TEXTURAL ANAYLSIS OF MAFIC ENCLAVES AS AN INSIGHT INTO MAGMA MIXING PROCESSES AT CHAOS CRAGS, LASSEN VOLCANIC CENTER, CALIFORNIA Crystal Dawn Hootman B.A., California State University, Sacramento, 2007 THESIS Submitted in partial satisfaction of the requirements for the degree of MASTER OF SCIENCE in GEOLOGY at CALIFORNIA STATE UNIVERSITY, SACRAMENTO SUMMER 2011 TEXTURAL ANAYLSIS OF MAFIC ENCLAVES AS AN INSIGHT INTO MAGMA MIXING PROCESSES AT CHAOS CRAGS, LASSEN VOLCANIC CENTER, CALIFORNIA A Thesis by Crystal Dawn Hootman Approved by: __________________________________, Committee Chair Dr. Lisa Hammersley __________________________________, Second Reader Dr. Diane Carlson __________________________________, Third Reader Dr. David Evans ____________________________ Date ii Student: Crystal Dawn Hootman I certify that this student has met the requirements for format contained in the University format manual, and that this thesis is suitable for shelving in the Library and credit is to be awarded for the thesis. __________________________, Department Chair ___________________ Dr. David Evans Date Department of Geology iii Abstract of TEXTURAL ANAYLSIS OF MAFIC ENCLAVES AS AN INSIGHT INTO MAGMA MIXING PROCESSES AT CHAOS CRAGS, LASSEN VOLCANIC CENTER, CALIFORNIA by Crystal Dawn Hootman The Chaos Crags are a series of volcanic domes located in Lassen Volcanic Center, southernmost Cascades Range. The six domes erupted approximately 1100 years ago. The host rock is dacite, which is compositional similar in all domes at (66-69 wt. % SiO2), and differs from the mafic enclaves that range in composition from (53-61 wt. % SiO2). The enclaves result from two distinct and thermally different magmas mixing and can provide an insight into the processes of magma mixing. Five texturally different enclaves types were identified. To determine abundance of the enclaves in each dome, 113 point count stations were completed in the dome complex talus slopes. Previously collected and new samples were photomircrographed and plagioclase crystals were hand traced to be processed in Crystal Size Distribution (CSD), which determines nucleation and growth time of crystals. Previously completed geochemical data was used to determine if the enclaves and host were similar or different in composition. The results of observation at Chaos Crags were 1) The total abundance of enclaves increases with eruption of domes. 2) There are distinctive abrupt increases in the total abundance of enclaves between eruption of domes B and C, domes C and D. iv 3) There are more modest increases in the total abundance of enclaves between eruption of domes A and B, domes E and F. 4) Although it seems likely that all enclave types are present in each dome, changes in distribution of enclave type seem to correlate with the increase in total abundance. 5) Host dacites show a narrow range in composition while enclaves show a mixing trend from a more mafic source toward the host dacite. 6) There is a clear link between enclave type and geochemistry. The most mixed enclave types are type 1. The least mixed are types 3 and 4. The magma mixing model proposed is one of repeated injections of small batches of mafic magma, either injected as a fountain or ponded at the base of the magma chamber. Also the mafic magma injections are the suggested cause of eruption. Enclaves present are directly related to the type of recharge event. Disaggregation of the enclaves occurred in the conduit during the eruption. This thesis was just an initial step using CSD and geochemical data leading to some surprising results and further research should be conducted. _______________________, Committee Chair Dr. Lisa Hammersley _______________________ Date v ACKNOWLEDGMENTS This thesis would have never been completed without the support of my graduate advisor, Dr. Lisa Hammersley. She was patient with me during the times I could not work on my thesis, yet would remind me that I could finish and remind me of my goal that I wanted that to teach. She helped my understanding of want it takes to be a scientist. I feel privileged to be Lisa’s first graduate student. I would also like to thank my committee members, Dr. Diane Carlson and Dr. David Evans, willing to take time out of their summer schedule to help me complete my Master’s Degree. I will always be appreciative, especially to Dr. Diane Carlson, throughout the last nine years, who would witness my change from being a horrible field mapper in Field Geology to signing off on my Master’s Degree. This project would have not even been started, if it were not for Dr. Michael Clynne of the USGS graciously allowing me to use his samples collected from Chaos Crags before I collected my own sample set. Christiana Stout, an undergraduate student at the time I started my project, introduced me to the Chaos Crags field area. Christine O’Neill was an undergraduate that also had a project at the Chaos Crags Jumbles and it was a privilege to spend time in the field and determining the enclave types with her. To my friend Melinda Fredericksen, she helped me complete field work and was excellent company out in the field. I appreciate the National Association of Geoscience Teachers for giving me a scholarship that helped with field work and tuition. Also I need to thank my son Justin, he means the world to me. Lastly, I need to thank my better half, Ed Shakespeare, he was encouraging me every day to finish my thesis, eventually it sunk in and I completed the task in hand. It feels incredible that my thesis is done! vi TABLE OF CONTENTS Page Acknowledgments.................................................................................................................... vi List of Tables ........................................................................................................................... ix List of Figures ............................................................................................................................ x Chapter 1. INTRODUCTION ……………...……………………………………………………….... 1 1.1 Purpose of the Study…………………...……………………………………….... 1 1.2 Geologic Setting of the Lassen Region ................................................................... 2 1.3 Geology of the Chaos Crags .................................................................................. 5 2. METHODS ......................................................................................................................... 8 2.1 Identification of Enclave Types ............................................................................. 8 2.2 Field Work ............................................................................................................. 8 2.3 Crystal Size Distribution Analysis (CSD) ........................................................... 11 3. RESULTS ......................................................................................................................... 16 3.1 Classification of Enclave Types ........................................................................... 16 3.2 Enclave abundance in the Chaos Crags ............................................................... 23 3.3 Crystal Size Distribution ...................................................................................... 27 3.4 Geochemistry ....................................................................................................... 33 4. DISCUSSION ................................................................................................................... 41 4.1 Magma Mixing Models........................................................................................ 41 4.2 The Lassen Peak Magma Mixing Model ............................................................. 42 4.3 Mixing studies at Chaos Crags………………………………………………… .. 45 4.4 Observations from Chaos Crags………………………………………………… 47 4.5 Chaos Crags Magma Mixing Model…………………………………………… 50 5. CONCLUSION………………………………………………………………………… 56 Appendix A. Point Count Locations ..................................................................................... 58 Appendix B. Hand Sample list .............................................................................................. 62 Appendix C. Theory of Crystal Size Distribution ................................................................. 64 Appendix D. Background to CSDCorrections Program ....................................................... 67 vii Appendix E. Individual Crystal Size Distribution Graphs .................................................... 68 References ............................................................................................................................. 166 viii LIST OF TABLES Page 1. Table 1 Total Enclave Abundance for each dome at Chaos Crags…………… 24 2. Table 2 Major Element Composition of Host and Enclave Rocks…………… 35 ix LIST OF FIGURES Page 1. Figure 1 Location map of the major volcanoes in Cascade Range.…………….. 3 2. Figure 2 Simplified Geologic Map of Chaos Crags, Lassen Volcanic Center, California…………………………...………………………………………….. 6 3. Figure 3 Point count grid with 361 points and example of point count sample location.………………………………...……………………………………… 9 4. Figure 4 Location map of point count locations for each of the domes at Chaos Crags…………………………..…..................................................................... 10 5. Figure 5 (A) Photomicrograph of sample #88-1283, which is enclave type 2 in Dome B. …………………………………………………...………………….. 12 6. Figure 6 An example of the CSDCorrections Program data