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Mylonite Zones in the Crystalline Basement Rocks of Sixmile Creek and Yankee Jim Canyon Park County Montana

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Mylonite Zones in the Crystalline Basement Rocks of Sixmile Creek and Yankee Jim Canyon Park County Montana University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1982 Mylonite zones in the crystalline basement rocks of Sixmile Creek and Yankee Jim Canyon Park County Montana Robert Robert Burnham 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 Burnham, Robert Robert, "Mylonite zones in the crystalline basement rocks of Sixmile Creek and Yankee Jim Canyon Park County Montana" (1982). Graduate Student Theses, Dissertations, & Professional Papers. 4677. https://scholarworks.umt.edu/etd/4677 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 COPYRIGHT ACT OF 1976 This is an unpublished manuscript in which copyright sub ­ s is t s . Any further reprinting of its contents must be approved BY THE AUTHOR. Mansfield Library U niversity of Montana Date: 19 8 2 MYLONITE ZONES IN THE CRYSTALLINE BASEMENT ROCKS OF SIXMILE CREEK AND YANKEE JIM CANYON, PARK COUNTY, MONTANA by Robert Burnham B.A., Dartmouth, 1980 Presented in partial fulfillment of the requirements for the degree of Master of Science UNIVERSITY OF MONTANA 1982 Approved by: UMI Number: EP40141 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. UMI Dissertation RybfeMfig UMI EP40141 Published by ProQuest LLC (2014). 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 ^ s f ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 ABSTRACT Burnham, Robert, M.S., Fall, 1982 Geology Mylonite Zones in the Crystalline Basement Rocks of Sixmile Creek and Yankee Jim Canyon, Park County, Montana Director: Donald Hyndman Yankee Jim Canyon and Sixmile Creek, approximately 50 kilometers south of Livingston, Montana, expose strongly deformed and meta­ morphosed Precambrian crystalline basement rocks. Mapping reveals two distinct packages of rocks: a medium-grained gneissic package, and a fine-grained schistose package. The gneissic package in Yankee Jim Canyon is subdivided into a mixed gneiss unit and a mylonitic potassium feldspar gneiss unit. In addition to the gneissic package, Sixmile Creek contains amphibole schist and micaceous schist of the schistose package. A northeast structural trend characterizes both map areas. The study area lies along a large northeast structural trend which extends from the southern Madison Range to the north Snowy block (Reid et a l., 1975; Erslev, 1981; Mogk, 1982). In Sixmile Creek, two ductile shear zones follow a similar northeast structural trend. Yankee Jim Canyon exposes only one ductile shear zone. In these ductile shears, the width of mylonitization varies, but probably does not exceed one-half kilometer. Adjacent to the mylonite zones, greenschist-facies retrograde metamorphism overprints amphibolite-facies prograde metamorphism only in the immediate vicinity of the mylonite zones. The localization of retrograde metamorphism may result from two processes: 1) Shear heating associated with mylonites may cause temperature increases in excess of one hundred degrees Celsius (Brun and Cobbold, 1980; Fleitout and Froidevaux, 1980). I f conduction or convection does not significantly dissipate the heat away from the mylonite zones, a retrograde aureole may form in areas of increased temperature. 2) Retrograde mineral assemblages develop more readily where an influx of water is sufficient to induce alteration. This water enters the system prior to or during mylonitization. Increased water content enhances mylonitization by increasing d u c tility . Water circulating along fractures may hydrate the rocks su fficie n tly to induce mylonitization along these zones of weakness. A combination of shear heating and increased water content may link mylonitization and retrograde metamorphism. This paper is dedicated to my mother and father for twenty five years of love and care. ACKNOWLEDGMENTS I would like to especially thank the three members of my committee, Prof. Donald Hyndman, Prof. David Fountain, and Prof. Keith Osterheld for their scie n tific insight and their c ritic a l review of this paper. Prof. John Wehrenberg assisted in x-ray diffraction for mineral identification. Shirley Pettersen typed several versions of this paper, and I might never have finished without her support. Sara Foland analyzed several samples for rare earth elements at Los Alamos Laboratories, New Mexico, and Dr. Peter Hooper analyzed further samples for whole-rock geochemistry at Washington State University. Dr. David Mogk of the University of Washington, offered well appreciated insight of the regional geology during a crucial point of the thesis. Paul Kuhn, Jeff Mauk, Rick Moore, and Sue Bloomfield provided assistance and encouragement during the past year. Finally, I would like to thank Lisa Conte for her moral support from afar. TABLE OF CONTENTS Page ABSTRACT................................................................. i i ACKNOWLEDGMENTS.................................................... i i i LIST OF TABLES ....................................................................................vi LIST OF FIGURES.................................................................................. v ii CHAPTER I. INTRODUCTION ........................................................................ 1 Background .................................................................. 3 II. ROCK DESCRIPTIONS............................................................ 6 Amphibole Schist ........................................................... 6 Micaceous Schist ........................................................... 10 Mixed Gneiss........................................................................10 Mylonitic Potassium Feldspar Gneiss .................... 15 Metamorphic Precursors -Schistose Package . 17 Metamorphic Precursors - GneissicPackage . 21 I I I . METAMORPHIC HISTORY ....................................................... 29 Prograde Metamorphism ................................................... 29 Retrograde Metamorphism .................................... 34 Spatial Association of Retrograde Metamorphism and Ductile Deformation ...................... 45 IV. STRUCTURE................................................................................49 V. COMPARISON OF YANKEE JIM CANYON AND SIXMILE CREEK TO THE SOUTHERN MADISON RANGE AND THE NORTH SNOWY BLOCK ................................................... 59 Southern Madison Range .................................... 59 North Snowy B lo c k ....................................................... 62 i v CHAPTER Page VI. CONCLUSION...............................................................................67 REFERENCES...............................................................................................70 APPENDIX I ...........................................................................................76 APPENDIX I I ...........................................................................................93 V LIST OF TABLES Table Page 1. Visually estimated modes of minerals in rocks of the schistose package ........................................................11 2. Visually estimated modes of minerals in rocks of the gneissic package ........................................................19 3. Neutron activation analyses for major and trace elements in amphibole s c h is t ................................ 20 4. X-ray flourescence analyses for major elements . 26 5. Direction of elongation of porphyroblasts i n myl oni tes . ....................................................................... 55 6. Comparison of some interpretations by Reid et a l. (1975) versus those by Mogk (1982) in the north Snowy b l o c k ..............................................................64 7. Comparison of deformational events versus metamorphic history for the study a re a .............................67 vi LIST OF FIGURES Figure Page 1. Location map ...................................................... 2 2. Geologic map of Yankee Jim C anyon ....................................7 3. Geologic map of Sixmile Creek ....................................... 8 4. Photomicograph of dislocated hornblende annealed by actinolite ....................................................... 9 5. Quartz-plagioclase-potassium feldspar diagram of samples from the micaceous s c h is t ............................12 6. Photograph of outcrop from the mixed gneiss unit . 9 7. Photograph of outcrop from the mylonitic potassium feldspar gneiss ............................................... 16 8. Photograph of outcrop from the mylonitic potassium feldspar gneiss ............................................... 16 9. Plot of Niggli c versus Niggli m g ..................................19 10.
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