Geology of a Portion of the Eocene Sylvan Pass Volcanic Center Absaroka Range Wyoming
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University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1977 Geology of a portion of the Eocene Sylvan Pass volcanic center Absaroka Range Wyoming Daphne D. LaPointe 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 LaPointe, Daphne D., "Geology of a portion of the Eocene Sylvan Pass volcanic center Absaroka Range Wyoming" (1977). Graduate Student Theses, Dissertations, & Professional Papers. 7097. https://scholarworks.umt.edu/etd/7097 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]. GEOLOGY OF A PORTION OF THE EOCENE SYLVAN PASS VOLCANIC CENTER, ABSAROKA RANGE. WYOMING by Daphne D. LaPointe A.B., Smith College, 1973 Presented in partial fulfillm ent of the requirements for the degree of Master o f Science UNIVERSITY OF MONTANA 1977 Approved by :hairman, Board^^f Examiners D ^n , Graduate School Date UMI Number: EP37898 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 Oi«seryAicm PuMiahmg UMI EP37898 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 Pro uesf ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 4 8 10 6 - 1346 LaPointe, Daphne D ., M.S., May, 1977 Geology Geology o f a Portion o f the Eocene Sylvan Pass Volcanic Center, Absaroka Range, Wyoming (60 pp.) D irector: Donald W. Hyndman Geologic mapping, thin-section petrography, and whole-rock chemistry were used to determine the Eocene volcanic and intrusive history of a portion of the Sylvan Pass volcanic center in the Absaroka Range of northwestern Wyoming. The layered volcanic rocks of the study area consist of over 600 meters of andesitic flows and flow breccias which interfinger to the west with heterolithologic debris-flow breccias and volcanogenic sediments. There is a pro gressive upward change in lithology from dominantly pyroxene andésite lower in the pile to dominantly hornblende andésite higher in the pile. The layered sequence is cut by numerous dikes of hornblende andésite and a few of biotite andésite and dacite. A breccia pipe containing fragments o f both Precambrian metamorphic rocks and Eocene volcanic rocks cuts the sequence. A small d io r it ic stock intrudes the layered sequence, hornfelsing and hydrothermally altering the adjacent rocks. Two episodes of faulting have affected the area: one approximately contemporaneous w ith the volcanic activity and one which postdates it. The Sylvan Pass rocks display chemical trends which are typical of the calc-alkaline rock association and similar to trends of other volcanic rocks of the Washburn and Thoroughfare Creek Groups o f the Absaroka Volcanic Supergroup. Overall variations in lithology and thickness of the un its indicate th a t during mid-Eocene time the study area was located on the west flank of an active volcanic center. Chemical and mineralogical variation of the eruptive rocks from more mafic to less mafic with time may support a possible comagmatic origin of the suite from a differentiating magma chamber. 11 ACKNOWLEDGMENTS I would lik e to extend my warmest thanks to the members o f my committee, Donald Hyndman, Graham Thompson, and Walter H ill fo r th e ir help in e d itin g th is thesis. Research was p a rtia lly funded by a G rant-in-Aid to Research from Sigma X i, the S c ie n tific Research Society of North America, to whom I am grateful. I would like to especially thank my husband, Tom Irwin, for his constant encourage ment and many helpful discussions of the thesis material. m TABLE OF CONTENTS Page ACKNOWLEDGMENTS ......................................................................................... i i i LIST OF TABLES.......................................................................................... vi LIST OF FIGURES........................................... vi LIST OF PLATES................................................................................................v li CHAPTER I. INTRODUCTION .............................................................................. 1 Previous Work ........................................................................ 1 Regional GeologicFramework ............................................. 4 I I . LOCAL GEOLOGY.............................................................................. 10 Layered Volcanic Rocks ...................................................... 10 Basal S e q u e n ce ................................................................ 10 Debris-Flow Sequence ................................................... 15 Upper S e q u e n ce ................................................................ 17 Breccia P ip e ........................................................................ 20 Intrusive Rocks ....................................................................... 20 D ik e s ..................................................................................... 20 D i o r i t e ................................................................................. 26 A l t e r a t i o n ............................................................................. 31 Volcanic Rocks ..................................................................... 31 Diorite Intrusive ............................................................ 32 IV CHAPTER Page S tr u c tu r e ......................................................................................32 Whole-Rock Chemistry ....................................................... 34 I I I . INTERPRETATIONS..............................................................................45 The Study Area as a Part of the Sylvan Pass Volcanic Center .................................................................... 45 Inferred Volcanic History of the Study Area . 48 Petrologic E volution ............................. 52 Regional Relationships ................................................... 53 IV. SUMMARY AND CONCLUSIONS ........................................................ 55 REFERENCES CITED ....................................................................................... 56 APPENDIX...........................................................................................................60 LIST OF TABLES Table Page 1. Modal percentages of minerals in diorite intrusive . 28 2. Whole-rock chemical analyses o f Sylvan Pass area r o c k s .....................................................................................................35 LIST OF FIGURES Figure Page 1. Generalized geologic map of Absaroka volcanic fie ld . 2 2. Location map of study area in Yellowstone National P a rk ............................................................... 3 3. Generalized stra tig ra p h ie sequence o f Absaroka Volcanic Supergroup .................................................................... 6 4a. Generalized geologic map o f study area ........................... 11 4b. Generalized geologic cross section of study area . 12 5. Stratigraphie section of study area units ................... 16 6. Q-A-P diagram showing d io rite in tru s ive samples. 29 7. Chemical variation diagrams for Sylvan Pass area rocks 37 8. Chemical va ria tio n diagram showing Peacock Index . 39 9. A-F-M plot of Sylvan Pass area rocks ...................................... 40 10. Plot of K 2O versus SiOg for rocks of Absaroka- Gallatin volcanic province ....................................................... 41 11. A-F-M plot for rocks of Absaroka-Gallatin volcanic province ........................................................................ 43 12. Hypothetical cross section o f a typ ica l Absaroka volcanic center ............................................................................. 46 v i LIST OF PLATES Plate Page 1. Zoned plagioclase la th ......................................................14 2. Flow breccia t e x t u r e .......................................................... ^4 3. Hypersthene-andesite f lo w ..............................................14 4. Roadcut o f well-bedded volcanogenic sediments. 14 5. Graded beds in volcanogenic sediments ........................... 19 6. Fragment o f garnet gneiss in breccia p i p e ............19 7. Fragment o f amphibolite in breccia p i p e ................ 19 8. Zoned hornblende phenocryst ................................................ 22 9. Hornblende glomerophenocryst ................................................ 22 10. Opaque-rimmed hornblende and non-rimmed augite . 22 11. Pseudomorph a fte r o liv in e .................................................... 22 12. Biotite-andesite d ik e ......................................................24 13. Diorite intrusive ....................................................................