PETROLOGY AND GEOCHEMISTRY OF THE LAZUFRE VOLCANIC COMPLEX: EVIDENCE FOR DIVERSE PETROGENETIC PROCESSES AND SOURCES IN THE ANDEAN CENTRAL VOLCANIC ZONE by Alicia Diane Wilder A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Earth Sciences MONTANA STATE UNIVERSITY Bozeman, Montana May 2015 ©COPYRIGHT by Alicia Diane Wilder 2015 All Rights Reserved ii ACKNOWLEDGEMENTS The National Science Foundation generously provided support for this project [NSF Grant EAR-0908140]. Additional support was provided by the Donald L. Smith Memorial Scholarship. I would like to thank Frank Ramos, TIMS Lab, New Mexico State University; Bob Jones, Microprobe Lab, Stanford University; and the Geoanalytical Lab, Washington State University for their analytical assistance. Thanks to the members of PLUTONS research team for insightful discussions and inspiration. I would like to extend my gratitude to my main thesis advisor, Dr. David Mogk, who has acted as my mentor and parent, offering me constructive criticism, unwavering support and thoughtful advice throughout my time at Montana State University. A special thank you to my committee members Drs. Colin Shaw, David Lageson, and Jim Schmitt for offering time, support and resources. I thank Drs. Paul Mueller and Sandy Underwood for assistance and insight. Thank you Gary Michelfelder for your guidance, support, and friendship. I am indebted to Arden Fortenberry and Melanie Baldwin for their assistance. Special thanks to Josh McFarland for his able and entertaining assistance in the field despite altitude, freezing temperatures and rat infestations. Thanks to the company of my friends and fellow students, especially Erin Moffett, Mikayla Fletcher, Ty Mack, Dan Jupka, and Robyn Wooldridge who reminded me that life exists outside of school and kept me smiling and laughing throughout most of my degree. Thank you to my family for supporting me and offering advice and encouragement along the way. Special thanks to my mom and dad, for instilling a curiosity and sense of adventure within me and providing me with unconditional love. iii TABLE OF CONTENTS 1. INTRODUCTION ...........................................................................................................1 2. GEOLOGIC SETTING ...................................................................................................5 Introduction ......................................................................................................................5 Evolution of the CVZ.......................................................................................................6 Geology of the CVZ.......................................................................................................10 Geology of the Lazufre Volcanic Complex ...................................................................11 3. ANALYTICAL METHODS .........................................................................................17 4. MORPHOLOGY OF THE LAZUFRE VOLCANIC COMPLEX ................................20 5. PETROGRAPHY AND MINERALOGY .....................................................................24 General Petrographic Overview of Lazufre ...................................................................24 Mineral Descriptions ......................................................................................................26 Plagioclase .............................................................................................................26 Orthopyroxene .......................................................................................................27 Clinopyroxene ........................................................................................................27 Amphibole..............................................................................................................28 Biotite .....................................................................................................................28 Olivine....................................................................................................................29 Quartz .....................................................................................................................29 Opaque Minerals ....................................................................................................29 6. WHOLE ROCK GEOCHEMISTRY .............................................................................43 Major Element Compositions ........................................................................................43 Trace Element Compositions .........................................................................................44 Radiogenic Isotopes .......................................................................................................46 Summary ........................................................................................................................47 7. PETROGENESIS ..........................................................................................................60 Introduction ....................................................................................................................60 Key Observations ...........................................................................................................63 Magmatic Processes .......................................................................................................64 Partial Crustal Melting ...........................................................................................64 Magma Mixing and Mingling ................................................................................65 Assimilation and Fractional Crystallization...........................................................67 iv TABLE OF CONTENTS – CONTINUED Petrogenetic Model ........................................................................................................68 Summary ........................................................................................................................70 8. DISCUSSION ................................................................................................................77 Volcanic Hazards ...........................................................................................................77 9. CONCLUSIONS............................................................................................................78 REFERENCES CITED ......................................................................................................80 APPENDICES ...................................................................................................................91 APPENDIX A: Sample Locations .........................................................................92 APPENDIX B: Modal Point Counting Data ..........................................................95 APPENDIX C: Feldspar Compositions .................................................................99 APPENDIX D: Pyroxene Compositions .............................................................112 APPENDIX E: Olivine Compositions .................................................................117 APPENDIX F: Major and Trace Element Concentrations ..................................119 APPENDIX G: Isotope Ratios .............................................................................123 v LIST OF TABLES Table Page 5.1 Representative plagioclase compositions ........................................................41 5.2 Representative clinopyroxene compositions ....................................................42 vi LIST OF FIGURES Figure Page 1. Shaded relief map of NE Chile and NW Argentina and the location of the Lazufre volcanic complex ............................................4 2.1. A. Schematic map of South America showing the three Volcanic zones, B. Volcanoes of the central Andes ......................................14 2.2. Schematic cross section through the CVZ ......................................................15 2.3. Geologic map and stratigraphy sequence of the Lastarria volcanic complex .............................................................................16 4.1. Representative views of Lazufre demonstrating varying morphology. .......................................................................................22 4.2. Aerial image of the Lazufre volcanic complex Illustrating lava flows, vents, and craters........................................................23 5.1. Locations of sampled lava flows, domes, and pyroclastic flows .............................................................................................31 5.2. Modal percent phenocrysts versus SiO2 for Lazufre flow rocks ....................32 5.3. Amphibole exhibiting a strong oxidation reaction rim ...................................33 5.4. Pumice sample exhibiting banding .................................................................34 5.5. Glomerocrysts in two different populations containing plagioclase, OPX, CPX, olivine, Fe-Ti oxides, biotite and hornblende ...............................................................................................35 5.6. Magmatic inclusion exhibiting a chilled margin within a lava flow............................................................................................36 5.7. Plagioclase within lava flows exhibiting zoned, unzoned, and sieved textures ..........................................................................................37