THE GEOCHEMICAL EVOLUTION OF THE CERRO UTURUNCU MAGMA CHAMBER, SW BOLIVIA AND ITS RELATION TO THE ANDEAN CENTRAL VOLCANIC ZONE by Gary Scott Michelfelder A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Earth Sciences MONTANA STATE UNIVERSITY Bozeman, Montana January, 2015 ©COPYRIGHT by Gary Scott Michelfelder 2015 All Rights Reserved ii DEDICATION This work is dedicated to my wife, Breanna, for the support, motivation, and insightful discussions. iii ACKNOWLEDGEMENTS First and foremost, I would like to thank my advisor, Todd C. Feeley, for his guidance, support and inspirational enthusiasm for the fields of igneous petrology and volcanology. My committee members, David Mogk, Colin Shaw, and Shanaka (Shan) de Silva provided insightful comments, suggestions and valuable reviews of this research, and participation of David Young (MSU Extended University) as graduate representative is gratefully acknowledged. Funding for this study was provided by the National Science Foundation Continental Dynamics Program administered by Dr. Feeley. This combined with generous support from the Department of Earth Sciences at MSU and the Veterans Memorial Scholarship from the Rocky Mountain Association of Geologists supported four years of enrollment in the doctoral program. I am thankful for the opportunity to focus on coursework and research and gain experience teaching in this time period and grateful to the people and professional organizations that made it possible. A special thanks to the other members of the PLUTONS Research Group for insightful discussions, help in the field, and overall collaboration during the project. Within the department of Earth Sciences, a special thank you my friends and colleagues for sharing their time and helping me in the labs and the field: Alicia Wilder, Jacob Thacker, Jamie Kern, Nick Atwood, Mort Larsen, Dan Ross, Eric Easley, Gillian Arnoux, Helen Lynn, and Whitney Treadway. iv TABLE OF CONTENTS 1. INTRODUCTION TO DISSERTATION ..........................................................................1 Research Goals ...................................................................................................................2 Manuscript Summary .........................................................................................................3 References ..........................................................................................................................8 2. BACKGROUND GEOLOGY OF THE ANDEAN CENTRAL VOLCANIC ZONE ..........................................................................................................10 The Andean Central Volcanic Zone .................................................................................10 Cerro Uturuncu .................................................................................................................13 References ........................................................................................................................16 3. GENERAL METHODS ...................................................................................................20 References ........................................................................................................................24 4. THE VOLCANIC EVOLUTION OF CERRO UTURUNCU: A HIGH-K, COMPOSITE VOLCANO IN THE BACK-ARC OF THE CENTRAL ANDES OF SW BOLIVIA ................ 26 Contribution of Authors and Co-Authors ........................................................................26 Manuscript Information Page ...........................................................................................27 Abstract ............................................................................................................................28 Introduction ......................................................................................................................29 Regional and Tectonic Setting ..........................................................................................30 Methods ............................................................................................................................30 Geology and Eruptive History of Cerro Uturuncu .............................................................30 Morphology and Volume ..........................................................................................32 Petrography ...............................................................................................................32 Magmatic Inclusions .................................................................................................33 Whole Rock Geochemistry ...............................................................................................36 Major and Trace Elements ........................................................................................36 Radiogenic Isotopes ..................................................................................................36 Petrogenesis of Cerro Uturuncu Magmas .........................................................................37 Volcanic Hazards ..............................................................................................................43 Conclusions .......................................................................................................................43 Acknowledgements ...........................................................................................................43 References .........................................................................................................................43 v TABLE OF CONTENTS- CONTINUED 5. CRUSTAL DIFFERENTIATION PROCESSES AT CERRO UTURUNCU, ANDEAN CENTRAL VOLCANIC ZONE, SW BOLIVIA ............................................48 Contributions of Authors and Co-Authors .........................................................................48 Manuscript Information Page ............................................................................................49 Abstract ..............................................................................................................................50 Introduction ........................................................................................................................51 Tectonic and Regional Background ...................................................................................53 Cerro Uturuncu ..........................................................................................................55 Results ................................................................................................................................56 Petrography and Bulk-Rock Geochemistry of Uturuncu Lavas and Domes ........................................................................................................................56 Magmatic Inclusions ..................................................................................................59 Nomarski Interferometry ...........................................................................................62 Electron Microprobe Analysis ...................................................................................63 87Sr/86Sr Crystal Isotope Stratigraphy ........................................................................64 Discussion ..........................................................................................................................66 Assimilation Fractional Crystallization and Magma Mixing/ Mingling Modeling .....................................................................................................66 Bulk-Rock Modeling ............................................................................................66 Origin of Major-Element Zonation in Plagioclase Phenocrysts .................................69 Origin of Single-Crystal Isotopic Variation ................................................................71 Crystal Growth in a periodically recharged chamber ............................................71 Crystal growth in magma progressively contaminated by country-rock ...............73 Self-mixing ............................................................................................................74 Timescales of Magmatic Processes at Cerro Uturuncu ..............................................74 Model of Contamination and Recharge ......................................................................76 Conclusions ........................................................................................................................80 Appendix 1. Methods .........................................................................................................80 Acknowledgements ............................................................................................................82 Figures................................................................................................................................84 Tables .................................................................................................................................99 References ........................................................................................................................103 6. MODIFICATION OF THE CONTINENTAL CRUST BY SUBDUCTION ZONE MAGMATISM AND VICE-VERSA: ACROSS-STRIKE GEOCHEMICAL VARIATIONS OF SILICIC LAVAS FROM INDIVIDUAL ERUPTIVE CENTERS IN THE ANDEAN CENTRAL VOLCANIC ZONE ........................................................................................................111