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Exploring Connections Between a Very Large Volume Ignimbrite and an Intracaldera Pluton: Intrusions Related to the Oligocene Wah Wah Springs Tuff, Western US

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Exploring Connections Between a Very Large Volume Ignimbrite and an Intracaldera Pluton: Intrusions Related to the Oligocene Wah Wah Springs Tuff, Western US Brigham Young University BYU ScholarsArchive Theses and Dissertations 2013-05-31 Exploring Connections Between a Very Large Volume Ignimbrite and an Intracaldera Pluton: Intrusions Related to the Oligocene Wah Wah Springs Tuff, Western US Chloe Noelle Skidmore Brigham Young University - Provo Follow this and additional works at: https://scholarsarchive.byu.edu/etd Part of the Geology Commons BYU ScholarsArchive Citation Skidmore, Chloe Noelle, "Exploring Connections Between a Very Large Volume Ignimbrite and an Intracaldera Pluton: Intrusions Related to the Oligocene Wah Wah Springs Tuff, Western US" (2013). Theses and Dissertations. 4041. https://scholarsarchive.byu.edu/etd/4041 This Thesis is brought to you for free and open access by BYU ScholarsArchive. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Exploring Connections Between a Very Large Volume Ignimbrite and an Intracaldera Pluton: Intrusions Related to the Oligocene Wah Wah Springs Tuff, Western US Chloe Skidmore A thesis submitted to the faculty of Brigham Young University in partial fulfillment of the requirements for the degree of Master of Science Eric H Christiansen, Chair Michael J. Dorais Bart J. Kowallis Department of Geological Sciences Brigham Young University June 2013 Copyright © 2013 Chloe Skidmore All Rights Reserved ABSTRACT Exploring Connections Between a Very Large Volume Ignimbrite and an Intracaldera Pluton: Intrusion Related to the Oligocene Wah Wah Springs Tuff, Western US Chloe Skidmore, Department of Geological Sciences, BYU Master of Science The Wah Wah Springs Tuff and the Wah Wah Springs Intrusive Granodiorite Porphyry (Wah Wah Springs Intrusion) both originated from the Indian Peak caldera complex, which was a major focus of explosive silicic activity in the middle Cenozoic Great Basin ignimbrite flareup. This caldera formed 30.0 Ma when an estimated 5,900 km3 of crystal-rich dacitic magma erupted to create the Wah Wah Springs Tuff. The Wah Wah Springs Intrusion later intruded the tuff, causing resurgence of the caldera. Field, modal, and geochemical evidence suggest the tuff and intrusion are cogenetic. The mineral assemblages of the two rocks are similar: both include similar proportions of plagioclase, quartz, hornblende, biotite, clinopyroxene, and Fe-Ti oxides, with trace amounts of titanite, apatite, and zircon. Whole rock geochemistry also matches, and both rocks have distinctively high Cr concentrations. Plagioclase, hornblende, and clinopyroxene have similar compositions but biotite and Fe-Ti oxides have been hydrothermally altered in the intrusion. Both hornblende and quartz provide clues to the magmatic evolution of the Wah Wah Springs Intrusion. Hornblende grains are either euhedral, have reaction rims, or are completely replaced by anhydrous minerals. Deterioration of hornblende was caused by decompression as the magma ascended and then stalled and solidified at shallow depths. Two stages of quartz growth are shown in cathodoluminescence (CL) imagery. Quartz first grew then was resorbed during eruption, then grew again at lower pressures indicated by CL-bright quartz rims and groundmass grains. The geochemical and mineralogical similarities, together with the distinctive hornblende and quartz characteristics suggest that after the Wah Wah Springs Tuff erupted, the unerupted mush rose to a shallow level where it crystallized at low pressure to form the Wah Wah Springs Intrusion. This indicates that the both rocks formed in the same chamber, and that tuffs and associated intrusions can be intimately related. Keywords: intracaldera pluton, magma chamber, ignimbrite, Wah Wah Springs Tuff ACKNOWLEDGEMENTS I would like to express my appreciation for all those who assisted in the completion of this work. Much appreciation goes to Dr. Eric Christiansen for answering my endless questions and providing guidance and direction. I am also grateful for the insight and comments of my committee, Dr. Bart Kowallis and Dr. Mike Dorais, with additional thanks to Dr. Dorais for his help with the electron microprobe. I would also like to acknowledge Dr. Myron Best and Kurtus Woolf for their previous research on the Wah Wah Springs Tuff, without which this project could not have been done. A special thanks also goes to my fellow students who helped me out a great deal: namely, Doug Johnson for his instruction on lab equipment use, and Kat Robertson and Audrey Warren for their help with field work. Last but not least, I’d like to thank Tanner Mills for always keeping me in good spirits while spending long hours in the grad cubes. TABLE OF CONTENTS ABSTRACT ............................................................................................... ii ACKNOWLEDGEMENTS ...................................................................... iii TABLE OF CONTENTS .......................................................................... iv LIST OF FIGURES .................................................................................... v LIST OF TABLES .................................................................................... vi INTRODUCTION ......................................................................................1 GEOLOGIC SETTING ...............................................................................3 METHODS ................................................................................................. 5 FIELD RELATIONS ..................................................................................7 RESULTS ...................................................................................................9 Whole Rock Analysis ..................................................................... 9 Petrography ...................................................................................10 Mineral Chemistry ........................................................................11 Feldspar ......................................................................................... 11 Hornblende ....................................................................................12 Pyroxene .......................................................................................14 Biotite ............................................................................................15 Fe-Ti Oxides .................................................................................16 Quartz ............................................................................................16 Thermobarometry .........................................................................19 DISCUSSION ........................................................................................... 22 Comparison of the Wah Wah Springs Intrusion to the Wah Wah Springs Tuff ....................................................... 22 Magma Decompression ................................................................25 Magma Chamber and Caldera Development ................................ 29 CONCLUSIONS....................................................................................... 31 REFERENCES .........................................................................................34 FIGURES ..................................................................................................41 TABLES ...................................................................................................56 APPENDIX A (Thin Sample Descriptions) ............................................. 63 APPENDIX B (Quartz Cathodoluminescence Images) ............................ 66 iv LIST OF FIGURES Figure 1. Index Map ..................................................................................41 Figure 2. Geologic Map of Indian Peak ....................................................42 Figure 3. Field Photographs ......................................................................43 Figure 4. Whole Rock Geochemistry Diagrams .......................................44 Figure 5. Modal Percentages Chart ...........................................................45 Figure 6. Feldspar Ternary Diagrams .......................................................46 Figure 7. Hornblende Microphotographs ..................................................47 Figure 8. Hornblende Classification Diagram ..........................................47 Figure 9. Hornblende Exchange Mechanisms ..........................................48 Figure 10. Pyroxene Classification Diagram ............................................49 Figure 11. Quartz Microphotographs ........................................................50 Figure 12. Quartz Cathodoluminescence Imagery ....................................51 Figure 13. Pressure-Temperature Diagrams .............................................52 Figure 14. Mineral Stability Diagrams .....................................................53 Figure 15. Ti in Quartz vs. Pressure Diagram ..........................................54 Figure 16. Diagram of Caldera Development ...........................................55 v LIST OF TABLES Table 1. Whole Rock Geochemistry ..........................................................56 Table 2. Representative Plagioclase Analyses ...........................................58 Table 3. Representative Hornblende Analyses ..........................................59 Table 4. Representative Pyroxene Analyses ..............................................60
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