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GEOLOGICAL SURVEY PROFESSIONAL PAPER 1.199 A-E Shorter Contributions to Isotope Research in the Western United States, 1980

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GEOLOGICAL SURVEY PROFESSIONAL PAPER 1.199 A-E Shorter Contributions to Isotope Research in the Western United States, 1980 GEOLOGICAL SURVEY PROFESSIONAL PAPER 1.199 A-E Shorter Contributions to Isotope Research in the Western United States, 1980 Archean Gneisses in the Little Rocky Mountains, Montana By ZELL E. PETERMAN Radioelement Distribution in a 3.06-Kilometer Drill Hole in Precambrian Crystalline Rocks, Wind River Mountains, Wyoming By CARL M. BUNKER and CHARLES A. BUSH Ages of Igneous Rocks in the South Park-Breckenridge Region, Colorado, and their Relation to the Tectonic History of the Front Range Uplift By BRUCE BRYANT, RICHARD F. MARVIN, CHARLES W. NAESER, and HARALD H. MEHNERT Potassium-Argon and Fission-Track Zircon Ages of Cerro Toledo Rhyolite Tephra in the Jemez Mountains, New Mexico By G. A. IZETT, J. D. OBRADOVICH, C. W. NAESER, and G. T. CEBULA Fission-Track Dating of the Climax and Gold Meadows Stocks, Nye County, Nevada By C. W. NAESER and FLORIAN MALDONADO GEOLOGICAL SURVEY PROFESSIONAL PAPER1199A-E UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1981 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary GEOLOGICAL SURVEY Doyle G. Frederick, Acting Director Catalog No. 81-600048 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, B.C. 20402 CONTENTS [Letters designate the chapters] Page (A) Archean gneisses in the Little Rocky Mountains, Montana, by Zell E. Peterman 1 (B) Radioelement distribution in a 3.06-kilometer drill hole in Precambrian crystalline rocks, Wind River Mountains, Wyoming, by Carl M. Bunker and Charles A. Bush................................................................................................................ 7 (C) Ages of igneous rocks in the South Park-Breckenridge region, Colorado, and their relation to the tectonic history of the Front Range uplift, by Bruce Bryant, Richard F. Marvin, Charles W. Naeser, and Harald H. Mehnert............. 15 (D) Potassium-argon and fission-track zircon ages of Cerro Toledo Rhyolite tephra in the Jemez Mountains, New Mexico, by G. A. Izett, J. D. Obradovich, C. W. Naeser, and G. T. Cebula .......................................................................................... 37 (E) Fission-track dating of the Climax and Gold Meadows stocks, Nye County, Nevada, by C. W. Naeser and Florian Maldonado ...................................................... 45 SYMBOLS AND ABBREVIATIONS USED IN THIS VOLUME °C Degrees Celsius CIPW Cross, Iddings, Pirsson, and Washington (igneous rock classification system) cm2 Square centimeters g Grams IUGS International Union of Geological Sciences km Kilometers m Meters mm Millimeters m.y. Million years n Neutrons ppm Parts per million RaeU Radium-equivalent uranium yr Years Xf Decay constant for spontaneous fission of 238U Xg Decay constant based on beta-particle emission Xe Decay constant based on orbital electron capture Microcalories per gram-year Micrometers (10~6 m) Pi Induced fission-track density p. Spontaneous (fossil) fission-track density a Standard deviation Neutron flux Archean Gneisses in the Little Rocky Mountains, Montana By ZELL E. PETERMAN SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES GEOLOGICAL SURVEY PROFESSIONAL PAPER 1199-A CONTENTS Page Abstract...................................................................................................................................................... 1 Introduction.............................................................................................................................................. 1 Acknowledgments ................................................................................................................................ 1 Samples and analytical methods ...................................................................................................... 2 Results ........................................................................................................................................................ 3 Regional considerations........................................................................................................................ 3 References cited........................................................................................................................................ 6 ILLUSTRATIONS Page FIGURE 1. Generalized geologic map of the Little Rocky Mountains, Montana .............. 2 2. Rb-Sr isochron plot for samples of gneiss and amphibolite from the Little Rocky Mountains ......................................................................................................... 4 3. Map showing regional Precambrian framework of north-central United States and adjacent Canada.................................................................................. 5 TABLES Page TABLE 1. Rb-Sr analyses of Precambrian rocks from the Little Rocky Mountains .... 3 SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH IN THE WESTERN UNITED STATES ARCHEAN GNEISSES IN THE LITTLE ROCKY MOUNTAINS, MONTANA By ZELL E. PETERMAN ABSTRACT hornblende and biotite respectively for Precambrian rocks just north of Zortman (fig. 1). These ages are in Rb-Sr dating of a layered complex of tonalitic and granitic the common range of K-Ar ages of basement rocks of gneisses and amphibolite in the Little Rocky Mountains of north- central Montana defines their age as late Archean. Previous K-Ar the western Canada sedimentary basin and of the ages of 1,710 and 1,750 m.y. on hornblende and biotite, Williston basin of western north Dakota and eastern respectively, reflect a widespread resetting of K-Ar and Rb-Sr Montana. The concordance of the ages from the mineral ages in the basement of the Williston basin and of the Little Rocky Mountains suggests that a specific western Canada sedimentary basin. Although exhibiting some thermal or metamorphic event is being recorded by scatter, the Rb-Sr data are consistent with an age of at least 2,550 m.y., and consideration of reasonable initial 87Sr/86Sr ratios the K-Ar systems. That these systems were indicates that the gneisses may be substantially older. Even apparently little affected by the Tertiary intrusion is though the Rb-Sr data do not provide an exact age, an Archean surprising in view of the association of the syenite age is firmly established. This age, coupled with determinations and the Precambrian rocks. The syenite intrudes on basement cores from southeastern Montana and northeastern Precambrian and possibly Cambrian rocks but is in Wyoming, extends the Archean terrane of the Wyoming age province well into the buried basement towards the Williston fault contact with younger rocks (Knechtel, 1959). basin. Continuity of the Archean of the Superior province and the Considering the relatively small areas of Wyoming age province in the subsurface of the Dakotas and Precambrian rocks, the thermal effects must have eastern Montana remains to be demonstrated, but the gap is been significant but not sufficient to reset the K-Ar narrowed considerably by the data reported here. systems. The K-Ar determinations provide a minimum age INTRODUCTION for the Precambrian rocks of the Little Rocky Mountains and identify them as Precambrian X or The Little Rocky Mountains of north-central older. The nearest Archean rocks crop out in the Montana are well known for excellent sections of Little Belt Mountains approximately 200 km Paleozoic and Mesozoic sedimentary rocks exposed southwest of the Little Rocky Mountains; they have in a domical uplift associated with several Tertiary been dated by Catanzaro and Kulp (1964), and are syenite porphyry stocks (fig. 1). Precambrian rocks part of the major Archean terrane of the Wyoming are also exposed in the area but as blocks within and age province. The extent of Archean rocks in the marginal to the intrusions. These blocks consist of buried basement of northern and eastern Montana is biotite schist and gneiss, quartzite, and amphibolite poorly defined. Gneisses exposed in the Little Rocky that are thought to represent a sequence of Mountains provide indirect access to this otherwise sedimentary and volcanic rocks metamorphosed to buried basement, and the results obtained from Rb- amphibolite grade (Knechtel, 1959). Hearn and others Sr dating of these gneisses are reported here. (1977) report K-Ar ages of 58 to 66 m.y. for the syenite and assign it to the Paleocene. The syenite was ACKNOWLEDGMENTS locally fractured, silicified, and altered during an episode of mineralization that produced gold-silver The technical support of G. T. Cebula and J. W. deposits in the area. Groen in the careful preparation of the rock samples Burwash, Baadsgaard, and Peterman (1962) and of Kiyoto Futa in the analytical work is determined K-Ar ages of 1,710 and 1,750 m.y. on gratefully acknowledged. SHORTER CONTRIBUTIONS TO ISOTOPE RESEARCH, WESTERN UNITED STATES 108°30' 48°00'- 10 KILOMETERS I EXPLANATION Tertiary syenite Mine Mesozoic sedimentary rocks Contact Paleozoic sedimentary rocks Fault Archean metamorphic rocks Sample locality FIGURE 1. Generalized geology of the Little Rocky Mountains, Montana (adapted from Knechtel, 1959). SAMPLES AND ANALYTICAL METHODS mine (fig. 1). A layered complex of gneiss and amphibolite has nearly horizontal banding and foliation at this locality. Fine-grained biotite gneiss is Rb and Sr analyses were completed following in bands from several centimeters to several tens of procedures slightly modified from those described centimeters thick. Biotite-hornblende gneiss occurs earlier (Peterman , and others, 1967). Ages are calcu­ in layers as much as a meter thick. Amphibolite
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