be of Archean or early Precambrian age. Both terms have now been aban- doned. Many of the rocks so mapped in the Blue Ridge are clearly of early Precambrian age, but there is no evidence that the rocks mapped by Keith as Carolina and Roan in the Inner Piedmont belt are of the same age. Recent work in central Virginia (Bloomer, 1950; Brown, 1958; Espenshade, 1954; Smith and others, 1964) has shown that large parts of the Piedmont are underlain by sedimentary and volcanic rocks of late Precambrian and early Paleozoic age. According to Kesler (1944) and Overstreet and Bell (1960; 1965, p. 100-102, 108-109), many of the rocks in the Inner Piedmont belt in North Carolina and South Carolina may be of late Precambrian or early Paleo- zoic age, and some of them may be equivalents of less metamorphosed Paleozoic rocks in the Kings Moun- tain belt and the Carolina slate belt. The layered rocks of the Inner Piedmont belt in the Grandfather Mountain area are therefore probably of late Pre- cambrian or early Paleozoic age, and rocks of both ages may be present. METAMORPHISM The layered rocks of the Inner Piedmont belt are of high metamorphic grade, except where they have been affected by shearing and retrogressive meta- morphism associated with the Brevard fault zone or by local shearing and hydrothermal alteration. They contain a complex array of mineral assemblages (ta- bles 18, 21, and 24), which occur in interlayered rocks and which have no conspicuous pattern of dis- tribution that would suggest a regional metamorphic gradient. It is therefore assumed that the high-grade regional metamorphism of the layered rocks took place under conditions that were rather uniform within the area studied. The diversity and complexity of the composition and mineralogy of the layered rocks and the lack of 135 detailed data on compositions of the coexisting min- erals precludes adequate representation of the min- eral assemblages on phase diagrams. A schematic ACF diagram (fig. 79), however, illustrates the general paragenetic relationships. The layered rocks contain mineral assemblages that in general correspond to the cooler part of the sillimanite zone and to the almandine-amphibolite facies of Turner (Fyfe and others, 1958), but the ACF diagram (fig. 79) does not agree closely with those for any of the subfacies described by Turner. The appearance of the apparently stable mineral pairs-sillimanite-muscovite, epidote-diopside, and epidcte-plagioclase-suggests pressures and temper- atures somewhat lower or P;,., somewhat higher than hose characteristic of Turner's sillimanite-al- mand ne subfacies, but the occurrence of sillimanite instead of kyanite and the absence of staurolite in rocks of suitable composition do not correspond to either the kyanite-muscovite-quartz or staurolite- quartz subfacies. Shearing and recrystallization of the rocks along the Brevard fault zone have produced polymeta- morphic rocks, many of which contain parts of two or more mineral assemblages. The widespread occurrence of recrystallized biotite, muscovite, oligo- clase, and epidote and the presence of porphyroclasts of staurolite and kyanite in some of the rocks sug- gest that the initial recrystallization was under me- dium-grade conditions, perhaps corresponding to the low-intensity part of the almandine-amphibolite facies of Turner. Later shearing, largely concen- trated closer to the Brevard fault zone, has resulted in further retrogression, producing low-grade assem- blages containing sericite, chlorite, epidote, and al- bite, which are characteristic of Turner's greenschist facies. HENDERSON GNEISS The Henderson Gneiss is a biotite quartz monzon- ite augen gneiss that forms pods and elongate lenses concordant with foliation and layering in the enclos- ing layered rocks (pl. 1). It is most common in the belt of polymetamorphic rocks southeast of the Bre- vard fault zone, but it also occurs in a few sinall bodies among the unsheared rocks farther southeast. The name Henderson Gneiss has been applied to the gneiss in the Grandfather Mountain area because of its strong lithologic similarities to and apparent con- tinuity with rocks mapped as Henderson Granite by Keith (1905, 1907b) in Henderson County, N.C., about 30 miles southeast. The lithologic designation 186 (Sillimanite) (Potassic-feldspar) Calc-silicateX¢//\sl \\\ rocks ////>§4/ \v_'\\\ GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE EXPLANATION Analyzed specimens Numbers in parentheses indicate rocks containing nonequilibrium assemblages M1) Sillimanite schist 8 @ Biotite gneiss .(2) Mica schist -5 Hornblende gneiss A2 Amphibolite Compositional fields of principal rock types >= ~ - \ (Almandine) \\ Biotite ~ \ 3‘-\ t _- -S- T7} = = <7 XIAI/Hom blende C FIGURE 79.-Schematic ACF diagram for layered rocks of the Inner Piedmont belt containing excess SiO,. Posi- tions of fields and tie lines largely schematic, but consistent with analyses of rocks studied and published analyses of minerals from rocks of similar metamorphic grade. Possible additional minerals are in parentheses. Compositional fields of the principal rock types outlined by short dashed lines; quartzite, quartz schist, and marble are omitted. Fields containing observed assemblages are numbered. Quartz is an additional phase in all assemblages; epidote appears in all assemblages; epidote appears in all observed fields but not in all assemblages. Garnet occurs in fields I and II in rocks where the molecular proportion of FeQ exceeds that was changed from granite to gneiss by Reed (1964b) to better describe the lithology of the unit. The gneiss is a fine- to medium-grained light- to medium-gray rock, containing abundant light-gray to white augen of potassic feldspar 0.5 to 3.0 cm long. The augen typically have single Carlsbad twins of MgO. Potassic feldspar occurs in fields I and II in rocks in which the molecular ratio of excess Al,O,:K,0 is less than 3. Sillimanite occurs in field I in rocks containing high proportions of Al,O,, but the ratios controlling its appearance in rocks of the Grandfather Mountain area are not determined by the data available. Analyses of similar rocks by Shaw (1956) indicate it is confined to rocks in which the molecular ratio Al,0,:Na,0 +K,0 + CaO exceeds 2. Occurrence of sphene is limited to field III. Other accesory minerals appear in all fields. Sillimanite schist analyses from table 24; biotite gneiss, table 20; mica schist, table 21; hornblende gneiss and amphibolite, table 23. and are rimmed by a thin chalky-white jacket of quartz and plagioclase. Where the rock has not been subjected to shearing associated with the Brevard fault zone, it is weakly foliated, and the augen are ovoid in outline and rudely alined or randomly ar- rayed. Most bodies of Henderson Gneiss, however, are in the belt of polymetamorphic rocks associated INNER PIEDMONT BELT with the Brevard fault zone (pl. 6C, D). There the gneiss is strongly foliated and lineated, and the feld- spar augen are reduced to lentil-shaped porphyro- clasts, flattened in the foliation and having their long axes alined with the cataclastic lineation. The gneiss is typically nonlayered in the centers of the larger bodies, but it commonly becomes rudely layered near the contacts and passes gradationally into layered gneisses containing scattered potassic feldspar augen. This transition is most conspicuous along strike from the ends of the larger lens-shaped bodies. Layered biotite gneiss near bodies of Henderson Gneiss commonly contains layers a few inches to sev- eral feet thick containing abundant augen of potassic feldspar similar to those in the Henderson but gener- ally in a finer grained and darker, more biotitic groundmass. Where the gneiss is unsheared, the matrix consists of an inequigranular mosaic of anhedral grains of sodic andesine and microcline 0.5 to 2 mm in diame- ter, strained quartz 0.1 to 2.0 mm in diameter, and rudely alined irregular flakes of reddish-brown bio- tite as much as 2.0 mm long. Apatite occurs as scat- tered prisms, irregular aggregates, and commonly as inclusions in biotite; allanite forms small inclusions surrounded by pleochroic halos in biotite. Other accessory minerals are sphene, zircon, and magne- tite. Sericite occurs as an alteration product in the feldspars and as fringes on the ends of biotite books. The feldspar augen are porphyroblasts of perthitic microcline, typically containing rectangular inclu- sions of plagioclase and amoeboid blebs of quartz. The microcline porphyroblasts are partially or en- tirely rimmed by myrmekite, which invades the po- tassic feldspar in wartlike indentations. A few speci- mens also contain porphyroblasts of andesine as much as 1.0 ecm in diameter. Where the gneiss has been sheared and recrystal- lized near the Brevard fault zone, it consists of an inequigranular granoblastic-lepidoblastic mosaic of quartz, plagioclase (chiefly calceic oligoclase), potas- sic feldspar, biotite, muscovite, epidote, and small amounts of chlorite, garnet, and green hornblende. The grains in the mosaic are 0.01 to 0.1 mm in diam- eter. Foliation is defined by segregation lamellae of quartz and feldspar and by diffuse folia of biotite and muscovite (fig. 804). Biotite occurs in small ir- regular flakes and grains and seems to be largely late synkinematic and partly postkinematic. Muscovite seems to be largely contemporaneous with the bio- tite, but a few specimens contain large bent porphy- roclasts of old muscovite. Epidote occurs as seattered grains and poikilitic grains and aggregates; some FIGURE 80.-Photomicrographs of Henderson Gneiss. A, Bio- tite quartz monzonite gneiss from east shore of Lake James about 1 mile west-northwest from River Valley Church (area E-9, pl. 1). Microcline and calcic oligoclase relicts of an earlier generation and recrystalized quartz, calcic oligoclase, biotite, and microcline. Some coarse-grained quartz in segregations. From near southeastern margin of belt of rock containing conspicuous polymetamorphic tex- tures. Outcrop has microcline porphyroclasts as long as 1 cm. Polarizers at 45°. B, Porphyroclastic epidote-biotite- muscovite quartz monzonite gneiss from West side of Upper Creek, 0.2 mile west of east edge of the Linville Falls quad- rangle (area F-8, pl. 1). Large and small porphyroclasts of microcline and smaller porphyroclasts of calcic oligoclase in a matrix of recrystallized quartz, microcline, calcie oligoclase, muscovite, biotite, and epidote. Typical jacket of myrmekite, quartz, and plagioclase on microcline porphyro- clast, which tapers off to spindle-shaped extension parallel with lineation. Polarizers at 45°. 138 TABLE 26.-Chemical analyses, modes, and norms of Hender- son Gneiss and biotite gneiss transitional into Henderson Gneiss [Analysis of sample 2 determined by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey. Other analyses, by standard chemical methods by C. L. Parker and D. F. Powers, U.S. Geol. Survey. Nd, not determined. Modes of anlyzed specimens determined by point counts; P, present but not intersected in counting; Tr, trace. Major oxides and CIPW norms given in weight percent; modes, in volume percent] 1 2 8 4 5+ d 28-1045 ' A-818 15-1061 21-2881 I-4061. 161247 H-8498. H-9482 Field Laboratory No..... Major oxides 67.4 70.37 Mo NE re ¥ as cane oke sls 15.4 14.59 s T2 1.30 3.8 1.48 11 . 61 2.8 1.58 3.8 3.33 2.6 4.95 .89 T2 13 "H1 82 47 38 12 10 .07 .05 .01 Nd Nd Nd Nd 100 99.71 Modes Quartz. .... 20 29 34 37 26.8 30.1 Plagioclase. .. . 40 42 24 31 27.5 82.9 Potassic feldspar 14 6 19 26 20.0 22.4 Biotite: ...... 20 20 10 3 12.3 10.1 Muscovite. 1.5 2.83 9 1.3 8.6 3.5 ATIDHIDOIG®: LOR YL PLD IL » eens EIL 2 Uu. th Tr Epidote. ._. 2.1 4 194 3 .3 Apatite. . .._. .8 .8 Tr v4 Zircon. . . P T $ Tr 1.0 18 Tr Tf Opaque minerals. ___. Tr .5 Tr (G&TREL L2 IR EIL CIU. Pech erik auks Tr Ir Points counted.... 662 600 600 610 5 AoE e- ences die . CIPW norms 1 Average of 50 random grains counted in each of 13 thin sections of fine-grained biotite gneisses containing conspicuous potassic feldspar augen and believed to be marginal phases of Henderson Gneiss. 2 Average of 50 random grains counted in each of 15 thin sections of typical Henderson Gneiss. NotE.-Minor-element analyses for samples, 1, 3, and 4 given in table 1. 1. Marginal facies of Henderson Gneiss. Strongly foliated and lineated non- layeged fine-grained medium-gray biotite-quartz-plagioclase gneiss con- taining abundant ovoid augen of pink potassic feldspar as much as 1 em long, elongated parallel to lineation. Augen are jacketed by a thin mantle of quartz and plagioclase. Texture is conspicuously cataclastic. The augen are porphyroclasts of microcline set in a matrix of recrys- tallized anhedral quartz and plagioclase (Ans?) 0.05 to 0.25 mm in di- ameter and stubby flakes of synkinematic and postkinematic brown bio- tite as much as 0.2 mm long. Plagioclase also occurs in faintly zoned porphyroclasts as much as 3 mm in diameter. Epidote occurs in irregu- lar grains and subhedral prisms less than 0.2 mm long. Muscovite forms small postkinematic flakes. Microcline porphyroclasts are jack- eted by myrmekite partly recrystallized to quartz and plagioclase. Roadcut on southeast side of county road southeast of Yadkin River, 1.0 mile S. 86° E. of village of Happy Valley (area J-6, pl. 1). 2. Typical Henderson Gneiss. Well-foliated nonlayered medium-grained biotite- quartz-plagioclase gneiss containing abundant potassic feldspar augen as much as 3 em long. Augen have single Carlsbad twins and conspicuous thin chalky-white jackets of quartz and plagioclase. Texture is not polymeta- morphic. Porphyroblasts of microcline and of plagioclase (Ans) as much as 0.4 mm in diameter are set in a matrix of inequigranular anhedral quartz 0.1 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE TABLE 26.-Chemical analyses, modes, and norms of Hender- son Gneiss-Continued to 2 mm in diameter and plagioclase (Anse) as much as 0.5 mm in diameter and randomly oriented irregular undeformed flakes of brown biotite as much as 1 mm long. Muscovite forms stubby primary books and scattered secondary small flakes and aggregates in feldpsar. Microcline porphyroblasts are sur- rounded and penetrated by myrmekite. Outcrop about 40 feet above south shore of Rhodiss Lake, 1.6 mile S. 75° E. of south end of Huffman Bridge (area 1-9, pl. 1) 3. Cataclastic Henderson Gneiss. Strongly foliated and lineated nonlayered fine- grained muscovite-biotite-plagioclase-quartz gneiss containing augen of pink potassic feldspar as much as 1 cm long elongated parallel to foliation. Texture is strongly cataclastic. Rock consists of crushed porphyroclasts of microcline and microperthite set in recrystallized matrix of granoblastic grains of quartz and plagioclase (Anis) and well-alined flakes of synkinematic and postkine- matic greenish-brown biotite and muscovite 0.01 to 0.1 mm long. Epidote forms granules and small sieve-textured grains. One sieve-textured grain of garnet 0.25 mm in diameter appears to be a porphyroblast. Roadcut on east side of dirt road on northeast side of Upper Creek, 0.9 mile S. 89° E. of Fair- view Church (area F-8, pl. 1). 4. Henderson Gneiss. Strongly foliated and lineated fine-grained biotite quartz monzonite gneiss containing lentil-shaped potassic feldspar ag- gregates elongated parallel to lineation. Texture is cataclastic. Rock consists of scattered porphyroclasts of microcline as much as 5 mm in diameter set in an inequigranular mosaic of recrystallized granoblastic quartz, plagioclase (An»), and microcline 0.1 to 1.0 mm in diameter and ragged grains of synkinematic and postkinematic olive-green bio- tite and muscovite 0.05 to 0.5 mm long. Garnet forms sieve-textured porphyroblasts as much as 5 mm in diameter, associated with irregular grains of green hornblende as much as 0.5 mm in diameter. Outcrop on south shore of Lake James, 0.4 mile S. 80° W. of Rock Hill Church (area D-9, pl. 1). grains have allanite cores. Some specimens contain small skeletal garnets. The feldspar porphyroclasts are chiefly microcline and microcline-microperthite, but some twinned pla- gioclase porphyroclasts occur. The potassic feldspar porphyroclasts have ragged outlines and are jack- eted by mosaics of quartz, plagioclase, myrmekite, and recrystallized potassic feldspar in which the grains are slightly coarser than in the adjacent groundmass (fig. 80B). Commonly, the porphyroclas- tic grains have been broken and the fractures healed by the mosaic. The chief accessory minerals in the gneiss are magnetite, sphene, apatite, and light-pink or orange zircon. The zircon occurs in slender euhedral prisms, some as long as 0.5 mm. Chemical analyses, modes, and norms of typical specimens of Henderson Gneiss and average modes of Henderson Gneiss and of biotite gneisses transi- tional into Henderson Gneiss are given in table 26. Ranges in proportions of the principal minerals are indicated by modal variation diagrams. (figs. 81 and 82). The Henderson Gneiss contains a much higher proportion of potassic feldspar than the layered bio- tite gneisses, but the proportions of the other miner- als are about the same. Chemically, the Henderson closely resembles biotite gneiss of similar SiO; con- tent, but it contains appreciably more K,O and therefore has a higher K,0/Na.0 ratio and a higher ratio of total alkalies to alumina. The gradational contacts between the Henderson Gneiss and the enclosing rocks and the compositional similarity between them suggest either that the Hen- derson Gneiss may have originated by recrystaili- INNER PIEDMONT BELT Biotite 139 Biotite Plagioclase Muscovite EXPLANATION x2 Chemically analyzed specimen Average typical Henderson Gneiss ® Average transitional Henderson Gneiss Biotite FIGURE 81.-Proportions of quartz, plagioclase, biotite, and muscovite in Henderson Gneiss and biotite gneisses transitional into Henderson Gneiss. Based on point counts of analyzed specimens and counts of 50 random grains in each of 13 thin sections of typical Henderson Gneiss and 11 thin sections of transitional gneisses. Contours 4, 7, 10, 14, and 18 percent. Numbers of analyzed specimens refer to analyses in table 26. 4 zation of slightly more potassium-rich rocks in the original volcanic and sedimentary sequence or that it was the product of the addition of KO to parts of the stratified sequence. The small bodies of un- sheared Henderson Gneiss outside the belt of poly- metamorphic rocks along the Brevard fault zone are all closely associated with rocks similar to the Toluca Quartz Monzonite. In the Causby quarry, on the east side of Hunting Creek, 0.9 mile S. 10° W. of the confluence of the Catawba River and Johns River (area H-9, pl. 1), biotite and biotite-amphibole gneiss adjacent to a small body of biotite granodior- ite has been converted to augen gneiss closely resem- bling the Henderson Gneiss, a fact suggesting that the Henderson Gneiss may be related to the Paleo- zoic granitic rocks. The larger and more abundant bodies of Henderson Gneiss in the belt of polymeta- morphic rocks along the Brevard fault zone are not closely associated with the granitic rocks, although they contain sheared pegmatite bodies containing heayy-mineral suites similar to those of the granitic rocks. The shape of the Henderson bodies in the belt of polymetamorphic rocks has apparently been greatly modified by shearing; many of these rocks 140 QUARTZ EXPLANATION *l Chemically analyzed specimen Average typical Henderson Gneiss it Average transitional Henderson Gneiss PLAGIOCLASE POTASSIC FELDSPAR FiGurE 82.-Proportions of quartz, plagioclase, and potassic feldspar in Henderson Gneiss and biotite gneisses transi- tional into Henderson Gneiss. Based on point counts of analyzed specimens and counts of 50 random grains in each of 13 thin sections of typical Henderson Gneiss and 11 thin sections of transitional gneisses. Contours 4, 7, 10, and 14 percent. Numbers of analyzed specimens refer to analyses in table 26. and the rocks now associated with them may be exotic and may have been carried for long distances during early movement along the Brevard fault zone. It therefore seems unlikely that the origin of the Henderson Gneiss can be satisfactorily determined from the evidence available in the Grandfather Mountain area. This problem might better be at- tacked farther southwest, where Keith (1905, 1907b) has mapped large areas of Henderson Gneiss many miles southeast of the Brevard fault zone. Keith (1905, 1907b) believed that the Henderson Gneiss was of early Precambrian (Archean) age, largely because of its relations to layered gneisses which he also considered to be of early Precambrian age. The enclosing rocks are now believed to be of late Precambrian or early Paleozoic age. Field rela- tions in the Grandfather Mountain area suggest that the Henderson is older than or related to the granitic rocks of early or middle Paleozoic age, and it is therefore believed to be of late Precambrian or early Paleozoic age. GRANITIC ROCKS AND MIGMATITE The layered rocks of the Inner Piedmont are in- vaded by extensive concordant bodies of granitic rocks ranging in composition from quartz diorite to quartz monzonite. The granitic bodies are flanked by diffuse zones of migmatite in the wallrocks, and their emplacement has been accompanied by emplacement GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE of myriad dikes, pods, and knots of pegmatite in the enclosing rocks. Strangely, granitic bodies have not been found in the belt of sheared and retrogressively metamorphosed rocks along the Brevard fault zone, although pegmatites are common in this belt and the large bodies of Henderson Gneiss are confined to it. The granitic rocks closely resemble the Toluca Quartz Monzonite of the Shelby area (Griffitts and Overstreet, 1952; Overstreet and others, 1963) in their general appearance, distribution, structural re- lations, and accessory mineral suite, but they differ from the Toluca in the preponderance of quartz dior- ite and granodiorite over quartz monzonite. The name Toluca Quartz Monzonite is therefore not for- mally applied to the granitic rocks in the Grand- father Mountain area, although the main belt of granitic rocks between Lenoir and Morganton can be traced southwestward from Morganton and is appar- ently continuous with rocks identified as Toluca by Overstreet and Griffitts (1955) along U.S. Highway 64 in the southeastern part of McDowell County. MEGASCOPIC FEATURES AND FIELD RELATIONS The granitic rocks are generally medium to coarse grained and light to medium gray; they are inequi- granular and locally contain porphyroblasts of mi- crocline as much as 2 em long. Many of them are strongly gneissic, but in some of the larger bodies they are massive or very weakly foliated. Foliation is commonly defined by subparallel dark biotitic streaks and by faint parallelism of micas. The folia- tion is generally rudely parallel to foliation and lay- ering in the enclosing rocks, but in some place cross- cutting dikes of granitic rock have planar flow structures parallel to their walls (fig. 83). Inclusions of amphibolite and hornblende gneiss are common in the more massive granitic rocks and are generally surrounded by thin reaction rims of biotite schist. Inclusions of other rocks are rare. Most of the larger granitic bodies are concordant or semiconcordant with the structures in the enclos- ing rocks, but locally the contacts are sharply discor- dant, and small disordant dikes are common (fig. 83A and D). Locally, the contacts are sharp, but the larger granitic bodies more commonly pass into the surrounding rocks through broad migmatitic zones composed of strongly gneissic granitic rock inter- leaved with folia of schist and layered biotite gneiss and containing abundant pods of amphibolite and biotite gneiss (fig. 83B). In the Lenoir quadrangle, the boundary between migmatitic granitic rocks and migmatitic wallrocks has been arbitrarily drawn INNER PIEDMONT BELT 141 FicurE 83.-Migmatite and granitic rocks. A, Contact of discordant dike of fine-grained muscovite-biotite quartz monzonite and related pegmatite cutting strongly foliated migmatitic biotite gneiss containing a pod of dark amphib- olite. Gneiss is feldspathized along joints extending into it from the granitic rock. Fresh rock exposed in abandoned quarry 200 feet southwest of U.S. Highway 321, 0.1 mile north of southern city limits of Lenoir (area J-7, pl. 1). B, Pods of amphibolite in migmatite composed of inter- leaved layered biotite gneiss and streaky amphibole-bearing biotite quartz diorite. Saprolite exposures on North Caro- lina Highway 18 bypass around Morganton, about 0.1 mile north of intersection with North Carolina Highway 181, on east bank of Catawba River (south edge of area G-9, pl. 1). C, Blocks of layered biotite-hornblende gneiss and amphibolite in streaky inequigranular quartz diorite or granodiorite and pegmatite. Saprolite exposure in road- cut on northwest side of North Carolina Highway 18, 0.2 mile southeast of the Burke-Caldwell County Line (area H-8, pl. 1). D, Thinly layered fine-grained biotite and biotite-hornblende gneiss cut by dike of streaky biotite quartz diorite having flow structure parallel to its walls. The layered gneiss contains concordant lenses and pods of granitic rock, some of which are cut by the dike and some of which may have been fed by the dike. Saprolite exposure in roadcut on north side of county road, 0.4 mile S. 17° E. of Littlejohn Church (area H-8, pl. 1). Width of view about 1.5 feet. 142 where the granitic rocks seem to make up about half the total volume. Muscovite-quartz-microcline-plagioclase pegmatite forms dikes, pods, and knots in the granitic rocks, migmatites, and in the nonmigmatitic rocks through- out the area, but the pegmatite bodies are most common in the migmatite zones adjacent to the gran- itic bodies. The pegmatite bodies range from a few inches to several tens of feet in thickness, and some are several hundred feet long. Some of them are closely associated with and pass irregularly into granitic rocks, but the relation between the granitic rocks and other pegmatites cannot be conclusively demonstrated, although all the pegmatite bodies sam- pled contain heavy-mineral suites similar to those of the granitic rocks. PETROGRAPHY The granitic rocks typically consist of an un- sheared inequigranular mosaic of anhedral grains of quartz, plagioclase, and microcline in various propor- tions and of scattered rudely alined or randomly ori- ented flakes of biotite and muscovite. The plagioclase is most commonly well-twinned and faintly zoned oli- goclase or sodic andesine in irregular grains 1 to 3 mm in diameter. It also forms porphyroblasts as much as 1 em in diameter in a few specimens. Com- positions of plagioclase in granitic rocks of various compositions are shown in figure 84. Plagioclase grains in many specimens are rimmed by thin films of albite. Quartz in the mosaic is generally smaller than and interstitial to the plagioclase, but in some specimens it also forms segregation laminae parallel to the foliation. Microcline forms small irregular grains intergrown with and intersitial to quartz and plagioclase in the mosaic, and in some specimens it occurs as porphyroblasts as much as 2 ecm long, simi- lar to those in the Henderson Gneiss. Myrmekite re- action rims are common at contacts between micro- cline and plagioclase and form wartlike projections into the potassic feldspar. Stubby irregular flakes of brown or reddish-brown biotite 0.1 to 1.0 mm long are randomly oriented or rudely alined. Commonly, they are arranged in streaks defining a rude foliation. Primary muscovite is subordinate to biotite in most rocks ; it forms slen- der flakes interleaved with biotite and commonly seems to have crystallized somewhat later. Second- ary muscovite forms fringes on the ends of biotite books and skeletal grains and sericite aggregates re- placing feldspars. Colorless epidote with moderate to high birefringence occurs as skeletal aggregates and clusters of anhedral grains, commonly in or adjacent GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE Granite and quartz monzonite (9 samples) 10 Granodiorite (9 samples) & 0 I Quartz diorite (21 samples) PERCENTAGE OF TOTAL SAMPLES 8 0 I 10 20 30 -. 40 50 PERCENTAGE OF ANORTHITE IN PLAGIOCLASE FIGURE 84.-Histograms showing percentage of granitic rocks containing various compositions of plagioclase whose in- dices of refraction were determined by oil-immersion methods. Some rocks contain plagioclase with 10-percent or greater range of anorthite content and plot in several ad- jacent bars; the total therefore is more than 100 percent. Based on 39 samples. to biotite. Some epidote grains have cores of meta- mict allanite. Garnet forms subhedral to anhedral equidimensional porphyroblasts as much as 0.25 mm in diameter in some specimens. Aggregates of small anhedral grains of sphene commonly lie along biotite cleavages. Magnetite in scattered anhedral grains and aggregates and apatite in small blunt prisms are common minor accessories. A few of the granitic rocks contain scattered skeletal grains of green horn- blende partly replaced by aggregates of biotite. Chlorite, carbonates, and clay minerals are common in slightly altered rocks. In addition to epidote, apa- tite, garnet, and magnetite, heavy-mineral suites panned from saprolitized granitic rocks contain abundant stubby prisms of colorless or light-pink zircon, anhedral orange grains of monazite, seattered euhedral pyramids of light-amber xenotime, and a few prisms of black tourmaline. The migmatites differ from the other granitic rocks only in their more gneissic textures and in their higher contents of biotite and lower contents of potassic feldspar. All gradations in texture between the layered biotite gneiss and the granitic rock are found. Mineralogically, they are apparently identical with the gra‘nitiq rocks. | COMPOSITION Chemical anaiyses, modes, and norms of selected specimens of giranitic rocks are given in table 27. Similar data for the migmatic rocks are given in table 28. A combined modal variation diagram show- ing ranges in proportions of quartz, plagioclase, bio- tite, and muscoxLite in granitic rocks and in migma- titic gneisses related to them is given in figure 85. Biotite INNER PIEDMONT BELT fl Quartz 143 The granitic rocks and the migmatites associated with them closely resemble the layered gneisses in their modal composition. They differ from the lay- ered gneisses only in containing slightly higher pro- portions of potassic feldspar and slightly lower pro- portions of biotite. A modal variation diagram show- ing the range in relative proportions of quartz, pla- gioclase, and potassic feldspar in the granitic rocks and migmatites is given in figure 86. A silica variation diagram (fig. 87) of the analyses from tables 27 and 28 shows that the similarity in Biotite Plagioclase Muscovite EXPLANATION 1 H Chemically analyzed specimen of migmatite 1 Chemically analyzed specimen of granitic rock ® Average migmatite © Average granitic rock Biotite FIGURE 85.—PrLJportions of quartz, plagioclase, biotite, and muscovite in migmatitic biotite gneisses and granitic rocks related to ¢he Toluca Quartz Monzonite. Based on counts of 50 random grains in each of 24 thin sections of mig- matitic gneisses and each of 26 thin sections of granitic rocks. Contours 2, 4, 8, 12, and 16 percent. Numbers of migmatite specimens refer to analyses in table 28; numbers of granitic specimens, to analyses in table 27. 144 TABLE 27.-Chemical analyses, modes, and norms of granitic rocks probably related to the Toluca Quartz Monzonite [Chemical analyses of samples 1 and 3 determined by standard chemical methods by D. F. Powers and C. L. Parker, U.S. Geol. Survey. Other analyses, by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, and Hezekish Smith, U.S. Geol. Survey. Nd, not determined. Modes of analyzed specimens determined by point counts; P, present but not intersected in count- ing; Tr, trace. Major oxides and CIPW norms given in weight percent; modes, in volume percent] 1 2 3 4 5 6 : Sield No___. 2583(L) 30-1395E3 B A-607 - 31-1669B ._________ Laboratory No..... I-4060 160146 H-3431 161249 100147 © Major oxides 64.69 68.5 71.83 74.2 75.5 16.53 17.7 14.35 15.3 14.4 53 .25 55 .30 14 3.89 .99 1.76 .64 108 1.79 .46 1.14 42 .85 4.32 1.4 2.04 3.2 1.8 2.98 3.8 3.38 4.9 3.2 3.32 6.0 8.80 .62 4.0 . 64 . 60 54 70 45 252 04 .038 .04 02 .04 70 22 .19 09 11 19 05 .08 08 01 10 00 .06 02 00 10 07 .09 05 05 07 Nd Nd Nd Nd 02 Nd Nd Nd Nd Total... 99.87 100 99.85 100 100 "%.. 20.00% Modes Quartz. ._... 24 17 37 BH 32 29.8 Plagioclase. . 46 40 35 60 39 44.8 Potassic feldspar. .. 8 31 18 " s 22 12.5 Biotite. ..... 20 8 T 6 4 8.9 Muscovite. .. 12 2.8 8 1.7 2.5 3.8 Hornblende._. Fd Mia rescan -i cnet bie be wean becuase on o Tr Epidote. .... .8 i1 <4 1.1 F .6 Apatite .._. Tem UP ASAT. e Penance 12 Tr oer aeg Tara sekt woes a's Ts Zircon. ..... as y Moe" are ell P Tr .2 rh rience cana we naw Tr Sphene__.. _. 1:0 0 e P cense ty 1 3 Tr Opaque minerals.. P seee ML Levee Lc Tr Points counted... _ 600 900 700 600 600... CIPW norms 21.24 19.54 30.88 35.72 1.06 2.70 1.37 .94 19.62 85.45 22.45 3.66 25.06 82.14 28.58 41.44 19.33 6.18 9.03 15.36 4.46 1.14 2.84 1.05 5.78 1.25 2.58 .82 x .36 .80 44 1.33 42 .36 17 45 12 19 07 Q tocol cnn ence ies eni ari be sance sss 28 .16 20 11 ' Total of 50 random grains counted in each of 26 thin sections of granitic rocks. NotE.-Minor-element analyses for samples 1 and 3 given in table 1. 1. Coarse-grained well-foliated streaky biotite granodiorite containing pods of coarse biotite pegmatite. Rock consists of unsheared inequigranular mosaic of anhedral grains of quartz as much as 4 mm long, plagioclase (Anso-wo) 1 to 5 mm long, and rudely alined irregular grains of brown biotite 0.5 to 2 mm long. Microcline forms small blebs in pla- glpclase and irregular grains interstitial to quartz and plagioclase; it is fringed with myremkite at contacts with plagioclase. Larger plagio- clase grains seem to be porphyroblastic. Green hornblende occurs in scattgrgd skeletal grains as much as 1 mm in diameter partly replaced by biotite. Epidote forms small scattered grains containing low bire- fringent brown cores. Muscovite occurs as very sparse secondary flakes in feldspar. Abandoned quarry on northwest side of McGalliard Creek, 2.0 miles N. 78° E. of Drexel (area I-9, pl. 1). 2. Coarse-grained streaky biotite quartz monzonite granodiorite containing scattered augen of potassic feldspar as much as 2 em long. Rock con- sists of an unsheared inequigranular mosaic of anhedral grains of quartz as much as 5 mm long and plagioclase (Anzo-so) as much as 5 mm long, intergranular films and irregular grains of microcline inter- stitial to quartz and plagioclase, and rudely alined irregular flakes of brown biotite 0.2 to 2 mm long. Plagioclase also forms zoned porphy- as much as 1 cm in diameter. Muscovite occurs as irregular flakes contemporaneous with biotite, as fringes on biotite, and as seri- cite in feldspars. Epidote forms prismatic grains as much as 0.2 mm long. North wall of Causby quarry, 0.9 mile S. 10° W. of confluence of Catawba River and Johns River (area H-9, pl. 1). 3. Biotite granodiorite. Medium-grained faintly foliated equigranular biotite granodiorite containing scattered porphyroblasts of potassic feldspar 1 to 3 inches long and a few streaks of coarse biotite schist. Rock con- GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE TABLE 21.-Chemical analyses, modes, and norms of gramitic rocks probably related to the Toluca Quartz Monzonite- Continued sists of unsheared mosaic of anhedral grains of quartz and plagioclase (Ans-»s) 2 to 3 mm in diameter and microcline as much as 5 mm in diameter and ragged flakes of dark-brown biotite 0.5 to 1.5 mm long interstitial to and included in quartz and feldspar. Plagioclase is twinned and faintly zoned and commonly passes into myrmekite at contacts with microcline. Muscovite occurs in ragged primary flakes and as secondary sericite in feldspars. Epidote forms irregular poikilitic grains and aggregates, commonly in biotite. Roadcut on east side of road between Oak Hill and Willow Tree school just south of bridge over Canoe Creek, 1.0 mile S. 27° W. of Oak Hill school (area F-9, pl. 1). 4. Medium- to coarse-grained light-colored quartz diorite containing pods of amphibolite. Rock consists of unsheared inequigranular mosaic of an- hedral grains of quartz and plagioclase (Anso-ss) as much as 5 mm in diameter and irregular nonalined flakes of partly chloritized brown bio- tite as much as 2 mm long. Muscovite forms fringes on biotite apd felted sericite aggregates in feldspars. Albite forms fringes on plagio- clase. Epidote forms irregular skeletal grains, some containing low bire- fringent brown cores, chiefly in biotite. Roadcut on south side of road, 2.0 miles S. 60° E. of school at Gamewell (area I-8, pl. 1). 5. Medium-grained streaky biotite quartz monzonite in 5-foot-long pqd in layered amphibole-biotite gneiss. Rock consists of unsheared mosaic of anhedral grains of quartz 1 to 3 mm in diameter, plagioclase (Aso-s) 1 to 5 mm in diameter, and microcline 1 to 5 mm in diame- ter and scattered irregular nonalined grains of brown biotite as much as 1 mm long. Sieve-textured porphyroblasts of microcline contain abundant inclusions of quartz and plagioclase. Myrmekite is common at contacts between microcline and plagioclase. Albite forms fringes on plagioclase. Muscovite forms spongy skeletal grains, fringes on biotite and sericite aggregates in feldspars, and is largely secondary. Roadcut on north side of county road on west side of Smoky Creek, 1.4 miles N. 55° E. of north end of Huffman Bridge (area I-9, pl. 1). mineral composition is concomitant with a close sim- ilarity in chemical composition. There is apparently no demonstrable difference in proportions of the major oxides between migmatitic rocks and nomig- matitic layered biotite gneisses of equivalent SiO» content. The granitic rocks also closely resemble the nomigmatitic biotite gneisses, but they have a gener- ally slightly higher alumina and slightly lower total iron content, and many of them have a somewhat higher KO content than nonmigmatitic gneisses of equivalent SiO; content. Figure 88 shows that the molecular proportions of Al,0O;, CaO, and K,0 +- Na,0 in the granitic rocks and migmatites are nearly identical with those in the layered biotite gneisses. The NaO:K;:0 ratio (hig. 89) of - most. of the migmatites is in the field of biotite gneisses, but a few of the migmatites and most of the granitic rocks have Na,0:K,0 ratios less than one. The only gra- nitic rock having a Na,0 :K,0 ratio greater than one is a plagioclase-rich quartz diorite lacking potassic feldspar and containing abundant inclusions of am- phibolite. ORIGIN The close chemical similarity between the granitic rocks and the enclosing biotite gneisses suggests that the granitic rocks could have been derived from the country rocks with only minor changes in bulk com- position. The generally conformable habit of the larger granitic bodies and the gradational contacts from granitic rocks through migmatites into non- ‘ INNER PIEDMONT BELT ‘ QUARTZ PLAGIOCLASE | POTASSIC FELDSPAR A EXPLANATION *1 Chemically analyzed specimen © Average 145 QUARTZ PLAGIOGCLASE _.POTASSIC FELDSPAR B FIGURE 86.—Prof)ortions of quartz, plagioclase, and potassic feldspar in migmatitic biotite gneisses and granitic rocks. A, Migmatitpc biotite gneisses. Based on point counts of analyzed rocks and counts of 50 random grains in each of 14 other thin sections. Contours 4, 8, 12, 16, and 25 percent. B, Granitic rocks. Based on point counts of analyzed rocks and count of 50 random grains in each of 19 other than sections. Contours 4, 8, 12, and 23 percent. Numbers of analyzed ‘specimens refer to analyses in tables 27 and 28. migmatitic gneisses suggests that a large part of the granitic material may have formed by in situ reerys- tallization of th? biotite gneiss of appropriate compo- sition, perhaps with local addition of KO. Discor- dant dikes of ‘pegmatite and granitic rocks are common, however, and the larger granitic bodies lo- cally contain rofiated inclusions of layered amphibol- ite and amphibole gneiss and in some places have sharp discordant contacts, showing that at least some of the graJnitilc material moved. The absence of migmatites and granitic rocks comparable in compo- sition with the hornblende gneisses and amphibolites and the preponderance of these rocks as inclusions is compatible with the hypothesis of origin of the grani- tic rocks by recrystallization and partial anatexis of the biotite gneisses, the interlayered hornblendic rocks being lefq as undigested relies. The general‘ structural conformity between the granitic rocks and migmatites and the enclosing rocks, the simiharity in mineralogy and mineral as- semblages between the granitic rocks, migmatites, and enclosing rocks, and the absence of metamorphic effects and chilled margins at the contacts of the granitic bodies indicate that the granitic rocks formed by recfystallization and partial anatexis of the country réycks during the climax of the high- grade regional metamorphism. The granitic bodies in this part of the Inner Piedmont belt have all the characteristics‘ of the "plutons of the catazone" de- scribed by Budrling‘ton (1959). There is as yet no unanimity of opinion on the correlation between metamorphic facies and the tem- perature and pressure ranges that they represent. Luth, Jahns, and Tuttle (1964, p. 760) indicated that water-saturated granitic magma could be formed by fusion of rocks of appropriate composition at tem- peratures of about 625° C under Py., of 10 kilo- bars, corresponding to depths of 35 to 40 km (kilo- meters), if Pu.o is assumed equal to lithostatic load (Hamilton, 1963, fig. 78), or at temperatures of about 700°C at depths of about 5 km. Recent recal- culation of the aluminum silicate triple point based on new experimental data (Newton, 1966) indicated that sillimanite is stable in the temperature range 600° to 700°C at pressures between about 2 and 7 kilobars, corresponding to depths of 7.5 to 26.5 km. These data probably provide a reasonable estimate of the possible range of temperatures and pressures that prevailed during the climax of regional meta- morphism and the formation of the granitic rocks in this part of the Inner Piedmont. AGE No stratigraphic evidence or geochronologic data are available that would unequivocally establish the age of the granitic rocks in the Inner Piedmont belt in the Grandfather Mountain area. The Toluca Quartz Monzonite, with which the granitic rocks in the Grandfather Mountain area are believed to be correlative, has been assigned an Ordovician age by Overstreet and Griffiths (1955) on the basis of lead- 146 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE TABLE 28.-Chemical analyses, modes, and norms of migmatitic biotite gneisses [Analysis of sample 1 determined by standard chemical methods by D. F. Powers, U.S. Geol. Survey. Other analyses, by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey. Nd, not determined. Modes of analyzed specimens determined by count of 600 points; P, present but not intersected in counting; Tr, trace. Major oxides and CIPW norms given in weight percent; modes, in volume percent] 1 2 3 4 5 6 T 8 9 10 T Ficdd els eens o wan aas oe 2028-L 30-1395F-1 28-1111A 33-2146B - A-580 - 32-2035B 33-2146D 80-1379B 28-1011C 29-121la _______. Laborstory ce} I-4062 _ 161334 - 160144 - 161245 161248 160155 161246 - 160145 161243 | 161244 | ________ Major oxides 66.6 66.9 68.0 69.6 69.8 70.4 73.8 74.0 ToB sl 15.1 17.0 14.9 14.9 14.6 14.9 13.8 18.2 12,0. . 66 .90 1.8 1.2 15 74 .85 . 84 N00 cus lilly 3.4 2.1 3.4 2.8 3.3 2.6 1.4 1.6 1.4 L:... 1.0 1.2 1.7 1.8 1.6 1.2 . 55 1.4 1.0. 3.6 4.2 8.8 2/0 2.6 2.6 1.2 3.0 4.0 3.0 4.0 3.4 3.6 8.1 3.3 2.9 8.8 BVB Suc el 3.9 1.6 2.1 2.0 2.8 3.3 4.8 1.0 PTB .66 . 80 . 84 .85 .85 .69 . 56 . 48 17 -a «sli... .05 .02 .07 .06 .04 . 04 .03 .02 (05 .86 .36 .20 .40 .62 .48 . 30 . 82 cad seals 4s .53 yal .20 . 09 .24 A2 A2 .09 09 22.4... .07 .05 . 08 . 08 . 06 .05 .02 .06 (04 2 .us cl.: . 48 <.05 ::~-«<.05 .05 -_ <©.05~ <.05 <;105.. '«.05 . «.05 ....:.... Nd Nd Nd Nd Nd Nd Nd Nd Nd ".: Nd Nd Nd Nd Nd Nd Nd Nd Nd .. 100 100 100 100 100 100 100 100 £00 4. tsun. cl Modes Quartz 22202: ITIL L_. 88 25 32 29 33 31 32 36 48 45 34.7 33 38 51 52 45 41 44 29 42 44 36.0 F otassic feldspar............... 2.2 8 t abasic s 02 9 a ad tata c lst { .2 Biotite: _.... re 28 22 15 15 19 25 12 10 9 8 14.9 0 els 4 1-8 2.8 .8 2.5 1:7 1.8 2:0: 4.4 A C arre on an alcian ss be ae sles T AA EAC r atea s 1.0 1.0 6:1 ck. 4 v2 .8 1.0 cs uss ues s se 2.2 1.6 1.1 ADAUILE : - 21. 23.00 2. n ele » iA 1.0 y o sa aan a - .8 vd ceils.... E elie ees unos Trp Chlorite® -c alu su. t Ulin ele. P.. ALLIE TL cca eas aon ce ae nent tbs yo ess Tr f iFCOR ses os ss sabe aL LL a eb aoe aii » Haale I n erik Ls SHe cease kene o an gid. & Tr .8 .8 Ea ele oo .2 e Ce an an ean ale s Tr seen T9 Air toons uk s reece shane seals 2 1 .2 2 Tr Opagtie minerals.. s.. .:} UPE SLA a .8 B BEEZ A i- rela Beau r= aks Tr Camels 22s lene lll p. oc nb Let :: 0! AD ee Heo cH ae our. s ah, 25.907 . 88.88... 26.75. 80.45 26.22 24.58. 82.14 28.08 ........ AMEN r eee ean a ev s tee 21.6¢_ 11.86. 19.46 - 15.06 ~ 13.80 11.838 12.11 5.17. 14.290 : 10.25 MME soc .R tU L 02 my aa sois ct to an La o TL s o an a areal os te a a aid dire tole en anl aoe ean o Aae a a ona a aot o aoi aan a l n heve OS Ee eR ULi -a are ck nae rmamnie -an niew oh abana inlet a a a aiclale aln u s R ee irene a n seneste t pilon cereal T.A7 2.49 2.99 4.28 4.48 8.98 2.99 1.97 3.48 | | pea rere ases eog 5.12 4.41 8.71 4.99 3.64 4.58 3.46 1:82 1.83 1.10 MG. 2 sr re ee be ule mid an amin 2.54 .96 1.30 1.88 1.74 1.09 1.07 ol 1.22 slr. $:: see RLE Pede wk 1.06 1.63 . 68 .38 76 1.18 .91 DT .61 (ob Ap eee ULM. o eines epa .28 1.26 . 50 A7 «af df. .28 .28 VB Tel s;. .l; Tes sesuai e ALEC Poa ial a aa a wile o fI Mee ddr o os an a e ut 2 ade a mile - = a o a aoe aa amie e alain ie ala hae aie ale a n aie Ue i and Praia aie arn nla Oc-! mda ie aan eL Panes .84 TOU l Le onus e caren Los o. - cloe banns anu can s ue an een ie's ia a aie nan s ao ule nle ia au hile i o ! Average of 50 random grains counted in each of 24 thin sections of migmatitic biotite gneisses. NotE.-Minor-element analyses for sample 1 given in table 1. Hornblende gneiss. Medium-grained well-foliated streaky dark-gray mig- matitic biotite gneiss containing light-colored quartz-feldspar folia and veinlets. Rock consists of mosaic of irregular interlocking grains of quartz 0.25 to 1.5 mm in diameter, clear twinned plagioclase (Anio) 0.25 to 1 mm in diameter, and well-alined undeformed flakes of brown biotite as much as 1 mm long. Microcline forms irregular grains in- tergrown with quartz and plagioclase and commonly is fringed with myrmekite. Green hornblende occurs as scattered irregular grains as much as 1.5 mm in diameter. Colorless epidote forms irregular grains and aggregates as much as 1 mm long. Roadcut on east side of U.S. Highway 321, 0.4 mile south of bridge over Lower Creek (area J-T, pl..1}. 2. Strongly foliated nonlayered biotite gneiss containing scattered potassic feldspar augen. This rock is apparently transitional between biotite- amphibole gneiss (specimen 7, table 23) and biotite quartz monzonite (specimen 2, table 27). Rock consists of inequigranular mosaic of ir- regular grains of quartz, plagioclase (Ansoss), microcline, and very irregular flakes of synkinematic and postkinematic brown biotite. Most quartz and feldspar grains are 0.1 to 0.5 mm, but plagioclase and microcline also form porphyroblasts as much as 5 mm in diame- ter, and quartz occurs as 4-mm grains in segregation folia. Muscovite occurs as skeletal grains replacing feldspars, and epidote, as small ir- regular grains in biotite, many with cores of brown allanite. Near northwest corner of Causby quarry, 0.9 mile S. 10° W. of confluence of Catawba River and Johns River (area H-9, pl. 1). 3. Layered medium-grained migmatitic biotite gneiss containing folded lay- ers of amphibolite and layers and streaks of biotite quartz diorite. Neither amphibolite nor quartz diorite are included in analyzed speci- men. Rock consists of unsheared mosaic of granoblastic quartz and plagioclase (Anss) in grains 0.5 to 1 mm in diameter and irregular flakes of undeformed reddish-brown biotite as much as 1 mm long. Plagioclase has thin albite fringes and contains a few irregular blebs of potassic feldspar. Epidote forms irregular to subhedral grains, some with allanite cores 0.01 mm in diameter. Muscovite occurs in small flakes probably derived from alteration of feldspar. North wall of abandoned quarry on east side of Hunting Creek 0.9 mile S. 12° W. of confluence of Johns River and the Catawba River (area H-9, pl 1). Coarse-grained streaky migmatitic quartz diorite gneiss (cf. specimen 7) interleaved with folia of coarse-grained biotite schist, fine-grained biotite gneiss, and amphibole-biotite gneiss (specimen 8, table 23). Rock consists of inequigranular mosaic of irregular grains of quartz as muchas 2 mm in diameter, plagioclase (Anss) as much as 8 mm in diaméter, and randomly oriented flakes of dark-brown biotite 0.25 to 1.5 mm long. Plagioclase is twinned and faintly zoned, and some has fringes of albite. Sericite forms fringes on biotite and aggregates ‘ INNER PIEDMONT BELT 147 TABLE 28.—Chemical\analyses, modes, and norms of megmatitic biotite gneisses-Continued in feldspar. Epidote‘ occurs as irregular aggregates, chiefly in biotite. Roadcut on east side of North Carolina Highway 18 bypass west of Lenoir, 1.1 miles S. ‘68° W. of Caldwell County Courthouse (area J-7, pl. 1). 6. Coarse-grained, streaky well-foliated migmatitic quartz diorite gneiss. Rock consists of inequigranular mosaic of grains of quartz as much as 3 mm long, plagioclase (An....) as much as 5 mm in diameter, and irregular alined flalrefs of olive-green biotite as much as 4 mm long. Plagioclase is twinned and faintly zoned, and some grains have fringes of albite. Some plagioclase twin lamellae and biotite flakes are warped. Potassic feldspar occurs in small irregular grains interstitial to quartz and plagioclase. Scattered poikilitic grains of blue-green hornblende are Daily replaced by biotite. Epidote occurs in skeletal aggregates and scattered subhedral grains in biotite. Roadcut on east §ide oil“ secondary road, 1.2 miles S. 74° E. of school at Gamewell (area -8, pL 1). 6. Coarse-grained, poorly foliated streaky migmatitic quartz diorite gneiss containing thin folia of biotite schist and boudinaged dikes of biotite pegmatite. Rock consists of an inequigranular mosaic of irregular grains of quartz 0.1 to 1.5 mm long, and of plagioclase (Anss) 0.5 to 2 mm long, and flakes of alined undeformed brown biotite 0.1 to 1.0 mm long. Plagioclase porphyroblasts as much as 5 mm in diameter are elongated parallel to foliation. Muscovite occurs in flakes synge- netic with biotite and as sericite from local alteration of plagioclase. Abandoned quarry on north side of Millers Creek, 1.55 miles S. 10° of Caldwell County Courthouse (area J-7, pl. 1). 7. Medium-grained, well-foliated streaky migmatitic granodiorite gneiss (ef. specimen 4) interleaved with folia of coarse-grained biotite schist, fine-grained biotite gneiss and amphibole-biotite gneiss (speci- men 8, table 23). Rock consists of inequigranular mosaic of irregular grains of quartz as much as 3 mm in diameter, plagioclase (Anso-ss) 1 to 4 mm in diameter, and stubby rudely alined grains of undeformed brown biotite as much as 3 mm long. Microcline forms irregular grains as much asInS mm long, elongated parallel to foliation, partly jacketed, and deeply penetrated by myrmekite. It also occurs in small irregular grains inferstitial to plagioclase. Muscovite is largely second- alpha age determinations on zircon and monazite that yielded dates of about 400 m.y. More recent iso- topic uranium-lead determinations on zircon from the Toluca in its type locality give discordant ages ranging from 405 to 485 m.y. (Davis and others, 1962). Biotite from the same rock gave a rubidium- strontium age of 250 m.y. Davis, Tilton, and Wether- ill (1962, p. 19915 interpreted these data as indicat- ing a minimum ‘age of 400 m.y. for the zircon, but pointed out that "it is not possible from this infor- mation to reach any conclusions about the time of intrusion of the rock itself." They believed that many of the ziicon crystals were inherited from sedimentary rocks. They attributed the biotite age of 250 m.y. to the effect of a later metamorphism. Even if the {like of 400 m.y. were accepted as the true age of the Toluca Quartz Monzonite, recent revi- sions of the absolute geologic time scale (Holmes, 1959; Kulp, 1961) indicate that the geologic age would be latest‘Silurian or Early Devonian rather than Ordovician. In view of the uncertainties in- volved in the interpretation of the available isotopic age data and in the correlation between isotopic and geologic ages, we suggest that the Toluca Quartz Monzonite be aéfsigned to the early or middle Paleo- zoic rather than specifically to the Ordovician. The correlative granitic rocks in the Inner Piedmont belt in the Grandfather Mountain area are also believed to be of early or middle Paleozoic age. A single un- published potassium-argon age determination by J L. Kulp (written commun., 1961) on muscovite from quartz monzonjte from a roadcut 0.5 mile N. 5° E. ary from alteration of plagioclase. Epidote forms rudely prismatic skeletal grains containing brown low birefringent cores. Same locality as specimen 4. 8. Medium-grained, strongly foliated migmatitic quartz monzonite contain- ing layers of fine-grained amphibole-biotite gneiss. Rock consists of inequigranular mosaic of irregular grains of quartz 0.1 to 1.0 mm in diameter, plagioclase (Ans) 0.25 to 1.0 mm in diameter, and alined stubby flakes and irregular grains of undeformed red-brown biotite 0.1 to 1 mm long. Plagioclase also occurs as porphyroblasts as much as 5 mm in diameter. Myrmekite occurs at contacts between plagioclase and microcline. Muscovite forms fringes on biotite and sericite clus- ters in feldspars and is apparently all secondary. Roadcut on west side of road north of Huffman Bridge, 0.4 mile N. 20° E. of south end of bridge (area I-9, pl. 1). 9. Medium-grained, layered, and well-foliated quartz diorite gneiss contain- ing streaks of biotite schist and amphibole-biotite gneiss and pods and blocks of amphibolite. Rock consists of an inequigranular mosaic of irregular grains of quartz 0.1 to 1.0 mm in diameter and plagioclase (Anxi) 0.5 to 4.0 mm in diameter, and rudely alined irregular flakes of brown biotite 0.1 to 1.5 mm long. Plagioclase is partly sericitized and contains blotches of albite. Bluish-green hornblende occurs in poikilitic grains as much as 1 mm long. Epidote forms irregular ag- gregates and small grains, many with brown weakly birefringent cores. East bank of the Catawba River 0.1 mile north of North Caro- lina Highway 181 (south edge of area G-9, pl. 1). 10. Coarse-grained, streaky migmatitic quartz diorite gneiss containing streaks and layers of amphibole quartz diorite. Rock is composed of inequigranular mosaic of irregular grains of quartz as much as 4 mm in diameter and plagioclase (Ans) 1 to 4 mm in diameter, and rudely alined irregular flakes of brown biotite as much as 1.5 mm long. Blue-green hornblende occurs in irregular poikilitic grains as much as 1 mm in diameter. Epidote forms irreguar aggregates and skeletal grains, some with light yellow weakly birefringent cores. South side of Warrior Fork, 0.4 mile N. 55° W. of confluence with the Catawba River (area G-9, pl. 1). of Willow Tree school (area F-9, pl. 1) is consistent with this age assignment and indicates that the mus- covite has not been affected by the metamorphism of 250 m.y. ago that affected the biotite in the Toluca Quartz Monzonite at its type locality. QUARTZ MONZONITE GNEISS On the southeast side of the Brevard fault zone in the southern Blowing Rock and northern Lenoir quadrangles (areas J-5 and I-6, pl. 1) is a long thin body of white fine- to medium-grained well-foliated and lineated quartz monzonite gneiss. Small lenses of quartz monzonite, most of which are too small to map, are strung out along the Brevard fault north- east and southwest of the main body, and some of the material mapped as blastomylonite to the southwest may have been derived from similar rock. Except for one lens in the Lenoir quadrangle, the bodies of light-colored quartz monzonite gneiss have sharp and very sheared contacts on the northwest side, whereas on the southeast side they are interlay- ered with gneiss of the Inner Piedmont belt. The rock ranges from blastomylonitic gneiss to blastomylonite and contains porphyroclasts of feld- spar and mica. Alined mineral grains form the linea- tion. The main body of quartz monzonite gneiss in- cludes only a few layers of other rock types, but to the southeast the white granitic gneiss forms layers in typical gneiss and schist of the Inner Piedmont. In one outcrop just west of Patterson (area I-6, pl. 1) the quartz monzonite gneiss contains septa of 148 is T T T T T T i 20 |- 5 AlsOs E ug wan 3 k: ©.. o h 3 151 \\°\®+ 0>\ 3 - mou @ 6 \< C z. 3 10 | | 1 10 e ZFeas [ K\\\\\ # FeO , |- Fue! 3 = 3 5|- - © ig =4 E *$* % EA.: d F T «ae. 9 ~s s I 1 I & t - fe 10 |- 3 & |- 7 Pee .g - §) ° c E & 22 a 3 5 - uel at ee & F §! _ g c ia r- fey" as>- 10 |- & cso: (~ " &, 8 5|- - - © & - \\\ $3.\\+\$¢\ - 3. 1 | zay. "{. _ Na,O 5|- fames -m c -p --] & Ca paige: 21. - of | 1 1 | | 1 3 + = g" -6 £ *%. 7 - o od e woah - Pow mene _- K,0::L (___fF\ o » gi oC 1 1 1 1 - 40 50 0 70 80 6 SiO,, IN WEIGHT PERCENT EXPLANATION @ Granitic rock Migmatite FigurE 87.-Si0, variation diagrams comparing granitic rocks and migmatites of the Inner Piedmont belt with layered biotite gneisses. Dashed line encloses field of biotite gneisses from figure 76. gneiss in which the layering is conformable with the cataclastic foliation of the granitic rock. The rock consists of porphyroclasts of microcline, plagioclase, quartz, and muscovite about 2 mm in diameter set in a mosaic-textured groundmass of re- erystallized quartz, plagioclase, microcline, sericite, biotite, epidote, and chlorite ranging from 0.01 to 0.2 mm in grain size (fig. 90). The plagioclase is sodic oligoclase and albite (about An,,). Some of the re- GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE AlzG; EXPLANATION + Granitic rock © Migmatite K20 Cao z Na20 FIGURE 88.-Molecular proportions of Al,O,, CaO, and total alkalies in granitic rocks and migmatites. Dashed line en- closes field of biotite gneisses from figure 78. 7r—~ 6 [- + «2 g. Biz 97 i 6 <> Lu O G Q. 4 k s ss + é ( +\e\ a el- \ "A \\ # \ x @ <. \ ¥ x /\ © a \\ 2|- \ / yi t 8 Sit ast A X \ il- / Ss No o Sa ) a. 18s | 1 | I | | 0 1 2 3 4 5 6 Na,0, IN WEIGHT PERCENT EXPLANATION n Migmatite Granitic rock FIGURE 89.-Na,0:K,0 variation diagram comparing granitic rocks and migmatites of the Inner Piedmont belt with layered biotite gneisses. Dashed line encloses field of layered biotite gneisses from figure 77. ‘ INNER PIEDMONT BELT FIGURE 90.—Photclmicrograph of quartz monzonite blasto- mylonite from east fork of Church Branch (area J-5, pl. 1). Small porphyroclasts of plagioclase (pseudomor- phosed by albite) and microcline in a groundmass of re- crystallized quartz, albite, microcline, muscovite, and sub- ordinate biotite and epidote. Section cut parallel to min- eral lineation. | sak i { crystallized biotite and epidote is as much as 1 mm long. Accessory minerals are garnet, zircon, apatite, and opaque minerals. The age of this granitic rock has not been estab- lished but it is younger than the country rock and older than the faulting and retrogressive meta- morphism. It, Jnerefore, may be anywhere from late Precambrian to middle Paleozoic in age. | _ ULTRAMAFIC ROCKS Serpentine schist and altered peridotite and dunite are intercalated with layered rocks of the Inner Piedmont belt in a few places. The bodies of ultra- mafic rocks rarlge from pods a few inches long to a lens 1,000 feet across and 4,000 feet long (area F-8, pl. 1). A series of disconnected ultramafic lenses as much as 200 Ffeet thick extends along strike for 149 nearly 2 miles between the Yadkin River and the head of Celia Creek (areas G-7 and H-7, pl. 1). The ultramafic bodies have been found only in the belt of polymetamorphic rocks southeast of the Brevard fault zone, and all occur along a well-defined trend parallel to the fault zone. Contacts of the ultramafic bodies are everywhere conformable with layering and foliation in the enclosing rocks, and most of the bodies are elongated parallel to the regional strike of the enclosing rocks. The ultramafic rocks commonly form resistant float and fresh natural outcrops, un- like most other rocks in the Piedmont, but contacts of most of the bodies are poorly exposed. The ultramafic rocks are chiefly lustrous, fine- to medium-grained, dark-gray-green magnesian schists, commonly having 0.5 to 1 em knots of light-green to gray amphibole on weathered surfaces. Schistosity is generally well developed, but in the centers of some of the larger bodies the rocks are nonfoliated and have textures resembling those of unaltered dunite. The rocks are principally composed of tabu- lar or felted aggregates of prochlorite(?) inter- leaved with magnetite and small flakes and aggre- gates of tale. Colorless to light-green amphibole occurs in ragged prisms and in poikilitic grains and tiny needles interwoven with chlorite. Relict poikilitic grains of clinopyroxene (diopside?) oc- cur in many specimens, and a few also contain relict grains of olivine partially replaced by antigorite(?). In the nonfoliated rocks having tex- tures relict from dunite, the olivine grains are en- tirely pseudomorphosed by antigorite or prochlorite set in a felted matrix of prochlorite, antigorite, and magnetite. Some of these rocks are strongly mag- netic in hand specimen. Veins of fibrous anthophyl- lite as much as 1 foot thick cut the unsheared ultra- mafic rocks in several of the larger bodies. One of these, 1.0 mile N. 86 W. of Conways Chapel (area G-7, pl. 1) has been mined for asbestos and is de- scribed by Conrad, Wilson, Allen, and Wright (1963). There is no direct evidence on the age of the ultra- mafic rocks in the Inner Piedmont belt. Reed (1964b) reported the occurrence of what he believed to be a biotized and feldspathized ultramafic rock adjacent to Henderson Gneiss in an outcrop on the south shore of Lake James, 1.5 miles southeast of the mouth of the Linville River (area E-9, pl. 1), and suggested that the ultramafic body was older than the Henderson Gneiss. On the other hand, the fact that the ultramafic bodies are apparently confined to the belt of sheared polymetamorphic rocks along the 150 Brevard fault zone and are alined with the fault zone may indicate a genetic relation. If so, the ultramafic rocks are either exotic lenses or are younger than the granitic rocks of early or middle Paleozoic age cut by the fault zone. Until the age relations between the ultramafic rocks and the granitic rocks are clearly established, the age of the ultramafic rocks is en- tirely undetermined. STRUCTURE OF ROCKS OF THE INNER PIEDMONT NOT SHOWING METAMORPHIC EFFECTS RELATED TO THE BREVARD FAULT ZONE Description and interpretation of the complex structure of the rocks of the Inner Piedmont belt is impeded by the lack of recognizable stratigraphic units, obliteration of original textures and struc- tures, and the difficulty of detailed geologic mapping. Deep weathering and extensive soil cover make it impossible to recognize structures intermediate in scale between those observed in single small outcrops and those inferred from the necessarily crude geo- logic maps. Therefore, statistical orientation dia- grams are necessary to describe the structural geom- etry and decipher the history of these rocks. Al- though it seriously handicaps geologic mapping, the deep weathering is not entirely disadvantageous, for in undisturbed saprolite, bedrock structures are al- most perfectly preserved and may be easily dissected with a pocketknife or entrenching tool for measure- ment and three-dimensional study. The general trends of structures in the Inner Pied- mont belt are shown on the foliation map (fig. 32), the lineation map (fig. 33), and on the geologic map (pl. 1). These maps show an obvious contrast in map pattern and structural fabric between the high-grade metamorphic rocks in the southeastern part of the area and the polymetamorphic rocks adjacent to the Brevard fault zone. Structures of the polymeta- morphic rocks are related to the Brevard zone and are discussed below. LAYERING AND FOLIATION Although no undisputed sedimentary features are preserved in the Inner Piedmont rocks, the strongly layered character of many of the gneisses and schists indicates that many of them were originally strati- fied rocks. The continuity of individual layers and groups of layers, the compositional contrasts be- tween adjacent layers, and the variety of lithologies represented strongly suggest that the layering is in- herited from bedding. Contacts between layers are largely tectonic, however, and flowage parallel to lay- GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE ering is indicated by attenuation of ductile mica-rich layers and boudinage of brittle amphibolite and quartzite layers to form detached pods and blocks, some of which are rotated with respect to the more ductile enclosing rocks. The arrangement of the var- ious individual lithologies in the layered rocks proba- bly has little or no relation to their original strati- graphic sequence. Foliation due to alinement of micaceous and pris- matic minerals and quartzo-feldspathic folia parallel to layering is conspicuous in most rocks. It is best developed in schists and the more micaceous gneisses and is least apparent in quartzo-feldspathic gneiss. Foliation due to parallel arrangement of biotite flakes and streaks and quartzo-feldspathic folia is well developed in migmatite and in many of the granitic rocks. Foliation in most of the migmatites and granitic rocks is parallel to the contacts of the rock bodies and to layering and foliation in the en- closing rocks. Some discordant dikes and a few larger granitic bodies have streaky planar flow structure parallel to their walls and at high angles to foliation and layering in the wallrocks (fig. 83D). FOLDS AND LINEATION Many outcrops of sillimanite schist have small crenulations in foliation which range in amplitude from fractions of an inch to several inches. Faint slip cleavage is developed parallel to the axial planes in many places (fig. 914 and B). Folds in competent layers of quartzite and gneiss interlayered with silli- manite schist have amplitudes of a few inches to several feet. They are sharp but generally not iso- clinal and have only minor thickening in the noses. Their axes and axial planes are parallel to those of crenulations in the enclosing sillimanite schist. Folds are rarely observed in the rocks khetween the silli- manite schist and the main belt of granitic rocks, but where they are present, they are generally more tightly appressed and have more pronounced thick- ening of ductile layers in their noses than do folds in the sillimanite schist. They closely resemble flexural flow folds described by Donath and Parker (1964). STRUCTURAL GEOMETRY Figure 4 on plate 5 shows the statistical distribu- tion of measurements of layering, foliation, linea- tion, fold axes, and axial planes in rocks of the Inner Piedmont belt in the area where no metamorphic ef- fects related to the Brevard fault zone were recog- nized (pl. 6D). This area is subdivided into three zones : the main belt of granitic rocks and migmatite ‘ INNER PIEDMONT BELT Muscovite porphyroblast 2 inches A FIGURE 91.-Folds in Inner Piedmont rocks. A, Crenulations in sillimanite schist. View on AC plane with north to the left, Small gram‘tic pods are conformable with folded folia- tion. Muscovite porphyroblasts are undeformed and cut across fold noses. Drawn from a hand specimen. South side of Rhodhiss Lake, 1.6 miles S. 82° W. of south end of Castle Bridge (area J-9, pl. 1). B, Folds in quartzite layers in sillimanite schist. Scale helow each fold represents 1 foot. All folds viewed on AC plane with north to the left. a. Roadcut through Cajah Mountain in area J-8, 1.3 miles which extends continuously northeastward from just west of Morganton through Lenior (pl. 5, fig 4, col. A) ; the area t? the southeast which is mapped as predominantly layered gneiss, but which also con- tains isolated boidoes of mica schist, migmatite, and granitic rocks (col. B); and the sillimanite schist and interlayered rocks in the southeastern corner of the Lenoir quadrangle (col. C). The structures in these belts are hiscussed in order of decreasing dis- tance from the Brevard zone. Separate plots show that, within any given area, layering and foliation in the layered rocks and folia- tion (other tharl obvious flow structures) in nonlay- ered rocks have orientation patterns that are statisti- cally indistinguishable, and these structures are 151 50 feet C N. 6° E. of school at Baton (area J-9, pl. 1). b. Same road- cut about 60 feet south of 1. c. South short of Rhodhiss Lake, 1.3 miles S. 80° E. of south end of Castle Bridge (area J-9, pl. 1). C, Folds in interlayered biotite gneiss, mica schist, and quartz schist (stippled at right side of drawing). Note isolated rotated blocks of amphibolite. Direction of view is northeast, approximately parallel to fold axes. Roadcut in saprolite on northeast side of North Carolina Highway 126, 0.6 mile S. 34° E. of Grandview Church (area F-9, pl. 1). therefore plotted together in the upper row of dia- grams. Layering and foliation in both groups of rocks southeast of the main belt of granitic rocks strike almost north-south and dip gently or moderately east. Map patterns, particularly that of the north- eastern contact of the sillimanite schist, suggest major open folds whose axes plunge gently east. Poles to layering and foliation cluster around single broad maxima (diagrams B-1 and C-1, fig. 4, pl. 5), with only the vaguest suggestion of girdles around east-plunging axes. Mineral lineation, marked by alinement of sillimanite needles and fibers in the sil- limanite schist and by hornblende needles and long dimensions of mica flakes and clusters and quartz- 152 feldspar aggregates in other rocks, plunges east- ward. Lineation projections (diagrams B-2 and C-2, fig. 4, pl. 5) are arrayed in partial girldes along the trace of the foliation and layering maxima and cul- minate in point maxima down the dip of the foliation and layering. Crenulations and minor folds observed in individ- ual outcrops have axes plunging gently east, parallel to the mineral lineation ; axial planes of most of the minor folds dip gently or moderately north or north- east, generally at angles slightly steeper than the foliation and layering (diagrams B-3 and C-3, fig. 4, pl. 5). Although metamorphic effects related to the Bre- vard fault zone are not recognized in the main belt of granitic rocks, the structural geometry of the rocks in this belt (col. A, fig. 4, pl. 5) is more like that of the polymetamorphic rocks to the northwest than that of the rocks to the southeast. Foliation and lay- ering strike northeast and dip consistently south- east; poles cluster symmetrically around a well-de- fined maximum (diagram A-1, fig. 4, pl. 5). Mineral lineation plunges northeast and forms a single high maximum in the projection (diagram A-2, fig, 4, pl. 5). Mineral lineation is absent in most of the gra- nitic rocks and is very rare in the migmatites. No major folds are apparent in the map pattern, but minor folds in layered gneiss and migmatite resem- ble those in the rocks to the southeast. Their axes plunge northeast, approximately parallel to the min- eral lineation, and their axial planes dip southeast, parallel to layering and foliation (diagram A-3, fig. 4, pl. 5). Two maxima appear in the fold axis orien- tations, one plunging very gently N. 45° E. and the second plunging more steeply northeast and lying on a partial small-circle girdle that extends to near the position of the fold-axis maxima in the rocks to the southeast (diagrams B-3 and C-3, fig. 4, pl. 5). This suggests that the second group of folds may have been rotated into their present orientation from an original orientation parallel to folds in the rocks to the southeast. AGE RELATIONS BETWEEN FOLDING, FORMATION OF MINERAL LINEATION, AND EMPLACEMENT OF GRANITIC ROCKS In the sillimanite schist, felted aggregates of ran- domly arrayed sillimanite fibers are bent around noses of crenulations and minor folds, whereas larger sillimanite needles are arranged parallel to fold and crenulation axes and form the mineral line- ation. Apparently, sillimanite began crystallizing GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE prior to folding and continued to grow parallel to fold axes during folding. Folding in the sillimanite schist is therefore inferred to have occurred during high-grade regional metamorphism, somewhat later than the beginning of crystallization of sillimanite. In other rocks, the age relations between high-grade regional metamorphism and folding are not as clear. However, the similarity in structural geometry be- tween the sillimanite schist and the rocks immedi- ately to the northwest and the parallelism between fold axes and mineral lineation formed by alinement of hornblende prisms, mica flakes, and quartz-feld- spar aggregates in layered gneiss, mica schist, and amphibolite suggest that folding was contempora- neous with high-grade regional metamorphism and that the lineation is due to preferential growth of minerals parallel to fold axes. Most mappable granitic bodies are concordant or semiconcordant in gross aspect; foliation in the granitic rocks and flanking migmatite generally is parallel to layering and foliation in the enclosing rocks. Minor folds commonly involve foliation in migmatite, and some dikes and pods of granitic rocks and pegmatite are apparently folded along with their wallrocks. In many places, however, dikes of granitic rocks are sharply discordant with folded layering and foliation in the enclosing rocks (fig. 924) and contain rotated inclusions, some of which have com- plex folds that must have formed before emplace- ment of the enclosing granitic rock (figs. 92B, 93). These relations indicate that most of the granitic rocks were emplaced during folding and high-grade regional metamorphism. Some granitic material, however, remained mobile long enough to be in- truded as discordant dikes and local crosscutting bodies after the peak of regional metamorphism and formation of the minor folds. BREVARD FAULT ZONE In the Grandfather Mountain area, the Brevard fault zone comprises a number of subparallel anasto- mosing faults marked by belts of mylonite, blastomy- lonite, and phyllonite intercalated with tectonic len- ses and slices of phyllonitic schist and gneiss, some of which are apparently derived from adjacent rocks and some of which are probably exotic. On the geo- logic map (pl. 1), rocks of the Brevard zone are sub- divided into blastomylonite and related siliceous mica-poor rocks and mica-rich phyllonitic schist and gneiss. ‘ BREVARD FAULT ZONE 153 FIGURE 92.-Field relations of granitic rocks. A, Dike of leucocratic granitic rock and pegmatite cutting layered biotite gneiss, biotite-hornblende gneiss, and fine-grained biotite schist and containing rotated and partly digested inclusions of the wallrocks. Biotite pegmatite forms a poorly defined zone along the contact. Drawn from a photo- graph. Roadcut in saprolite along farm road, 2.2 miles S. FIGURE 93.—Fold‘ in layered biotite and hornblende-biotite gneiss cut by dire of light-colored granitic rock containing of layered gneiss. Diagonal dark streaks are iron-stained zones along joints. Saprolite exposure in borrow pit in area H-8 on west side of road, 1.7 miles N. 16° E. of school at Chesterfield (area H-9, pl. 1). Direction of view is east, approximately parallel with fold axis. F Rocks in the Blue Ridge thrust sheet and Inner Piedmont belt adjacent to the Brevard fault zone blocky inclusion 77° E. of school at Gamewell (area I-8, pl. 1). B, Inclusion of complexity folded interlayered biotite and biotite-horn- blende gneiss and amphibolite in leucocratic granitic rock and migmatite. Drawn from a photograph. Roadcut in saprolite in area J-6, 300 feet south of Zacks Fork Creek, 3.25 miles N. 36° E. of courthouse in Lenoir (area J-7, pl. 1). t have structures and textures indicating that they | have been pervasively sheared and retrogressively metamorphosed during movement along faults in the Brevard zone. Unlike most of the rocks in the Bre- vard zone itself, however, these polymetamorphic rocks can be identified with appropriate map units in the flanking tectonic blocks. The polymetamorphic rocks are therefore mapped and described with the rocks of the Blue Ridge thrust sheet, Inner Piedmont belt, and Grandfather Mountain window, but their structure is discussed in conjunction with the struc- ture of the rocks in the Brevard fault zone. The slice of layered gneiss in the Brevard zone in the Linville Falls quadrangle (areas E-8 and F-8, pl. 1) is so like polymetamorphic layered gneiss adjacent to the zone in the Inner Piedmont that it is not described separately, although it is designated by a different pattern and symbol (gn) on plate 1. ROCK UNITS BLASTOMYLONITE Blastomylonite interleaved with mylonite, phyllon- ite, and phyllonitic schist and gneiss forms a narrow 154 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE FIGURE 94.-Photomicrographs of blastomylonite. A, Por- phyroclastic blastomylonite probably derived from Hender- son Gneiss, outcrop along Roses Creek about 1 mile south- east of the Linville Falls fault (area F-8, pl. 1). A por- phyroclast of perthitic microcline in a matrix of reerys- tallized biotite, epidote, sericite, quartz, and feldspar. Late quartz veinlet. Section perpendicular to mineral lineation. B, Blastomylonite from same outcrop as A. Epidote, seri- cite, microcline, albite, and quartz. C, Mylonite breccia from outcrop on point on north shore of Lake James, south of mouth of Dales Creek (area C-9, pl. 1). Fragments of mylonite in a matrix of recrystallized introduced (?) quartz. but continuous belt in the Brevard fault zone in the Linville Falls quadrangle and southwestward beyond the southwestern end of the Grandfather Mountain window. Blastomylonite also occurs in small lenses elsewhere along the Brevard fault zone and as thin layers and zones parallel to foliation in schist and gneiss on both sides, but these bodies are too small to distinguish on the geologic map (pl. 1). The blastomylonite is a very fine grained or aphanitic flinty rock, generally gray, greenish gray, buff, or pink, and commonly it has a dull waxy lus- ter. Some types superficially resemble quartzite or altered volcanic rock. Porphyroclasts of microcline, patch perthite, and plagioclase 0.5 to 1 em in diame- ter are common and, in some places, are larger (fig. 94A). The rock is composed of fine-grained recrys- tallized quartz, albite, and untwinned potassic feld- spar and scattered flakes and wavy folia of sericite, biotite, and chlorite (fig. 94B). Foliation is defined by lentils of coarser and finer grained mosaic and quartz segregation laminae, as well as by micaceous folia. Porphyroclasts of muscovite are common. Some of the rock is a microbreccia in which angular fragments of microcyrstalline material containing wavy bands of pseudotachylyte are set in a recrystal- lized matrix. The microcrystalline fragments and the coarser granoblastic mosaic (fig. 94C) are cut by an- astomosing quartz-feldspar mortar and microbrec- cia. Blastomylonite derived from mafic rocks con- tains abundant chlorite and epidote, and some of it has small porphyroclasts of hornblende and sphene. Locally, the blastomylonite is silicified and consists almost entirely of secondary quartz. The blastomy- lonite weathers to a structureless buff or cream-col- order clayey saprolite, locally containing irregular zones of chippy-weathering silicified blastomylonite. Foliation is commonly well developed in the blasto- mylonite and is generally parallel to foliation in the encosing rocks and to foliation in interleaved phyl- lonitic rocks. Contacts between blastomylonite and adjacent rocks are gradational through distances ranging from a few inches to hundreds of feet. PHYLLOLITIC SCHIST AND GNEISS Most of the Brfivard fault zone in the eastern half of the Grandfather Mountain area consists of a nar- row belt of phyllonitic schist interlayered with phyl- lonitic and blastomylonitic gneiss and phyllonite. The belt extends southwestward to Lake James and northeastward out of the map area. | gray, gray-green, or blue-green chlorite-sericite schist containing conspicuous bent porphyroclasts of muscovite as much as 2 em in diameter, partly re- placed by sericitefi and commonly containing porphy- roclasts of garnet 0.5 to 2 em in diameter, partially or entirely altere+i to chlorite. The muscovite porphy- roclasts and the sericite aggregates replacing them give much of the schist a curly, sealy, lensy, or wavy appearance ; in soils derived from weathering of the schist they form chips resembling buttons or large fish scales. With ldecrease in the size and abundance of muscovite porphyroclasts, the phyllonitic schist passes into lustrgus gray or greenish-gray chlorite- The phyllonitiflfi schist is a lustrous fine-grained sericite phyllonite. The phyllonitic schist commonly contains layers of light- to medium-gray phyllonitic or blastomylonitic gneiss ; locally, a few layers of amphibolite as much as 20 feet thick fire interlayered with the schist and gneiss. In a few places, the gneiss layers are clearly beds, and locally they have graded bedding. Small stringers and SZEregation lenses of finely granular quartz are abundant in the schist and gneiss; the texture suggests that the quartz has been sheared and recrystallized. The phyllonitihf schist resembles some of the poly- metamorphic schists in both the Blue Ridge thrust sheet and the Inner Piedmont belt, but it is richer in sericite, chlorittfi plagioclase, and garnet. Pegmatite has not been found in phyllonitic schist or interlay- ered rocks in the Brevard zone, although small peg- matite bodies are ubiquitous in the schist and gneiss in adjacent parts of the Inner Piedmont and the Blue Ridge thrust sheet. gneiss chiefly in the proportion of micaceous miner- als. In both rocks, recrystallized quartz and seattered grains of plagioclase and epidote form a mosaic of grains 0.01 to 0.2 mm in diameter. Plagioclase is gen- erally albite, bu£in a few rocks, the plagioclase has The phyllonitij: schist differs from the interlayered not been completely decalcified. Synkinematic and postkinematic flakes of sericite and chlorite 0.05 to 0.2 mm long are and form wisps scattered through the quartz mosaic and laminae which define the folia- BREVARD FAULT ZONE 155 tion. A few rocks contain synkinematic brown bio- tite, but in most, biotite is entirely replaced by chlor- ite. Muscovite porphyroclasts are very abundant in the phyllonitic schist, and a few occur in the inter- layered gneiss. In cross section they have the outline of flattened parallelograms with the long axes 0.5 to 1 ecm long and short axes 1 to 2 mm long. The long axes are alined with the foliation. Cleavage in some of the porphyroclasts is undeformed, but in many it is gently warped, and in a few it is sharply bent (fig. 95). Muscovite porphyroclasts are partly sericitized in many rocks, and in some, they are entirely re- placed by sericite aggregates, which, with more shearing, are distributed so that no trace of the large muscovite remains. Garnet crystals range from skeletal to euhedral, and many contain rotated helicitic trains of magne- tite and quartz inclusions. Muscovite porphyroclasts are bent around garnets, and in some rocks the gar- nets seem to be rounded. Most of the garnets are at least partly replaced by chlorite, and in some rocks they are entirely pseudomorphosed or remain only as scattered relicts in chlorite aggregates. In many rocks it is not clear whether the garnets are porphy- roblasts or porphyroclasts, but locally they are clearly porphyroclastic, and their alteration to chlor- ite suggests that they are all porphyroclastic, at least in relation to the latest recerystallization. FIGURE 95.-Photomicrograph of typical phyllonitic mica schist from roadcut on U.S. Highway 321 at south edge of Blowing Rock quadrangle (south edge of area J-5, pl. 1). Bent porphyroclasts of white mica and partly chloritized garnet in a matrix of quartz and white mica. 156 Magnetite is the principal opaque mineral and occurs as scattered small grains and aggregates which constitute several percent of some rocks. Small synkinematic and postkinematic needles of tourmaline are widespread ; the tourmaline is black in hand specimen and green or dark blue green in thin section. Other typical accessory minerals are zircon, apatite, and sphene. Locally, the schist con- tains scattered relicts of staurolite 0.2 to 0.5 mm in diameter embedded in sericite aggregates that were probably derived from staurolite porphyroclasts. A chemical analysis of a typical specimen of phyl- lonitic schist (table 29) shows Na,0, K.0O, and Al,0O; contents compatible with the amounts of chlorite and micas in the mode, assuming that the chlorite is aluminous and that some of the white mica is paragonite. X-ray study by A. J. Gude, 3d, of two TABLE 29.-Chemical analysis, mode, and norm of typical phyllonitic mica schist [Analysis determined by rapid methods by Paul Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, and Hezekiah Smith, U.S. Geol. Survey, 1962. Mode, by count of 600 points; P, present but not intersected in counting. Major oxides and CIPW norms given in weight percent; modes, in volume percent] Field Field 25-237 Laboratory 160198 - Laboratory No________.. 160198 Major oxides era c reals an ass 58.7. 2.4 MsO;-. :. c 28.0 8.3 F9203 ________________ 5.8 HQO raed R auth t u le .09 e 5.0 : 2.22..." 1.8 2/4. .34 lilt ec. ii. @MnO:=:-:;..1...... 26 1:9; <.05 Total:.}........ 100 Mode Muscovite and Epidote....:.<...... 5 paragonite....._._.._ 50 < 24 ...... .8 18 Tourmaline.'._...... .2 Opaque 5 R CGarmnet - 1.5. P CIPW norm :... lel s. 28,10 5.98 sis cas o 1680 Es 2.11 Ort. eral l .> 14;18 8.41 Abse: RA alle as 16.07 2.47 Ans? e Us ane un aaa ae 1:30 . 80 NotE.-Description of analyzed specimen follows: Lustrous green chlorite schist with silvery muscovite aggregates forming knots as much as 1 inch in diameter. Porphyroclasts of white mica as much as 1 cm in diameter are bent, folded, and partly granulated. Laminae formed by fine- grained white mica alternating with mosaic-textured quartz (grain size averaging 0.1 mm but reaching as much as 0.5 mm). MgFe chlorite is derived from alter- ation of garnet and possibly of biotite. Subhedral to euhedral garnet porphyro- clasts 0.5 to 1 mm in diameter partly altered to chlorite; some have rotated helicitic structure. Anhedral to euhedral magnetite 0.1 to 0.7 mm in diameter and finer grained opaque mineral included in mica. Staurolite in porphyroclasts as much as 0.3 mm long. From southwest side of Coffey Creek 0.7 mile northwest of North Carolina Highway 90 (area H-6, pl. 1). GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE other similar specimens of phyllonitic schist indi- cates that no albite is present and that about half of the white mica identified as sericite is paragonite. No other rocks in the Grandfather Mountain area are known to contain significant amounts of paragonite. The absence of pegmatite in the phyllonitic schist and gneiss in the Brevard fault zone and the miner- alogical differences between these rocks and polymet- amorphic schists and gneisses in adjacent parts of the Blue Ridge thrust sheet and Inner Piedmont belt suggest that the schist and gneiss in the fault zone form an exotic tectonic slice derived from outside the Grandfather Mountain area. In texture and mineral- ogy the rocks resemble parts of the Candler Forma- tion in the James River synclinorium in Virginia (Redden, 1963), but further detailed mapping along the Brevard fault zone northeast and southwest of the Grandfather Mountain area will be necessary to establish the source of the slice and the age and cor- relation of the rocks in it. STRUCTURE OF ROCKS OF THE BREVARD FAULT ZONE AND FLANKING POLYMETAMORPHIC ROCKS The geologic contrasts between the Blue Ridge belt northwest of the Brevard fault zone and the Inner Piedmont belt southeast of the zone are obvious on the generalized geologic map (pl. 1) and on the structure maps (figs. 32 and 33). The abrupt litho- logic change between rocks of the Blue Ridge belt and rocks of the Inner Piedmont belt is marked by the phyllonitic and mylonitic rocks of the Brevard. The changes in structural geometry are less abrupt and take place in a broader zone, mostly in the flanking rocks having metamorphic effects related to the Bre- vard fault zone. The orientation of lineation (fig. 33) in the Blue Ridge thrust sheet and Grandfather Mountain win- dow swings clockwise and becomes nearly horizontal and trends northeast in the fault zone and in the polymetamorphic rocks of the Inner Piedmont belt to the southeast. Southeast of the belt of polymeta- morphic rocks, the lineation changes abruptly to the east-west trend characteristic of the high-grade met- amorphic rocks of the Inner Piedmont. Statistical diagrams (fig. 5, pl. 5) summarize the structural geometry of rocks of the Brevard fault zone and of the adjacent rocks affected by shearing and retrogressive metamorphism related to it. Autochthonous basement rocks in the Grandfather Mountain window all have a uniform structural pat- tern. Cataclastic foliation formed during low-grade regional metamorphism strikes northeast and gener- BREVARD FAULT ZONE ally dips 40° to 60° SE. (fig. 32). Cataclastic linea- tion plunges southeast, nearly down the dip of the foliation planes (fig. 338). Diagrams A-1 and A-2 (fig. 5, pl. 5) show the orientation of foliation and lineation in the autochthonous basement rocks in an arbitrary belt 2 miles wide along the southeast edge of the window. Poles to foliation cluster around a single high maximum corresponding to a stike of N. 55° E. and dip of 45° SE. Projections of lineations form a point maximum plunging 45° S. 25° E. This pattern is consistent with the structural pattern in the autochthonous basement rocks elsewhere in the Grandfather Mountain window and is unrelated to the Brevard zone. Most of the basement rocks are nonlayered and consequently no folds can be seen, but the layered phase of the Wilson Creek Gneiss near the Linville Falls fault in the eastern part of the window (pl. 1) has tight and isoclinal folds with axial planes paral- lel to cataclastic foliation. Axes of most of these folds plunge southeast, parallel to cataclastic mineral lineation (diagram A-3, fig. 5, pl. 5), but some axes plunge gently south or southwest, forming a partial girdle in the orientation diagram. Structures in the Tablerock thrust sheet and in the higher thrust slices in the southwestern part of the Grandfather Mountain window are more closely re- lated to those of the overriding Blue Ridge thrust sheet than to those in the autochthonous rocks in the window. The geometry of structures in the Table- rock thrust sheet and higher thrust sheets in the win- dow in an arbitrary belt 2 miles along the southeast- ern edge of the window is summarized in the dia- grams in column B of figure 5, pl. 5. Bedding is nearly parallel to foliation in rocks of the Chilhowee Group, and bedding and foliation poles are therefore both plotted in diagram B-1, figure 5, plate 5. They are scattered around a single point maximum, in- dicating a more easterly strike and gentler dip than foliation in the autochthonous basement rocks along the southeastern edge of the window. Mineral line- ation generally plunges gently to moderately south and forms a partial girdle in the diagram (B-2, fig. 5, pl. 5), in a manner that corresponds to the con- spicuous swing from southeast plunges parallel to the regional cataclastic lineation in the Grand- father Mountain window to south and southwest plunges near the Brevard zone (fig. 33). The change in direction of mineral lineation in the Tablerock thrust sheet and higher thrust sheets occurs within 2 miles of the southeastern boundary of the window, but it is not apparent in autochthonous basement 157 rocks in the window at corresponding distances from the window boundary and the Brevard fault zone. Axes of crenulations and isoclinal and subiso- clinal folds in rocks of the Tablerock thrust sheet are parallel or subparallel to mineral lineation and have a similar distribution. Axial planes of the folds are nearly parallel to foliation (diagram B-3, fig. 5, pl. 5). The orientation pattern of crenulation axes and of fold axes and axial planes closely resemble that in the layered Wilson Creek Gneiss in the northeast- ern part of the window (diagram A-3, fig. 5, pl. 5). Diagrams in column C of figure 5, plate 5 show orientation of structures in rocks of the Blue Ridge thrust sheet in the narrow belt between the south- eastern side of the Grandfather Mountain window and the Brevard fault zone. Most of these rocks were originally of medium or high metamorphic grade, but most of them have been sheared and retrogres- sively metamorphosed under low-grade conditions during movement along the Linville Falls fault and the Brevard fault zone. Some of the medium-grade rocks in the northeastern part of the belt (pl. 6) have been remetamorphosed at medium grade, but some are apparently not polymetamorphic. Layering and foliation in the rocks of this belt are nearly ev- erywhere parallel; the poles are distributed in a single maximum reflecting strikes parallel to the Brevard zone and dips averaging about 50° SE. Cata- clastic mineral lineations formed during shearing and retrogressive metamorphism are arrayed in a partial great-circle girdle (diagram C-2, fig. 5, pl. 5) corresponding to the clockwise swing from moderate southward plunges near the Linville Falls fault to subhorizontal northeast trends near the Brevard fault zone (fig. 33). Axes of small isoclinal and subi- soclinal folds (diagram C-3, fig. 5, pl. 5) plunge gen- tly northeast or southwest; axial planes are subpar- allel to layering and foliation, but many are more steeply dipping. Medium-grade rocks in the north- eastern part of the belt have many sharp crinkles with subhorizontal axes (figs. 6A, 96) trending northeast parallel to minor folds elsewhere in the belt. Axial planes of the crinkles are marked by a steeply dipping slip cleavage in the nonpolymeta- morphic rocks which is parallel to cataclastic folia- tion in the polymetamorphic medium-grade rocks to the southeast. Foliation and layering of phyllonitic and blasto- mylonitic rocks in the Brevard fault zone strike northeastward, parallel to the trend of the zone, and generally dip steeply southeastward, but vertical or even northwestward dips are found locally. Poles of GEOLOGY, GRANDFATHER MOUNTAIN FIGURE 96.-Crinkles with steep axial planes and northeast- trending axes in biotite-muscovite schist and gneiss of the Blue ridge thrust sheet southeast of the Grandfather Mountain window. At altitude of 1,710 feet near head of Cove Branch (area J-5, pl. 1). foliation and layering (diagram D-1, fig. 5, pl. 5) cluster around a single broad maximum similar to the foliation and layering maximum in the Blue Ridge thrust sheet northwest of the fault zone. Mineral lineation is conspicuous. It is marked by alinement of recrystallized mineral grains and ag- gregates, elongation of porphyroclasts, and streak- ing and grooving on foliation planes and is indistin- guishable from lineation in low-grade polymeta- morphic rocks northwest of the fault zone. Most line- ations are subhorizontal and trend northeast, par- allel to lineation in polymetamorphic rocks southeast of the zone, but many plunge southwest or south, forming a poorly defined partial great-circle girdle in the statistical diagram (diagram D-2, fig. 5, pl. 5). No large folds are apparent in the rocks of the Brevard zone, but minor folds and crenulations are common, especially in phyllonitic schist and layered gneiss in tectonic slices within the zone. The minor folds in the gneissic rocks are isoclinal or subiso- clinal and have steeply dipping axial planes; they commonly have slip cleavage parallel to axial planes. Layers maintain approximatey uniform thicknesses parallel to axial planes. Fold axes show wide varia- tion in attitude within single outcrops, and some in- dividual axes are visibly curved. Many of the phyllonitic rocks have steeply dipping slip cleavage planes which offset and crenulate the WINDOW, NORTH CAROLINA AND TENNESSEE older foliation. Crenulations range in amplitude and wavelengths from a fraction of an inch to several inches. Their axes are approximately parallel to the other fold axes, and they show similar variation in attitude in single outcrops. The attitudes of the minor folds and crenulations (diagram D-3, fig. 5, pl. 5) have a distribution pattern similar to that of the mineral lineation. The shape of the folds and the distribution of their axes in the spherical projection in a great circle rather than in a single maximum indicate that they are passive flow folds (Donath and Parker, 1964). The geometry of the structures in the belt of rocks in the Inner Piedmont having polymetamorphic fea- tures related to the Brevard fault zone is summa- rized in the diagrams in columns E and F of figure 5, pl. 5. The diagrams in column E show structures in rocks thoroughly sheared and completely recrystal- lized under medium- or low-grade conditions adja- cent to the fault zone. The diagrams in column F show orientation of structures in the rocks partially sheared and recrystallized under medium-grade con- ditions along the southeast side of the belt of poly- metamorphic rocks. Foliation and layering strike northeast and dip southeast at moderate to steep angles which increase as the fault zone is approached; poles cluster sym- metrically around high point maxima (diagrams E-1 and F-1) which are more sharply defined than those in the nonpolymetamorphic rocks to the southeast. Mineral lineation is much more conspicuous in the polymetamorphic rocks than in the rocks to the southeast. It is marked by streaking of flakes and clusters of new micas and recrystallized quartz-feld- spar aggregates on foliation planes and by alinement of long dimensions of feldspar porphyroclasts. The lineation lies nearly parallel to the strike of the folia- tion planes. Projections of lineation in the statistical diagrams (EF-2 and F-2) show nearly symmetrical concentrations around high point maxima having slight northeast plunges. No major folds are suggested by the map pat- terns; bodies of mica schist and Henderson Gneiss form long strips and concordant lenses that resemble megaboudins in the layered gneiss country rock. Minor folds (fig. 97) are subisoclinal and have axes trending northeastward, parallel to mineral linea- tion, and axial planes that generally dip steeply to moderately southeast. Near the Brevard fault zone, axial planes are steep or vertical, and the folds are more tightly appressed. Many layers maintain rather uniform thicknesses in these folds, although most in." ar'qdfn 4 ; ia B, . NBG ~~ CAP & fd tne sats \ ym Aly C \ ® Muscovite pegmatite layers are conspicuously thickened in the apices, but have approximately uniform thickness measured parallel to the trace of the axial plane. In some lay- ers, slip cleavage is locally developed parallel to axial BREVARD FAULT ZONE 159 FIGURE 97.-Folds in polymetamorphic rocks of the Inner Piedmont belt near the Brevard fault zone. A, Folds in interlayered fine-grained dark-gray biotite gneiss and medium-grained Henderson Gneiss containing potassic feld- spar augen elongated parallel to the fold axes. Direction of view is northeast, approximately parallel to fold axes. Note incipient fracture cleavage parallel to axial planes. Saprolite exposure on shore of Lake James, 0.85 mile N. 88° W. of Linville Church (area E-9, pl. 1). B, Folds in interlayered light- and dark-gray biotite gneiss and white quartzo-feldspathic gneiss containing boudins of amphib- olite and pods of cataclastic muscovite pegmatite. Direc- tion of view is northeast, parallel to fold axes. Roadcut in saprolite on northeast side of road 1.2 miles S. 55° E. of Fairview Church (area F-8, pl. 1). planes. Boudins and blocks of pegmatite, amphibol- ite, and other less ductile rocks show no consistent geometric relation to fold noses and must have been produced by shearing parallel to layering prior to formation of the folds. RELATION BETWEEN STRUCTURES IN THE BREVARD FAULT ZONE AND STRUCTURES IN ROCKS TO THE NORTHWEST The northwest-trending cataclastic mineral linea- tion in the rocks of the Grandfather Mountain win- dow and in the low-grade polymetamorphic rocks near the sole of the Blue Ridge thrust sheet (fig. 33) is in the a direction, parallel to tectonic transport of the Blue Ridge and Tablerock thrust sheets. Similar cataclastic lineation in the Brevard fault zone and in polymetamorphic rocks to the southeast is subhori- zontal and trends northeast, parallel to the strike of the zone. It is believed to be an a lineation formed during strike-slip movement along the Brevard fault zone (Reed and Bryant, 1964b). Previously we interpreted the swing of the cata- clastic lineation in the rocks of the Blue Ridge belt near the Brevard zone as the result of drag along the Brevard and concluded that movement along the Brevard was therefore right lateral and younger than thrusting (Reed and Bryant, 1964b). However, the statistical orientation diagrams (fig. 5, pl. 5) show that the geometry of the structures in the thrust sheets northwest of the Brevard is similar to that in the Brevard zone itself and that the change in direction of the cataclastic lineation is not as appar- ent in autochthonous basement rocks as in the thrust sheets in the Grandfather Mountain window near the Brevard zone. Rotation of the cataclastic linea- tion by drag would require concomitant rotation of planes containing the lineation, unless, by coinci- dence, the rotation occurred around an axis exactly perpendicular to the planes. The planes containing the lineation strike consistently northeast and dip southeast and show no evidence of rotation by drag. 160 In the orientation diagrams (fig. 5, pl. 5), poles to foliation and layering in the Brevard zone (diagram D-1) and in the Blue Ridge thrust sheet between the Brevard and the Grandfather Mountain window (diagram C-1) form single high maximums in about the same position as poles to foliation in autochthon- ous basment rocks in adjoining parts of the window (diagram A-1). In the Tablerock thrust sheet, the cataclastic mineral lineation lies on bedding planes and foliation planes parallel to bedding; poles of these planes cluster around a point maximum in a different position than the foliation and layering maximums in the adjoining rocks (diagram B-1). Mineral lineation in the Tablerock thrust sheet is involved in the swing in regional lineation and forms a partial great-circle girdle in the diagram (diagram B-2) similar to the lineation girdles in the Blue Ridge thrust sheet (diagram C-2) and the Brevard zone (diagram D-2). If the swing in lineation in diagram B-2 were due to rotation around an axis normal to bedding and foliation in the Tablerock thrust sheet (diagram B-1), it should have rotated foliation and layering planes in the Blue Ridge thrust sheet and produced a small-circle girdle in the foliation (diagram C-1). Conversely, if the line- ation swing in the Blue Ridge thrust sheet (dia- gram C-2) were due to rotation around the corre- sponding layering and foliation maximum (diagram C-1), it should have produced a small-circle girdle in the bedding and foliation diagram for the Tablerock thrust sheet (diagram B-1). As neither diagram B-1 nor C-1 shows any indication of such rotation, it seems unlikely that the lineation swing is due to sim- ple rotational drag. There is no indication that the northeast-trending cataclastic lineation in the Brevard zone has over- printed an older northwest-trending lineation in the rocks to the northwest. All the lineations are petro- graphically similar, and nowhere are diversely ori- ented lineations superimposed. The smooth swing in lineation trends and the resulting continuity in the girdles in diagrams B-2, C-2, and D-2 could hardly have resulted from overprinting an older lineation by a younger one. The similarity in lineation pattern between the Brevard zone (diagram D-2) and the adjacent Blue Ridge thrust sheet (diagram C-2) is particularly striking. This similarity could not be the result of rotation of older lineation or of the eradica- tion of older lineation and formation of new linea- tion during movement along the Brevard, for in the Brevard zone itself the old lineation should have been either entirely rotated or completely destroyed by the intense shearing and almost complete reerys- GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE tallization that took place, as shown by the blastomy- lonitic and phyllonitic rocks in the Brevard zone. The northeast-trending cataclastic a lineation along the Brevard zone is, therefore, probably of the same age as northwest-trending cataclastic a linea- tion in rocks of the Blue Ridge thrust sheet and Grandfather Mountain window. The swing in linea- tion trends in the thrust sheets northwest of the Bre- vard must be due to change in direction of tectonic transport, from nearly horizontal and northeast along the southeast side of the Brevard to northward and upward near the northwest side of the zone to northwestward in the Tablerock and Blue Ridge thrust sheets farther from the Brevard zone. Thus, the lineation pattern indicates that strike-slip move- ment along the Brevard fault zone was contempora- neous with northwestward thrusting of rocks of the Blue Ridge. Axes of folds and crenulations in the Brevard zone and in the flanking rocks are generally parallel or subparallel to the cataclastic lineation. The close sim- ilarity between the orientation pattenrs of the fold axes (diagrams A-3 through F-3, fig. 5, pl. 5) and of the lineation (diagrams A-2 through F-2) indicates that most of the fold axes are in the a direction and that they formed at the same general time as the lineation. Many folds in the Blue Ridge thrust sheet north and west of the Grandfather Mountain win- dow also have axes parallel to cataclastic mineral lin- eation (fig. 1, pl. 5, diagram A-4). The consistent parallelism between fold axes and cataclastic linea- tion in the zone between northwest-trending linea- tion in the Blue Ridge and Tablerock thrust sheets and northeast-trending lineations in the polymeta- morphic rocks southeast of the Brevard zone is con- sistent with the conclusion that the two lineations are contemporaneous. RELATION BETWEEN STRUCTURES IN POLYMETAMORPHIC ROCKS SOUTHEAST OF THE BREVARD ZONE AND STRUCTURES IN OTHER ROCKS OF THE INNER PIEDMONT Shearing and recrystallization during retrogres- sive metamorphism have produced a structural pat- tern in the polymetamorphic rocks of the Inner Piedmont adjacent to the Brevard fault zone that is quite different from that of the unaffected rocks farther southeast. Although the geometry and sym- metry of the two groups of structures is similar, those in the polymetamorphic rocks are apparently related to movement along the Brevard fault, whereas those in the rocks to the southeast date from the earlier high-grade regional metamorphism. DIABASE GRANDFATHER MOUNTAIN WINDO! | | | | | | | | zBUGEE BREVARD [THRUST | SHEET asc Nand IpoLymetamorpHic ? | | Rocks OF THE | INNER PIEDMONT & | NONPOLYMETAMORPHIC f | rocks OF THE INNER j PIEDMONT | | | | AND LAYERING & | TO NUMBER OF MEASUREMENTS OF FOLIATION RATIO OF NUMBER OF LINEATION MEASEREMENTS | | | | | 9 3 os 6 - a4 | | | | = sl | I AND LAYERING 0.05 - x~ RATIO OF NUMBER OF FOLD AND CRENULATION AXES TO NUMBER OF MEASUREMENTS OF FOLIATION A B C D= ~~E F A B € FIGURE 98.-Graphs showing ratios of numbers of measure- ments of fold and crenulation axes and mineral lineation to numbers of measurements of layering and foliation in rocks of the Brevard fault zone and adjacent parts of the Grandfather Mountain window, Blue Ridge thrust sheet, and Inner Piedmont belt. Letters under first six graphs, A-F, correspond to areas for which orientation diagrams are given in figure 5, plate 5; letters under last three graphs, A-C, a? figure 4, plate 5. ured in almost every outcrop of bedrock or saprolite, the degree of development of mineral lineation and the relative abundance of minor folds can be esti- mated by comparing the number of attitudes of min- As attitudes if layering or foliation can be meas- eral lineation or fold axes measured in a given area with the number of measurements of foliation and layering in the same area. Figure 98 shows relative degree of lineation development and abundance of minor folds in aFeas corresponding to the columns of 161 figures 4 and 5, plate 5. Mineral lineation is most strongly developed northwest of the Brevard zone, but it is far more conspicuous in the Brevard zone and in the polymetamorphic rocks of the Inner Pied- mont adjacent to it than in the nonpolymetamorphic rocks farther southeast. Clearly, the northeast-trend- ing mineral lineation in the rocks near the Brevard zone cannot be explained by simple rotational drag of older east-trending mineral lineation of the Pied- mont rocks. The lineation map (fig. 33) suggests that locally some rotation of the old mineral lineation in the Inner Piedmont rocks may have occurred, but most, and probably all, of the northeast-trending mineral lineation in the polymetamorphic rocks is younger. Nowhere, however, have both lineations been recognized in the same outcrop. Minor folds are most abundant, or at least most easily measured, in the sillimanite schist of the Inner Piedmont, and they are considerably less conspicuous in the other nonpolymetamorphic Piedmont rocks. They are least abundant along the southeast edge of the belt of polymetamorphic rocks and become more common as the Brevard zone is approached. This dis- tribution suggests that the east-trending folds in the Piedmont rocks were obliterated during shearing and retrogressive metamorphism along the Brevard zone and that continued movement in the polymeta- morphic rocks produced new folds, with northeast- trending axes, which increase in abundance and in- tensity closer to the fault zone. Only locally (dia- gram A-3, fig. 4, pl. 5) is there evidence that some of the older east-trending folds are rotated, but these rotated folds were obliterated in the more strongly sheared rocks to the northwest. The inferred age relation between the two sets of folds and the lineations associated with them is sup- ported by their relations to the granitic rocks and pegmatites. Discordant granitic and pegmatitic dikes cut across east-plunging folds in the nonpolymeta- morphic rocks in the Inner Piedmont, but similar pegmatites were sheared and boudinaged before for- mation of northeast-trending folds in the polymeta- morphic rocks. DIABASE A remarkably straight and continuous dike of un- metamorphosed olivine diabase a few feet to several hundred feet thick enters the Grandfather Mountain area near Drexel (area I-9, pl. 1) and extends north- westward for more than 15 miles. It passes without deflection from the Inner Piedmont belt across the Brevard fault zone and the Linville Falls fault and FIGURE 99.-Dike of unmetamorphosed diabase of Late Triassic(?) age in sheared and retrogressively meta- morphosed gneiss of the Inner Piedmont in the Brevard fault zone. Note spheroidal weathering. Roadcut in saprolite on east side of Wilson Creek southwest of Adako (area G-7, pl. 1). into the basement rocks in the Grandfather Moun- tain window. Smaller disconnected diabase dikes lo- cally parallel the main dike, and a few are found along the same trend in the basement rocks at sev- eral places along Wilson Creek as far northwest as the mouth of Harper Creek (area F-6, pl. 1). Keith and Sterrett (1954) mapped a large and nearly con- tinuous diabase dike along the same trend for at least another 15 miles southeast of the Grandfather Mountain area. Diabase has been found nowhere else in the Grandfather Mountain window or in the Inner Piedmont part of the area, but a few dikes and a sill too small to map have been found in rocks of the Blue Ridge thrust sheet near the mouth of Fall Branch (area C-5). The diabase is a fine- to medium-grained dark- blue-gray rock which weathers to shades of ochre on GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE exposed surfaces. It is more resistant to weathering than most of the enclosing rocks and forms abundant round boulders and cobbles in the soil and a few natural outcrops along streams (fig. 99). The diabase has ophitic textures in which slender laths of twinned and strongly zoned plagioclase (generally bytownite) are enclosed in poikilitic grains of augite and olivine or intergranular tex- tures in which olivine and augite grains are intersti- tial to the plagioclase laths (fig. 100). In a few specimens, augite and olivine also form anhedral to subhedral phenocrysts. The rock consists of 40 to 60 percent plagioclase, 20 to 35 percent augite, 1 to 5 percent magnetite, 0 to 25 percent olivine, and lo- cally contains a few grains of basaltic hornblende and biotite. In the centers of the larger dikes, the diabase is medium grained and generally has ophitic textures in which the plagioclase laths are 1 to 2 mm long and the poikilitic augite and olivine grains are 1 to 2 mm in diameter (fig. 100). In the small dikes and in chilled margins within 1 or 2 feet of the contacts of the larger bodies, the rock is finer grained and tex- tures are more commonly intergranular. Plagioclase laths are 0.2 to 0.5 mm long, and intergranular grains of augite and olivine are 0.1 to 0.2 mm in diameter. The unmetamorphosed character of the diabase, and the undeflected northwest trend of the principal dike athwart all other structures clearly shows that FIGURE 100.-Photomicrograph of diabase. Unmetamorphosed diabase from roadcut on North Carolina Highway 18 (area H-8, pl. 1) 1 mile northeast of Chesterfield. Ophitic tex- ture. Laths of bytownite enclosed in grains of augite and olivine. From interior of dike at a place where the dike is about 100 feet thick. MAJOR FAULTS BOUNDING TECTONIC UNITS the diabase was emplaced after all the metamorph- isms and major deformations of the enclosing rocks. The sharp chilled contacts of the dikes show that the diabase was intruded into relatively cool wallrocks ; lack of offset of layers in the country rocks across the dikes shows that no faulting was involved during their emplacement. The trend of the principal diabase dike in the Grandfather Mountain area is parallel with similar dikes elsewhere in the Piedmont in North Carolina, South Carolina, and Georgia (King, 1961; Lester and Allen, 1950). These dikes are generally consid- ered to be of Late Triassic age because of their rela- tions to rocks of the Upper Triassic Newark Group, but they may be as young as Early Cretaceous (Reinemund, 1955). This dike is clearly part of the same dike swarm, and the diabase in the Grandfather Mountain area is therefore probably of Late Triassic age but is possibly younger. MAJOR FAULTS BOUNDING TECTONIC UNITS THRUST FAULTS LINVILLE FALLS FAULT The Linville Falls fault forms the boundary of the Grandfather Mountain window. Along it Pre- cambrian crystalline rocks of the Blue Ridge thrust sheet are carried over autochthonous Precambrian plutonic rocks and metamorphosed upper Pre- cambrian sedimentary and volcanic rocks and over Cambrian and Cambrian(?) rocks in the Tablerock thrust sheet. Branches of the Linville Falls fault cut the Tablerock fault east of Shortoff Mountain (area E-8, pl. 1). The imbricate faults in areas B-9 and C-9 and the faults marked by slices of Chilhowee quartzite in the Blue Ridge thrust sheet on the northwest side of the window are somewhat older subsidiary faults of the Linville Falls fault, as the map pattern shows that they are cut by it. The Ta- blerock fault may also be an older subsidiary thrust. On the southeast side of the Grandfather Moun- tain window, the Linville Falls fault dips southeast, parallel to foliation in adjacent parts of the window and the Blue Ridge thrust sheet. On the north and west sides of the window, there is marked structural discordance between the fault and the cleavage in the autochthonous window rocks (fig. 32) ; locally, folia- tion in the nearby window rocks is dragged in a sinistral sense into parallelism with the fault. The fault plane is parallel to cataclastic foliation in the overriding rocks. 163 Where the Linville Falls fault overlies the Table- rock thrust sheet, no discordance is apparent between foliation in the Blue Ridge thrust sheet above the fault and cleavage and bedding in the Tablerock thrust sheet beneath it, although mapping shows dis- cordance in the gross structure. The Linville Falls fault is best exposed on the west side of the Linville River 100 yards upstream from the end of the National Park Service trail to the head of Linville Falls (area D-6, pl. 1). There, rudely layered Cranberry Gneiss overlies green seri- cite quartzite of the Tablerock thrust sheet (fig. 101). The contact, which dips gently west, is marked by 6 to 18 inches of white to green finely laminated blastomylonite (fig. 102) which is parallel to bedding in the quartzite and to foliation in the gneiss. Sev- eral other thin blastomylonite layers occur in the quartzite 100 to 200 feet south of the exposure of the main fault. The fault plane separating Cranberry Gneiss above from Shady Dolomite beneath is exposed near the base of a prominent cliff on the west side of the FIGURE 101.-Linville Falls fault at its type locality. Expo- sure on west side of Linville River about 100 yards up- stream from end of National Park Service trail to head of Linville Falls (area D-6, pl. 1). Massive rock is coarse- grained blastomylonitic quartz monzonite gneiss of the Blue Ridge thrust sheet. It overlies less resistant zone of blastomylonite, a layer of which interfingers with the quartz monzonite. Trash in foreground lies on outcrop of quartzite of the Chilhowee Group in the Tablerock thrust sheet. 2 mm FIGURE 102.-Photomicrograph of blastomylonite from Lin- ville Falls fault at Linville Falls (fig. 101). Partly re- crystallized mylonite containing recrystallized quartz and iron-rich muscovite and some unrecrystallized quartz and feldspar. Crinkles trend northeast, parallel with late folds in nearby rocks of the Tablerock thrust sheet (fig. 67). Section cut parallel to mineral lineation. valley of the north fork of the Catawba River about 0.3 mile north of Linville Caverns (area C-7, pl. 1). Near the fault, the gneiss has been reduced to a dark-green siliceous blastomylonite. The fault is marked by a 6-inch quartz vein. The Shady Dolomite is shattered and silicified in a 1- to 2-foot zone below GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE the fault. The fault plane is parallel to layering and foliation in the gneiss; bedding is not apparent in the dolomite. The fault is also well exposed on the north side of the window in a roadcut along North Carolina High- way 194 about half a mile east of Bowers Gap (area E-3, pl. 1; fig. 103). There, blastomylonitic Cran- berry Gneiss with cataclastic foliation dipping north overlies metasiltstone of the Grandfather Mountain Formation, which has east-dipping cleavage. The rocks are separated by a gouge zone half an inch thick, and the cleavage in the underblock is disturbed in a zone 2 to 6 feet thick below the fault. The fault dips 35° N. The fault is locally exposed on the southeast side of the window on the steep slopes of Stone Mountain at an altitude of 2,450 feet, S. 72° E. of Harris Gap (area J-4, pl. 1). There, garnet-biotite-muscovite schist, containing porphyroclasts of muscovite as much as 7 mm in diameter, overlies a few feet of amphibolite and strongly sheared layered quartz- feldspar and biotite-quartz-feldspar gneiss. The schist near the contact has fragments of biotite FIGURE 103.-Linville Falls fault in roadcut along North Carolina Highway 194 about one-half of a mile east of Bowers Gap (area E-3, pl. 1). Cataclastic foliation in Blue Ridge thrust sheet is parallel with fault plane, which dips about 35° N. Trace of east-dipping cleavage in silt- stone of the Grandfather Mountain Formation shown. Direction of view is northeast. Photograph by Frank G. Lesure, U.S. Geological Survey. MAJOR FAULTS BOUNDING TECTONIC UNITS schist 1 to 2 inches long that appear to be tectonic inclusions. Recrystallized plagioclase in rocks on ei- ther side of the contact is An;;. In several places along this segment of the Linville Falls fault, similar exposures occur. Amphibolite is commonly found at the fault and ranges from a few inches to 50 feet thick. Farther southwest, the fault is exposed on the southeast side of the window in a small quarry and in an adjacent roadcut (area F-8, pl. 1) along North Carolina Highway 181 on the east side of Steels Creek about 0.7 mile northwest of Smyrna Church. In the quarry, layered biotite gneiss overlies felsic metavoleanic rocks. The fault plane is marked by a 10- to 20-foot quartzite slice; several other thin slices are intercalated with the gneiss of the overriding block within a few feet of the fault. In the roadcut, a single 2- to 3-foot quartzite slice separates gneiss from the underlying felsic volcanic rock. In many places along the fault, lenses of quartzitic blastomylonite or arkosic quartzite a few feet thick resembling rocks of the Chilhowee Group are found with blastomylonite and mylonite derived from feld- spathic gneiss. The sheared rocks along the fault all have strong lineation parallel to cataclastic lineation in nearby parts of the Blue Ridge thrust sheet. In the south- western part of the window (pl. 1), the fault is lo- cally vertical or overturned, and the lineation along it is horizontal, indicating that in that area the latest displacement along the fault was strike-slip asso- ciated with movement along the Brevard fault zone. Roadcuts along North Carolina Highway 80 by the Lake Tahoma Dam (area A-10) expose biotite-bear- ing arkosic quartzite of the Chilhowee Group and associated blastomylonite and mylonite along the steeply dipping segment of the fault on the west side of the window. STONE MOUNTAIN FAULT The Stone Mountain fault carries rocks of the Blue Ridge thrust sheet over rocks of the Unaka belt in northeastern Tennessee. It occupies a position analo- gous to the Great Smoky fault to the southwest, but the connection shown on plate 3 is only an interpre- tation ; detailed published maps are lacking between northeast Tennessee (King and Ferguson, 1960), the Hot Springs window (Oriel, 1950), and the Great Smoky Mountains (Hamilton, 1961; Hadley and Goldsmith, 1963; King, 1964b; Neuman and Nelson, 1965). 165 In the northwest part of the Grandfather Moun- tain area, the Cranberry Gneiss overrides similar but less metamorphosed plutonic rock and little met- amorphosed Cambrian sedimentary rocks along the Stone Mountain fault and related faults (pl. 1). The relations between the faults are complex and some- what obscure because their exact positions and the configuration of their intersections are obscured by colluvium. A reexamination of parts of the area, however, suggests a somewhat different interpreta- tion than that of King and Ferguson (1960) or Rodgers (195832). The gross pattern of faults (pl. 1) resembles that mapped by Rodgers (19532) more than that by King and Ferguson (1960). Rodgers' map (fig. 1044) shows the basement rocks in the vicinity of Dark Ridge Creek (area C-2, pl. 1) within the Mountain City window. He inferred that the Unaka Mountain and Stone Mountain faults were overridden by crys- talline rocks along the Snow Mountain fault. King and Ferguson (1960, p. 77-78, fig. 18) offered a "speculative explanation" of the more complex map pattern revealed by further mapping (fig. 104B). They suggested a well-defined sequence of faults ; the rocks are broken successively along the Stone Moun- tain fault, the Poga fault, and the Unaka Mountain fault. They believed the strip of basement rocks at the southern foot of Little Stone Mountain (area C-1, pl. 1) to be unconformably beneath rocks of the Chilhowee Group. We believe that the Unaka Mountain and Snow Mountain faults are connected as shown in figure 104C and that the Stone Mountain and Poga faults of King and Ferguson (1960, fig. 18) are an upper branch of the same fault. We have termed the entire system of faults the Stone Mountain fault. The Chil- howee Group rocks on Little Stone Mountain are in- terpreted as a subsidiary slice below the main fault. The basement rocks in the Dark Ridge Creek area are believed to have been overridden by the Blue Ridge Creek thrust sheet along the lower branch of the Stone Mountain fault and to have overridden the rocks of the Chilhowee Group in the Doe River inner window of the Mountain City window along the com- plex group of thrusts south of Little Stone Mountain. Thus, they occupy an intermediate sheet between the main Blue Ridge thrust sheet and the Mountain City window. The long westward-protruding tongue of basement rocks shown in the northern part of figure 104C is believed to have been carried farther north- west than the main mass of the Blue Ridge thrust 166 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE EXPLANATION 2 Rome Formation and Allochthonous Precambrian Shady Dolomite plutonic rocks Chilhowee Group Autochthonous Precambrian plutonic rocks Contact C& _& __ lll Thrust fault Sawteeth on upper plate. Dashed where approximately located; dotted where projected; queried where doubtful odgers (1953a, plate 6) POGA FAULT STONE MOUNTAIN ~, FAULT } STONE _ MOUNTAIN 4 / s FAU LT STONE MOU NT ATN POGA Faure... [AY STONE MOUNTAIN ULT a STONE MOUNTAIN STONE, MQLNIA! N FAULT FAULT lower branch)A 4 STONE MOUNTAIN STONE MOUNTAIN FAU LT FAULT (upper branch) ..' ‘~,§Iower branch) £ nfanigpatd via '\I\/\:‘\‘/. 1 This paper 1 0 e 2 3 4 MILES KCucild | | | ] FIGURE 104.-Map and sections of the northwest corner of the Grandfather Mountain area showing various interpretations ofthe structures on the northwest margin of the Blue Ridge thrust sheet. MAJOR FAULTS BOUNDING TECTONIC UNITS sheet by somewhat later movement along the upper branch of the Stone Mountain fault. The upper branch of the Stone Mountain fault ex- tends southeastward into the Linville quadrangle and apparently dies out north of Long Ridge (area E-2, pl. 1). It is marked by slices of Chilhowee quartzite along much of its length, and where it crosses Beech Creek (area D-1, pl. 1), it dips north rather than south as shown by King and Ferguson (1960, pl. 1 and fig. 18). The same is true at several other places where its dip can be determined. The Snow Mountain fault of Rodgers (19532) con- nects with the Unaka Mountain fault of Rodgers 19532) and King and Ferguson (1960, fig. 18) near the site of the former Dark Ridge School (area D-2, pl. 1) and extends north to Flat Springs Branch (area D-1) in the Elk Mills 74-minute quadrangle. This extension cannot be traced with certainty, but the fault is generally marked by phyllonite and lo- cally, by slices of Chilhowee quartzite. The nature of the intersection between this fault and the upper branch of the Stone Mountain fault is uncertain. There is no marked difference in lithology across the segment of the Stone Mountain fault between Dark Ridge school and Flat Springs Branch (Snow Moun- tain fault of Rodgers), but west of the fault, shear- ing is generally confined to phyllonite zones, and some of the intervening rock is not affected by shear- ing and retrogressive metamorphism. Lineation is much less conspicuous in this block than in the Blue Ridge thrust sheet (fig. 33). We were unable to recognize the difference in the basement rocks immediately south of Little Stone Mountain and those farther south. We therefore be- lieve that the fault separating the Chilhowee rocks on Little Stone Mountain from the basement rocks to the south lies along the contact between the Chil- howee and the basement rocks, rather than in the basement as shown by King and Ferguson (1960, pl. 1). Map relations near the head of Flat Springs Branch indicate that this fault dips north (fig. 104C) rather than south as they believed (fig. 104B). No complete exposures of the Stone Mountain fault were found in the Grandfather Mountain area, but there are many exposures of blastomylonite and phyllonite in the fault zone along the steep north side of Dark Ridge (area C-2, pl. 1) and on the west slope of Dividing Ridge (area D-2). Near the north- west corner of the Linville quadrangle, the fault zone contains breccia composed of fragments of sheared quartzite and perthite in a matrix of green iron-rich chlorite (fig. 105). 167 FIGURE 105.-Photomicrograph of breccia from Stone Moun- tain fault zone 1.0 mile S. 77° E. of northwest corner of the Linville quadrangle (area C-2, pl. 1). Fragments of sheared quartzite and perthite in a matrix of chlorite. Section cut parallel to mineral lineation. Northwest of Dark Ridge School (area C-2, pl. 1), the Stone Mountain fault dips 5° to 20° SW., except at the Elk River where the dip appears to be 'as steep as 60°. Discordance between gently south- dipping cataclastic foliation in the Beech Granite above the fault and east-dipping bedding and folia- tion in rocks beneath the fault is conspicuous along this segment. North of the point where the Stone Mountain fault leaves the contact of the Beech Gran- ite (area D-2), it dips 45° to 70° SE., parallel to foliation in the flanking rocks. The faults between the basement rocks of the Dark Ridge Creek area and the Doe River inner win- dow (fig. 104C) dip from about 40° SE. to nearly vertical. They are marked by phyllonite, phyllonitic gneiss, and slices of quartzite in the crystalline rocks but are more difficult to locate precisely in the quartz- ites of the Chilhowee Group. EXTENT OF THRUSTING The Grandfather Mountain window lies near the midpoint of the belt of extensive thrusting along the northwestern edge of the Blue Ridge that extends from central Virginia to northern Alabama, where the Appalachian belt is covered by younger sedimen- tary rocks (fig. 1). The window is perhaps the best available indicator of the minimum amount of north- westward transport of the Blue Ridge thrust sheet. At least 35 miles of movement is required to bring 168 the plutonic rocks of the northwest edge of the Blue Ridge thrust sheet from southeast of the Grand- father Mountain window. Discordant relations be- tween medium- and high-grade metamorphic rocks and the structurally underlying low-grade plutonic rocks around the Grandfather Mountain window suggest the presence of another major fault along which the overlying higher grade rock in the Blue Ridge thrust sheet moved at least an additional 20 miles northwestward. Little information is available on the amount of transport of the Blue Ridge thrust sheet elsewhere along strike. The map pattern south of Roanoke, Va. (Woodward, 1932), indicates a minimum displace- ment of 8 miles across an allochthonous footwall block. Estimates based on detailed mapping in the Great Smoky Mountains indicate a minimum of 10 miles displacement on the Great Smoky fault (Neu- man and Nelson, 1965, p. 52) and total postmeta- morphic northwestward movement of the Blue Ridge thrust sheet of 12 to 24 miles (Hamilton, 1961, p. 46; King, 1964b, p. 121). The northwestward bulge of the Blue Ridge belt in the Great Smoky Mountain region (fig. 1), however, suggests as much as 50 miles of transport. The greatest concentration of thrust faults mapped in the southern Appalachians lies northwest of the Great Smoky Mountains (fig. 1) and may indicate that maximum northwestward transport of the Blue Ridge thrust sheet took place there. Kessler (1950, p. 30-33) concluded that no major thrust occurs along the margin of the Blue Ridge province in northwest Georgia, but other work just to the north shows that the Great Smoky fault ex- tends into Georgia, where it separates metamorph- osed rocks of the Ocoee Series from nonmetamorph- osed Paleozoic rocks (Salisbury, 1961, p. 49-50). One of the thrust faults along the western margin of the Blue Ridge belt in Alabama is estimated to have a minimum displacement of 15 miles (Butts, 1940a). DIRECTION OF THRUSTING The pervasive northwest-trending lineation in the rocks of the Grandfather Mountain window and the overlying Blue Ridge thrust sheet is interpreted as an a lineation, parallel to the direction of tectonic transport, because of its relation to other structures in the Grandfather Mountain area. This interpretation is strengthened by regional geologic relations which require northwestward transport of the crystalline rocks of the Blue Ridge thrust sheet over the nonmetamorphosed Paleozoic GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE rocks of the Valley and Ridge belt. To the northwest, basement rocks are concealed beneath nonmetamor- phosed Paleozoic sedimentary rocks, so that a large sheet of Precambrian crystalline rocks metamor- phosed during the Paleozoic could not have been de- rived from the northwest. Movement northeastward or southwestward perpendicular to the pervasive lin- eation and parallel to the regional structural trend would not allow Precambrian rocks metamorphosed to medium or high grade during the Paleozoic to be superposed over Precambrian, upper Precambrian, and lower Cambrian rocks of low metamorphic grade in the Grandfather Mountain window. Minor folds in the window rocks and the later set of minor folds and crinkles in the Blue Ridge thrust sheet are generally asymmetric and overturned to the northwest, and all indicate yielding of the rocks in that direction. AGE OF THRUSTING Direct geologic evidence in the Grandfather Moun- tain area shows only that thrusting occurred after deposition of the Lower Cambrian Shady Dolomite and before emplacement of the diabase dike of Triassic( ?) age. Geologic evidence in other areas and radiometric measurements, however, suggest more precise ages for the thrusts. West of the Great Smoky Mountains, 60 miles west of the Grandfather Mountain area, movement along the Great Smoky thrust apparently took place during Mississippian time or later (Hadley and oth- ers, 1955, p. 406-407). The regional map pattern (pl. 3) suggests that the faults bounding the Shady Val- ley and Bald Mountain thrust sheets are either ap- proximately the same age as or older than the Great Smoky and that they are in turn overridden by the Stone Mountain fault. If so, the Stone Mountain fault is the youngest fault in the Unaka belt, and movement along it must have occurred in the late Paleozoic or later. The Stone Mountain fault may be equivalent to the Linville Falls fault, although the relations between them may be more complicated. Medium- and high-grade metamorphism of the Blue Ridge thrust sheet took place about 350 m.y. ago (latest Devonian according to Holmes, 1959). The fact that the sedimentary and volcanic rocks in the Grandfather Mountain window never underwent metamorphism of that grade indicates that the main period of transport over the Grandfather Mountain window took place after the metamorphism of 350 m.y. ago. MAJOR FAULTS BOUNDING TECTONIC UNITS ORIGIN OF THE GRANDFATHER MOUNTAIN WINDOW Any theory of the origin of the Grandfather Mountain window must explain the following facts: (1) It is the only known window completely sur- rounded by Precambrian crystalline rocks of the Blue Ridge belt, (2) cleavage in the window dips to the southeast in the northwest part of the window as steeply as or more steeply than cleavage elsewhere in the window, and (3) no pervasive cleavage of similar orientation cuts the rocks of the Blue Ridge thrust sheet. If the structural high represented by the win- dow were due to warping of the thrust sheet, the cleavage in the window should have gentler south- east dips on the west side of the window than else- where, because the Blue Ridge thrust sheet and the Linville Falls fault dip as much as 40° NW. on the west side. If the deformation were due to slip folding along the pervasive cleavage in the window, rocks of the Blue Ridge thrust sheet should be cut by simi- larly oriented cleavage. In the absence of such cleav- age in the Blue Ridge thrust sheet, the different ori- entations of cleavage in the window and of cataclas- tic foliation in the Blue Ridge thrust sheet appar- ently indicate that folding and formation of cleavage in the window rocks and arching of the thrust sheet over the window took place during the main episode of movement along the Linville Falls fault. One can speculate why the Grandfather Mountain window is unique. The thick sequence of upper Pre- cambrian rocks in the northwest part of the window probably thins both to the northeast and southwest beneath the Blue Ridge thrust sheet. The window may exist because this thick sequence formed an original structural high that was emphasized by movements along the Brevard fault and perhaps by later doming. Some late deformation is indicated by the gentle northwest-trending fold in the northwest- ern part of the window and in the overlying Blue Ridge thrust sheet. BREVARD FAULT Reed and Bryant (1964b) have discussed the Bre- vard fault zone and have summarized evidence that it marks a strike-slip fault of great magnitude. The following section is largely summarized from that paper, but a different interpretation of the relation- ship between thrusting and strike-slip faulting is set forth. The Brevard zone is a narrow belt of low-grade metamorphic rocks that emerges from beneath the Coastal Plain deposits northeast of Montgomery, Ala., and has a remarkably straight and continuous 169 trace northeastward for at least 325 miles, passing just southeast of the Grandfather Mountain window (fig. 1). It undoubtedly extends northeastward into Virginia, but it has not yet been traced in detail. The Brevard zone marks the southeastern edge of the Blue Ridge geologic belt and separates it from the Inner Piedmont belt to the southeast (King, 1955). Keith (1905, 1907b) named the low-grade meta- morphic rocks in the zone the Brevard Schist and believed that they were of Cambrian age and occu- pied a narrow synclinal infold in flanking Pre- cambrian rocks of higher metamorphic grade. Jonas (1932) recognized the continuity and unity of the belt and pointed out that many of the low-grade rocks were products of retrogressive metamorphism of the flanking rocks. She concluded that the belt marked a great overthrust fault that carried rocks of the Piedmont northwestward over rocks of the Blue Ridge. We concur with Jonas that the Brevard is a major fault zone, but believe that it is a strike-slip fault rather than a thrust because of its long straight trace, the width of the belts on either side showing structural and metamorphic effects related to it, tec- tonic lenses of exotic rocks in the zone, the contrast between rocks on opposite sides of the zone, and the subhorizontal cataclastic a lineation in the zone. Be- cause no sundered geologic features can be matched across the fault in the 135-mile segment that we ex- amined between central Georgia and the Grand- father Mountain area, we inferred that displacement along it must exceed 135 miles and may be much more. In our previous paper (Reed and Bryant, 1964b, p. 1188), we interpreted the Brevard fault as younger than the Linville Falls fault; the abrupt clockwise swing from northwest-trending cataclastic lineation in the Blue Ridge thrust sheet and Grandfather Mountain window to subhorizontal northeast-trend- ing lineation in the polymetamorphic rocks along the Brevard zone was attributed to drag. We therefore concluded that movement along the Brevard was right lateral and suggested that the parallelism be- tween the Linville Falls fault and the Brevard zone was due to deformation of the older structure during strike-slip movement along the Brevard. Burchfiel and Livingston (1967) have recently called attention to the close analogy between the Bre- vard zone and the root zones of the alpine nappes such as the Urseren zone, the Pusteria-Insubric line and others. They infer that the thrust sheets of Blue 170 Ridge are rooted in the Brevard zone rather than being truncated by it as we had suggested. Reevaluation of our data and more thorough anal- ysis of the structural geometry indicate, however, that the northwest-trending cataclastic a lineation in the rocks northwest of the Brevard is contempora- neous with the cataclastic lineation in the Brevard zone itself and in the polymetamorphic rocks to the southeast. This suggests that northwestward move- ment of the Blue Ridge and Tablerock thrust sheets was largely concurrent with and directly related to movement along the Brevard. Apparently the thrust faults are rooted along the northwest side of the Bre- vard zone. King (1964a, p. 12-14) has pointed out that major srtike-slip faults like the Brevard parallel the struc- GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE tural grain of major mountain systems in many parts of the world and has suggested that there may be a genetic connection between strike-slip faulting and structures generally attributed to lateral com- pression. Reed, Bryant, and Myers (1970) have suggested a possible reinterpretation of the relations between strike-slip faulting and thrusting in the Grandfather Mountain area (fig. 106). This interpretation envi- sions the Brevard as a first-order wrench fault (Moody and Hill, 1956, p. 1213) developed in re- sponse to drift of the crustal block southeast of the fault toward the north, in the direction of the heavy arrows on the top surface of the block (fig. 106). The original position of the fault "', fig. 106) was an unknown distance southeast of its present location \\\\ B ) \ FIGURE 106.-Diagrammatic sketch illustrating posible relation between strike-slip faulting and thrusting. TECTONICS (B-B'). Northward movement of the drifting block | by an amount m would result in left-lateral strike- slip movement (s) along the Brevard zone and would requie northwestward migration of the entire zone by an amount t. Northwestward migration of the zone is accomplished by downbuckling and thicken- ing of the stationary block and by northwestward thrusting of material from along its southeast edge. Once these thrust sheets reach the crest of the tec- tonic high northwest of the Brevard, further north- westward movement is facilitated by gravity sliding. The swing in the trend of the cataclastic lineations in the rocks immediately northwest of the Brevard is due to the change from predominately strike-slip mo- tion near the zone to predominately thrust motion farther to the northwest. The trend of the lineation at any point (light lines on the fault surfaces, fig. 106) is in the direction of the vector sum of the local strike-slip (s) and thrust (¢) components. The total cross-sectional area of the thrust sheets, the amount of thickening of the stationary block, and the depth to the detachment zone at the base of the drifting block must be known in order to calculate the amount of northwestward migration of the Bre- vard zone necessary to explain the thrusting in the Grandfather Mountain area. In the line of the sec- tion on plate 4, the thrust sheets in the Blue Ridge are about 60 km wide and probably at least 5 km thick. If the depth of the detachment zone below the drifting block is assumed to be 20 km and thickening of the stationary block is neglected, only 15 km of northwestward migration of the Brevard is required to explain the thrusting; if the detachment zone were at the base of the crust (about 35 km), less than 9 km of migration would be necessary. This model of simultaneous strike-slip movement and thrusting requires left-lateral displacement along the Brevard, rather than right-lateral as we previously suggested. There is as yet no conclusive evidence as to the sense of movement along the zone, but the reinterpretation seems more nearly compati- ble with the observed structural relations in the Grandfather Mountain area than our previous inter- pretation. An interesting corollary to this hypothesis is that the diabase dike that crosses the Brevard in the Len- oir quadrangle lies in expected orientation of the ro- tated shear set, conjugate to the left-lateral strike- slip fault (Cloos, 1955, p. 244-245). Swarms of simi- larly oriented diabase dikes occur southeast of the Brevard in Alabama, Georgia, and South Carolina (King, 1961) ; some cut the Brevard zone, and sev- £71 eral extend entirely across it, but the dike swarms are absent northwest of the zone. If the above interpretation of the structural rela- tion between the Brevard fault and the northwest- ward thrusting of the Blue Ridge thrust sheet is correct, most of the movement on the Brevard fault would have been concomitant with the thrusting and could have started in the middle Paleozoic after the - climax of regional metamorphism about 350 m.y. ago. However, the Brevard fault may have been active even during early Paleozoic. Movement along the zone must have ended before the emplacement of the diabase dike, presumably in the Late Triassic. TECTONICS sUMMARY OF METAMORPHIC AND STRUCTURAL HISTORY Table 30 is an outline of the structural and meta- morphic history of the Grandfather Mountain area. The chronologic order of the principal events that have affected rocks of each of the major tectonic blocks is fairly well established on the basis of struc- tural and petrographic relationships discussed in de- tail in earlier parts of this paper. However, the dates of many events and the correlations between the his- tories of the tectonic units prior to their juxtaposi- tion by faulting depend largely on radiometric dat- ing of minerals and on inferences drawn from broader regional relationships that are discussed below. A brief summary of the metamorphic and struc- tural history of the southern Blue Ridge by Bryant and Reed (19702) was based on some of the conclu- sions below but encompassed a larger region. REGIONAL SYNTHESIS Plate 4 is a cross section of the western part of the Appalachians extending through the Grandfather Mountain area and northwestward across the Unaka and Valley and Ridge belts into the Appalachian Pla- teau. It shows what we believe to be the most reason- able structural interpretation in the light of present knowledge. The segment of the section northwest of the Grandfather Mountain area is based principally on Rodgers (19532); King and Ferguson (1960) ; Butts (1940b); Virginia Division of Mineral Re- sources (1963) ; and on regional syntheses and inter- pretations by King (1951, 1955, 1959, 19642) ; Rodg- ers (1950, 1953b, 1964) ; and Colton (1961). Modern geologic information southeast of the Grandfather Mountain area is not yet sufficient to allow extension 172 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE TABLE 30.-Structural and metamorphic history of the Grandfather Mountain window and vicinity Grandfather Mountain window Age Blue Ridge thrust sheet Inner Piedmont Major fault movements Tablerock thrust sheet Autochthonous rocks € Thrusting | Brevard 92,2 fault £3 P4 % Intrusion of diabase. Intrusion of diabase. Intrusion of diabase. Gentle warping of thrust Formation of north- sheet northwest and west-trending joints northeast of window(?). «in Brevard fault zone. Formation of northwest- & trending goints in Brevard fault zone (and elsewhere?). Formation of northeast- Formation of joints. Formation of joints, slip Latest movement & g trending (and north- cleavage, and small- along Brevard fault; 8 west-trending?) joints. scale folds. formation of blas- g tomylonite and is Incomplete low-grade High-angle faulting partial recrystalliza- e- metamorphism south- in southwestern tion at low meta- & east of window. extension of window. morphic grade along i narrow belt. Sl Formation of open folds E and crinkles. Northwestward migra- M “I tion of Brevard fault. 2 j 8 Low-grade partial Formation of mineral e- Reactivation of all Shearing, recrystalliza- & metamorphism of lineation and late | existing cleavage; tion, folding, and a plutonic rocks. open folds and M formation of mineral formation of linea- a crenulations; | lineation. tion under medium- § Formation of mineral deformation of grade conditions H lineation. earlier fold axes. e Medium-scale folding; associated with &: wil Cataclasis and recrys- tallization at medium grade southeast of window. Formation of isoclinal folds Metamorphism at low grade. Formation of tightly appressed folds and associated cleavage. Middle Paleozoic Early Paleozoic -- Metamorphism? Medium-grade dynamo- thermal metamorphism and folding at about 350 m.y. ago. Intrusion of granodiorite and pegmatite no later than 350 m.y. ago and possibly as early as 450 m.y. Metamorphism climaxed $50 m.y. ago but may have lasted from 450 to 250 m.y. formation and deformation of folds in layered gneiss Formation of phyllonite zones in granitic rocks and formation of new cleavage; obliteration of early cleavage in northwestern part of window; possible reactivation of cleavage in southern part and in basement rocks. Formation of large- scale folds that control present trends of map units in Grandfather Mountain Formation; formation of early cleavage. movement along Brevard fault. ? ? ? H later than 350 m.y. Probably not earlier than 450 m.y. or Emplacement of granitic rocks, migmatite (and, possibly, Henderson Gneiss); folding and metamorphism to sillimanite grade. Emplacement of ultra- mafic rocks(?). Cambrian(?) and Cambrian Deposition an unknown distance southeast of present position. Late Precambrian Intrusion of Bakersville Gabbro and other mafic dikes. Deposition of sediments and volcanics on older Precambrian rocks virtually in present position. Be.» 7-19 Emplacement of ultramafic rocks- Deposition of sedi- ments and volcanics. Early Precambrian y. 1100-1000 m. { Emplacement of Beach & Granite and other | _ intrusive granites- &+ _ possibly 800-900 m.y. | _ ago. e- | Metamorphism of sedi- ments and volcanics; { formation of Cranberry Gneiss and migmatitic features in gneiss southeast of window. Emplacement of ultra- mafic rocks(?). Deposition of sediments and volcanics younger than 1,270 m.y.(?) old. 1100-1000 ml.y. { Intrusion of Brown Mountain Granite. Emplacement of Wilson Creek Gneiss and Blowing Rock Gneiss. Folding and metamor- phism of preexisting rocks. Emplacement of diorite and gabbro. Deposition of mafic volcanics(?) and associated sediments. TECTONICS of the section southeastward across the remainder of the exposed width of the Appalachians. The approxi- mate thickness of the crust is from Pakiser and Steinhart (1964). Above the geologic section is plotted the gravity profile taken from the Bouguer gravity map of the United States (Am. Geophys. Union, Spec. Comm. Geophys. and Geol. Study Continents, 1964). The broad gravity low in the Blue Ridge and Unaka belts in the line of section is part of a well-defined nega- tive anomaly that extends along the Appalachian miogeosyncline from Vermont to central Virginia where it begins to transgress southeastward into the crystalline rocks and reaches the Piedmont in west- ern North Carolina. Individual lows on the regional gravity low in western North Carolina and eastern Tennessee are among the lowest negative anomalies in the eastern United States. King (1964a) has pointed out coincidence between these large negative gravity anomalies and the major windows in the southern Appalachians-the Mountain City window, the Hot Springs window, and the Grandfather Moun- tain window. The average density of rocks exposed in the Grandfather Mountain window is 2.73 (weighted av- erage of 75 determinations by Zui Yuval, U.S. Geol. Survey). There is apparently no significant differ- ence in density between the upper Precambrian sedi- mentary and felsic volcanic rocks and the underlying plutonic basement rocks. The average density of rocks in the Blue Ridge thrust sheet in the Grand- father Mountain area is 2.80 (weighted average of 51 determinations), significantly higher than that of rocks in the window. Evidently, the pattern of grav- ity anomalies in this part of the Blue Ridge belt is strongly influenced by the thickness of the Blue Ridge thrust sheet. Near Asheville, N.C., 40 miles southwest of the Grandfather Mountain window, the lowest negative anomalies are only -50 milligals. The low ridge separating gravity lows over the Grandfather Mountain and Mountain City windows may be due to large bodies of mafic volcanic rocks (density about 2.99) in the Grandfather Mountain Formation in the northwestern part of the Grand- father Mountain window. The increase in Bouguer gravity values northwest- ward across the Valley and Ridge belt is probably due to decreasing depth to basement, as average densities of the unmetamorphosed miogeosynelinal rocks are presumably less than those of the underly- ing basement rocks. 173 The average density of exposed rocks in the Inner Piedmont belt in the Grandfather Mountain area is 2.74 (weighted average of 21 determinations), nearly the same as that of rocks exposed beneath the Blue Ridge thrust sheet in the Grandfather Moun- tain window. It appears that the regional gravity profile in the Piedmont must be influenced by crustal thickness, basement lithology, or other factors not obvious from the surface geology. PRINCIPAL TECTONIC EVENTS PRECAMBRIAN The earliest recorded metamorphic event in the southern Appalachians was an episode of plutonic activity about 1,100 m.y. ago during which the com- plex of granitic, migmatitic, and metamorphic rocks now exposed in the Blue Ridge belt was formed. The basement now concealed beneath Paleozoic sedimen- tary rocks northwest of the Blue Ridge was presum- ably subjected to the same plutonic episode, for simi- lar dates have been recorded for minerals from base- ment rocks penetrated by deep wells as far west as the crest of the Cincinnati arch and from basement rocks exposed in Virginia, Maryland, New Jersey, and New York (Tilton and others, 1960). Some granitic plutons such as the Beech Granite and the small intrusive body near Crossnore (Davis and oth- ers, 1962, p. 1990) may have been emplaced in the basement complex as recently as 800 to 900 m.y. ago. The lithology of the basement rocks in the north- western part of the line of section (pl. 4) is entirely unknown, for the basement is nowhere exposed at the surface nor has it been penetrated by drilling. Basement rocks now exposed in the Grandfather Mountain window are more uniformly granitic than the rocks in the overriding Blue Ridge thrust sheet, a fact suggesting that plutonic activity generally dim- inished southeastward, but the original position and orientation of the boundary between plutonic and nonplutonic rocks is unknown. In late Precambrian time, sedimentary and vol- canic rocks of the Grandfather Mountain Formation and the Mount Rogers Formation were deposited in one or more deep local basins. Farther southeast and south, rocks to the Ocoee Series were laid down in a more extensive basin, which was probably not con- nected with the Grandfather Mountain basin in the line of section but may have connected farther south. Siltstone and crossbedded sandstone of the Snowbird Group were deposited near the northwest margin of the Ocoee basin, whereas father southeast, conglom- 174 erates, sandstones, siltstones, and shales of the Great Smoky Group were laid down. Volcanic rocks are lacking in the Ocoee Series, but rocks of the Snow- bird Group somewhat resemble sedimentary rocks in the Grandfather Mountain Formation. The Great Smoky Group commonly has graded bedding and was probably deposited in deeper water than either the Snowbird Group or the Grandfather Mountain For- mation. Clearly, these deposits record extensive tec- tonic activity, but no radiometric age or structural evidence suggests the date or type of tectonism that resulted in deposition of the upper Precambrian rocks. Sedimentation probably occurred in the Piedmont in late Precambrian time, for metasedimentary rocks are cut by subsequently metamorphosed plutonic rocks having lead-alpha ages of 505 to 565 my. (Overstreet and Bell, 1965, p. 100). EARLY CAMBRIAN TO EARLY ORDOVICIAN Sandstones, shales, and conglomerates of the Chil- howee Group are the basal clastic deposits of the sequence of miogeosynclinal Paleozoic rocks north- west of the Blue Ridge. The stratigraphy of these rocks is exceedingly complex in detail, but gross stratigraphic units can be recognized for hundreds of miles along strike and for tens of miles across strike, even where facies have been telescoped by thrusting. In a few places, the uppermost beds of the Chilhowee Group contain identifiable Lower Cambrian fossils, but great thicknesses of beds con- formably below the lowest known fossiliferous hori- zons are classed as Lower Cambrian(?). The Chil- howee Group rests nonconformably on plutonic base- ment rocks that are 1,100 m.y. old, but no uncon- formity has been proven to exist between the Chil- howee Group and the upper Precambrian sedimen- tary and volcanic rocks. Differences in stratigraphy and environment of deposition suggest that at least local disconformities must be present. The Chilhowee Group forms a clastic wedge that thickens southeastward from a few hundred feet in deep wells near the northwestern end of the section (pl. 4) to more than 5,000 feet in the Shady Valley thrust sheet in the line of section. As much as 7,500 feet of Chilhowee beds are preserved elsewhere in the Shady Valley thrust sheet (King and Ferguson, 1960, p. 33). The presence of rocks of the Chilhowee Group and of the conformably overlying Shady Dolomite in the Tablerock thrust sheet in the Grandfather Mountain window shows that Chilhowee deposition must have GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE extended southeastward, completely across the pres- ent site of the Blue Ridge belt and at least 12 miles southeast of the present position of the Tablerock thrust sheet. Bloomer and Werner (1955, p. 599) suggested that the Chilhowee Group in Virginia may pass southeastward into eugeosynelinal rocks in the Piedmont. If such a transition occurred in the line of our cross section, it took place southeast of the pres- ent position of the Brevard zone, and all evidence for it has been obscured by subsequent deformation and metamorphism and by movement along the Bre- vard fault. None of the radiometric mineral ages from the Piedmont or Blue Ridge indicate tectonic activity corresponding to deposition of the Chilhowee clastic wedge, nor is there any geological evidence that such an event affected any pre-Chilhowee rocks now ex- posed. Hadley (1964, p. 41) inferred that the Chil- howee sediments were derived from basement rocks to the northwest that are now concelaed beneath the Appalachian Plateau and pointed out that the lack of evidence for tectonic activity during latest Pre- cambrian and Early Cambrian time in the Piedmont and Blue Ridge supports this view. Cambrian and Lower Ordovician deposits overly- ing the Chilhowee Group in the Valley and Ridge and Unaka belts are chiefly carbonate rocks interbedded with shale and locally, with sandstone. They were deposited under stable conditions, probably in a slowly subsiding trough ; the clastic rocks were ap- parently derived from the continental interior (Rodgers, 1953a). The occurrence of Shady Dolomite in the Tablerock thrust sheet indicates that at least the lower part of the Cambrian and Ordovician car- bonate sequence was deposited at least as far south- east as the present site of the Brevard zone. This occurrence and the lack of clastic rocks derived from the southeast in the Cambrian and Ordovician car- bonate sequence in the Valley and Ridge belt suggest that rocks now exposed in the Blue Ridge were not subjected to deformation or metamorphism during Cambrian or Early Ordovician time. MIDDLE ORDOVICIAN TO EARLY SILURIAN A thick wedge of marine clastic rocks of Middle Ordovician age rests discomformably on the Cambrian and Ordovician carbonate sequence in the Valley and Ridge belt and furnishes the first strati- graphic evidence of Paleozoic tectonism southeast of the Appalachian miogeosyncline. Most of the wedge in the line of cross section con- sists of sales and sandstones that thin rapidly north- TECTONICS westward and intertongue with carbonate rocks. Red beds are extensive in the upper part of the Middle Ordovician clastic wedge. The southeasternmost ex- posures of Middle Ordovician clastic rocks in the line of section are just northwest of and below the Hol- ston Mountain fault, where more than 5,000 feet of Middle Ordovician shale and sandstone are preserved in a synclinorium below the Shady Valley thrust sheet (King and Ferguson, 1960, p. 57). The upper part of the Middle Ordovician sequence there con- tains conglomerate beds and lenses as much as 30 feet thick. Pebbles in the conglomerates are poorly sorted and variously rounded; they are predomi- nantly derived from Ordovician and Cambrian car- bonate rocks but also include pebbles of quartzite probably derived from the Chilhowee Group and pebbles of vein quartz, feldspar, and volcanic rocks (Kellberg and Grant, 1956). Similar congolmerates occur elsewhere in Tennessee, Virginia, and Georgia. The Middle Ordovician clastic rocks must have been derived from land to the southeast. King and Ferguson (1960, p. 61) concluded that the conglom- erates were originally deposited near shore but were later carried into their present position by turbidity currents flowing down an oversteepened submarine slope. The conglomerates must ultimately have been derived from an area where the Chilhowee Group and older rocks were exposed to erosion. Accumulation of clastic rocks, chiefly shales and sandstones, continued in the miogeosyneline in Late Ordovician and Early Silurian time, but these depos- its are very thin in the line of cross section. These later clastic deposits are largely derived from sources to the northeast and represent the distal edge of a thick clastic wedge centered in southern Pennsylvania. Rodgers (1953a, p. 94) referred to the tectonism indicated by the Middle Ordovician clastic deposits as the Blountian phase and to the Upper Ordovician and Lower Silurian clastic rocks as representing the main phase of the Taconian orogeny. Current geologic time scales (Holmes, 1959 ; Kulp, 1961) indicate that the Ordovician and Silurian clastic rocks were deposited during the time span from about 460 to 420 m.y. ago. Zircon from benton- ite interbedded with Middle Ordovician rocks in Ten- nessee and Alabama has U®#%/Pb°" ages of about 445 m.y.; biotite from the same rocks has similar ages (Kulp, 1961). There is no definite indication that rocks now ex- posed in the Grandfather Mountain window were 175 subjected to deformation or metamorphism during this interval, although some of the early folds and cleavage in the Grandfather Mountain Formation may have started to form at this time. During the Taconian orogeny, rocks in the Blue Ridge thrust sheet lay many miles southeast of their present posi- tion. A few mineral ages from basement rocks in the Blue Ridge thrust sheet suggest that they may have undergone shearing and metamorphism 420 to 450 m.y. ago (Hadley, 1964). In the eastern Great Smoky Mountains, folding of the Ocoee Series, formation of foliation in basement rocks, and extensive thrusting occurred prior to Late Devonian regional meta- morphism, probably during the Ordovician (Hadley and Goldsmith, 1963, table 16). Biotite in metamor- phosed rocks of the Qcoee Series at Ducktown, Tenn., has an apparent potassium-argon age of 435 m.y., suggesting that at least part of the Ocoee Se- ries was subjected to metamorphism at about this time (Long and others, 1959). Scattered isotopic uranium-lead age determina- tions on zircon indicate emplacement of granitic rocks in the Inner Piedmont at about this time. Zircon from the Toluca Quartz Monzonite at its type locality in the Piedmont of North Carolina has dis- cordant ages of 405 to 480 m.y. (Davis and others, 1962) ; zircon from granitic rocks in Georgia gives discordant ages of 415 to 490 m.y. (Grunenfelder and Silver, 1958). Farther east, in the Carolina slate belt, recent lead-alpha determinations on zircon from felsic vol- canic rocks give ages of 440 to 470 m.y. (White and others, 1963), showing that at least some of the vol- canic, pyroclastic, and sedimentary rocks probably accumulated during the Taconian orogeny. Sedimen- tation was, at least in part, tectonically controlled (Conley and Bain, 1965, p. 133-134). Thus, during the Taconian orogeny, which re- sulted in deposition of a thick wedge of Middle Or- dovician through Lower Silurian clastic rocks in the miogeosyncline, rocks in the Blue Ridge thrust sheet (which then lay southeast of the present position of the Grandfather Mountain window) were probably subjected to shearing and metamorphism. Still far- ther southeast, granitic rocks now exposed in the Inner Piedmont belt were emplaced in the deeper parts of the eugeosyncline. At the same time, vol- canic and sedimentary rocks were being deposited in higher parts of the eugeosyncline; these rocks now compose the Carolina slate belt. 176 LATE DEVONIAN AND EARLY MISSISSIPPIAN The next major episode of orogenic activity is re- corded in the miogeosyncline by clastic deposits of Late Devonian and Early Mississippian age. These deposits rest on a regional unconformity that bevels beds ranging in age from Early Devonian to Early Ordovician (Rodgers, 195382). The Chattanooga Shale, which rests on the unconformity in northeast- ern Tennessee, is largely of Late Devonian age. The Chattanooga and equivalent shales, siltstones, and sandstones in southwestern Virginia represent the distal edge of the Catskill delta, the great clastic wedge of Middle and Late Devonian age centered in New York and Pennsylvania. This wedge marks the Acadian orogeny of the central and northern Appa- lachians. Lower Mississippian clastic rocks are chiefly shale, siltstone, and sandstone apparently de- rived from the southeast. They indicate a period of uplift in the crystalline belt of the southern Appa- lachians somewhat later than the Acadian orogeny farther north. Potassium-argon and rubidium-strontium ages on biotite from a bentonite layer in the Chattanooga Shale and whole rock uranium-lead ages on the shale establish the absolute age of the Chattanooga Shale at about 350 m.y. (Faul, 1960; Kulp, 1961). Radi- ometric mineral ages in the range 335 to 350 m.y. seem to record metamorphic and plutonic events in the Blue Ridge and Piedmont during deposition of the Upper Devonian and Lower Mississippian clastic rocks in the miogeosyncline. A rubidium-strontium age on biotite from Wilson Creek Gneiss (Davis and others, 1962) indicates the low-grade retrogressive metamorphism and formation of cataclastic foliation in basement rocks in the Grandfather Mountain win- dow took place about 350 m.y. ago. Geologic evidence shows that the late Precambrian rocks were folded, sheared, and metamorphosed at about the same time (table 30). Mica ages (Long and others, 1959; Kulp and Eckelmann, 1961) indicate widespread medium- grade regional metamorphism of rocks in the Blue Ridge thrust sheet during this interval. Granodiorite and pegmatite in the Spruce Pine district were em- placed prior to or during this metamorphism. Scattered radiometric ages of minerals from rocks in the Inner Piedmont belt fall in the 335 to 350 m.y. range and suggest at least some metamorphic and plutonic activity at that time. Hart (1964) found that potassium-organ and ru- bidium-strontium ratios are affected by reacting to temperatures as low as 200°C in an environment of eoutact metamorphism in already crystalline rocks. GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE Hadley (1964) has pointed out that mineral ages based on these ratios may therefore record the last time the rock cooled below some critical temperature, rather than the date of crystallization or meta- morphism. Thus, the 335 to 350 m.y. dates may record a period of uplift, erosion, and cooling follow- ing an episode of metamorphism, rather than the date of the climax of the metamorphism. LATE MISSISSIPPIAN, PENNSYLVANIAN, AND PERMIAN STRATIGRAPHIC AND GEOCHRONOLOGIC RECORD Upper Mississippian and Pennsylvanian clastic rocks form a wedge which thickens and coarsens southeastward in the line of section (pl. 4). The Upper Mississippian clastic rocks are chiefly varicol- ored marine shale and sandstone but locally include thin beds of limestone. They rest conformably on older Mississippian limestones and shales and are ov- erlain by continental clastic rocks of Pennsylvanian age. In northeastern Tennessee, there is no evident disconformity between Mississippian and Pennsyl- vanian rocks, but to the north and northeast the Pennsylvanian rocks rest disconformably on Missis- sippian strata (Rodgers, 1953a, p. 125). The Mississippian and Pennsylvanian clastic rocks constitute the last preserved stratigraphic record in the miogeosyncline in the Southern Appalachians; farther north, in Pennsylvania and. West Virginia, continental clastic rocks as young as early Permian are preserved. Recently revised geologic time scales (Holmes, 1959; Kulp, 1961) indicate that the Upper Missis- sippian clastic rocks began to accumulate about 320 m.y. ago, and the youngest preserved Permian clas- tic rocks may be as young as 260 m.y. Published radiometric age determinations do not suggest that metamorphism or plutonism affected rocks in the Blue Ridge at this time, but structural evidence (see below) indicates that much of the major thrusting took place during this interval. Many micas from metamorphic and igneous rocks in the eastern Piedmont have potassium-argon and ru- bidium-strontium ages in the range from 310 to 240 m.y.; Long, Kulp, and Eckelmann (1959) have sug- gested that these ages indicate a major episode of metamorphism and plutonic activity that reached a peak about 250 m.y. ago in the eastern Piedmont in Georgia, South Carolina, and North Carolina. Alter- natively, these dates may reflect regional uplift and cooling at that time. As Hadley (1964) has pointed TECTONICS out, many of the youngest mica ages in the Piedmont are younger than the youngest preserved deposits in the carboniferous clastic wedge. Perhaps the pre- served clastic deposits represent material removed by erosion which eventually allowed rocks now exposed in the eastern Piedmont to cool below the critical temperature for retention of argon and strontium in micas. STRUCTURAL EVENTS Evidence from geochronology indicates a long and complex history of deformation and metamorphism for the rocks of the Blue Ridge and Piedmont belts during the Devonian and Mississippian, but tectonic events in these areas did not directly affect the partly synchronous miogeosynclinal rocks to the northwest until late in the Paleozoic. Prior tectonic activity in the Blue Ridge and Piedmont is recorded in the rocks of the miogeosynecline by unconformities and by wedges of clastic deposits derived from the southeast, but the rocks themselves were apparently little deformed, although local stratigraphic varia- tions suggest that some folds formed in them during Paleozoic sedimentation (Cooper, 1964). Thrust faults in the Cumberland Plateau and in the northwestern part of the Valley and Ridge belt involve beds as young as Early Pennsylvanian. Lat- est movement on these faults is thus later than Early Pennsylvanian, but how much later is not estab- lished. Thrust faults in the southeastern part of the Valley and Ridge belt and in the Unaka and Blue Ridge belts involve only older rocks. In the Grand- father Mountain area, the latest movement on the Linville Falls fault was probably pre-Late Triassic, for the fault is cut by a diabase dike of Late Triassic (?) age. The available stratigraphic and structural evi- dence thus serves only to set broad upper or lower limits on the time of latest movement of a few of the major thrust faults. In no case is the time of latest movement of any single fault closely dated, and no evidence is available as to the date of initial move- ment or the span of time involved in thrusting. As King (1964a, p. 25) has pointed out, deformation must have proceeded for a long time, and initial movement on the thrusts may have begun well before deposition of the youngest beds known to be in- volved. Nevertheless, the major thrust faults shown on plate 4 were apparently all formed during an oro- genic episode that reached a climax in late Paleozoic or earliest Mesozoic time. Imbricate thrust faults that displace rocks in the Valley and Ridge belt (pl. 4) are probably all closely 177 related in age and origin. They generally dip south- eastward; some extend along strike for several hundred miles and remain nearly parallel to bedding in the overriding block but commonly cut across for- mations in the rocks beneath. Incompetent shales of Middle and Early Cambrian age (Conasauga Group and Rome Formation) are brought to the surface along the soles of these thrusts for many miles, but no- where do older rocks appear at the surface. This has led to the interpretation (Rich, 1934; Miller, 1945; Rodgers, 1953b, 1964) that the thrust faults in the Valley and Ridge belt are rooted in a décollement horizon in the Paleozoic sequence and that older rocks below are not involved in the thrusting, al- though they may be somewhat deformed. Geophysi- cal work just south of the line of section in Tennes- see and Kentucky (Watkins, 1962, 1964) and north- east of the line of section near Blacksburg, Va. (Sears, 1964), supports the hypothesis that base- ment rocks are not involved in the imbricate thrust- ing of Paleozoic sedimentary rocks. Beneath the Valley and Ridge belt, the décollement horizon is thought to be incompetent shales in the Conasauga Group and Rome Formation; northwest of the Powell Valley anticline, the principal décolle- ment takes place in the Chattanooga Shale, which forms the sole of the Pine Mountain fault. The décollement must continue southeast of the Valley and Ridge belt, but its location is uncertain. It may continue in the Rome Formation and emerge as an unrecognized folded bedding-plane thrust as shown on plate 4, or it may turn downward into basement rocks beneath the Unaka and Valley and Ridge belts. Gwinn (1964) has recognized a similar detachment thrust beneath the central Appalachians and has suggested analogous alternatives for its southeastward continuation. Unfortunately, at pres- ent, no geological or geophysical evidence tends to favor one or the other of these alternatives. The Shady Valley and Buffalo Mountain thrust sheets (pl. 3) are the principal thrust masses in the Unaka belt. They cannot have been rooted in a dé- collement within the Paleozoic sequence, for they are composed principally of rocks older than the Rome Formation and contain late Precambrian rocks and older plutonic basement rocks in their lower parts. In the line of cross section (pl. 4), the Shady Valley thrust sheet is preserved in the Shady Valley syn- cline, a shallow syncline younger than the thrusting; southwest of the line of section, the Buffalo Moun- tain thrust sheet occupies the trough of the syncline. In the cross section, it is tentatively suggested that 178 the Spurgeon and Bristol faults are continuous be- neath a similar syncline northwest of the Shady Val- ley syncline, but no modern mapping is available there to support this interpretation. The rocks in the Shady Valley and Buffalo Moun- tain thrust sheets are unmetamorphosed and were not deformed prior to thrusting (King and Fergu- son, 1960, p. 85). Southeast-dipping cleavage on the southeast side of the Buffalo Mountain thrust sheet is younger than thrusting (Ordway, 1959). Base- ment rocks in the thrust sheets show no evidence of the pervasive cataclasis that affected rocks in the Blue Ridge thrust sheet and Grandfather Mountain window. The thrust sheet rests on unmetamorphosed rocks as young as Middle Ordovician that were folded prior to or during emplacement of the thrust sheet. Northeast of the Great Smoky Mountains (pl. 3), the Shady Valley thrust sheet overrides the Pulaski fault, the southeasternmost of the imbricate thrusts in the Valley and Ridge belt. The Pulaski fault in- volves rocks as young as Early Mississippian in Vir- ginia, but it is probably of the same general age as the thrusts farther northwest which involve Early Pennsylvanian rocks. Whether or not there are any systematic age relations among the imbricate thrusts in the Valley and Ridge belt has not yet been deter- mined. In any case, the Shady Valley and the tecton- ically higher Buffalo Mountain thrust sheets arrived in their present position in post-Early Mississippian time. i King and Ferguson (1960, p. 79, 83) have shown that the Shady Valley thrust sheet originated south- east of the Mountain City window. Rocks in the Shady Valley and Buffalo Mountain thrust sheets are unmetamorphosed and were not deformed prior to thrusting ; clearly, they were not subjected to folding and low-grade metamorphism that affected rocks now exposed in the Grandfather Mountain window about 350 m.y. ago (table 30). In the Shady Valley thrust sheet, rocks of the Chilhowee Group are in stratigraphic contact with plutonic basement rocks. The thrust sheet, therefore, could not have origi- nated in the area of the Grandfather Mountain win- dow, where a thick sequence of upper Precambrian rocks lies between the Chilhowee and the basement. Its source must have been either in the area between the Grandfather Mountain and Mountain City win- dows or southeast of the Grandfather Mountain win- dow, beyond the southeastern edge of the basin of deposition of the Grandfather Mountain Formation. GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE Derivation of the Shady Valley thrust sheet from southeast of the Grandfather Mountain window re- quires more than 50 miles of northwestward trans- port. In addition, it requires that the rocks in the thrust sheet must have somehow escaped deforma- tion and metamorphism during the tectonic event 350 m.y. ago. It therefore seems more likely that the source of the thrust sheet was between the unmeta- morphosed rocks now exposed in the Mountain City window and the metamorphosed rocks exposed in the Grandfather Mountain window. The present distance between the two windows is less than the exposed width of the Shady Valley thrust sheet, but the dis- tance between them may have been telescoped by a major fault now hidden beneath the Blue Ridge thrust sheet. The principal objection to this hypothesis is that at least 20 miles of displacement is required along such a fault to make room for the rocks of the Chil- howee Group in the thrust sheets of the Unaka Belt, yet the fault must bring rocks of the Grandfather Mountain Formation over basement rock. If the fault has a rather low dip, the Grandfather Moun- tain Formation may override basement rock that was immediately below Chilhowee Group rocks and structurally higher than the Grandfather Mountain Formation before faulting. It is even possible that lower Paleozoic rocks underlie the fault locally. The relations between the Linville Falls fault and any fault hidden beneath the Blue Ridge thrust sheet are unknown. If the underlying fault is later than the Linville Falls fault, then the Linville Falls and Stone Mountain faults are not correlative, and the age relations between the Linville Falls fault and faults of the Unaka belt are unknown. The Unaka belt faults might even be splits off the Linville Falls fault which were subsequently overridden by the Stone Mountain fault. In any case the Blue Ridge thrust sheet overrides the thrust sheets of the Unaka Belt along the Stone Mountain and equivalent faults. The presence in the Grandfather Mountain win- dow of overridden upper Precambrian and Cambrian rocks beneath the Blue Ridge thrust sheet indicates at least 35 miles of northwestward thrusting of the crystalline rocks of the Blue Ridge. If the thrust sheets in the Unaka belt came from between the Grandfather Mountain and Mountain City windows, an additional 20 miles of northwestward displace- ment of the Grandfather Mountain window and overlying Blue Ridge thrust sheet are required. King (1964a, p. 18) has pointed out the possibility "* * * that the whole body of Precambrian rocks in TECTONICS this segment of the Blue Ridge is allochthonous," an inference that is strongly supported by the amount of thrusting demonstrated in the Grandfather Moun- tain area. Latest movement along the Stone Mountain fault system at the northwest margin of the Blue Ridge thrust sheet was post-Early Mississippian, later than the latest movement along faults in the Unaka belt and therefore, later than the latest movement along the Pulaski fault. Movement probably ceased prior to emplacement of the Upper Triassic(?) diabase dike that cuts the Linville Falls and Brevard faults in the Grandfather Mountain area. Unlike rocks in the thrust sheets to the northwest, rocks in the Blue Ridge thrust sheet were deformed and metamor- phosed prior to thrusting, and movement of the thrust sheet was accompanied by shearing and re- crystallization of rocks near its sole and by the for- mation of cataclastic lineation in the direction of movement. Rocks of the Chilhowee Group and the overlying Shady Dolomite in the Tablerock thrust sheet were deformed and metamorphosed prior to or during the early stages of thrusting (table 30). Structures within the Tablerock thrust sheet closely resemble those in the Blue Ridge thrust sheet and indicate that it is a tectonic slice carried from southeast of the Grandfather Mountain window along the sole of the overriding Blue Ridge thrust sheet. Small slivers and slices of rocks of the Chilhowee Group occur along branches of the Linville Falls fault northwest of the Grandfather Mountain window (pl. 1) and along faults of the Stone Mountain family south and southeast of the Mountain City window (King and Ferguson, 1960). These bodies are analogous in tec- tonic position to the Tablerock thrust sheet and are probably similar in origin. The Blue Ridge and Tablerock thrust sheets are rooted along the northwest side of the Brevard zone rather than being truncated by it as we have pre- viously suggested (Reed and Bryant, 1964b). North- westward movement of these thrust sheets was ap- parently concurrent with and possibly a direct result of strike-slip movement along the Brevard, although movement on the Brevard may have lasted somewhat longer than movement on the Linville Falls fault and have caused local truncation at the southwest corner of the Grandfather Mountain window. According to the interpretation given above, the Blue Ridge thrust sheet is composed of rocks that originally lay southeast of the present position of the Brevard, but northwest of the position of the Bre- 179 vard prior to thrusting. Rocks now exposed in the Blue Ridge thrust sheet were probably originally ov- erlain by metamorphosed eugeosynclinal rocks of late Precambrian and Paleozoic age similar to those now exposed in the Piedmont. Upper Precambrian rocks of the Ocoee Series exposed in the higher parts of the Blue Ridge thrust sheet could, despite their lack of volcanic rocks, represent the basal parts of the Piedmont eugeosynclinal sequence that now has largely been removed by erosion. Movement along the inferred fault now concealed beneath the Blue Ridge belt could have been directly related to the same tectonic mechanism. If the con- cealed fault dips southeastward below the Grand- father Mountain window, it must either connect with the Brevard at depth or descend to a level in the crust where movement along it is distributed as plas- tic flow. In either case, northward movement of the crustal block southeast of the Brevard could have caused northwestward thrusting of the block be- tween the Brevard and the fault underlying the win- dow. The interpretation of the thrust faults in the Val- ley and Ridge belt according to this mechanism de- pends in large part on the position of the southeast- ern extension of the décollement zone in which they are rooted. If it turns downward into basement rocks beneath the Unaka belt, the faults in the Valley and Ridge belt may be analogous to those in the Unaka belt, and they too may be due to compression during northwestward migration of the Brevard. If, on the other hand, the décollement remains in the lower part of the Paleozoic sequence and is pres- ent as an unrecognized folded bedding-plane thrust in the rocks below the Shady Valley thrust sheet, the décollement and the structures rooted in it may be due to gravity sliding off a tectonic high raised dur- ing early stages of thrusting. If so, the breakaway zone and the training edge of the sliding block have subsequently been folded and overridden by the thrusts in the Unaka belt. Rocks overridden by faults of the Unaka belt are exposed in the Mountain City window. The Rome Formation crops out over broad areas, but nowhere in the window are rocks preserved that are younger than the Rome. King and Ferguson (1960, p. 83) believed that : The rocks that once overlay the Mountain City window prob- ably formed a thrust in the Appalachian Valley similar to those now preserved there, but it was probably higher and has been long since eroded. 180 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE Is it possible, however, that the higher rocks are ab- | tion somewhere northwest of the Shady Valley syn- sent in the Mountain City because they slid north- | cline, perhaps in the sheet bounded by the Spurgeon westward along the décollement before the arrival of | and Bristol faults (pls. 3, 4) which may be a more the Shady Valley thrust sheet in its present position ? northwesterly part of the Shady Valley thrust sheet. If so, they are to be found above the Rome Forma- TABLE 31.-Location of typical exposures of rock units Area on Map unit Quadrangle plate 1 Location Mountain City window Chilhowee Group....:......_/__ C-2 South side of Nowhere Ridge along road between Poga School and the Elk River, and along Dark Ridge Creek. Shady ::...... Linville and Elk C-1; C-2._:::0. Valley of Elk River. Mills 714. Rome Formation...:..._..:...... Ek: Mills....:..._... COAs cd. ees. Many exposures in roadcuts. Blue Ridge thrust sheet Biotite-muscovite schist. ___ Linville Falls______. ari. Roadcuts along Blue Ridge Parkway. Linville:._..__.~__. CB.. Valley of Plumtree Creek. Blowing Rock and J-2, K-8..:..... North side of Valley of Elk Creek. Grandin 714 min. Blowing J-5.... Along Buffalo Cove Road. Amphibolite and hornblende Linville. Along Squirrel Creek. schist. Blowing Rock...... G-2, H-2:...._. South slopes of Howard Knob, Rich Mountain, and Snakeden Mountain. Mixed rocks:....._........._... ee Along North Toe River between Minneapolis and New- and. Blowing Rock...... tsa csa Along north fork of Elk Creek between altitudes of 2,600 and 2,800 feet. adore Along road northeast of the New River south of U.S. Highway 421. Beech Granite_...;...:..._...._.. Linville:...:......; C-2, D-2....... Upper reaches of Beech and Buckeye Creeks and along the Elk River. Quartz monzonite __do__-._...___:___ D-8::2.0.0.2.. Roadcuts along North Carolina Highway 194 along Elk River and along secondary road along east fork of Curtis Creek. Aegerine-augite granite gneiss____ __ l. Inactive quarry along road between Crossnore and Mt Pleasant, 0.7 mile northwest of Crossnore. Bakersville Gabbro......_...... __ do- see k. C-8, C-4111____ Slopes of Hump Mountain. Chilhowee Group- ____________ __ do- s eile. Near head of Cooper Branch. ; 0 eee arse ne el D-8, F-8....... Along North Carolina Highway 194 northwest of White- head Creek and along strike in the Elk River. Roadcut on North Carolina Highway 80 south of Lake Tahoma dam. 4 : Linville Falls_______ Roadcut along a quarry south of North Carolina Highway 181 at the southeast boundary of the Grandfather Mountain window. Layered gneiss southeast of Blowing Rock______. Along Buffalo Creek Road and in valley of Licklog Branch. Grandfather Mountain e Along North Carolina Highway 90 west of Collettsville. window. Linville Falls. __.___._ Along North Carolina Highway 181 near Smyrna Church. C do Llc rsn cc.. sis.. Along Linville River west of Lake James. Cranberry Gnelgs...__..__._.____ Blowing Rock______. es Along Elk River. $ Linville_.;;:;;_ 2: E- g, F-2, and Along Watauga River and Elk River. 3. Linville Falls_ ____._. sisi. Along Blue Ridge Parkway northeast of Humpback Ultramafic rocks (see table 11). Granodiorite and pegmatite. Linville Falls. _._... Quartz monzonite and pegmatite Blowing Rock...... in Deep Gap area. Mountain and along U.S. Highway 221 south of village of Linville Falls. C-6.-.. 0a l.. Saprolite exposures in clay pits in valley of Brushy Creek. ss... cise. Mines and prospects in valley of Brushy Creek. Ia $ Roadcuts along Blue Ridge Parkway south of Deep Gap. TECTONICS 181 TABLE 31.-Location of typical exposures of rock units-Continued Map unit Quadrangle 13:5: 1°“ Location Autochthonous rock units, Grandfather Mountain window Metagabbro and ._ Blowing Rock.... G-4, G-5._____._ West side of Billy's Knob and in vicinity of Upton. Exposures poor. Wilson Creek Gneiss-________- --- Linville Falls_._..... Roadcuts along North Carolina Highway 181 south of road-metal quarry. Linville Falls, F-5, F-6_...... Roadcuts along Wilson Creek Road. Linville Wilson Creek Gneiss Blowing Rock.____.. Along Buffalo Creek Road. (layered phase). dors In roadcuts along U.S. Highway 821. Wilson Creek Gneiss Linville, F-5,F-0....... Along North Harper Creek below uranium prospect. (phyllonite phase). Linville Falls. s Along lower reaches of Rockhouse Creek. Linville Falls_...... E-7, F-1....... In roadcuts along North Carolina Highway 181 in vicinity of bench mark 1661 (weathered exposures). Wilson Creek Gneiss Blowing Rock...... Along Yadkin River, Bailey Camp Branch, and Dennis (blastomylonite phase). Creek. Blowing Rock Gneiss________--- -- do --cz isl calan, H-8, H-4...... Many large exposures along U.S. Highway 321 both north and south of village of Blowing Rock. Brown Mountain Granite Lenoir.;......_i.l.. RZ uk. ces Large continuous exposures along Wilson Creek for about 2 miles upstream from Brown Mountain Beach. Grandfather Mountain Forma- Linville: <5 sis also Along Blue Ridge Parkway east of Pineola. tion (arkose units). cll. 00k... E-4, F-4.._._._. Along U.S. Highway 221 on southeast flank of Grandfather Mountain. Grandfather Mountain Forma- Blowing Rock.-___--- Valley of Flannery Fork. tion (siltstone units). Linville.:..........> Roadcuts along North Carolina Highway 105 in vicinity of Foscoe. Grandfather Mountain Forma- Blowing Roadcut along North Carolina Highway 105, 0.2 mile tion (felsic volcanic rocks). south of bridge across Watauga River. r= nnd: H-8 : .:....c... Roadcuts along Blue Ridge Parkway near first overlook Grandfather Mountain Forma- tion (mafic volcanic rocks). Linville Falls_ __.... -~ MO occ aso Blowing Rock.-_._-. Obi leaesegnecks Linville Falls. __... northeast of the Arts and Crafts center northwest of Blowing Rock. Cuts along old railroad grade on east side of Wilson Creek opposite bench mark 1410. Saprolite exposures in roadcuts along North Carolina Highway 181 north of bench mark 1195; fresh exposures along Steels Creek a few hundred yards to the west. G-4_ L zl Along Phillips Creek about 0.9 mile southeast of Pack Hill School. Chiefly flows. : Roadcuts northeast of Flattop Mountain. Tuffs and inter- bedded sediments. Qutcrops on east side of Wilson Creek about 0.1 mile upstream from bench mark 1410. Mafic flows (not mapped separately) interbedded with felsic volcanic rocks. Grandfather Mountain Forma- Linville. D-4, D-5. ..... Outerops in vicinity of village of Montezuma. Type locality. tion (Montezuma Member). 2 cn cu Qutcrops and roadcut along North Carolina Highway 194 about 0.6 mile southeast of Banner Elk. Linville ' _do..........L.... Outerops along U.S. Highway 221 south of Pilot Knob. ~ F-4:°........... Ouécrops along North Carolina Highway 105 at Linville ap. Tablerock thrust sheet Chilhowee Group Linville Falls.... Roadcuts along North Carolina Highway 183, 0.8 to 1.7 (lower quartzite unit). miles northeast of bridge over Linville River above Linville Falls. doz [Pil .. Cliffs at Wisemans view. Chilhowee Group E enlil rcs Roadcut along North Carolina Highway 183 about 0.8 (phyllite unit). mile northeast of bridge over Linville River above Linville Falls. 182 GEOLOGY, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA AND TENNESSEE TABLE 31.-Location of typical exposures of rock units-Continued Map unit Area on Quadrangle plate 1 Location Tablerock thrust sheet-Continued Chilhowee Group (upper quartzite unit). Shady . Linville Falls______. D-6..:}2:000.0. Roadcuts along North Carolina Highway 183, 0.4 and 0.8 mile northeast of bridge across Linville River above Linville Falls. sedo.-Lt cer Outcrops along National Park Service trails in vicinity of Linville Falls. ado: co 000 West side of North Fork of Catawba River in vicinity of Linville Caverns and for 1 mile to north. Little Switzerland 004.00 Road-metal quarry east of U.S. Highway 221 about 0.5 714. mile south of Woodlawn. Inner Piedmont belt Biotite Micaschist.:..:: Sillimanite schist.._....._.._...-_ Henderson Gneiss. _____________ Granitic ":. Quartz monzonite gneiss_____ ___. Ultramafic rocks.._>........~:-. Linville Falls_ ___... Saprolite exposures in wave-cut cliffs on both sides of northeastern arm of Lake James. Lendirs":: cc s Al... Roadcuts along country road from Lenoir to Collettsville for about 1 mile norhtwest of Abingdon. Linville Falls_______ D>-9:. .s _ Outcrops on both sides of Catawba River arm of Lake James south of South Mountain Institute. Lenoir. .:.sc..:..__ Roadcuts on east side of Husband Creek near bench mark 1080 southeast of Oakwood Church. vedo 1-9... }}: Cliffs on north shore of Rhodhiss Lake east of Huffman Bridge. S -_. 1-9..:: Roadcut on south side of Cayah Mountain, 1.4 miles north of Baton. Linville Falls_______ Outcrops along southeast side of arm of Lake James south of Lake James Church. Lenoir. >...}. :l: Outecrops and roadcuts along road over Indian Grove Gap, both north and southwest of gap. Linville Falls. _____. Roadcuts south of Canoe Creek on road from Oak Hill to Willow Tree School. .. I= Tins lasts e Roadcuts along bypass road west of Lenoir in valley of Spainhour Creek. . +9. Abandoned quarry on west side of McGalliard Creek south of Lakeview Church. racers .css lts Saprolite exposures in roadcuts along North Carolina Highway 18 bypass west of Morganton. nize s Roadcuts along U.S. Highway 321 (business route) near southern city limit of Lenoir and in abandoned quarry to west. Blowing Rock._______ 1-5-2 2 Roadcut along road on west side of Yadkin River at south edge of quadrangle. Lenoir... rsp... Asbestos prospect on southeast side of Johns River, 0.4 mile northeast of confluence with Wilson Creek. Brevard belt Phyllonitic schist and gneiss _ ___ .. B-10___________ Cuts along Clinchfield Railroad southeast of Hankins. 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T Aegirine-augite granite gneiss, Blue Ridge thrust 41 Amphibolite, Blue Ridge thrust sheet __12, 17 Inner Piedmont belf _____._______-_._____- 122 Anthophyllite gneiss, Inner Piedmont belt 131 Arkose, Grandfather Mountain Formation _ 75 Artis, Lowell, analyst _____ 15, 18, 19, 26, 40, 70, 80, 87, 90, 102, 120, 125, 129, 138, 144, 146 Ashe Formation _ 36, 58 Bakersville Gabbro ___- __20, 41, 57 Barlow, I. H., analyst ____ cane Barnett, Paul R., analyst _______._____._ 8 Beech Granite ___28, 29, $6, 42, 45, 53, 60, 167 Biotite gneiss, Inner Piedmont belt ____ 116 Blastomylonite, Brevard Fault zone _____ 153 Wilson Creek Gneiss ______________ 65 Blowing Rock Gneiss _____ 63, 65, 67, 71, 114 Blue Ridge front 2.L .... 5, 60 Blue Ridge thrust sheet, rock units _____ 12 Cen cnn d olo nene ee eo ee aeons 58 Blue Ridge upland, physiography _____- 8 Botts, Samuel, analyst ____15, 18, 19, 26, 40, 70, 80, 87, 90, 102, 120, 125, 129, 138, 144, 146 Brevard fault zone _____ 10, 21, 26, 53, 105, 115, 116, 122, 124, 130, 131, 135, 140, 147, 149, 158, 161, 169, 174 Brown Mountain Granite __________ 71, 89, 114 Buffalo Mountain thrust sheet ________ 103, 177 Cale-silicate rocks, Inner Piedmont belt 130 Candler: Formation i.__________._____.-__. 156 Carolina Gneiss _____ Carolina slate belt __ Catoctin Formation _ Chattanooga Shale 176, 177 Chilhowee Group ___10, 59, 78, 95, 98, 105, 106, 115, 178, 179 Correlation -1c. cl Lool lo ns 48 correlation and regional relationships 103 slices north of the Grandfather Moun- tain window _c. c- 45 slices southeast of the Grandfather Mountain window __________ 47 Stone Mountain fault ______________ 165 structural history L-_.__-._______.__ 174 Chloe, Gillison, analyst ____15, 18, 19, 26, 40, 70, 80, 87, 90, 102, 120, 125, 129, 138, 144, 146 Cleavage, Grandfather Mountain window 106 (CUMSLE 225 2-0... LOLEL _L i 420 00m bates cL 4 - GTOUD: 177 Cranberry Gneiss ____14, 18, 20, 26, 37, 39, 40, 41, 42, 45, 46, 49, 58, 55, 59, 65, 163, 165 Disbase (202... ... ooc conduits. 161 Doe River inner window _______.. 10, 103, 165 Elmore, Paul, analyst ___.15, 18, 19, 26, 40, TO, 80, 87, 90, 102, 120, 125, 129, 188, 144, 146 INDEX [Italic page numbers indicate major references] Page Brwin Formation -. 11 Faults, bounding tectonic units _________ 163 Grandfather Mountain window ____ 105 Field studies T Fisttop Schist _.-...~...._..__.~ 68 Folds, Blue Ridge thrust sheet ________ 55 Grandfather Mountain window ______ 107 Grandfather Mountain Formation __ 108 Inner Piedmont belt +_..____..__.____ 150 Tablerock thrust sheet _____________ 110 Wilson Creek Gneiss. _________.___. 107 Foliation, Blue Ridge thrust sheet 53 Brevard fault sone ___.c_.______._. 157 Inner Piedmont belt _____________ 150, 152 Fries FAUIE N ouch ene ne lone ee s 58 Prost, 1. C., analyst 87 Geography | 2 Gossan Lead district _ 19 Gossan Lead fault 58 Grandfather Mountain Formation ___10, 46, 48, 59, 60, 64, 71, 72, 105, 106, 114, 178 Folle !.. __ CE Tua - oh seas 108 Precambrian deposition _______ -~ 478 Granite rocks, Inner Piedmont belt _____ 140 Granodiorite and pegmatite, Blue Ridge thrust sheet ._._________L_.. 49 Granofels, Blue Ridge thrust sheet _____ 19 Great Smoky fault 168 Great Smoky Group 96 Precambrian deposition ____________ 174 Hamilton, John C., analyst ____________ 8 Hampton Formation _..___._..________ 11, 78 Helenmode Member, Erwin Formation 11, 100, 105 Henderson gneiss ___________ 26, 185, 142, 149 Hillebrand, W. F., analyst ______.._____ 90 Holston Mountain fault ___________ 175 Hornblende, Blue Ridge thrust sheet _____ 20 Hornblende gneiss, Inner Piedmont belt 122 Hornblende schist, Blue Ridge thrust sheet - 17 Inner Piedmont belt, rock units _____ 115, 116 Keith, Arthur, pioneer mapping ________ 6 Kerr, W. C., quoted 73, 98 Layered gneiss, Blue Ridge thrust sheet 21 Layered rocks, Inner Piedmont belt ____ 116 Layering, Brevard fault zone __ a... MBT Inner Piedmont belt _________ 150, 152 Lineation, Blue Ridge thrust sheet ______ 53 Brevard fault sone ____________. 158, 159 Grandfather Mountain window ____ 106 Inner Piedmont belt _..._.__._..... 150 Linville Falls fault ____26, 29, 40, 45, 47, 538, 58, 71, 84, 88, 104, 105, 106, 108, 112, 114, 115, 161, 163, 168, 169, 177, 178, 179 Linville Metadiabase _______ 57, 60, 78, 96, 115 LOCRLIOR ... Jc .- s 2 Lynchburg (Oneles ) -_ oc cus coccal oles. 19 Page Maclure, William, quoted ______________ 98 Mapping ._ ___. ee ULU OAU eaves T Marble, Inner Piedmont belt _________- 131 Metadiorite, Grandfather Mountain win- NOW | 59 Metagabbro, Grandfather Mountain win- (OW "clo .. 22 bores o Lens 59 Metamorphism, Blue Ridge thrust sheet 55 Grandfather Mountain window _____ 114 Mountain City window _____________ 11 Mica gneiss, Blue Ridge thrust sheet ___ 12 Blue Ridge thrust sheet, age ________ 19 Mica schist, Blue Ridge thrust sheet ___ 12 Blue Ridge thrust sheet, age _ 19 Inner Piedmont belt _______ 122 Migmatite, Inner Piedmont belt _________ 140 Mineral studies, reports ._._.__._._____. 4 Mixed rocks, Blue Ridge thrust sheet __ 20 Montezuma Member, Grandfather Moun- tain Formation ____73, 78, 88, 98, 96, 98, 115 Morganton basin, physiography ________ 5 Mount Rogers Formation ______________ 10, 96 Precambrian deposition ____________ 173 Mountain City window ________________ 10 Ocoee Series _________. 10, 48, 96, 98, 168, 175 Precambrian deposition ____________ 173 Origin, Grandfather Mountain window 169 Parker, C. L., analyst ___45, 70, 72, 80, 87, 90, 138, 144 Pegmatite, Deep Gap area ______________ 53 Petrographic nomenclature _____________ 9 Phyllite, Chilhowee Group ______- -. 99 Phyllonite, Wilson Creek Gneiss ________ _ 65 Phyllonitic schist and gneiss, Brevard Pault sone coon 220.1 ALL _L 155 Pine Mountain feult 177 Plutonic rock, defined 9 Poga fault --..} .c lc cll cus 165 Porphyroclast, defined __________________ 9 Powers, Dorothy F., analyst ___40, 42, 80, 90, 120, 138, 144, 146 Previous. investigations .__._.__________ 6 Pulaski Fatt) c... 0 ue ucc uled 00 A2 ence 178 Quartz monzonite, Deep Gap area _____ 58 Wilson Creek Gneiss ______________ 65 Quartz monzonite gneiss, Blue Ridge thrust sheet 40 Inner Piedmont belt 147 Quartz porphyry, Blue Ridge thrust sheet 45 Quartz schist, Inner Piedmont belt _____ 129 Quartzite, Inner Piedmont belt ________ 129 Roads Roan Gneiss _____ Rome Formation ___.____________ 11, 171, M9 Scolithus 11, 98, 99, 103, 105 Sedimentary rocks, Grandfather Mountain Formation " 2030. T5 Shady Dolomite ___. 11, 59, 98, 99, 104, 163, 179 structural history _.___..LLLL.___L__ 174 189 190 Page Shady Valley thrust sheet ______ 96, 103, 177 Sillimanite schist, Inner Piedmont Belt _ 128 Siltstone, Grandfather Mountain Forma- MON C22: 81 Smith, Hezekiah, analyst ____15, 18, 19, 26, 40, 70, 80, 87, 90, 102, 120, 125, 129, 138, 144, 146 Smith, Vertie C., analyst ______________ 40, 70 Snow Mountain fault 165 Snowbird Group, Precambrian deposition 173 Spruce Pine district ____6, 35, 45, 48, 49, 58, 176 Stokes, Ruth H., analyst._-.____.___.__ _15, 18 Stone Mountain fault ____38, 39, 165, 168, 178, 179 Stratigraphic record cc___..___.________ 176 INDEX Page Structure, Blue Ridge thrust sheet ______ 58 Brevard fault sone ___L_____.__:--- 156 Grandfather Mountain window _____ 105 history, nee cn lo Rens Inner Piedmont belt ___ Mountain City window ____________ 11 Synthesis, Grandfather Mountain area __ 171 Tablerock fault 105, 108, 114 Tablerock thrust sheet ______________ 98, 110 Tectonics, Grandfather Mountain area _ 171 Thrust faults, Grandfather Mountain area 163 Toluca Quartz Monzonite _______ 140, 145, 175 Trace elements % 8 Ultramafic rocks, Blue Ridge thrust sheet - 48 Inner Piedmont belt _______________ 149 Page Unaka belt ____10, 55, 95, 98, 103, 165, 168, 173 Unaka Mountain fault _____________- 165, 167 Unicoi Formation __11, 36, 48, 60, 73, 103, 105 Valley and Ridge belt ._.__.___.__...._. 177 Vegetation s . .. 5 Volcanic rocks, Grandfather Mountain Formation 88 Grandfather Mountain - Formation, age and correlation _______ 95 Wilson Creek Gneiss ____47, 60, 71, 105, 106, 114, 115, 176 folds 4 -... 2... o oo aL Leu 107 Yuval, Zui, rock density determinations 173 * U. S, GOVERNMENT PRINTING OFFICE : 1971 O - 382-126 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL sURVEY MOUNTAIN CITY WINDOW pm _J Rome Formation Shady Dolomite Lower Cambrian Arcs CAMBRIAN | - C2 Erwin Formation J B & > $1 ? a \ o s | $ w- 'S ks 2 é 2) Hampton Formation T i= > v § )| 2 m 3 < J fs f U _ Unicoi Formation § 82°00° 45" 36°15" STONE M AULT Upper branch STONE MOUNTAIN FAULT Lower branch + of uncertain correlation 36°00 35°45" E X PL A NA TIO N Mountain City Window Includes minor thrust slices beneath the Stone Mountain fault ~ Blue Ridge thrust sheet Includes basement rocks of the Park Ridge Creek area Tablerock thrust sheet sheet Autochthonous (?) rocks Grandfather Mountain Window Brevard fault zone Inner Piedmont belt 35°45 BLUE RIDGE THRUST. SHEET Granodiorite and Quartz monzonite pegmatite and pegmatite Mapped locally in the Deep Gap area Shady Dolomite PALEOZOITC Lower Cambrian Ultramafic rocks Chilhowee Group €cu, upper quartzite unit €cp, phyllite unit €cl, lower quartzite unit OR OLDER Lower Cambrian(?) MIDDLE PALEOZIC MIDDLE (?) and Lower Cambrian Chilhowee Group Thin tectonic slices of quartzite intercal- ated with rocks of the Blue Ridge thrust sheet along faults northwest and south- east of the Grandfather Mountain window Bakersville Gabbro CAMBRIAN (?) AND - CAMBRIAN g H_JK___Y___J Upper Precambrian(?) PRECAMBRIAN (?) pEb Quartz monzonitic Aegirine-augite gneiss granite gneiss Beech Granite Cranberry Gneiss p€c, layered cataclastic gneiss. Gramitic layers predominantly quartz monzonite p€cq, predominantly quartz dioritic gneiss p€cop, mafic porphyroclastic gneiss Lines indicate areas containing many phyllonite zones Layered gneiss southeast of the Grandfather Mountain window Mica schist, mica gneiss, and amphibolite p€ms, biotite-muscovite schist and gneiss p€a, amphibolite and horn- blende gneiss # Lower Precambrian A. ay! PRECAMBRIAN Mixed rocks Contact Dashed where approximately located, gradational, or CAMBRIAN CAMBRIAN (?) AND CAMBRIAN Pleistocene(?) and Holocene A Upper Triassic(?) Flood-plain alluvium Gravel, sand, silt, and clay in valley bottoms in places overlain by red clay colluvium along valley sides. Contains rare lenses of peaty sand sand. 0 to at least 50 feet thick. and clay. 0 to at least 20 feet thick E Xx P EL iA N A F | O - N Qal Tertiary age High-level gravels Terrace and pediment deposits, deeply dissected and locally thoroughly weathered. Generally overlain by red colluvial clay. On the Linville Falls quadrangle, an overprint pattern on other map units indicates areas of scattered gravel in float. Locally may in- clude deposits of Tertiary age Diabase GRANDFATHER MOUNTAIN WINDOW Alluvial-fan and colluvial deposits Angular to rounded boulders in matrix of crudely stratified or unstratified clay, silt, and pebbly Locally may include deposits of Linville Metadiabase f QUATERNARY TRIASSIC (?) Porphyroclastic granite gneiss Grandfather Mountain Formation p€gs, siltstone pEga, arkose pEgm, Montezuma Member p€gt, felsic volcanic rocks p€gum, mafic volcanic rocks Brown Mountain Granite Blowing Rock Gneiss abundant phyllonite Wilson Creek Gneiss pE€w, nonlayered quartz mon- pEwl, layered gneiss p€ wb, blastomylonite and pE wd, dioritic gneiss, com- monly sheared to blasto- mylonite and phyllonite pE wm, massive light-colored Limes indicate areas of abun- oe c Z i 2 m C. Quartzite and > 3 © ( greenschist 6 E q, quartzite Lu U g, greenschist C y t. O R g is -S § 8 § R & & $ ~ & -3 £ ast i S s o $5 zonitic gneiss A, 5 ® g phyllonite K] quartz monzonite dant phyllonite 12 flog 4 Overturned anticline Horizontal Inclined Metadiorite and gabbro 35 +- An Vertical Generalized Overprint indicates areas of inferred; dotted where concealed van) paua: s Fault Dashed where approximately located or inferred; queried where doubtful; dotted where concealed mgr gph Thrust fault Dashed where approximately located or inferred; dotted where concealed; queried where doubtful. Sawteeth on upper plate. Crossbars indicate segments along which latest movement was strike-slip e Anticline Showing trace of axial plane and direction of plunge; dotted where concealed M-}. _.... Syncline Showing trace of axial plane and direction of plunge; dotted where concealed Overturned syncline Showing trace of axial plane and direction of plunge; dotted where concealed PLANAR FEATURES &© < S Horizontal Inclined Vertical £35 _125 MS Overturned Top uncertain Generalized Strike and dip of beds =} 28 4+ s Horizontal Inclined Vertical Generalized Strike and dip of compositional layering Strike and dip of crystallization foliation in medium- grade metamorphosed rocks, cataclastic foliation in low-grade metamorphosed rocks, and cleavage in bedded rocks #4 -o- Inclined Vertical Strike and dip of foliation in phyllonite zones tn 10 7 voles Mii Isoclinal Nonisoclinal Strike and dip of axial plane of medium- or small- scale fold LINEAR FEATURES May be combined with any of the planar features «<-> 2022 <-> /O<fi Horizontal Plunging Blowing Rock quadrangle Horizontal Plunging Bearing and plunge of mineral alinement, stretching, Includes higher thrust slices be- neath the Blue Ridge thrust Showing trace of axial plane and direction of plunge; dotted where concealed 82°00" \ ~: S615" 3 81°30" 15 MILES ] streaking, or grooving BREVARD FAULT ZONE fs O Lo £ hb LN p LU ye LOz= Blastomylonite and g'fl €0 related rocks a - Q. x 0 O00 20 Phyllonitic schist | TW Bt and gneiss g 2' s Q. < 8 LU £5 C B w PRECAMBRIAN 5 «-> Se- <--» <4-- Horizontal Plunging Linville quadrangle Bearing and plunge of axis of minor fold or crenulation Horizontal Plunging 40 Horizontal Plunging Blowing Rock quadrangle Bearing and plunge of intersection of S-planes Horizontal Plunging % X R ~ Prospect or Prospect or - Mine or _ Mine or small mine _ small mine, _ quarry, _ quarry, inactive active _ inactive A *A Gravel or clay pit, Gravel or clay pit, active inactive A, anthophyllite Pb, lead As, asbestos Rm, road metal Au, gold S, soapstone Cu, copper St, building stone F, feldspar Ti, titanium Fe, tron U, uranium M, mica Zn, zinc Mn, manganese TRIASSIC (?) Relative age uncertain PROFESSIONAL 615 PLATE 2 INNER PIEDMONT BELT Granitic rocks and migmatite Red pattern shows migmatitic phase in Lenoir quadrangle. Migmatitic phase not distinguished in Linville Falls quadrangle Brown pattern shows areas where pods and blocks of amphibolite are abundant f Sm) f Quartz monzonite gneiss LOWER OR MIDDLE PALEOZOIC hg A Henderson Gneiss BC C Ultramafic rocks Sillimanite schist Red pattern shows areas where schist con- tains abundant porphyroclasts of potassic feldspar and folia, knots, and layers of quartz monzonite Brown pattern shows areas where schist con- tains layers and pods of amphibolite PRECAMBRIAN OR LOWER PALEOZOITC Gneiss and schist gs, predominantly layered biotite gneiss ms, predominantly mica schist gnu, undivided gneiss, schist and associated rocks in the Marion and Marion East quad- rangles Red pattern shows areas of migmatitic gneiss and schist containing abundant folia, pods, and sill-like bodies of quartz monzonmite. Contacts with migmatitic phase of quartz monzonite generally gradational Brown pattern shows areas of schist and gneiss containing abundant pods and layers of amphibolite 2 s6°:00' 45" INTERIOR-GEOLOGICAL SURVEY, WASHINGTON, D.C. -1970 -G68225 81°3305' is Surficial deposits not shown. Patterns show generalized trends of bedding, cleavage, and foliation co 08 3 > ‘k\\/ X09 A7/<é\ 8 00 50 EXPLANATION, FENCE DIAGRAM, AND GENERALIZED TECTONIC MAP OF THE GRANDFATHER MOUNTAIN WINDOW AND VICINITY, NORTH CAROLINA AND TENNESSEE UNITED STATES DEPARTMENT OF THE INTERIOR PROFESSIONAL PAPER 615 GEOLOGICAL SURVEY PLATE 3 37° 83° YZ 82° a 37° 1 NL /\>|:—’ v; \ > D s! ‘ Q \V 0 Q Qflfi", z SS ~- p = €*®>~ cin w oris} ‘YVG 7;'\~\7\//\1\~':1, / z € 1 (a‘ljifiydr / -~ ® > > EN, / 11, mes- thm sys I/\,I~,‘_\/\f_\l / e sez" f . Preparreisnimeis seney) .* "4 S" }. p 3 _|/\r.,7_\/<| \—;:‘]_\ <7>\__ 'l\ ~ ue -L a rf Mount -/ i/ }} 4 r ini pean # _ t e ar 13 By ¢8 4 " AZ Pars W '>\'—‘\/\‘/\— C LSA ~\1/2 x~ > §7 \’/,(| \/\——I/|\\'-l~/\/\ 17-\/\’ /\I\:l/|\‘\\ ~ f_ ME ert TT -----> VIRGINIA A1v> ry< L1pAVrL1r 45‘ 4A ¥A<:\l; 7/03 :’:/‘\:T\‘_(zl\":’|>\ we enn ene es paces Pup d Pud -- i w TENNES: € T y ‘vEJLr‘3rqA X./:|‘,\_}_5 YZ X~ NZ | SEE » nls Jad MML -z," ~nAY< kes pi a 2 VIRGINIA > / si Pg uma nnis Quds yey nay AGV / " DT NeS U AE F, te d 2191 s uk z¢ I-1> ~> IAN yy IA le hes u Si Hiver pic ial eas leven 5a e % t- 4 VI A z* Q<’,\i’\\\/1\\’\‘,q‘fl;¢\’ / / =~ / 4 ~1 " e A Lite DY vito". // / F SIS E Tig. e. A 4 hy T, ‘\|/‘/_\I\\/\q)1 “QQJ o Jc» aa 9; Knol nob b \ —,\’/(/ X :’//\f\“’\\/§'\7 / / / ~ a ¢ 2 et a o Pri / “7 ’/ / / / / / / BAC p / / / / o \j / 6 ; / / Aj) 3 $ # .~ 1 "/“l\\/( / lA bes! / C e .:: ox | \ / #] - N rd L . C atea sk mice ns PAre f a4 m He tm Ais sis £ « (91 4 fi‘ Pardee Poth/Q‘il 3 his ///\fi\:l"~l\\ » // / a -/ pm f -t Y2i" , 8 = a _» \ w- mew« 15 »6«\"/~’\/’\II<\\’\\"\'7‘\'\:/l:l/\:’\\ a> \/_\,’\\<,\/ (1:1); ~ / Yami Af / £ ‘l Pied > S Ns v8 /O Sug! i ACLVDAA /\I/\\l_ Ll ,)>’-Lf\-‘,.\\ \\\/\\’I Z ASV £ I tA, a & f \’1 71‘\\ '\ \ x7 35 INNEREWINDO MZ (SAX - {rye, Traks alts (s )a fe fop - EXPLANATION ,)'727\‘§‘|-’ (ast2; + T $ Lif / 15,2 CG; _ Paleozoic plutonic rocks UNAKA BELT ty M- & f SLX? a a BRANC as > 2a \V\//|N|56 ~ (15915 Pg 1 Y! (Pf: \)> $a $ _ f Buffalo Mountain thrust sheet p I Mescie aleozoic sedimentary rocks _\/|\ ’\| /f} /7:\ vy‘ >a wat ® f:} / 4 -~. Chilhowee Group 1 ¢‘\ # ~- ore : Sy -/ Sw~ - Morganton Ocoee Series §\\,(P‘\VX sY, 8 *. '. %*. = ." =a s= 7st 1" #4, =[» a 7 & e s ago :f L & st ® 52g 2 =t s = = - [~* oi fies is- = = s ='1"« ,*! ==." /I Walden Creek Group ~~§==,1L4\\=3\\m=_\\=<¢§=:= 6 OA *F 13! AG s aat" -*- § = = = % = __~,/,,;“,’=l\\=\\§¢a\ . Q, restr A /ll¢§l"¢ Il\=\\\\’/§l/4///’\= yf LS, vu//“:\\“’/,/f\\\¢" Pay l:', \4;\\§4 21 =+ = % = "z = . ~* Prema t n ty "Aff ele < sa is T ~ Mount Rogers Formation Great Smoky Group as =, ~ «* 44\‘\‘\\\ #5 I I yan! Il=¢¢“4 $4 =" s I5 q ,f9;fi < 'L‘>7=\\ Sa d= * +: i- ¢" %" and Grandfather Moun- P>) suiting a 3 *. ~AFiZ".*'L < +2" s + / stay Le a*," S% == J # 2} x*. tain Formati wk: ngman 33% =7 924 of S = *« = ".* * . # 4 _ $ Des a=", =* Domes ~ 4 *= 1 a=} = *," p" h= # =//\\/\\ n_\\\\’\\ 2 § = ="% "~ 1",= i an z £4 ist e's" gp a! Snowbird Group BuT 4 = % a § \\§I/\\Z {i \ '# a" 7 2 #4 8 f o u 7\’_‘1\J—\ Pifis=s p =u 2 =3 = L F ## n ts ~a Le \ * 1 #4, =" = -_ 1 Z F F ¥1 = +41 Precambrian plutonic rocks n +4 in“ =4* = 14 “gut/0‘\ $ = ee Miata ¥ L. x2 nz Py a s 1 = 4 \ # #8 Y ueze1z =* Precambrian metamorphic rocks 144 1p 3 a ”22.“ SC 125 Z?“ CZ Contact 1y3 > 4_\— i 2 mer mafic = 2+ .~ {inst < Thrust fault ~~~ Dash . y // ashed where approximately located; s awteeth on upper plate __ __ errr mec mmen mames Po 3 _ Fault Dashed where approximately located; queried where doubtful. Arrows in- dicate relative horizontal movement 35° L_ as Shady Valley thrust sheet Hot Springs and Hampton thrust sheets Limestone Cove, Doe River, and Doe Branch inner windows BLUE RIDGE BELT Blue Ridge thrust sheet GRANDFATHER MOUNTAIN WINDOW Tablerock thrust sheet Autochthonous (?) rocks Brevard fault zone Inner Piedmont belt o 82° GEOLOGIC AND TECTONIC MAP OF THE BLUE RIDGE AND VALLEY AND RIDGE BELTS IN PARTS OF NORTH CAROLINA, TENNESSEE, AND SOUTHERN VIRGINIA SCALE 1:500 000 10 0 10 20 30 40 50 MILES ) £00000 00 S n s aged . 4 1 1 I 1 10 0 10 20 30 40 50 KILOMETERS ] L6 A ia 1 1 1 | 1 81° INTERIOR-GEOLOGICAL SURVEY, WASHINGTON, D.C. -1970-- 668225 Blue Ridge and Unaka belts modified from a map compiled by P. B. King. Valley and Ridge belt modified from Rodgers (1953a) and Virginia Div. Mineral Resources (1963) UNITED STATES DEPARTMENT OF THE INTERIOR PROFESSIONAL, PAPER 615 GEOLOGICAL SURVEY PLATE 4 MILLIGALS 0 l 0 MILLIGALS | 50 50 100 100 SIMPLE BOUGUER GRAVITY ANOMALY. PROFILE Data from Am. Geophys. Union Spec. Comm. Geophys. and Geol. Study Continents (1964) ~ = i% sos 7 sa% a A 28 | as s § 4 o o & fb 9 OA Z = nto P co 17 en cn 00 a JT TS] Z 4 Aid BREVARD FAULT © m + 112 MOUNTAIN CITY 2 (m 2 % z © KILOMETERS 5- $ 5 a 1 - winbow glg @ss s ] P al %, % BUFFALO MOUNTAIN $4 > >18 * "Sop * THRUST SHEET ""%! “5.4100” BLUE RibeE"i$tHrUst - shHEET $% 5 f t+ re eg 74; ""7.f’.’/vp < GRANDFATHER MOUNTAIN winpow 2 ~ € I ij&’§70 o .,F,‘_\P.L.T.......: "ax-F404 SHADY VALLEY THRUST SHEET "flag-fl?“ £ O€ca 'she & cet A es aaa asl tig g Northwestward extent gis ”(it $A TAs ra tye of upper Precam- fas 4T. 2%, brian rocks uncertain = ! Sug : AS g 4 2 a shd tI * f d siss/ a/ sat iz ‘“/‘\’4’,‘,l as' e o acte on a toata /\',\‘F‘\’/,\’fz\ = , 7 Ridge belt may turn downward into . _'7 ' ~/<' fart If yal faa a aA yin ene yor o o tote ue Sate t coe onde i>'the basement in this area rather}'.~,~' - than continuing southeastward ~ '-in Rome Formation as shown 10 - Occurrence of ultramafic rocks along southeast side of the Brevard fault *a zone suggests that the structure < may extend to the base of the crust 20 - - 20 30 - 30 Crust & Mantle (> Crust | Mantle Crust Mantle fs" GEOLOGIC SECTION AND GEOLOGIC SECTION EX-P L "A. N-~A T 1 O N Upper Precambrian f—A—_——fi - MDca O€ca €ca Clastic rocks Clastic rocks Predominantly carbonate Clastic rocks Clastic rocks Predominantly carbonate Rome Formation Carbonate rocks Chilhowee Group Sedimentary and Basement rocks rocks rocks volcanic rocks Includes Chattanooga Shale Includes Conasauga Group at base at base C & a. y ep AK - sd _______ X_ g- rakes. - A PA < Pe y MISSISSIPPIAN MISSISSIPPIAN DEVONIAN: ORDOVICIAN, ORDOVICIAN CAMBRIAN CAMBRIAN CAMBRIAN PRECAMBRIAN AND AND SILURIAN, AND AND PENNSYLVANIAN MISSISSIPPIAN AND ORDOVICIAN CAMBRIAN (?) DEVONIAN ya y ont n> Gneiss and schist Mois s ovale. .l fee 4 PRECAMBRIAN AND(OR) LOWER PALEOZOIC SIMPLE BOUGUER GRAVITY ANOMALY PROFILE OF THE WESTERN PART OF THE See plate 3 for location of section 5 0 7) 10 1|5 2|O | 1IO 1l5 2,0 215 KILOMETERS Granitic rocks Nomen maine e MIDDLE (?) PALEOZOIC SOUTHERN APPALACHIANS 215 MILES UNITED sTATES DEPARTMENT OF THE INTERIOR PROFESSIONAL PAPER 613 GEOLELOGICAE SURVEY PELATE.-53 Axial pl f tight A f tigh Fold and crenulation Foliation and layering Mineral lineation a“? gangs ff II? >'' 5 - £ o & he & 2 A o $8 €. A $ 5 $5 o 5 c $ 5 3 > 6 3 f ga 3 6 n a 3 Contours 0.5, 1, 2, 5, and 10 A-1 A-2 A-3 A-4 27 Acé #72 9,1. 42,9, percent Contours 0.5, 3, 5, 10, 15, and 20 percent Contours 1.5, 3, 5, 10, and 15 percent Contours 1.5, 3, 5, and 10 percent Contours 1, 2, 5, 10, and 15 percent Contours 1, 5, 10, 15, and 20 percent Contours 5, 10, 15, 20, and 25 percent n=651 n=327 n=B1 n=47 #=756 n=83 ? contours = 28 a C Sg o 3 2.9 9 £5 7 3 < 2.0 cp 2 3 '= (€ fore 5 » D ® 5 o z ke oa < 5 ~ - tD < 0c.) O 0 :C 0 $ O <+ B 3 n B G .S g 5 Say' on C 8 b EE s 5. x 6 € © r4 2; a E 5 2 w e 2 in + £6 $2 & 0 8 w © * 5 Contours 0.5, 1, 2.5, 5, and 10 percent B-1 | Contours 1, 2.5, 5, and 10 percent B-2 | Contours 2, 5, and 10 percent B-3 Contours 2, 5, and 10 percent B-4 Contours 1, 2, 5, and 10 percent B-1 | contours 1, 5, 10, 15, and 20 percent B-2 | contours 5, 10, 15, 20, 25, and 30 percent B-3 Axial planes (dashed contours) and axes ne-2054 #=54 mense ee (solid contours) of open folds and crinkles £ Haxes=bO1 M. axial plant—35:59 8 .5 is] G = 42 0 £ >n = wi o C S £ § & ID 2 ¢ C C o m i $ 2 2 r o sg 8 O a 11:9 kel - ® m.E o €) 22 G 2-0 oo 51 El. 5 | B §) 2 2 w & y > '% #: g a © % a a $ */ 17 in x. G € 2 é § 30 O g 8 & B-2 32 e: B- Solid contours 4, 10, Dashed contours 10, p-3 s Contours 0.5, 1, 5, 10, 15, and 20 percent and 15 percent 20, and 30 percent Dashed contours 1, 5, 10, 15, 20, and 25 percent Contours 1, 5, 10, 15, 20, 25, and 30 percent Contours 1, 5, 10, and 15 percent C-1 Contours 1, 2, 5, 10, and 15 percent B-1 |_ contours 1, 2, 5, 10, 15, and 20 percent B-2 B-3 Ficur® 3.-Tablerock thrust sheet. n=370 n=184 fold axes Axial planes (solid contours) (dashed contours) 0 C A= R to. & x LU c am 3 g & =R | / 0 o = 2 = & # EXP LA N A T1.:0O N EEE C we All diagrams are equal-area projections in Lu 5 S 7 the lower hemisphere with the plane of 8 C % y £ ? z on projection horizontal and north at the top. &C "' < Only areas within three highest contours u- " § are patterned, »= number of points 3° © co 0 3 Me i o l & C-3 Solid contours 2, 5, Dashed contours 5, 10, >30 2530 20-25 15-20 Contours 1, 2, 5, 10, and 15 percent C-1 Contours 1, 2, and 5 percent C-2 and 10 percent 15, and 20 percent ........ n=233 m=50 ........ 1; ........ 12 o bp $5 f C LJ CG Z 42 N +y" N rs) / S n 3 2 # § ~. |P e o 3 § ,> £ 2g CC co 0 >n E 2 w s G Contours 1, 2, 5,10, and 15 percent D-1 Contours 2, 5, and 10 percent D-2 D-3 Fold and crenulation Axial planes P p= n=341 ? I.. £ n=1,484 k axes (solid contours) (dashed contours) Ev; n=59 n=48 £50 3 @ s s ? a 9 R 2 3 - f a 9 3 S= £ | 5.4 5 < C S= >. ® 4 < E 5 E 1 CONTOUR DIAGRAMS SHOWING ORIENTATION OF STRUCTU 2 | $56 f G WING 0] CTURES 5 | if" / ) si |- 95g Q 5 5. 62 GRANDFATHER MOUNTAIN AND VICINITY § 2ff , i & 3a S y eae NORTH CAROLINA AND § s;: TENNESSEE § Z p =. C E-3 $s s “it" § ~ Solid contours 5, 10, Dashed contours 2, 5, - Contours 1, 2, 5, 10, 15, and 20 percent E-4 Contours 1, 5, 10, and 15 percent E-2 and 15 percent and 10 percent LL ~ |E C n=743 n= 174 & |» S 213% i, 80 9 | § 3 TC ~. a- U an \ § f S ae | < 53 s £ an 5 = l > {20 31. p' 4 xy | o px" 2 |283 A 8 5 5 i_ I o * $ a ope D0 l 5 ee. Com s . 4 PN Contours 1, 5, 10, 15, 20, and 25 percent Contours 1, 5, 10, 15, 20, 25, and 30 percent Kz8 Ficur® 5.-Southeastern part of the Grandfather Mountain window, the Blue Ridge thrust sheet southeast of the Grandfather Mountain window, the Brevard fault zone, and polymetamorphic rocks in the Inner Piedmont belt southeast of the Brevard fault zone. UNITED STATES DEPARTMENT OF THE INTERIOR PROFESSIONAL GEOLOGICAL SURVEY PEATE 6 82°00" » 82°00" 81°45" fags 5C E \ 36°15" - -==- here is somas &" + if fea Bayt b Ls ; /|\/\/\/l\\_/_\/\l\L\,T\/\/l/\:I> R IM La d) ++ e |\/—\ soix 1/‘\4\/\/\>|f/\/\,\:/\l ,‘\/\//\’/\|’|' $ I. ne al o oll =>, '/\7\,\‘/_\/\/.‘;'/‘\,/\/_.\/\J_\//\L\//’\\lJ r 1:53 of f msi ANS E, a s 1" yay * x- +# + J+ 7 l\/—-\|: SLN Af 1g - >:,_ § 'as 4% 7+ = Q3 1s > R \\\/I\ s' s & - v8 § /\/\‘//\/\l/\, j AX* I* ") \d joust d al x12 Al $114 1 \ v¥§#7 «* $ $400 7. 36°00" = % fl" y ai 112 * x4 = 1 -> 7 AS I- 7 - A \l7|\ o U \/\ sos e v3 = €. (J/Ql— 7 11.4 #, A4, ROCKS METAMORPHOSED OR EMPLACED DURING THE EARLY PRECAMBRIAN 30°45 s 7 T~etof +*+ # t//\/"\//: Mle 4 alc tp ~\ 7: 21 \j + % % a> 133 ."; . fl ss Nonlayered granitic rocks Metamorphic rocks Granitic rocks and migmatite Rocks metamorphosed 450 Rocks metamorphosed Emplaced during or after regional metamor- and (or) 350 m. y. ago 350 m .y. ago phism 450 or 350 m.y. (million years) ago §*A#7 #3 c * *y * A ra s/2 4 xs * aa i Rocks metamorphosed 450 and Rocks metamorphosed after 350 m.y. ago Layered granitic rocks Younger rocks and migmatites Granitic rocks possibly 350 m.y.ago and possibly after 450 m.y. ago Emplaced during or before regional metamor- phism 450 or 350 m.y. ago EX ~ P L.-A N. A -IT 1! -O -N Contact Thrust fault Unclassified fault Dashed where approximately located Sawteeth on upper plate 82°00" 82°00" No data 81°45, 81030! P 81°30 ‘ 36°15 30°15'|-- 36°00 36°00" J s | 35°45" ........ > C, ROCKS METAMORPHOSED DURING THE {y asf ) LATE -PALEOZOIC TO EARLY MESOZOIC I), PRESENT METAMORRPHLC Medium-grade rocks containing zones with Rocks partly or completely metamorphosed conspicuous polymetamorphic texture to low grade along Brevard fault zone High Low Unmetamorphosed sedimentary Sillimanite-muscovite Biotite-albite-chlorite and plutonic rocks Medium-grade rocks with polymetamorphic Rocks partly metamorphosed to low grade; Medium Incomplete low textures throughout relation to Brevard fault zone uncertain Kyanite-staurolite Chlorite MAPS SHOWING METAMORPHIC HISTORY AND GRADE OF THE GRANDFATHER MOUNTAIN WINDOW AND VICINITY, NORTH CAROLINA AND TENNESSEE 6 0 5 10 15 MILES | I $ 0 6 10 15 KILOMETERS | I ee. UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY PROFESSIONAL PAPER 615 PLATE 7 $2100: |- _ 36°15" ---|_--- EXPLANATION Contact -A___A__ A.. A.A.. Al Thrust fault Sawteeth on upper plate Boundary of sector Boundary between the northern and southern areas of the Grandfather Mountain Formation GRANDFATHER MOUNTAIN 29°45 FORMATION TABLEROCK THRUST SHEET Sector Number Contours 1 Sectot Number Contours of poles (percent) eeto of poles (percent) I 173 0.5, 1.7, 4.0 A 149 1.3, 6.6, 13.0 II 41 2.5, 1.2, 12.0 B 149 0.6, 2.0, 13.0 III T7 12,52, 10.0 C 81 1,2,,12.0, 31.0 IV 170 1:1;2.2, A:T D 160 06.6, : 8.0, 16.0 V 52 2.0, 4.0, 8.0 E 102 1.0, 5.0, 20.0 VI 49 2.0, 4.0, 8.2 F 89 1.0, 4:0, 18.0 Cai 102 2.0,5.0, 9.0 G 101 1.0, 3.0, 10.0 ¥HT 20 3.5, 7.0, 14.0 | H 26 4.0, - 8.0, 88.0 IX | 27 4.0, 7.0, 11.0 I 89 1.0. 55.320 X | 49 2.0, 4.0, 8.0 J, 155 0.6, 8.2, 29.0 XI 23 4.2,8.1, 12.5 K 60 1.6, ©$3,25.0 XII 111 2.0, 4.0, 7.0 All diagrams are equal-area projections XIII 25 4.0, 8.0, 16.0 in lower hemisphere with planes of pro- XIV 70 3.0, 6.0. 12.0 jection horizontal and north at top XV 96 1.0, 5.0, 30.0 2 4 6 SMILES | 1 | ] O 3 _ y CONTOUR DIAGRAMS OF POLES TO BEDDING IN THE GRANDFATHER MOUNTAIN FORMATION AND THE TABLEROCK THRUST SHEET, GRANDFATHER MOUNTAIN AND VICINITY, NORTH CAROLINA AND TENNESSEE