Volcanology and Petrology of Volcán Miño, Andean Central Volcanic Zone

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Volcanology and Petrology of Volcán Miño, Andean Central Volcanic Zone AN ABSTRACT OF THE THESIS OF Claire M. McKee for the degree of Master of Science in Geology presented on June 29, 2001. Title: Volcanology and Petrology of VolcáIi Miño, Andean Central Volcanic Zone. Redacted for Privacy Anitá'L. Grunder Volcán Miño (21011'S) is located on the westernmost periphery of a long- lived complex of stratovolcarioes and domes called the Aucanquilcha Complex. The Aucanquilcha Complex ranges in age from 11 Ma to 1-lolocene and lies along the main N-S trending axis of Quaternary volcanoes in the Andean Central Volcanic Zone (CVZ). Volcán Aucanquitcha lies at the center of the complex and forms a ridge extending 10 km in an east-west direction; defined by a distinct cluster of andesite and dacite stratocones, dacite domes and a prominent collapse structure and two debris avalanche deposits. In contrast to the main edifice, Volcán Miño (5611 m) is a steep-sided, symmetric andesitic stratovolcano. Volcán Miño lavas range in age from 3.0 to 3.7 Ma and eruptive products are dominantly two-pyroxene ± hornblende andesites. Basaltic andesites and dacites are rare. - ----- - ------ ---I, ..--.- __, IuIVULJ, LII Volcán Miño lavas conform to regional med- to high-potassium caic-alkaline trends and are characterized by subduction-related light rare earth and large ion lithophile-element enrichments and high field strength element depletions. Miño lavas are distinctive in that they display a restricted range in whole-rock composition, 60±2 weight percent Si02. Despite this whole- rock compositional homogeneity, lavas are texturally and mineralogically diverse as evidenced by variations of proportions and textures of clinopyroxene, orthopyroxene, and amphibole in assemblages with similar weight percent Si02. Volcán Miño lavas exhibit textural evidence for both thermal and chemical disequilibrium including mixed phenocryst populations, xenocrysts, and amphibole breakdown textures. Petrographic observation suggests that these lavas have undergone complicated magmatic histories involving combined mixing, assimilation, fractionation, and hybridization. Major and trace element modeling and petrographic interpretation of disequilibrium phenocryst assemblages elucidate the importance of magma mixing in buffering whole-rock composition in an open magmatic system. Amphibole disequilibrium textures preserve important information about the pre- eruptive conditions of Volcán Miño's magma chamber and are used here to constrain the range in pressure, temperature, f02, and water concentration responsible for the generation of Miño andesites. Volcanology and Petrology of Volcán Miño, Andean Central Volcanic Zone by Claire M. McKee A THESIS submitted to Oregon State University In partial fulfillment of the requirements for the degree of Master of Science Completed June 29, 2001 Commencement June 2002 Master of Science thesis of Claire M. McKee presented on June 29, 2001: APPROVED: Redacted for Privacy Major Professor, reprenting Geology Redacted for Privacy Chair of Department of Geogciences Redacted for Privacy Dean of Grad I understand that my thesis will become part of the permanent collection of Oregon State University libraries.I hereby authorize release of my thesis to any reader upon request. Redacted for Privacy Claire'M. McKee, Author ACKNOWLEDGEMENTS Funding for this thesis was provided under a grant awarded to A.L. Grunder by the National Science Foundation. Additional field supportwas provided in collaboration with the Chilean Geo'ogical Survey, SERNAGEOMIN. I would like to extend my gratitude to my main thesis advisor,Anita Grunder, who has acted as both my mentor and friend, offeringme constructive criticism and thoughtful advice throughoutmy master's program. Roger Nielsen has been a constant help to me, offeringme support and encouragement throughout themany stages of my degree. Thank you Roger, for spending endless hours at the Beanerysifting through microprobe data and graph after graph after graph. A specialthank you to Randy Keller, for his willingness and excitementto act as a member on my thesis committee. Special thanks to my field assistants Charles Lindsay,Jorge Lemp, and Tuco Diaz. Without them, my field work would havebeen not nearly as successful, and not to mention not nearlyas entertaining. Thank you to Moyra Gardeweg, Constantino (Cocho) Mpodozis,Andy Tomlinson, Edmundo Polanco, and John Black for their hospitalityand assistance while in Santiago and in the field. I would like to thank Diane Johnson, GeoAnalyticalLab, Washington State University; Bob Duncan, Noble Gas Lab,Oregon State University; the Radiation Center; Oregon State University; Ed Kohut and Roger Nielsen, Microprobe Lab, Oregon State University; and Vaughn Baizer for their analytical assistance. The company of my roommates and fellow students, especially Dr. Anthony Cheng, Derek Sowers, Becky Ashton, Erik Klemetti, Mike Appel, Martin Hannigan, Stacy Wagner, and Heather Petcovic, kept me smiling and laughing through most of my degree. Thank you to my family for supporting me and offering advice and encouragement from so far away. My mother, Ellen Mckee, has been my greatest inspiration.I dedicate this thesis and all my successes in life to her. TABLE OF CONTENTS CHAPTER 1. Introduction I CHAPTER 2. Regional Tectonic and Geologic Setting 5 2.1 Introduction 5 2.2 Evolution of the CVZ 7 2.3 Geology of the CVZ 12 2.4 CVZ Petrology 14 2.5 Geology of the Aucanquilcha Complex 17 CHAPTER 3. Geology of Volcán Miño 24 3.1 Introduction 24 3.2 Lithologic Character of Map Units 29 3.2.1 Jkv-Jurassic Rhyolite Porphyry 30 3.2.2 Tvs-Tertiary volcaniclastic sediments 30 3.2.3 Tsic- Tertiary silicic ignimbrite; Carcote 31 3.2.4 Tpv- Tertiary Pliocene volcanics 31 3.2.5 Qgd- Quaternary Glacial Deposits 32 3.2.6 Qt and Qc- Quaternary talus and colluvium 32 3.2.7 Qal- Quaternary Alluvium 32 3.2.8 Sn- Snow 33 3.3 4OAr/39Ar Dates 35 CHAPTER 4. Analytical Methods and Facilities 39 CHAPTER 5. Results 43 5.1 General petrographic overview of Volcán Miño lavas 43 5.1.1 Plagioclase 49 5.1.2Pyroxene 50 5.1.3Opaques 50 5.1.4 Olivine and Quartz 51 5.1.5 Amphibole 51 TABLE OF CONTENTS (Continued) 5.2 Mineral Composition 57 5.2.1 Olivine 57 5.2.2 Pyroxene 57 5.2.3 Opaques 64 5.2.4 Plagioclase 68 5.2.5 Amphibole 74 5.3 Whole-Rock Major and Trace Element Compostions 78 CHAPTER 6. Discussion 86 6.1 Introduction 86 6.2 Interpretation of whole-rock composition 86 6.3 Amphibole 96 6.4 Estimation of magmatic intensive parameters 100 6.4.1 Thermometry 102 6.4.2 Pressure and Depth 103 6.4.3 Water Content 104 CHAPTER 7. Comparison with Other CVZ Volcanoes 106 CHAPTER 8. Conclusion 111 References 114 Appendices 122 LIST OF FIGURES Figure Page 1. The three active volcanic zones in the Andes and volcanoes of the Central Andes 6 2. Schematic cross-section of the Central Andes subduction zone 8 3. The Aucanquilcha Complex and Vicinity 18 4. Generalized map of the Aucanquilcha Complex 20 5. Picture of Volcán Miño 25 6. Landsat thematic mapper image and topographic map of Volcán Miño and vicinity 27 7. C-C' and D-D' cross-sections of Volcán Miño 28 8. Schematic geologic map of Volcán Miño 34 9. Argon plateau ages for Miño lavas 38 10. Total alkalies vs. Si02 for Volcán Miño lavas 44 11. 1<20 vs.Si02for Volcân Miño lavas 45 12. Photomicrographs of a Volcãn Miño basaltic andesite and a 2-pyroxene andesite 46 13. Histograms which illustrate mineralogic and textural variation for Volcán Miño andesites and dacites 53 14. Photomicrographs of homblende-rich, 2-pyx andesite and black-type amphibole 54 15. Photomicrographs of gabbroic-type amphibole breakdown 55 LIST OF FIGURES (cont.) Figure Page 16. Photomicrographs of inverted gabbroic-type amphibole 56 17. Range in forsterite composition for olivine 58 18. Ti02 and Al203 vs. Mg# for clinopyroxene and orthopyroxene 61 19. En-Wo-Fs ternary projections for Miño pyroxene 62 20. Fe203-Ti02-FeO ternary projections for Miño Fe-Ti oxides 65 21. Cr203 and TiO2. Mg# for Miño pyroxene 66 22. Ab-An-Or ternary projections for Miño plagioclase 69 23. Variation in Sr and Ba vs. An for Miño plagioclase 71 24. Al"1versus AI' for Miño amphibole 76 25. Major element Harker variation diagrams for Miño lavas 81 26. Trace element compositions versus Si02 82 27. MORB-Normalized incompatible element spider diagram 85 28. Cr and Ni vs. Si02; Rb and Ba vs. CaO 93 29. Magma mixing, AFC, hybridization, and fractionation active at Miño 94 30. Generalized stability for amphibole in caic-alkaline andesites and dacites 101 31. Isotope DataforMiño lavas 110 32. Process vs. Time for Miño lavas 112 LIST OF TABLES Table Page 1. Representative analyses of olivine in Miño lavas 59 2. Representative analyses of pyroxene in Miño lavas 63 3. Representative analyses of Fe-Ti oxides in Miño lavas 67 4. Representative analyses of plagioclase in Miño lavas 72 5. Representative analyses of amphibole in Miño lavas 77 6. Representative major and trace element analyses for Miño lavas 79 7. Major Element Fractional Crystallization Models 89 8. Trace Element Distribution Coefficients 92 APPENDICES Appendix Page I A; All major and trace element data for representative analyses of Miño lavas 123 2 B;AgedatesforMinolavas 132 3 C; Sample locations for representative samples 133 4 D; Petrography for representative samples 134 5 E; Feldspar Microprobe Data 144 6 F; Pyroxene Microprobe Data 178 7 G; Fe-Ti oxide Microprobe Data 212 8 H; Amphibole Microprobe Data 240 9 I; Olivine Microprobe Data 252 VOLCANOLOGY AND PETROLOGY OF VOLCAN MINO, ANDEAN CENTRAL VOLCANIC ZONE CHAPTER 1: INTRODUCTION Processes involved in magmagenesis along subduction zones and the interactions between the crust and mantle at volcanic arcs continue to be a focusofmuch research (Tatsumi and Eggins, 1995). In the past, studies of subduction zones have focused on across-arc and along-arc variations in composition and crustal and mantle contributions to magmatism (Hildreth and Moorbath, 1988), yet the geology and petrology of individual volcanic centers have largely been overlooked.It is in this respect that the Central Andes represent an ideal laboratory in which to study both the interactions between the crust and the mantle and the evolutionofindividual eruptive centers.
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