7 DAYS EAR! 0-H pc* \J. I Introduction to Correlation of Precambrian Rock Sequences) Introduction to Correlation of Precambrian Rock Sequences By JACK E. HARRISON and ZELL E. PETERMAN CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO Edited by JACK E. HARRISON and ZELL E. PETERMAN GEOLOGICAL SURVEY PROFESSIONAL PAPER 1241-A An introduction to interregional correlation, emphasizing the need for charts based on isotopic data UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1984UNITED STATES DEPARTMENT OF THE INTERIOR WILLIAM P. CLARK, Secretary GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data Harrison, Jack Edward, 1924- Introduction to Correlation Precambrian Rock Sequences. (Correlation of Precambrian Rocks of the United States and Mexico) (Geological Survey Professional Paper 1241-A) Bibliography: 7 p. Supt. of Docs, no.: I 19.16:1241-A 1. Geological time. 2. Stratigraphic correlation. 3. Geology, Stratigraphic—Precambrian. 4. Geology—United States. 5. Geology—Mexico. I. Peterman, Zell E. II. Title. III. Series. IV. Series: Geological Survey Professional Paper 1241-A For sale by the Branch of Distribution, U.S. Geological Survey 604 South Pickett Street, Alexandria, VA 22304CONTENTS Page Abstract ........................................................................................................................ A1 General statement................................................................................................................. 1 Acknowledgments............................................................................................................... 1 The problem...................................................................................................................... 1 Preparation of detailed charts.................................................................................................... 4 Generalized correlation chart.................................................................................................. 5 Concluding remarks................................................................................................................ 7 References cited.................................................................................................................. < ILLUSTRATIONS Page Plate 1. Generalized correlation chart for the Precambrian of the United States and Mexico in pocket Figure 1. Generalized outcrop map of Precambrian rocks of Canada, the United States, and Mexico......................................... A2 IIICORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO INTRODUCTION TO CORRELATION OF PRECAMBRIAN ROCK SEQUENCES By Jack E. Harrison and Zell E. Peterman ABSTRACT This introductory chapter on correlation of Precambrian rocks of the United States and Mexico outlines the problem of correlations and emphasizes the need for charts based on isotopic data. The groundrules adapted for preparation of the correlation charts are given and apply to all charts in Professional Paper 1241. A generalized correlation chart for the Precambrian of the United States and Mexico presents state-of-the-art knowledge on interregional correlations and serves to illustrate some of the difficulties in deriving an acceptable time scale for the Precambrian on a continent-wide or world-wide basis. GENERAL STATEMENT This and subsequent chapters of Professional Paper 1241 summarize present knowledge of Precambrian rock sequences of the United States and Mexico, with emphasis on their dating and correlation. The reports have been prepared by the Working Group on the Precambrian for the United States and Mexico, a formal committee of the Subcommission on Precambrian Stratigraphy of the International Union of Geological Sciences (IUGS) Commission on Stratigraphy. This Working Group is one of several groups established throughout the world in 1975 by the Chairman of the IUGS Subcommission, H. L. James, to aid the Subcommission in its analysis of the Precambrian terranes of the world and in its effort to devise an internationally acceptable time scale and nomenclature for the Precambrian. The character and distribution of major Precambrian units of the conterminous United States have been described by King (1976). Reports from the Working Group are principally concerned with evaluations of fundamental geologic and age data that serve as a basis for correlation, that identify which parts of the Precambrian record are preserved, and that show how much of the record is missing. In addition, the economic importance of various Precambrian rock units is briefly noted, and first summaries of the Precambrian of Alaska and Mexico are presented. ACKNOWLEDGMENTS Working sessions and preparation of charts and reports by the Working Group were supported financially by the U.S. Geological Survey and by grant number 78-095-E from the U.S. Department of Energy. Guidance on stratigraphic questions was generously supplied by Marjorie E. MacLachlan of the U.S. Geological Survey, Denver. M. J. Frarey, Chairman of the IUGS Working Group on the Precambrian for Canada, participated in some of our Working Group sessions. We are also pleased to acknowledge technical advice from two of our Canadian consultants, G. H. Eisbacher, Geological Survey of Canada, Vancouver, British Columbia, and Harold Williams, Memorial University, St. John’s, Newfoundland. THE PROBLEM Precambrian time encompasses the first seven-eighths of Earth history. A series of crust-forming, tectonic, magmatic, sedimentational, biologic, and ore-forming events—some of which are unique in Earth history—are recorded in rocks of the Precambrian. Ordering of these events and understanding of the geologic processes that formed the rocks is essential to comprehension both of the architecture and of the resources of the Earth. Study of the Precambrian in the United States and Mexico is complicated not only by the changes produced by deformation and metamorphism but also by the extensive cover of Phanerozoic rocks that obscure large terranes of Precambrian from direct view. The general isolation of individual terranes of Precambrian rocks is illustrated on the tectonic map of North America (King, 1969) and by figure 1 of King’s (1976) summary of the Precambrian geology of the United States. A simplified geologic map of the United States, Canada, and Mexico (fig. 1) also shows how disconnected the Precambrian AlPACIFIC Figure 1.—Precambrian exposures of North America (modified from King and Edmonston, 1972), showing regions discussed in succeeding chapters of this report. Precambrian regions: 1, Eastern United States; 2, Lake Superior region; 3, Central Interior; 4, Rocky Mountains and Black Hills; 5, Western and Southwestern United States; 6, Northwestern Mexico; 7, Eastern and Southern Mexico; 8, Alaska. co INTRODUCTION TO CORRELATION OF PRECAMBRIAN ROCK SEQUENCESA4 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO exposures are outside of the shield area of Canada. Summaries of the Precambrian of Canada are contained in reports of the Geological Survey of Canada, notably Douglas (1970). Though relative ordering of the Precambrian rock units and events in many of the isolated Precambrian outcrop areas can be accomplished through careful geologic mapping, correlation and chronologic ordering among isolated Precambrian terranes in a region is almost totally dependent on credible isotopic dating of rock units. This dependence on isotopic dating stems from the complexities of many highly deformed metamorphic and plutonic terranes. Such terranes cannot be analyzed by normal stratigraphic procedures that depend on superposition of strata for ordering and on homotaxis for correlation between separated areas of exposure. Most Precambrian rocks, particularly the rocks of mid-Proterozoic (about 1600 m.y.) age or older, lack fossils that have meaningful stratigraphic application. Correlation of terranes on a continental or global scale must be done by comparing isotopic ages of rock sequences, or more rarely, by comparing fossil or paleomagnetic data that have been calibrated in isotopi-cally well dated rock sequences. Understanding of the geologic history of the Precambrian requires placing each of the isolated terranes in its proper time frame, and then comparing terranes to establish the full extent of regions that have been subjected to similar geologic processes and sequences of events. The time frame of seemingly parallel sequences of events may differ from region to region. On a global scale, for example, the termination of -sequences of events that characterize the older parts of shield areas and that have been used to define the Archean actually range in age from about 2800 m.y. in some shields to about 2300 m.y. in others (Cloud, 1976). Thus Archean as a time term as used in the past was found to inhibit orderly scientific communication, as the time span intended by the user could vary by 500 m.y. To avoid such confusion, the IUGS Subcommission has recommended a boundary at 2500 m.y. to separate the Archean from the younger Proterozoic (James, 1978). Continent-wide correlation of the Precambrian has also been fraught with severe problems. Time terms such as early and late Precambrian have been used to identify relative ages of rock assemblages in local terranes. As the number of isotopically dated rocks has increased, it has become evident that such terminology is more confusing than helpful in understanding Precambrian history, because the so-called late Precambrian of one area has turned out to be the so-called early Precambrian in another. In addition, workers have established some time brackets by defining a tectono-magmatic event in a local area, or a time boundary has been established at the termination of such an event, and then a time term has been extended to any rocks of that age whether they participated in that event or not. Such a procedure tends to obscure rather than elucidate Precambrian geologic history. Detailed correlation charts for the Precambrian of the United States and Mexico presented in subsequent chapters of Professional Paper 1241 represent the state of knowledge on chronometric dating of the major Precambrian terranes. Some reports incorporate previously unpublished data, and all represent an intensive review of the literature and the best judgment of the authors on the quality of geologic and geochronologic data currently available. The charts and reports should serve not only as a source of current data, but also as a tool for testing geologic hypotheses and developing meaningful time terms for the Precambrian of the area covered. PREPARATION OF DETAILED CHARTS For the purpose of describing time-rock relations of the Precambrian, seven more or less coherent regions containing Precambrian terranes (fig. 1) were defined, and small groups of specially qualified geologists and geochronologists were enlisted to prepare the reports. Participants in the regional compilations were as follows: Eastern United States—D. W. Rankin, James McClelland, R. E. Zartman, A. L. Odom, and T. W. Stem Lake Superior Region—G. B. Morey, S. S. Gol-dich, P. K. Sims, and W. R. Van Schmuss Central Interior—R. E. Denison, E. G. Lidiak, M. E. Bickford, and E. B. Kisvarsanyi Rocky Mountains and Black Hills—C. E. Hedge, R. L. Houston, Ogden Tweto, Z. E. Peterman, J. E. Harrison, and R. R. Reid Western and Southwestern United States—L. T. Silver, C. A. Anderson, M. D. Crittenden, Jr., J. C. Crowell, and J. M. Robertson Northwestern Mexico—T. H. Anderson and L. T. Silver Eastern and Southern Mexico—Fernando Ortega G., T. H. Anderson, L. T. Silver, and Jose Guerrero Alaska—G. D. Eberlein and M. A. Lanphere Boundaries of the regions (fig. 1) are principally geologic in the sense that they correspond to boundaries of younger geologic provinces, but some reflect political or geographic distinctions, and some are merely convenient from the standpoint of compiling information. TheINTRODUCTION TO CORRELATION OF PRECAMBRIAN ROCK SEQUENCES A5 boundaries should not be interpreted as fundamental geologic features within the Precambrian basement. The correlation charts are intended to depict objectively the current state of knowledge of Precambrian rocks and history. Time-rock columns for specific areas portray the record as established from isotopically dated rock bodies and geologically determined sequences. In some areas, one or both of these elements may be lacking, and some units are provisionally placed in the charts through homotaxial or other stratigraphic inferences. Anticipation of any international definition of a time scale for the Precambrian was purposely avoided. Nor were any constraints imposed on the compilations and interpretations from classifications currently in use for Precambrian rocks of North America (for example, James, 1972, or Stockwell, 1973). The radiometric ages used for constructing the charts and cited in the reports are recalculated, as necessary in terms of the isotopic and decay constants recommended by the IUGS Subcommission on Geochronology (Steiger and Jager, 1977): Uranium X(238U)=1.55125 xlO*10/yr atomic ratio X(235U)=9.8485x 10'lo/yr 238U/235U = 137.88 Thorium X(282Th)=4.9475x 10'n/yr Rubidium X(87Rb)=1.42x 10'n/yr atomic ratio 8ERb/87Rb=2.59265 Strontium atomic ratios 86Sr/88Sr=0.1194 84Sr/86Sr=0.056584 Potassium X(40Kp-)=4.962xlO'10/yr isotopic abundances X(4OKt)+X(4OKc)=0.581xl0_1°/yr 39K=93.2581 atom percent 40K=0.01167 atom percent 4,K=6.7302 atom percent Argon atomic ratio 4 o N o Absence of beds INTRUSIVE x COMPLEX * '/ x X x /W 450±10 m.y. * V (oldest phase) (U-Pb) I X X x Base not exposed x \ x/x v v V v v i A A A A A <3 O & -'r EXPLANATION MAFIC VOLCANIC ROCKS pillowed FELSIC VOLCANIC ROCKS—Includes metakeratophyre and metarhyolitic tuffs and flows BEDDED MAFIC BRECCIA AND TUFF SANDSTONE AND SILTSTONE BANDED MUDSTONE, ARGILLITE, AND SHALE LIMESTONE AND MARBLE INTERBEDDED MUDSTONE, LIMESTONE, AND SANDSTONE CONGLOMERATE AND SEDIMENTARY BRECCIA SCHIST, SEMISCHIST, PHYLLITE, AND SLATE MAFIC AND FELSIC INTRUSIVE ROCKS GRAPTOLITES LAND PLANTS Figure 4.—Generalized stratigraphic relations of the Wales Group, southeastern Alaska. Queries in section indicate uncertainty of extent and stratigraphic and structural relations of discontinuous deposits. Modified from Churkin and Eberlein (1977). the zircon populations. Subsequent analyses of a hornblende diorite yielded concordant ages within and between multiple zircon fractions, indicating a crystallization age of 450±10 m.y. (Saleeby and Eberlein, 1981). On the basis of stratigraphic evidence, the intrusive complex is known to be of pre-middlePRECAMBRIAN ROCKS OF ALASKA B13 Lower Devonian (Pragian) age, because clasts of the complex are a major constituent of an overlying sedimentary breccia that in turn is conformably overlain by calcareous siltstone, conglomerate, and black slaty shale that contains Monograptus pad ficus. Zircon from a deformed dike in a gneissic phase of the Wales Group at Sunny Cove, Prince of Wales Island, yielded a concordant U-Pb age of 525 m.y. (Jason Saleeby and G.D. Eberlein, unpub. data, 1977). If one assumes an age of about 570 m.y. for the base of the Cambrian, then given the apparent stratigraphic thickness of the Wales Group it seems likely that at least part of the Wales Group is Precambrian in age. Mineral resources.—The Wales Group has served as the host for a wide variety of mineral deposits, including polymetal veins, copper-iron skarns, stratabound volcanogenic massive sulfide deposits, dolomite replacements, and mineralized shear zones. Most mines and known occurrences are clearly related to the emplacement of Cretaceous (about 102 m.y.) granitic plutons. However, certain stratabound deposits may be Precambrian in age and of volcanogenic origin. At the Khayyam mine on eastern Prince of Wales Island, lenses of massive iron and copper sulfide minerals, with subordinate zinc, silver, and gold, are aligned parallel to the layering of enclosing gneissic amphibolite host rock and define a mineralized zone 3-8 m wide that can be traced for a distance of about 340 m (Fosse, 1946). Similar relations occur at the Mammoth (Stumble-on) prospect about 2.4 km on strike to the northwest. The Polymetal zinc-lead prospect, located on the west side of the South Arm, Cholmondeley Sound, eastern Prince of Wales Island, reportedly occurs in banded chloritic quartz-sericite schist similar to metarhyolitic(?) host rock in the lenticular volcanogenic massive sulfide deposits of Niblack Anchorage (Brooks, 1902, p. 87; Peeke, 1975). However, recent geologic mapping by one of us (Eberlein) suggests that the Niblack assemblage is of early Paleozoic age, although the evidence is not unequivocal. The lenticular banded chalcopyrite-pyrite-sphalerite massive sulfide deposits of Corbin and Copper City on the east side of Hetta Inlet, western Prince of Wales Island, also occur in chloritic quartz-sericite schist of the Wales Group, along with locally inter-layered metakeratophyre. The settings strongly suggest a syngenetic origin in a submarine felsic volcanogenic environment, although most descriptions in the literature emphasize the relation of the deposits to faults and shear zones. At Lime Point, the eastern headland of Hetta Inlet, lenses and veinlets of barite are interlayered with and transect dolomite and marble of the Wales Group (Twenhofel and others, 1949; Eberlein, unpub. field data, 1971). Although field relations indicate that the deposit was probably formed by selective replacement of the original limestone, a premetamorphic origin is suggested by the fact that the barite shows local evidence of folding. Thus, the deposit is considered stratabound and of possible Precambrian age. EAST-CENTRAL ALASKA (YUKON-TANANA UPLAND) The Yukon-Tanana upland, lying between the Yukon and Tanana Rivers in east-central Alaska (fig. 1, pi. 1, J), is mainly a region of complexly deformed, largely polymetamorphic rocks intruded by batholiths of Mesozoic age and smaller plutons of Mesozoic and Tertiary age (Foster and others, 1973). The metamorphic rocks are of both igneous and sedimentary origin, ranging in grade from greenschist to upper-amphibolite facies. These rocks extend southeastward into Canada, where they constitute part of the Yukon crystalline terrane (Templeman-Kluit, 1976). To the northwest, in the Livengood-Crazy Mountains region discussed previously (fig. 1, pi. 1, F), they pass into or are thrust over relatively little-metamorphosed sedimentary rocks of early Paleozoic and (or) late Precambrian age (Foster and others, 1973). These same metamorphic rocks are separated from the unmetamorphosed Tindir Group to the north (fig. 1, pi. 1, B) by the Tintina fault zone. The various metamorphic rocks of the Yukon-Tanana upland and the adjacent Yukon Territory were assigned a number of stratigraphic names by United States and Canadian geologists in the late 1800’s and early 1900’s. Mertie (1937), in his comprehensive review of the geology of the Yukon-Tanana upland, named and described the Birch Creek Schist, a name he applied to what he considered to be the oldest rocks in the region. He, in effect, equated these rocks with the equally ill defined Yukon Group of Yukon Territory (Cairnes, 1914a). Mertie considered the Birch Creek Schist to be of early Precambrian age, primarily because of drastic lithologic differences between it and the Tindir Group, which he considered to be of Precambrian and Early Cambrian age. It is now evident that the two units are juxtaposed along the Tintina fault zone (Templeman-Kluit, 1976) so that their lithologic differences have no bearing on the age relations inferred by Mertie. Foster and others (1973) abandoned the name “Birch Creek Schist” because of its doubtful value as a stratigraphic unit. Similarly, Templeman-Kluit (1976) called attention to the stratigraphic limitations of the name “Yukon Group” and recommended either that it be discontinued or that its meaning be restricted.B14 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO Wasserburg and others (1963) reported whole-rock Rb-Sr ages on schist ranging from 1,170 to 664 m.y. and discordant younger ages on biotite and muscovite from schist of the Yukon-Tanana upland. These results were reported before the problem of redistribution of radiogenic 87Sr during metamorphism was fully appreciated. The currently preferred interpretation of these data supports a pervasive Mesozoic metamorphic event. The schist terrane may include a component of inherited, perhaps even Precambrian material, but this possibility has not yet been definitely proved. Several K-Ar mineral ages have been measured on metamorphic rocks of the Fairbanks district; the oldest age is 470±35 m.y. on hornblende (Foster and others, 1973). McCulloch and Wasserburg (1978) reported Sm-Nd (samarium-neodymium) and Rb-Sr model ages on a single sample of schist, called by them the Birch Creek Schist, from the Alaska Range. They correlated this sample on the basis of lithologic similarity with schist of the Yukon-Tanana upland. The Sm-Nd model age on the sample is 2,330±50 m.y., which McCulloch and Wasserburg interpreted as evidence that the sample was derived from an ancient Precambrian source, probably the Canadian Shield. The Rb-Sr model age of 714±8 m.y., which is similar to the total rock Rb-Sr ages of Wasserburg and others (1963), does not uniquely prove a Precambrian depositional or metamorphic age for the schist. More recently, U-Pb-Th analyses of zircons from an orthoaugen gneiss intrusive body in the Yukon-Tanana upland of east-central Alaska have provided substantiating evidence for the presence of Early Proterozoic material in that area. Uranium-lead data reported by Aleinikoff and others (1981) define a chord that intersects concordia at approximately 2,300 m.y. and 345 m.y. Two alternate interpretations are suggested: (1) the protolith was emplaced during the Proterozoic and subsequently metamorphosed in Paleozoic time, or, more likely, (2) the protolith was emplaced in the Paleozoic and entrained material of Proterozoic age. These authors also presented a 1,880±80 m.y. Sm-Nd model age on a whole-rock sample of the augen gneiss, which may be considered additional evidence for the presence of a Proterozoic component in the gneiss. Aleinikoff, Foster, and others (1984) subsequently confirmed Early Proterozoic sources for sedimentary rocks in the Yukon-Tanana upland through isotopic analyses of detrital zircons from quartzite sampled at several widely separated localities in the upland. A best-fit line for U-Pb data for detrital zircon from quartzite and orthoaugen gneiss (Aleinikoff and others, 1981) intercepts with concordia at 346±38 m.y. and 2,232±34 m.y. These data were interpreted as indicating incorporation of detritus from Early Proterozoic rocks in sedimentary rocks of latest Proterozoic and (or) earliest Paleozoic age. There is no definitive evidence for the age of the quartzites, though one locality is considered to be Cambrian(?) because of the occurrence of the trace fossil Oldhamia (Churkin and Brabb, 1965). The lower concordia intercept was interpreted as the age of partial melting of sedimentary rocks to produce a magma or, alternatively, assimilation of sedimentary rocks into a magma that formed the plutonic protolith of the augen gneiss (Aleinikoff, Foster, and others, 1984). Sillimanite gneiss in Big Delta quadrangle, some of the highest-grade metamorphic rock in the Yukon-Tanana upland, yielded similar U-Pb results. Selected U-Pb data for zircon from the gneiss he along a chord that intersects concordia at ages of 302± 156 m.y. and 2,383±398 m.y. (Aleinikoff, Dusel-Bacon, and Foster, 1984). The interpretation of the data was similar to the quartzite study above—deposition before mid-Paleozoic time of detrital material from an Early Proterozoic source followed by dynamothermal metamorphism that produced the sillimanite gneiss. For the examples just discussed, the U-Pb and Sm-Nd data show a similar feature that may characterize the Yukon-Tanana upland in general. This feature is the evidence for a component of detrital material derived from an Early Proterozoic source terrane in the meta-sedimentary rocks of the upland. Direct evidence of the depositional age of the protolith of the metasedimentary rocks is still lacking, though the protolith may likely be Late Proterozoic in age. Metavolcanic rocks in the Mount Hayes quadrangle provide the most persuasive evidence for Precambrian rocks in the Yukon-Tanana upland. Uranium-lead analyses of zircon from metarhyodacite yielded 207Pb/206Pb ages of about 2,000 m.y. (Aleinikoff and Nokleberg, 1984). The zircons appear to be igneous, not detrital like the zircons in quartzite discussed from Aleinikoff, Foster, and others (1984). The distribution of Early Proterozoic rocks in the Yukon-Tanana upland and the detailed Precambrian history of the upland remain to be determined. TERRANES THAT CONTAIN ROCKS OF POSSIBLE PRECAMBRIAN AGE At least one other region of low- to high-grade metamorphic rocks is considered Precambrian by some workers. However, little compelling stratigraphic evidence for a Precambrian age assignment has beenPRECAMBRIAN ROCKS OF ALASKA B15 found, and any isotopic evidence for a Precambrian history has been obliterated by tectonomagmatic events during the Mesozoic and Tertiary. The terrane in question is the orthogneiss and paragneiss belt of the Coast Range, southeastern Alaska (fig. 1, pi. 1, K). SOUTHEASTERN ALASKA (COAST RANGE ORTHOGNEISS AND PARAGNEISS BELT) The Coast Plutonic complex, which extends the entire length of the Coast Range for a distance of about 1,770 km, underlies much of the eastern part of southeastern Alaska. The complex is mainly composed of foliated and nonfoliated granitoid plutonic rocks but also contains large areas of metamorphic rocks, including quartz-mica-feldspar-hornblende orthogneiss and paragneiss, which have been assigned to the Central Gneiss Complex of the Prince Rupert region in British Columbia, Canada (Hutchinson, 1970). Although the age of at least some of these rocks and their metamorphism could conceivably be Precambrian, evidence in support of this assignment is at best only permissive and is mainly based on relations found in Canada. In the Tulsequah area of Canada, north and east of Juneau, Alaska, Middle Triassic and older strata were regionally metamorphosed during late Middle Triassic time. The presence of crystalline clasts in the lower part of the Stuhini Group indicates that parts of the Coast crystalline belt and its gneissic components were emergent at the beginning of Late Triassic volcanism in early Karnian time (Souther and Armstrong, 1966; Souther, 1971). Some 300 km to the southeast, relatively unmetamorphosed Permian(?) limestone overlies the Central Gneiss Complex, which suggests that the metamorphic rocks may be of pre-Permian age and that the metamorphism itself may also be pre-Permian (Hutchinson, 1970). The only known isotopic data on postulated Precambrian units in the Central Gneiss Complex are U-Pb ages on zircon from leucogneiss near the eastern margin of the belt in the Prince Rupert -Terrace area, British Columbia. Highly discordant isotopic ratios obtained on two zircon size-fractions lie on a discordia line that projects to an upper concordia intercept of 425 or 700 m.y., depending on the choice of time at which lead is assumed to have been removed from the zircon (that is, 0 or 75 m.y., the average of U-Pb ages obtained on neighboring rocks; Wanless and others, 1975). We question the suggestion of Wanless and others that the occurrence of a “pre-Devonian” and possibly “pre-Ordovician” metamorphic complex on Prince of Wales Island and vicinity in southeastern Alaska is evidence for an early metamorphic event that could be related to the age of gneiss of the Central Gneiss Complex (Hutchinson, 1970, p. 384). Wanless and others (1975) apparently referred to the Wales Group, for which we have seen that good evidence of a Precambrian age exists. However, the Wales Group is part of the Alexander terrane, which is probably allochthonous and may therefore be a totally unrelated assemblage. Precambrian rocks may possibly occur in orthogneiss and paragneiss of the Central Gneiss Complex, but limiting stratigraphic evidence indicates only that these rocks are pre-Late Triassic or perhaps pre-Permian in age. SUMMARY AND CONCLUSIONS Our evaluation of currently available information on the Precambrian of Alaska leads to the conclusion that no Archean (>2,500 m.y.) rocks are present, and it is likely that most of the Precambrian rocks are younger than Late Proterozoic. The oldest known rocks occur in the Kilbuck terrane of southwestern Alaska and the Yukon-Tanana upland of east-central Alaska. In the Kilbuck terrane the protolith of granitic orthogneisses evidently crystallized about 2,050 Ma (Early Proterozoic). Uranium-lead analyses also indicate that these rocks were subjected to amphibolite to granulite facies metamorphism about 1,770 Ma. Field and geophysical evidence indicates that this terrane is rootless and may have moved substantially, but the direction and amount of movement are unknown. Metavolcanic rocks in the Yukon-Tanana upland have zircon 207Pb/2o6Pb ages of about 2,000 m.y. Other rocks in the upland contain inherited zircon that was derived from a slightly older, Early Proterozoic source. If the correlation of the Tindir Group with parts of the Belt and Purcell Supergroups is correct, it may be inferred that a maximum older limit for the age of the Tindir is about 1,500 m.y. Metamorphic and (or) plutonic events in the range of 1,000 to 600 Ma have been isotopically documented for gneiss of the Seward Peninsula, the belt of schist that rims the northeastern apex of the Cretaceous Kuskokwim basin in central Alaska, and probably for the schist belt of the southern central and western Brooks Range. Evidence bearing upon the geologic evolution of Alaska within a framework of global tectonics is accumulating rapidly, but more field data are required before more than general conclusions can be drawn regarding the role of Precambrian lithologic assemblages. It now appears that the only truly autochthonous terranesB16 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO containing Precambrian rocks are in eastern Alaska (the Tindir Group) and perhaps in the eastern part of the Brooks Range (the Neruokpuk Quartzite and related strata); all the other terranes are considered to some extent allochthonous. The Tindir Group of east-central Alaska is interpreted as having been deposited near the shelf edge of the North American continental margin in relatively shallow water, perhaps in part under intertidal to sub-tidal conditions. A westward transition into deeper water, hemipelagic deposits and turbidites is evidenced by an argillite-quartzite sequence of comparable age in the Mount Schwatka-Crazy Mountains region 160-290 km to the west. The presence of Oldhamia, a Nereites infaunal type of trace fossil, is believed to signify a deep-water, bathyal-abyssal environment (Seilacher, 1964, 1967; Chamberlain, 1971a, b). It has not yet been possible to relate the Neruokpuk Quartzite and underlying units to other sequences of comparable age within the North American craton because the units cannot be traced eastward beyond the British and Barn Mountains of Northwestern Canada. The Precambrian-early Paleozoic sequence in the eastern Brooks Range to which the Neruokpuk Quartzite belongs is possibly part of an ancient borderland ter-rane similar to that of central and eastern Ellesmere Island in the Canadian Arctic Archipelago. The Precambrian terranes of the Seward Peninsula and the northeastern Kuskokwim Mountains, as well as the low-grade schist terranes of the southern Brooks Range and the Ruby geanticline, probably were imbricated and shuffled rather than moved great distances, but we lack data bearing on the distances of transport. The striking similarities within the geologic sequences on either side of the Bering Strait suggest that the Seward Peninsula and northeastern Chukotka, U.S.S.R., may have been connected since Precambrian time (Gnibidenko, 1969; Churkin, 1970). The movement and collision of terranes, which probably took place during the Mesozoic era, have markedly degraded the isotopic evidence on the age of Precambrian rocks. In southeastern Alaska, the Wales Group, of probable Precambrian age occurs in the Alexander terrane, which on the basis of geologic and paleomagnetic evidence did not originate at its present position. North-weird movement across latitude lines from a paleo-position of about lat 40° N., long 120° W., during the Mesozoic, and a post-Carboniferous 35° counterclockwise rotation of the terrane (Van der Voo and others, 1978) are favored by existing geologic and paleomagnetic evidence. REFERENCES CITED Aleinikoff, J.N., Dusel-Bacon, C., and Foster, H.L., 1984, Uranium-lead isotopic ages of zircon from sillimanite gneiss and implication for Paleozoic metamorphism, Big Delta quadrangle, east-central Alaska, in Coonrad, W.L., and Elliott, R.L., eds., The United States Geological Survey in Alaska—Accomplishments During 1981: U.S. Geological Survey Circular 868, p. 45-48. Aleinikoff, J.N., Dusel-Bacon, Cynthia, Foster, H.L., and Futa, Kiyoto, 1981, Proterozoic zircon from augengneiss, Yukon-Tan ana upland, east-central Alaska: Geology, v. 9, no. 11, p. 469-473. 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Gnibidenko, G.C., 1969, Metamorficheskie komplexsy v stukturakh severozapadnogo sektora kikhookeau-skogo poyasa (Metamorphic complexes in structures of the northwestern sector of the Pacific belt]: Akademiia Nauk SSSR Sibirskoe Otdelenie, Nauka, Moscow, 143 p. Griscom, Andrew, 1978, Aeromagnetic map and interpretation of the Goodnews and Hagemeister Island quadrangles region, southwestern Alaska: U.S. Geological Survey Open-File Report 78-9-C, 22 p. Hoare, J.M., Forbes, R.B., and Rimer, D.L., 1974, Precambrian rocks in southwest Alaska, in Carter, Claire, ed., United States Geological Survey Alaska program, 1974: U.S, Geological Survey Circular 700, p. 46. Hummel, C.L., 1961, Regionally metamorphosed metalliferous contact-metasomatic deposits near Nome, Alaska: U.S. Geological Survey Professional Paper 424-D, p. 198-199. Hutchinson, WW„ 1970, Metamorphic framework and plutonic styles in the Prince Rupert region of the Central Coast Mountains, British Columbia: Canadian Journal of Earth Sciences, v. 7, na 2, p. 376-405. Jones, D.L., Irwin, W.P., and Ovenshine, A.T., 1972, Southeastern Alaska—a displaced continental fragment?, in Geological Survey research 1972: U.S. Geological Survey Professional Paper 800-B, p. B211-B217. Jones, D.L., Silberling, N.J., Berg, H.C., and Plafker, G., 1981, Tfec-tonostratigraphic terrane map of Alaska: U.S. Geological Survey Open-File Report 81-792, 20 p. Kimball, A.L., 1969, Reconnaissance of Thtonduk River red beds: U.S. Bureau of Mines Open-File Report 1-69, lip. Kline, Gary, 1977, Earliest Cambrian (Tbmmotian) age of the upper Tindir Group, east-central Alaska: Geological Society of America Abstracts with Programs, v. 9, na 4, p. 448. Lanphere, M.A., 1965, Age of Ordovician and Devonian mafic rocks in northern Alaska, in Geological Survey research 1965: U.S. Geological Survey Professional Paper 525-A, p. A101-A102.B18 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO Leffingwell, E. de K., 1919, The Canning River region, northern Alaska: U.S. Geological Survey Professional Paper 109, 251 p. McCulloch, M.T., and Wasserburg, G.J., 1978, Sm-Nd and Rb-Sr chronology of continental crust formation: Science, v. 200, no. 4345, p. 1003-1011. Mertie, J.B., Jr., 1933, The Tatonduk-Nation district: U.S. Geological Survey Bulletin 836-E, p. 347-443. ______1937, The Yukon-Tanana region, Alaska: U.S. Geological Survey Bulletin 872, 276 p. Patton, W.W., Jr., and Dutro, J.T., Jr., 1979, Age of the metamor-phic complex in the northern Kuskokwim Mountains, west-central Alaska, in Johnson, K.M., and Williams, J.R., eds., The United States Geological Survey in Alaska—Accomplishments during 1978: U.S. Geological Survey Circular 804-B, p. B61-B63. Patton, W.W., Jr., Moll, E.J., Dutro, J.T., Jr., Silberman, M. L., and Chapman, R.M., 1980, Preliminary geologic map of the Medfra quadrangle, Alaska: U.S. Geological Survey Open-File Report 80-811A, 1 sheet, scale 1:250,000. Payne, M. W., and Allison, C.W., 1978, Precambrian-Cambrian sedimentary rocks from east-central Alaska [abs.]: American Association of Petroleum Geologists Bulletin, v. 62, no. 3, p. 553. Payne, T.G., and others, 1952, Geology of the Arctic slope of Alaska: U.S. Geological Survey Oil and Gas Investigations Map OM-126, scale 1:1,000,000, 3 sheets. Peeke, B.C., 1975, Geology and mineral deposits of the Niblack Anchorage area, Prince of Wales Island, Alaska: Fairbanks, Alaska, University of Alaska M.S. thesis, 50 p. Reiser, H.N., 1970, Northeastern Brooks Range—a surface expression of the Prudhoe Bay section, in Adkison, W.L., and Brosg6, M.M., eds., Proceedings of the geological seminar on the North Slope of Alaska: American Association of Petroleum Geologists, Pacific Section, p. K1-K13. Reiser, H.N., Brosge, W.P., Dutro, J.T., Jr., and Detterman, R.L., 1980, Geologic map of the Demarkation Point quadrangle, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-1133, scale 1:250,000. Reiser, H.N., Norris, D.K., Dutro, J.T., Jr., and Brosg6, W. P., 1978, Restriction and renaming of the Neruokpuk Formation, northeastern Alaska, in Sohl, N.F., and Wright, W.B., eds., Changes in stratigraphic nomenclature by the U.S. Geological Survey, 1977: U.S. Geological Survey Bulletin 1457-A p. A106-A107. Sainsbury, C.L., 1969, Geology and ore deposits of the central York Mountains, western Seward Peninsula, Alaska: U.S. Geological Survey Bulletin 1287, 101 p. Sainsbury, C.L., 1974, Geologic map of the Bendeleben quadrangle, Seward Peninsula, Alaska: Anchorage, Alaska, The Mapmakers, 31 p., 1 sheet, scale 1:250,000. Sainsbury, C.L., 1975, Geology, ore deposits, and mineral potential of the Seward Peninsula, Alaska: U.S. Bureau of Mines Open-File Report 23-75, 107 p. Sainsbury, C.L., Coleman, R.G., and Kachadoorian, Reuben, 1971, Blueschist and related greenschist facies rocks of the Seward Peninsula, Alaska, in Geological Survey research 1970: U.S. Geological Survey Professional Paper 700-B, p. B33-B42. Saleeby, Jason, and Eberlein, G.D., 1981, An ensimatic basement complex and its relation to the early Paleozoic volcanic-arc sequence of southern Prince of Wales Island, southeastern Alaska: Geological Society of America Abstracts with Programs, v. 13, no. 2, p. 104. 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Young, G.M., 1982, The late Proterozoic Tindir Group, east-central Alaska—Evolution of a continental margin: Geological Society of America Bulletin, v. 93, p. 759-783. * U.S. GOVERNMENT PRINTING OFFICE: 1988-573-047/66,102 REGION NO. 87 DAYS Geology and Geochronology of Precambrian Rocks in the Central Interior Region of the United States LIBRARY UNIVERSITY pi CA'.IFORNIA Geology and Geochronology of Precambrian Rocks in the Central Interior Region of the United States By RODGER E. DENISON, E. G. LIDIAK, M. E. BICKFORD, and EVA B. KISVARSANYI CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO Edited fry JACK E. HARRISON and ZELL E. PETERMAN GEOLOGICAL SURVEY PROFESSIONAL PAPER 1241-G Lithology, distribution, correlation, and isotope ages of Precambrian terrane for five subareas between the Appalachians and the Rocky Mountains UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1984DEPARTMENT OF THE INTERIOR WILLIAM P. CLARK, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data Main entry under title: Geology and geochronology of Precambrian rocks in the central interior region of the United States. (Geological Survey Professional Paper ; 1241-C) Includes bibliographic references. Supt. of Docs. No.: I 19.16:1241-C 1. Geology, Stratigraphic—Precambrian. 2. Stratigraphic correlation. 3. Geology—United States. I. Denison, Rodger E. II. Series. QE653.G476 1984 551.7'1'0973 83-600332 For sale by the Branch of Distribution U.S. Geological Survey 604 South Pickett Street Alexandria, VA 22304CONTENTS Page Abstract .................................................... Cl Introduction................................................. 1 Acknowledgments........................................... 2 Area I—North and South Dakota................................. 2 Archean time.............................................. 2 Gneiss................................................ 2 Greenstone............................................ 4 Granite and granodiorite ............................. 4 Proterozoic time (interval occurring 1,600-2,500 m.y. ago) 4 Metamorphic rocks..................................... 4 Granites.............................................. 4 Silicic volcanic rocks................................ 5 Proterozoic time (interval occurring 900-1,600 m.y. ago) . 5 Granite............................................... 5 Mafic and ultramafic rocks............................ 5 Sioux Quartzite....................................... 5 Area II—Nebraska, Iowa, Northern Missouri, Northern Kansas, and Eastern Colorado .................................... 5 Archean time.............................................. 5 Proterozoic time (interval occurring 1,600-2,500 m.y. ago) 5 Metavolcanic, metasedimentary, and foliated granitic rocks formed 1,650-1,800 m.y. ago....... 7 Proterozoic time (interval occurring 900-1,600 m.y. ago). 7 Anorogenic plutonic rocks formed about 1,450-1,480 m.y. ago.......................................... 7 Anorthosites in southwestern Nebraska....... 7 Mafic volcanic and hypabyssal rocks and related arkosic sedimentary rocks associated with the Central North American rift system............................... 8 Metamorphism.......................................... 8 Page Area III— Southern Missouri, Southern Kansas, Oklahoma, and Northwestern Arkansas.............................. C8 Proterozoic time (interval occurring 900-1,600 m.y. ago) . 8 Formation of rhyolitic to dacitic volcanic rocks and associated epizonal plutons 1,485 to 1,350 m.y. ago . 8 Mesozonal granite rocks along the Nemaha Ridge in Kansas and Oklahoma, and in the eastern Arbuckle Mountains, Oklahoma............................... 10 Phanerozoic time (Paleozoic Era, Cambrian Period) ... 11 Area IV—Texas and Eastern New Mexico.......................... 11 Proterozoic time (interval occurring 1,600-2,500 m.y. ago) . 11 Torrance metamorphic terrane and “older granitic gneisses”........................................ 11 Proterozoic time (interval occurring 900-1,600 m.y. ago) . 11 Rocks formed 1,200 to 1,400 m.y. ago.............. 11 Rocks formed 1,000 to 1,200 m.y. ago............... 13 Proterozoic time (interval occurring 600-900 m.y. ago) . 14 Metarhyolite of Devils River uplift ................ 14 The Van Horn Sandstone................................ 14 Phanerozoic time (Paleozoic Era, Cambrian Period) ... 14 Area V—Eastern Midcontinent ................................... 14 Proterozoic time (interval occurring 900-1,600 m.y. ago) . 14 Granite-rhyolite terrane.............................. 14 Sedimentary rocks..................................... 16 Basaltic rift zones.................................. 16 Rift-related sedimentary rocks........................ 16 Subsurface Grenville province ........................ 16 Metallogenic significance of the Precambrian basement ... 17 References cited............................................... 18 ILLUSTRATIONS Page Plate 1. Correlation chart for Precambrian rocks of the Central Interior region, United States. In pocket Figures 1-5. Sketch maps showing geology of basementrocks in: 1. North and South Dakota........................................................................................C3 2. Nebraska, Iowa, and parts of Missouri, Kansas, and Colorado................................................... 6 3. Oklahoma and parts of Kansas, Missouri, and Arkansas.......................................................... 9 4. Texas and eastern New Mexico..................................................................................42 5. Central Interior region east of the Mississippi River.........................................................15 illCORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO GEOLOGY AND GEOCHRONOLOGY OF PRECAMBRIAN ROCKS IN THE CENTRAL INTERIOR REGION OF THE UNITED STATES By Rodger E. Denison,1 E. G. Lidiak,2 M. E. Bickford,3 and Eva B. Kisvarsanyi4 ABSTRACT Rocks of the buried Precambrian crust in the Central Interior region range from more than 2,700 to less than 1,000 million years in age and from granite and granulitic gneiss to gabbro and basalt in rock type. The oldest rocks occur in the Dakotas and clearly are buried portions of the Canadian Shield; they are mostly older than 2,700 million years, and some may be as old as 3,600 million years. The central part of this region, including Nebraska, northern Missouri, and northern Kansas, is underlain by diverse igneous and metamorphic rocks whose ages are mostly 1,600 to 1,800 million years; scattered anorogenic granitic plutons whose ages are about 1,400-1,500 million years are also known in this terrane. The most distinctive feature of the Continental Interior is the great terrene of felsic igneous rocks that makes up the basement from Ohio and Wisconsin across southern Missouri and Kansas and into the Texas Panhandle and far western Texas. These rocks, which include abundant rhyolite and mesozonal and epizonal granitic bodies, range in age from 1,500 to 1,200 million years, and the general tendency is for ages to decrease from northeast to southwest; older rocks are not known anywhere within this terrene. Toward the east in Ohio, eastern Kentucky, and eastern Tennessee, and toward the south in central Texas, the basement terrene consists of medium-grade metamorphic rocks and associated granitic plutons that formed mainly 1,000-1.100 million years ago. A belt of basalt, interflow arkosic sandstone and siltstone, and related mafic intrusive rocks can be traced with the aid of geophysical data from the Lake Superior region southward into central Kansas. This feature, the Central North American rift system, is widely believed to be an abortive continental rift that formed about 1,100 million years ago. Geophysical data suggest that other areas in the eastern part of the interior are also underlain by rift basalts and related rocks. The Central Interior region was dominated by eugeosynclinal sedimentation and orogenic tectonics prior to about 1,600 million years ago. After that time the region apparently stabilized, and the sedimentation was characterized by the deposition of sheets of quartzose sandstone about 1,600 million years ago. Subsequent igneous activity, sedimentation, and tectonics have been dominantly anorogenic except along the margins of the stable interior. ‘One Energy Square, Dallas, TX 75206. ^Department of Geology and Planetary Sciences, University of Pittsburgh, Pittsburgh, PA 15620. ‘Department of Geology, University of Kansas, Lawrence, KS 66045. ‘Missouri Department of Natural Resources, Rolls, MO 65401. INTRODUCTION Our understanding of the Precambrian in the Central Interior region is based upon widely separated outcrop areas and samples from irregularly distributed but numerous wells drilled largely in search of oil and gas. Flawn (1956) showed that it was possible to make a map of the buried Precambrian terrane based on drillhole samples, and the larger scale study of Muehlberger and others (1967) led to publication of the basement rock map of the United States (Bayley and Muehlberger, 1968). These works remain the foundation of our present understanding. The geology and geochronology of the scattered surface exposures are now better known, but the geochronology of the subsurface has received little attention, and the basic reference work is the series of reports by Goldich and his coworkers (1966). The rocks in the Central Interior region are here divided into four general types: (1) deep-seated granitic and metamorphic rocks similar to those exposed in the shield areas; (2) anorogenic mesozonal and epizonal granite; (3) rhyolite and epizonal granite; and (4) basalt and gabbro of “rift” type. The first type is typical of rocks exposed in the Precambrian shields. These are diverse and strongly deformed rocks together with undeformed massive plutons that are characteristically older than about 1,600 m.y. (million years). The marked density and magnetic contrasts of these rocks allow extrapolation by geophysical methods in areas where drill control is lacking. About 18 percent of the Central Interior region is underlain by rocks of this type. The second type is characterized by anorogenic mesozonal to epizonal granitic plutons, formed 1,300 to 1,500 m.y. ago, and associated with relatively minor metasedimentary and metaigneous rocks. Silver and CiC2 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO others (1977) and Emslie (1978) have discussed the importance of these rocks, which form a discontinuous band from northeastern New Mexico to central Missouri and probably eastward to the Grenville Front in the Central Interior region. We estimate that 13 percent of the Continental Interior is underlain by these rocks. The third type is characterized by large tracts of rhyolite and associated epizonal granite. After these tracts were recognized in the subsurface (Muehlberger and others, 1966; Muehlberger and others, 1967), additional drilling showed that much of the area east of the Mississippi River and west of the southward extension of the Grenville province is underlain by similar rocks. Although the origin of the epizonal granite-rhyolite terrane remains unclear, certain conclusions may be drawn from our present understanding: (1) The rocks are preserved in structural depressions; surrounding rocks represent deeper crustal levels of emplacement. (2) The rhyolites are invariably associated with coeval hypersolvus granites that commonly display micrographic quartz-perthite intergrowths. (3) The rhyolites are not known to be associated with any significant volume of other volcanic or sedimentary rocks. (4) The rhyolites are essentially undeformed and only locally recrystallized. (5) The several tracts of rhyolite-granite are similar but not the same in age; the ages show no simple pattern of variation. The rhyolite-epizonal granite association underlies an estimated 52 percent of the Continental Interior, although much of this area is east of the Mississippi River where drill-hole control is poor. The abundance of these rocks is the major difference between the buried Precambrian of the Continental Interior and the exposed shield areas. Gravity and magnetic observations are of limited value in extrapolating these rocks into areas where drill control is poor. The extension of Keweenawan basaltic and gabbroic rocks from the Lake Superior region into the Central Interior region along the Central North American rift system has yielded the fourth major rock association. Basaltic rocks and related arkose can be traced as far as east-central Kansas on the basis of scattered well samples and gravity and magnetic data. That smaller areas east of the Mississippi River are also underlain by similar mafic igneous rocks can be inferred from geophysical measurements. These rocks are possibly time correlative with the Keweenawan associations. We estimate that about 10 percent of the interior is underlain by rocks of this type. Perhaps the single most significant feature of the Precambrian rocks of the Central Interior region is the great preponderance of granite and related volcanic rocks. These rocks, which generally have petrographic features indicating that they were emplaced at shallow to intermediate crustal levels, make up about two-thirds of the Continental Interior. Mafic rocks occur mostly along the Central North American rift system. Igneous rocks of intermediate composition are exceptionally rare. The greenstone belts that so characterize the older shield areas are confined to the buried extensions of the shield in Area I, and thus make up only about 1 percent of the Central Interior region. Sedimentary rocks are also notably rare. Shelf-type sedimentation evidently began about 1,700 m.y. ago, but the major deposits were of clean sandstone. Carbonate rocks are virtually unknown, and all the sedimentary rocks make up only an estimated 7 percent of this great region. ACKNOWLEDGMENTS The cooperation of the various State geological surveys, mining companies, and oil companies in providing access to samples has been essential to this report. James D. Hansink of Rocky Mountain Energy provided an opportunity for Denison and Lidiak to review the basement of the Continental Interior in 1976, and this work has been an important base for the results presented here. W. R. Muehlberger and W. R. Van Schmus reviewed the manuscript and gave many helpful comments. Financial support was provided by the U.S. Geological Survey and the U.S. Department of Energy. Lidiak is pleased to acknowledge support from NASA grant NSG-5270. AREA I—NORTH AND SOUTH DAKOTA Characterization of the buried basement complex in North and South Dakota is based on gravity and magnetic anomalies and on lithologic study of several hundred basement well samples (Muehlberger and others, 1967). Lithology and ages of rock units are shown on plate 1. These data indicate that the eastern Dakotas are mainly part of the subsurface extension of Archean rocks of the Canadian Shield. The western Dakotas are a continuation of mainly Proterozoic rocks of the Canadian Shield. ARCHEAN TIME GNEISS Gneiss of Archean age is apparently widespread in eastern North and South Dakota. The oldest rocks (fig. 1) are granitic and granulitic gneisses that crop out in the Minnesota River valley (Goldich and others, 1961;GEOLOGY AND GEOCHRONOLOGY OF PRECAMBRLAN ROCKS C3 104° 49“n— l' I * ■ EXPLANATION PROTEROZOIC SIOUX QUARTZITE GRANITE SILICIC VOLCANIC ROCKS METAMORPHIC BELTS ARCHEAN GRANITES AND GRANODIORITIES GREENSTONE BELTS GRANITIC AND GRANULITIC GNEISSES CONTACT OR INFERRED BASEMENT CONTACT FAULT OR INFERRED FAULT AREA OF EXPOSED ROCKS 200 KILOMETERS _l Figure 1.—Sketch geologic map of basement rocks in North and South Dakota. Unpattemed areas have no control. Goldich and others, 1970) and extend along a series of prominent gravity and magnetic anomalies from near Duluth, Minn., west-southwestward to east-central South Dakota (Lidiak, 1971; Morey and Sims, 1976). Detailed radiometric studies of the Minnesota River valley rocks (Goldich and others, 1970; Goldich and Hedge, 1974) have shown that they are at least 3,500 m.y. old and that one phase appears to be 3,700 m.y. old. Metamorphism and granite emplacement about 2,700 m.y. ago and a thermal event about 1,800 m.y. ago have partly obliterated the earlier geologic history of the gneisses. Granitic gneiss is shown on figure 1 as the predominant rock type in five other areas of northeastern South Dakota and in a large area of northeastern North Dakota. The belts in South Dakota are associated with west-southwest-trending gravity lows and magnetic highs; five wells to basement encountered granitic and gneissic rocks. In North Dakota the gravity anomalies are less distinct, but 22 wells to basement demonstrate the granitic and gneissic character of the terrane. Mineral assemblages in the gneisses indicate widespread metamorphism to the amphibolite facies. Sparse radiometric data indicate that the gneisses are of probable Archean age (Burwash and others, 1962). A further indication of age is the continuation of geophysical anomalies associated with Archean gneisses of the Canadian Shield into the eastern Dakotas. The higher grade of metamorphism in the gneisses compared to Archean greenstones suggests that at least some of the gneisses predate the 2,700-m.y.-old Algoman orogeny. Rocks of Archean age are also present in the Black Hills (Zartman and Stem. 1967; Ratt£ and Zartman,C4 COKRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO 1970; Kleinkopf and Redden, 1975). Two granulites from near the center of the Williston Basin may also be Archean in age because they occur along a prominent linear gravity high, which can be traced northward to the Nelson River high of Innes (1960). The Nelson River high has been correlated with Archean granulitic gneisses that were involved in both the Keno-ran and Hudsonian orogenies (Patterson, 1963; Bell, 1966; Gibb, 1968; Komik, 1969). GREENSTONE Belts of greenstone and related rocks are extensive in the basement of the eastern Dakotas. Seven belts are shown on figure 1; they are characterized by gravity highs and less pronounced but linear magnetic highs. Study of 18 samples indicates that amphibole schists and gneisses are dominant; serpentinite is present in one basement well. Modal compositions suggest mafic and ultramafic igneous antecedents. A staurolite schist in northeastern South Dakota indicates that metasedimentary rocks are associated with the greenstones. The rocks are characterized by metamorphism to the greenschist or lower amphibolite facies. No age determinations have been published for any of the supracrustal rocks in the Dakotas. They are regarded as being of Archean age because the associated gravity and magnetic anomalies continue into northern Minnesota, where they coincide with greenstone and iron-formation of the Keewatin Group, dated at about 2,700 m.y. (Hart and Davis, 1969). GRANITE AND GRANODIORITE Coarse-grained, two-feldspar granite (21 wells) is interpreted as the principal rock type in large areas of the eastern Dakotas (fig. 1). Granodiorite and trondhje-mite (seven wells) and amphibolite-grade gneiss (seven wells) are also present but are apparently subordinate to granite. These rocks lie in the subsurface extension of the Superior province of Canada and are characterized by gravity and magnetic lows. The granite and related rocks in the eastern part of the area of figure 1 have the same general trend as the greenstones and are interpreted as being part of an Archean greenstone-granite terrane that was involved in the Algoman orogeny. K-Ar (potassium-argon) and Rb-Sr (rubidium-strontium) ages on minerals from both granite and gneiss reflect mainly the widespread igneous activity and metamorphism that occurred during this orogeny (Burwash and others, 1962; Peterman and Hedge, 1964; Goldich and others, 1966). PROTEROZOIC TIME (INTERVAL OCCURRING 1,600-2,500 M.Y. AGO) MET AMORPHIC ROCKS The east-northeast-trending anomalies of the Superior province are terminated in the central Dakotas by northwest-trending anomalies of the Churchill province (Muehlberger and others, 1967; Lidiak, 1971). The alinement of gravity and magnetic anomalies implies a northwest structural trend of the basement rocks. Three metamorphic belts are inferred. The belt in the central Dakotas is marked by magnetic highs and both highs and lows in gravity. The few wells to basement suggest that the area is underlain by mafic and silicic schist and gneiss. The belt in south-central South Dakota continues into Nebraska and coincides with magnetic and gravity lows. The rocks are dominantly silicic schists (Lidiak, 1972). The third metamorphic belt trends through the Black Hills and continues into Montana. This belt also coincides with a magnetic low. The rocks are mainly medium grade metasedimentary rocks, locally intruded by granite. Gough and Camfield (1972) suggested that graphitic schist may be abundant. The presence of Archean granitoid rocks in the Black Hills suggests that these metamorphic belts probably developed on a sialic crust within a craton rather than along a continental margin. The time of deposition is inferred to have been about 1,900-2,100 m.y. ago. Goldich and others (1966) concluded that the rocks in the western Dakotas were involved in orogeny l, 700-1,900 m.y. ago. Most of the ages are of minerals from metamorphic rocks. The basement may include older rocks whose ages were reset by younger metamorphism as well as rocks formed at that time. The metamorphic belts possibly date from earlier Proterozoic time, but this dating can be demonstrated only in the Black Hills, where Archean granite gneiss (Zartman and Stem, 1967) is unconformably overlain by a thick metasedimentary succession that was folded, metamorphosed, and intruded by granite 1,700-1,900 m. y. ago during the Black Hills orogeny (Goldich and others, 1966). GRANITES Granite (11 wells) occurs at scattered localities in the western Dakotas. Apparent radiometric ages on minerals and whole rock samples are in the range 1,660-1,810 m.y. (Goldich and others, 1966). The granites probably formed during the major period of orogeny in the western Dakotas.GEOLOGY AND GEOCHRONOLOGY OF PRECAMBRIAN ROCKS C5 SILICIC VOLCANIC ROCKS Silicic volcanic rocks are present in eastern South Dakota. These rocks are essentially unmetamorphosed and apparently overlie the older plutonic complex. Three determinations yield ages of 1,680-1,700 m.y. (Goldich and others, 1966). Petrographically similar rhyolitic volcanic rocks occur in adjacent northwestern Iowa (included in undifferentiated felsic rocks on fig. 2). PROTEROZOIC TIME (INTERVAL OCCURRING 900-1,600 M.Y. AGO) GRANITE Four Rb-Sr whole rock or feldspar ages on granite from southern South Dakota and adjacent Nebraska are in the range 1,480-1,510 m.y. (Goldich and others, 1966). These rocks are probably related to the anorogenic granites discussed in the following section on Nebraska, Iowa, and surrounding area. MAFIC AND ULTRAMAFIC ROCKS Diabase, diorite, gabbro, and pyroxenite occur in scattered wells in North and South Dakota. Except for deuteric alteration in the diabases, the rocks are unaltered and thus intrusive into the plutonic complex. Their age is unknown, but they probably reflect several intrusive episodes. They are tentatively regarded as postdating regional metamorphism and thus being less than 1,750 m.y. old. SIOUX QUARTZITE The Sioux Quartzite is a uniform, mildly folded, subhorizontal formation that is nonconformable on the underlying plutonic complex. It is extensively developed in the surface and subsurface of southeastern South Dakota and extends into adjacent Minnesota, Iowa, and Nebraska. The formation is composed mainly of silicified quartz sandstone that is conglomeratic near the base, and minor thin beds of red shale and argillite. The presence in the essentially undeformed quartzite of diaspore plus quartz and pyrophyllite plus quartz (Berg, 1938) suggests hydrothermal or burial metamorphism under static conditions. Pebbles of iron-formation in the Sioux Quartzite indicate that the unit may be no older than about 1,900 m.y. (Goldich, 1973), and a Rb-Sr age determination on a rhyolite from Sioux County, Iowa, suggests that it may be at least 1,520 m.y. old (Lidiak, 1971). A nearby well is reported to have penetrated alternating layers of rhyolite and quartz sandstone (Beyer, 1893). Similar silicic volcanic rocks in South Dakota yield ages of 1,680-1,700 m.y.; the Baraboo Quartzite of Wisconsin, often considered to be correlative with the Sioux Quartzite, rests upon rhyolite whose U-Pb zircon age is 1,760 ± 10 m.y. (Van Schmus, 1978). AREA II—NEBRASKA, IOWA, NORTHERN MISSOURI, NORTHERN KANSAS, AND EASTERN COLORADO Basement rocks in Area II include a variety of igneous, metamorphic, and sedimentary rocks whose ages range from at least 1,800 m.y. to about 1,000 m.y. The distribution and petrography of these rocks have been determined primarily from study of cuttings and cores from deep drilling, but geophysical data have also been used to extend terranes mapped on the basis of well samples. Large numbers of well samples are available in Nebraska and Kansas because of oil and gas exploration; these have been studied by Lidiak (1972) in Nebraska and by Scott (1966) and Bickford and others (1979) in Kansas. The Missouri basement is reasonably well known because of drilling for minerals and has been studied by Kisvarsanyi (1974, 1975). Relatively little is known about the Precambrian rocks of Iowa and eastern Colorado, because a smaller number of wells have penetrated the basement there. ARCHEAN TIME No rocks of Archean age are known in Area II, although we infer such rocks to underlie parts of northern Iowa because of the proximity of the ancient rocks that are exposed in the Minnesota River valley (Goldich and others, 1970; Goldich and Hedge, 1974). PROTEROZOIC TIME (INTERVAL OCCURRING 1,600-2,500 M.Y. AGO) Only one radiometric age greater than 1,800 m.y. has been reported for any rocks from Area II, and most are 1,700 m.y. or less (Goldich and others, 1966). We have, however, indicated on the chronometric chart (pi. 1) that both sedimentary and volcanic rocks may have formed as early as 2,000 m.y. ago. This speculation is based upon the presence of silicic metavolcanic rocks and various metasedimentary rocks (schists, quartzites) in the basement of both Kansas and Nebraska. These are associated spatially with gneissoid granitic rocks that have yielded ages of about 1,700 m.y. If metamorphism occurred later than 1,700 m.y. ago, it94 O 05 FELSIC ROCKS UNDIFFERENTIATED GRANITE RHYOLITE GABBRO BASALT METAMORPHIC ROCKS— Mostly metasedimentary; grade variable, mostly low to medium SIOUX QUARTZITE KEWEENAWAN AGE ROCKS Sedimentary rocks ------ CONTACT OR INFERRED BASEMENT CONTACT ■?---- FAULT OR INFERRED FAULT— Queried where extension uncertain Figure 2.—Sketch geologic map of basement rocks in Nebraska, Iowa, and parts of Missouri, Kansas, and Colorado. Unpatterned areas have no well control. CNARS, Central North American rift system. CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICOGEOLOGY AND GEOCHRONOLOGY OF PRECAMBRIAN ROCKS C7 did not result in lowering of Rb-Sr whole-rock or feldspar ages of at least some of the gneissoid rocks, and we consider it more likely that metamorphism either preceded or accompanied the synkinematic emplacement of the granitic rocks about 1,700 m.y. ago. This event would thus be correlative with the Boulder Creek event, which has been well documented in the northern Front Range of Colorado (Peterman and Hedge, 1968; Stem and others, 1971). MET A VOLCANIC, METASEDIMENT ARY, AND FOLIATED GRANITIC ROCKS FORMED 1,650-1,800 M.Y AGO Much of Area II is underlain by the gneissoid granitic rocks (fig. 2) mentioned in the preceding section. Some of these rocks have been determined to be 1,600 to l, 800 m.y. old by either Rb-Sr or U-Pb (uranium-lead; zircon) methods (Goldich and others, 1966; Bickford and others, 1981). These rocks are commonly granitic to granodioritic in composition and are characterized by slightly to moderately developed foliation caused by pervasive shearing and cataclasis. Metasedimentary and metavolcanic rocks are distributed throughout the area either in fairly well defined belts or in small patches only a few kilometers in diameter. Metavolcanic rocks that were evidently originally rhyolitic to dacitic are known in western Kansas, north-central Missouri, and in northern and southwestern Nebraska, but they are not as widely distributed as metasedimentary rocks. Metamorphic rocks include fairly abundant muscovite and biotite schist, minor amphibolite, and abundant quartzite; the quartzite forms prominent basement-surface highs in southwestern Nebraska (Lidiak, 1972), to the south on the Central Kansas uplift (Walters, 1946), and on the Central Missouri high (Kisvarsanyi, 1974). The Sioux Quartzite (age and extent discussed previously) extends as far south as extreme northern Nebraska in the subsurface. It is known to rest nonconformably upon the underlying igneous-metamorphic complex, but it is not known whether the patches of metavolcanic and metasedimentary rocks that are known elsewhere throughout Area II lie upon the 1,600-1,800-m.y.-old granitic rocks or are older pendants and inclusions within them. It seems clear that a widespread period of pervasive shearing and cataclasis occurred between 1,800 m.y. ago (the age of the oldest rocks dated) and about 1,480 m. y. ago, the oldest age determined from a widespread suite of nonfoliated anorogenic plutons that occur within the older terrane (Goldich and others, 1966; Har-rower, 1977; Bickford and others, 1981). Because the age of the metavolcanic and metasedimentary rocks relative to that of the foliated granitic rocks is not known, it cannot be determined whether a single period of pervasive regional metamorphism, reaching amphibolite facies in parts of the area, affected all these rocks, or whether the metasedimentary and metavolcanic rocks were formed by a metamorphic episode earlier than the period of shearing and cataclasis that affected the granitic rocks. PROTEROZOIC TIME (INTERVAL OCCURRING 900-1,600 M.Y. AGO) ANOROGENIC PLUTONIC ROCKS FORMED ABOUT 1,450-1,480 M.Y. AGO Granitic to tonalitic plutons having ages in the range 1,450-1,480 m.y. are known in Nebraska, northern Kansas, and northern Missouri. These rocks are generally not foliated and thus presumably were intruded into the older terrane after the pervasive shearing event. Because these rocks are not accompanied by associated volcanic rocks in this region, and because they are not deformed, they are assumed to have been emplaced anorogenically. These rocks are evidently a part of the great belt of anorogenic plutons of this age which are known from Labrador to California (Silver and others, 1977; Emslie, 1978). Where they have been well studied at the surface—for example, the Wolf River batholith, Wisconsin (Van Schmus and others, 1975; Anderson and Cullers, 1978) and the St. Francois Mountains batholith, Missouri (Bickford and Mose, 1975)—they are seen to be characterized by rapakivi texture and silicic-alkalic chemistry. In Nebraska the age of these plutons is known mainly from Rb-Sr measurements of total rock samples, but zircons separated from a core from southwestern Nebraska yielded a U-Pb age of 1,445±15 m.y., reported by Harrower (1977). Harrower also determined a similar age for zircons from a core in north-central Kansas. As will be seen in the discussion of Area III, plutons of the anorogenic type occur to the south in southern Kansas, southern Missouri, and Oklahoma in association with extensive rhyolitic volcanic rocks. There, however, the age of the plutons is about 1,380 m.y., except in southeastern Missouri, where plutons and volcanic rocks are about 1,480 m.y. old (Bickford and Mose, 1975). ANORTHOSITES IN SOUTHWESTERN NEBRASKA A complex of anorthositic rocks occupies an area of about 400 km2 in southwestern Nebraska (gabbro on fig. 2). The rocks, ranging in composition from anorthosite to anorthositic gabbro, have been subjected to cataclasis and to incipient greenschist facies metamorphism. No direct radiometric age measurement is available for these rocks, but Lidiak (1972) has inferredC8 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO that they are younger than schists in the area because they lack metamorphism of amphibolite facies that may have occurred about 1,800 m.y. ago, and that they are older than a period of cataclasis and greenschist facies metamorphism that is inferred to have occurred about 1,200 m.y. ago. MAFIC VOLCANIC AND HYPABYSSAL ROCKS AND RELATED ARKOSIC SEDIMENTARY ROCKS ASSOCIATED WITH THE CENTRAL NORTH AMERICAN RIFT SYSTEM A major belt of mafic volcanic and hypabyssal igneous rocks coincides with pronounced positive magnetic and gravity anomalies in Kansas, Nebraska, and Iowa (King and Zietz, 1971; Woollard, 1943; Lyons, 1950; Thiel, 1956). Flanking basins containing immature sedimentary rocks are associated with negative anomalies on both sides of the belt of mafic rocks. This feature, the Central North American rift system (Ocola and Meyer, 1973; Chase and Gilmer, 1973), can be traced northwards into Minnesota where both the mafic volcanic rocks and the flanking arkosic sedimentary rocks appear at the surface in the Lake Superior region. There the age of the mafic volcanism has been determined to be about 1,100 m.y. (Goldich and others, 1961; Silver and Green, 1963, 1972; Goldich and others, 1966; Chaudhuri and Faure, 1967; Van Schmus, 1971); a well sample from Nebraska also yielded a K-Ar whole-rock age of 990 m.y. (Goldich and others, 1966). The continuity of these rocks in a belt more than 1,500 km long and about 65 km wide implies that they formed about the same time during a late Proterozoic rifting event. In Nebraska the mafic igneous rocks include both hypabyssal types and extrusive basalts, and similar types have been observed in Kansas. The relatively small number of basement wells in Iowa precludes much detailed knowledge of these rocks there, but both mafic igneous rocks and arkosic sedimentary rocks have been encountered along the trend of the geophysical anomalies. Sedimentary rocks in both Kansas and Nebraska are mostly arkosic, but subarkose, argillaceous wackes, and red siltstones are also present. In Kansas, Scott (1966) called these rocks the Rice Formation. METAMORPHISM Lidiak (1972) noted the widespread occurrence of metamorphism in greenschist facies in rocks in the Nebraska basement, and inferred that this event occurred about 1,170 m.y. ago on the basis of numerous K-Ar and Rb-Sr ages of micas that fall within about ±100 m.y. of this age. That these mica ages record a metamorphic event is indicated by the fact that many of them are from rocks for which whole-rock or feldspar ages are significantly greater. Lidiak (1972) also observed low-grade metamorphic mineral assemblages in the basaltic rocks of the Central North American rift system. These assemblages, including pumpellyite, laumontite, epidote, and chlorite, are indicative of metamorphism under conditions commonly attributed to simple burial metamorphism. AREA III—SOUTHERN MISSOURI, SOUTHERN KANSAS, OKLAHOMA, AND NORTHWESTERN ARKANSAS Area III (fig. 3) is underlain almost entirely by an extensive terrane of silicic volcanic rocks and associated epizonal and mesozonal granitic plutons. These rocks were formed in the interval 1,300-1,500 m.y. ago; rocks older than about 1,500 m.y. are not known anywhere in the area. Moreover, Precambrian mafic and intermediate igneous rocks are quite rare and, except for small areas in Missouri and Kansas, sedimentary or metasedimentary rocks are not known. The igneous rocks of the Wichita Mountains in south-central Oklahoma (fig. 3) include basalt, rhyolite, epizonal granite plutons, and a large body of gabbroic rocks (Ham and others, 1964). Most of these rocks yield K-Ar and Rb-Sr ages in the range 510-530 m.y. (Tilton and others, 1962; Muehlberger and others, 1966; Burke and others, 1969) and thus constitute an anomalously young part of the crystalline crust in this area. PROTEROZOIC TIME (INTERVAL OCCURRING 900-1,600 M.Y. AGO) FORMATION OF RHYOLITIC TO DACITIC VOLCANIC ROCKS AND ASSOCIATED EPIZONAL PLUTONS 1,485 TO 1,350 M.Y. AGO One of the major events in the formation of the central part of the continent occurred during the interval 1,485-1,350 m.y. ago, when an extensive terrane of silicic volcanic and plutonic rocks formed. This terrane extends across the midcontinent region from western Ohio at least into the Oklahoma Panhandle; similar rocks occur in the Texas Panhandle and New Mexico, but these appear to be somewhat younger. This terrane is notable for its scarcity of intermediate to mafic rocks. In the St. Francois Mountains of southeast Missouri, about 900 km2 of an extensive terrane of alkali rhyolitic ash-flow tuff, trachyte, trachyandesite, and a number of granitic plutons are exposed (Tolman and Robertson, 1969; Anderson, 1970; Berry and Bickford, 1972; Kis-varsanyi, 1972). This igneous terrane underlies an area of at least 40,000 km2 in southeast Missouri (Kisvar-sanyi, 1974). The exposed part of this terrane includes part of the volcanic roof that is several kilometers thickminor mesozonal granite RHYOLITE MESOZONAL GRANITE mu EPIZONAL GRANITE— May locally include EXPLANATION GABBRO METAMORPHIC ROCKS— Mostly metasedimentary, grade variable, mostly low to medium CONTACT OR INFERRED BASEMENT CONTACT FAULT OR INFERRED FAULT AREA OF EXPOSED BASEMENT ROCKS Figure 3.—Sketch geologic map of basement rocks in Oklahoma and parts of Kansas, Missouri, and Arkansas. O 50 GEOLOGY AND GEOCHRONOLOGY OF PRECAMBRIAN ROCKSCIO CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO and a complex of subvolcanic and epizonal plutons. Plutonic rocks are exposed principally in the northeastern part of the area, whereas to the southwest, volcanic rocks are exposed, suggesting that some plutons are tilted to the southwest (Bickford and others, 1977). Contacts between the plutons and the volcanic roof, as well as chemical, mineralogical, and textural variations within the plutons, indicate that some of them are sheetlike. Some plutons, however, are cylindrical and like a cone-sheet in form, as suggested by subsurface and geophysical data (E. B. Kisvarsanyi, unpub. data, 1981). This terrane has clearly not been subjected to penetrative deformation. The exposed rocks of the St. Francois terrane have been dated by Rb-Sr whole-rock and by U-Pb (zircon) methods by Bickford and Mose (1975). The Rb-Sr system has evidently been disturbed, for the ages reported range from about 1,380 m.y. to as low as 1,200 m.y., and the age data and field relations sometimes contradict each other. U-Pb measurements on zircons, however, yield consistent ages of about 1,485 m.y. for four major plutons and for one of the major volcanic units; one small sill or stock, the Munger Granite Porphyry, yields a U-Pb zircon age of about 1,385 m.y., which may indicate a younger igneous event in this area. A granite core from a buried part of the St. Francois terrane also yields a U-Pb age of 1,485 m.y. (Bickford and others, 1981). Rocks entirely similar to those of the St. Francois terrane extend in the subsurface across southern Missouri and northern Arkansas into southern Kansas, Oklahoma, and the Texas Panhandle. Studies of these rocks, mainly from cuttings and cores returned from deep drilling, include those of Denison (1966) in northeastern Oklahoma, southeastern Kansas, and southwestern Missouri; Muehlberger and others (1967) over a large region including parts of Texas and New Mexico as well as the region considered here; Kisvarsanyi (1974) in Missouri; and Bickford and others (1979) in Kansas. The ages of these rocks have been studied in northeastern Oklahoma and southwestern Missouri by Muehlberger and others (1966), Denison and others (1969), and Bickford and Lewis (1979), and in the Kansas basement by Bickford and others (1981). Zircons from the Spavinaw Granite, which is exposed in northeastern Oklahoma, and from a granite in the subsurface in southeastern Kansas (Bickford and others, 1981) yield U-Pb age determinations indicating that they are 1,375 m.y. old. These ages are in reasonable agreement with the Rb-Sr isochron age of about 1,300 m.y. determined by Denison and others (1969) from subsurface samples in northeastern Oklahoma. MESOZONAL GRANITE ROCKS ALONG THE NEMAHA RIDGE IN KANSAS AND OKLAHOMA, AND IN THE EASTERN ARBUCKLE MOUNTAINS, OKLAHOMA A terrane of mesozonal granitic rocks is known in southern Kansas along the northeast-southwest trend of the Nemaha Ridge (Bickford and others, 1979), and it extends southwesterly into Oklahoma at least as far as Oklahoma City (Denison, 1966). The age of these rocks is not well known, but their mesozonal character and their occurrence along the Nemaha Ridge suggest that they are somewhat deeper portions of the continental crust that were brought up by fault movements on the Nemaha structure and exposed by erosion prior to Late Cambrian sedimentation. The only other place in Area III where more deep seated igneous rocks are known is in the eastern Ar-buckle Mountains of southeastern Oklahoma (Ham and others, 1964). There, four extensive plutons are exposed in the core of the Tishomingo-Belton anticline (Denison, 1973). Rocks exposed include an unnamed granodiorite, the Troy Granite, the Tishomingo Granite, and the gneiss of Blue River, an informal name. The Troy Granite intrudes the unnamed granodiorite and is intruded by the Tishomingo Granite; the Blue River gneiss is intruded by the Tishomingo Granite, but its age relationships with the Troy Granite and the unnamed granodiorite are not known because the Tishomingo Granite separates it from those rock bodies. All these rocks are medium to coarse grained and have petrographic features suggesting mesozonal emplacement. Bickford and Lewis (1979) have determined the U-Pb ages of zircons from the Tishomingo Granite (1,374 ±15 m.y.), the Troy Granite (1,399 ±95 m.y.), and the gneiss of Blue River (1,396 ±40 m.y.). The rocks are therefore the mesozonal age equivalents of the epizonal granophyres and rhyolites in southern Kansas and northeastern Oklahoma. Several types of dikes intrude the granitic rocks of the eastern Arbuckle Mountains. The most common dikes are diabasic, whereas dikes of microgranite porphyry, granite, and rhyolite porphyry are less common. On the basis of unpublished age determinations, it appears that some of the diabase dikes and the granite and microgranite porphyry dikes are approximately the same age as their granitic host rocks, about 1,350-1,400 m.y.; the rhyolite porphyry dikes and the other diabase dikes are of Cambrian age. All the dikes have a strongly developed preferred strike direction near N. 60° W., which is parallel to the major Pennsylvanian deformational axes.GEOLOGY AND GEOCHRONOLOGY OF PRECAMBRIAN ROCKS Cll PHANEROZOIC TIME (PALEOZOIC ERA, CAMBRIAN PERIOD) The igneous rocks of the Wichita Mountains consist of a bimodal suite of silicic and gabbroic rocks. The silicic rocks, consisting of epizonal granite plutons and rhyolite, and some of the gabbroic rocks have yielded ages in the range 510 to 530 m.y. (Tilton and others, 1962; Muehlberger and others, 1966). Paleomagnetic data (Roggenthen and others, 1976) and geologic considerations (Powell and Phelps, 1977) have suggested that the oldest rock unit, the layered series of the Raggedy Mountain Gabbro, is of Precambrian age. The K-Ar ages of these rocks, however, suggest a Cambrian age (Burke and others, 1969), and the age must be considered uncertain. The Raggedy Mountain Gabbro is the only large layered gabbroic mass exposed in the Continental Interior. The geologic relations and structural framework for southern Oklahoma that were outlined by Ham and others (1964) appear to be essentially correct. However, the rhyolite terrane in extreme southwestern Oklahoma, which was considered to be of Cambrian age by Ham and others, is now thought to be an outlier of the Panhandle rhyolites of Precambrian age (see discussion of Area IV) on the basis of unpublished age determinations. The Tillman Metasedimentary Group is most probably Precambrian, as suggested by Muehlberger and others (1967), although parts of this unit may indeed be of Cambrian age as argued by Ham and others. AREA IV—TEXAS AND EASTERN NEW MEXICO This area, shown on figure 4, was the subject of the first successful study of the buried basement rocks of a large region. Flawn (1956) was able to show that consistently mappable units could be recognized over large areas by the petrographic study of well samples. Virtually no isotopic ages were available at that time, and the sequence of events and relative ages of the units were later modified when ages became available. The dating of both surface and subsurface samples by Was-serburg and others (1962) and Muehlberger and others (1966) made possible the determination of the sequence of events. Later, largely unpublished geochronological work on well samples has refined this timing of igneous and metamorphic activity, but no major modifications of the published data are justified at this time. PROTEROZOIC TIME (INTERVAL OCCURRING 1,600-2,500 M.Y. AGO) TORRANCE METAMORPHIC TERRANE AND “OLDER GRANITIC GNEISSES” The oldest isotopic ages from the area shown on figure 4 have been reported from eastern New Mexico where Muehlberger and others (1966) determined Rb-Sr ages in excess of 1,600 m.y. for a whole-rock sample and for a feldspar from granitic gneisses. Micas from both of the rock samples studied yielded metamorphic ages of about 1,350 m.y. The granitic gneisses are associated with metasedimentary and metavolcanic rocks that are probably equivalent to the sequence found in outcrop along the Los Pinos-Manzano trend (Stark and Dapples, 1946; Stark, 1956). Long (1972, p. 3425) reported ages of “about 1,600 m.y. or older” for metavolcanic rocks northward along this trend. The relationship between gneisses and the supracrustal rocks cannot be determined on the basis of the available information, but the mature character of the metasedimentary rocks suggests that they were originally shelf deposits upon sialic crust. These two units (the Torrance metamorphic terrane, and the “older granitic gneisses” of Muehlberger and others, 1967) have been extended by us with considerable trepidation in the subsurface on the basis of petrography. PROTEROZOIC TIME (INTERVAL OCCURRING 900-1,600 M.Y. AGO) ROCKS FORMED 1,200 TO 1,400 M.Y. AGO Granitic gneisses, found through much of southeastern New Mexico, were grouped into the Chaves granitic terrane by Muehlberger and others (1967). It now seems desirable to extend this unit into the Texas Panhandle on the basis of scattered unpublished age determinations (by Mobil Research and Development Corp.) and petrographic similarities of rocks encountered. These rocks have yielded ages in the 1,400 m.y. range, the oldest ages known in Texas. Some of the ages measured reflect periods of metamorphism, but others are probably close to the time of original intrusion. Differentiation of rock units within the area is not justified on the basis of the available data. The Sierra Grande terrane of northeastern New Mexico and the Texas Panhandle (Muehlberger and others, 1967) is the oldest of the large areas underlain by anorogenic granite. These rocks are distinguished from older units by the absence of metamorphic features and by their more silicic chemical composition.C12 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO CHAVES' GRANITE AND GRANITIC GNEISS TORRANCE' METAMORPHIC ROCKS GRANITIC GNEISS INFERRED BASEMENT CONTACT—Dashed where questionable AREA OF EXPOSED ROCKS 1060-1200 m.y. RAGGEDY MOUNTAIN GABBRO DEVILS RIVER UPLIFT METARHYOLITE FRANKLIN MOUNTAINS IGNEOUS ROCKS DEBACA-SWISHER' METASEDIMENTARY AND BASALTIC ROCKS LLANO PROVINCE ROCKS 1300 m.y. RED RIVER UPLIFT-TILLMAN METAMORPHIC GROUP PANHANDLE’ RHYOLITES AMARILLO' AND RELATED GRANITES CARRIZO MOUNTAIN GROUP METAMORPHIC ROCKS SIERRA GRANDE' AND RELATED ANOROGENIC GRANITES 1400 m.y. (some possibly older) 1600 m.y. 1600 m.y. VT77777/7A 'Names of terranes Figure 4.—Sketch geologic map of basement rocks in Texas and eastern New Mexico. Unpatterned areas have no control. Well control essentially that of Bayley and Muehlberger (1968).GEOLOGY AND GEOCHRONOLOGY OF PRECAMBRIAN ROCKS C13 Rb-Sr data from several published and unpublished determinations of whole rocks, feldspars, and micas form an isochron indicating an age of about 1,300 m.y. Apparent ages on some rhyolites that are petrographically and geographically inseparable from the younger Panhandle volcanic terrane indicate that volcanism also occurred during this period. The metamorphic rocks called the Red River mobile belt by Flawn (1956) and the Tillman Metasedimentary Group by Ham and others (1964) remain an unresolved problem. Rocks grouped under these names have yielded metamorphic ages from 1,380 m.y. to about l, 000 m.y. (Wasserburg and others, 1962; and R. E. Denison, unpub. data, 1981). The rocks around the Muenster arch are now believed to be related in time of metamorphism to those in the Llano province. Rocks to the west, along the Red River uplift, are older, but their relationship to surrounding rocks is not known. ROCKS FORMED 1,000 TO 1,200 M.Y. AGO A sequence of rhyolites and comagmatic granites was extruded and emplaced over much of the Texas Panhandle and far eastern New Mexico about 1,180 ±20 m. y. ago. The age of this extensive rhyolite field, the Panhandle volcanic terrane, is known from Rb-Sr whole-rock isochron studies. It covers more than 52,000 km2 despite considerable diminution by erosion and partial covering by younger rocks. Many of the rhyolites preserve delicate ignimbritic features. The associated granites, grouped into the Amarillo granite terrane, are typical hypersolvus epizonal intrusives; micrographic textures are common. The rocks are leucocratic and are composed almost entirely of quartz and perthite. A wide variety of Precambrian metamorphic and igneous rocks is exposed in the Llano uplift of central Texas. These rocks and their subsurface equivalents are here called the Llano province. This suite of rocks can be traced in the subsurface with some degree of confidence nearly 300 km north of the uplift. The boundary to the west is difficult to define because of sparse control and other complications. To the south and east the Precambrian is buried beneath thick Paleozoic rocks of the Ouachita foldbelt. The geology of the Precambrian rocks in the Llano uplift has been summarized by Clabaugh and McGehee (1962) and Garrison and others (1978). The geochronology of certain of the rock units has been studied by Z art man (1964, 1965), Delong and Long (1976), and Garrison and others (1979). Three major rock units constitute the Llano province. The oldest of two metamorphic units is the VaUey Spring Gneiss. It is overlain by the Packsaddle Schist, which has a measured thickness of 7,330 m and is composed of hornblende, graphite, biotite, muscovite, and actinolite schists; marble and various leptites also make up a substantial part of the Packsaddle section. These two units form the country rock for the third unit, which is composed of a variety of granitic intrusions. The VaUey Spring Gneiss has yielded an age of about 1,160±30 m.y. (Zartman, 1965). Foliated granitic intrusive rocks (Big Branch Gneiss and Red Mountain Gneiss) that cut the VaUey Spring and lower Packsaddle have yielded a Rb-Sr isochron age of 1,167 ± 12 m.y. (Garrison and others, 1978; Garrison and others, 1979). These results suggest that the Packsaddle Schist was deposited dining the relatively narrow time span between 1,190 and 1,155 m.y. ago. AU these older rocks were intruded by massive plutonic granites such as the Town Mountain Granite about 1,060 m.y. ago, near the end of an episode of regional metamorphism (Zartman, 1964). The Van Horn area of western Texas is underlain by a wide variety of metaigneous and metasedimentary rocks (King and Flawn, 1953; King, 1965). Dating of these rocks, the Carrizo Mountain Group, indicates a period of regional metamorphism about 1,000 m.y. ago with the development of pegmatites (Denison and others, 1971). The age of deposition of the Carrizo Mountain Group has not been clearly defined. The metarhyoUtes in the Van Horn area may not be as old as the calculated Rb-Sr age of about 1,280 m.y. (Denison and Hetherington, 1969), but they appear to be dis-tinguishably older than the rhyohtes in the Franklin Mountains, on the basis of comparative Rb-Sr and 207Pb/206Pb ages. (See also Wasserburg and others, 1962.) The DeBaca terrane and the Swisher diabasic terrane appear to be isochronous. Muehlberger and others (1967) and Denison and Hetherington (1969) have reviewed previous information and correlations from the outcrop into the subsurface. Outcrops of these metasedimentary and basaltic rocks are found in the northern Van Horn area, the Franklin Mountains, and in smaU outcrop areas in southeastern New Mexico. In far western Texas the metasedimentary rocks are in part demonstrably of marine origin. Northward into the subsurface the rocks become increasingly arkosic and are probably nonmarine. Basaltic rocks associated with the metasedimentary units are more common northward. The time of sedimentation has not been strictly determined. In the Franklin Mountains metasedimentaryC14 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO rocks are conformably overlain by rhyolites and intruded by granites that have ages near 1,000 m.y. It seems probable that the original sedimentary rocks were deposited just prior to the extrusion of the rhyolites, perhaps in the interval between 1,100 and 1,000 m.y. ago. The Franklin Mountains have some exceptionally fine exposures of Precambrian igneous and metamorphic rocks (Harbour, 1972). Nearly 460 m of rhyolite flows overlies metasedimentary rocks of the DeBaca terrane and is in turn intruded by diverse granitic stocks and sills. The ages of rhyolites and granites in the Franklin Mountains all fall into a rather narrow range around 1,000 m.y. (Denison and Hetherington, 1969). These ages indicate that this is the youngest of the Precambrian rhyolite-epizonal granite associations. These igneous rocks appear to be rather limited in areal extent. Small isolated outcrops are found about 100 km to the east and some 150 km to the north of the Franklin Mountains. This igneous activity evidently did not cover as great an area as the older rhyolite fields. Perhaps a smaller volume of magma erupted; but the rocks may also have been removed by erosion, or they may extend to the south into north-central Mexico where no information is available. PROTEROZOIC TIME (INTERVAL OCCURRING 600-900 M.Y. AGO) METARHYOLITE OF DEVILS RIVER UPLIFT A few wells along the Devils River uplift southwest of the Llano uplift in Texas have penetrated metarhyolite. The few isotopic measurements available from these rocks suggest late Precambrian or Early Cambrian ages (Nicholas and Rozendal, 1975; Denison and others, 1977). The rhyolites are underlain in part by massive granitic rocks about 1,250 m.y. old that have been penetrated in only one well. The best interpretation of the isotopic data is that the rhyolites were extruded not less than 725 m.y. ago. The extent of this unit and its significance are not known because of sparse control from drill holes; it would appear to be the youngest Precambrian igneous rock found in the area. Micas from the metarhyolites yield mid to late Paleozoic ages, indicating that they have been strongly affected by younger metamorphism. THE VAN HORN SANDSTONE This sedimentary rock unit may be late Precambrian or earliest Paleozoic in age. It is geographically restricted to an area north of Van Horn in far western Texas. McGowan and Groat (1971) have shown that the unit was deposited as an alluvial fan upon strongly folded, faulted, and dissected Precambrian rocks that were metamorphosed about 1,000 m.y. ago. The Van Horn Sandstone is overlain by the Bliss Sandstone, which is of Ordovician age in this area. Thus the Van Horn must have been deposited between about 1,000 and 480 m.y. ago. The available data do not permit extension of this unit into the subsurface. PHANEROZOIC TIME (PALEOZOIC ERA, CAMBRIAN PERIOD) Areas underlain by the subsurface extension of the Wichita province igneous rocks (see discussion of Area III) are found in adjacent parts of the Texas Panhandle. Muehlberger and others (1966) reported a rhyolite yielding an apparent Cambrian age in the central Texas Panhandle, well away from the principal exposures and subsurface extent in southern Oklahoma. AREA V—EASTERN MIDCONTINENT Area V (fig. 5) is underlain by two widespread basement rock terranes. In the eastern part of the area is the subsurface extension of the Grenville province of Canada. To the west of the Grenville province is a terrane that consists predominantly of granite, rhyolite, trachyte, basalt, and related rocks of middle and late Proterozoic age. PROTEROZOIC TIME (INTERVAL OCCURRING 900-1,600 M.Y. AGO) GRANITE-RHYOLITE TERRANE Southern Wisconsin, Illinois, Indiana, and the western parts of Ohio, Kentucky, and Tennessee are underlain by a vast terrane of essentially unmetamorphosed rhyolitic and trachytic volcanic rocks and epizonal granitic rocks. These rocks represent an apparent continuation of the anorogenic terrane to the north in central Wisconsin. Van Schmus (1978) found that the Precambrian basement in central Wisconsin consists in part of 1,780-1,800-m.y.-old rhyolitic ignimbrites, grano-phyric granites, and porphyritic granites intruded by l,500-m.y.-old rapakivi-type granites. The anorogenic terrane is extensively developed in the eastern and south-central midcontinent. It apparently extends from central Wisconsin southwestward to northern New Mexico and Arizona (Bass, 1960; Zietz and others, 1966; Muehlberger and others, 1967; Bickford and Mose, 1975; and others).GEOLOGY AND GEOCHRONOLOGY OF PRECAMBRIAN ROCKS C15 Lake Ontario. Lake Michigan ->C-Vv- WISCONSIN \/>’3)i S ’ ILLINOIS A/ ' A INDIANA’Y'\W ■// •. ■ \. • \—/ \—/ \ —/ r V. vi^<■' (\ / N'T/// ; 8°x^/\V /.v/n >• y y ,n a •< \r»< ?i/\^/v\ <0®4ZM A______- T 200 KILOMETERS EXPLANATION BBS III SUBSURFACE GRENVILLE PROVINCE SEDIMENTARY ROCKS MAFIC IGNEOUS ROCKS BASALTIC RIFT ZONE GRANITE-RHYOLITE PROVINCE FELSIC ROCKS (Undifferentiated) ----------- INFERRED BASEMENT CONTACT ----------- FAULT ZONE -----------HIGH-ANGLE FAULT -----?-----INFERRED BASEMENT FAULT— Queries indicate possible extension -*■--*---*— THRUST FAULT— Sawteeth on upthrown side Figure 5. Sketch geologic map of basement rocks in the Central Interior region east of the Mississippi River. Unpatterned areas have no control.C16 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO The granite-rhyolite terrane of the eastern midcontinent is characterized by an overall homogeneity and relatively subdued magnetic anomaly pattern, suggesting that the area forms a distinct crustal unit of essentially undeformed volcanic rocks and mainly epizonal granitic rocks. The western boundary of the terrane is drawn near the Iowa-Illinois State line along a distinct change in magnetic anomaly pattern (Zietz and others, 1966). The apparent age and petrographic character of the granite-rhyolite terrane were described by Lidiak and others (1966). Granitic igneous activity appears to have occurred between 1,200 and 1,500 m.y. ago. Most of the available age determinations, however, are Rb-Sr and K-Ar dates on micas, and thus they may be minimum ages that have been reduced by later igneous or metamorphic activity. An unpublished Rb-Sr measurement on a micrographic granite from Fulton County, northern Indiana, yielded an apparent age of 1,480 ±40 m.y. This age corresponds to the age of the Wolf River batholith of central Wisconsin (Van Schmus and others, 1975) and suggests that granitic igneous activity was widespread at this time. The age of the volcanism is also uncertain. Apparent ages from rhyolite and trachyte are between 1,250 and 1,350 m.y. (Lidiak and others, 1966). However, the volcanism may be older: it may date from 1,500 m.y. to possibly 1,760 ±10 m.y. ago. SEDIMENTARY ROCKS Unmetamorphosed to slightly metamorphosed sedimentary rocks of pre-Late Cambrian age occur in widely spaced wells in the eastern midcontinent. At least some of the rocks are of middle Proterozoic age. For example, the Baraboo Quartzite and related rocks of south-central Wisconsin (Dott and Dalziel, 1972) were deposited later than 1,760 m.y. ago. The Baraboo Quartzite and the previously described Sioux Quartzite of South Dakota are probably correlative and represent widespread sedimentation. Similarly, the quartzites and slates in the subsurface of southern Wisconsin (Thwaites, 1931) may have been deposited during this period of time. BASALTIC RIFT ZONES A series of north- to northwest-trending basement rift zones occurs in the granite-rhyolite terrane (fig. 5). The rifts, which are probably underlain by mafic igneous rocks, are delineated mainly by linear gravity and magnetic anomalies and by sparse basement well control. None of these rocks have been dated, but they are tentatively assigned a middle Keweenawan age (1,000-1,200 m.y.) because of their general similarities to rocks of the Central North American rift system. The best documented of these inferred rifts occurs in the Michigan Basin. Hinze and others (1975) made a detailed study of the linear gravity and magnetic anomalies and concluded that they represent a middle Keweenawan rift zone. A recent deep test drilled near the center of the basin encountered mafic igneous rocks beneath a thick section of red clastic sedimentary rocks (Bradley and Hinze, 1976; Van Der Voo and Watts, 1976), thus supporting the rift zone interpretation. The linear gravity high in eastern Indiana and western Ohio is also regarded as indicating a rift zone. Three of the six wells to basement along this structure bottomed in basalt; the other three bottomed in felsic igneous rocks. Immediately to the south of the rift, two micrographic granites have Rb-Sr ages on feldspars of about 1,125 m.y. The relation of these felsic rocks to the basalts is not established, but the ages suggest that felsic igneous activity also occurred during the development of the rift zones in middle Keweenawan time. Northwest-trending gravity and magnetic highs outline an inferred belt of basalt or gabbroic igneous rocks in eastern Illinois. Rudman and others (1972) recognized an area of magnetic and gravity highs immediately to the south in southwestern Indiana and concluded that the area is underlain by basalt. No basement well control presently exists for this proposed structure in Illinois or for its possible extension into Indiana. The inferred north-trending rift zone in Kentucky, Tennessee, and Alabama also coincides with gravity and magnetic highs. The geophysical anomalies suggest the presence of a thick sequence of mafic igneous rocks. RIFT-RELATED SEDIMENTARY ROCKS Unmetamorphosed sedimentary rocks that are apparently associated with the basaltic rift zones have been encountered in the Michigan Basin (Bradley and Hinze, 1976), western Ohio, and northern Kentucky. These rocks occur beneath Upper Cambrian strata and are of probable late Proterozoic age. They are inferred to have been deposited during formation of the rift zone. Other sedimentary rocks of pre-Late Cambrian age are also present in western Tennessee and Kentucky and in southern Illinois near the center of the Illinois Basin. They are apparently not associated with basalts and may be as old as latest Proterozoic. SUBSURFACE GRENVILLE PROVINCE The Grenville province of Canada extends into the subsurface of the United States near the west end of Lake Erie along a series of prominent south-trendingGEOLOGY AND GEOCHRONOLOGY OF PRECAMBRIAN ROCKS C17 gravity and magnetic highs. These anomalies appear to cut and thus postdate the northwest-trending anomalies associated with the rift zone in the Michigan Basin (Hinze and others, 1975). The south-trending anomalies continue into Ohio and form a sharp gradient, separating a series of positive magnetic and gravity highs on the east from broader, less intense anomalies on the west (Zietz and others, 1966). Petrographic study and age determinations (Lidiak and others, 1966) show that this sharp gradient coincides with the boundary between a granite-metamor-phic complex on the east and an older, less deformed terrane on the west. East of the boundary in eastern Ohio and West Virginia, the basement rocks consist mainly of mica and hornblende schist and gneiss, two-feldspar granite, and less commonly marble and calc-silicate rock. Most of the metamorphic rocks are of amphibolite grade. Age determinations on micas from gneiss, schist, and granite in parts of Michigan, Ohio, Pennsylvania, and West Virginia are in the range 800-1,000 m.y. These ages are in good agreement with mica ages determined from the Grenville province in Canada. Most of the mica ages do not date the main period of orogeny, but instead reflect later tectonic or thermal disturbance and probably deep burial and subsequent uplift. The last main period of metamorphism occurred about 1,100 m.y. ago. (Compare Lidiak and others, 1966.) More recent studies by Krogh and Davis (1969) indicate that regional metamorphism and formation of paragneiss in the northwest Grenville area occurred between 1,500 and 1,900 m.y. ago. Other periods of Grenville regional metamorphism occurred about 1,300 and 1,100 m.y. ago. The Grenville Front extends southward into Kentucky and Tennessee (Lidiak and Zietz, 1976). The predominant rock types east of the front are granite gneiss, two-feldspar granite, medium-grade metamorphic rock, and anorthosite. Trachyte, rhyolite, basalt, and weakly metamorphosed sedimentary rocks are the characteristic rocks west of the front. Locally felsic volcanic rocks also occur immediately east of the front. The Grenville Front is tentatively shown extending into Alabama on figure 5. The only available isotopic age determinations on the subsurface rocks of the Grenville province are the previously mentioned K-Ar and Rb-Sr ages on micas. Consequently, the period or periods of sedimentation, anorthosite intrusion, and granitic plutonism that are shown on the chronometric chart have been inferred. They are based mainly on regional correlations and extrapolations from outcrops. MET ALLOGENIC SIGNIFICANCE OF THE PRECAMBRIAN BASEMENT Better understanding of the Precambrian geology of the Central Interior region is a key to understanding the evolution and distribution of its resource systems. The shallow volcanic-plutonic complexes are potentially the most important tectonic and metallogenic units of the region. The St. Francois terrane of southeastern Missouri constitutes an iron metallogenic province and has in fact been the source of iron production for more than 150 years. Kiruna-type iron (apatite) and iron-copper deposits, some of them rare-earth enriched, are associated with the silicic volcanic rocks of the terrane. Marginal manganese mineralization has occurred in the volcanic rocks; hypo-xenothermal veins of W-Pb-Ag-Sn occur in at least one of the plutonic bodies; and late-stage two-mica granites of the terrane are among the most uraniferous granites of North America (Malan, 1972). The metallogenesis of the volcanic-plutonic complexes, as suggested by observations in the St. Francois terrane, is intimately related to the complex magmatic-tectonic processes that produced this extensive anorogenic suite of rocks. Two lines of metallogenic evolution are indicated and have the potential for enrichments in ore deposits: (1) the ferrous metals, related to alkaline intermediate magmatism, and (2) W, Ag, Sn, Pb, U, Th, and F in the late granites (Kisvar-sanyi, 1976). The metallogenesis of the older metamorphic basement is not known because of lack of outcrops and no proven ore bodies. By analogies with Canadian Shield provinces, however, it may contain complex and varied mineral deposits. The resource potential in the basement complex of Missouri has recently been evaluated on the basis of drill-hole data (Kisvarsanyi and Kisvar-sanyi, 1977). Among the most interesting possibilities are the layered mafic intrusions, which have a potential for Fe-Ni-Cu-Co and Pt- Cr-Ti mineralization. The Central North American rift system is a favorable site for rift-related metallogeny. Another important metallogenic aspect of the Precambrian basement is its tectono-morphologic control on the emplacement and localization of ore bodies in the overlying sedimentary rocks. Major mineral districts in Missouri, Oklahoma, Kansas, and Illinois are located over Precambrian topographic and structural highs and ancient fracture zones (Snyder, 1970; Kisvarsanyi, 1977). Although the metals may have been derived from multiple sources, at least some of the metals may have been recycled from a Precambrian source and redistributed into flanking sedimentary basins.C18 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO As near-surface resources are depleted, the vast resource potential inherent in the buried basement rocks becomes of increasingly greater interest, particularly where depth to basement is not prohibitive to mining. REFERENCES CITED Anderson, J. L., and Culler, R. L., 1978, Geochemistry and evolution of the Wolf River Batholith, a Late Precambrian rapakivi massif in north Wisconsin, U.S.A.: Precambrian Research, v. 7, p. 287-324. Anderson, R. E., 1970, Ash-flow tuffs of Precambrian age in southeast Missouri (Contributions to Precambrian geology no. 2): Missouri Geological Survey and Water Resources Report of Investigations 46, 50 p. Bass, M. N., 1960, Grenville boundary in Ohio: Journal of Geology, v. 68, p. 673-677. Bayley, R. W., and Muehlberger, W. R., compilers, 1968, Basement rock map of the United States, exclusive of Alaska and Hawaii: U.S. Geological Survey, 2 sheets, scale 1:2,500,000. Bell, C. K., 1966, Churchill-Superior province boundary in northeastern Manitoba: Geological Survey of Canada Paper 66-1, p. 133-136. Berg, E. L., 1938, Notes on catlinite and the Sioux quartzite: American Mineralogist, v. 23, p. 258-268. Berry, A. W., Jr., and Bickford, M. E., 1972, Precambrian volcanics associated with the Taum Sauk caldera, St. Francois Mountains, Missouri. U.S. A.: Bulletin of Volcanology, v. 36, p. 308-318. Beyer, S. W., 1893, Ancient lava flows in the strata of northwestern Iowa: Iowa Geological Survey, v. 1, p. 165-169. Bickford, M. E., Harrower, K. L., Hoppe, W. J., Nelson, B. K., Nusbaum, R. 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W., 1970, Age of the Morton and Montevideo gneisses and related rocks, southwestern Minnesota: Geological Society of America Bulletin, v. 81, p. 3671-3695. Goldich, S. S., Lidiak, E. G., Hedge, C. E., and Walthall, F. G., 1966, Geochronology of the Midcontinent region, United States, 2, northern area: Journal of Geophysical Research, v. 71, p. 5389-5408 Goldich, S. S., Nier, A. O., Baadsgaard, H., Hoffman, J. H., and Krueger, N. W., 1961, The Precambrian Geology and Geo-GEOLOGY AND GEOCHRONOLOGY OF PRECAMBRIAN ROCKS C19 chronology of Minnesota: Minnesota Geological Survey Bulletin 41, 193 p. Gough, D. I., and Camfield, P. A., 1972, Convergent geophysical evidence of a metamorphic belt through the Black Hills of South Dakota: Journal of Geophysical Research, v. 77, p. 3168-3170. Ham, W. E., Denison, R. E., and Merritt, C. A., 1964, Basement rock and structural evolution of southern Oklahoma: Oklahoma Geological Survey Bulletin 95, 302 p. Harbour, R. 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B., 1965, Geology of the Sierra Diablo region, Texas: U.S. Geological Survey Professional Paper 480, 185 p. King, P. B., and Flawn, P. T., 1953, Geology and mineral deposits of Precambrian rocks of the Van Horn area, Texas: Texas University Bureau of Economic Geology Publication 5301, 218 p. Kisvarsanyi, E. B., 1972, Petrochemistry of a Precambrian igneous province, St. Francois Mountains, Missouri (Contribution to Precambrian Geology No. 4): Missouri Geological Survey and Water Resources Report of Investigations 51, 103 p. ------1974, Operation basement—Buried Precambrian rocks of Missouri—their petrography and structure: American Association of Petroleum Geologists Bulletin, v. 58, p. 674-684. ------1975, Data on Precambrian in drill holes of Missouri including rock type and surface configuration: Missouri Geological Survey and Water Resources Report of Investigations 56, 20 p. Kisvarsanyi, Geza, 1976, Precambrian metallogenesis in the St. Francois Mountains igneous province, southeast Missouri, in Kisvarsanyi, E. B., ed., Studies in Precambrian geology of Missouri: Missouri Department of Natural Resources Division of Geology and Land Survey, Contributions to Precambrian Geology 6, Report of Investigations 61, p. 164-173. ------1977, The role of the Precambrian igneous basement in the formation of the stratabound lead-zinc-copper deposits in southeast Missouri: Economic Geology, v. 72, p. 435-442. Kisvarsanyi, Geza, and Kisvarsanyi, E. B., 1977, Mineral-resource potential of the basement complex in Missouri: Missouri Academy of Sciences Transactions, v. 10-11, p. 16-43. Kleinkopf, M. D., and Redden, J. A., 1975, Bouguer gravity, aeromagnetic, and generalized geologic maps of part of the Black Hills of South Dakota and Wyoming: U.S. Geological Survey Geophysical Investigations Map GP-903. Komik, L. J., 1969, An aeromagnetic study of the Moak Lake-Setting Lake structure in northern Manitoba: Canadian Journal of Earth Sciences, v. 6, p. 373-381. Krogh, T. E., and Davis, G. L., 1969, Geochronology of the Grenville Province: Carnegie Institute of Washington Yearbook 67, p. 224-230. Lidiak, E. G., 1971, Buried Precambrian rocks of South Dakota: Geological Society of America Bulletin, v. 82, p. 1411-1420. ------1972, Precambrian rocks in the subsurface of Nebraska: Nebraska Geological Survey Bulletin 26, 41 p. Lidiak, E. G., Marvin, R. F., Thomas, H. H., and Bass, M. N., 1966, Geochronology of the midcontinent region, United States, Part 4, eastern area: Journal of Geophysical Research, v. 71, p. 5427-5438. Lidiak, E. G., and Zietz, I., 1976, Interpretation of aeromagnetic anomalies between latitudes 37°N and 38°N in the eastern and central United States: Geological Society of America Special Paper 167, 37 p. Long, L. 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F., 1972, Precambrian geophysical provinces in Indiana: Indiana Academy of Sciences, v. 81, p. 223-228.C20 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO Scott, R. W., 1966, New Precambrian (?) formation in Kansas: American Association of Petroleum Geologists Bulletin, v. 50, p. 380-384. Silver, L. T., Bickford, M. E., Van Schmus, W. R., Anderson, J. L., Anderson, T. H., and Medaris, L. G., Jr., 1977, The 1.4-1.5 b.y. transcontinental anorogenic plutonic perforation of North America: Geological Society of America Abstracts with Programs, v. 9, p. 1176. Silver, L. T., and Green, J. C., 1963, Zircon ages for Middle Keweenawan rocks of the Lake Superior region [abs.]: American Geophysical Union Transactions, v. 44, p. 107. ------1972, Time constants for Keweenawan igneous activity: Geological Society of America Abstracts with Programs, v. 4, p. 665. Snyder, F. G., 1970, Structural lineaments ana mineral deposits, eastern United States, in Raush, D. 0., and Mariacher, B. 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Walters, R. F., 1946, Buried Precambrian Hills in northeastern Barton County, central Kansas: American Association of Petroleum Geologists Bulletin, v. 30, p. 660-710. Wasserburg, G. J., Wetherill, G. W., Silver, L. T., and Flawn, P. T., 1962, A study of the ages of the Precambrian of Texas: Journal of Geophysical Research, v. 67, p. 4021-4047. Woollard, G. P., 1943, Transcontinental gravitational and magnetic profile of North America and its relationships to geologic structure: Geological Society of American Bulletin, v. 54, p. 747-789. Zartman, R. E., 1964, A geochronologic study of the Lone Grove pluton from the Llano uplift, Texas: Journal of Petrology, v. 5, p. 359-408. ------1965, Rubidium-strontium age of some metamorphic rocks from the Llano uplift, Texas: Journal of Petrology, v. 6, p. 28-36. Zartman, R. E., and Stem, T. W., 1967, Isotopic age and geologic relationships of the Little Elk granite, northern Black Hills, South Dakota: U.S. Geological Survey Professional Paper 575-D, p. D157-D163. Zietz, I., King, E. R., Geddes, W., and Lidiak, E. G., 1966, Crustal study of a continental strip from the Atlantic Ocean to the Rocky Mountains: Geological Society of America Bulletin, v. 77, p. 1427-1448. U.S. GOVERNMENT PRINTING OFFICE: 1984-576-049/29,003 REGION NO. 87 DAYS Al-T) The Precambrian of the Rocky Mountain Region —— U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1241-D U.S. QEFOS* AUG 7 198b The Precambrian of the Rocky Mountain Region By CARL E. HEDGE, ROBERT S. HOUSTON, OGDEN L. TWETO, ZELL E. PETERMAN, JACK E. HARRISON, and ROLLAND R. REID CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO Edited fry JACK E. HARRISON and ZELL E. PETERMAN U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1241-D Lithology, distribution, correlation, and isotope ages of exposed Precambrian rocks in the Rocky Mountain region UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1986DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data The Precambrian of the Rocky Mountain region. (Correlation of Precambrian rocks of the United States and Mexico) (U.S. Geological Survey professional paper ; 1241-D) Bibliography: p. 1. Geology, Stratigraphic—Pre-Cambrian. 2. Geology—Rocky Mountain Region. I. Hedge, Carl E. II. Series. III. Series: Geological Survey professional paper ; 1241-D QE653.P734 1986 551.7T0978 86-600168 For sale by the Branch of Distribution Books and Open-File Reports Section U.S. Geological Survey Federal Center Box 25425 Denver, CO 80225CONTENTS Page Abstract .................................................................................. 1 Introduction .............................................................................. 1 Archean rocks of Wyoming and southern Montana.............................................. 3 Early Proterozoic metasedimentary rocks of southeastern Wyoming and the Black Hills, South Dakota.......................................................................... 5 Proterozoic rocks of Colorado and southern Wyoming....................................... 6 Crystalline basement rocks of northern Utah and Idaho.................................. 9 Post-1,700 million year supracrustal rocks .............................................. 10 Tectonics................................................................................. 12 Summary................................................................................... 14 References cited.......................................................................... 14 ILLUSTRATIONS Page Plate 1. Correlation chart for Precambrian rocks and events in the Rocky Mountain region. In pocket Figure 1. Generalized geologic-geochronologic map of the Precambrian of the Rocky Mountain region............................... 2 2. Diagram of zircon data for the Beartooth Mountains, Montana............................................................. 4 3. Geologic map of the Precambrian of Colorado and southernmost Wyoming.................................................... 8 4. Diagram of ages of approximately l,670-m.y.-old plutons in Colorado..................................................... 9 5. Diagram of estimated ages of some events in the formation of the Belt Supergroup, Montana........................... 11 6. Map showing some Precambrian tectonic features in the Rocky Mountain region............................................ 13 hi CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO THE PRECAMBRIAN OF THE ROCKY MOUNTAIN REGION By Carl E. Hedge, Robert S. Houston1, Ogden L. Tweto, Zell E. Peterman, Jack E. Harrison, and Rolland R. Reid2 ABSTRACT Precambrian crystalline rocks of the Rocky Mountain region of the United States represent two age provinces. An Archean province (older than 2,500 million years) occupies Wyoming and adjacent parts of Utah, Montana, and South Dakota. A Proterozoic province (about 1,600 to 1,800 million years old), is represented by only sparse exposures west and northwest of the older terrane, and by extensive exposures to the south. The Archean province is mostly felsic gneisses and associated metasedimentary rocks that were metamorphosed about 2,800 million years ago. Tonalitic to granodioritic plutons were emplaced in this terrane 2,500 to 2,760 million years ago. In Colorado, a thick sequence of volcanic and sedimentary rocks was deposited between 2,000 and 1,750 million years ago. These rocks were metamorphosed and intruded by numerous granodioritic plutons about 1,700 million years ago. This province was invaded by granitic plutons 1,400 million years ago and again, in central Colorado, 1,015 million years ago. Shelf-type sedimentary sequences were deposited on the older crust during the interval from 2,500 to 1,700 million years ago and are preserved in a belt from southern Wyoming to the Black Hills. A younger sequence, 1,460 to 1,600 million years in age, is preserved only as the Uncompahgre Formation in southwestern Colorado. A still younger sequence, the miogeoclinal Belt Supergroup, 850 to 1,500 million years in age, is preserved in western Montana and northern Idaho. Rocks roughly equivalent to but isolated from the Belt Supergroup include the Yellowjacket Formation and Lemhi Group of Idaho and the Uinta Mountain Group of northeastern Utah and northwestern Colorado. Eugeoclinal rocks, including diamictites, were deposited west of the miogeoclinal rocks beginning approximately 850 million years ago. INTRODUCTION Precambrian rocks are exposed in numerous areas in the Rocky Mountain region, mostly in the cores of uplifted mountain blocks (fig. 1). The rocks range from unmetamorphosed late Proterozoic sedimentary rocks to Archean gneisses. The state of knowledge of the geology and geochronology of the rocks varies widely through the region. In Colorado, abundant geo-chronologic data and extensive detailed field studies make possible the assignment of almost every Precambrian rock unit to a specific time period (Tweto, 1979). 'University of Wyoming, Laramie, Wyo. 2University of Idaho, Moscow, Idaho. In contrast, the history of the pre-Belt basement rocks of western Montana and Idaho is only beginning to come into focus. The earliest geologic work in the region established that the relatively unmetamorphosed sedimentary rocks, such as the Belt Supergroup and the Uinta Mountain Group, are younger than the crystalline complexes in many of the mountain ranges of Wyoming and Colorado. The advent of radiometric dating made possible studies that showed that the Precambrian crystalline rocks of most of Wyoming are older than those of Colorado. By the early 1960’s, enough radiometric ages were available to show that the framework Precambrian rocks of Wyoming are of Archean age and are approximately equivalent to those of the Superior province of the Canadian Shield. Condie (1969) referred to the Archean terrane exposed mainly in Wyoming and southern Montana as the Wyoming province. In his reconstruction of the growth of the North American continent, Engel (1963) connected the Canadian and Wyoming terranes. Subsequent data from wells that penetrated basement rocks in the midcontinent region (Goldich and others, 1966) revealed, however, that younger rocks intervene between the two older terranes in the basement of western North and South Dakota. The Archean of the Wyoming province extends into bordering States to the east, north, and west. Zartman and others (1964) demonstrated that Archean gneiss underlies the Precambrian metasedimentary rocks of the Black Hills, and Armstrong and Hills (1967) and Compton and others (1977) identified an Archean basement in northwestern Utah and south-central Idaho. Catanzaro and Kulp (1964) found the gneisses in the Little Belt Mountains of Montana to be of Archean age, and Peterman (1981) has demonstrated the extension of the Archean to the Little Rocky Mountains of north-central Montana and into the basement of northeastern Wyoming. To the south of the Wyoming province, no Archean 1CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO 116° 114° 112° 110° 108° 106° 104° EXPLANATION Upper and middle Proterozoic supracrustal rocks Proterozoic plutonic and metamorphic rocks Lower Proterozoic supracrustal rocks Archean plutonic and metamorphic rocks 400 KILOMETERS Figure 1.—Generalized geologic-geochronologic map of the exposed Precambrian rocks of the Rocky Mountain region.THE PRECAMBRIAN OF THE ROCKY MOUNTAIN REGION 3 rocks have been recognized. Extensive dating in Colorado has yielded no ages greater than about 1,850 m.y. (million years), and isotopic data strengthen the conclusion that the Precambrian of Colorado (and probably of large areas to the south and east) does in fact represent major new additions to the continent during the early Proterozoic. The thick Precambrian sedimentary sections that occur along the western flank of the Rocky Mountains have been difficult to date, but considerable progress has been made, particularly with the Belt Supergroup, which was apparently deposited between 1,500 and 850 m.y. ago (Harrison, 1972). Figure 1 provides a general overview of the time-space relationships of the exposed Precambrian rocks of the Rocky Mountain region. Plate 1 summarizes the chronologic knowledge of various areas or mountain ranges within the region. ARCHEAN ROCKS OF WYOMING AND SOUTHERN MONTANA The Archean terrane of the Wyoming province, although exposed mainly in the cores of mountain ranges in Wyoming and Montana (fig. 1), extends at least as far east as the Black Hills, as far west as the northwest comer of Utah, and northward to north-central Montana. In southeastern Wyoming, the Archean is overlapped by early Proterozoic supracrustal rocks and intruded by early and middle Proterozoic igneous rocks. Chronostratigraphic relations for Precambrian rocks exposed in the various ranges of the Wyoming province are shown on plate 1. In most of these areas, late Archean granites were intruded into greenstone terranes and (or) gneiss complexes between about 2,500 and 2,700 m.y. ago. Earlier metamorphic and igneous events are recognized in some areas. In comparison with the Archean of the Superior province of northern Minnesota and adjacent Ontario, the Wyoming province contains a much lower proportion of greenstone sequences. The best preserved greenstone terrane crops out at the southeast end of the Wind River Range (Bayley and others, 1973). Greenstones are also recognized in the Owl Creek Mountains (Granath, 1975), the Seminoe Mountains (Bayley, 1968), the central Laramie Range (Graff and others, 1982), and possibly in the Granite Mountains (Peterman and Hildreth, 1978). Gneiss complexes occur throughout the Wyoming province and are the principal units in some of the ranges. These complexes include a variety of rock types commonly at a medium to high grade of metamorphism. Some complexes are banded gneisses derived from stratified rocks, and others are massive and are probably orthogneisses. In some areas, interlayers of amphibolite, quartzite, and marble are common. In southwestern Montana, a gneiss complex contains abundant schistose rocks as well as quartzite, marble, and iron-formation. The oldest rocks thus far reported from the Wyoming province are in the Beartooth Mountains of south-central Montana (pi. 1, G, H). The Beartooth Mountains consist of an older metamorphic complex, a younger group of metasedimentary rocks and more or less metamorphosed granitic rocks, the Stillwater Complex, and finally quartz monzonitic intrusions. Zircons from several of the older metamorphic units gave U-Pb (uranium-lead) isotopic results that define a good chord indicating an age of about 3,070 m.y. (fig. 2). This age is difficult to interpret: it may approximate the age of the source terranes from which the sediments were derived, or it may be the time of metamorphism. Mueller and others (1976) have interpreted U-Pb zircon data to indicate a history extending back to more than 3,300 m.y. in the eastern Beartooth Mountains. Metamorphism of the younger sedimentary sequence and intrusion of granitic rocks occurred about 2,750 m.y. ago in the Beartooth Mountains (Reid and others, 1975). The Stillwater Complex, a layered mafic complex, was emplaced just prior to 2,700 m.y. ago (Nunes and Tilton, 1971). Page (1977) concluded that a block containing the Stillwater Complex moved with respect to a southern block along a major fault; quartz monzo-nite dated at 2,700 m.y. is believed to have been injected along the fault zone. The pre-Beltian rocks of all of southwestern Montana (pi. 1, F) probably have a history as complex as that of the Beartooth Mountains, but attempts to unravel the pre-2,730 m.y. history have failed thus far (Mueller and Cordua, 1976; James and Hedge, 1980). James and Hedge concluded that the terms Cherry Creek Group and Pony Group are of no value because both names are applied to rocks that can only be classified as parts of an Archean metamorphic complex. Although the Precambrian rocks of the Bighorn Mountains (pi. 1, L) have thus far yielded no pre-3,000 m.y. ages, Arth and others (1979) have dated a 3,000-m.y. trondhjemitic intrusion and a 2,950-m.y. granodiorite in the southwestern part of the range. Banks and Heimlich (1976) and Heimlich and Banks (1968) reported an age of 2,850 m.y. for granites from the northern part of the range, and Stueber and Heimlich (1977) obtained an age of 2,760 m.y. for diabase dikes that cut these rocks.Pb/ 4 0.7 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO Figure 2.—Zircon data for the Beartooth Mountains, Mont. Data from Catanzaro and Kulp (1964), Nunes and Tilton (1971), and Reid and others (1975). Numbers on curve in millions of years. In the Teton Range (pi. 1, I), Reed and Zartman (1973) assigned an age of metamorphism of 2,820 m.y. to the Webb Canyon Gneiss. They also reported an age of 2,440 m.y. for the Mount Owen Quartz Monzonite, but this latter age is suspect because of the improbably high initial 87Sr/86Sr ratio. Two plutons in the Wind River Range (pi. 1, J), the Bears Ears pluton (Popo Agie batholith) and the Louis Lake Granodiorite, have been dated at 2,560 m.y. and 2,640 m.y. respectively (Naylor and others, 1970). The Louis Lake Granodiorite intrudes the greenstone belt at South Pass, in the southeastern Wind River Range. A Rb-Sr (rubidium-strontium) isochron age of 2,790 m.y. was obtained on metagraywacke and metavolcanic rocks within the greenstone belt, but the age has a large uncertainty (Z. E. Peterman, unpub. data, 1973). Similarly, five granitic gneiss samples from a gneiss complex in the central Wind River Range define a Rb-Sr isochron age of 2,760 m.y. (Z. E. Peterman and Fred Barker, unpub. data, 1975). An amphibolite-grade sedimentary-volcanic sequence in the Granite Mountains (pi. 1, M) of central Wyoming was metamorphosed at 2,860 m.y. (Peterman and Hildreth, 1978), and this unit may be equivalent to the greenstones at South Pass. The major granite batholith in the Granite Mountains has been dated at 2,550 m.y. by Rb-Sr methods (Peterman and Hildreth, 1978) and at 2,590 m.y. by U-Pb methods on zircon (Ludwig and Stuckless, 1978). A small body of foliated granite gave an older U-Pb age of 2,640 m.y. (Ludwig and Stuckless, 1978). The ages of the granitic intrusions are remarka- bly similar to those from the southeastern Wind River Range and suggest two discrete periods of regional plutonism. Three whole-rock Rb-Sr isochron ages have been reported (Johnson and Hills, 1976) for various gneiss units in the Laramie Mountains (pi. 1, N). These ages range from 2,700 m.y. to 2,960 m.y. A major batholith of granite was emplaced in the Laramie Mountains 2,510 m.y. ago (Hills and Armstrong, 1974; Johnson and Hills, 1976). In the central Laramie Mountains (pi. 1, O), the Elmers Rock greenstone belt (Graff and others, 1982) comprises late Archean metasedimentary and metavol-canic rocks. Zircons from metadacite are dated at about 2,720 m.y. (Z. E. Peterman, unpub. data, 1983). A similar sequence of metasedimentary and metavolcanic rocks in the Hartville uplift (pi. 1, R) is also late Archean and may be correlative with the sequence in the central Laramie Mountains (Snyder, 1980; Snyder and Peterman, 1982; Peterman, 1982). In the Hartville uplift, the supracrustal rocks are intruded by both late Archean and early Proterozoic rocks including the Rawhide Buttes Granite (2,580 m.y.), the Flattop Buttes Granite (1,980 m.y.), the Twin Hills Diorite (1,740 m.y.), and the Haystack Range Granite (1,720 m.y.). Archean rocks are also exposed in the northern parts of the Medicine Bow Mountains and in the Sierra Madre of south-central Wyoming (pi. 1, P, Q). Again, an older gneissic complex was intruded by granites; however, the published ages are somewhat younger than those for similar rocks farther north. Hills and others (1968) reported a Rb-Sr age of 2,500 m.y. for the granitic gneisses in the Medicine Bow Mountains, whereas Divis (1976) obtained a single zircon age of 2,630 m.y. in the Sierra Madre. Three ages on the Bag-got Rocks Granite (Hills and others, 1968; Divis, 1976) range from 2,360 to 2,500 m.y. We are not certain whether these ages from southern Wyoming represent the true ages of the rocks, or whether the ages have been lowered somewhat by the metamorphic and igneous events just to the south. A number of important mineral deposits are in these Archean rocks of Wyoming and Montana. These deposits include chromium, platinum, and nickel deposits of the Stillwater Complex, iron-formation in many of the greenstone belts, and gold in quartz veins and shear zones of the greenstone belts. Iron-formation described by Bayley and others (1973) is currently being mined in the Atlantic City region in the southeastern Wind River Range.THE PRECAMBRIAN OF THE ROCKY MOUNTAIN REGION 5 EARLY PROTEROZOIC METASEDIMENT ARY ROCKS OF SOUTHEASTERN WYOMING AND THE BLACK HILLS, SOUTH DAKOTA Variably metamorphosed sedimentary rocks are preserved in a string of synclinoria or basins along the southern and eastern margins of the Wyoming Archean province (Hills and Houston, 1979). The most complete and best studied sequence preserved in Wyoming is in the Medicine Bow Mountains (Houston and others, 1968). The supracrustal rocks in the Medicine Bow Mountains consist of two sequences of strata separated by an angular unconformity. The lower sequence, which has been named the Deep Lake Formation, consists of at least 6,000 m of quartzite, metaconglomerate, chlorite schist, amygdular metabasalt, metatuff(?), and siliceous marble (Houston and others, 1968). Sericitic quartzite predominates. This very thick and composi-tionally variable formation contains strata representing many geologic environments, including probable glacial deposits as well as deposits of marine and fluviatile environments. The Deep Lake Formation, currently being restudied, will be divided into two and possibly three subdivisions (Karlstrom and Houston, 1978; Houston and others, 1977). Deposition is cyclic, and the cyclic units resemble those described by Frarey and Roscoe (1970) in the Huronian Supergroup of Canada. In fact, radioactive quartz-pebble conglomerate has been recognized in rocks of the Deep Lake Formation (Houston and others, 1977; Graff and Houston, 1977) that may have been deposited in an oxygen-deficient environment like that suggested for radioactive quartz-pebble conglomerate of the Matinenda Formation of the Huronian Supergroup (Roscoe, 1973). The Libby Creek Group, which consists of approximately 6,800 m of strata, overlies the Deep Lake Formation unconformably. The lower 3,700 m of this group is clastic sedimentary rock, of which about 60 percent is quartzite (in large part orthoquartzite) that has crossbeds and ripple marks, and most of the remainder of the group is micaceous quartzite and quartz-rich schist and phyllite. The basal formation of the Libby Creek Group, the Headquarters Schist, includes layers of fine-grained schist and phyllite that contain scattered boulders, cobbles, and pebbles. This formation may be partly of glacial origin (Houston and others, 1968). Above the quartz-rich lower part of the Libby Creek Group is the Nash Fork Formation, consisting of inter- bedded dolomitic marble that commonly contains algal structures (Knight and Keefer, 1966), and black phyl-lites and slates, some of which are pyritic and belong to the sulfide facies of iron-formation as defined by James (1954). The Nash Fork Formation and all lower formations in the Libby Creek Group are clearly of shallow-water origin and possibly of glacial origin in part, and we interpret them to be a shelf sequence. The Towner Greenstone and the French Slate, the upper two formations of the Libby Creek Group, are of less certain interpretation; perhaps they record the beginning of a change in sedimentologic conditions along the edge of the formerly stable shelf. The Towner Greenstone consists mainly of chlorite-amphibolite schist in which rare thin beds of sandstone have been reported. Its origin is enigmatic, but it may consist of metamorphosed basic volcanic flows, basic pyroclastics, or both. The French Slate is a gray, pyritic slate or phyllite whose environment of deposition also is not well known. The metasedimentary rocks in the other synclinoria are not as well known as those in the Medicine Bow Mountains. In the Sierra Madre, west of the Medicine Bow Mountains, a thick (probably more than 3,000 m), unnamed sequence of mainly clastic metasedimentary rocks has been mapped (Houston and others, 1975). Micaceous quartzites predominate, but slate and phyllite are also abundant, and metavolcanic rocks, metalimestone, and metaconglomerate are present. Some of the metaconglomerates are possibly tillite, and some have the appearance of dropstone conglomerates. These strata probably correlate in part with the Deep Lake Formation of the Medicine Bow Mountains (Houston and others, 1968; Houston and others, 1975). Cyclic units like those identified in the Deep Lake Formation have been recognized in equivalent^) rocks of the Sierra Madre, and radioactive quartz-pebble conglomerate has also been identified in these rocks (Graff and Houston, 1977), reinforcing the correlation with the Deep Lake Formation. The metasedimentary rocks of the Black Hills (pi. 1, S) are more highly deformed and are of a higher metamorphic grade than the rocks of the Sierra Madre and Medicine Bow Mountains of Wyoming. Although many problems of correlation and structural complexity still exist, two maps that show subdivisions of the Pre-cambrian of the entire Black Hills have been published (Redden and Norton, 1975; Kleinkopf and Redden, 1975). The lower part of the metasedimentary section is rich in quartzite but also contains metaconglomerate, schist, marble, and taconite iron-formation. These rocks6 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO are overlain by sequences that contain abundant amphibolite (metabasalt) and metagraywacke, which suggest a transition from a shelf-type environment to a more eugeoclinal type. What are apparently the youngest parts of the sequence are dominantly phyllites or schists and quartzites. It is perhaps significant that radioactive quartz-pebble conglomerate has been reported in the lower shelf-type environment of the Black Hills succession (Hills, 1977), but how these rocks may relate to similar shelf-type units of southeastern Wyoming or the Lake Superior region is not clear. All that is known for certain about the age of this belt of supracrustal rocks is that it is younger than the Archean crystalline basement (2,500 m.y.) and older than about 1,700 m.y. This later age was suggested as the time of metamorphism in the Medicine Bow Mountains (Hills and others, 1968) and is the age of the Harney Peak Granite in the Black Hills (Riley, 1970). Hills and Houston (1979) have correlated various units of this supracrustal sequence with rocks of the Lake Superior region. The early Proterozic metasedimentary rocks contain a number of major mining districts, and the potential for undiscovered mineral deposits is good. The extent of beds of iron-formation in the Black Hills, in particular, suggests that mining of these units will some day be feasible. Although it has been a point of long-standing controversy, recent studies concluded that the major Black Hills gold deposits are of Precambrian age (Redden and Norton, 1975; Rye and others, 1974). Copper was mined from deposits in metasedimentary rocks of the Sierra Madre of Wyoming at the turn of the century (Spencer, 1904). As mentioned earlier, this entire belt of metasedimentary rocks has potential for uranium. Radioactive quartz-pebble conglomerate has been found in the Black Hills, Sierra Madre, and Medicine Bow Mountains, and uranium is known to occur in faults and shear zones within the metasedimentary succession (Houston and others, 1977). PROTEROZOIC ROCKS OF COLORADO AND SOUTHERN WYOMING The Precambrian of Colorado consists mainly of a Proterozoic metamorphic complex extensively intruded by granitic rocks of three general age groups. The Archean rocks that underlie the Uinta Mountain Group in northeastern Utah extend only slightly into Colorado; all the other Precambrian rocks of Colorado are significantly younger. The Precambrian metamorphic complex has been divided into two units, one consisting dominantly of metamorphosed sedimentary rocks and one consisting dominantly of metamorphosed volcanic rocks (Tweto, 1979). The metasedimentary rocks are mostly siliceous graywackes and shales, but they contain minor calcareous rocks and sandstone. The metavolcanic rocks seem to have been a bimodal suite of tholeiitic basalt and rhyodacitic tuffs and lavas. The metasedimentary and metavolcanic rocks are interlayered and intertongue on both regional and local scales (Tweto, 1977); however, the metavolcanic rocks predominate in southern and northern Colorado, and the metasedimentary rocks predominate in the central area (fig. 3). The metamorphic rocks belong to the upper amphibolite facies over most of the State, but facies as low as lower amphibolite or as high as granulite exist locally. The pervasive regional metamorphism makes dating of the parent rocks difficult if not impossible. Silver and Barker (1968) obtained a U-Pb age of 1,800 m.y. on zircons from a metamorphosed volcanic rock in the Needle Mountains, and Hedge (unpub. data, 1979) got an identical age by Rb-Sr on an only slightly metamorphosed rhyodacite tuff near Salida, Colo. Other attempts to obtain ages on the metamorphosed volcanic rocks have yielded figures of 1,700 to 1,750 m.y. using both Rb-Sr and U-Pb methods. These apparent ages are indistinguishable from the time of regional metamorphism, and we believe that they are partly or totally reset ages. Dating the time of deposition of the original sedimentary rocks is difficult because of the added possibility of an inherited age component. Apparent Rb-Sr ages of these rocks are mostly 1,700 to 1,750 m.y., but they are as much as 1,950 m.y. (Hedge and others, 1967; Peterman and others, 1968; Hansen and Peterman, 1968; and C. E. Hedge, unpub. data, 1979). Though many of these ages probably reflect the time of metamorphism, accumulation of the pile must have occurred sometime in the interval of 1,750 to 1,950 m.y., unless some metamorphic mechanism caused massive removal of radiogenic strontium from the entire complex on a regional scale. Several stages of the major period of metamorphism and deformation are recognized in most Colorado localities. These cannot be differentiated by radiometric dating techniques, and they are inferred to have been closely spaced in time. During the later stages of early Proterozoic regional metamorphism, numerous batholiths and smaller plu-tons were emplaced throughout the State. These range in composition from gabbro to granite, but granodiorite predominates. Most of these plutons are foliated, with internal structures parallel to those of the metamorphic wall rocks. Varied relations of the igneous bodies to the structure of the enclosing gneisses indicate emplacement over a period of time during the waning stages of metamorphism and deformation. RadiometricTHE PRECAMBRIAN OF THE ROCKY MOUNTAIN REGION 7 dating has failed to resolve any differences in the ages of the plutons, however, and all of them appear to have been emplaced about 1,670 m.y. ago (fig. 4). One of the youngest phases of this early plutonic event is represented by lamprophyre dikes in the Front Range west of Denver. These dikes are undeformed, are only slightly metamorphosed, and are cut by undeformed pegmatites that have been dated at 1,620 m.y. (C. E. Hedge, unpub. data, 1979). After these intrusive and metamorphic events and before another intrusive episode about 1,430 m.y. ago, sediments of the Uncompahgre Formation were deposited in southwestern Colorado. The sediments were folded and metamorphosed to slates, phyllites, and quartzites and were then intruded by granite of 1,435 m.y. age. Rocks equivalent to the Uncompahgre are not known elsewhere in the Rocky Mountain region, but abundant clasts of chlorite phyllite in the Pennsylvanian Mintum Formation suggest that some may have existed in the west-central part of the Front Range as late as Pennsylvanian time. Throughout the rest of the State, the geologic record is missing for a period of 200-250 m.y. prior to about 1.430 m.y., when a major period of granite emplacement began. Plutons of this period were apparently intruded at a higher crustal level than those of the 1,670-m.y. period (Hutchinson and Hedge, 1967). They occur in a wide belt that extends from northwestern Mexico to the Great Lakes region. Within Colorado the granites are of two different types, which occupy distinct geographic areas (fig. 3), and seem to be slightly different in age. The first type is exemplified by the Sherman Granite of northern Colorado and southern Wyoming and the Eolus Granite of southwestern Colorado. These rocks are massive, are generally red or pink, contain blocky microcline crystals, and commonly contain hornblende as well as biotite. In several localities these granites are associated with syenites, and the 207Pb/206Pb age of the Sherman Granite is indistinguishable from that of the adjacent Laramie Anorthosite. The Sherman Granite is 1,435 m.y. old (Subbarayudu and others, 1975). A similar rock, the granite of the Mount Ethel pluton in the Park Range, yielded an age of 1,440 m.y. (Snyder and Hedge, 1978). The Eolus Granite in southwestern Colorado has been dated at 1,435 m.y. (Bickford and others, 1969), and similar rocks in west-central Colorado have given an age of 1.430 m.y. (Bickford and Cudzilo, 1975). The second type of approximately 1,430 m.y. granite is represented by the Silver Plume Granite, which occurs in the central part of the State (fig. 3). This rock is gray to light pink and is characterized by two micas and by microcline crystals that are distinctly tabular. Rocks of this type have yielded ages of about 1,400-1,410 m.y. (Peterman and others, 1968; Doe and Pearson, 1969; Hansen and Peterman, 1968; and Stern and others, 1971). The Pikes Peak batholith was emplaced 1,015 m.y. ago (Hedge, 1970; Barker and others, 1976). It is a large (3,400 km2) composite batholith that consists principally of potassic granite but also contains fayalite granite, syenite, and gabbro and associated small plutons and ring complexes (Hutchinson, 1976; Wobus, 1976). Unlike the granites of the two earlier periods of plutonism, the Pikes Peak Granite occurs only in a single batholith, and except for a few dikes no other rocks of this age are exposed in the State. The Pikes Peak batholith was probably emplaced at even shallower levels than the 1,400-m.y. plutons (Hutchinson and Hedge, 1976). Throughout most of the State, no geologic record is preserved for the interval between 1,400 m.y. and l, 015 m.y. The single exception is in extreme northwestern Colorado, where the Uinta Mountain Group extends in from Utah. This thick sequence of quartzite, shale, and conglomerate was apparently deposited between 1,440 and 950 m.y. ago; it is discussed later in this report. An important economic aspect of the Precambrian rocks in Colorado is the role of northeast-trending shear zones in setting the stage for Laramide and younger mineralization of the Colorado mineral belt (Tweto and Sims, 1963). In addition, many kinds of economic mineral occurrences of Precambrian age exist. None of these have thus far been highly productive, but they point to the possibility of the existence of more significant deposits. Noteworthy is the recognition of gahnite, the zinc spinel, as a metamorphic component of gneisses in several localities (Sheridan and Raymond, 1977). The gahnite sometimes occurs with metamorphic base-metal sulfides but also is found in the absence of sulfides, principally in metavolcanic rocks. Numerous Precambrian sulfide deposits have been worked or prospected in the State (Tweto, 1968). In addition to base metals, these deposits contain gold and silver, and some contain molybdenum, tungsten, or nickel. Metamorphic scheelite and powellite occur in calc-silicate rocks in several localities (Tweto, 1960). Rutile occurs in economic amounts (as much as 5 percent) in sillimanitic topaz-quartz gneiss in a lengthy belt in the east-central Front Range (Marsh and Sheridan, 1976). Xenotime and monazite constitute as much as 5 percent by volume of migmatized biotite gneiss in the Central City area of the Front Range (Young and Sims, 1971). Uraninite is disseminated in migmatized biotite gneiss in the west-central Front Range and is of about the same age as nearby granite of the 1,400- m. y.-age group (Young, 1975). Pegmatites valuable forCOLORADO 8 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO 0 100 KILOMETERS 1 __________________i___________________I Figure 3.—Geologic map of the Precambrian of Colorado and southernmost Wyoming. Numbers refer to radiometrically dated 1,670-m.y. plutons shown on figure 4. THE PRECAMBRIAN OF THE ROCKY MOUNTAIN REGION 9 MAP NO. UNIT OR LOCALITY REFERENCES 1 Rawah batholith 1 2 Northern Front Range 2 3 Boulder Creek Granodiorite 2. 3 4 Mount Evans 4 5 Byers Canyon 5 6 Buffalo Pass 6 7 Cross Creek Granite 6 8 Glenwood Canyon 6 9 White River uplift 6 10 Uncompahgre uplift 7 1 1 Black Canyon 8 12 Salida 6 13 Arkansas River 6 14 Needle Mountains 9, 10 AGE 1500 I--- 1600 ---1--- 1700 1--- 1800 m.y. V////A I777777777I r/7777 tZZZZZZZZZZA X////A ZZZZZZZA WS7/A Figure 4.—Ages of approximately 1,670 m.y. plutons in Colorado. Bar indicates range. Localities are shown on figure 3. References are: 1, McCallum and Hedge (1976); 2, Peterman and others (1968); 3, Stem and others (1971); 4, Bryant and Hedge (1978); 5, Izett (1968); 6, Snyder and Hedge (1978); 7, C. E. Hedge, unpub. data, 1979; 8, Hedge and others (1968); 9, Hansen and Peterman (1968); 10, Bickford and others (1969). minerals such as beryl, mica, and spodumene are associated with granites of the 1,400-m.y. group, and beryllium-bearing greisen pipes accompany a late phase of the Pikes Peak batholith. Thorium veins and vanadium-bearing titaniferous magnetite are associated with an alkalic intrusive center in southwestern Colorado that is on the borderline between Precambrian and Cambrian in age (Olson and others, 1977). CRYSTALLINE BASEMENT ROCKS OF NORTHERN UTAH AND IDAHO The crystalline basement rocks of northern Utah and Idaho are poorly understood because of the very limited exposures and the intensity of younger geologic events. (See pi. 1, A-C.) Archean rocks occur as far west as northwestern Utah and south-central Idaho, where they appear in the cores of gneiss domes (Armstrong, 1968; Compton and others, 1977). In northeastern Utah the Uinta Mountain Group was deposited on the Red Creek Quartzite. The Red Creek is dominantly quartzite, but schist and amphibolite are also common. These rocks have been metamorphosed to amphibolite facies. Muscovite from the Red Creek Quartzite has given a Rb-Sr age of 2,400 m.y. (Hansen, 1965), and because this may be a minimum age (the muscovite gave a K-Ar (potassium-argon) age of 1,500 m.y.), the Red Creek should probably be assigned to the Archean. Only K-Ar mineral dates are available for the Precambrian crystalline rocks of the Wasatch Mountains and Antelope Island, in the Great Salt Lake; these are no older than about 1,600 m.y. (Whelan, 1969). However, Pb-isotopic data presented by Stacey and others (1968) for ore leads in this region afford conclusive evidence of the existence of an Archean basement. Unpublished data (C. E. Hedge, J. S. Stacey, and B. R. Bryant, 1980) indicate that the Wasatch Mountains contain Archean gneisses and an l,850-m.y.-old granite. In central Idaho, along the eastern margin of the Idaho batholith, Belt rocks have been metamorphosed to a high grade and tectonically mixed with the pre-Belt basement rocks. Neither the geologic nor the geo-chronologic data are yet adequate to separate the mixed rocks with certainty at many exposures. The pre-Belt Boehls Butte Anorthosite has yielded a U-Pb zircon age of 1,625 m.y., and the Boehls Butte Formation, which the anorthosite intrudes, gave a zircon age of 1,665 m.y. (Reid and others, 1973); these data were interpreted as being minimum ages for these units. Armstrong (1975) concluded that the orthogneiss along the Salmon River is at least 1,500 m.y. old. Clark 1 (1973) reported an age of 1,540 m.y. for orthogneisses10 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO in northern Idaho that are also interpreted as being pre-Belt. POST-1,700 MILLION YEAR SUPRACRUSTAL ROCKS The west edge of the Rocky Mountain region became the western continental margin about 1,700-1,600 m.y. ago. Three sedimentary sequences were deposited westward into probable marine basins underlain by rocks as old as 2,400+ m.y. and as young as 1,700 m.y. The oldest sequence, the Uncompahgre Formation of southwestern Colorado (pi. 1, V) was deposited between 1,600 and 1,450 m.y. ago (Barker, 1969). The erosional remnant of the formation has a relatively small areal extent (fig. 3). Barker (1969) described the rocks as consisting mostly of crossbedded quartzite plus minor amounts of conglomerate, slate, phyllite, and schist that have a total thickness of perhaps 2,450 m. The depositional environment of the formation is not certain. The rocks were metamorphosed and folded before being intruded by granite 1,435 m.y. ago. The next younger sedimentary-rock sequences of the Rocky Mountain region are the Uinta Mountain Group of northeastern Utah and northwestern Colorado (fig. 3) and the Belt Supergroup and equivalents of western Montana, northern Idaho, and northeastern Washington (fig. 1). These and other sedimentary sequences in Alaska, Canada, and Western and Northwestern United States appear to have been deposited in response to tectonic-magmatic events about 1,400-1,500 m.y. ago. This sedimentation cycle is punctuated by igneous activity that produced small amounts of anorogenic granite, basic sills in extension fractures, and minor amounts of lava at about 1,100 m.y.; the cycle terminated about 850 ±50 m.y. ago in most areas. Within the Rocky Mountain region, the southernmost sedimentary sequence of this cycle is the Uinta Mountain Group (pi. 1, A). Rocks of the group are exposed along an east-trending Phanerozoic arch and continue eastward in the subsurface. The rocks are almost unmetamorphosed and undeformed, and they consist principally of about 7,900 m of red to brown slightly feldspathic quartzites, olive-drab shales, and small amounts of red or black shale (Hansen, 1965). They represent fluvial sediments transported from the north and northeast across a strandline, along with nearshore marine and deltaic deposits formed by westward transport in the basin (Wallace and Crittenden, 1969). The north edge of the basin is reasonably well defined and may represent a Precambrian fault that was reactivated in the Phanerozoic, helping to form the prominent west-trending Uinta Mountains; the south edge of the Precambrian basin is eroded and cannot be defined. The age of the Uinta Mountain Group is bracketed between 1,440 and 950 m.y. In northeastern Utah, the basal rocks were deposited unconformably on the Red Creek Quartzite. Muscovite from the quartzite gives a Rb-Sr age of 2,400 m.y. (Hansen, 1965), but K-Ar ages also record another event at about 1,500 m.y. An even younger maximum age is suggested by the undeformed rocks in the subsurface that extend eastward into Colorado almost to the Park Range, where a large pluton of l,440-m.y.-old granite is exposed. Thus it seems unlikely that the deposition of the undeformed Uinta Mountain Group began before 1,440 m.y. ago. A minimum age of 950 m.y. for the group is given by a whole-rock Rb-Sr age determination on the uppermost formation (Red Pine Shale) (Crittenden and Peterman, 1975). The most extensive exposures of sedimentary rocks deposited during the cycle from 1,500 to 850 m.y. ago are of the Belt Supergroup (Purcell Supergroup of Canada) in western Montana, northern Idaho, and eastern Washington, and their probable correlatives in east-central Idaho (pi. 1, B-D). Belt sediments at least 20 km thick accumulated in an epicratonic reentrant presumably connected to the Cordilleran miogeocline on the west (Harrison and others, 1974). The rocks are relatively undeformed except near the edges of the exposed Belt terrane; metamorphism ranges from almost none at the top of the section, to biotite grade at the bottom, to amphibolite grade near plutons of Cretaceous-Tertiary age. Sedimentation within the Belt basin was in four major cycles of marine or marginal marine clastic and carbonate sediments that accumulated without apparent widespread interruptions (fig. 5). Early deposits (Prichard Formation) contain abundant turbidites, and the first cycle was terminated by a black shale (Edmunds, 1973). Subsequent deposits are dominantly red-bed sequences and carbonates. The second cycle (Ravalli Group) is represented by a deltaic sequence prograded from the south (Hrabar, 1971) that interfingers with a probable delta complex built southwest-ward from the Canadian Shield. The third cycle is a carbonate sequence that contains shelf-type carbonates and evidence of periodic hypersaline conditions (Eby, 1977) in the east and northeast. These rocks grade westward into a more clastic carbonate-bearing section that contains a zone of slope breccias and large slumps; this section had a southern source terrane. The fourth cycle (Missoula Group) consists mostly of clastic red-bed sequences prograded into the basin from both the south and the northeast. The middle part of this cycle has been interpreted by Winston (1973) as deposits of braided streams, alluvial fans, and associated shallow-water facies. A general history of Belt events (fig. 5) was pre-THE PRECAMBRIAN OF THE ROCKY MOUNTAIN REGION 11 GEOLOGIC AGE UNCONFORMITIES (m.y.) SEDIMENTARY DEPOSITS AND THICKNESS MAGMATIC EVENTS TECTONIC EVENTS METAMORPHIC EVENTS 700 Windermere System of Canada (6,700 + m) Volcanics Gabbroic sills East Kootenay orogeny Purcell anticlinorium East Kootenay event— biotite-grade regional metamorphism at depth 800 Windermere 900 Upper part of Missoula ? Group (4,000 + m) 1000 1100 1200 1300 1400 McNamara Bonner Shepard Purcell Lava Snowslip Helena-Wallace St. Regis-Spokane Upper part of the lower part of Missoula Group (3,300 m) Lower part of the lower part of Missoula Group (2,000 m) Middle Belt carbonate unit (4,400 m) Ravalli Group (5,600 m) Lower Belt (6,700+ m) ? Purcell Lava; gabbroic sills Coeur d'Alene lead and uranium veins (calculated ages may be too old) Granodiorite at Hellroaring Creek Gabbroic sills Minor folding and tilting along east edge Major change in basin shape. Possible faulting and folding in Coeur d'Alene area Warping to form upper Ravalli basin Minor faulting Coeur d'Alene event (?)— high grade to south, biotite grade in basin Regional metamorphism affecting Prichard near Alberton, Montana 1500 Granitic intrusions, now augen gneisses, in Elk City and Priest River areas, Idaho (pre-Belt?) Elk City event (?) (pre-Belt?) Elk City event (?) (pre-Belt?) 1600 1700 Pre-Belt magmatic and metamorphic events Figure 5.—Estimated ages of some events in the formation of the Belt Supergroup, Montana. Modified from Harrison (1972). Extent of unconformities: solid line, basinwide; long dashed line, local; short dashed line, inferred. Queries indicate uncertainty in either occurrence or age of an event. sented by Harrison (1972) and is only slightly modified as reproduced here. Many of the problems of dating events and rock bodies within this sequence are still unresolved. The maximum age of the Belt Supergroup, as indicated by mica ages from the basement in Montana and Canada, is about 1,700 m.y. In Idaho, rocks as young as 1,500 m.y. may be the basement upon which the Belt was deposited. The minimum age is loosely established by a series of magmatic and tectonic events (the East Kootenay orogeny) ranging in age from about 870 to 725 m.y. Probable correlatives of Belt rocks in east-central Idaho include about 12,000 m of clastic rocks called the Yellowjacket and Hoodoo Formations, the Lemhi Group, and the Swauger Formation. Although the Yellowjacket Formation resembles the Prichard Formation, the lower formations of the Lemhi Group resemble the Ravalli Group, and the Swauger resembles some12 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO rocks of the Missoula Group, the middle 6,000 m only vaguely resemble Belt rocks and no major carbonate unit is present. Ruppel (1975) suggested that although the rocks may be Belt in age, the Lemhi Group and Swauger Formation lie on thrust plates that have been translated to the east perhaps as much as 160 km and therefore represent deposits of the Cordilleran miogeocline now telescoped into juxtaposition with rocks of the Belt basin. Zircon suites from cogenetic plutons that intrude the upper Yellowjacket Formation yield an age of 1,370 ± 10 m.y., which suggests that the Yellowjacket is a time correlative of the Prichard (Evans, 1981). Rocks of the cycle from 1,500 to 850 m.y. ago host a variety of ore deposits of Precambrian age. The unique lead-silver veins of the Coeur d’Alene mining district, Idaho, are on the west end of a zone of crustal weakness that was active as a trough during Belt sedimentation and was the site of some local faulting (Chevillon, 1977). Ores that include a few early uranium-bearing veins were emplaced after tight upright to overturned folding occurred (Hobbs and others, 1965), called by some the Coeur d’Alene event (fig. 5). Although isotopic data have been interpreted to show that the ores are about 1,200 m.y. old (Zartman and Stacey, 1971), no evidence of a major disturbance has been found in the sedimentation record for that time as yet, and a permissive isotopic age of about 850 m.y. cannot yet be excluded. Stratabound ores of two types are known in Belt age rocks. The most famous are Sullivan-type lead-zinc ores that occur in Canada in the lower Belt equivalent (Aldridge Formation). Recent sulfur isotope studies (Campbell and others, 1978) combined with previous geologic studies indicate that the ore was deposited from upwelling metalliferous solutions entering restricted seafloor basins formed by faulting penecontem-poraneous with sedimentation. Such euxinic basins are apparently characteristic of the first cycle of Belt and Yellowjacket sedimentation, and the search for geologic characteristics indicative of penecontemporaneous faulting during deposition has become a major prospecting tool in these units. Copper-silver stratabound ores and occurrences are common in the clastic rocks of the 1,500-850 m.y. ago cycle. The most abundant occurrences have been described by Harrison (1974) as limited to green beds in red-bed sequences and to nonoxidized quartzites of the Belt Supergroup. Although copper deposits were formed during diagenesis, Harrison (1974) suggested that subsequent remobilization from the original sedimentary traps was essential to form ores in permeable strata. Subeconomic occurrences of copper and silver are also known from the reduced (green or black) clastic strata of the Lemhi and Uinta Mountain Groups. The final cycle of the late Proterozoic deposition lasted from about 950 m.y. through the Early Cambrian (about 540 m.y.). The rock sequences are characterized particularly by diamictites and eruptive volcanics as well as marine elastics and carbonates, and they were deposited unconformably on and seaward of the previous rock sequences. Very limited exposures of these rocks occur in the Rocky Mountain region, principally in eastern Washington and southeastern Idaho; they and their equivalents are correlated with the Winder-mere Supergroup of Canada. The rock sequence has been interpreted as representing initial deposits in a rift formed by continental separation that began about 850 m.y. ago (Stewart, 1976). An alternative hypothesis is that these and other late Proterozoic rocks of the previous cycle were deposited on a stable platform (trailing edge) in response to dominantly vertical tectonic movements (Harrison and Reynolds, 1976). TECTONICS Interpretations of Precambrian tectonic activity in the Rocky Mountain region are constrained by the limited and isolated exposures of Precambrian rocks and by the fact that these exposures result from Laramide tectonics that commonly were superimposed on reactivated Precambrian features. In places, even the Laramide tectonics are not yet completely understood. All the Precambrian metamorphic rocks were complexly folded and many show multiple deformations. No regional stress directions are obvious from this folding. Some foldings or refoldings clearly were in response to local stresses around intruding granitic plutons and are not necessarily related to regional patterns. The importance of Precambrian faults and shear zones on subsequent geologic history cannot be overemphasized, and this subject deserves much more work. Many of these Precambrian tectonic elements have now been identified (fig. 6) and for some of them the time and sense of movement are known. Much of the Laramide-age Colorado mineral belt (fig. 6, A) parallels a system of Precambrian shear zones (Tweto and Sims, 1963). These shear zones were active at least from the time of the 1,430-m.y. plutonic event onward, though they could have had an earlier history. Aside from the shear zones, the Precambrian fracture pattern is dominated by faults of north-northwest trend. These faults created a grain that persists to this day in the geology and topography of the mountain province in Colorado. Some of the faults were in existence before the 1,670-m.y. granites were emplaced, and some are occupied by mafic to intermediate dikes of THE PRECAMBRIAN OF THE ROCKY MOUNTAIN REGION 13 Figure 6.—Some Precambrian tectonic features in the Rocky Mountain region. Pattern, exposed Precambrian. A, Shear zone that defined trend of Colorado mineral belt (Tweto and Sims, 1963); B, Precambrian shear zones (Snyder, 1978); C, Shear zone that controls minor Tertiary igneous activity and mineralization (Abbott, 1962); D, Ilse fault zone (Scott and others, 1976); E, Discontinuity in K-Ar ages (Peterman and Hildreth, 1978); F, Shear zone separating Wyoming and Colorado provinces (Hills and others, 1968); G, Hinge line bounding northern margin of Uinta basin (Hansen, 1965); H, Willow Creek fault (Harrison, 1972); I, Hinge line bounding east side of Belt basin (Harrison, 1972); J, Coeur d’Alene trough (Harrison, 1972); K, Faults bounding Stillwater block (Page, 1977).14 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO approximately 1,400 and 1,000 m.y. ages. Many of the faults were repeatedly reactivated in Phanerozoic time. About 1,600 m.y. ago the southern one-third of Wyoming moved vertically relative to the area to the north (Peterman and Hildreth, 1978). Movement on this fault zone (fig. 6, E) was undoubtedly a controlling factor in the deposition and preservation of the early Proterozoic sediments in this region. A shear zone separates the Archean province of Wyoming from the middle Proterozoic rocks of southeastern Wyoming and Colorado (fig. 6, F). Hills and Houston (1979) suggested that the present shear zone marks the location of a collision between an Atlantic-type continental margin and a volcanic arc about 1,700 m.y. ago. The post-1,700 m.y. supracrustal sediments were deposited in basins controlled, at least in part, by vertical movements along faults in the older crust. Although sedimentation in the Rocky Mountain region proceeded more or less continuously for hundreds of millions of years, local events in individual basins apparently were not synchronous (pi. 1). The rapid subsidence at the Uinta basin (Hansen, 1965) seems to require a sharp time hinge line or fault (fig. 6, G). The Willow Creek fault (fig. 6, H) bounded a part of the Belt basin during early deposition of coarse elastics carried northward from the older Precambrian terrane about 1,500 m.y. ago, and subsidence along a basin margin hinge line (fig. 6, I) as well as minor faulting and subsidence of a basinal trough (fig. 6, J) has been discussed by Harrison (1972). The basinal trough not only was active during Belt sedimentation but also was part of a zone of extensive shear and high-angle faults known as the Lewis and Clark line, along which movement has occurred intermittently at least through the Tertiary (Harrison and others, 1974). SUMMARY The earliest events in the Precambrian history of the Rocky Mountain region are ill defined, and much more work is needed to establish a time framework. In southern Montana and probably in northern Wyoming, silicic rocks existed well before 3,000 m.y. ago. Sediments derived from this ancient source seem to have been metamorphosed as early as 3,100 m.y. ago. In southern Montana, this metamorphism was followed by an intense metamorphic and plutonic episode, including emplacement of the Stillwater Complex at about 2,700 m.y. ago. Sedimentation may have occurred between 2,700 and 3,100 m.y. ago. An earlier period (or periods) of regional metamorphism is dated between 2,800 and 3,000 m.y. in the Bighorn Mountains, Laramie Mountains, Granite Moun- tains, and Teton Range. The apparent absence of this event elsewhere may be due to a lack of data. Periods of granitic plutonism in central Wyoming are well defined in the interval of 2,550-2,650 m.y. ago. In the southeastern Wind River Range, the 2,650-m.y. age of the Louis Lake batholith is a minimum age for the economically important greenstone belt of the South Pass region. Present data do not provide an unequivocal age for the greenstone belt, but it may have been metamorphosed approximately 100 m.y. earlier. Gneiss and granite in southern Wyoming have commonly yielded ages 100 to 150 m.y. younger than similar rocks in central Wyoming. Whether these ages indicate a true decrease in age of events to the south or are the result of later metamorphism is uncertain. Three analytically precise ages of 2,360 m.y., 2,420 m.y., and 2,500 m.y. for the same geologic unit, the Baggot Rocks Granite, in southern Wyoming, suggest that several geologic problems have not been fully resolved. At best, two of the three ages are wrong; possibly all have been lowered as a consequence of nearby events during the early and middle Proterozoic. The predominance of metasedimentary rocks in southwestern Montana, the character of the Red Creek Quartzite in northeastern Utah, and the possibly Archean metasedimentary rocks in the Laramie Mountains all suggest the possibility of sedimentation around the margins of the Wyoming province in late Archean time. In any case, at least the southern and eastern margins were the site of thick accumulations of shelf-type sediments in early Proterozoic time. In part contemporaneous with this shelf-type sedimentation on the flank of the Archean craton, the continent in Colorado was being formed by the accumulation of volcanic and sedimentary rocks. Regional metamorphism of these rocks took place 1,700 m.y. ago; they were intruded by granitic rocks at 1,670, 1,400-1,430, and again at 1,015 m.y. ago. 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E., Hedge, C. E., and Braddock, W. A., 1968, Age of Precambrian events in the northeastern Front Range, Colorado: Journal of Geophysical Research, v. 73, p. 2277-2296. Peterman, Z. E., and Hildreth, R. A., 1978, Reconnaissance geology and geochronology of the Precambrian of the Granite Mountains, Wyoming: U.S. Geological Survey Professional Paper 1055, 22 P- Redden, J. A., and Norton, J. J., 1975, Precambrian geology of the Black Hills, in Mineral and water resources of South Dakota: South Dakota Geological Survey Bulletin, v. 12, p. 21-28.THE PRECAMBRIAN OF THE ROCKY MOUNTAIN REGION 17 Reed, J. C., Jr., and Zartman, R. E., 1973, Geochronology of the Precambrian rocks of the Teton Range, Wyoming: Geological Society of America Bulletin, v. 84, p. 561-582. Reid, R. R., McMannis, W. J., and Palmquist, J. C., 1975, Precambrian geology of the North Snowy block, Beartooth Mountains, Montana: Geological Society of America Special Paper 157, 135 P- Reid, R. R., Morrison, D. A., and Greenwood, W. R., 1973, The Clearwater orogenic zone—A relict of the Proterozoic orogeny in central and northern Idaho, in Belt Symposium: Idaho Bureau of Mines and Geology Special Publication, v. 1, p. 10-56. Riley, G. H., 1970, Isotopic discrepancies in zoned pegmatites, Black Hills, South Dakota: Geochimica et Cosmochimica Acta, v. 34, p. 713-725. Roscoe, S. M., 1973, The Huronian Supergroup, a Paleoaphebian succession showing evidence of atmospheric evolution: Geological Association of Canada Special Paper 12, p. 31-48. Ruppel, E. T., 1975, Precambrian Y sedimentary rocks in east-central Idaho: U.S. Geological Survey Professional Paper 889-A, 23 P- Rye, D. M., Doe, B. R., and Delevaux, M. H., 1974, Homestake gold mine, South Dakota—II, Lead isotopes, mineralization ages and source of lead in ores of the northern Black Hills: Economic Geology, v. 69, p. 814-822. Scott, G. R., Taylor, R. B., Epis, R. C., and Wobus, R. A., 1976, Geologic map of the Pueblo l°x2° quadrangle, south-central Colorado: U.S. Geological Survey Miscellaneous Field Studies Map MF-775. Sheridan, D. M., and Raymond, W. H., 1977, Preliminary data on some Precambrian deposits of zinc-copper-lead sulfides and zinc spinel (gahnite) in Colorado: U.S. Geological Survey Open-File Report 77-607, 27 p. Silver, L. T., and Barker, F., 1968, Geochronology of Precambrian rocks in the Needle Mountains, southwestern Colorado, part 1, U-Pb zircon results [abs.]: Geological Society of America Special Paper 115, p. 204. Snyder, G. L., 1980, Map of Precambrian and adjacent Phanerozoic rocks of the Hartville Uplift, Goshen, Niobrara, and Platte Counties, Wyoming: U.S. Geological Survey Open-File Report 80-779, 11 p., 2 sheets. Snyder, G. L., and Hedge, C. E., 1978, Intrusive rocks northeast of Steamboat Springs, Park Range, Colorado: U.S. Geological Survey Professional Paper 1041, 42 p. Snyder, G. L., and Peterman, Z. E., 1982, Precambrian geology and geochronology of the Hartville uplift, Wyoming: 1982 Archean Geochemistry Field Conference, Part 1, Guide to Field Trips, p. 64-94. Spencer, A. C., 1904, The copper deposits of the Encampment District, Wyoming: U.S. Geological Survey Professional Paper 25, 107 p. Stacey, J. S., Zartman, R. E., and Nkomo, I. T., 1968, A lead isotope study of galenas and selected feldspars from mining districts in Utah: Economic Geology, v. 63, p. 796-814. Stern, T. W., Phair, George, and Newell, M. F., 1971, Boulder Creek Batholith, Colorado, part II, Isotopic age of emplacement and morphology of zircon: Geological Society of America Bulletin, v. 82, p. 1615-1634. Stewart, J. H., 1976, Late Precambrian evolution of North America—Plate tectonics implication: Geology, v. 4, p. 11-15. Stueber, A. M., and Heimlich, R. A., 1977, Rb-Sr isochron age of the Precambrian basement complex, Big Horn Mountains, Wyoming: Geological Society of America Bulletin, v. 88, p. 441-444. Subbarayudu, G. V., Hills, F. A., and Zartman, R. E., 1975, Age and Sr isotopic evidence for the origin of the Laramie anorthosite-syenite complex, Laramie Range, Wyoming: Geological Society of America Abstracts with Programs, v. 7, p. 1287. Tweto, Ogden, 1960, Scheelite in the Precambrian gneisses of Colorado: Economic Geology, v. 55, p. 1406-1428. ------1968, Geologic setting and interrelationships of mineral deposits in the mountain province of Colorado and south-central Wyoming, in Ore deposits of the United States, 1933-1967, Volume 1: American Institute of Mining, Metallurgical and Petroleum Engineers, p. 551-588. ------1977, Nomenclature of Precambrian rocks in Colorado: U.S. Geological Survey Bulletin 1422-D, 22 p. ------1979, Geologic map of Colorado: U.S. Geological Survey, Special Geologic Map, scale 1:500,000. Tweto, Ogden, and Sims, P. K., 1963, Precambrian ancestry of the Colorado mineral belt: Geological Society of America Bulletin, v. 74, p. 991-1014. Wallace, C. A., and Crittenden, M. D., Jr., 1969, The stratigraphy, depositional environment, and correlation of the Precambrian Uinta Mountain Group, western Uinta Mountains, Utah, in Lindsay, J. B., ed., Geologic guidebook of the Uinta Mountains, Utah’s maverick range: Intermountain Association of Geologists 16th Annual Field Conference, p. 126-141. Whelan, J. F., 1969, Geochronology of some Utah rocks, in Guidebook of northern Utah: Utah Geological and Mineralogical Survey Bulletin 82, p. 97-104. Winston, Donald, 1973, The Bonner Formation as a Late Precambrian pediplain: Northwest Geology, v. 2, p. 53-58. Wobus, R. A., 1976, New data on potassic and sodic plutons of the Pikes Peak batholith, central Colorado: Colorado School of Mines Professional Contributions 8, p. 57-67. Young, E. V., 1975, An occurrence of disseminated uraninite in Wheeler Basin, Grand County, Colorado: U.S. Geological Survey Journal of Research, v. 3, no. 3, p. 305-311. Young, E. V., and Sims, P. K., 1961, Petrography and origin of xenotime and monazite concentrations, Central City district, Colorado: U.S. Geological Survey Bulletin 1032-F, p. 273-299. Zartman, R. E., Norton, J. J., and Sterns, T. W., 1964, Ancient granite gneiss in the Black Hills, South Dakota: Science, v. 145, p. 479-481. Zartman, R. E., and Stacey, J. S., 1971, Lead isotopes and mineralization ages in the Belt Supergroup rocks, northwestern Montana and northern Idaho: Economic Geology, v. 66, p. 849-860. * U.S. Government Printing Office 1986: 676-047/26030in ^ Z days '^'^Correlation Chart for Precambrian Rocks of the Eastern United States GEOLOGICAL SURVEY PROFESSIONAL PAPER 124 1-E NOV 21193 Correlation Chart for Precambrian Rocks of the Eastern United States By D. W. RANKIN, T. W. STERN, JAMES McLELLAND, R. E. ZARTMAN, and A. L. ODOM CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO Edited by J. E. HARRISON and Z. E. PETERMAN GEOLOGICAL SURVEY PROFESSIONAL PAPER 12 4 1-E Lithology, distribution, correlation, and isotope ages of exposed Precambrian rocks in Eastern United States UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1983UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging-in Publication Data Main entry under title: Correlation chart for Precambrian rocks of the eastern United States. (Correlation of Precambrian rocks of the United States & Mexico) (Geological Survey professional paper ; 1241-E) Bibliography: p. Supt. of Docs, no.: 119.16:1241-E 1. Geology, Stratigraphic-Precambrian. 2. Stratigraphic correlation—United States. I. Rankin, Douglas W. II. Series. III. Series: Geological Survey professional paper ; 1241-E. QE653.C68 551.7T0973 81-607917 AACR2 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402CONTENTS Page Abstract___________________________________________________ El Introduction________________________________________________ 1 Design of the correlation chart_____________________________ 2 Geologic chronometric data__________________________________ 3 Adirondacks_____________________________________________ 3 Taconide zone___________________________________________ 4 Blue-Green-Long axis____________________________________ 4 Older Precambrian rocks_____________________________ 4 Stratified rocks along the Blue-Green-Long axis___ 6 West limb_______________________________________ 6 East limb_______________________________________ 7 Intrusive rocks younger than 1 b.y._________________ 7 Allochthons of eastern rocks____________________________ 7 Inner Piedmont and Smith River allochthons__________ 7 Maryland and northern Virginia Piedmont allochthons_______________________________________ 8 Western gneiss domes____________________________________ 8 Pine Mountain belt________________________________ 8 Page Geologic chronometric data—Continued Western gneiss domes—Continued Kings Mountain belt________________________________ E9 Sauratown Mountains anticlinorium___________________ 9 Baltimore Gneiss domes______________________________ 9 Manhattan Prong_____________________________________ 9 Central belt____________________________________________ 9 Charlotte belt______________________________________ 10 Bronson Hill-Boundary Mountain anticlinorium______ 10 Merrimack synclinorium______________________________ 10 Avalonian zone__________________________________________ 10 Carolina volcanic slate belt________________________ 10 Southeastern New England____________________________ 11 Southeastern metamorphic belt_______________________ 12 Mineral resources__________________________________________ 13 Acknowledgments_____________________________________________ 13 References cited____________________________________________ 13 ILLUSTRATIONS Page Plate 1. Correlation chart for Precambrian rocks of Eastern United States_________________________________________________ In pocket 2. Map of terranes of Precambrian and selected Cambrian rocks of Eastern United States and adjacent Canada--------- In pocket TABLE Table 1. Ages of rocks from the Hudson Highlands Page E5 illCORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO CORRELATION CHART FOR PRECAMBRIAN ROCKS OF THE EASTERN UNITED STATES By D. W. Rankin, T. W. Stern, James McLelland1, R. E. Zartman, and A. L. Odom2 ABSTRACT In the Eastern United States, Precambrian rocks are exposed in the Adirondack massif and in the Appalachian orogen. Rocks dated at l, 300-1,000 m.y. occur as outliers of the Grenville province of Canada. These rocks constitute a western basement to the Appalachian orogen and appear in the Adirondack massif, in anticlinoria along the Blue-Green-Long axis, and in gneiss domes farther east. The Chain Lakes massif in Maine, about 1,500 m.y. old, may represent a different block of continental crust. Rifting of the Grenville continental mass, accompanied by anorogenic igneous activity, began about 820 m.y. ago. An eastern basement (750-650 m.y. old) has been identified in the Avalonian zone of the Appalachian orogen in southeastern New England. It is intruded by calc-alkaline granitic rocks roughly 600-640 m. y. old that are nonconformably overlain by Lower and Middle Cambrian strata. Extensive Precambrian stratified rocks (mostly younger than 610 m.y. but possibly including rocks as old as 725 m.y.) crop out in the Carolina volcanic slate belt and probably in the higher grade metamorphic belt east of that. These rocks include large volumes of felsic volcanic rocks and probably were deposited on a continental crust. The eastern terranes were sutured to ancestral North America during the Appalachian orogenic events that closed the Iapetus Ocean basin. The location of the suture is uncertain, but it probably lies just east of the gneiss domes containing rocks older than 1 b.y. INTRODUCTION Precambrian rocks are exposed in the Adirondack Mountains, N.Y., in the core of a large dome flanked by lower Paleozoic sedimentary rocks. These high-grade metamorphic and intrusive rocks are contiguous with similar rocks of the Grenville structural province in Canada. The Eastern United States is dominated by the Paleozoic Appalachian orogen, the product of collisional events between at least two continental masses. Differences between the Cambrian fauna on opposite sides of the orogen are important evidence for the former existence of an ocean basin called Iapetus, between the two flanks. These Cambrian faunal realms are referred to as Pacific or Ollenelus on the west and as Acado-Baltic or Paradoxides on the east (Theokritoff, 1968; Wilson, 1969). Oceanic crust is preserved as ophiolites within the lower Paleozoic rocks. Rocks stratigraphically beneath Lower Cambrian strata are recognizable on each flank of the orogen. A 1 Colgate University, Hamilton, NY 13346 2 Florida State University, Tallahassee, FL 32306 pre-Appalachian crystalline basement consists of rocks metamorphosed by, or formed during, pre-Appalachian orogenic events. In the Adirondack massif and along the Blue-Green-Long axis, these rocks have isotopic ages typically ranging from 1,250 to 1,000 m.y. old. Along the eastern margin of the exposed Appalachian orogen in southeastern New England, the crystalline basement is dated as 750-650 m.y. old. An eastern basement has not been identified with certainty south of the Potomac River. In places along the western margin of the Appalachian metamorphic belt (the Blue-Green-Long axis), a thick-to-thin sequence of stratified rocks lies uncon-formably on Precambrian crystalline basement and is overlain conformably by fossiliferous Lower Cambrian strata. These upper Precambrian and overlying Cambrian stratified rocks are interpreted as marking the ancient eastern continental margin of North America and recording the breakup of a larger continental mass by rifting and the formation of the Iapetus Ocean basin (Rankin, 1975). Prominent bends in the structural trends of the Appalachian orogen are thought to be inherited from the irregularities developed at the time of this late Precambrian continental breakup (Rodgers, 1975). Because upper Precambrian or Eocambrian rhyolites are restricted to three bends convex toward the craton, Rankin (1976) suggested that the bends developed at triple junctions over hot spots. Thomas (1977) amplified Rodgers’ (1975) suggestion that the irregularities originated as a rift broken by transform faults. He called the bends that were convex toward the craton reentrants, rather than salients, and the bends that were concave toward the craton promontories, rather than recesses. As a framework in which to discuss the Precambrian rocks, we accept that a rifting event in late Precambrian-early Paleozoic time created an ocean basin (Iapetus) in eastern North America. The ocean basin probably developed to the east of a series of early ensialic rifts, much as the present Atlantic opened to the east of the earlier ensialic Late Triassic-Early Jurassic rifts still exposed in eastern North America. The Appalachian orogen was formed by the closing of the Iapetus Ocean basin and the subsequent continent-continent collision. The ElE2 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO continental mass that collided with the North American craton, however, may not have been the same continental mass that separated during the formation of the Iapetus Ocean. The location of the suture between the continental masses is one of the unresolved problems of Appalachian geology. The Adirondack massif is entirely west of the area affected by Appalachian (Paleozoic) metamorphic events. Potassium-argon mica ages from the Adirondacks reflect uplift and cooling following metamorphic events > 1 b.y. ago. The metamorphic and deformational history of the Appalachian orogen is complex, commonly polyphase. The grade of Paleozoic metamorphism increases east-wardly into the Appalachian orogen from little-recrystallized rocks of the Valley and Ridge belt and western Taconide zone to grades as high as sillimanite along the Blue-Green-Long axis (see Morgan, 1972; Zen, 1974). This westernmost Paleozoic metamorphism is of Ordovician age (about 430-460 m.y. ago) and is related to the Taconic orogeny. In New England, the Taconic metamorphism is overprinted to the east by a later Acadian event (about 380-400 m.y. ago), and rocks as young as Early Devonian are metamorphosed to grades as high as sillimanite-potassium feldspar. Because fossiliferous rocks younger than Ordovician have not been identified in the crystalline Appalachians south of the New Jersey Highlands, the presence of an Acadian event is less easy to identify in the central and southern Appalachians. Nonetheless, polyphase metamorphism and isotopic ages suggest that a younger event roughly coeval with the Acadian also occurred in the Blue Ridge, Inner Piedmont, and Charlotte belt (see Butler, 1972; 1973). The area comprising Rhode Island, eastern Connecticut, and southeastern Massachusetts forms a distinctive block separated from the rest of New England by a major fault system. This block, which contains the Acado-Baltic faunal province, shows no conclusive evidence of either the Taconic or Acadian metamorphic events. In southwestern Rhode Island, rocks as young as Pennsylvanian have been buried and deformed at stau-rolite and kyanite grade before crystallization of sillimanite by contact metamorphism adjacent to the 276-m.y.-old Narragansett Pier Granite (Grew and Day, 1972). The metamorphic history of the eastern part of the southern Appalachians is not well understood. The grade of Paleozoic metamorphism decreases eastward through the Carolina volcanic slate belt, suggesting that the axis of the orogen (the suture) lies to the west. Yet a terrane of medium- to high-grade metamorphic and plutonic rocks, the southeastern metamorphic belt, crops out to the east adjacent to the Coastal Plain overlap. Several postorogenic plutons in and adjacent to this belt from Virginia to Georgia yield ages of about 300 m.y. (Fullagar, 1971; Secor and Snoke, 1978). On the other hand, we cannot rule out that parts of the southeastern metamorphic belt represent a higher grade ter- rane older than the Carolina volcanic slate belt; that is, an eastern basement. Finally, the Coastal Plain of southwesternmost Georgia and northern Florida is underlain by nonfolded and nonmetamorphosed clastic rocks containing fossils ranging in age from Early Ordovician to probably Middle Devonian (Rodgers, 1970). These rocks have not been deformed by Appalachian orogenic events, and they contain a pelecypod fauna closely resembling that of central Bohemia and Poland but also with similarities to that of Nova Scotia, North Africa, and South America (Pojeta and others, 1976). Samples of crystalline rocks recovered from bore holes in Florida have yielded ages as old as 520 m.y. (Bass, 1969). These rocks, which may extend downward into the Precambrian, are not discussed further. At least for that part of the Appalachian orogenic belt in which billion-year-old basement has been identified, the direction of tectonic transport was to the west or northwest. This Paleozoic transport may further complicate interpretation of Precambrian geology. Recent work by the Consortium for Continental Reflection Profiling (COCORP) suggests that the Blue Ridge and at least part of the Piedmont have been thrust westward at least 150 km in northeastern Georgia (Cook and others, 1979). DESIGN OF THE CORRELATION CHART The Appalachian orogen is characterized by subparallel relatively narrow belts differing in rock type, structure, and physiography. Some, but not all, of these belts are recognizable for the entire length of the orogen from Alabama and Georgia to Newfoundland. The correlation chart (pi. I)3 is organized as a traverse across these belts from the craton in the Adirondack massif to the opposite side of the orogen in the subsurface of Florida. These belts reflect the previously outlined plate-tectonic history; that is, the opening and closing of the Iapetus Ocean. In constructing the chart, we have sought to consider all pertinent isotopic ages published through August 1979 and some unpublished ages. Ages shown in italics in the text (all ages on the chart) were calculated using the constants and abundances given in the introductory chapter of thisreport(Harrison and Peterman, in press). The recalculated ages are given to the nearest million years but do not necessarily imply that precision. The ages that appear opposite the time-rock columns on the chart are only Rb-Sr whole-rock isochrons or the upper intercept of U-Pb discordia curves. In general, the Rb-Sr whole-rock-isochron ages are given only for intrusive and extrusive igneous rocks or their metamorphic equivalents. If the same rock body has been dated by more than one method, both ages are indicated. This “Synthesized stratigraphic columns to which the isotopic ages are tied are identified by circled capital letters on pi. 1. Related stratigraphic columns are indicated by subscripts. The generalized geographic locality of every constructed or synthesized stratigraphic column is shown on pi. 2 but because of space constraints not all are shown on pi. 1.CORRELATION CHART FOR PRECAMBRIAN ROCKS OF THE EASTERN UNITED STATES E3 data set is the first attempt to portray all available ages using a fixed set of decay constants and abundances. We hope that the reader will recognize our intent to provide a consistent data set rather than immutable ages. From a review of papers published by many authors over a considerable period of time, it is difficult to evaluate the analytical errors. Typically, a 2- to 3-percent uncertainty at the 95-percent confidence level can be expected for modern determinations. If the scatter of the data or the authors’ assessment suggests an uncertainty of greater than 6 percent, the age is not given on the chart. For these determinations and for mineral ages and ^Pb^Pb ages (hereafter referred to as Pb-Pb ages), the position of the footnote number on the chart indicates the published age on the vertical axis (see pi. 1). Potassium-argon mica ages were used only for some pegmatites in the Adirondacks. We have used the subdivisions of the Cambrian suggested by the Holmes Symposium (Geological Society of London, 1964), with the base of the Cambrian at 570 m.y. The correlation chart, therefore, includes at least the lower part of Cambrian sections that are stratigraphically tied to Precambrian rocks. GEOLOGIC-CHRONOMETRIC DATA ADIRONDACKS The Adirondack massif is a dome of multiply deformed Precambrian rocks that were regionally metamorphosed about 1 b.y. ago during the Grenvillian orogeny. A northeast-trending mylonite belt separates granulite facies rocks of the Adirondack Highlands (A2) from the dominantly amphibolite facies rocks of the northwestern Lowlands (Aj) (pi. 2). The Highlands and Lowlands consist of similar rocks: (1) a basal sequence of principally charnockitic quartzo-feldspathic gneisses, (2) several sequences of stratified units including quartzites, marbles, calc-silicates, and quartzofeldspathic gneisses, (3) at least two thick and commonly massive units of quartzofeldspathic gneisses within the stratified sequences, and (4) metaigneous rocks of the anorthosite-charnockite suite and crosscutting pegmatites, granitic rocks, and olivine meta-gabbros. Group 2 dominates in the Lowlands, whereas groups 1, 3, and 4 dominate in the Highlands. Isachsen and others (1975) suggested that the anorthosite intruded the stratified sequence. Walton and de Waard (1963) envisaged the anorthosites as part of a pre-“Grenvillian” basement upon which the stratified rocks were unconformably deposited. De Waard (1970) and de Waard and Romey (1969) presented evidence for a comagmatic evolution of all the rocks in the anorthosite-charnockite suite, although Buddington (1972) disputed these conclusions and favored separate intrusions of magma. Isachsen and others (1975) favor a crustal anatexis origin for the charnockitic rocks. Silver (1969) reported a U-Pb zircon age of 1,111 m.y. for charnockites of the Ticonderoga dome and other localities. Textural evidence suggests that this age dates the time of crystallization of these charnockites and closely associated anorthosites. A slightly younger age of 1,070 m.y. obtained directly on Highland anorthosite and norite is interpreted to date the peak of granulite facies metamorphism in the region. Hills and Gast (1964) determined a Rb-Sr whole-rock-isochron age of 1,071 m.y. for charnockites in the Lake George pluton, which may be either an intrusive age or a metamorphic age of strontium isotopic homogenization. Nearby paragneisses have similar, albeit poorly constrained, ages suggesting that such homogenization did occur on a large scale, at least in the eastern Adirondacks. Although Spooner and Fairbairn (1970) reported a Rb-Sr whole-rock-isochron age of 1,39\±1M m.y. on charnockites in the Snowy Mountain dome, Hills and Isachsen (1975) obtained an age of 1,173 m.y. on samples from the same locality. Bickford and Turner (1971) obtained Rb-Sr whole-rock-isochron ages of 1,095 m.y. and 1,120 m.y. for rocks believed to be of anatectic origin from two granitic domes. These ages for the period of anatexis are indistinguishable from the age of metamorphism determined by Silver (1969). A paragneiss also dated by Bickford and Turner yielded an age of 1,18.4 m.y., which is only slightly older than the time of anatexis in this region. In general, investigators have found little geochronologic evidence supporting the concept of an older basement complex in the Adirondacks. Several K-Ar ages of micas obtained from pegmatites are generally younger, in the range of 900 m.y.-l,000 m.y., and reflect cooling of this terrane following the major metamorphic events. In southern Ontario, supracrustal rocks of the Grenville Supergroup lie unconformably on the 1,400- to l,500-m.y.-old Algonquin batholith (Lumbers, 1979; written commun., 1979). Volcanic rocks within the carbonate sequence of the supracrustal rocks are dated at about 1,300 m.y. (Silver and Lumbers, 1966) and are related to intrusive rocks dated at about 1,280 m.y. Thus, evidence from Canada suggests that the Adirondack stratified rocks, at least those directly across the Frontenac axis in the Lowlands, accumulated 1,400-1,250 m.y. ago. The Adirondack rocks are deformed by four major foldsets whose mutual interference determines outcrop patterns over large areas. High-grade metamorphism appears to have continued throughout Fi and F2 folding, at pressures corresponding to a depth of burial of 25 km. Because seismic studies by Katz (1955) indicate that, at present, the M-discontinuity is at a depth of 36 km, a crustal thickness of about 60 km for the Adirondacks is implied at the time of the orogenic event 1.0-1.1 b.y. ago. McLelland and Isachsen (1980) discussed two models forE4 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO generating this double crustal thickness and the structural framework of the Adirondacks. Continental col-lison and some degree of continental underthrusting toward the northwest are involved in both models. They suggested that the New York-Alabama geophysical lineament of King and Zietz (1978) and the, as yet, undefined northeastern continuation may be the suture resulting from this continental collision (pi. 2). If that interpretation is correct, the billion-year-old suture is roughly parallel to Appalachian structures. In fact, we cannot rule out, at present, the possibility that the billion-year-old suture was even farther southeast along the same line now represented by the Taconian suture. TACONIDE ZONE The east-derived Taconic allochthon (B) of eastern New York and western New England consists of at least six structural slices that overlap eastward so that the highest structural level is at the eastern edge of the composite allochthon (Zen, 1967; Ratcliffe and others, 1975). Fossils from the allochthon range in age from Early Cambrian to Middle Ordovician. No Precambrian rocks have been dated isotopically, but a thick section of green and purple slate, graywacke, conglomerate, and minor quartzite (for example, the Nassau Formation) lies conformably beneath the West Castleton Formation, which contains Early Cambrian fossils. The Nassau is of inferred late Precambrian age and makes up a large part of the structurally higher slices. The Rensselaer Graywacke Member of the Nassau has been interpreted as a graben facies deposited during the initial opening of the Iapetus Ocean basin (Bird, 1975). BLUE-GREEN-LONG AXIS OLDER PRECAMBRIAN ROCKS The older Precambrian rocks of the axis from the Reading Prong north to the Green Mountains more closely resemble the rocks of the Adirondack massif than do those farther south. These northern massifs include paragneiss, marble, calc-silicate gneiss, hyper-sthene leucogneiss, and syenitic and granitic gneiss with relatively minor clearly identifiable intrusive rocks. South of Pennsylvania, intrusive orogenic granitic rocks (quartz monzonite to diorite) predominate over paragneiss. Along the western margin of the Blue Ridge where the Paleozoic metamorphic overprint is relatively minor, the older basement is comprised of both paragneiss and orthogneiss. Zircons from samples of paragneiss at Pardee Point, Tenn., and Deyton Bend, N.C. (pi. 2, Dla), yield a discordia intercept age of 1,280 m.y. (Davis and others, 1962). Fullagar and Odom (1973) obtained a Rb-Sr whole-rock-isochron age of 1,225 m.y. for layered biotite-muscovite gneiss (Paleozoic amphibolite facies terrane) from central Ashe County, N.C. (Ji). Although the older Precambrian orthogneisses constitute large masses along much of the Blue Ridge anti-clinorium, toward the southwestern end they crop out only in the cores of smaller second-order folds within terrane of younger(?) layered gneiss and schist. The southwesternmost known areas of older Precambrian rocks are two bodies of foliated granite near Carters-ville, northwestern Georgia (C4), which dating confirms as being at least 1 b.y. old. Farther northwest in terrane of higher grade Paleozoic metamorphism, Kish and others (1975) report a 1.1-b.y. Rb-Sr whole-rock-isochron age from the augen gneiss in the core of the Bryson City dome (C3). Above the Fries-Hayesville fault in northeastern Georgia, the Wiley (augen) Gneiss dated at about 1,190 m.y. old crops out in the cores of refolded folds around the Tallulah Falls dome (Hatcher, 1976) (Ii). The Toxaway Gneiss in the core of the Toxaway dome (I2) has a Rb-Sr whole-rock-isochron age of about 1,190 m.y. (Fullagar and others, 1979). Basement plutonic rocks in northwestern North Carolina were informally named the Elk Park plutonic group by Rankin and others (1973).4 The protoliths of this orogenic calc-alkaline suite were fine-grained to coarsely porphyritic diorites to quartz monzonites. Stratigraphic names in common usage for these rocks include the Cranberry Gneiss (Di, Ji) of the Blue Ridge thrust sheet in northwestern North Carolina and the Blowing Rock (augen) and Wilson Creek Gneisses (D2) within the Grandfather Mountain window. The Max Patch Granite (C3) of the Great Smoky Mountains is probably correlative. The name Grayson Granodiorite Gneiss (DO was retained by Fullagar and Odom (1973) for similar rocks in Grayson, County, Va., although Rankin and others (1972) preferred to extend the name Cranberry Gneiss into that area. Davis and others (1962) published zircon analyses from three samples of rocks of the Elk Park Plutonic Suite, and four additional samples of the unit not previously reported were collected by Rankin and analyzed by Stern. Although the seven rocks were collected from localities as far apart as 150 km—from the Blue Ridge thrust sheet, as well as the Grandfather Mountain window—the geologic interpretation is that the rocks are correlative and yield a pooled discordia intercept age of 1,079 m.y. Fullagar and Odom (1973) published three Rb-Sr whole-rock-isochron ages for older plutonic rocks in northwestern North Carolina and vicinity. (1) A flaser gneiss phase of the Cranberry Gneiss from western Watauga County, N.C., and eastern Johnson County, Tenn., yielded an age of 1,0U1 m.y. (2) The Blowing Rock Gneiss yielded an age of 1,005 m.y. (D2). (3) The Grayson Granodiorite Gneiss in the Blue Ridge thrust sheet in Grayson County, Va. (DO, yielded an age of 1,U9 m.y. and may be somewhat older than the other units. The Elk Park is herein adopted as a formal term and designated Elk Park Plutonic Suite.CORRELATION CHART FOR PRECAMBRIAN The next large area of older Precambrian basement to the northeast forms the core of the Virginia Blue Ridge from Floyd County, Va., to near Frederick, Md., a distance of about 380 km (D3, J2). No unambiguous para-gneiss unit has been identified in this part of the Blue Ridge anticlinorium. The assemblage of plutonic rocks, collectively called the Virginia Blue Ridge Complex, is much like those of the North Carolina Blue Ridge except that charnockitic rocks are more abundant. Tilton and others (1960) reported a Pb-Pb age of 1,130 m.y. for zircons from the hypersthene granodiorite at Mary’s Rock Tunnel in Shenandoah National Park. Lukert and others (1977) gave unspecified zircon ages for rock units toward the north end of the anticlinorium core. The next Precambrian massif to the northeast is the Reading Prong-Hudson Highlands. The southwestern end of the massif in Pennsylvania (Ej) is allochthonous (Drake, 1969). The northeast end, in New York and Connecticut (E2), is probably parautochthonous (Harwood and Zietz, 1974). This latter terrane can be broken down further into the western and the eastern (Hudson) Highlands divided by the Ramapo-Canopus fault zone (Hall and others, 1975). The effects of Paleozoic deformation and metamorphism are much more intense in the eastern Highlands (Long and Kulp, 1962; Dallmeyer and Sutter, 1976). The Reading Prong and western Highlands consist mostly of high grade quartzofeldspathic metasedimen-tary and metavolcanic rocks interlayered with smaller amounts of amphibolite and marble and associated with sodic granitic rocks, hornblende granite, and alaskite (Drake, 1969). Within the Reading Prong, Long and others (1959) obtained Pb-Pb ages on uraninite from a marble near Phillipsburg, N.J., and monazite from the Losee Gneiss near Chester, N.J., of 898 and 883 m.y., respectively. Many Precambrian rocks have been dated from the Hudson Highlands, and these results are summarized in table 1. Rocks of the Housatonic Highlands lying strati-graphically beneath the Poughquag Quartzite (Hall and others, 1975) in western Connecticut have not been ROCKS OF THE EASTERN UNITED STATES E5 dated, but they are lithologically similar to those of the Berkshire massif and presumably of similar age. The older Precambrian rocks of the Berkshire massif (F and K) are largely quartz and feldspar-rich graphitic and nongraphitic metasedimentary rocks and lesser amounts of felsic and mafic metavolcanic rock and one distinctive calc-silicate unit (Ratcliffe and Zartman, 1976). A distinctive blue-quartz-bearing graphitic gneiss and biotite gneiss, the Washington Gneiss, is intruded by coarsely blastoporphyritic Tyringham Gneiss in broadly concordant sills. The Berkshire massif is allochthonous and was transported at least 21 km from the east at the time of Taconic metamorphism. Uranium-lead ages for zircons from the Washington Gneiss and Tyringham Gneiss are only slightly discordant, and the intercepts of discordia lines are 1,022 and 1,060 m.y. Ratcliffe and Zartman (1976) favored the interpretation that either the Tyringham Gneiss was intruded during the same dynamothermal event that reset the Washington Gneiss zircons or that the zircons from both the Tyringham and Washington Gneisses record a meta-morphic age. Mose (1975; written commun., 1980) determined a Rb-Sr whole-rock-isochron age of 1,01+5 ± 128 m.y. for the Tyringham Gneiss. The older Precambrian rocks of the Green Mountain massif (G and L) were collectively called the Mount Holly Complex by Doll and others (1961). The complex consists largely of fine- to medium-grained biotite gneiss, locally muscovitic, massive and granitoid in some areas, and compositionally layered in others. Amphibolite, hornblende gneiss, mica schist, quartzite, calc-silicate granulite, and marble are present, as well as numerous bodies of pegmatite and foliated granitic rock. Except for a Rb-Sr age of 1,01+7 m.y. on muscovite from a pegmatite at Buttermilk Falls (Naylor, 1975), the only pertinent age determinations are lead-alpha determinations (Faul and others, 1963). STRATIFIED ROCKS ALONG THE BLUE-GREEN-LONG AXIS At placed along the western flank of the Blue-Green-Long axis, thick sections of stratified rocks are present Table 1.—Ages of rocks from the Hudson Highlands Number of Reference Recalculated System samples (footnote number age (m.y.) used if known from pi. 1) Rock type West of or along the Ramapo-Canopus fault zone 9 IS+90 Rb-Sr 7 17 Partial melt from paragneiss of the Canada Hill type 996 + 91+ Rb-Sr 4 55 Late tectonic alaskite 1,106 + 90 Rb-Sr 12 55 Hornblende-granite gneiss at Bear Mountain 1.068 + AO Rb-Sr 6 41 Canopus diorite-monzonite pluton 1,11+7 + 86 Rb-Sr 7 17 Paragneiss at Bear Mountain 1,139 + 50 Rb-Sr 13 55 Metavolcanics west of Bear Mountain 1.11.9+15 U-Pb 2 50 Storm King Granite and Canada Hill Granite Gneiss East of Ramapo-Canopus fault zone 1,281 + 196 Rb-Sr 4 55 Reservoir Granite Gneiss, J series 1,225 + 60 Rb-Sr 10 55 Reservoir Granite Gneiss, R seriesE6 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO between the fossiliferous Lower Cambrian rocks and the 1.0- to 1.3-b.y.-old basement. Only a few of the stratified rocks have been dated isotopically. The available time interval for formation of many of the stratified rocks is about 500 m.y. Fossils have not been identified on the east flank, so that the younger age limit, other than by very long distance correlation, is really established only by the age of the Paleozoic metamorphism or intrusive rocks. WEST LIMB The Talladega Group (Ci) is a thick section of weakly metamorphosed mostly fine-grained clastic rocks that crop out in Alabama and Georgia southwest of Carters-ville, Ga. (Alabama Geological Society, 1973). The group is coextensive with the Talladega block, bordered on the northwest by sedimentary rocks of the Valley and Ridge province and on the southeast by the metamorphosed stratified rocks of the crystalline Appalachians. The block appears to be bounded in Alabama on both sides by northwest-directed thrust faults (Tull, 1978). Most of the Talladega terrane is unfossiliferous, although the Jemison Chert at the extreme southwestern end of the block contains Early Devonian fossils. The relation of this unit to the rest of the Talladega Group has not been established. One traditional interpretation is that these rocks are, at least in part, correlative with the Ocoee Supergroup (see Hadley, 1970). The Ocoee Supergroup is a great mass of volcanic-free clastic sedimentary and metasedimentary rocks that crop out over a large area from Cartersville, Ga., northeastward along the Tennessee-North Carolina border to the Nolichucky River gorge, a distance of about 290 km. The supergroup extends across strike about 100 km and is present in several major tectonic units separated by thrust faults. The total thickness of the Ocoee has been estimated as 12 km (Hadley, 1970), and as much as 7 km of section are present in a single thrust sheet (Rodgers, 1972). The great thickness of the Ocoee and the evidence for rapid deposition suggest accumulation in one or more large block-faulted basins. The Ocoee Supergroup is considered to be of Precambrian age although no pertinent isotopic ages are available from it. Along the northwest edge of the Great Smoky Mountains (C2J, the Sandsuck Formation, defined as the upper unit of the Walden Creek Group, is overlain, with no obvious discordance, by the basal Cochran Formation of the Chilhowee Group.5 Early Cambrian ostracods occur in the upper part of the Chilhowee Group (Murray Shale) in the foothills belt of the Smokies. 'Note added in press. Knoll and Keller (1979) report morphologically distinct and stratigraphically useful acritarchs from three formations spanning most of the Walden Creek Group. Their work confirms the latest Precambrian age for the Walden Creek Group. Southeast of the Greenbrier fault, the only stratigraphic unit known to overlie the Ocoee is the Murphy Belt Group (C3), which McLaughlin and Hathaway (1973) report as containing Ordovician brachiopods and gastropods. The contact between the Ocoee and Murphy Belt Group was described as conformable by Power and Forest (1973) and as gradational by Mohr (1973). Northeast of the Ocoee basins, volcanic rocks, largely subaerial, characterize the upper Precambrian stratified rocks on the west limb of the Blue Ridge anticlinor-ium. These volcanic rocks are assigned, from southwest to northeast, to: (1) the Grandfather Mountain Formation, exposed only in the Grandfather Mountain window (D2), (2) the Mount Rogers Formation of southwestern Virginia (Di), and (3) the Catoctin Formation north of Roanoke, Va. (D3). The volcanic rocks, together with consanguineous intrusive rocks, constitute the bimodal Crossnore Plutonic-Volcanic Complex.6 Metamorphosed basalt is ubiquitous, but rhyolite of peralkaline affinity is present in the Grandfather Mountain and predominates in the Mount Rogers and in the Catoctin Formation at South Mountain, Pa. Laminated pebbly mudstone and associated diamictite at the top of the Mount Rogers Formation and laminated pebbly mudstone near the highest exposed part of the Grandfather Mountain Formation may indicate an episode of late Precambrian glaciation. Zircons from five samples of rhyolite (Grandfather Mountain, Mount Rogers, and Catoctin Formations) give a discordia intercept age of 810 m.y. (Rankin and others, 1969). The Grandfather Mountain Formation is structurally isolated from stratigraphically younger rocks, but the Mount Rogers and Catoctin Formations are overlain, with no obvious structural or metamorphic break, by clastic rocks of the Chilhowee Group. The contact between the Mount Rogers and Chilhowee is disconform-able (Rankin, 1970). Scolithus is present locally in formations as low as the middle of the Chilhowee Group, and an Ollenelus fauna has been reported from a few localities at the top of the group. Stratified rocks of definite latest Precambrian age are not found northeast of South Mountain, Pa., on the northwest flank of the Blue-Green-Long axis. Stratified rocks that crop out between the sparsely fossiliferous basal Lower Cambrian quartzitic sandstones and the older metamorphic complex are unnamed in the Reading Prong but are called the Dalton Formation (F) in Massachusetts and southern Vermont and the Pinnacle Formation (G) in central and northern Vermont. The Dalton and Pinnacle have traditionally been assigned an Early Cambrian(?) age, but they are not fossiliferous and may be totally, or in part, of late Precambrian age. "This name was defined but informally designated “Crossnore plutonic-volcanic group” by Rankin and others (1973). The name Crossnore Plutonic-Volcanic Complex is herein adopted as a formal term.CORRELATION CHART FOR PRECAMBRIAN ROCKS OF THE EASTERN UNITED STATES E7 EAST LIMB No volcanic rocks on the east limb have been dated by the zircon U-Pb or whole-rock-isochron Rb-Sr methods, nor are Early Cambrian fossils known to exist in the area. A Precambrian age is suggested for the units shown on the chart on the basis of regional correlations. The supposition is strongest for northern Virginia and may be valid for northern and central Vermont. In both areas, stratified units of probable Precambrian age can be traced around the noses of anticlinoria exposing basement rocks. In northern Virginia, rocks of the Chil-howee Group overlie the upper Precambrian stratified rocks on the east limb of the Blue Ridge anticlinorium. Stratified rocks of presumed late Precambrian age east of the Blue-Green-Long axis are thicker and contain a higher percentage of clastic sedimentary rocks, largely metamorphosed graywacke and shale, than do their counterparts west of the axis (Ocoee excepted), which contain a higher percentage of volcanic rocks. A Precambrian age for the Lynchburg, Ashe, and Tallulah Falls Formations, the Great Smoky Group southeast of the Hayesville fault, and the Ashland, Wedowee, and Heard Groups presents some problems. These units contain numerous pods of ultramafic rock that range in size from a few meters to several kilometers. The ultra-mafic pods were emplaced prior to the major pulses of Ordovician(?) regional metamorphism, but their mode of emplacement is still debated. They are probably either fragments of obducted ophiolites emplaced in a sedimentary melange or along thrust faults or are diapiric. Similar ultramafic pods along the east flank of the Blue-Green-Long axis in New England and near Washington, D.C., are interpreted as dismembered ophiolites obducted during the closing of the Iapetus Ocean and subsequently emplaced within the sedimentary pile (Rolfe Stanley, written commun., 1978; Drake and Morgan, 1981). In Vermont, the absence of pods in rocks younger than the Middle Ordovician Moretown Formation suggests that obduction had occurred by that time. Evidence from Newfoundland suggests that the obduction began in the Early Ordovician (Williams and Talk-ington, 1977). Reasons for suggesting a Precambrian age for these strata on the east limb of the Blue Ridge anticlinorium include (1) the stratigraphic location of the Lynchburg Formation (J2) beneath mafic rocks mapped as Catoctin and (2) the lithologic similarity of metamorphosed clastic rocks on both sides of the Hayesville-Fries fault (both called Great Smoky). A late Precambrian age is reasonable for the Fauquier and Catoctin Formations (J3) of northern Virginia that lie stratigraphically beneath the Chilhowee on the east limb of the Blue Ridge anticlinorium. The Fauquier belt is on strike with the Lynchburg belt to the southwest, and the two are generally correlated. A thick greenstone overlies the Lynchburg east of Charlottesville and generally is assigned to the Catoctin. Alternatively, the Lynchburg may not be correlative with the Fauquier but, along with the Ashe, Heard, and Ashland, may be significantly younger, perhaps even as young as Ordovician. This age would be compatible with the origin of the Blue Ridge ultramafic pods as dismembered obducted ophiolites emplaced either as tectonic or as sedimentary melanges during the closing of the Iapetus Ocean. The younger age, however, would require the transportation of large volumes of clastic sediments across the carbonate bank lying west of the Blue Ridge. INTRUSIVE ROCKS YOUNGER THAN 1 BILLION YEARS Malfic and felsic dikes, sills, and plutons of the Cross-nore Plutonic-Volcanic Complex, intrusive into the basement crystalline rocks and the Grandfather Mountain and Mount Rogers Formations, are particularly common near the Mount Rogers reentrant. Zircons from five granite plutons of the Crossnore have a discordia upper intercept age of 824 m.y., which is in good agreement with the 810-m.y. U-Pb zircon age for comagmatic rhyolites (Rankin and others, 1969). Two of the samples used to determine the discordia curve for the granites are from earlier work by Davis and others (1962), and the other four are new analyses by T. W. Stern and M. F. Newell. In contrast, Odom and Fullagar (1971) reported a Rb-Sr whole-rock-isochron age of 672 m.y. for a composite isochron based on samples from the Beech, and Striped Rock Granites and the algirine-augite granite gneiss near Crossnore, N.C. A peralkaline granite of Amissville in northern Virginia has a Pb-Pb zircon age of 655 m.y. with nearly concordant U-Pb ages (T. W. Stern, in Rankin, 1975). Lukert and others (1977) reported a U-Pb zircon age of 732 m.y. for the Robertson River Formation, and they interpret the Robertson River to be intrusive into the Virginia Blue Ridge Complex. These granites are presumably related to the late Precambrian breakup of eastern North America, although uncertainties remain as to the relation of these ages to the Crossnore plutons and to the rhyolite of the Catoctin Formation at South Mountain, Pa. Bodies of foliated granite crop out near Kennesaw Ridge, Austell, and Hightower within the belt of the Ashland and Heard Groups in Georgia (H). This terrane lies northwest of the Brevard zone and along the east limb of the Blue-Green-Long axis. A tentative age of about 560 m.y. for these granitic gneisses is anomalous because nowhere else along the Blue-Green-Long axis do plutons of this age occur. ALLOCHTHONS OF EASTERN ROCKS INNER PIEDMONT AND SMITH RIVER ALLOCHTHONS Between the Blue Ridge anticlinorium and structural domes to the east, which expose rocks older than 1 b.y., are terranes that are poorly understood in terms of tectonic setting, age of rocks, and geologic history. TheE8 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO largest of these, the Inner Piedmont (Mi), consists mostly of amphibolite-facies gneiss, schist, and amphibolite intruded by a variety of granitic rocks. Migmatite is characteristic of much of the terrane. Rankin (1975) suggested that the entire block is allochthonous and was transported from the southeast side of the Pine Mountain-Kings Mountain-Sauratown Mountains terrane. The new COCORP data, as interpreted by Cook and others (1979), also suggest that the Inner Piedmont is allochthonous. Only two rock types from the Inner Piedmont have yielded isotopic ages of interest, although numerous granitic rocks have been dated as Paleozoic. The Henderson Gneiss (Mia), a major unit of the western Inner Piedmont in North and South Carolina, is largely an augen gneiss of quartz monzonite composition. Most workers have interpreted the protolith of the Henderson to be metavolcanic (Bryant and Reed, 1970; Espen-shade, in Rankin and others, 1973) or metaarkose (Hatcher, 1971). The Caesars Head Quartz Monzonite (Hadley and Nelson, 1971), a closely related and more massive rock, may be an intrusive rock. The Toluca Quartz Monzonite, the other dated rock, generally has been interpreted as an intrusive body. The Rb-Sr whole-rock-isochron age of 524 m.y. for the Henderson agrees well with a zircon discordia intercept age of 526 m.y. on the same unit (Odom and Fullagar, 1973). A single zircon age for the Toluca by Davis and others (1962) falls essentially on the same discordia line and suggests a similar age (Odom and Fullagar, 1973). Hatcher (1971) considered the Henderson to be near the top of the stratigraphic section in the Inner Piedmont. The rocks of the Smith River allochthon are much like those of the Inner Piedmont and, presumably, at least overlap in age. MARYLAND AND NORTHERN VIRGINIA PIEDMONT ALLOCHTHONS Recent work by Crowley (1976), Drake and Morgan (1981), and Drake and others (1979) supports the interpretation that large masses of rock, including ultramafic-mafic complexes northwest of the Baltimore-Washington anticlinorium, were transported from southeast of the Baltimore Gneiss terrane. Detailed mapping by Drake and others (1979) in Fairfax County, Va., has delineated several lithotectonic units (N). Some rocks within these units may be of late Precambrian age, but solid evidence is lacking. This recent work supports the earlier suggestion by Rankin (1975) that parts of the Maryland and northern Virginia Piedmont are allochthonous, derived from southeast of the Baltimore Gneiss terrane, and perhaps are part of an eastern continent or continental fragment. Higgins and others (1977) reviewed the problem of interpreting the existing isotopic ages for the Piedmont of the central Appalachians. From the data presented in their paper, we have selected the zircon ages from samples of rocks whose correlation is strongest—the metavolcanic rocks of the Chopawamsic and James Run Formations and the Baltimore paragneiss (Tilton and others, 1970; Higgins and others, 1977). The six zircon samples gave a recalculated discordia intercept age of 528 m.y. Between Washington, D.C., and Fredericksburg, Va., the Chopawamsic apparently is intruded by two plutons, the Occoquan Adamellite and the Dale City Quartz Monzonite, for which a discordia intercept age of 561 m.y. has been reported by Seiders and others (1975). Both the Chopawamsic and the Dale City Quartz Monzonite are unconformably overlain by the Ordovician Quantico Slate (Pavlides and others, 1980). WESTERN GNEISS DOMES Rocks older or probably older than 1 b.y. are exposed east of the Blue-Green-Long axis in the cores of a series of uplifts from Alabama (Ox) to Vermont (06). The old core rocks are interpreted as exposures of autochthonous or parautochthonous rocks of the North American craton (Naylor, 1975; Rankin, 1975), although Williams (1978) suggested that, south of latitude 36°, the old crystalline rocks may belong to the eastern continent. Typically, the western gneiss domes consist of a crystalline complex of rocks older than 1 b.y., which is over-lain nonconformably by a younger metasedimentary sequence. The mantling rocks commonly consist of quartzite, dolomitic marble, and schist. None of the metasedimentary units are fossiliferous, and none have been dated isotopically. The sequences, however, are very similar to the transgressive sequences of Cambrian and Ordovician age along the western flank of the Blue-Green-Long axis. Rankin (1975) suggested that the western gneiss domes may have originated as horsts on the eastern margin of a late Precambrian rift system. PINE MOUNTAIN BELT The Pine Mountain belt of Alabama and Georgia (Ox) is separated from the Inner Piedmont on the northwest by the northwest-dipping Towaliga fault and is separated on the southeast from the Uchee belt by the southeast-dipping Goat Rock fault. Rocks of the Pine Mountain belt consist of an older crystalline basement, predominantly orthogneiss, called the Wacoochee Complex and the younger metasedimentary Pine Mountain Group (Bentley and Neathery, 1970). Both the Wacoochee and the Pine Mountain are polydeformed and have undergone kyanite-grade Paleozoic metamorphism. Within the Wacoochee Complex, the Woodland Gneiss of Hewett and Crickmay (1937) is probably equivalent to Clarke’s (1952) Jeff Davis Granite. The predominant lithology is a biotite-garnet gneiss of quartz-monzonite composition; other rocks of the complex, such as the Cunningham Granite, are hypersthene bearing. A Rb-Sr-isochron age of 1051 m.y. has beenCORRELATION CHART FOR PRECAMBRIAN ROCKS OF THE EASTERN UNITED STATES E9 obtained for the Woodland, and a zircon discordia intercept age of 111# m.y. for the Cunningham. KINGS MOUNTAIN BELT The inclusion of the Kings Mountain belt (02), along strike to the northeast, in the group of western gneiss domes is problematical. No rocks demonstrably older than 1 b.y. have been identified, and the stratified rocks are distinctive because they include a significant volcanic component. Nonetheless, the position of the Kings Mountain belt along the Bouguer gravity gradient between the Inner Piedmont and the Charlotte belt and lithologic similarities argue that the belt forms a link between the Pine Mountain belt and the Sauratown Mountains anticlinorium. SAURATOWN MOUNTAINS ANTICLINORIUM Rocks in the core of the Sauratown Mountains anticlinorium (03) were interpreted by Espenshade and others (1975) to consist of an older suite of layered fine-grained biotite-quartz-plagioclase gneiss containing either muscovite or amphibole, biotite schist, and minor impure marble intruded by foliated granitic rocks (granite to diorite in composition) and augen gneiss. Espenshade and others (1975) included the plu-tonic rocks in the Elk Park Plutonic Suite. Foliated biotite-quartz monzonite near Pilot Mountain, N.C., has a zircon Pb-Pb age of 1,172 m.y. (T. W. Stern, in Rankin and others, 1973). Distinctive foliated biotite-quartz monzonite, commonly containing fluorite, mesoperthite megacrysts, and, less commonly, dark-blue-green amphibole, is present also in the core of the Sauratown Mountains anticlinorium, as are foliated aegirine-bearing aplite dikes. These alkalic rocks are considered to be related to the late Precambrian rifting of the ancient North American craton. Zircons from two bodies of the alkalic rocks have been analyzed by Stern (Pb-Pb ages of 85k m.y. and 790 m.y.). BALTIMORE GNEISS DOMES Foliated granitic rocks, augen gneiss, layered gneiss, and minor amphibolite, collectively called Baltimore Gneiss (Crowley, 1976), crop out in the cores of a number of foliation folds (mostly domes) in the area of Baltimore, Md., and West Chester, Pa. (04). Pelitic rocks surrounding the domes have undergone Paleozoic metamorphism of sillimanite-muscovite grade. The Baltimore Gneiss from several domes yielded ages in the range of 1-1.2 b.y. Samples of Baltimore Gneiss from the Phoenix, Towson, and Woodstock domes yielded a Rb-Sr whole-rock-isochron age of 1,028 ±k0 m.y. (Wetherill and others, 1968). Grauert (1974) published two precise discordia lines from the Baltimore area that suggest that the Baltimore Gneiss of the Towson dome is somewhat older (1,180 ±25 m.y.) than that of the Phoenix dome (1,080 ±20 m.y.). Grauert and others (1973) and Grauert (1974) showed that zircons from different facies of Baltimore Gneiss granulites from the West Chester Prong and the Avondale anticline yielded zircon discordia intercept ages of 980 and 1,060 m.y. MANHATTAN PRONG The basement complex of the Manhattan Prong (05) includes the Fordham Gneiss, Yonkers Gneiss, and Pound Ridge Granite Gneiss, all having undergone Paleozoic metamorphism of sillimanite grade. The Ford-ham is a heterogeneous unit consisting largely of gray biotite-quartz-feldspar gneiss; calc-silicate granulite and amphibolite are locally present (Hall, 1976). The Yonkers is a relatively homogeneous commonly pinkish biotite-ferrohastingsite-quartz-feldspar gneiss (Hall, 1976), and the Pound Ridge is a quartz-microcline-microperthite granite gneiss containing small amounts of biotite and muscovite (Mose and Hayes, 1975). The Yonkers and Pound Ridge occur as structurally concordant lenses within the Fordham. Hall (1976) interpreted the Fordham and Yonkers to be a metamorphosed eugeosynclinal sequence of sedimentary and volcanic rocks that contains some intrusive rock. Zircons from the Fordham show a large spread in Pb-Pb ages, ranging from 800 to 1,300 m.y. (Grauert and Hall, 1973). They interpreted the data to indicate a 100-to 200-m.y. orogenic event, which ended about 980 m.y. ago. They also suggested that the zircon population contained a significant number of inherited zircons with “primary” ages of 1,600 to 1,700 m.y. Rb-Sr whole-rock-isochron ages ranging between 1,100 and 1,330 have been obtained for the Fordham Gneiss at three different localities (Mose and Hall, written commun., 1980). The Yonkers Gneiss and Pound Ridge Granite Gneiss, on the other hand, yielded much younger ages. Long (1969) determined a Rb-Sr whole-rock-isochron age of 563 ± 35 m.y. from the south end of the Yonkers outcrop area. Mose and Hall (written commun., 1980) obtained a Rb-Sr-isochron age of 528 ± 82 m.y. from the north end of the Yonkers outcrop area. Mose and Hayes (1975) determined a Rb-Sr whole-rock-isochron age for the Pound Ridge of 583 ±25 m.y. If the determined ages approximate the actual ages of the rocks and if the field relations have been interpreted correctly, an intrusive origin for the Yonkers and Pound Ridge is most likely. This extrusive or intrusive igneous event clearly is tied spatially to the western basement and may be related to the opening of the Iapetus Ocean. CENTRAL BELT The Central belt is, in many respects, a subdivision of convenience that has little continuity of geology along the orogen. The suture zone, representing the closing of the Iapetus Ocean, is interpreted to lie west of this belt, at least as far north as central Massachusetts.E10 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO CHARLOTTE BELT The Charlotte belt (P) is characterized by an abundance of plutonic rocks including many mafic plutons that intrude stratified rocks of high metamorphic grade for which no systematic stratigraphy has yet been established. Tobisch and Glover (1971) concluded that, near the Virginia-North Carolina State line, the eastern edge of the Charlotte belt decreases sharply but continuously in metamorphic grade into the Carolina volcanic slate belt. Farther southwest, however, a fault separates the two belts. The stratified rocks may be, in part, of Precambrian age, particularly if they are higher grade equivalents of the Carolina volcanic slate belt. Uranium-lead ages of the paragneiss are in the range 606-725 m.y. (Glover and others, 1971). Fullagar (1971) published Rb-Sr whole-rock data suggesting that some plutons may be as old as 500-600 m.y. BRONSON HILL-BOUNDARY MOUNTAIN ANTICLINORIUM Mantled gneiss domes that penetrate nappes derived from the east form a belt roughly parallel to and east of the Connecticut River extending from Long Island Sound north-northeastward to the Maine-New Hampshire boundary near Berlin, N.H. (Thompson and others, 1968). These domes constitute the Bronson Hill anticlinorium, which has been interpreted as a mobilized island-arc terrane. The oldest mantling strata are metamorphosed volcanic rocks, the Ammonoosuc Vol-canics, of Middle Ordovician age. The core rocks consist of massive gneisses that may be intrusive and layered gneisses that may be metamorphosed sedimentary and volcanic rocks. Collectively, the core rocks are referred to as the Oliverian Plutonic Series and range in composition from quartz diorite to granite. Most of the core rocks that have been dated have Ordovician ages of about 450 m.y. (Naylor, 1975). Naylor and others (1973) reported a Pb-Pb age of 56k m.y., however, for the Dry Hill Gneiss of the Pelham dome, possibly an Oliverian dome on the west flank of the anticlinorium in Massachusetts (Qi). This westernmost appearance of the eastern basement may indicate that the suture representing the closure of the Iapetus Ocean lies between the Pelham dome and the Athens and Chester domes (06). The Boundary Mountains anticlinorium, an Acadian structure along the Maine-Quebec boundary, lies en echelon to the northwest of the Bronson Hill anticlinorium. An older complex, the Chain Lakes massif (Q2), has been identified in the core of the anticlinorium. The stratigraphic succession that overlies the massif with probable unconformity is not agreed upon but contains rocks at least as old as Ordovician (Boudette and Boone, 1976). Interpretations differ also as to the proto-liths of the massif, but some of the massif appears to be strongly retrograded and fragmented gneiss and mig-matite. Naylor and others (1973) reported Pb-Pb ages of l,k76 and l,k86 m.y. for zircons from the massif. If these ages correctly date the Chain Lakes massif, that terrane would be the oldest identified in the Appalachians. MERRIMACK SYNCLINORIUM The Merrimack synclinorium of central New England consists largely of Silurian and Devonian rocks but locally exposes older rocks. High-grade gneisses and schists are exposed discontinuously along the southeastern margin of the synclinorium from north-central Massachusetts to south-central Maine. Besancon and others (1977) reported U-Pb ages for zircons from orthogneisses of the Massabesic Gneiss (Ri) near Manchester, N.H., in the range of 600-620 m.y. Aleinikoff and others (1979) suggested that the paragneiss of the Massabesic may be volcaniclastic in origin, with zircons yielding a minimum Pb-Pb age of 6k6 m.y. The Cushing Formation of southwestern Maine (R2) is lithologically similar to parts of the Massabesic Gneiss, as well as the Nashoba Formation of northeastern Massachusetts (U2). The Cushing cannot be traced continuously into either unit but may be of early Paleozoic or late Precambrian age (Osberg, 1979). Farther northeast is the Passagassawakeag Gneiss in the vicinity of the Penobscot River, Maine (R3), for which Stewart and Wones (1974) suggested a possible Precambrian age. The Grand Pitch Formation of north-central Maine (R4) consists of gray, green, and red slate and siltstone interlayered with vitreous quartzite and lesser amounts of graywacke and tuff (Neuman, 1967). The formation contains the trace fossil Oldhamia and is probably of late Precambrian or Early Cambrian age. It is overlain unconformably by rocks as old as Early or early Middle Ordovician. AVALONIAN ZONE Rocks of the Carolina volcanic slate belt and southeastern New England have much in common with rocks of the Avalon Peninsula of Newfoundland. We extend the name Avalonian zone into the Appalachians for these rocks, as did Williams (1978). CAROLINA VOLCANIC SLATE BELT Abundant mafic to felsic volcanic rocks, together with pelitic sedimentary rocks generally of low metamorphic grade, characterize the Carolina volcanic slate belt that extends from central Georgia to southern Virginia. Isotopic dating suggests that rocks toward the northern end of the belt may be older than those at the southern end. The largest area of the Carolina volcanic slate belt for which detailed mapping reveals a consistent stratigraphy lies in central North Carolina between Albemarle and Asheboro (S2) (Seiders and Wright, 1977). There, the metamorphic grade is low (chlorite and biotite zones), and the folds are broad and open. Volcanic andCORRELATION CHART FOR PRECAMBRIAN ROCKS OF THE EASTERN UNITED STATES Ell volcaniclastic rocks, overwhelmingly felsic, constitute most of the older part of the section (the Uwharrie Formation). The upper part of the section (the Albemarle Group) comprises mostly sedimentary rocks and lesser amounts of largely mafic volcanic rocks. The Mil-lingport, the uppermost formation of the Albemarle, is the youngest unit preserved in this part of the Carolina volcanic slate belt. IParadoxides carolinaensis has been identified in a float piece of laminated argillite, probably from the Floyd Church Member, the lowest of two members of the Millingport (St. Jean, 1973). Felsite near the top of the Uwharrie Formation has a U-Pb discordia intercept age of 581+ m.y. (Wright and Seiders, 1981). Hills and Butler (1969) reported a Rb-Sr whole-rock-isochron age for rhyolite, also from the Uwharrie Formation, of 551+ ±50 m.y. An andesitic tuff that D. J. Milton (oral commun., 1980) places in the Floyd Church Member of the Millingport has an Rb-Sr whole-rock-isochron age of 51+0 ±7 m.y. (Black, 1978). R. H. Carpenter, A. L. Odom, and M. E. Hartley (written commun., 1978) described a generally similar stratigraphy along the Georgia-South Carolina boundary (SO about 260 km southwest of Albemarle. The basal Lincolnton Metadacite is overlain by a felsic pyroclastic sequence, which is, in turn, overlain by an upper sedimentary sequence, mainly banded argillite and thin in-terlayered mafic volcanic rocks. The workers suggested the correlation of the Lincolnton with the Uwharrie Formation and the correlation of the banded argillites of the upper sedimentary sequence with similar rocks of the Tillery Formation. They reported a Rb-Sr whole-rock-isochron age for the Lincolnton Metadacite of 51+7 ± 91+ m.y., in reasonable agreement with a U-Pb zircon discordia intercept age of 566 ±15 m.y., based on four zircon fractions of two of the same samples. In the Roxboro-Durham area (S3) along the eastern edge of the Charlotte belt, mafic and felsic gneisses of amphibolite facies (unit I of Glover and Sinha, 1973) are overlain with apparent conformity by unit II (Carolina volcanic slate belt) that consists mainly of felsic tuff and lapilli tuff with subordinate pyroclastic rocks and lavas of intermediate and mafic composition. Near Durham, N.C., rocks probably correlative with unit II contain impressions of worm-like forms, Vermiforms antiquo Cloud, n. gen., n. sp. (Cloud and others, 1976). Unit II is overlain by tuffaceous epiclastic rocks and reworked tuffs of unit III. The youngest stratified unit preserved, unit IV, consists of mostly mafic volcanic rocks to the north but felsic volcanic rocks to the south, all overlain by thin-bedded mudstone. This stratified sequence had been folded into a major syncline, and the synclinal axis offset more than 16 km along a left-lateral strike-slip fault (the Virgilina deformation) prior to the intrusion of the Roxboro Granite batholith. Glover and others (1971) and Glover and Sinha (1973) reported that the gneisses of unit I may be as old as 725 m.y., based upon zircon analyses. Felsic tuff breccia near the top of map unit II has a U-Pb zircon discordia intercept age of 606 ±20 m.y. (Glover and Sinha, 1973). High level plutons of the Flat River Complex (also called the Moriah pluton, Cloud and others, 1976) that may be intrusive equivalents of unit II have a zircon age of 650 ±30 m.y. (McConnell and others, 1976). Finally, the U-Pb discordia intercept age for the Roxboro Granite, based on two nearly coincident points (Glover and Sinha, 1973), is 561+ m.y. These data imply that the stratified rocks of units I to IV were deformed after the extrusion of unit II 606 ±20 m.y. ago but prior to the intrusion of the Roxboro Granite about 561+ m.y. ago. This orogenic event has not been identified in the central and southern parts of the Carolina volcanic slate belt, from which stratified rocks, in general, yield younger ages. Wright and Seiders (1981) suggest that the Virgilina deformation was synchronous with the deposition of the upper part of the Albemarle Group but that the deformation did not extend into central North Carolina. Rb-Sr whole-rock-isochron ages reported by Black and Fullagar (1976) for rocks near Chapel Hill are not easily related to the ages just discussed. They interpret the age of the Virgilina deformation to be 613 m.y. and theorize that plutons 638 and 705 m.y. old intrude dacite metatuffs (Efland Formation) that were affected by the Virgilina deformation. SOUTHEASTERN NEW ENGLAND Calc-alkaline plutonic rocks, such as the Dedham Granodiorite, are widespread in southeastern New England. At Hoppin Hill, near North Attleborough, Mass. (Ti), fossiliferous Lower Cambrian slates and limestones of the Hoppin Formation lie nonconformably upon coarse-grained igneous rocks similar to the Dedham Granodiorite. Billings (1929) suggested a Precam-brian age for the Dedham Granodiorite on the basis of the relation at Hoppin Hill. Isotopic age studies now have established that an area bounded on the north and west by the Bloody Bluff, Lake Char, and Honey Hill fault zones contains large areas of upper Precambrian plutonic and volcanic rocks. This terrane had largely stabilized prior to deposition of Lower Cambrian sedimentary rocks and, thereafter, except for Alleghenian deformation, was involved only peripherally in Paleozoic penetrative deformation that strongly affected rocks immediately to the north and west. The southwestern part of this terrane, such as the Stony Creek dome near New Haven, Conn. (T3), did undergo a considerable degree of Acadian metamorphism. Included in the suite of upper Precambrian (600- to 650-m.y.-old) calc-alkaline plutonic rocks are the Stony Creek Granite of south-central Connecticut, the Scitu-ate Granite of the Sterling Plutonic Group, the Esmond and Milford Granites and Ponaganset Gneiss of easternE12 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO Connecticut and western Rhode Island (T2), the granitic gneiss of the Willimantic dome, Connecticut (V), the Bulgarmarsh Granite of southeastern Rhode Island (W2), and the Dedham Granodiorite and related rocks of eastern Massachusetts (Ui). R. E. Zartman and Naylor (unpublished data) have determined a Rb-Sr whole-rock-isochron age for the Milford Granite of 591 ± 50 m.y. and a U-Pb zircon discordia intercept age of 630 ±15 m.y. for the Milford Granite and Dedham Granodiorite. Kovach and others (1977) determined a Rb-Sr whole-rock-isochron age of 595 ± 16 m.y. for the Dedham Granodiorite. Smith and Giletti (1978) reported a Rb-Sr whole-rock-isochron age of 603 ± 14 m.y. for the porphyritic granite of Aquidneck Island, R.I. (W^. The Rb-Sr whole-rock-isochron age of 516 ± 13 m.y. determined by Galloway (Murray and Skehan, 1979) for the Bulgarmarsh Granite of southeastern Rhode Island may be reset partially by later metamorphism. The calc-alkaline suite is intrusive into an older sequence of metasedimentary (quartzite, marble, schist, and gneiss) and metavolcanic rocks, the Blackstone Group and equivalents. The Plainfield Formation and Absalona Gneiss lying east of the Lake Char fault and south of the Honey Hill fault in Connecticut (T2) and the Westboro Quartzite and Middlesex Fells Volcanic Complex in eastern Massachusetts (Ui) are probably roughly correlative with the Blackstone Group of Rhode Island (Quinn, 1971). Sedimentary and volcanic rocks of the Boston basin (the Boston Bay Group) are younger than the Dedham Granodiorite that generally forms a basement for the basin. Until recently, the Boston Bay Group was considered to be of Carboniferous age. A study of the Matta-pan Volcanic Complex beneath the Boston Bay Group has resulted in a precisely defined U-Pb-zircon discordia intercept age of 602 ±3 m.y. (Kaye and Zartman, 1980). This age and new field observations suggest that deposition of the Boston basin sequence began in late Precambrian time and progressed more or less continuously into the Middle Cambrian (Braintree Argillite). Similar rocks in the smaller Woonsocket and North Scituate basins to the southwest also may be of Proterozoic age (Richard Goldsmith, oral commun., 1980). In eastern Massachusetts, a fault-bounded terrane of poorly understood high-grade (sillimanite) metamor-phic and plutonic rocks lies between the Avalonian zone on the southeast and the Merrimack synclinorium on the northwest (U2) (Cameron and Naylor, 1976). The dominant stratified unit in this belt is the Nashoba Formation, composed mostly of felsic biotite gneiss, with lesser amounts of interlayered amphibolite, calc-silicates, and pelitic or quartzofeldspathic schists. Olszewski (1978) identified two zircon populations from this terrane. The first group consists of euhedral to sub-hedral cyrstals of presumed volcanic origin that give a U-Pb intercept age of about 750 m.y. The second group is made up of rounded and subrounded detrital zircons that have an upper discordia intercept age of about 1.55 b.y. The presence of volcanic zircons suggests that these strata formed during a late Precambrian episode of sedimentation and volcanism, whereas the presence of detrital zircon implies a significantly older source area for the terrigeneous metasedimentary rocks. Terranes comparable to those of eastern Massachusetts may be present in south-central and coastal Maine. The Passagassawakeag Gneiss has already been noted in the section on the Merimack synclinorium. Stewart (1974) reported small areas of schist, quartzite, marble, and amphibolite intruded by pegmatite on Seven Hundred Acre Island and other small islands in Islesboro Township in Penobscot Bay, Maine (X). Brookins (1976) obtained a Rb-Sr whole-rock-isochron age of 731* ± 100 m.y. on samples of the metamorphic rocks and a Rb-Sr mineral-isochron age of 606 ±20 m.y. on the pegmatite. SOUTHEASTERN METAMORPHIC BELT South of the Potomac River, a belt of medium- to high-grade metamorphic and plutonic rocks crops out adjacent to the Coastal Plain overlap. These rocks may be separated from the rest of the Piedmont to the northwest by the eastern Piedmont fault system of Hatcher and others (1977). The southernmost segment in Georgia and adjacent Alabama is called the Uchee belt (Yi). The dominant lithology, migmatitic granitic gneiss, was called the Phenix City Gneiss by Bentley and Neathery (1970). Four zircon samples from the Phenix City were analyzed by G. S. Russell and Odom (written commun., 1978), and the best-fit discordia line to the data yielded an upper intercept age of 566 m.y. However, such a line does not give a positive lower intercept, and it seems clear that not all the data form a single array. The Pb-Pb ages, most of which lie between 58h and 619 m.y., are probably a better estimate of the actual age than is the intercept of the chord with concordia. The southeastern metamorphic belt in South Carolina is called the Kiokee belt (Y2); it is characterized by amphibolite facies metasedimentary and metavolcanic rocks and stratiform granitic masses of orthogneiss. Secor and Snoke (1978) noted the similarity of the Kiokee and Carolina volcanic slate belts and suggested that they are, in part, correlative. The Kiokee belt is unusual in that it appears to have undergone amphibolite facies regional metamorphism in the late Paleozoic (Hercynian). Rocks as high grade as kyanite crop out in the Raleigh belt (Y3) east of the Deep River Triassic-Jurassic basin in North Carolina. According to Parker (1977), the Bar-rovian metamorphism is related to emplacement of the late Paleozoic (Hercynian) Rolesville batholith. Green-schist facies rocks south and east of the higher grade rocks are known as the eastern slate belt and correlateCORRELATION CHART FOR PRECAMBRIAN ROCKS OF THE EASTERN UNITED STATES E13 in general with the Carolina volcanic slate belt west of the Deep River basin. This terrane is, thus, much like the Kiokee belt and probably includes rocks of latest Precambrian age metamorphosed in late Paleozoic time. Parker (1977) implied that an older terrane also may exist in Wake County, N.C., where felsic quartzofeldspathic gneiss may lie unconformably beneath rocks correlated with the Carolina volcanic slate belt. Pavlides (1976) reported a complexly folded terrane of schist, gneiss, and granite east of the outcrop belt of the Quantico Slate of Ordovician age. He referred to these rocks as the Fredericksburg Complex (Y4); this complex corresponds, in part, with the area shown as Baltimore Gneiss by the Virginia Geological Survey (1928). He reported a Pb-Pb age of 59-4 m.y. for zircons from a hornblende-biotite paragneiss. Parts of the Fredericksburg Complex may be older. Glover and others (1978) reported a 1.0-b.y. age (Rb-Sr whole-rock-isochron) for a pluton near Richmond, Va., that intrudes rocks considered to be a southern extension of the Fredericksburg Complex (Louis Pavlides, oral commun., 1979). MINERAL RESOURCES The Adirondack province has been a fairly productive area of both metallic and nonmetallic minerals. Metallic deposits include the Tahawus-Sanford Lake magnetite-ilmenite deposit (Highlands), the largest titanium deposit in the United States; the Benson Mines magnetite-hematite deposit and similar but small deposits at Port Henry and Lyon Mountain (Highlands); syngenetic zinc deposits of Balmat-Edwards (Lowlands) that supply about 10 percent of the U.S. zinc production; and a number of small pyrite deposits in the Adirondack Lowlands. Nonmetallic deposits are also important in the Adirondacks. Graphite was produced from the Ticonderoga area of the Highlands. Each of the following mines is the largest producer of its kind in North America: the Barton Garnet mine of the Gore Mountain garnet deposit, which is a hydrous phase of an olivine metagabbro (Highlands); the Willsboro wolla-stonite deposit, a skarn deposit near Lake Champlain (Highlands); and talc-tremolite deposits in the complexly deformed carbonate-rich sequence of the Lowlands in the Balmat-Edwards district. Rocks older than 1 b.y. in the Appalachians (the western basement) contain mineral deposits not unlike those of the Adirondacks. Historically, probably the most important are the unique zinc-manganese deposits in marble near Franklin, N.J. Other deposits in the western basement are relatively minor but include magnetite deposits in New York, New Jersey, and North Carolina, gold in North Carolina, and titanium associated with anorthosite near Roseland, Va. Nickel was produced from ultramafic rocks within the basement of the Mine Ridge anticline, Pennsylvania. Exploration for uranium is continuing in the billion-year-old basement terrane of the Grandfather Mountain window, North Carolina. Mineral deposits in the stratified rocks of the Blue-Green-Long axis include minor native copper deposits in the greenstones of the Catoctin Formation, Va., and the massive sulfide deposit of the Great Smoky Group, Ducktown, Tenn., currently worked for sulfur, with copper as a byproduct. Similar deposits (Elk Knob, Ore Knob, Gossan Lead, Toncray) occur in the Ashe Formation of North Carolina and Virginia, although these host rocks may not be of Precambrian age. Other mineral occurrences in the stratified Precambrian(?) rocks on the east limb of the Blue-Green-Long axis include gold from the Dahlonega district, Georgia, mica pegmatite (of Paleozoic age) in Georgia, North Carolina, and Virginia, and minor precious and semiprecious stones (emerald, ruby, rhodelite), mostly in North Carolina. Few mineral resources, other than stone, are known in Precambrian rocks between the Blue-Green-Long axis and the Avalon zone. Gold and relatively minor amounts of base metals were produced from the area of the Gold Hill fault zone, which is roughly the boundary between the Charlotte belt and the Carolina volcanic slate belt in southern North Carolina (Sg). Tungsten was produced from the Hamme district of the Virgilina area. Numerous pyrophyllite deposits occur in the Carolina volcanic slate belt. ACKNOWLEDGMENTS Numerous colleagues have contributed information and offered helpful suggestions. We wish to specifically acknowledge help from J. G. Arth, A. A. 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Virginia Geological Survey, 1928, Geologic map of Virginia: Charlottesville, Va., scale 1:500,000.E18 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO Walton, M. S., Jr., and de Waard, Dirk, 1963, Orogenic evolution of the Precambrian in the Adirondack Highlands—a new synthesis: Koninklijke Nederlandse Akademie van Wetenschappen, Proceedings Series B, Amsterdam, no. 66, p. 98-106. Wasserburg, G. J., 1961, [Discussion of] Isotopic ages from northern New Jersey and southeastern New York by L. E. Long, in Kulp, G. L., ed., Geochronology of rock systems: New York Academy of Science Annals, v. 91, p. 406. Wetherill, G. W., Davis, G. L., and Lee-Hu, C., 1968, Rb-Sr measurements on whole rocks and separated minerals from the Baltimore Gneiss, Maryland: Geological Society of America Bulletin, v. 79, p. 757-762. Wetherill, G. W., Tilton, G. R., Davis, G. L., Hart, S. R., and Hopson, C. 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Zen, E-an, 1967, Time and space relationships of the Taconic allochthon and autochthon: Geological Society of America Special Paper 97, 107 p. -----1972, The Taconide zone and the Taconic orogeny in the western part of the northern Appalachian orogen: Geological Society of America Special Paper 135, 72 p. -----1974, Prehnite and pumpellyite-bearing mineral assemblages, west side of the Appalachian metamorphic belt, Pennsylvania to Newfoundland: Journal of Petrology, v. 15, p. 197-242. *U.S. GOVERNMENT PRINTING OFFICE:IS83-381-419/117*(T> (ACT 7 DAYS m\- f Correlation of Precambrian Rocks of the Lake Superior Region, United States U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1241-F U.Ji. DEPOSITORY DEC 2 01988Correlation of Precambrian Rocks of the Lake Superior Region, United States By G.B. MOREY and W.R. VAN SCHMUS CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO Edited by JACK E. HARRISON and ZELL E. PETERMAN U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1241-F Lithology, distribution, correlation, and isotope ages of exposed Precambrian rocks in the Lake Superior region of the north-central United States UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1988DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging-in-Publication Data Morey, G.B. Correlation of Precambrian rocks of the Lake Superior region, United States. (Correlation of Precambrian rocks of the United States and Mexico) (U.S. Geological Survey professional paper ; 1241-F) Bibliography: p. Supt. of Docs, no.: I 19.16:1241-F 1. Geology, Stratigraphic—Pre-Cambrian. 2. Stratigraphic correlation—Superior, Lake, Region. I. Van Schmus, W.R. II. Title. III. Series. IV. Series: Geological Survey professional paper ; 1241-F. QE653.M67 1988 551.71 '097749 87-600313 For sale by the Books and Open-File Reports Section, U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225 Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Geological SurveyCONTENTS Page Abstract ............................................................................ FI Introduction ........................................................................ 1 Geologic and chronometric relations................................................... 1 Archean rocks ........................................................................ 2 Gneiss terrane.................................................................... 2 Stratigraphy and geochronology................................................ 4 Greenstone-granite terrane ....................................................... 7 Stratigraphy and geochronology................................................ 9 Proterozoic rocks.................................................................... 11 Pre-Penokean, Early Proterozoic rocks ........................................... 11 Stratigraphy and geochronology .............................................. 13 Post-Penokean, Early Proterozoic rocks .......................................... 16 Rhyolites and granites of the Fox River Valley type.......................... 16 Sedimentary rocks of the Baraboo-Sioux type.................................. 17 The 1,630 m.y. metamorphic event............................................. 17 Middle Proterozoic rocks......................................................... 17 Wolf River batholith and Wausau Syenite Complex.............................. 17 Midcontinent rift system .................................................... 18 Summary ............................................................................. 23 References cited..................................................................... 23 ILLUSTRATIONS Page Plate 1. Correlation chart for Precambrian rocks of the Lake Superior region. In pocket Figure 1. Location of the Lake Superior region as related to the Canadian Shield and to known or inferred Precambrian basement rocks of the North American craton........................................................................ F2 2. Generalized bedrock geologic map of the Lake Superior region...................................................... 3 3. Geologic map of the Minnesota River Valley, southwestern Minnesota................................................ 5 4. Geologic map of a part of the Vermilion district and the Vermilion Granitic Complex............................... 8 5. Idealized sections showing evolution of stratigraphic nomenclature in the Archean greenstone-granite terrane of north- ern Minnesota and adjoining Ontario....................................................................... 9 6. Generalized geologic map of the Lake Superior region showing distribution of Early Proterozoic rocks............. 12 7. Correlation chart for Early Proterozoic strata in the Animikie basin............................................. 14 8. Generalized geologic map of the western Lake Superior region showing distribution of Middle Proterozoic rocks associated with the Midcontinent rift system......................................................................... 20 9. Correlation chart for Middle Proterozoic rocks associated with the Midcontinent rift system, Lake Superior region 22) < i I «CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO CORRELATION OF PRECAMBRIAN ROCKS OF THE LAKE SUPERIOR REGION, UNITED STATES By G.B. Morey1 and W.R. VAN SCHMUS2 ABSTRACT Precambrian rocks in the Lake Superior region underlie parts of Minnesota, Wisconsin, and Michigan, very near the geographic center of the North American continent. The region contains two contrasting basement terranes of Archean age—a gneiss terrane and a greenstone-granite terrane. The more southerly gneiss terrane records one of the earliest identifiable events in Earth history, for the protoliths of some of the gneisses are about 3,600 million years old. The gneiss terrane remained tectonically active throughout much of Precambrian history, as indicated by a succession of events that have been dated at 3,000-2,900 m.y., 2,600 m.y., 2,400-2,300 m.y., 2,100-2,000 m.y., l, 900-1,800 m.y., about 1,760 m.y., about 1,630 m.y., and 1,500-1,450 m. y. ago. In contrast, the second Archean terrane—the greenstone-granite terrane—formed within an interval of only a few hundred million years approximately 2,700 million years ago, and has remained essentially stable since about that time. Two distinctly different sequences of stratified rocks were formed during Early Proterozoic time. The older sequence, deposited between 2,100 m.y. and 1,900 m.y. ago, consists predominantly of clastic rocks of geosynclinal affinity and includes the great iron-formations of the region. These rocks are bounded on the south, in Wisconsin, by a somewhat younger sequence of dominantly mafic volcanic rocks. Sedimentation and volcanism were either terminated or closely followed by several tectonic, metamorphic, and igneous events that span the interval from 1,890 m.y. to 1,770 m.y. ago and that collectively are referred to as the Penokean orogeny. Post-Penokean, Early Proterozoic rocks include felsic volcanic and epizonal granitic rocks, approximately 1,760 m.y. old, which are overlain by quartzitic red beds of fluvial to shallow-water marine origin. These rocks were locally deformed and metamorphosed at approximately 1,630 m.y. ago and were intruded by Middle Proterozoic alkalic and alkaline igneous rocks approximately 1,500 m.y. ago. In late Middle Proterozoic time, during the interval 1,200-1,000 m.y. ago, a sequence of dominantly mafic volcanic, hypabyssal, and plutonic igneous rocks and derivative red beds was formed as part of the Midcontinent rift system. This major tectonic feature extends from the Lake Superior region to near the Kansas-Oklahoma border in Central United States. Although some rocks in the interval 1,000-900 m.y. ago may be present, no rocks of definite Late Proterozoic age have been recognized in the Lake Superior region. Minnesota Geological Survey, St. Paul, MN 55114. 2University of Kansas, Lawrence, KS 66045. INTRODUCTION Precambrian rocks in the Lake Superior region underlie parts of Minnesota, northern Wisconsin, and northern Michigan, very near the geographic center of the North American continent. With respect to surface exposures, the region lies at the southern extremity of the Canadian Shield, but if the subsurface geology is considered, the region is located near the center of the Precambrian basement of the North American craton (pi. 1, index; fig. 1). Because of their position at the edge of the Canadian Shield, the Precambrian rocks are buried to the east, south, and west by increasing thicknesses of Phanerozoic strata. However, Precambrian rocks also are exposed as inliers in Phanerozoic strata in southwestern Minnesota and south-central Wisconsin. Additionally, much of the bedrock is covered by a considerable thickness of Quaternary glacial and postglacial materials. GEOLOGIC AND CHRONOMETRIC RELATIONS The geologic map of the Lake Superior region (fig. 2) and the correlation chart (pi. 1) emphasize a wide diversity of superposed rock types and events, ranging in age from about 3,600 m.y. to 1,000 m.y. The rationale for dividing this long span of time into the Archean and Proterozoic Eons, with a boundary at 2,500 m.y. ago, is discussed by the International Union of Geological Sciences Working Group on the Precambrian for the United States and Mexico (Harrison and Peterman, 1982). The Archean is subdivided into Early, Middle, and Late Archean Eras by time boundaries at 3,400 and 3,000 m.y., and the Proterozoic into Early, Middle, and Late Proterozoic Eras by time boundaries at 1,600 and 900 m.y. FIF2 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO Figure 1.—Location of the Lake Superior region as related to the Canadian Shield and to known or inferred Precambrian basement rocks of the North American craton (Morey, 1978b). The chronometric aspects of the correlation chart were constructed using procedures and conventions given in the introductory chapter of this series (Harrison and Peterman, 1984). The correlations are based on geologic relations and radiometric data published through early 1984. These data indicate that the Archean rocks in the Lake Superior region can be divided into two distinct suites—a gneiss terrane and a greenstone-granite terrane—that differ in age, rock assemblage, structural style, and metamorphic grade (Morey and Sims, 1976; Sims, 1980). The Late Archean (2,750-2,600 m.y.) greenstone-granite complexes that occur in the northern part of the region are typical of much of the Superior structural province (Stockwell and others, 1970); the southern part of the region is underlain by a complex assemblage of gneiss, amphibolite, and granite that is in part 3,600 m.y. old. Because these terranes have had profoundly different geologic histories and significant effects on the Early Proterozoic geology, they have been used to subdivide the correlation chart (pi. 1) into two discrete geographic entities, a northern segment underlain by greenstone-granite and a southern segment underlain by gneiss. ARCHEAN ROCKS The gneiss terrane and the greenstone-granite terrane are juxtaposed along a boundary named the Great Lakes tectonic zone (Sims and others, 1980). Although the location of the zone is fairly well defined (fig. 2), the manner in which the rocks of the two terranes are interrelated along and within the zone is not well understood. Seismic reflection profiling in east-central Minnesota (Gibbs and others, 1984) has defined a concentration of north-dipping seismic planes that project to the surface near the trace of the Morris fault, which Sims and others (1980) used to define the south edge of the tectonic zone. The moderate dip of these planes (about 30 °), their continuity over tens of kilometers, and their persistence throughout the crust in this region led Gibbs and others (1984) and Pierson and Smithson (1984) to suggest that the seismic planes correspond to thrust-fault zones that originated in Late Archean time and along which rocks of the greenstone-granite terrane were thrust over rocks of the gneiss terrane. Modeling of aeromagnetic and gravimetric data across the zone (Southwick and Chandler, 1983) also has delineated two layers separated by a northward-dipping boundary that is truncated on the north by a high-angle fault. This high-angle fault corresponds to the northern edge of the Great Lakes tectonic zone in Minnesota. The upper layer appears, from limited shallow drilling in the area, to consist largely of folded low-grade metavolcanic and metasedimentary rocks of Late Archean and (or) Early Proterozoic age. The lower layer has geophysical attributes similar to those associated with rocks in the gneiss terrane, but these rocks have not yet been drilled. Geologic relations in and around the Great Lakes tectonic zone are further complicated in Minnesota (Morey, 1983a,b), and in Wisconsin and Michigan (Sims and Peterman, 1981, 1983; Sims and others, 1984), by Late Archean and Early Proterozoic tectonic events that produced structures oriented subparallel to the boundary between greenstone-granite and gneiss terranes. Therefore, the Great Lakes tectonic zone is a fundamental crustal feature that was tectonically reactivated a number of times in the Precambrian and Phanerozoic. GNEISS TERRANE The gneiss terrane, as defined in the Lake Superior region, is that segment of Archean crust composed of basement gneisses that are in part Early and Middle Archean (or older than 3,000 m.y.) in age, have ubiquitous amphibolite to granulite metamorphic grade, and a distinctive structural style that is characterized in many areas by moderately dipping foliations and lowprecambrian rocks of the lake superior region F3 EXPLANATION Phanerozoic rocks, undivided Middle Proterozoic Midcontinent rift system (Age, ~ 1,200-1,000 m.y.) Wolf River batholith and Wausau Syenite Complex (Age, «1,500 m.y.) Early Proterozoic Quartzite, rhyolite, and epizonal granite (Age,»1,760-1,630 m.y.) Granitoid rocks (Age, 1,890-1,770 m.y.) Granitoid and volcanic rocks of the Wisconsin magmatic terrane (Age, « 1,890-1,825 m.y.) Stratified rocks of the Animikie basin—Includes the major iron-formations of the region (Age, « 2,100-1,860 m.y.) ► Figure 2.—Generalized bedrock geologic map of all Precambrian rocks of the Lake Superior region (modified from Morey and others, 1982, and Mudrey and others, 1982). Ym Yg;. Xq rrTTTTN] .-Wgm / Late Archean greenstone-granite terrane (Age, 2,750-2,650 m.y.) Agn Archean gneiss terrane (Age, ^3,600-2,600 m.y.) ----------Contact ----------Fault ---------Location of Great Lakes tectonic zone ! I LAKE MICHIGANF4 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO to moderately plunging fold axes. The terrane also contains some apparently younger (2,800-3,000 m.y.) migmatitic gneisses (Van Schmus and Anderson, 1977). In places, such as in east-central Minnesota (Southwick and Chandler, 1983), at Watersmeet, Mich. (Sims, 1980; Sims and others, 1984), and in Dickinson County, northern Michigan (James and others, 1961), older gneisses are unconformably overlain by Late Archean supra-crustal rocks and intruded by Late Archean plutonic rocks. Exposures of Early and Middle Archean rocks in the gneiss terrane are limited by an extensive cover of Proterozoic and Phanerozoic rocks and very young glacial debris, and accordingly, the geographic extent of the gneiss terrane is poorly known. The terrane is inferred, however, to continue eastward into the Lake Huron region of Ontario (Sims and others, 1981) and westward an unknown distance beneath Phanerozoic cover in South Dakota. The southern extent of the terrane into Iowa also is obscured by a thick Proterozoic and Phanerozoic cover (Anderson and Black, 1983). STRATIGRAPHY AND GEOCHRONOLOGY The gneiss terrane is best known from the Minnesota River Valley (fig. 3) where a grossly conformable sequence of interlayered migmatitic gneisses, apparently a few thousand meters thick (Grant, 1972), is intruded by younger granitic and pegmatitic rocks. The gneisses are folded on east-trending, gently plunging axes, and have mineral assemblages characteristic of upper amphibolite- and granulite-facies metamorphism (Bauer, 1980; Himmelberg and Phinney, 1967). The best exposures are in the Granite Falls-Montevideo (Himmelberg, 1968; Goldich, Hedge, and others, 1980) and Sacred Heart-Morton areas (Grant, 1972; Goldich, Wooden, and others, 1980; Wooden and others, 1980; Goldich and Wooden, 1980). In the Granite Falls-Montevideo area, the gneisses are metamorphosed to granulite grade and consist dominantly of a quartzofeldspathic gneiss called the Montevideo Gneiss (Goldich and others, 1970). The Montevideo Gneiss is a medium-grained, equigranular, leucocratic rock consisting of alternating layers of a gray, foliated, granodioritic paleosome and a red granitic neosome. The red granitic neosome occurs both concordantly and discordantly and as veins in the granodiorite. The gray granodioritic phase yields Rb-Sr whole-rock ages of about 3,680±70 m.y., whereas the red granite phase yields an age of 3,045±32 m.y. (Goldich, Hedge, and others, 1980). The terrane also contains layers and lenses of garnet-biotite gneiss, amphibolite, and metagabbro. These rocks record a high-grade metamorphic event at about 2,600 m.y. (Wilson and Murthy, 1976), but presumably all these units are about 3,500 m.y. old. Postmetamorphic mafic dikes and a small pluton called the granite of section 28 emplaced at 1,840±50 m.y. ago (Doe and Delevaux, 1980) intrude gneisses near Granite Falls. In the Morton-Sacred Heart area, Grant (1972) has delineated four rock units. The three lower units are quartzofeldspathic gneisses individually characterized by abundant, common, and rare rafts of amphibolite. The middle unit in this succession is the Morton Gneiss (Goldich and others, 1970). Where not migmatized, the quartzofeldspathic gneisses are tonalitic or granodioritic in composition and generally are compositionally layered. The uppermost stratigraphic unit in the area is composed of two kinds of biotite gneiss and amphibolite. One of the biotite gneisses contains the mineral association biotite-cordierite-garnet-anthophyllite in addition to quartz and plagioclase, and the other contains sillimanite and potassium feldspar and, locally, garnet and cordierite. Amphibolite also occurs in the biotite gneisses as discontinuous layers, lenses, and boudins. Rubidium-strontium and U-Pb data suggest that the tonalitic and granodioritic gneisses and the associated amphibolite lenses in the Morton-Sacred Heart area are 3,500 m.y. or more old (Goldich and Wooden, 1980). Also, zircon dating by ion microprobe methods indicates that the paleosome is 3,535±45 m.y. old (Williams and others, 1984), an age consistent with a Sm-Nd model age of 3,580±30 m.y. from a saprolitic clay developed from the Morton Gneiss (McCulloch and Wasserburg, 1978). Where migmatized, the neosome is granite and, locally, pegmatite, which formed during two later episodes. An older deformed granite was emplaced about 3,043±26 m.y., and a younger, largely undeformed granite was emplaced 2,555±55 m.y. ago (Goldich and Wooden, 1980). The gneissic rocks are intruded by the Sacred Heart Granite of Lund (1956), a medium-grained, generally homogeneous to weakly foliated rock that yields a Pb-Pb age of 2,605±6 m.y., which Doe and Delevaux (1980) interpreted as the time of emplacement. The Sacred Heart Granite is late-tectonic or possibly posttectonic. In east-central Minnesota gneisses of possible Early or Middle Archean age include the Richmond and Sartell Gneisses of Morey (1978a), the Hillman Migmatite of Morey (1978a) and the McGrath Gneiss of Woyski (1949). The first three units form a basement that lies to the west of an extensive Early Proterozoic cover. These gneisses have not been dated radio-metrically, but they resemble those in the Minnesota River Valley in having been metamorphosed to the upper amphibolite or granulite facies and folded into generally open, moderately plunging antiforms andPRECAMBRIAN ROCKS OF THE LAKE SUPERIOR REGION F5 95°45' 95°30' 95°15' 95°00' EXPLANATION GRANITE FALLS-MONTEVIDEO AREA Early Proterozoic granite of section 28 Archean Biotite-garnet gneiss Interlayered gneisses Hornblende-pyroxene gneiss Granitic gneiss with rafts of amphibolite—Includes Montevideo Gneiss SACRED HEART-MORTON AREA Early Proterozoic gabbro-granophyre rocks of the Cedar Mountain Complex Quartzofeldspathic gneiss Quartzofeldspathic gneiss with amphibolitic rafts— Includes the Morton Gneiss Interlayered quartzofeldspathic gneiss and amphibolite Approximate contact—Dashed where inferred Fault—Dashed where inferred Antiform—Showing crest line and direction of plunge Synform—Showing trough line and direction of plunge Archean Foliated quartz monzonitic and granitic rocks of the Sacred Heart batholith Aluminous biotite gneiss and amphibolite Approximate edge of valley Figure 3.—Geologic map of Minnesota River Valley, southwestern Minnesota, showing major rock units and structural attributes as mapped by Grant (1972).F6 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO synforms (Dacre and others, 1984). The McGrath Gneiss forms several gneiss domes surrounded by superjacent metasedimentary and metavolcanic strata of Early Proterozoic age (Morey, 1978b). It is a coarsegrained, locally migmatitic, augen gneiss of quartz mon-zonitic composition. It yields a minimum Rb-Sr whole-rock isochron age of 2,700 m.y. (Stuckless and Goldich, 1972). The original age of the McGrath, however, is obscured by a pervasive metamorphic and cataclastic event at about 1,770 m.y. ago (Keighin and others, 1972). In the western part of northern Michigan, near the Michigan-Wisconsin boundary, Archean gneiss forms several domes. Although not all the domes have been studied in detail, the rocks in one dome—informally called the gneiss at Watersmeet (Sims and Peterman, 1976)—are an augen gneiss of tonalitic to granodioritic composition, which is intruded by younger granitic rocks. Radiometric studies of the gneisses indicate an Early Archean age. 207Pb/206Pb ages on severed fractions of zircon range from 3,199 to 3,411 m.y., and a primary age of 3,562±39 m.y. is indicated by a Concordia plot (Peterman and others, 1986). Zircon dating by the ion microprobe has yielded a somewhat older age of approximately 3,650 m.y. (Williams and others, 1984), which is consistent with a Sm-Nd age of about 3,620±30 m.y. (McCulloch and Wasserburg, 1980). In contrast, whole-rock Rb-Sr systems in the dome are highly disturbed and define secondary isochrons of about 1,750 m.y., recording reactivation during Early Proterozoic time (Sims and others, 1984). At Watersmeet, the gneiss is unconformably overlain by amphibolite and biotite gneiss which yields a U-Th-Pb age of approximately 2,640 m.y. The amphibolite and gneiss are cut by a 2,590-m.y. leucogranite (Sims and others, 1984). As in the Watersmeet area, the geology in southern Marquette and Dickinson Counties, in upper Michigan (fig. 2), consists of islandlike masses of Archean gneiss surrounded by tightly folded supracrustal strata of Late Archean or Early Proterozoic age. Gneissic rocks are exposed over a broad area south of the Marquette district in the informally designated Southern complex of Van Hise and Bayley (1895), in the Amasa uplift (Gair and Wier, 1956), a large northwest-trending gneiss dome located about 15 km west of the Southern complex, and in several other unnamed, fault-bounded antiformal blocks in Dickinson County (James and others, 1961). The Southern complex consists dominantly of the Bell Creek Gneiss (Cannon and Simmons, 1973), whereas the core of the Amasa uplift is composed of the Margeson Creek Gneiss of Gair and Wier (1956). These similar and possibly equivalent gneisses are very coarse grained megacrystic rocks of granitic to granodioritic composition. The Bell Creek Gneiss is intruded by parts of the Compeau Creek Gneiss of Gair and Thaden (1968) along the north edge of the Southern complex. The latter is dominantly tonalitic to granodioritic in composition and contains lenses and layers of amphibolite. Rubidium-strontium data indicate that the Bell Creek and Compeau Creek Gneisses are at least 2,550 m.y. old, and more likely are about 2,750 m.y. old (Van Schmus and Woolsey, 1975; Hammond and Van Schmus, 1978). The gneisses have been affected by several later events as indicated by Rb-Sr ages on biotite of 1,630±40 m.y. (Van Schmus and Woolsey, 1975), whole-rock Rb-Sr ages of about 1,850 m.y., and local anatexis in the Compeau Creek Gneiss at about 2,350 m.y. ago (Hammond and Van Schmus, 1978). Strongly deformed and cataclastic gneissic rocks in Dickinson County, Mich., include the granite gneiss of the Norway Lake area of James and others (1961) and the Carney Lake Gneiss of Bayley and others (1966). The former is a porphyroclastic augen granitic gneiss containing inclusions of mafic metavolcanic material, schist, and quartzite, whereas the latter consists dominantly of granitic gneiss with lesser interlayered amounts of hornblende-biotite gneiss and biotite-rich gneiss. The granite gneiss of the Norway Lake area is unconformably overlain by metamorphosed strata assigned to the Dickinson Group (James, 1958). The uppermost units of this group, which consist dominantly of arkose, metagraywacke, schist, and amphibolite, grade laterally and vertically into a belt of granitic gneiss, which in turn apparently grades into the gneiss at Granite Bluffs of Aldrich and others (1965) as well as into several other masses of unnamed granitic gneiss. Thus, at least two gneiss-forming events may have taken place in Dickinson County. Attempts to date the granite gneiss of the Norway Lake area have been only partially successful, mainly because of multiple later events. Banks and Van Schmus (1971, 1972) reported that zircons from the gneiss yield an age of about 2,400 m.y; Rb-Sr data, however, indicated an older parentage. More recent zircon analyses for the granite gneiss at Norway Lake (Van Schmus, unpub. data, 1986) confirm that it is about 2,600 to 2,800 m.y. old, but indicated also that it has been altered by subsequent events. Post-Dickinson granitic gneisses yield zircons with ages of 2,600 m.y., but Rb-Sr data for these rocks indicate a major event causing redistribution of Rb and Sr about 1,965 ±65 m.y. ago (Van Schmus and others, 1978). Detrital zircons from the East Branch Arkose of the Dickinson Group yield Pb-Pb ages of about 2,900 m.y. (Banks and Van Schmus, 1972) which, together with the presence of inclusions of schist and quartzite in the granite gneiss at Norway Lake, imply the existence of a terrane older than 2,900 m.y. in the region. iPRECAMBRIAN ROCKS OF THE LAKE SUPERIOR REGION F7 Zircons from the Carney Lake Gneiss in Dickinson County, the southernmost exposed Archean unit in Michigan, indicate an age of about 2,750 m.y. (Aldrich and others, 1965; Van Schmus and others, 1978). These data are difficult to interpret in terms of specific events; they clearly show that the Archean rocks of this region were extensively reworked during Proterozoic time. Gneissic rocks in central Wisconsin are separated from those in northern Michigan by a belt of Early Proterozoic volcanic and plutonic rocks, named the Wisconsin magmatic terrane (fig. 2). The gneissic terrane consists dominantly of amphibolite, gametiferous hornblende gneiss, and schist, all intruded by mafic rocks, quartz diorite, and several kinds of granitic rocks and their cogenetic pegmatites (Myers, 1974; Cummings, 1984). Their age has not been determined. Zircons from migmatitic gneisses in the Pittsville area and along the Black River define a concordia-intercept age of approximately 2,800 m.y. (Van Schmus and Anderson, 1977; DuBois and Van Schmus, 1978). Although the Rb-Sr systematics observed for whole-rock samples from the Pittsville area clearly reflect the event at 2,800 m.y., they also can be interpreted as indicating a protolith age in the range of 3,000 m.y. to 3,200 m.y. for the gneisses (Van Schmus and Anderson, 1977). Recent U-Pb results on zircons from a small remnant of Archean gneiss near Marshfield, Wis., indicate that the protolith to the gneiss is about 3,000 m.y. old (Van Schmus, unpub. data, 1986). Van Schmus and Anderson (1977), Maass and others (1980), and Van Schmus (1980) have shown that the gneisses were intruded by several kinds of tonalitic and granodioritic rocks as young as 1,830±15 m.y.; some of these younger rocks have a secondary foliation similar to that in the gneisses, which implies that the terrane was extensively deformed during Early Proterozoic time. Consequently, it is difficult to determine whether the Precambrian rocks in central Wisconsin are an Archean gneissic basement with many Early Proterozoic plutons or a plutonic complex with many remnants of an Archean gneissic crust. GREENSTONE-GRANITE TERRANE Late Archean rocks of the greenstone-granite terrane constitute nearly all the bedrock in northern Minnesota (fig. 2); but these rocks are well exposed only in the Vermilion district of northeastern Minnesota (Sims, 1976a), along the International Boundary in the Rainy Lake district (Goldich and Peterman, 1980), and in the Birchdale-Indus area (Ojakangas and others, 1977). Rocks of the greenstone-granite terrane also are exposed locally south of Lake Superior along the Wisconsin-Michigan border and in northern Michigan just south of Lake Superior. In these areas, the Archean rocks occur as inkers surrounded and partly overlain by strata of Early Proterozoic age. The greenstone-granite terrane forms the southern part of the Superior province of the Canadian Shield. In Canada the terrane has been divided into several large, east-northeast-trending volcanic-plutonic belts or subprovinces that alternate with belts of migmatite and gneiss (Goodwin, 1978). Three of these belts extend westward into Minnesota; from north to south they are the Wabigoon volcanic-plutonic belt, represented by exposures in the Birchdale-Indus and Rainy Lake districts; the Quetico gneiss'belt, represented by exposures in the Vermilion Granitic Complex; and the Shebandowon or Wawa volcanic-plutonic belt, represented by exposures in the Vermilion district proper. The Late Archean rocks that crop out south of Lake Superior are most likely part of the Wawa volcanic-plutonic belt, although those in central Wisconsin could be part of yet another migmatite-gneiss belt. In northern Minnesota, the volcanic-plutonic belts are characterized by overlapping piles of mafic volcanic rocks and related synvolcanic intrusions (Schulz, 1980, 1982). Lenses of banded iron-formation occur throughout and are common in the upper parts of the dominantly mafic successions (Morey, 1980). The mafic rocks pass abruptly upward and laterally into dacitic to rhyodacitic volcaniclastic rocks, which in turn grade upward and laterally into dominantly volcanogenic graywacke-shale sequences derived from the volcanic centers (Ojakangas, 1972). Beds of chert, carbonaceous shale, and rare siliceous marble also are present in parts of the graywacke-shale sequences (Morey and others, 1970; Green, 1970). The complexly interlayered volcanic and sedimentary rocks have been folded several times (Hooper and Ojakangas, 1971; Hudleston, 1976) and metamorphosed to the greenschist facies contemporaneously with diapiric rise of granitic bodies of batho-lithic dimensions. Because of the characteristic greenschist-facies metamorphism, the volcanicsedimentary successions are commonly called greenstone belts. Most of the plutonic igneous rocks that bound the several greenstone belts in northern Minnesota are younger than the volcanism and sedimentation, although one pluton—the Saganaga batholith in the Vermilion district—was emplaced and unroofed more or less contemporaneously with sedimentation. The Giants Range batholith along the south side of the Vermilion district is divided into several older syntectonic units of tonalitic to granitic composition and younger posttectonic rocks of monzonitic to quartz monzonitic composition (Sims, 1976a). Posttectonic rocks also form several small plutons scattered widely along the lengths of the greenstone belts.F8 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO Late Archean migmatitic gneiss belts are represented in northern Minnesota (fig. 4) by the Vermilion Granitic Complex (Southwick and Sims, 1979). The interior portions of this fault-bounded structural entity consist entirely of granite—previously named the Vermilion Granite (Grout, 1925), but now called the Lac La Croix Granite (Southwick and Sims, 1979). Most of the complex, however, consists of biotite schist and amphibolite, metamorphosed generally to the upper greenschist facies (garnet grade) and in places (particularly near larger bodies of igneous rock) to the upper amphibolite facies (sillimanite grade). The gneisses also have had a complex history of injection, anatexis, metasomatism, and deformation. The Lac La Croix Granite was more or less passively emplaced into previously folded stratified rocks, most likely at depths somewhat greater than plutonic activity in adjoining volcanic-plutonic belts (Southwick, 1972, 1976; Day, 1983). 47°45' 0 10 20 30 40 50 KILOMETERS 1 -----1______I_______I______I_______I EXPLANATION Late Archean Syenitic plutons Giants Range batholith Monzonite and quartz monzonite Granite-tonalite Vermilion Granitic Complex Saganaga Tonalite Newton Lake Formation Knife Lake Group Lake Vermilion Formation Ely Greenstone Upper member Soudan Iron-formation Member Lower member ----- Contact ----- Fault Figure 4—Geologic map of a part of the Vermilion district (modified from Sims, 1976a) and the Vermilion Granitic Complex (modified from Southwick and Sims, 1979). The Vermilion fault separates the Vermilion district to the south from the Vermilion Granitic Complex to the north. This break also corresponds with the boundary between the Wawa volcanic-plutonic belt to the south and the Quetico gneiss belt to the north.PRECAMBRIAN ROCKS OF THE LAKE SUPERIOR REGION F9 STRATIGRAPHY AND GEOCHRONOLOGY The stratigraphic succession of Late Archean rocks astride the International Boundary between Minnesota and Ontario contains many stratigraphic units prominently mentioned in the geologic literature (fig. 5). Lawson (1888, 1913) distinguished two periods of deformation and magmatic activity which he called the Laurentian and the Algoman. In his view, the Laurentian rocks were intruded into a dominantly metasedimentary sequence (called the Coutchiching Series), which in turn was overlain by a dominantly metavolcanic sequence (called the Keewatin Series). After a period of erosion, a second metasedimentary sequence (called the Seine Series) was deposited, and this second sequence and also the older rocks were deformed a second time when the Algoman intrusions were emplaced. The concept of a two-fold subdivision separated by a post-Laurentian unconformity persisted well into the Keewatin greenstones -(major unconformity) Coutchiching metasedimentary rocks ? ? ? (major unconformity) Coutchiching metasedimentary rocks ? ? ? D Giants Range granite Knife Lake slates ooooooqpooo Ogishke conglomerate (major _ unconformity) Saganaga granite ^Soudan iron-bearing formation \ Ely greenstone ' ? ? ? Giants Range and 00000,000 _ Knife Lakes series (1 8 members) Saganag Granite of (major unconformity) Vermilion Granite of Algoman series Laurentian series Soudan formation Ely greenstone ? ? ? Algoman batholithic Knife Lake Group (major unconformity) \Soudaji iron^bearing_member LaurentianV" Qf £|y Greenstone intrusions \ Giants Range and Vermilion Granite (Algoman Orogeny) Knife Lake Group (major unconformity) (minor unconformity) Ely Greenstone 'RanananASoudari Iron-Formation Granite ' (Laurentian Orogeny) N Coutchiching metasedimentary rocks (major unconformity) (minor unconformity) Figure 5—Idealized sections showing evolution of stratigraphic nomenclature in the Archean greenstone-granite terrane of northern Minnesota and adjoining Ontario. A, Rainy Lake district (Lawson, 1885, 1888); B, Rainy Lake district (Adams and others, 1905; Lawson, 1913); C, Vermilion district (Clements, 1903); D, Eastern Vermilion district (Gruner, 1941); E, Northern Minnesota (Grout and others, 1951); F, Northern Minnesota (Goldich and others, 1961); G, Vermilion district (Morey and others, 1970; Sims, 1976a). Keewatin GroupF10 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO middle of the 20th century (for example, Grout and others, 1951). However, as pointed out first by Goodwin (1968) and since then by many others, any volcanicsedimentary assemblage in the Superior province may consist of a single mafic-to-felsic-to-sedimentary sequence, or it may consist of several of these sequences that are repeated in space and time. The resulting stratigraphic succession can be complex, particularly near the fringes of individual volcanic centers where any stratigraphic interpretation is complicated by the interfingering of diverse volcanic and sedimentary rock types. All the major rock-iinning events in the Rainy Lake area occurred between approximately 2,700 and 2,600 m.y. ago (Tilton and Grunenfelder, 1968; Hart and Davis, 1969; Peterman and others, 1972; Goldich and Peterman, 1980). Hart and Davis (1969) obtained several whole-rock Rb-Sr isochron ages in the interval 2,700-2,500 m.y. ago that were numerically consistent with the stratigraphic succession, as described by Lawson. However, zircon samples from all the units collectively define a U-Pb age of about 2,710±30 m.y.; Peterman and others (1972) also obtained a Rb-Sr age for the so-called Algoman granites of 2,485±90 m.y., which is approximately 185 m.y. younger than a corresponding U-Pb zircon age of 2,670 m.y. Such discrepancies between older U-Pb, and apparently younger Rb-Sr whole-rock, potassium-argon, and Rb-Sr mineral ages (Hanson and others, 1971) are common in the greenstone-granite terrane of northern Minnesota. Peterman and others (1972) have suggested that the rocks behaved as partially open systems during one or more periods of subsequent low-grade metamorphism, hydrothermal alteration, or cataclasis. An interlayered sequence of metavolcanic and metasedimentary rocks also occurs in the Vermilion district of northern Minnesota (fig. 5G), where the Ely Greenstone interfingers with and is overlain by the Lake Vermilion Formation or the Knife Lake Group (Morey and others, 1970). A second volcanic unit, the Newton Lake Formation, which also contains several tabular, ultramafic intrusions, overlies the Knife Lake Group (Green, 1970). This simple stratigraphic succession is complicated at the east end of the district by the Saganaga batholith, a classic example of a “Laurentian batholith.” (See Grout and others, 1951.) This pluton intrudes mafic volcanic rocks apparently equivalent to the Newton Lake Formation, whereas adjacent Knife Lake strata lie on its eroded surface. Therefore, part of the Knife Lake Group is older and part of it is younger than the Saganaga batholith. Igneous rocks of the Giants Range batholith intrude all the stratified rocks and appear to represent the last major rock-forming event in the greenstone-granite terrane. Radiometric dating has delineated only one major period of plutonic igneous activity and metamorphism in the Vermilion district. Rocks of the Saganaga batholith have yielded a U-Pb age of 2,650±50 m.y. (Anderson, 1965) and two Rb-Sr whole-rock isochron ages of 2,650 m.y. and 2,630 m.y. (Hanson and others, 1971). The batholithic rocks are cut by the posttectonic Icarus pluton, which has yielded a similar Rb-Sr isochron age of approximately 2,630 m.y. (Hanson and others, 1971). The Giants Range batholith, along the south side of the Vermilion district, and the Lac La Croix Granite along the north side have yielded Rb-Sr isochron ages of 2,610±65 m.y. (Prince and Hanson, 1972), and 2,620±95 m.y. (Peterman and others, 1972), respectively. Supracrustal rocks in the district have Rb-Sr isochron ages of 2,590±110 to 2,630±180 m.y. (Jahn and Murthy, 1975). Posttectonic rocks dated by U-Pb methods include the Snowbank Lake pluton, emplaced at approximately 2,700 m.y. ago (Catanzaro and Hanson, 1971). Thus a significant igneous-tectonic event occurred in the greenstone-granite terrane of northern Minnesota at about 2,700 m.y. ago. Stratigraphic relations in the greenstone-granite terrane south of Lake Superior have not been studied in detail. However, in northwestern Wisconsin and adjoining parts of Michigan, pillowed mafic lavas and mafic to felsic pyroclastic rocks assigned to the Ramsay Formation are intruded by the Puritan Quartz Monzonite (Schmidt, 1976), a plutonic body of batholithic dimensions that has yielded a Rb-Sr whole-rock isochron age of 2,650±140 m.y. (Sims and others, 1977). However, zircons from a nearby, related granite in the Great Lakes tectonic zone gave a concordia intercept age of 2,745±65 m.y., which probably also is the best estimate of the age of the Puritan Quartz Monzonite (Peterman and others, 1980). Similar rocks also assignable to the greenstone-granite terrane constitute the Northern complex of the Marquette district in northern Michigan (Van Hise and Bayley, 1895). The oldest rock unit, called the Kitchi Schist (Gair and Thaden, 1968; Morgan and DeCristoforo, 1980) consists of mafic flows and tuffs which grade upward into a coarse volcanic breccia of felsic composition. The lower part of the Kitchi contains a large serpentinized ultramafic body, the Deer Lake Peridotite of Morgan and DeCristoforo (1980). The Kitchi Schist is overlain in turn by the Mona Schist, a sequence of mafic flows with iron-formation near the top. The metavolcanic rocks have been intruded and metamorphosed by granitic rocks assigned to the Com-peau Creek Gneiss (Gair and Thaden, 1968) and by other unnamed tonalitic to granodioritic plutons. These units, as well as the volcanic rocks, yield U-Pb ages of 2,750 to 2,700 m.y. (Peterman and others, 1980; HammondPRECAMBRIAN ROCKS OF THE LAKE SUPERIOR REGION Fll and Van Schmus, 1978). Thus, when all the radiometric data are considered collectively, the greenstone-granite terrane in the Lake Superior region formed in a remarkably short span of time about 2,700 m.y. ago. PROTEROZOIC ROCKS Rocks of Early and Middle Proterozoic age are extensive in the southern part of the Lake Superior region (fig. 2). The Early Proterozoic rock record consists of two sedimentary-volcanic supracrustal sequences separated in time by a number of discrete tectonic-thermal events that occurred during the interval of 1,850±30 m.y. ago (Van Schmus and Bickford, 1981), except in Michigan, where events possibly as young as 1,750 m.y. have been recognized (Sims and others, 1984). These events have been referred to collectively as the Penokean orogeny (Goldich and others, 1961; Cannon, 1973). The pre-Penokean rocks of Early Proterozoic age are a “geosynclinal sequence,” composed mainly of clastic sedimentary rocks, with lesser but significant amounts of mafic volcanic rocks, and subordinate chemical sediments, chiefly iron-formation. The post-Penokean sequence consists dominantly of subaerial rhyolite and coeval granitic rocks overlain by quartz-rich sedimentary rocks of fluvial origin. The upper part of the post-Penokean sequence also contains units of black shale and iron-formation-bearing dolomite of possibly marine origin. The post-Penokean rocks were affected by a tectonic-thermal event at about 1,630 m.y. ago that also disturbed the isotopic systems of many of the older rocks in the Lake Superior region. Rock-forming events in Middle Proterozoic time included the intrusion of the Wolf River batholith and several syenitic complexes in south-central Wisconsin and the formation of the Midcontinent rift system, a major structure that extends southward from the Lake Superior region to at least the southern part of Kansas. PRE-PENOKEAN, EARLY PROTEROZOIC ROCKS Pre-Penokean, Early Proterozoic rocks constitute a discontinuous linear foldbelt some 1,300 km long extending from northern and east-central Minnesota, through northern Wisconsin, and across much of the southern part of northern Michigan. These rocks compose the major part of the Southern province (Stockwell and others, 1970) or the Hudsonian fold belt along the United States-Canada border on the tectonic map of North America (King, 1969). In addition to abundant clastic rocks, this sequence contains nearly all the commercially exploited iron-formations in the region (fig. 6). A second and possibly younger sequence of dominantly volcanic rocks forms a separate easttrending belt that extends across northern Wisconsin. The pre-Penokean stratified rocks of the Lake Superior region were deposited in the Animikie basin (Morey, 1983a), one of several basins that formed over and approximately parallel to the Great Lakes tectonic zone (Sims and others, 1981). The strata form a generally southward-thickening wedge that can be divided into two facies on the basis of pronounced differences in rock type and thickness: (1) a relatively thin succession (1.5-2.0 km) of predominantly sedimentary rocks deposited on greenstone-granite terrane north of the Great Lakes tectonic zone; and (2) a much thicker succession of intercalated sedimentary and bimodal basaltic and rhyolitic volcanic rocks to the south (Sims and others, 1981). The rocks of the Animikie basin in Michigan are fault bounded on the south against a thick, stratigraphical-ly complex sequence of volcanic rocks, including basalt, andesite and rhyolite, and lesser amounts of sedimentary rocks of clastic and chemical origin. This dominantly volcanic sequence forms the Wisconsin magmatic zone (or terrane), which occurs as an east-trending belt extending across much of northern Wisconsin (Greenberg and Brown, 1983; May and Schmidt, 1982). Although extensively developed in Wisconsin, correlative volcanic rocks have not been recognized along the south side of the Animikie basin in east-central Minnesota. Because of this and because a fault (Niagara fault) separates volcanic rocks from the rocks in the eastern segment of the Animikie basin, the stratigraphic relations of the volcanic sequence to the better known dominantly sedimentary succession are not clearly understood. The stratified rocks of the Animikie basin were extensively deformed and metamorphosed during the Penokean orogeny (Cannon, 1973; Klasner, 1978; Morey, 1978b). The degree of deformation varies considerably from north to south and appears related at least in part to contrasting kinds of Archean basement rocks. Thus, the stratified rocks can be divided into two broad zones on the basis of contrasting styles of deformation and grades of metamorphism—a northern stable cratonic zone and a southern deformed zone termed the Penokean fold belt (Sims and others, 1980; Sims and Peterman, 1983). The tectonic front separating the two zones coincides approximately with the inferred northern edge of the Great Lakes tectonic zone. North of the tectonic front, the Proterozoic supracrustal rocks unconformably overlie the Archean greenstone-granite basement and are virtually undeformed and unmetamorphosed (Morey, 1983a). South of the tectonic front, supracrustal rocks within theF12 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO GUNFIINT RANGE Marquette CUYUNA RANGE .IRON RIVER-CRYSTAL^ FALLS jf RANGE/V nfegl lASA UPLIFT S/JStA 200 KILOMETERS EXPLANATION Rocks younger than Early Proterozoic in age—Undivided except for Ygs, Wausau Syenite Complex, and Ygw, Wolf River batholith - Xv Early Proterozoic Granitoid rocks of tonalitic to granitic composition—Age, 1,770-1,890 m.y Volcanic and granitoid rocks of the Wisconsin magmatic terrane— Includes metavolcanic rocks, older granitic gneiss, and younger grano-dioritic and granitic rocks Stratified rocks of the Animikie basin—Includes major iron-formations shown as screened Late Archean rocks of the greenstone-granite terrane, undivided Archean rocks of the gneiss terrane, undivided Contact -----------Fault Figure 6.—Generalized geology of Lake Superior region showing distribution of Early Proterozoic rocks referred to in text (modified from Morey and others, 1982).PRECAMBRIAN ROCKS OF THE LAKE SUPERIOR REGION F13 foldbelt display two contrasting tectonic styles. The rocks in Minnesota and in a broad subzone 60-70 km wide between the tectonic front and the Niagara fault zone in Wisconsin and Michigan are intensely deformed (for example, Cannon, 1973; Klasner, 1978) and are characterized by a nodal distribution of metamorphic zones (James, 1955). The volcanic rocks south of the Niagara fault zone in Wisconsin also are intensely deformed, but the metamorphic patterns have a linear rather than a nodal geometry (Sims and Peterman, 1983). The volcanic rocks south of the fault zone also contain numerous mesozonal granitic plutons of Early Proterozoic age (Sims and Peterman, 1980; Van Schmus, 1980). All supracrustal rocks within the Animikie basin are presumed to lie unconformably on Archean rocks, and Early or Middle Archean gneisses are exposed in the cores of several gneiss domes or upraised fault blocks. The volcanic rocks also lie on Archean gneisses in places, but in other places the basement gneisses are of Early Proterozoic age (Sims and others, 1982). STRATIGRAPHY AND GEOCHRONOLOGY The Animikie basin is separated into two distinct segments by Middle Proterozoic and younger rocks (fig. 6). Rocks of the northwest segment contain the sequences of the Mesabi and Cuyuna iron ranges of northern and east-central Minnesota and of the Gunflint range of Ontario; and the southeast segment contains those of the Gogebic, Marquette, Menominee, and Iron River-Crystal Falls ranges of northern Wisconsin and Michigan (fig. 7). Because they are physically separated, the rocks in the northwest segment have been assigned to the Mille Lacs and Animikie Groups (Morey, 1978a) and those in the southeast segment to the Marquette Range Supergroup (Cannon and Gair, 1970). The Marquette Range Supergroup is much thicker and more diverse than the Mille Lacs and Animikie Groups and is interrupted by unconformities that serve to divide it into the Chocolay, Menominee, Baraga, and Paint River Groups (James, 1958). A major stratigraphic problem concerning the stratified rocks of the Animikie basin has been their correlation with the partly glaciogenic Huronian Supergroup (Robertson and others, 1969), on the north shore of Lake Huron in Ontario. (See Young, 1966; Church and Young, 1970; and Young, 1983, for a discussion.) The basal part of the Early Proterozoic succession at places in Michigan contains coarse clastic deposits, named the Reany Creek and Fern Creek Formations, that may be partly glacial in origin (Pettijohn, 1943; Puffett, 1969; Gair, 1975, 1982; Ojakangas, 1984; for a contrary view see Larue, 1981a, or Mattson and Cambray, 1983). Since the work of James (1958), most correlations place the Reany Creek and Fern Creek Formations in the Chocolay Group. However, Pettijohn (1943), Tyler and Twenhofel (1952), and Dutton and Bradley (1970) have suggested that they are separated from overlying Chocolay strata by an unconformity. Regardless of whether the unconformity exists, the Reany Creek-Fern Creek strata could be correlative with glacial units in the upper part of the Huronian Supergroup in Canada. Young (1983) further extended the correlation of Marquette Range and Huronian Supergroup strata by correlating the entire Chocolay Group with the upper part of the Huronian Supergroup. The present chronometric data seem to indicate that at least parts of the Marquette Range Supergroup are younger than the Huronian Supergroup. In the Felch trough area of Michigan, the basement rocks were affected by a metamorphic episode about 2,000 m.y. ago that did not affect the unconformably overlying Marquette Range rocks (Van Schmus and others, 1978). Similarly, the Mille Lacs Group in east-central Minnesota unconformably overlies a migmatitic terrane yielding an apparent Rb-Sr age of 2,100 to 2,000 m.y. (Goldich, 1973). In northern Minnesota, the somewhat younger Animikie Group unconformably overlies dike rocks 2,120±67 m.y. old (Beck and Murthy, 1982). These events correspond to the approximate age of the Nipissing Diabase dikes which intrude Huronian strata in Canada (Van Schmus, 1965, 1976; Fairbairn and others, 1969; Gibbons and McNutt, 1975), thus making the Huronian older than 2,100 m.y. These relationships imply that the Mille Lacs and Animikie Groups and at least most of the Marquette Range Supergroup are distinctly younger than the Huronian Supergroup. Although the actual depositional age is unknown, Banks and Van Schmus (1971, 1972) have reported a U-Pb age of 1,910±10 m.y. for zircon from a rhyolite in the Marquette Range Supergroup. The rhyolite unit is part of the Hemlock Formation, which traditionally has been placed in the Baraga Group (James, 1958). However, Prinz (1976) suggested that the Hemlock Formation correlates with the Negaunee Iron-formation of the Menominee Group, a correlation diagrammed in figure 7. Lithostratigraphic correlations within and between the individual iron-mining districts within the Animikie basin are well established, and the Marquette Range Supergroup generally is interpreted as recording a complete transition from a “stable craton” to a “eugeosyn-clinal” environment (James, 1954; Bayley and James, 1973; Larue and Sloss, 1980; Morey, 1983a). Evidence for such a transition is especially clear in the northwest segment of the Animikie basin, where the sedimentary record can be divided into five depositional phasesNorthwest segment Southeast segment Gunflint range, Minnesota-Ontario Mesabi Range, Minnesota Cuyuna range, Minnesota Western part Gogebic range, Wisconsin Eastern part Gogebic range, Michigan Western Marquette range, Michigan Eastern Marquette range, Michigan Amasa uplift, Michigan Iron River-Crystal Falls range, Michigan Menominee range, Wisconsin Goodwin, 1956 White, 1954 Morey, 1978a Aldrich, 1929 Sims and others, 1 984 Cannon, 1986 Gair and Thaden, 1 968 Cannon, 1 986 Cannon, 1 986 Bayley and others, 1968 Fortune Lakes Slate Stambaugh Formation Hiawatha Graywacke Riverton Iron-formation Dunn Creek Slate j Paint River Group Marquette Range Supergroup | Animikie Group Rove Formation Virginia Formation Rabbit Lake Formation Tyler Formation X. Michigamme \lFormation Copps Formation Michigamme Formation Badwater Greenstone Badwater Greenstone Badwater Greenstone | Baraga Group; Michigamme Slate Michigamme Formation Michigamme Slate Goodrich Quartzite Goodrich Quartzite Gunflint Iron-formation Biwabik Iron-formation Trommald Formation Iron wood Iron-formation Emperor / Volcanic Complex/giajr /Z Creek // Formation Negaunee Iron-formation Negaunee Iron-formation Amasa and Fence River Formations Amasa and Fence River Formations Vulcan Iron-formation Menominee Group Basal .conglomerate Pokegama Quartzite Mahnomen Formation Palms Formation Palms Formation Siamo Slate Siamo Slate Hemlock Formation Hemlock Formation Fetch Formation ^-N_^nember Ajibik Quartzite Ajibik Quartzite Wewe Slate * Chocolay Group \ Mille Lacs Group ! Trout Lake Formation , \ Chocolay Group ] Bad River Dolomite Bad River Dolomite Kona Dolomite Saunders Formation Randville Dolomite Little Falls Formation Glen Township Formation Denham Formation Sunday Quartzite Mesnard Quartzite Sturgeon Quartzite Enchantment Lake Formation Reany Creek Formation Fern Creek Formation Archean rocks, undivided Figure 7.—Correlation chart for Early Proterozoic strata in the Animikie basin (modified from Morey, 1983b). F14 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICOPRECAMBRIAN ROCKS OF THE LAKE SUPERIOR REGION F15 (Morey, 1983b). The first two phases constitute a miogeosynclinal sequence during which sediments were derived from both north and south of the basin (see also Ojakangas, 1983). The third phase represents a shelf upon which iron-rich strata were precipitated, whereas the fourth phase forms a transitional sequence marked by rapid subsidence of the shelf and concurrent deposition of black, carbonaceous mud. The fifth phase is a southward-thickening flysch deposited by southwardflowing turbidity currents associated with submarine fan complexes. Stratified rocks over the greenstone-granite basement in the southeast segment of the Animikie basin are similar to those in the northwest segment. For example, the Chocolay and Menominee Groups along the Gogebic Range in Michigan are strikingly similar to the Mille Lacs and Animikie Groups in Minnesota. Traditionally such lithologic similarities provide much of the evidence for correlation of strata between the northwest and southeast segments of the basin. The stratigraphic succession is much more diverse and complex elsewhere in the southeast segment where the basement is Archean gneiss. The Marquette Range Supergroup can be divided into three, generally fining upward depositional cycles (Sims and others, 1981). The first cycle consists mostly of shallow-water sediments, stromatolitic dolomite, and locally, slate, which collectively constitute the Chocolay Group (Larue, 1981a). The second cycle consists of the Menominee Group and contains the major iron-formations of the region (Larue, 1981b). Although the iron-formations over the greenstone-granite terrane record nearly contemporaneous transgressive sedimentation over large parts of the Archean craton, those over the gneiss terrane differ greatly in thickness, stratigraphic detail, and depositional facies. Moreover, the Menominee Group contains appreciable quantities of bimodal mafic and felsic volcanic rocks. The third cycle, starting at the base of the Baraga Group, was a period of pronounced crustal disturbance characterized by differential subsidence that culminated with deposition of a thick graywacke-slate sequence. Volcanic rock units in the southern part of the basin are lenticular and locally as much as 3 km thick. Gab-broic dikes and sills were emplaced at this time as sub-volcanic equivalents of the basaltic lavas (Cannon, 1973). A similar, unstable tectonic regime persisted through deposition of the Paint River Group in the southern part of the basin. ‘ Inasmuch as depositional patterns in the southeast segment are very similar to those in the northwest segment, it seems certain that both segments are part of the same intracontinental basin. This basin formed by rifting processes akin to those associated with proto-ceanic rift systems of Phanerozoic age (Cambray, 1978). The Niagara fault system along the southern edge of the Animikie basin, in northeastern Wisconsin, juxtaposes dominantly sedimentary rocks of the basin against a nearly 200-km-wide sequence of dominantly volcanic rocks to the south (May and Schmidt, 1982; Greenberg and Brown, 1983). The volcanic rocks are truncated to the west by Middle Proterozoic rocks of the Midcontinent rift system, for correlative rocks have not been recognized west of the Middle Proterozic Midcontinent rift system in central Minnesota. However, Early Proterozoic plutonic rocks are extensive in east-central Minnesota. The volcanic belt in Wisconsin is intruded by a variety of plu tonic rocks of Early Proterozoic age, and has been called the Penokean volcano-plutonic belt (Van Schmus, 1976) or the Wisconsin magmatic zone (Sims and Peterman, 1984). Although inferred to underlie a large area, the volcanic rocks of the zone have been studied at only a few places (Greenberg and Brown, 1983). In northeastern Wisconsin, the volcanic rocks are assigned to a number of units including the Quinnesec Formation, a mainly basaltic unit with intercalated andesite, rhyolite, and iron-formation (Cummings, 1978; Dutton, 1971; Schulz and Sims, 1982; Schulz, 1983). In the Rhinelander-Crandon area to the west, a mixed unit of mafic flows and felsic to intermediate tuffs and breccias is overlain by a sequence of dominantly mafic flows (Schmidt and others, 1978). An even more felsic succession occurs near Ladysmith at the western end of the volcanic belt, where a sequence of dacitic to rhyolitic crystal tuffs and massive andesitic, dacitic, and rhyolitic flows has been studied in some detail (May, 1977). Similar volcanic rocks also have been mapped in central Wisconsin, where an older succession of volcanic rocks metamorphosed to the amphibolite facies is un-conformably overlain by a younger basalt-rhyolite sequence metamorphosed to the greenschist facies (LaBerge and Myers, 1984). The felsic parts of the volcanic sequence at Ladysmith, Crandon, and several other places contain massive copper- and zinc-bearing sulfide deposits similar to those observed in stratabound, volcanogenic massive sulfide deposits in greenstone belts of Archean age in Canada (May, 1977; Schmidt and others, 1978). After Van Schmus (1976) established that the volcanic rocks were Early Proterozoic in age, it was suggested that they were correlative with the upper part of the Marquette Range Supergroup. However, a felsic unit from the Quinnesec Formation has yielded a zircon U-Pb age of about 1,869±25 m.y. (Banks and Rebello, 1969, recalculated). Mafic rocks from the same formation have yielded a Sm-Nd age of 1,871 ±57 m.y. (Beck and Murthy, 1984). Model lead ages from two of the massive sulfide deposits in the volcanic sequence alsoF16 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO are about 1,830± 150 m.y. old (Stacey and others, 1977; Afifi and others, 1984). The model lead age has been interpreted as a primary age, mainly because the contained sulfide deposits are believed to be synvolcanic in origin (Sims, 1976b). Thus, the chronometric data indicate the presence of two distinct sequences of volcanic rocks in the eastern segment of the Lake Superior region, with those in Wisconsin perhaps being as much as 50 m.y. younger than those in the Marquette Range Supergroup. This distinction has been further substantiated by Schulz (1983), who has shown that the volcanic rocks of the Marquette Range Supergroup have trace-element patterns characteristic of continental tholeiites, whereas the volcanic rocks of the Wisconsin magmatic zone have a more oceanic chemical affinity. The volcanic rocks of the Wisconsin magmatic zone are intruded by a variety of plutonic rocks ranging in composition from quartz diorite to granite. Granitic units at the west end of the zone have yielded a composite Rb-Sr isochron age of 1,885±65 m.y. (Sims and Peterman, 1980). Uranium-lead zircon ages on several plutons from the east end of the zone range from 1,890 m.y. to 1,820 m.y. (Banks and Cain, 1969; Van Schmus, unpub. data, 1980); the younger rocks tend to be less deformed than the older plutons (Maass and others, 1980). Thus, it appears that igneous activity culminated in Wisconsin about 1,850±30 m.y. ago. This age is similar to: (1) the metamorphic age of approximately 1,820 m.y. (Aldrich and others, 1965) for the Peavy Pond Complex in northern Michigan; (2) the ages of a number of small granite and pegmatite bodies in northern Michigan (Sims and Peterman, 1984; Sims and others, 1977); (3) the probable age (1,800 m.y.-l,700 m.y.) of plutonic rocks in east-central Minnesota (Spencer and Hanson, 1984); and (4) the age (1,840±50 m.y.) of an Early Proterozoic granite stock that cuts the Archean gneiss terrane in the Minnesota River Valley (Doe and Delevaux, 1980). POST-PENOKEAN, EARLY PROTEROZOIC ROCKS Events of post-Penokean, Early Proterozoic age in the Lake Superior region include: (1) extrusion and intrusion of rhyolitic and granitic rocks of the “Fox River valley type” at 1,760±10 m.y. followed by (2) deposition of texturally mature quartz-rich red beds of the “Baraboo-Sioux type,” and (3) subsequent low-grade, regional metamorphism at about 1,630 m.y. RHYOLITES AND GRANITES OF THE FOX RIVER VALLEY TYPE Volcanic and granitic rocks of dominantly felsic composition crop out as small, scattered inliers in south-central Wisconsin (fig. 2), mainly as mounds or topographic highs surrounded by Phanerozoic strata (Mudrey and others, 1982). Most of the known exposures occur along the valley of the Fox River (Asquith, 1964; Smith, 1978a) and around the margins of the Baraboo district (Dalziel and Dott, 1970). However, the rhyolitic rocks were once more widely distributed, because several outliers of flat-lying, felsic extrusive rocks unconformably over he folded Early Proterozoic strata in east-central Minnesota (Van Hise and Leith, 1911; Morey and others, 1981), and large rhyolite clasts are present near the inferred base of the Sioux Quartzite in southwestern Minnesota (Weber, 1981). The rhyolitic rocks along the Fox River appear to be subaerial in origin and include various proportions of intercalated porphyritic and nonporphyritic flow units, ash-fall tuff and breccia, ignimbrite, and mud-flow breccia, exposed in a series of open, north-northeasttrending antiforms and synforms (Smith, 1978b). Flow directions imply that the eruptive centers were to the northwest and that flow was predominantly to the southeast (Smith and Hartlaub, 1974). Chemical data (Smith, 1978b; 1983) indicate that the rhyolites and the associated granites are coeval and of both peraluminous and meta-aluminous affinity. The granites of the Fox River valley are epizonal in character and apparently were intruded into their own volcanic roof rocks. However, several synchronous quartz monzonitic plutons of mesozonal character have been identified to the north in the Wisconsin magmatic zone (Van Schmus, Thurman, and Peterman, 1975; Van Schmus, 1980), including the Amberg Quartz Monzonite of Medaris and Anderson (1973), the granite near Monico, and the porphyritic granite at Radisson. Judged from limited chemical data (Anderson and others, 1980), the plutons in northern Wisconsin appear to be the deep-seated equivalents of the granitic rocks in the Fox River valley. The rhyolitic and granophyric rocks along the Fox River valley yield U-Pb concordia ages of about 1,760±10 m.y. (Van Schmus, Thurman, and Peterman, 1975; Van Schmus, 1978, 1980). Similar U-Pb concordia ages of 1,760± 10 m.y. have been obtained from the mesozonal plutons that crop out in northern Wisconsin. A small, epizonal, gabbroic-granophyric stock, the Cedar Mountain Complex of Lund (1956), which cuts the Archean gneisses in the Minnesota River Valley, has a similar age of 1,750 m.y. (Goldich and others, 1961; Hanson, 1968). Although no substantive evidence indicates that the epizonal rocks of the Minnesota River Valley or the mesozonal rocks of northeastern and central Wisconsin are related petrogenetically to the rhyolitic and granitic rocks of the Fox River valley type, the radiometric data imply that igneous activity was widespread in the Lake Superior region about 1,760 m.y. ago.PRECAMBRIAN ROCKS OF THE LAKE SUPERIOR REGION F17 SEDIMENTARY ROCKS OF THE BARABOO-SIOUX TYPE Sedimentary rocks of the Baraboo-Sioux type are exposed in only a few areas (fig. 2) but are widely distributed in the subsurface over much of the southern part of the Lake Superior region (Dott, 1983). In addition to the Baraboo Quartzite in southern Wisconsin and the Sioux Quartzite in southwestern Minnesota and adjoining States, the sequence includes the Waterloo Quartzite in southern Wisconsin (Dott and Dalziel, 1972), and the Barron and Flambeau Quartzites in northwestern Wisconsin (Dott, 1983; Campbell, 1981). As the names imply, the sequence, which has been inferred to be as much as 2,000 m thick in places, is dominated by red beds of braided fluvial origin, derived dominantly from the north and northwest (Weber, 1981; Morey, 1983c; Southwick and Mossier, 1984). The quartzite in the Baraboo area is overlain by 100 m of black argillaceous strata called the Seeley Slate, which in turn is overlain by 300 m of dolomite and iron-formation called the Freedom Formation (Weidman, 1904; Leith, 1935; Schmidt, 1951). Dott (1983) has suggested that the Seeley Slate and Freedom Formation were deposited by shallow marine processes. Erosion subsequently removed an unknown amount of the Freedom Formation before additional quartzite and shale—the Dake Quartzite and Rowley Creek Slate— were deposited. The sedimentary rocks of the Baraboo-Sioux type have been variably deformed and metamorphosed. The northernmost Barron Quartzite is only broadly warped and contains kaolinite. The Sioux Quartzite also is broadly folded and contains kaolinite and diaspore. However, the bottom part of the formation, at least locally, contains pyrophyllite, which implies a burial metamorphic event (Vander Horck, 1984). In contrast, both the Baraboo and Waterloo Quartzites have been complexly folded; the former contains pyrophyllite and minor muscovite, and the latter contains andalusite and abundant muscovite (Geiger and others, 1982). In southwestern Minnesota, the Sioux Quartzite rests on Archean gneiss that was affected by a thermal event around 1,800 m.y. ago (Goldich and others, 1970): in-terbedded argillaceous rocks yield a K-Ar mineral age of approximately 1,200 m.y. (Goldich and others, 1961). Therefore, the age of the Sioux Quartzite is not well constrained. In contrast, the Baraboo Quartzite overlies 1,760±10 m.y.-old rhyolitic rocks, and some geologic evidence implies that the two may be locally contemporaneous (Greenberg and Brown, 1984). The quartzitic rocks at Waterloo are in turn intruded by pegmatite which has yielded mineral ages of about 1,400 m.y. (Bass, 1959; Goldich and others, 1966). These pegmatites are probably closer to 1,470-1,500 m.y. in age, corresponding to times of other major igneous activity in the region (Van Schmus, Thurman, and Peterman, 1975). The volcanic rocks near Baraboo have yielded a Rb-Sr whole-rock isochron age of about 1,630±40 m.y. (Van Schmus, Thurman, and Peterman, 1975), an age that may reflect a period of folding and shearing (Smith, 1978a; Van Schmus, 1976). Because folding and shearing in the rhyolitic rocks probably were contemporaneous with folding of the quartzitic rocks, Van Schmus (1980) has suggested that the Baraboo Quartzite was deposited in the interval 1,760±10 m.y. to 1,630±40 m.y. THE 1,630 M.Y. METAMORPHIC EVENT Although a metamorphic event that reset the Rb-Sr systematics of the volcanic and quartzitic rocks at about 1,630 m.y. ago is well defined in southern Wisconsin, it is not geographically limited to that area. Van Schmus and Woolsey (1975) have shown that the Archean and Early Proterozoic rocks over a large area of northern Michigan underwent a major period of Rb-Sr reequilibration at 1,630±40 m.y. ago. Early Proterozoic rocks of the Animikie Group in northern Minnesota have yielded Rb-Sr whole-rock isochron ages of 1,624 m.y. (Keighin and others, 1972). A similar Rb-Sr age of 1,600±24 m.y. has been obtained from the correlative Gunflint Iron-formation in Canada (Faure and Kovach, 1969). Igneous rocks having primary ages of about 1,630 m.y. have not been found in the Lake Superior region. Therefore, the widespread resetting of the Rb-Sr systems of a variety of rocks at that time must have been accomplished primarily by a low-grade metamorphic event, possibly accompanied by folding and shearing. Most likely these processes were the northern manifestation of a more extensive orogenic event that occurred to the south (Van Schmus and Bickford, 1981). MIDDLE PROTEROZOIC ROCKS Middle Proterozoic events in the Lake Superior region include the emplacement of the Wolf River batholith and the syenitic complexes of the Wausau type in Wisconsin, and the development of the Midcontinent rift system, a narrow belt of dominantly mafic rocks flanked by half-graben wedges of derivative sedimentary rocks, which extends from the Lake Superior region southward at least to Kansas. WOLF RIVER BATHOLITH AND WAUSAU SYENITE COMPLEX Rocks assigned to the Wausau Syenite Complex and to the Wolf River batholith underlie parts ofF18 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO south-central Wisconsin (figs. 2, 6). These rocks are representative of a large number of anorogenic, calcalkaline to alkaline plutons that invaded a broad zone of preexisting continental crust extending from Labrador to California and Wyoming to Mexico about 1,450 m.y. ago (Silver and others, 1977). In Wisconsin, the syenitic complexes form two small calderalike plutons that are generally circular in plan view, and whose structures are accentuated by the concordant distribution of numerous large, vertically dipping xenoliths. The larger, Wausau pluton (LaBerge and Myers, 1973; Myers, 1976) consists of two partially overlapping segments. The older segment contains pyroxene- and amphibole-bearing syenite and the younger segment is differentiated inward from a pyroxene-bearing syenite to a core of biotite-bearing granite. The smaller Stettin pluton (Myers, 1973a, b) is more alkalic and is differentiated inward from a rim of biotite-bearing syenite, nepheline-syenite gneiss, or syenite, through intermediate zones of amphibole- and pyroxene-bearing syenite and magnetite-rich, nepheline-hedenbergite-fayalite-bearing syenite, to a core of pyroxene-bearing syenite (Myers, 1976). The Wolf River batholith is an epizonal, composite unit of alkaline affinity. It consists of at least 10 separate plutons that are mostly of quartz monzonitic to granitic composition, but units of monzonite, syenite, quartz porphyry, feldspar porphyry, and anorthosite also have been mapped (Medaris and others, 1973). Individual plutons within the batholith formed by differentiation from early granite to late quartz monzonite, most likely by the progressive fusion of a crustal source (Anderson and Cullers, 1978). The anorthosite units appear to represent large xenolithic blocks or roof pendants of an older unit caught up during emplacement of the batholithic rocks. Zircons from the Wausau pluton yield a U-Pb Concordia intercept age of 1,520±15 m.y. (Van Schmus, 1980), and zircons from several plutons of the Wolf River batholith yield an age of 1,485±15 m.y. (Van Schmus, Medaris, and Banks, 1975). Both yield apparent Rb-Sr isochron ages of approximately 1,435±34 m.y. (Van Schmus, Medaris, and Banks, 1975), and apparently were affected by the same post-crystallization event. A pegmatite dike that cuts the Waterloo Quartzite yields a similar Rb-Sr age of approximately 1,410 m.y. (Bass, 1959), and muscovite from a phyllite in the Waterloo Quartzite yields a minimum K-Ar age of 1,410 m.y. (Goldich and others, 1966). These ages probably represent slight degradation of systems somewhere near 1,480 m.y., rather than discrete younger events. MIDCONTINENT RIFT SYSTEM The final event in the formation of the Precambrian crust in the Lake Superior region was the Midcontinent rift system (King and Zietz, 1971). Rifting was accompanied by massive upwelling of magma, solidification of mafic plutonic rocks at depth, and widespread volcanism and clastic sedimentation at the surface. Rocks associated with this period of rifting contain large stratabound deposits of native copper and copper sulfides and comprise a classical sequence of rocks called the Keweenawan system (Goldich and others, 1961), Keweenawan series (White, 1972), or Keweenawan supergroup (King, 1976). Unfortunately none of these stratigraphic names has been formally defined (Morey and Green, 1982). Rocks of the Midcontinent rift system crop out along the shores of Lake Superior and extend from near the western end of Lake Superior to the south-southwest for a distance of about 160 km, to a place where they pass beneath overlapping Paleozoic strata (fig. 8). Geophysical evidence indicates that this arm of the rift system—generally referred to as the Midcontinent geophysical anomaly—extends southward beneath the Paleozoic strata for another 1,600 km to central Kansas (King and Zietz, 1971) and possibly to the Kansas-Oklahoma border (Yarger, 1983). A second arm of the rift system extends southeastward from the eastern end of Lake Superior (Hinze and others, 1975; Catacosinos, 1981) beneath the Paleozoic strata of the Michigan basin to approximately the buried continuation of the Grenville Front. At least two locations in Canada have been suggested for the third possible arm of this rift system. One extends from north of Isle Roy ale to the general vicinity of Lake Nipigon. This half-graben-like structure is filled by sedimentary rocks of the Sibley Group. The other proposed third arm, the Kapuskas-ing structural zone, extends from very near the eastern end of Lake Superior north-northeastward into Canada (Percival and Card, 1983). Rocks of undisputed Keweenawan affinity have not been defined in the Isle Royale-Lake Nipigon structure and occur only to a very limited extent in the Kapuskasing structural zone (Percival and Krogh, 1983), and thus there may be no well-defined third arm. In early reports on Keweenawan geology (summarized in Morey and Green, 1982) it was noted that igneous activity—represented by lava flows and mafic intrusions—was preceded and followed by deposition of dominantly clastic strata. Therefore, the Keweenawan succession was divided into a lower, a middle, and an upper part (Van Hise and Leith, 1911). More recent geologic and paleomagnetic data, however, indicate that igneous activity and sedimentation were partly contemporaneous and that boundaries separating volcanic and sedimentary rocks are not everywhere synchronous (for example, White, 1972). Thus, a recent trend has beenPRECAMBRIAN ROCKS OF THE LAKE SUPERIOR REGION F19 towards subdividing the Keweenawan in terms of magnetopolarity units. At least two reversals of magnetic polarity (DuBois, 1962; Books, 1968,1972) occur within the Keweenawan sequence (fig. 9). An older normal-to-reverse magnetic reversal has been identified in volcanic sequences that crop out only in northern Wisconsin and Michigan. Evidence for a younger reverse-to-normal magnetic reversal is much more widespread, but unfortunately the reversal took place during a depositional hiatus represented by an unconformity. Although both reversals have limited chrono-stratigraphic value, they provide a means of broadly correlating rock units of diverse distribution in the region. Quartz-rich sandstone, siltstone, and conglomerate of the Bessemer Quartzite, Puckwunge Sandstone, and Nopeming Formation (fig. 9) are the oldest unequivocal Keweenawan rocks in the region (Ojakangas and Morey, 1982). Although these formations have similar litho-topes and occupy a stratigraphic position beneath lava flows, they are not correlative. The Bessemer Quartzite is unconformably overlain by normally polarized volcanic rocks, whereas those over the Nopeming Formation and Puckwunge Sandstone are reversely polarized (Books, 1972; Green and Books, 1972). The chronometric ages of the basal Keweenawan clastic units are poorly constrained. However, the Puckwunge Sandstone of northern Minnesota has been traditionally correlated with the Sibley Group in Canada (Robertson, 1973; DuBois, 1962), which has yielded a Rb-Sr isochron age of 1,340±33 m.y. (Wanless and Loveridge, 1978). This age is considerably greater than those typically associated with the Midcontinent rift system. However, the data are highly scattered and the behavior of the Rb-Sr system in such sedimentary rocks is uncertain. Therefore the age may be of uncertain value. Because of the problematic value of the reported age and because the Sibley Group lacks any evidence of a volcanic component, Green (1983) and Halls and Pesonen (1982), among others, have suggested that the Sibley Group formed at some time prior to the onset of sedimentation and volcanism related to the Midcontinent rift. The earliest unequivocal Keweenawan eruptive rocks occur in northern Michigan and Wisconsin, where thay are assigned to the Siemens Creek Formation of the Powder Mill Group (Hubbard, 1975a). The lowermost one or two flows are pillowed, but most of the formation, which is as much as 1,300 m thick, formed under subaerial conditions. Subaqueous and subaerial flows. in the lowermost 100 m or so of the formation are normally polarized, whereas the remainder is reversely polarized (Books, 1968, 1972). The Kallander Creek Formation overlies the Siemens Creek Formation in the Powder Mill Group. This formation formed under subaerial conditions, is as much as 4,500 m thick (Green, 1977), and is reversely polarized (Books, 1972). Other reversely polarized volcanic rocks (Green and Books, 1972) include the Grand Portage and Hovland Lavas in the basal part of the North Shore Volcanic Group in northeastern Minnesota (Green, 1972) and the Elys Peak Basalts near Duluth (Kilbury, 1972). Hypabyssal intrusions emplaced during this interval of reversed magnetic polarity include the dikes from Marquette and Baraga Counties in northern Michigan (Pesonen and Halls, 1979) and the intrusions near Logan (Jones, 1984; Weiblen and others, 1972; DuBois, 1962) in northern Minnesota. The plutonic-gabbroic rocks of Nathan’s (1969) so-called “layered series of the Gunflint prong” of the Duluth Complex also were emplaced at this time. The chronometric age of the reversely polarized volcanic rocks in the Powder Mill Group has not been established, but Silver and Green (1972), using U-Pb zircon ages, have suggested that the magnetically normal units from the group and from throughout the region formed in the 1,110±10 m.y. range. However, Hanson (1975) reported 40Ar/39Ar ages of 1,147 to l, 172 m.y. for plagioclase separates from the Logan intrusions. York and Halls (1969) have reported a K-Ar age of approximately 1,210 m.y. from a Logan-type intrusion, and Hanson and Malhotra (1971) have reported an even older K-Ar age of 1,305±65 m.y. for a chilled margin of a Logan-type intrusion. Although these ages are imprecise, as is a Rb-Sr isochron age of 1,242±200 m. y. (Wanless and Loveridge, 1978) from sedimentary rocks metamorphosed by a Logan-type intrusion, they collectively suggest that Keweenawan igneous activity began considerably before 1,110±10 m.y. ago. Contact relationships between reversely polarized and overlying normally polarized parts of the North Shore Volcanic Group are covered or obscured by intrusive rocks. Regional geologic map patterns (Green, 1982) and detailed aeromagnetic data (Chandler, 1983) imply that the contact is a low-angle unconformity. The normally polarized parts of the North Shore Volcanic Group consist dominantly of basalt, but also contain intermediate and felsic lavas (Green, 1977, 1982, 1983; Basaltic Volcanism Study Project, 1981) and minor quantities of interlayered sedimentary rocks (Merk and Jirsa, 1982; Jirsa, 1984). The volcanic rocks overlie and were the roof for mafic plutonic rocks of the Duluth and Beaver Bay Complexes (Weiblen and Morey, 1980; Weiblen, 1982). Cogenetic zircons from felsic rocks in the volcanic sequence and the Duluth Complex yield U-Pb ages of 1,110±10 m.y. (Silver and Green, 1963, 1972). This age agrees closely with a Rb-Sr isochron age of 1,090±30 m.y. for the Duluth Complex (Faure andF20 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES nm „ b and Mexico 92° Figure 8 (above and facing page).—Generalized geology of western Lake Superior region showing distribution of Middle Proterozoic rocks associated with the Midcontinent rift system (Morey and Green, 1982). others, 1969). The volcanic rocks also are intruded by a number of hypabyssal intrusions that are comagmatic with parts of the Duluth Complex (Weiblen and Morey, 1980). These include rocks in northeastern Minnesota that are correlative with the intrusions near Pigeon River of Geul (1970) in Canada (Mudrey, 1976, 1977). The intrusions near Pigeon River have yielded a K-Ar age of approximately 1,150 m.y. (York and Halls, 1969) and a 40Ar/39Ar age of 1,145±10 m.y. (Hanson, 1975). Still younger dikes and sills also occur in northeastern Minnesota. These include the Endion sill at Duluth, which has produced a Rb-Sr isochron age of 1,068±30 m.y. (Faure and others, 1969). Two other normally polarized eruptive units (Books and Green, 1972; Books, 1972) crop out to the south of Lake Superior. These are the Portage Lake Volcanics along the Keweenaw Peninsula (White, 1960, 1966) and on Isle Roy ale in the northern part of Lake Superior (Huber, 1973), and the Chengwatana Volcanic Group in east-central Minnesota (Morey and Mudrey, 1972) and adjoining parts of Wisconsin (White, 1978). Of the two, the Portage Lake Volcanics are much better known, mainly because of their contained native copper deposits (White, 1968). Although neither continuous nor entirely correlative (for example, White, 1978), both consist predominantly of basalt and both lack rocks of intermediate chemical composition (Cornwall, 1951; Jolly and Smith, 1972; Morey and Mudrey, 1972; Green, 1977, 1982). Both units also occupy uncertain stratigraphic positions within the normally polarized epoch, mainly because their contacts are either faulted or covered by younger materials. However, near Mellen, Wis., the Chengwatana Volcanic Group overlies the Powder MillPRECAMBRIAN ROCKS OF THE LAKE SUPERIOR REGION F21 CORRELATION OF MAP UNITS Post-Middle Proterozoic rocks, undivided ► Middle Proterozoic Pre-Middle Proterozoic rocks, undivided LIST OF MAP UNITS Bayfield Group, undivided, and Hinckley Sandstone and Fond du Lac Formation in Minnesota Oronto Group, undivided Unnamed formation of White (1972) Copper Harbor Conglomerate Chengwatana Volcanic Group and related rocks Portage Lake Volcanics Mellen Intrusive Complex Nonmagnetic rocks of Hubbard (1975a) and Jacobsville Sandstone North Shore Volcanic Group Volcanic rocks of generally mafic composition having normal paleomagnetic polarity Volcanic rocks of generally mafic composition having reverse paleomagnetic polarity and the Nopeming Sandstone and the Puckwunge Sandstone Duluth and Beaver Bay Complexes Troctolitic and gabbroic series and related felsic rocks Anorthositic series and related ultramafic and felsic rocks Powder Mill Group, undivided, and Bessemer Quartzite Olivine diabase intrusions Intrusions near Logan, northern Minnesota - Contact Fault—Dashed where approximate; dotted where concealed by water Axis of syncline Group (White, 1978) and is separated from it by sedimentary rocks that Aldrich (1929) called the conglomerate of Davis Hill. The correlation of that conglomerate with other units of the region is unknown. To the east, Portage Lake rocks in Michigan are similarly juxtaposed against a sequence of “nonmagnetic rocks” that appear to be sedimentary in origin (King, 1975) and that overlie the Powder Mill Group. Hubbard (1975a) suggested that the sedimentary rocks may be a western facies of the Jacobsville Sandstone and that they are unconformably overlain by the Portage Lake Volcanics. Most interpretations, however, infer that the Portage Lake and sedimentary rocks are juxtaposed in this area by the Keweenaw fault (Kalliokoski, 1982). Stratigraphic relations between the Portage Lake Volcanics and the North Shore Volcanic Group are equally uncertain. However, the nearly linear northwest shoreline of Lake Superior may represent an exhumed erosion surface formed on rocks of the North Shore Volcanic Group before the Portage Lake Volcanics were erupted (White, 1966). This unconformity, however, may represent a relatively short hiatus inasmuch as U-Pb ages from the two units appear to be indistinguishable (Silver and Green, 1972). Many of the reported Rb-Sr results from felsic units in the Portage Lake Volcanics also are concordant, within stated uncertainty limits, with the U-Pb ages of Silver and Green (1972), but other Rb-Sr ages are significantly discrepant (Van Schmus and others, 1982). Reported Rb-Sr ages range from 1,075±25 m.y. to 1,016±30 m.y. (Chaudhuri, 1973, 1975; Chaudhuri and Brookins, 1969; Chaudhuri and Faure, 1967). The slightly younger Rb-Sr ages are consistent with the stratigraphic succession, but the more discrepant ages may reflect disturbed Rb-Sr isotopic systems. As with data from the volcanic rocks, Rb-Sr results from the intrusive rocks must be interpreted with caution. Intrusive rocks that were emplaced more or less contemporaneously with the Portage Lake Volcanics include the Mellen Intrusive Complex of Hubbard (1975a), the Mineral Lake intrusion of Olmsted (1968), and the Rearing Pond intrusion of Olmsted (1979), all in the general vicinity of Mellen, Wis. Gabbro from the Mellen Complex yields a Rb-Sr isochron age of 1,075±25 m.y. (Chaudhuri and others, 1969), whereas granite from the Mineral Lake intrusion gives an age of 1,042±30 m.y. (Chaudhuri and Brookins, 1969). The Chengwatana Volcanic Group is conformably overlain by sedimentary rocks of the Oronto Group in Wisconsin and the Solor Church Formation in Minnesota (Morey and Ojakangas, 1982). The Oronto Group also overlies the Portage Lake Volcanics in parts of Michigan, but in other places, the two are separated by yet another normally polarized volcanic unit of generally felsic composition, the unnamed formation of White (1972). The unnamed formation is interlayered with the Copper Harbor Conglomerate, formerly considered to be the lowermost unit of the Oronto Group. Therefore,F22 CORRELATION OF PRECAMBRIAN ROCKS OF THE UNITED STATES AND MEXICO Figure 9.—Correlation chart for Middle Proterozoic rocks associated with the Midcontinent rift system in the Lake Superior region (modified from Green, 1977). a Rb-Sr age of 1,072±25 m.y. (Chaudhuri, 1972) from a rhyolite flow approximates both the time volcanism ended and the time clastic sedimentation started in this part of Michigan. The Oronto Group, as redefined by White (1972), consists of the Nonesuch Shale and the overlying Freda Sandstone. The Copper Harbor Conglomerate as well as the rocks of the Oronto Group all exhibit normal magnetic polarity (Books, 1972; Henry and others, 1977; Halls and Pesonen, 1982). The Copper Harbor Conglomerate and Freda Sandstone are dominantly red colored clastic units of lithic sandstone, shale, and conglomerate deposited by alluvial and fluvial processes (Daniels, 1982; Elmore, 1984), whereas the Nonesuch Shale is an unoxidized sequence of dark-gray to black siltstone, shale, and mudstone deposited by lacustrine processes. The Nonesuch is mineralized with copper sulfides over a wide area; the only economic concentrations occur near White Pine, Mich. (Ensign and others, 1968). The Solor Church Formation in southern Minnesota physically resembles the Copper Harbor Conglomerate and Freda Sandstone, consisting of lithic sandstone, silt-stone, and shale deposited by fluvial processes (Morey, 1974). Although the Solor Church Formation and the Oronto Group were never continuous units, Morey and Ojakangas (1982) have suggested on the basis of lithologic similarity that both are part of the same deposi-tional episode. The time of this depositional episode in Michigan is bracketed by the 1,072±25 m.y. age from the unnamed formation and a Rb-Sr isochron age of 1,052±50 m.y. from the Nonesuch Shale (Chaudhuri and Faure, 1967). The signifance of the latter age has been questioned (Van Schmus and others, 1982) mainly because the Nonesuch was derived from a volcanic source of that age. However, Ruiz and others (1984) have reported a Rb-Sr age of 1,047±35 m.y. from calcite veins associated with the copper mineralization in the Nonesuch. Therefore it seems likely that the Oronto Group was deposited between approximately 1,100 and 1,000 m.y. ago, an age further substantiated by a K-Ar age of 1,062±34 m.y. (M.A. Lanphere as quoted in White, 1968) from a biotite-bearing rhyolite stock near Bear Lake, Mich., that intrudes the Freda Sandstone. 1 PRECAMBRIAN ROCKS OF THE LAKE SUPERIOR REGION F23 A number of felsic intrusions whose ages cluster around 930± 35 m.y. (such as a granite stock just north of Mellen) also have been recognized in Wisconsin and Michigan (Chaudhuri, 1972, 1976). Compared with the felsic extrusive rocks, these intrusions have significantly higher initial 87Sr/86Sr ratios, implying a previous crustal history (Chaudhuri and others, 1969). If the Rb-Sr isotopic systems are undisturbed in these rocks, the reported ages define the end of igneous activity in the Lake Superior region. However, the biotite-bearing rhyolite stock near Bear Lake that was dated by M.A. Lanphere at 1,062±34 m.y. also yielded an apparent Rb-Sr isochron age of 985±25 m.y. (Chaudhuri, 1975). This suggests that the Rb-Sr isotopic system, at least in this one rock, has been disturbed. Thus all the very young felsic intrusions may be much older than their apparent Rb-Sr ages. In that regard, the paleomagnetic pole positions of these rocks are not significantly different from those associated with other normally polarized extrusive rocks (Van Schmus and others, 1982). Therefore, although it has been suggested that igneous rocks as young as 980-900 m.y. may exist in the Lake Superior region, their presence has not been firmly established. The youngest Precambrian sedimentary rocks in the Lake Superior region are assigned to the Bayfield Group (Thwaites, 1912). In Wisconsin, the group consists of three formations, from oldest to youngest the Orienta, Devils Island, and Chequamegon Sandstones (Morey and Ojakangas, 1982; Ostrom, 1967). Correlative rocks in Minnesota include the Fond du Lac Formation (Morey, 1967) and the overlying Hinckley Sandstone (Tryhorn and Ojakangas, 1972). The Bayfield Group and its Minnesota equivalents are typical red-bed sequences of fluvial and lacustrine origin (Morey and Ojakangas, 1982). They differ from the older Oronto Group in that they lack primary igneous material, are less well lithified, and are more arkosic or quartz rich in composition. The age of the Bayfield Group is uncertain: it has been assigned to the Late Proterozoic (for example, King, 1976) or to the Early-Middle Cambrian (for example, Ostrom, 1967). However, the Fond du Lac Formation has a paleomagnetic pole position akin to those associated with other Middle Proterozoic rocks in the rift system (Watts, 1981). A third alluvial-fluvial red-bed sequence—the Jacobs-ville Sandstone of Lane and Seaman (1907)—crops out over a wide area in the southeastern part of northern Michigan (Hamblin, 1958; Kalliokoski, 1982). This sequence consists of lithic to arkosic sandstone, conglomerate, and some shale. It forms a wedge that dips and thickens toward the northwest and unconformably over-lies magnetically reversed volcanic rocks of the Powder Mill Group or still older pre-Keweenawan basement rocks (Bacon, 1966; Hubbard, 1975a). Because much of its northern limit coincides with the Keweenaw fault, the stratigraphic position of the Jacobsville Sandstone is uncertain. On the basis of regional considerations, it has been correlated with the Bayfield Group (White, 1966), the Oronto Group (Ostrom, 1967), rocks of the Cambrian System (Hamblin, 1958), and with a sequence possibly older than the Portage Lake Volcanics (Babcock, 1975, 1976; Hubbard, 1975a). However, paleomagnetic data suggest that the Jacobsville is only slightly younger than the Freda Sandstone of the Oronto Group (Roy and Robertson, 1978). SUMMARY Several conclusions regarding the geologic history of the Lake Superior region can be made from the stratigraphic and chronometric data summarized on the geochronometric correlation chart (pi. 1). The Archean gneiss terrane records a long span of geologic history extending back to about 3,600 m.y. ago, which is nearly a billion years before any major datable geologic event in the Late Archean greenstone-granite terrane. Additional major geologic events took place in the gneiss terrane almost a billion years after formation of the greenstone-granitic terrane. The gneiss terrane was tectonically mobile throughout much of Precambrian history as indicated by a succession of events that have been dated at about 3,600 m.y., 3,000-2,900 m.y., 2,600 m.y., 2,400-2,300 m.y., 2,100-2,000 m.y., 1,900-1,800 m.y., 1,760 m.y., 1,650 m.y., and 1,500-1,450 m.y. In contrast, the greenstone-granite terrane formed within a narrow range of a few hundred million years approximately 2,700 m.y. ago and has remained essentially stable since that time, except for several periods of low-grade metamorphism and dike emplacement. Lastly, both crustal segments were transected and affected when the Midcontinent rift system formed 1,200-1,000 m.y. ago. Although some rocks in the 1,000-900 m.y. interval may be present, no rocks of definite Late Proterozoic age have been recognized in the Lake Superior region. REFERENCES CITED Adams, F.D., Bell, R., Lane, A.C., Leith, C.K., Miller, W.G., and Van Hise, C.R., 1905, Report of the Special Committee for the Lake Superior region: Journal of Geology, v. 13, p. 89-104. Afifi, A., Doe, B.R., Sims, P.K., and Delevaux, M.H., 1984, U-Th-Pb isotope chronology of sulfide ores and rocks in the early Proterozoic metavolcanic belt of northern Wisconsin: Economic Geology, v. 79, p. 338-353. Aldrich, H. 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Dutton, C.E., and Bradley, R.E., 1970, Lithologic, geophysical, and mineral commodity maps of Precambrian rocks in Wisconsin: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-631. Elmore, R.D., 1984, The Copper Harbor Conglomerate—A late Precambrian fining-upward alluvial fan sequence in northern Michigan: Geological Society of America Bulletin, v. 95, p. 610-617. Ensign, C.O., Jr., White, W.S., Wright, J.C., Patrick, J.L., Leone, R.J., Hathaway, D.J., Tranmell, J.W., Fritts, J.J., and Wright, T.L., 1968, Copper deposits in the Nonesuch Shale, White Pine, Michigan, in Ridge, J.D., ed.. Ore deposits of the United States, 1933-1967 (Graton-Sales Volume), volume 1: New York, American Institute of Mining, Metallurgical, and Petroleum Engineers, p. 460-488. 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Southwick, D.L., 1972, Vermilion granite-migmatite massif, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota—A centennial volume: Minnesota Geological Survey, p. 108-119. ______1976, High-grade metamorphism associated with the Vermilion batholith, Minnesota-Ontario: Proceedings of the 22nd Annual Institute on Lake Superior Geology, Minnesota Geological Survey, St. Paul, p. 61. Southwick, D.L., and Chandler, V.W., 1983, Subsurface investigation of the Great Lakes tectonic zone, west-central Minnesota: Geological Society of America Abstracts with Programs, v. 15, p. 692. Southwick, D.L., and Mossier, J.H., 1984, The Sioux Quartzite and subjacent regolith in the Cottonwood County basin, Minnesota: Minnesota Geological Survey Report of Investigations, 32, p. 17-44. Southwick, D.L., and Sims, P.K., 1979, The Vermilion Granitic Complex—A new name for old rocks in northern Minnesota: U.S. Geological Survey Professional Paper 1124-A, p. Al-All. Spencer, K.J., and Hanson, G.N., 1984, Pb, Nd isotope and REE constraints on east-central Minnesota Proterozoic granites—Evidence for newly derived crustal components [abs.]: Eos, v. 65, no. 16, p. 292. Stacey, J.S., Doe, B.R., Silver, L.T., and Zartman, R.E., 1977, Plum-botectonics IIA, Precambrian massive sulfide deposits, in Karpenko, S.F., ed., Geochronology and problems of ore formation: U.S. Geological Survey Open-File Report 76-476, p. 93-106. Stockwell, C.H., McGlynn, J.C., and Emslie, R.F., 1970, Geology of the Canadian Shield, in Douglas, R.J.W., ed., Geology and economic minerals of Canada: Geological Survey of Canada Economic Geology Report 1, p. 44-150. Stuckless, J.S., and Goldich, S.S., 1972, Ages of some Precambrian rocks in east-central Minnesota, Part 1 [abs.]: Proceedings of the 18th Annual Institute on Lake Superior Geology, Michigan Technological University, Houghton, paper 31. Taylor, G.L., 1972, Stratigraphy, sedimentology and sulfide mineralization of the Kona Dolomite: Houghton, Mich., Michigan Technological University Ph. D. thesis, 111 p. Thwaites, F.T., 1912, Sandstones of the Wisconsin coast of Lake Superior: Wisconsin Geological and Natural History Bulletin 25, 117 p. Tilton, G.R., and Grunenfelder, M.H., 1968, Sphene—Uranium-lead ages: Science, v. 159, p. 1458-1461. Trent, V.A., 1976, The Emperor Volcanic Complex of the east Gogebic range, Michigan, in Cohee, G.V., and Wright, W.B., eds., Changes in stratigraphic nomenclature by the U.S. Geological Survey, 1975: U.S. Geological Survey Bulletin 1422-A, p. A69-A74. Tryhorn, A.D., and Ojakangas, R.W., 1972, Sedimentation and petrology of the upper Precambrian Hinckley Sandstone of east-central Minnesota, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota—A centennial volume: Minnesota Geological Survey, p. 431-435. Tyler, S.A., and Twenhofel, W.H., 1952, Sedimentation and stratigraphy of the Huronian of upper Michigan, Parts I and II: American Journal of Science, v. 250, p. 1-27 and 110-151. Van Hise, C.R., and Bayley, W.S., 1895, Preliminary report on the Marquette iron-bearing district of Michigan: U.S. Geological Survey 15th Annual Report, p. 485-650. Van Hise, C.R., and Leith, C.K., 1911, The geology of the Lake Superior region: U.S. Geological Survey Monograph 52, 641 p. Van Schmus, W.R., 1965, The geochronology of the Blind River-Bruce Mines area, Ontario, Canada: Journal of Geology, v. 73, p. 755-780. ______1976, Early and middle Proterozoic history of the Great Lakes area, North America: Philosophical Transactions of the Royal Society of London Series A, v. 280, p. 605-628. ______1978, Geochronology of the southern Wisconsin rhyolites and granites: Geoscience Wisconsin, v. 2, p. 19-24. ______1980, Chronology of igneous rocks associated with the Penokean orogeny in Wisconsin, in Morey, G.B., and Hanson, G.N., eds., Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield: Geological Society of America Special Paper 182, p. 159-168. Van Schmus, W.R., and Anderson, J.L., 1977, Gneiss and migmatite of Archean age in the Precambrian basement of central Wisconsin: Geology, v. 5, p. 45-48. Van Schmus, W.R., and Bickford, M.E., 1981, Proterozoic chronology and evolution of the Midcontinent region, North America, in Kroner, A., ed., Precambrian plate tectonics, Developments in Precambrian geology: New York, Elsevier, p. 261-296. Van Schmus, W.R., Green, J.C., and Halls, H.C., 1982, Geochronology of Keweenawan rocks of the Lake Superior region—A summary, in Wold, R.J., and Hinze, W.J., eds., Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 165-171. Van Schmus, W.R., Medaris, L.G., and Banks, P.O., 1975, Geology and age of the Wolf River batholith, Wisconsin: Geological Society of America Bulletin, v. 86, p. 907-914. Van Schmus, W.R., Thurman, M.E., and Peterman, Z.E., 1975, Geology and Rb-Sr chronology of middle Precambrian rocks in eastern and central Wisconsin: Geological Society of America Bulletin, v. 86, p. 1255-1265. Van Schmus, W.R., and Woolsey, L.L., 1975, A Rb-Sr geochronology study of the Republic Metamorphic node, Marquette County, Michigan: Canadian Journal of Earth Sciences, v. 12, p. 1723-1733.PRECAMBRIAN ROCKS OF THE LAKE SUPERIOR REGION F31 Van Schmus, W.R., Woronick, R.E., and Egger, N.L., 1978, Geochronologic relationships in the Carney Lake Gneiss and other basement gneisses in Dickinson County, upper Michigan [abs.]: Proceedings of the 24th Annual Institute on Lake Superior Geology, Wisconsin Geological and Natural History Survey, Madison, p. 37. Vander Horck, M.P., 1984, Diagenesis in the Sioux Quartzite: Minneapolis, Minn., University of Minnesota M.S. thesis, 106 p. Wanless, R.K., and Loveridge, W.D., 1978, Rubidium-strontium isotopic age studies, Report 2 (Canadian Shield): Geological Survey of Canada Paper 77-14, 70 p. Watts, D.P., 1981, Paleomagnetism of the Fond du Lac Formation and the Eileen and Middle River sections with implications for Keweenawan tectonics and the Grenville problem: Canadian Journal of Earth Sciences, v. 18, p. 829-841. Weber, R.E., 1981, Petrology and sedimentation of the Upper Precam-brian Sioux Quartzite, Minnesota, South Dakota and Iowa: Duluth, Minn., University of Minnesota M.S. thesis, 151 p. Weiblen, P.W., 1982, Keweenawan intrusive igneous rocks, in Wold, R.J., and Hinze, W.J., eds., Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 57-82. Weiblen, P.W., Mathez, E.A., and Morey, G.B., 1972, Logan intrusions, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota—A centennial volume: Minnesota Geological Survey, p. 394-406. Weiblen, P.W., and Morey, G.B., 1980, A summary of the stratigraphy, petrology and structure of the Duluth Complex: American Journal of Science, v. 280-A, p. 88-133. Weiblen, P.W., Morey, G.B., and Mudrey, M.G., Jr., 1971, Guide to the Precambrian rocks of northwestern Cook County as exposed along the Gunflint Trail: Proceedings of the 17th Annual Institute on Lake Superior Geology, Duluth, University of Minnesota-Duluth, p. 97-127. Weidman, Samuel, 1904, The Baraboo iron-bearing district of Wisconsin: Wisconsin Geological and Natural History Survey Bulletin 13, 190 p. White, D.A., 1954, Stratigraphy and structure of the Mesabi range, Minnesota: Minnesota Geological Survey Bulletin 38, 92 p. White, W.S., 1960, The Keweenawan lavas of Lake Superior, an example of flood basalts: American Journal of Science, Bradley Volume, v. 258-A, p. 367-374. ______1966, Tectonics of the Keweenawan basin, western Lake Superior region: U.S. Geological Survey Professional Paper 524-E, p. E1-E23. ______1968, The native-copper deposits of northern Michigan, in Ridge, J.D., ed., Ore deposits of the United States, 1933-1967 (Graton-Sales Volume), volume 1: New York, American Institute of Mining, Metallurgical, and Petroleum Engineers, p. 303-326. ______1972, The base of the upper Keweenawan, Michigan and Wisconsin: U.S. Geological Survey Bulletin 1354-F, 23 p. ______1978, A theoretical basis of exploration for native copper in northern Wisconsin: U.S. Geological Survey Circular 769, 19 p. Williams, I.S., Kinny, P.D., Black, L.P., Compston, Wm., Froude, D.O., and Ireland, T.R., 1984, Dating Archean zircon by ion microprobe—New light on an old problem [abs.], in Workshop on the early Earth, April 23-25,1984: Lunar and Planetary Institute, Houston, Tex., p. 79-81. Wilson, W.E., and Murthy, V.R., 1976, Rb-Sr geochronology and trace element geochemistry of granulite facies rocks near Granite Falls, in the Minnesota River Valley [abs.]: Proceedings of the 22nd Annual Institute on Lake Superior Geology, Minnesota Geological Survey, St. Paul, p. 69. Wold, R.J., and Hinze, W.J., eds., 1982, Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, 280 p. Wooden, J.L., Goldich, S.S., and Suhr, N.H., 1980, Origin of the Morton gneiss, southwestern Minnesota—Part 2, Geochemistry, in Morey, G.B., and Hanson, G.N., eds., Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield: Geological Society of America Special Paper 182, p. 57-75. Woyski, M.S., 1949, Intrusives of central Minnesota: Geological Society of America Bulletin, v. 60, p. 999-1016. Yarger, H.L., 1983, Regional interpretation of Kansas aeromagnetic data: Kansas Geological Survey Geophysical Series 1, 35 p. York, D., and Halls, H.C., 1969, K-Ar dating of dike swarms from the north shore of Lake Superior [abs.]: Proceedings of the 15th Annual Institute on Lake Superior Geology, Oshkosh, University of Wisconsin-Oshkosh, p. 44. Young, G.M., 1966, Huronian stratigraphy of the McGregor Bay area, Ontario—Relevance to the paleogeography of the Lake Superior region: Canadian Journal of Earth Sciences, v. 3, p. 203-210. ______1983, Tectono-sedimentary history of early Proterozoic rocks of the northern Great Lakes region, in Medaris, L.G., Jr., ed., Early Proterozoic geology of the Lake Superior region: Geological Society of America Memoir 160, p. 15-32. * U.S. GOVERNMENT PRINTING OFFICE: 1988—673-047/86,016 REGION NO. 8: i 4 1 1 1 1UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY V- 4- PROFESSIONAL PAPER 1241-A PLATE 1 ALASKA MEXICO WESTERN AND SOUTHWESTERN UNITED STATES ROCKY MOUNTAINS AND BLACK HILLS CENTRAL INTERIOR Subsurface of Blue-Green- Western Central belt Avalonian zone Grenville province Minnesota Wisconsin Michigan Adirondacks Long axis West | East gneiss domes Carolinas New England LAKE SUPERIOP REGION EASTERN UNITED STATES 2 O Northeast Central North- west Accreted blocks in south and southeast Alaska East and south jy 20 o ' c £-9 J o c < CO O) .c a CD (5 XT ti cv. rv. 1 1 UM E cy* cv-n. £ isp i pm 0.(\f ^Y/6-. * |- $ J> ''iv.VMH r <1 - fY. fY.fY. o II Wyoming and Black Hills N t y. <2 CD I}' li o CD o c t'5 co ♦; $ CD _U_ CD CD 4-1 CD II O o ,T £*r —CW-; o .« c c < 0) >. (0 [E m Q. 3 o k_ o c 3 a> 1 CO 3 a 2 l» |1 8 ■o -c a) co GC a 3 O 5 c 3 O 5 3 a CJ ** ^ 2 50 1800- 2000- 2 111 CO cc Q. GC o (/) QC < 2200 ~ to 05 z: a> "z: -c~ E e-e 5 o o>, ^ Q_ . .cy co ' co CO 13 — Kh CD Ui CL Q3 i2 • - c 2 £ CD V4 1 1? « CD CO , a I™ 05 Q. 3 (/) "5 GO s teOi l|f *:b Iijij-ii JVL |s|' CY»X| pY.. L §4# {' i: t i: 3 i« ^ I! 1 0<>V ! m i |x , Tl £> I N .S i l. 2 -a ® to ■= 1^3 to :| < O w III y-t; ^5 co S ii 2 < o I! c «5 O CD 05 ..rr-Q5 05 C » .- v:- > CD CD {•• ■m CC % b: a. (x- 4 ^ CO C O CD GC A 05 < e 4 :T T i I I 14 YAl c 05 O "c CO _o o > CD 05 E -OCY.' c CD CD C 05 E T> 0) CO CD 0) 2 lO E i_ CD T3 05 a 3 0) (Y. CY/fs 2400 05 05 (/) z o °nV 2600- c o 'a < 05 > GC r CD GC WA CJ *o 05 GC CY. CY- C o > -c CJ 05 !C Q. 2 E 05 O E o E u. CD §41 E o £2 t xl T»t l l !■ 1° ! 1 4-* |o T 1 I l ,s 1 V/ o Q o jC a. Iff o c < c I m m 'it r- IM |<4 l 3 " ^ Q 8 Tf1 o c CD il CD 1 i O ■o = C 'Z CD CD II m <3 Y A 5 1 1 1 f 1 X X S ‘ n X X 1 1 j*' . CY. [■’ M ® « i TC t. coO); t5|: CDJ^l CY. fY. P| m Mi m co b 05 41 IJ u •41 41 01 : a CD Kv i I. I n e E 1 "O CO E .E ^ w 1 c ^ 3 05 O 25 o ^ 1> O QC 05 1 ^.1 A* K A CO 1E.E 02 .sj I O o t- o -Q X 10 ‘ GC § p- ° m 6 s q k_ 05 T'mCJ 05 a. x 1- TO. i e »l.2 o oc o 600 « S X: *iEl = kfl sV o a- 2 3 J2 o OQ 800 1000 -1200 -1400 -1600 1800 2800- EXPLANATION 3000 3200 2 < LLI I CJ DC < >4 v p>Srj', 4 v 4 . l~ . XXX XXX 3400- 3600 SEDIMENTARY ROCKS—Includes low-grade (green-schist facies) metamorphic rocks where appropriate FELSIC VOLCANIC ROCKS MAFIC VOLCANIC ROCKS FELSIC INTRUSIVE ROCKS MAFIC INTRUSIVE ROCKS—Half-width columns indicate poorly dated dikes or sills AMPHIBOLITE FACIES METAMORPHIC ROCKS GRANULITE FACIES METAMORPHIC ROCKS Note: Two or more rock symbols may be combined; half-width adjacent columns indicate geologic relations among rock units are uncertain. All touching or overlapping columns indicate rock units are in same small geographic area MAXIMUM AND (OR) MINIMUM AGE KNOWN—May represent either length of time for a single rock-forming event or analytical uncertainty ROCK BODY OR BODIES FALL WITHIN RANGE SHOWN—May not represent a single or continuous rock-forming event T I X X I I J J AGE OF ROCK BODY OR BODIES PROBABLY FALLS WITHIN RANGE SHOWN—Isotopic ages scattered and do not define sharp age limits: queries indicate uncertainty in limit shown AGE OF ROCK BODY UNCERTAIN, MAY EXTEND BEYOND RANGE SHOWN—Rock body placed on chart by homotaxial correlation or use of nondefinitive isotopic data FIRST APPEARANCE OF SHELLY FOSSILS STROMATOLITES AGE LT u ff;l ti MAXIMUM AND (OR) MINIMUM METAMORPHISM KNOWN1 OF METAMORPHIC EVENT FALLS WITHIN RANGE SHOWN1 METAMORPHIC EVENT PROBABLY FALLS WITHIN RANGE SHOWN1 METAMORPHIC EVENT KNOWN AND POSSIBLY FALLS WITHIN RANGE SHOWN1 'Symbol applies only to columns to left of symbol in a given subarea 3800 (Y.(Y, 1 I £5 TO O C © 05 C E.9>J2 “<50 05 05 ^ 05 P CD c I" ^ CD TJ 05 C ^ CD k- o Z W o A TJ — O TO C ^ O 2 °> (_ CD 05 CD k-CD CD 05 C © “E o c ~ CD L. — 05 CD *-> 2000 2200 w CD a. cc O V) cc < UJ >- 2400 ul O w 2 O 2600 2800 -3000 -3200 -3400 3600 -3800 GENERALIZED CORRELATION CHART FOR THE PRECAMBRIAN OF THE UNITED STATES AND MEXICO Compiled for the Working Group on the Precambrian by J. E. Harrison and Z. E. Peterman, 1980MILLIONS OF YEARS BEFORE PRESENT DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY 1976-1985 I I X. I SEDIMENTARY ROCKS—Includes low-grade (green-schist facies) metamorphic rocks where appropriate FELSIC VOLCANIC ROCKS MAFIC VOLCANIC ROCKS FELSIC INTRUSIVE ROCKS MAFIC INTRUSIVE ROCKS AMPHIBOLITE FACIES OR HIGHER GRADE METAMORPHIC ROCKS AGE OF ROCK BODY FALLS WITHIN RANGE SHOWN—May not represent a single or continuous rock-forming event AGE OF ROCK BODY PROBABLY FALLS WITHIN RANGE SHOWN—Isotopic ages, if available, do not define sharp age limits; queries indicate uncertainty in limit shown AGE OF ROCK BODY UNCERTAIN, MAY EXTEND BEYOND RANGE SHOWN—Rock body placed on chart by homotaxial correlation or use of nondefinitive isotopic data FIRST APPEARANCE OF SHELLY OR TRACE FOSSIL MICROFOSSIL A STROMATOLITES METAMORPHIC EVENT PROBABLY FALLS WITHIN RANGE SHOWN1—Queries indicate greater un-I certainty 921 AGE IN MILLIONS OF YEARS DETERMINED K“Ar RADIOMETRICALLY BY METHOD INDICATED— K-Ar, potassium-argon; U-Pb, uranium-lead; Rb-Sr, rubidium-strontium; brace shows uncertainty Symbol applies only to column to left of symbol in a given subarea dec 1 - 1988 ... EXPLANATION" Note: Lithologic symbols may be combined PROFESSIONAL PAPER 1241-B PLATE 1 I I 175° ?0° 170“ 165° 160° 155° 150° 145° 140° 135° 130° 125° i E OCEAN ARCTIC circle --it .V gr Ml ALASKA EXPLANATION TERRANE INCLUDING ROCKS OF PRECAMBRIAN, PROBABLE PRECAMBRIAN, OR POSSIBLE PRECAMBRIAN AGE AREA DISCUSSED ON MAP AND (OR)INTEXT—Area K is omitted 0 200 from correlation chart I----1---L_ __________I________________I_______________I_ 400 KILOMETERS CORRELATION CHART FOR PRECAMBRIAN ROCKS OF ALASKADEPARTMENT OF THE INTERIOR U. S. GEOLOGICAL SURVEY AREA I. NORTH AND SOUTH DAKOTA AREA II. NEBRASKA, NORTHERN KANSAS, IOWA, NORTHERN MISSOURI, EASTERN COLORADO AREA III. SOUTHERN KANSAS, OKLAHOMA, SOUTHERN MISSOURI AREA IV. TEXAS, EASTERN NEW MEXICO Q^S IA 12^1-C PROFESSIONAL PAPER 1241-C PLATE 1 AREA V. EASTERN MIDCONTINENT Sedimentary rocks 500 600 700 800 900 1000 1100- 1200 1300- 1400- 1500- OC £L £ 1600 S2 1700 < ID >- li. O if) z o 3 I- • ° fV O I;- O W 1800- 1900 2000 2100 2200 - 2300 2400- 2500 3 pi » o pi 5 2600 2700 2800 2900 — 03 n CD c o to c 03 CD (5 iiil Volcanic rocks o if) r7- Jr* k'P llv 2 V 03 \* * §f*. I L\ Ik* u I* * Plutonic rocks I u-JS »- I X I ^ O lx x| c -Hs 11 li I! xRi s XN E XN “ SJ to Metamorphism Sedimentary rocks :5. CO JZ Q. c-O E CD 03 E CO Q lQ I 0 I I g IS ®! H Volcanic rocks ■h 1 ♦ I1 o £ * +1 — § .* £ V _Hl-Q 0 tvl y\ s r,;ls h/JS tvl s Fvl-2 r v • CO viz Plutonic rocks Metamorphism Sedimentary rocks Ms VI M vis ixxxi g |V|* |XXX| C lx x1 CD l/Jj? |xM S |xXx|-g l*Mz K-' O K./'rJ- -C LC' £ f\7 ° I',1 V s t,V' 2 t-COi -o Li'I ® 3 CO CD C^- ■o 1 1 CO 2 I IS | ||S t— I - CD Q) fv. CD ■*" CJ 'I £E t I 01 I s IS O -D a E < II Volcanic rocks ♦| ^ J if) I o >* -C QC c o (J So i co c — CD ^ O 1 LD (/) uJ ^ Plutonic rocks Sedimentary rocks m I •v.yJ ■yi CD «lf.>1 *'///' CO //< O ///. CO /// 13 IO O1 CD |//'| J I ///I ■O Lt (0 CD 1 W CJ Evi hxx «51 r^-; | •aj-c-c0*' O < O ' i "? 5*^ ^ „ i%js! 0 0 U Eoc !^1 /// I/// |// >- 4— >x, C CC 0) n CJ 03 Lj i - T/kt?1 r I rJ * CO I^J "q. I = ® ■> c* ! c If: J IM| t tt> V Ml> J CD I j t f v h- L* -f *■ r1 Plutonic rocks Sedimentary rocks Hi n \* c — C/3 IV V 1 if) c CD , j ** i" r V P 3 l c/> C jS X X X X X X LL. X X X X X 03 > r J r- V , J* r x* -o c CO .c ^ V - ^ « 4 r- i .i r O r ‘O CD f X X X X X X X X n _o n CD 1 ? X X X X X X X X CL < < 1 o c o (V. fV. ^ i 11*1 d e K:;*i | o m m ii. ■ |lf::-*l S 3b:Jc2 o I'.:: - 5 I X I CO* x| IK-I o ix xx! Sf;;*’j! I# ii m i p m > Is: A Eastern New Mexico B Texas Volcanic rocks cu o c <- S.s 4- c ^+- CD E-E o > !ir-" I4MI- o- — c c"1 J — CD ' ’ J d1-'xM s ^ ^ '-I > [ESE CD CO s| Occ Plutonic rocks .m, I Ms “r>~ x r- o bl'Ti m ^ C /> 1 ^ c Bg c ° H Metamorphism w !E 3 ID 2 A Anorogenic igneous terrane (Western region) B Subsurface Grenville province (Eastern region) 500 600 -700 -800 900 1000 1100 -1200 1300 1400 1500 2 UJ w UJ GC CL 1600 £ O -1700 £ < UJ > u. 1800 ° (b 1900 ^ -2000 EXPLANATION —X--X--JT X X X X x X X X ^ * S"dcV 'V/'l -vV/'ivV V/////. IGNEOUS ROCKS GRANITE RHYOLITE BASALT GABBRO METAMORPHIC ROCKS HIGH GRADE MEDIUM GRADE LOW GRjADE SEDIMENTARY ROCKS QUARTZITE ARKOSIC UNDIFFERENTIATED MARINE ROCKS OF AREA IV NONMARINE ROCKS OF AREA IV 7 T J. | MAXIMUM AND (OR) MINIMUM AGE KNOWN AGE OF ROCK BODY OR BODIES FALLS WITHIN RANGE SHOWN—May not represent single or continuous rock-forming event AGE OF ROCK BODY OR BODIES PROBABLY FALLS WITHIN RANGE SHOWN AGE OF ROCK BODY OR BODIES UNCERTAIN—May extend beyond range shown 1 Tillman Metasedimentary Group— Red River mobile belt 2Navajoe Mountain Basalt Group 3Wichita igneous province 4Names of terranes 5Carrizo Mountain Group 6Packsaddle Schist 7 Raggedy Mountain Gabbro 8Llano province ft i, POORLY DATED MAFIC DIKES AGE OF METAMORPHISM—Dashed where metamorphism probably falls within range shown, queried where age uncertain and may fall outside of range shown A a STROMATOLITES IN SECTION I E.G. Lidiak II M E. Bickford, E.G. Lidiak, E.B. Kisvarsanyi III M E. Bickford, R.E. Denison, E.B. Kisvarsanyi IV R E. Denison V E.G. Lidiak INDEX TO AREAS AND COMPILATION RESPONSIBILITY 2100 -2200 -2300 -2400 2500 2600 2700 2800 2900 Compiled by R.E. Denison, EG. Lidiak, Mi. Bickford, and EB. Kisvarsanyi, 1980 CORRELATION CHART FOR PRECAMBRIAN ROCKS OF THE CENTRAL INTERIOR REGION, UNITED STATESDEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1241-D PLATE 1 CORRELATION CHART FOR PRECAMBRIAN ROCKS AND EVENTS IN THE ROCKY MOUNTAIN REGION EXPLANATION Two or more rock symbols may be combined where geologic relations known but uncertainties exist in age overlap + + + + + + + + + + X X X X X X X X X SEDIMENTARY ROCKS MAFIC VOLCANIC ROCKS GRANITIC INTRUSIVE ROCKS MAFIC INTRUSIVE ROCKS METAMORPHIC ROCKS / • MAXIMUM AND (OR) MINIMUM AGE KNOWN AGE OF ROCK BODY— Showing length of time for rockforming event AGE OF ROCK BODY FALLS WITHIN RANGE SHOWN I J AGE OF ROCK BODY PROBABLY FALLS WITHIN RANGE SHOWN AND PROBABLY WAS A CONTINUOUS ROCK-FORMING EVENT J ! AGE OF ROCK BODY PROBABLY FALLS WITHIN RANGE SHOWN; MAY OR MAY NOT BE A CONTINUOUS ROCK-FORMING EVENT J J AGE OF ROCK BODY UNCERTAIN, MAY EXTEND T T BEYOND RANGE SHOWN \{,\ POORLY DATED MAFIC DIKES I A ▲ FIRST APPEARANCE OF SHELLY FOSSILS1 nn STROMATOLITES1 1-1 MAXIMUM AND (OR) MINIMUM AGE OF METALS MORPHISM KNOWN METAMORPHIC EVENT FALLS WITHIN RANGE SHOWN METAMORPHIC EVENT INFERRED TO FALL WITHIN RANGE SHOWN A MAXIMUM AND (OR) MINIMUM AGE OF TECTONIC V EVENT KNOWN TECTONIC EVENT FALLS WITHIN RANGE SHOWN TECTONIC EVENT INFERRED TO FALL WITHIN RANGE SHOWN 'Only relative position in column, not age, is impliedUNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY PROFESSIONAL PAPER 1241-E PLATE 1 V o o N o cc O cc CL ADIRONDACK PROVINCE LOWLANDS HIGHLANDS Geologic reference Buddington, 1972 McLelland, 1979 fc Theokritoff, 1968 Silver. 1969 Younger metamorphic events t o t tl TACONIDE ZONE TACONIC ALLOCHTHON Ratcliffe and others, 1975 Zen. 1967 Palmer, 1971 :■ |ii I I '■y-n I-. -.15 i ■ - J si r ■ •4 ”1 jV"?l ' JjC. '■ ?l "■ BLUE-GREEN-LONG AXIS WESTERN LIMB AND CORE © TALLADEGA © GREAT SMOKY MOUNTAINS NW OF THE GREENBRIER FAULT, TN.-N.C. SE OF THE GREENBRIER FAULT, TN.-N.C. © (NW GEORGIA) MT. ROGERS REENTRANT VA.-N.C.-TN. (BLUE RIDGE CENTRAL VA.) © GRANDFATHER MOUNTAIN WINDOW, N.C. © READING PRONG, PA.-N.J. (RP) HUDSON HIGHLANDS, N.Y.-CT. (HH) © BERKSHIRE HIGHLANDS CT.-MA. © GREEN MOUNTAINS, VT. Alabama Geol Soc., 1973 King and others, 1968 Hadley and Nelson, 1971 King and others, 1968 McLaughlin and Hathaway, 1973 Espenshade, 1970 King and Ferguson, 1960 Rankin and others, 1973 Bryamt and Reed, 1970 Rankiin and others, 1973 Drake. 1969 Hall and others, 1975 k Palmer, 1976 Ratcliffe and Zartman, 1976 Doll and others, 1961 k Palmer. 1971 II E 1 t £ o 1 H 1 0 1 If: ij: I ; 1. . . iil 1 •Yi I A y :A Chilhowee ( • . ’•< Group j • . \ 'i II II © feat "fci II m » (IV) s i ® i ' . J • * I '.'•I . I ; • •. j L+ + +I e e « 5 © © ©P? ©© =1 s^®lj V u >yi |X Xiftyo; 1149 L X v ©Layered gneiss of Pardee Point and Deyton Bend (D,a) © V/' ® \ 4 0 EASTERN LIMB AND CORE FRIES-HAYESVILLE FRIES-HAYESVILLE FRIES-HAYESVILLE THRUST BLOCK © AL.-GA. THRUST BLOCK © NORTHEAST GA. ©© NORTH CAROLINA THRUST BLOCK © N.C.-VA. © (CENTRAL VA.) © BERKSHIRE HIGHLANDS, CT.-MA. © GREEN MOUNTAINS VT. Bentley and Neathery, 1970 Hurst, 1973 Hatcher, 1976 Fullagar and others. 1979 Hadley and Nelson, 1971 Hall, 1976 r . . Harwood, 1975 Espenshade, 1970 Hatch and Stanley. ,973 Rankin and others. 1973 Rstc|iff, and Zartman, 1976 Doll and others, 1961 (IV) t. „ t= © ©@r i x * I ?! o ©" (j)l225- /\ Va o ALLOCHTHONS (?) OF EASTERN ROCKS INNER PIEDMONT AL.-GA.-S.C.-N.C. SMITH RIVER, N.C.-VA. ® MARYLAND AND NORTHERN VIRGINIA ALLOCHTHONS Bentley and Neathery, 1970 Hatcher, 1971 Rankin, 1975 Pavlides and others. 1980 Drake and Morgan, 1981 © eo. WESTERN GNEISS DOMES PINE MOUNTAIN BELT, AL.-GA. © (KINGS MOUNTAIN BELT, S.C.-N.C.) CENTRAL BELT NEW ENGLAND SAURATOWN MOUNTAINS ANTICLINORIUM N.C.-VA. © BALTIMORE GNEISS DOMES MD.-DE.-PA. MANHATTAN PRONG, N.Y.-CT. Clarke, 1952 Hewett and Espenshade and Crickmay, 1937 others, 1975 Schamel and others. 1976 Rankin and others. 1973 Horton and Butler, 1977 Higgins, 1972 Crowley. 1976 Hall, 1976 Hall and others. 1975 (IV) (HI) ^i|f, -p © © 563 - 583 ® X/j Pound Ridgi Granite Gn T- lx xl „ ©lxXxlol uVil I x . A © m Hatcher and others. 1977 Parker, 1977 and 1968 Pavlides. 1976 Secor and Snoke. 1978 *$!i I iii 1 KlifI i-’vi; 6 "il IvVll l n INTERIOR-GEOLOGICAL SURVEY. RESTON. VA.-1983-G81244 E X PLAN A T 1 1 0 N STRATIFIED ROCKS INTRUSIVE ROCKS METAMORPH 1 C ROCKS Paleozoic overprint Precambrian metamorphism (where dearly recognizable) E3 153 112 ES) Sedimentary rocks Felsic Mafic Sedimentary and volcanic rocks Felsic |y Q| Mafic m E3 Amphibolite Granulite facies facies Y/X kVI Amphibolite Granulite facies facies Volcanic rocks Age of rock body, showing length of time of rockforming event Combined with to indicate paleontological control i i i i i i i i Age of rock body probably falls within range shown and was a continuous rock-forming event Age of rock body or bodies falls within range shown, but bar may not represent a single or continuous rock-forming event i i i i i i Age of rock body or bodies probably falls within range shown i i T T T T Age of rock body or bodies uncertain and may even extend beyond range shown Neither isotopic nor paleon-tologic data available t t Stratigraphic section continuous upward and includes rocks youngerthan 500 million years t t Stratigraphic section continuous upward and probably includes rocks younger than 500 million years First appearance of shelly fossils * 563--- K-Ar age Age determined from Rb-Sr (Adirondacks only) whole-rock isochron or U-Pb discordia curve Unconformity Thrust fault © Source of isotopic data (see footnotes). Where standing alone without age, placed opposite published (recalculated using current constants and isotopic abundances, if necessary) age. Position is not known to represent a well-determined whole-rock isochron or discordia curve @ Identifying symbol of constructed stratigraphic section. Location shown on geologic map (Plate 2). Column may be used for more than one location. Additional locations indicated by parentheses. M E T A M ORPHIC EVENTS Event M.Y.A. References (III) Parentheses indicate T], Event affects all col- VI Late Paleozoic - 200-250 © @ events younger umns to left of bar V Hercynian ~ 300 ® © than 500 million r-p and to right of the IV Acadian - 380-400 Q@©@@© years i1 next bar to the left. III Taconic 430-480 ©@©@@© i Uncertainty of II Virgilina-Avalonian 1 range indicated by 1 "Grenville" dashed bar Sources of isotopic data 1. Bass, 1969. Two Rb-Sr mineral isochrons. 2. Besancon and others [abs.], 1977. U-Pb zircon ages in the range of 600-620 m.y. 3. Bickford and Turner, 1971. Three Rb-Sr whole-rock isochrons. 3a. Black [abs.], 1978. Rb-Sr whole-rock isochron. 4. Brookins, 1976 Rb-Sr muscovite and whole-rock isochron. 5. Davis and others, 1962. U-Pb zircon; see text. 6a. Doe, 1962. K-Ar phlogopite (zoned pegmatite). 6b. Doe, 1962. Rb-Sr muscovite (unzoned pegmatite). 7. Faul and others, 1963. Pb-alpha zircon. 8. Fullagar, 1971. Rb-Sr whole-rock isochrons on several plutons. 9. Fullagar and Odom, 1973. Four Rb-Sr whole-rock isochrons used herein. 9a. Fullagar and others, 1979. Recalculated Rb-Sr whole-rock isochron of 1189±59 m.y., Toxaway Gneiss (lb). Recalculated Rb-Sr whole-rock isochron of 1214±83 m.y., migmatite, Mars Hill quadrangle (Ic). 10. Geological Society of London, 1964. Cambrian time divisions. 11. Fisher and others, 1970. Rb-Sr muscovite. 12. Glover and Sinha, 1973. U-Pb zircon. 13. Glover and others [abs.], 1971. U-Pb zircon ages. 14. Grauert, 1974. U-Pb zircon. 15. Grauert and others, 1973. U-Pb zircon ages of different facies of granulites from West Chester-Avondale area. 16. Grauert and Hall, 1973. U-Pb zircon. 17. Hall and others, 1975. Rb-Sr whole-rock isochrons. Ages recalculated by D. G. Mose (written commun., 1980) from original data. See Table 1. 18. Heath and Fairbairn, 1969. Rb-Sr whole-rock isochron; seven samples of syenitic gneiss. 19. Higgins and others, 1977. U-Pb zircon. 20. Hills and Butler [abs.], 1969, Rb-Sr whole-rock isochron, six samples. 21. Hills and Dasch, 1972. Ambiguous Rb-Sr whole-rock isochron. 22. Hills and Gast, 1964. Rb-Sr whole-rock isochron, eight samples. 23. Hills and Isachsen [abs.], 1975. Rb-Sr whole-rock isochron; eight samples. 24. Kish and others, 1975. Rb-Sr whole-rock isochron; no data. 25. Kovach and others, 1977. Rb-Sr whole-rock isochron; samples from nine localities. Supersedes ages published by Fairbairn and others, 1967. 26. Long, 1969. Rb-Sr whole-rock isochron; seven samples of Yonkers Gneiss. 27. Long and others, 1959. U-Pb uraninite and monazite. 28. Long and Kulp, 1962. Pb-Pb zircon age, Fordham Gneiss (1113 m.y.). K-Ar biotite from pegmatite, Speculator, N.Y. 29. Lukert and others, 1977. U-Pb zircon; no data. Ages stated are Robertson River Formation (732 m.y.), Flint Hill Gneiss (1081 m.y.), and Old Rag Granite (1140 m.y.). 30. McConnell and others [abs.], 1976. U-Pb zircon. 31. Mose, 1975, and written commun., 1980. Rb-Sr whole-rock isochron; 11 samples of Tyringham Gneiss. 32. Mose and Hayes, 1975. Rb-Sr whole-rock isochron; six samples of Pound Ridge Granite Gneiss. 33. Naylor, 1975. Rb-Sr muscovite. In a subsequent paper, Naylor (1976) noted that an earlier reported Pb-alpha age of 1100 m.y. from the Green Mountains (Faul and others, 1963) is actually from detrital zircon in the Hoosac Formation. 34. Naylor and others [abs.], 1973. Pb-Pb zircon. 35. Odom and Fullagar [abs.], 1971. Rb-Sr whole-rock isochron (15 samples) 36. Odom and Fullagar, 1973. Rb-Sr whole-rock isochron (18 samples) and U-Pb zircon (four samples). 37. Odom and others [abs.], 1973. U-Pb zircon ages of greater than 1 b.y. reported for Woodland Gneiss and charnockite of the Pine Mountain belt and Corbin Granite of northwest Georgia. 37a. Pavlides, 1976. Pb-Pb zircon, Fredericksburg Complex. 38. Rankin, 1975. Pb-Pb zircon; one sample. 39. Rankin and others, 1973. Pb-Pb zircon; one sample. 40. Rankin and others, 1969. U-Pb zircon; five samples of rhyolite. 41. Ratcliffe and others, 1972. Rb-Sr whole-rock isochron. 42. Ratcliffe and Zartman, 1976. U-Pb zircon; limiting intercepts of discordia lines. 44. Seiders and others, 1975. U-Pb zircon; four samples. 45. Silver [abs.], 1964. U-Pb zircon; syenite and alaskitic granite. 46. Silver [abs.], 1965. U-Pb zircon; hornblende-biotite-quartzite-plagioclase gneisses from several localities. Rocks interpreted as dacitic volcanic rocks interstratified with metasedimentary layers. 47. Silver, 1969. U-Pb zircon. 48. Spooner and Fairbairn, 1970. Rb-Sr whole-rock isochrons. Two isochrons recalculated by J. G. Arth (written commun., 1978) from data given. A six-point isochron of 1303±136 was obtained for charnockite gneiss from Crane Mountain. A five-point isochron of 1394±444 was obtained for pyroxene gneiss from Indian, Blue Mountain, and West Canada Lake quadrangles. 49. Tilton and others, 1970. U-Pb zircon age from Woodstock dome plots very close to the discordia curve for layered gneiss of the North Carolina-Tennessee Blue Ridge of Davis and others (1962). 50. Tilton and others, 1960. U-Pb zircon. Blue Ridge: Pb-Pb age of 1130 m.y. from hypersthene granodiorite from Mary's Rock tunnel, Shenandoah National Park, Virginia. Hudson Highlands: two-point U-Pb age of 1149 m.y. calculated from data given for Storm King Granite and Canada Hill Granite Gneiss; Pb-Pb ages are 1040 m.y. and 1150 m.y. respectively. 51. Tilton and others, 1958. Pb-Pb zircon ages, Phoenix and Towson domes. Superseded by work of Grauert (1974). 52. Wasserburg, 1961. K-Ar muscovite, pegmatite, Whippoorwill Corners, N.Y. 53. Wetherill and others, 1968. Rb-Sr whole-rock isochron (five samples). 54. Wright and Seiders, 1980,. U-Pb zircon; four samples. Chord recalculated. 55. Mose, D. G., written commun., 1980. Rb-Sr whole-rock isochron. 55a. Mose, D. G., and Hall, L. M., written commun., 1980. Rb-Sr whole-rock isochron. 56. Odom, A. L., unpublished data. Rb-Sr whole-rock isochron and U-pb zircon. Wiloy Gneiss: two nearly concordant U-Pb ages about 1190 m.y. Corbin Granite: Rb-Sr whole-rock isochron of 1003±50 m.y.; one nearly concordant U-Pb age about 1080 m.y. Salem Church Granite: Pb-Pb ages of 1030 m.y. Granite and gneiss of Kennesaw Mtn., Austell, and Hightower: U-Pb age of about 560 m.y. (one sample each). 57. Carpenter, R. H., Odom, A. L., and Hartley, M. E., written commun., 1978. Rb-Sr whole-rock isochron (recalculated by J. G. Arth, written commun., 1980, from data given) and U-Pb zircon. 58 Odom, A. L., and Kish, S. A., unpublished data. Rb-Sr whole-rock isochron and U-Pb zircon. 59. Russell, G. S., and Odom, A. L., unpublished data U-Pb zircon. 60. Stern, T. W., and Espenshade, G. H., unpublished data. Pb-Pb zircon, granite at Danbury, N.C. 61. Stern, T. W., and Rankin, D. W., unpublished data. U-Pb zircon (see text). Pb-Pb zircon, granite at Flat Rock, N.C. 62. Zartman, R. E., and Naylor, R. S., unpublished data. Rb-Sr whole-rock isochron and U-Pb zircon. 64. Olszewski [abs.], 1978. U-Pb zircon. 65. Smith and Giletti [abs.], 1978. Rb-Sr whole-rock isochron; four samples. 66. Kaye and Zartman, 1980. U-Pb zircon. 67. Aleinikoff and others, 1979. U-Pb zircon and monazite. 68. Black and Fullagar [abs.], 1976. Rb-Sr whole-rock isochron. 69. Lumbers, 1979, and written communication, 1979; Silver and Lumbers, 1966. Ages of volcanic rocks within the carbonate section and granitic rocks which intrude the stratified rocks in southern Ontario, Canada. 69a. Glover and others [abs.], 1978. Rb-Sr whole-rock isochron. References applying only to age of Paleozoic metamorphism: 70. Butler, 1972. 71. Butler, 1973. 73. Dallmeyer and Sutter, 1976. 74. Moench and Zartman, 1976. 75. Naylor, 1971. 76. Odom and Russell [abs.], 1975. 77. Wetherill and others, 1966. 78. Zen, 1972. 79. Grew and Day, 1972. 80. Secor and Snoke, 1978. CORRELATION CHART FOR PRECAMBRIAN ROCKS OF THE EASTERN UNITED STATESUNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY % -o -4 © p S' o' pi p PROFESSIONAL PAPER 1241-^ PLATE 2 OHIO KENTUCKY CANADA PENNSYLVANIA WEST VIRGINIA 9e°Phy; 'nearrie ALABAMA NEW YORK VERMONT Province MAINE NEW HAMPSHIRE p MARYLAND .CONNECTICUT GEORGIA SS^CHUSE NEW JERSEY DELAWARE RMODF' ISLAND SOUTH CAROLINA NORTH CAROLINA EXPLANATION Eastern plate Plutons younger than about 550 m.y. make up a considerable percentage of the Piedmont in places. They are ignored on this map. FLORIDA North American plate Terrane that consists largely of rocks whose age is resonably certain Terrane that consists of rocks whose age assign ment is poorly known Rocks about 600 to 550 m.y. old Rocks about 600 to 500 m.y. old Only basal Cambrian clastic sequence shown along NW limb of Blue-Green-Long axis. Elsewhere may contain rocks as young as 450 m.y. Eastern basement Rocks metamorphosed or emplaced during a Pre-cambrian orogeny Rocks about 630 to 600 m.y. old SCALE 1:2 500 000 Calc-alkaline plutonic rocks Rocks probably 800 to 630 m.y. old Rocks about 820 to 600 m.y. old Layered rocks Age of rocks on SE limb of Blue-Green-Long axis and Talladega belt very poorly controlled. May contain younger rocks TERRANES OF PRECAMBRIAN AND SELECTED CAMBRIAN ROCKS OF EASTERN UNITED STATES AND ADJACENT CANADA Western basement Rocks older than 1000 m.y. Rocks metamorphosed or emplaced during a Precambrian orogeny Modified from Georgia Geological Survey (1975), Harwood (1979), and King and Beikman (1974) Contact Dashed where gradational or approximately located Fault Dashed where approximately located; dotted where concealed Terrane of constructed stratigraphic section (see correlation chart) INTERIOR—GEOLOGICAL SURVEY. RESTON, VA —1983— G81244 4 4 3>o 4 4 4 4 * 4 Ko *>o Base from U.S. Geological Survey, 1982'R DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY 600 800- 1000- 1200- 1400- 1600- NORTHERN SEGMENT—UNDERLAIN BY GREENSTONE AND GRANITE Northern and northeastern Minnesota 1800 C/3 LU £ 2000 2400- 2600- 2800 3000 3200- 3400- 3600 3800 W t/3 :*£< + Q. D O i_ CJ ;A%* 0 yry.n: ooo E y> °o§0 ‘c < Northwestern Wisconsin and parts of northern Michigan cn u SOUTHERN SEGMENT—UNDERLAIN BYGNEISSIC ROCKS Eastern part of northern Michigan Northeastern, central, and southern Wisconsin O J&:03 r- ■ 0 4- 4-+ +j Z5 a i xxx C/) P 4- 4- c 0 xxx _c + 4- t xxx ^ 1 ^ < o xxx 4- + z cc CP co Southwestern and east-central Minnesota -600 - 800 - 1000 O _Q O o __ U 0 CD 0 CD CJ u CP o > p > T3 0 J ro ^ DC CO 1 CJ ■a a_ o cj - 1200 - 1400 1600 co ® -q E o o o CJ CJ CO 1800 CO LU 2000 £ 03 -2200 > -2400 -2600 — CP xxx 1 0 o xxx ■ti 0 1 c -a xxx xxx J 2 ■> L xxx - 3000 CP CP PROFESSIONAL PAPER 1241-F PLATE 1 EXPLANATION SEDIMENTARY ROCKS—Includes low-grade metamorphic rocks where appropriate; open circles, conglomerate FELSIC VOLCANIC ROCKS MAFIC VOLCANIC ROCKS FELSIC INTRUSIVE ROCKS MAFIC INTRUSIVE ROCKS AMPHIBOLITE FACIES METAMORPHIC ROCKS GRANULITE FACIES METAMORPHIC ROCKS Note: Two or more rock symbols may be combined MAXIMUM AND (OR) MINIMUM AGE KNOWN —May represent either length of time for a single rock-forming event or analytical uncertainty ROCK BODY OR BODIES FALL WITHIN RANGE SHOWN—May not represent a single or continuous rock-forming event AGE OF ROCK BODY OR BODIES PROBABLY FALLS WITHIN RANGE SHOWN— Isotopic ages scattered and do not define sharp age limits; queries indicate uncertainty in limit shown j AGE OF ROCK BODY UNCERTAIN—Rock ! body placed on chart by homotaxial correlation or use of nondefinitive isotopic data ^ ^ STROMATOLITES a FIRST APPEARANCE OF SHELLY FOSSILS MAXIMUM AND MINIMUM AGE OF METAMORPHISM KNOWN1 METAMORPHIC EVENT FALLS WITHIN RANGE SHOWN1 METAMORPHIC EVENT PROBABLY FALLS WITHIN RANGE SHOWN1 METAMORPHIC EVENT KNOWN AND POSSIBLY FALLS WITHIN RANGE SHOWN1 ’Symbol applies only to columns to left of symbol in a given subarea Note that the magnetic polarity of rocks in the Middle Proterozoic Midcontinent rift system is given in figure 9 - 2800 3200 3400 -3600 - 3800 0 100 200 KILOMETERS Index map showing outline of the two basement terranes in the Lake Superior region CORRELATION CHART FOR PRECAMBRIAN ROCKS OF THE LAKE SUPERIOR REGION, UNITED STATES Compiled by G. B. Morey and D. J. Bergstrom, 1984