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Using Micro Ct Imaging to Understand K-Feldspar Megacryst Origin, Crystallization, and Textural Coarsening

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Using Micro Ct Imaging to Understand K-Feldspar Megacryst Origin, Crystallization, and Textural Coarsening USING MICRO CT IMAGING TO UNDERSTAND K-FELDSPAR MEGACRYST ORIGIN, CRYSTALLIZATION, AND TEXTURAL COARSENING Justine G. Grabiec A thesis submitted to the faculty at the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Master of Science in the Department of Geological Sciences. Chapel Hill 2019 Approved by: Drew S. Coleman Ryan D. Mills Sarah C. Penniston-Dorland © 2019 Justine G. Grabiec ALL RIGHTS RESERVED ii ABSTRACT Justine G. Grabiec: Using Micro CT Imaging to Understand K-Feldspar Megacryst Origin, Crystallization, and Textural Coarsening (Under the direction of Drew S. Coleman) Three K-feldspar megacryst populations from the Tuolumne Intrusive Suite, California, were scanned using micro CT, producing three-dimensional models of the megacrysts’ internal structure and chemical zonation. Zones were mapped for each megacryst. Comparisons of core- to-rim zone-pattern transects from the same megacryst have highly correlated cores and poorly correlated rims (R2). Mafic mineral inclusions are concentrated in the core, are depleted in the rim, and do not represent modal proportions of minerals in the surrounding granodiorite. The data are inconsistent with megacrysts growing while lofted and mixed in a magma chamber. Rather, they suggest that megacrysts crystallize in individual melt zones experiencing μm- to m- scale chemical heterogeneities in a static crystal matrix. Downward-trending temperature oscillations may result in megacryst coarsening via dissolution-reprecipitation. Megacrysts are interpreted to have igneous cores and metaigneous to metamorphic rims, making them neither phenocrysts nor porphyroblasts, but metaigneous crystals. iii For Christina and Jeffrey Grabiec iv ACKNOWLEDGEMENTS I would like to thank Drew Coleman for indispensable discussions and support while keeping me grounded during particularly stressful periods. Thank you to Ryan Mills for insightful input, valuable perspectives, and lighthearted jokes. I want to thank Sarah Penniston- Dorland for sparking my passion for petrological fieldwork and for offering invaluable advice. Thank you to Allen Glazner for initial project ideas and construction, and field assistance along with Bryan Law and the GEOL72H first-year seminar students from 2018 and 2019. Thank you to Sean Gaynor and Joshua Rosera for fresh pots of office coffee, Lauren Graniero and Madelyn Percy for welcoming me to Chapel Hill with open arms, and Adam Nicastro for midday laughs. This study was funded by the Martin Trust, administered by the Department of Geological Sciences at the University of North Carolina at Chapel Hill. Additional funding was provided by the Lipman Research Award (Geological Society of America Grant 12049-17). A portion of this research was conducted at the Duke University Shared Materials Instrumentation Facility (SMIF), a member of the North Carolina Research Triangle Nanotechnology Network, which is supported by the National Science Foundation (Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure. Special thanks to Justin Gladman at SMIF for assistance during data collection and reconstruction. Continual thanks to Christina, Jeffrey, Jeremy, Mary Ann, and Josie Grabiec for unwavering love and support throughout my academic endeavors. I am particularly grateful for v strength, support, and coffee from Jack Pugh. Additional thanks to friends and graduate and undergraduate students whose enthusiasm for earth science is shared and much appreciated. vi TABLE OF CONTENTS LIST OF TABLES ......................................................................................................................... ix LIST OF FIGURES ........................................................................................................................ x LIST OF ABBREVIATIONS AND SYMBOLS………………………………………………...xi INTRODUCTION .......................................................................................................................... 1 Pluton Formation ......................................................................................................................... 2 K-Feldspar Megacrysts in Granitoids ......................................................................................... 4 End-Member Hypotheses ............................................................................................................ 5 Phase Equilibria........................................................................................................................... 8 Dissolution-Reprecipitation ........................................................................................................ 9 Testing the Hypotheses………………………………………………………………………..11 GEOLOGIC SETTING ................................................................................................................ 12 Tuolumne Intrusive Suite .......................................................................................................... 12 K-Feldspar Megacrysts in Tuolumne ........................................................................................ 15 METHODS ................................................................................................................................... 17 RESULTS………………………………………………………………………………………..22 Map Correlations………………………………………………………………………………24 vii Skeleton Plot Correlations ......................................................................................................... 25 Microstructures and Inclusions ................................................................................................. 27 DISCUSSION ............................................................................................................................... 29 Inter- and Intracrystalline Zone Patterns ................................................................................... 29 Local Heterogeneities ................................................................................................................ 32 Evidence for Dissolution-Reprecipitation ................................................................................. 33 Mineral Inclusions ..................................................................................................................... 34 The Static Crystal Matrix .......................................................................................................... 36 Model for Megacryst Formation ............................................................................................... 37 CONCLUSIONS........................................................................................................................... 39 REFERENCES…………………………………………………………………………………..41 viii LIST OF TABLES Table 1: Summary of megacryst sample populations……………………………………………18 Table 2: Descriptions of mineral inclusions in thin section……………………………………...30 ix LIST OF FIGURES Figure 1: Cut megacryst hand sample and megacryst mosaic in glacial slab ……………………..5 Figure 2: Schematic of two end-member hypotheses. .................................................................... 7 Figure 3: Dissolution-reprecipitation schematic. .......................................................................... 10 Figure 4: Epidote-filled veins. ...................................................................................................... 11 Figure 5: Map of Tuolumne Intrusive Suite ................................................................................. 13 Figure 6: Modified version of Figure 5 from Johnson and Glazner (2010) .................................. 18 Figure 7: Micro CT slice and three-dimensional reconstructions ................................................. 19 Figure 8: Mapping process............................................................................................................ 20 Figure 9: Megacryst maps. ............................................................................................................ 21 Figure 10: Original skeleton plots................................................................................................. 22 Figure 11: Length-normalized skeleton plots ............................................................................... 23 Figure 12: Altered skeleton plots .................................................................................................. 24 Figure 13 Boxplots of R2 values from map transects ................................................................... 25 Figure 14: Boxplots of R2 values of skeleton plot transect pairs. ................................................. 26 Figure 15: Plots of R2 versus percentage of core-to-rim transect ................................................. 27 Figure 16: Grayscale variations within individual zones in a single megacryst ........................... 28 Figure 17: Microstructures in micro CT scans ............................................................................. 29 Figure 18: Asymmetric zone patterns in sample PHD4................................................................ 31 x LIST OF ABBREVIATIONS AND SYMBOLS (100) Miller index for crystallographic plane [100] Miller index for crystallographic rotation axis cm centimeter CSD crystal size distribution °C degrees Celsius g gram Ka thousands of years/thousands of years before present LP sample prefix for megacryst population 1 from Cathedral Peak Granodiorite m meter Ma millions of years/millions of years before present
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