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Characterizing the Earliest Stages of Partial Melting: a Study of the Pyrometamorphic Aureole of Miocene Mt

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Characterizing the Earliest Stages of Partial Melting: a Study of the Pyrometamorphic Aureole of Miocene Mt UNLV Theses, Dissertations, Professional Papers, and Capstones May 2016 Characterizing the Earliest Stages of Partial Melting: A Study of the Pyrometamorphic Aureole of Miocene Mt. Perkins Pluton, Northwest Arizona Mathew Beshears University of Nevada, Las Vegas Follow this and additional works at: https://digitalscholarship.unlv.edu/thesesdissertations Part of the Geochemistry Commons, and the Geology Commons Repository Citation Beshears, Mathew, "Characterizing the Earliest Stages of Partial Melting: A Study of the Pyrometamorphic Aureole of Miocene Mt. Perkins Pluton, Northwest Arizona" (2016). UNLV Theses, Dissertations, Professional Papers, and Capstones. 2639. http://dx.doi.org/10.34917/9112031 This Thesis is protected by copyright and/or related rights. It has been brought to you by Digital Scholarship@UNLV with permission from the rights-holder(s). You are free to use this Thesis in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s) directly, unless additional rights are indicated by a Creative Commons license in the record and/ or on the work itself. This Thesis has been accepted for inclusion in UNLV Theses, Dissertations, Professional Papers, and Capstones by an authorized administrator of Digital Scholarship@UNLV. For more information, please contact [email protected]. CHARACTERIZING THE EARLIEST STAGES OF PARTIAL MELTING: A STUDY OF THE PYROMETAMORPHIC AUREOLE OF MIOCENE MT. PERKINS PLUTON, NORTHWEST ARIZONA By Mathew Beshears Bachelor of Science in Geosciences Indiana University – Purdue University Indianapolis December 2011 A thesis submitted in partial fulfillment of the requirements for the Master of Science – Geoscience Department of Geoscience College of Sciences The Graduate College University of Nevada, Las Vegas May 2016 Thesis Approval The Graduate College The University of Nevada, Las Vegas April 13, 2016 This thesis prepared by Mathew Beshears entitled Characterizing the Earliest Stages of Partial Melting: A Study of the Pyrometamorphic Aureole of Miocene Mt. Perkins Pluton, Northwest Arizona is approved in partial fulfillment of the requirements for the degree of Master of Science – Geoscience Department of Geoscience Rodney V. Metcalf, Ph.D. Kathryn Hausbeck Korgan, Ph.D. Examination Committee Chair Graduate College Interim Dean Terry Spell, Ph.D. Examination Committee Member Jean Cline, Ph.D. Examination Committee Member George Rhee, Ph.D. Graduate College Faculty Representative ii Abstract Partial melting is an important process in the evolution of continental crust. Due to its progressive nature, evidence of the early stages of partial melting is often overprinted by later, advanced stages of partial melting. Experimental work has provided a model for the early stages of partial melting, but is limited to timescales of hours to weeks. Pyrometamorphic environments within the contact aureole of mafic intrusions provide natural laboratories for the study of the earliest stages of partial melt production on geologic time scales of 102 to 104 years. Samples collected along two traverses of the tilted pyrometamorphic aureole of the Mt. Perkins pluton provide an opportunity to quantify partial melt volume percent and chemistry at 2 kbar. Using titanium-in-quartz geothermometry, EPMA techniques to obtain mineral chemistry, and a point- counting method using imaging software, melt volume percent, whole-rock, melt, and residual chemistry and percent minerals in each category were quantified. Melt volume percent and temperature decrease with distance from the pluton. Melt geochemistries plot near the cotectic in the quartz-(Ab+Or)-An system. Plaigoclase anorthite components were quantified and compared with eachother based on textures. The earliest stages of partial melting are recorded in samples that experienced a lower heat budget distal to the pluton. These data confirm hypotheses that earliest melts are silica rich and become progressively enriched in other cations as melting continues. iii Table of Contents Abstract iii Table of Contents iv List of Tables vii List of Figures xiii 1. Introduction 1 2. Background 3 2.1 Recognizing Pseudomelt Pockets 3 2.2 Current Model for Early Stages of Partial Melt (Low Melt Percent) 4 2.3 The Plagioclase Problem 5 2.4 Melt mobility 7 3. Geologic Setting – Mt. Perkins pluton 9 4. Study Design 12 4.1 Hypotheses 12 4.2 Predictions 13 5. Methods 16 5.1 Sample Collection 16 iv 5.2 Point counting method 18 5.3 Analytical Methods 21 5.4 Geothermometry 22 6. Results 25 6.1 Mineralogy and Textures 25 6.2 Point Count Results 26 6.3 Melt connectivity 27 6.4 Plagioclase chemistry with respect to textures. 28 6.5 A and B traverse plagioclase. 30 6.6 Calculated whole-rock, melt, and residual geochemistry 31 6.7 Melt, residual, and whole rock geochemistry in the Q-An-(Ab+Or)-H2O system33 6.8 Melt volume percent 34 6.9 TitaniQ geothermometry 34 7.1 Discussion 36 7.1 Melt Textures 36 7.2 Melt Reaction 36 7.3 Melt Volume 38 7.4 Geochemical evolution 39 7.5 Plagioclase geochemistry 40 v 8. Conclusions 42 Appendix A 81 Appendix B 101 Appendix C 167 References 218 Cirriculum Vitae 222 vi List of Tables Table 1 List of minerals found in various textures…………………………….…………76 Table 2 Modal mineral abundances for each sample divided into three types: melt, residual and whole rock, and distance from pluton……………..…….…………77 Table 3 Average EPMA analysis of plagioclase within each sample………….…………78 Table 4 Whole rock geochemistry for each sample separated into melt, residual, and whole rock as well as distance from pluton and titanium-in-quartz geothermemetry………………………….............................................................79 Table 5 CIPW norm results from point counting divided into three types: melt, residual and whole rock ……………………………..........................................................80 Table A1 Point count results from the pilot study on sample 925-1.....................................81 Table A2 Whole rock, melt and residual geochemistries......................................................82 Table A3 Average mineral composition used to calculate residual geochemisty for sample 136-1A...................................................................................................................83 Table A4 Average mineral composition used to calculate melt geochemisty for sample 136- 1A...........................................................................................................................84 Table A5 Average mineral composition used to calculate whole rock geochemistry for sample 136-1A.......................................................................................................85 Table A6 Average mineral composition used to calculate residual geochemistry for sample 136-6A...................................................................................................................86 Table A7 Average mineral composition used to calculate melt geochemistry for sample 136-6A...................................................................................................................87 vii Table A8 Average mineral composition used to calculate whole rock geochemistry for sample 136-6A.......................................................................................................88 Table A9 Average mineral composition used to calculate residual geochemistry for sample 136-11A.................................................................................................................89 Table A10 Average mineral composition used to calculate melt geochemistry for sample 136-11A.................................................................................................................90 Table A11 Average mineral composition used to calculate whole rock geochemistry for sample 136-11A.....................................................................................................91 Table A12 Average mineral composition used to calculate residual geochemistry for sample 136-2B....................................................................................................................92 Table A13 Average mineral composition used to calculate melt geochemistry for sample 136-2B....................................................................................................................93 Table A14 Average mineral composition used to calculate whole rock geochemistry for sample 136-2B.......................................................................................................94 Table A15 Average mineral composition used to calculate residual geochemistry for sample 136-8B....................................................................................................................95 Table A16 Average mineral composition used to calculate melt geochemistry for sample 136-8B....................................................................................................................96 Table A17 Average mineral composition used to calculate whole rock geochemistry for sample 136-8B.......................................................................................................97 Table A18 Average mineral composition used to calculate residual geochemistry for sample 136-14B..................................................................................................................98 viii Table A19 Average mineral composition used to calculate melt geochemistry
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