Precambrian Geology of the Boulder River Area Beartooth Mountains Montana
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University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1980 Precambrian geology of the Boulder River area Beartooth Mountains Montana Gary Weeks The University of Montana Follow this and additional works at: https://scholarworks.umt.edu/etd Let us know how access to this document benefits ou.y Recommended Citation Weeks, Gary, "Precambrian geology of the Boulder River area Beartooth Mountains Montana" (1980). Graduate Student Theses, Dissertations, & Professional Papers. 7451. https://scholarworks.umt.edu/etd/7451 This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. COPYRIGHT ACT OF 1976 Th is is an unpublished manuscript in which copyright sub s is t s . Any further r e p r in t in g of it s contents must be approved BY THE AUTHOR. Ma n s fie ld Library Un iv e r s it y of Montana Date: 19 8 0 PRECAriBRIAN GEOLOGY OF THE BOULDER RIVER AREA, BEARTOOTH MOUNTAINS, MONTANA by Gary Weeks B.S. in Forestry, University of Montana, 1975 Presented in partial fulfillment of the requirement for the degree of Master of Science UNIVERSITY OF MONTANA 1980 Approved by: Chairman, Board of Examiners Dean, Graduate Sclwol / Date UMI Number: EP38252 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. UMT Oiewftation PuNi«hing UMI EP38252 Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code P r o Q ^ s f ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346 ABSTRACT Weeks, Gary L. , M.S., Winter, 1980 Geology Precambrian Geology of the Boulder River Area, Beartooth Moun tains, Montana Director: David Fountain ^1). The Boulder River area lies along the Boulder River within the Archean core of the Beartooth Mountains. The area contains two major lithologies: 1) a medium-grained massive to slightly folia ted granite; and 2) biotite-rich metasediments. The biotite rich metasediments are mapped as regionally metamorphosed schists bear ing the assemblage quartz + biotite ± cordierite ± staurolite ± anthophyllite and massive hornfels within the Stillwater Complex thermal aureole. The hornfels lacks staurolite and may have pyro xene. The schist mineral assemblage indicates amphibolite facies metamorphism of the low pressure intermediate (Buchan) type. The thermally metamorphosed rocks are in the hornblende and pyroxene hornfel facies. The medium-grained granite covers at least 150 sq. km and in trudes the biotite schists. The granite and the metasedimentary rocks have undergone a later,low grade metamorphism which pro duced a chlorite and epidote assemblage. Minor lithologies in the Boulder River area are quartzo-feldspathic gneiss, amphibolite, quartzite, quartz-magnetite rock, and tonalité. Foliation in the schists indicates open folds plunging gently northward. This is the same structural pattern as the terrain to the east of the Boulder River which is known as the Beartooth Block. It differs from the northeast striking fold pattern of the rocks to the west in the North Snowy Block. The granite and schists also resemble rocks mapped to the east along the S till water River. Major events in Precambrian geologic history of the Boulder River area are: 1) deposition of schist protoliths; 2) regional amphibolite facies metamorphism (~2.7 b.y.); 3) intrusion of the Stillwater Complex and formation of hornfels (~2.7 b.y.); 4) gra nite intrusion (~2.7 b.y.); and 5) greenschist facies metamorphism (<1.9 b.y. ). 11 ACKNOWLEDGMENTS I owe thanks to Dr. David Fountain, Dr. Tom Nimlos, Dr. Don Hyndman, and Dr. Graham Thompson for their guidance and expertise. Special thanks to Gisela Toth for the more artistic line drawings. The field work was partly supported by the Bendix Field Engineering Corporation. 111 TABLE OF CONTENTS Page ABSTRACT. .............................................................. ii ACKNOWLEDGMENTS..................... i i i LIST OF FIGURES .............................................................. vi LIST OF TABLES. .......................................................................................... CHAPTER I. INTRODUCTION....................................................................................... 1 Location and Access ................................................ 7 Previous Studies. ............................. 7 Present Work.................................................................................. 8 I I. IGNEOUS ROCKS .................................................................................. 9 Granitic plutons ......................................................................... 9 Petrology .................................................... 10 Field Relationships............................................................. 16 Metamorphism.............................................................................. 18 Origin........................................................................................... 18 Age ..............oo.oeo#.. 20 Diabase Dikes ............................................................ 22 I I I . METAMORPHIC ROCKS .......................................................................... 23 Biotite Schists ......................................................................... 23 Hornfels........................................................................................... 32 1 V CHAPTER Origin of the Schists and Hornfels ................................ 33 Quartzo-feldspathic Gneiss .................................................... 36 Origin of the Quartzo-feldspathic Gneiss.................. 37 Amphibolites. « ................................................................. 38 Metamorphorism. ................................................................. 38 A ge................................................................................................... 42 IV. STRUCTURE........................................................................................... 45 V, HISTORICAL SUMMARY.......................................................................... 48 REFERENCES CITED. 53 APPENDIX 1............................................................................................................ 58 LIST OF FIGURES Figure Page 1 The Archean Wyoming Province. ............ 2 2 Sketch map of the Beartooth Mountains .................................. 3 3 Geologic map of the Boulder River area............................ 6 4 Q-A-P ternary plot of the Boulder River granite .... 12 5 Myrmekite and microc11ne in the granite ............................. 14 6 Antiperthitic plagioclase ........................................................ 14 7 Muscovite inclusions in plagioclase ...................................... 15 8 Biotite, magnetite, muscovite, and epidote .................. 15 9 Granite intruding schist .............................................................17 10 Granite intruding tonalité........................................................ 17 11 Knobby surface of cordierite bearing schist ..................... 24 12 Altered cordierite poikiloblasts. .................................. 30 13 Foliation wrapping around cordierite piokiloblasts. 30 14 Staurolite, anthophyllite, and biotite inclusions in a cordierite poikiloblast ................................................................. 31 15 Biotite clot from a hornfels. ................................................31 16 Estimated conditions of metamorphism...................................39 17 AFMK plot of the schists and hornfels ..............................40 18 Equal-area plot of poles to foliation and fold axes . 46 VI LIST OF TABLES T a b le Page 1 Modal analyses of granitic rocks ............................................. 11 2 Chemical analyses of granitic rocks .................................... 19 3 Radiometric ages of granites from the Beartooth Mtns. 21 4 Modal analyses of metamorphoric rocks ................................ 26 5 Chemical analyses of schists and hornfels ....................... 34 6 Comparison of chemical analyses of Beartooth schists and hornfels with other rock types.............................. 35 7 Precambrian history of the Boulder River area * . 49 VI 1 CHAPTER I INTRODUCTION The Beartooth Mountains are a rectangular block of Precambrian rock uplifted along reverse faults in Laramide time. The range lies along the Montana-Wyoming border near the northern lim it of a gorup of basement-cored uplifts called the Wyoming Province (Fig. 1). North of the Colorado-Wyoming border, these uplifts yield 2.6 - 2.7 billion year (b.y.) Rb-Sr whole rock ages and so are referred to as the Archean Wyoming Province (Condie, 1976). In the Beartooth Mountains, the event that set the Rb-Sr whole-rock systems at 2.6 - 2.7 b.y. is called the Beartooth orogeny (Skinner and others, 1969).