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University of Nevada, Reno a Study of Pleistocene Volcano Manantial

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University of Nevada, Reno a Study of Pleistocene Volcano Manantial University of Nevada, Reno A study of Pleistocene volcano Manantial Pelado, Chile: Unique access to a long history of primitive magmas in the thickened crust of the Southern Andes A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology by Heather Winslow Dr. Philipp Ruprecht, Thesis Advisor May 2018 THE GRADUATE SCHOOL We recommend that the thesis prepared under our supervision by HEATHER WINSLOW Entitled A Study Of Pleistocene Volcano Manantial Pelado, Chile: Unique Access To A Long History Of Primitive Magmas In The Thickened Crust Of The Southern Andes be accepted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Philipp Ruprecht, Ph.D., Advisor Wenrong Cao, Ph.D., Committee Member Adam Csank, Ph.D., Graduate School Representative David W. Zeh, Ph.D., Dean, Graduate School May, 2018 i ABSTRACT Textural and geochemical analysis of lavas and tephra from a poorly studied, glacially dissected, mafic, stratocone, Manantial Pelado, in the Southern Andean Volcanic Zone was collected to characterize the volcano’s petrogenesis and assess its primitive nature. Manantial Pelado lies within the transitional segment of the Southern Volcanic Zone (35.5°S) amidst thickened crust (~55 km) while surrounded by extensive silicic volcanism such as the Descabezado Grande-Cerro Azul Volcanic Complex. How mafic magmas reached the surface through thickened continental crust is a larger question at hand, but prior to addressing broader processes at work, initial geochemical characterization is necessary. Understanding the full extent of its primitive nature is crucial for broader insight of proximal vent interactions and relationships as well as insight towards magma genesis. A combination of the whole-rock and mineral-scale data reveals initial primitive characterization may not accurately represent the initial compositions and that their signature is truly primitive. Textural and zonation patterns of olivine, the presence of cr-spinel within olivine cores, and elevated Fo and Ni content within olivine cores provides evidence toward a more primitive signature for these lavas. This led to further investigation of petrogenetic processes such as diffusive equilibration. Mineral-melt relationships also provided magmatic reservoir constraints through the use of geothermometers and hygrometers to estimate crystallization temperatures, oxygen fugacity, and initial water content of these lavas. A potential variation in source of melting was identified as well as varying water contents. ii ACKNOWLEDGEMENTS There are many people I would like to thank for their unending support through this entire process. Primarily, I am grateful for my advisor, Dr. Philipp Ruprecht, for the continual guidance and support through this project. This has been a massive learning experience and he never failed to challenge me as a scientist and writer while also acting as my biggest fan. I would also like to thank Ellyn Huggins and Max Gavrilenko for their amazing help and constant patience as I tried to navigate through masses of data (and also hijacking our weekly meetings). A big thanks to Joel Desormeau who provided an abundance of knowledge on analytical instruments as well as his support and encouragement. A special thanks to my committee members, Dr. Wenrong Cao and Dr. Adam Csank, for their guidance and revisions, as well as my fellow graduate students (Scott Feehan, Michelle Dunn, Emma McConville, Kelley Shaw, Sarah Trubovitz, Gabe Aliaga, Elizabeth Hollingsworth, and Curtis Johson) for keeping me sane throughout the past two years. And finally, I would like to thank my parents and friends back home who have always been my biggest support system and motivators. I could not have done it without all of you! iii TABLE OF CONTENTS ABSTRACT…………………………………………………………………….………... i ACKNOWLEDGEMENTS……………………………………………………………... ii LIST OF TABLES………………………………………………………………………. v LIST OF FIGURES……………………………………………………………………... vi INTRODUCTION……………………………………………………………………….. 1 GEOLOGIC SETTING………………………………………………………………….. 7 Southern Volcanic Zone Geologic Setting……………………………………… 8 Geochemical Characterization of SVZ………………………………………….. 9 Silicic Activity near Manantial Pelado………………………………………… 10 Sample Location………………………………………………………………………... 11 Stratigraphic Description………………………………………………………………. 11 ANALYTICAL METHODS…………………………………………………………… 14 Whole-rock analyses…………………………………………………………… 14 Calculations for plotting……………………………………………………….. 14 Mineral Analyses………………………………………………………………. 14 Geochronology………………………………………………………………… 17 RESULTS……………………………………………………………………………… 17 Whole-rock data………………………………………………………………... 17 Petrographic Description………………………………………………………. 24 Mineralogical Data……………………………………………………………... 29 Order of Crystallization………………………………………………………... 29 DISCUSSION…………………………………………………………………………...33 Closed System Dynamics and Long-lived Storage…………………………….. 34 Source Variation………………………………………………………………... 41 Melt Composition Calculation…………………………………………………. 42 Oxygen Fugacity………………………………………………………………...45 iv Hygrometers……………………………………………………………………. 48 MELTS…………………………………………………………………………. 55 CONCLUSION………………………………………………………………………… 56 REFERENCES…………………………………………………………………………. 60 APPENDIX……………………………………………………………………………...68 v LIST OF TABLES APPENDIX 68 Table A1. XRF+ICPMS Major and Trace Elements 69 Table A2. OSU EMP Olivine 73 Table A3. OSU EMP Pyroxene 77 Table A4. OSU EMP Plagioclase 79 Table A5. WUSTL EMP Olivine 83 Table A6. WUSTL EMP Oxide 91 Table A7. WUSTL EMP Spinel 97 A8. EarthChem References 101 vi LIST OF FIGURES INTRODUCTION Figure 1A. Location map for Manantial Pelado study area 5 Figure 1B/C. Map of sample locations 6 GEOLOGIC SETTING Figure 2. Field photos of the cinder cone, dike, and unconformable flow 12 Figure 3. Field photos of main lithologies 13 RESULTS Figure 4. Subdivision of Subalkaline rocks 19 Figure 5. Major elements vs SiO2 20 Figure 6a. Spider diagram of Manantial Pelado lavas 21 Figure 6b. Extended REE diagram with Manantial Pelado and global data 22 Figure 6c. Regional extended REE diagram 23 Figure 7. BSE images of olivine textures 26 Figure 8. BSE image of olivine and transect diagram 27 Figure 9. BSE images of plagioclase textures 28 Figure 10. Trace elements vs SiO2 32 DISCUSSION Figure 11. Fo (olivine) vs. Mg# (whole rock) 38 Figure 12. Fo (olivine) vs Ni (olivine) 39 Figure 13. MgO (wt%, WR) vs Ni (ppm, WR) 40 Figure 14. Oxygen Fugacity diagram 47 Figure 15. Ca-in-olivine hygrometer 54 CONCLUSION Figure 16. Manantial Pelado magmatic reservoir schematic 59 1 INTRODUCTION Manantial Pelado is a Pleistocene stratocone in the Southern Volcanic Zone (SVZ) of the Andes (Fig. 1A) and resides among extensive, regional, silicic volcanism as a result of thickened crust. The SVZ is littered with active volcanic centers ranging from smaller monogenetic cones, large stratocones, and large caldera forming eruptions (Stern, 2004). Large volumes of high-silica rhyolites have erupted from Pleistocene to recent years from caldera-forming systems such as Laguna del Maule (Anderson et al. 2017), Puyehue-Cordon Caulle (Singer et al. 2008), Maipo (Stern et al. 1984a), and Calabozos (Hildreth et al. 1984; Grunder, 1987). Proximal to Manantial Pelado (15 km S), sits the Descabezado-Cerro Azul Volcanic Complex (Fig. 1A). Volcán Quizapú lies on the northern flank of Cerro Azul and has had the two largest historical eruptions both emitting >9.5 km3 (Hildreth & Drake, 1992; Ruprecht et al. 2012). The SVZ is highly active and with extensive valleys and rivers providing channelized pathways for deposits to reach the largest city in Chile, Santiago, ~100 km away, it poses significant natural hazards (Stern, 2004). Mafic and more primitive magmas are the building blocks to highly evolved systems such as the vents densely populating the SVZ (Sigurdsson et al. 2015). Mafic magmas contribute heat and mass to sustain the presence of a long-lived shallow magma systems as well as volatile elements such as H2O, CO2, and SO2 to an emerging gas phase. As a result, the mafic input modulates magma compositions in the volcano’s subsurface, alter the eruptibility and the eruptive behavior of stored magmas, and affects the frequency of eruption. Therefore, it is vital to characterize the mafic magma input into the crust and its evolution as it 2 interacts with magmas already stored in the crust, ultimately aiding the forecasting of volcanic unrest and activity. In areas of highly evolved volcanism, a chance to look back into its history and origin of magma genesis could provide substantial insight into how the system has evolved temporally, what to expect in other locations with similar compositions, and insight on magmas that have fed the upper crustal system. Volcán Manantial Pelado, is a poorly-studied basaltic andesite to andesite stratocone within the transitional segment of the Southern Andean Volcanic Zone (SVZ, 33-37° S). It is one of the few mafic to intermediate centers in an area otherwise surrounded by voluminous silicic magmatism. The most recent regional silicic activity, including historic eruptions at Quizapú (Hildreth & Drake, 1992; Ruprecht et al. 2012), occurs within the Descabezado Grande-Cerro Azul (DG-CA) Volcanic Cluster (Fig. 1A), but many other silicic centers exist within about a 50 km radius. Especially high-silica rhyolite eruptions in Holocene and Pleistocene times have been studied East and Southeast of Manantial Pelado. The Calabozos Caldera produced at least three large caldera-forming eruptions (each >100
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