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The Behavior of Zircon During Partial Melting in Anatectic Migmatites

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The Behavior of Zircon During Partial Melting in Anatectic Migmatites UNLV Theses, Dissertations, Professional Papers, and Capstones 5-1-2012 The behavior of zircon during partial melting in anatectic migmatites: Insights to metamorphism, deformation and tectonism in the central Santa Fe Range, northern New Mexico Michelle Renee Stropky University of Nevada, Las Vegas Follow this and additional works at: https://digitalscholarship.unlv.edu/thesesdissertations Part of the Geochemistry Commons, Geology Commons, and the Tectonics and Structure Commons Repository Citation Stropky, Michelle Renee, "The behavior of zircon during partial melting in anatectic migmatites: Insights to metamorphism, deformation and tectonism in the central Santa Fe Range, northern New Mexico" (2012). UNLV Theses, Dissertations, Professional Papers, and Capstones. 1636. http://dx.doi.org/10.34917/4332617 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]. THE BEHAVIOR OF ZIRCON DURING PARTIAL MELTING IN ANATECTIC MIGMATITES: INSIGHTS TO METAMORPHISM, DEFORMATION AND TECTONISM IN THE CENTRAL SANTA FE RANGE, NORTHERN NEW MEXICO By Michelle Renee Stropky Bachelor of Science Environmental Geology University of Nevada, Las Vegas December 2008 A thesis submitted in partial fulfillment of the requirements for the Master of Science Degree in Geoscience Department of Geoscience College of Sciences The Graduate College University of Nevada, Las Vegas May 2012 Copyright by Michelle Renee Stropky, 2012 All Rights Reserved THE GRADUATE COLLEGE We recommend the thesis prepared under our supervision by Michelle Renee Stropky entitled The Behavior of Zircon During Partial Melting in Anatectic Migmatites: Insights to Metamorphism, Deformation and Tectonism in the Central Santa Fe Range, Northern New Mexico be accepted in partial fulfillment of the requirements for the degree of Masters of Science in Geoscience Department of Geoscience Rodney Metcalf, Ph.D., Committee Chair Terry Spell, Ph.D., Committee Member Eugene Smith, Ph.D., Committee Member Vernon Hodge, Ph.D., Graduate College Representative Ronald Smith, Ph. D., Vice President for Research and Graduate Studies and Dean of the Graduate College May 2012 ii ABSTRACT THE BEHAVIOR OF ZIRCON DURING PARTIAL MELTING IN ANATECTIC MIGMATITES: INSIGHTS TO METAMORPHISM, DEFORMATION AND TECTONISM IN THE CENTRAL SANTA FE RANGE NORTHERN NEW MEXICO By Michelle Renee Stropky Dr. Rodney V. Metcalf, Examination Committee Chair Professor of Geology University of Nevada, Las Vegas Three lithologic assemblages exposed in the central Santa Fe Range, northern New Mexico include (1) a septum of Proterozoic low temperature (~700oC, 5.5 kbars), semi-pelitic anatectic migmatites surrounded by (2) deformed megacrystic biotite granite that is intruded by (3) discrete plutons of largely undeformed hornblende biotite tonalite. Metamorphic fabrics in the migmatite record three deformation events (D1 – D3); a composite S1/S2 fabric (D1 and D2) that formed contemporaneous with anatexis and is folded about a southwest plunging map-scale (D3) antiform. The megacrystic granite exhibits a pervasive S1 subsolidus foliation that parallels the S1/S2 fabric in the migmatite and is cut by discordant leucosomes. The hornblende tonalite cross- cuts both the S1 foliation of the megacrystic granite and the S1/S2 foliation of the migmatite. New U-Pb SIMS ages were collected on zircon from the two plutonic units and from a stromatic metasedimentary migmatite (leucosome-melanosome-paleosome). Megacrystic granite yielded a concordant U-Pb age of 1633 ± 12 Ma (2σ) and hornblende tonalite yielded a concordant U-Pb age of 1395 ± 7 Ma (2σ); both plutonic units have Th/U >0.1. The evaluation of migmatite zircon from leucosome-melanosome-paleosome have highly variable Th/U values (>0.1 to <0.01) and uranium concentrations (200 – 9,000ppm). Migmatite zircon with Th/U >0.1 yielded two age iii populations: protolith cores with a Pb-Pb age = 1634 ± 16 Ma and new growth with a Pb-Pb age of 1398 ± 17 Ma. Three zircon analyses yield older Pb-Pb ages of 1791 ± 25 Ma. Migmatite zircon with Th/U values <0.1 yield mixed Pb-Pb ages between 1.6 – 1.4 Ga that have elevated Ca, Fe, and Al concentrations, suggesting zircon recrystallization from fluids and/or melts; such zircon is primarily found in leucosome and may represent fluid released during melt crystallization. Older migmatite zircon ages are interpreted as being inherited from a metasedimentary protolith and younger migmatite zircon ages at 1399 Ma represents the age of anatexis and deformation (this includes deformation of the 1633 Ma megacrystic biotite granite). Statistical correlation of both the migmatite and tonalite ages suggests mafic magmatism enhanced regional metamorphism and supports studies documenting metamorphism, deformation and tectonism across the Laurentian margin at ~1.4 Ga. iv ACKNOWLEDGEMENTS I would first like to thank all of the institutions and organizations for their financial support towards this project. Without their funding this project would not have been possible. Much appreciation is made to the University of Nevada, Las Vegas Geoscience Department for funding two term scholarships, the UNLV Graduate and Professional Student Association grant, the Geological Society of America grant, and the Mineralogical Society of America grant in providing financial support for this project. I would also like to extend many thanks to Dr. Rodney Metcalf for his advisement and project development. Without your advisement and faith in me this project would not have been possible, you knew when to push me and were always there during the ups and downs. I cannot thank you enough for all of your encouragement; this experience has allowed me to grow as both a person and as a scientist and will be carried with me throughout the rest of my life. I also want to extend my thanks towards all the great geoscientists that assisted with data collection and project collaboration. Thanks is made to my advisory committee Dr. Terry Spell, Dr. Eugene Smith, and to Dr. Vernon Hodge. Appreciation is also made to Dr. Sean Mulcahy for Cathodoluminescence imagery and zircon sample preparation; to Christopher Adcock for Electron Backscatter Detection analyses; and to Axel Schmitt for Secondary Ion Mass Spectrometry analysis and zircon sample preparation. Lastly I cannot thank my friends and family enough for their encouragement and support throughout this entire process, especially to Robert E. Davis. Without your support and love this never would have been possible and I thank you for that. v TABLE OF CONTENTS Abstract …………………………………………………………………………………… iii Acknowledgements ……………………………………………………………………………… v List of Tables …………………………………………………………………………………… vii List of Figures …………………………………………………………………………………… viii Chapter 1 Introduction …………………………………………………………………… 1 Chapter 2 Geologic Setting ……………………………………………………………… 3 Chapter 3 Migmatites ………………………………………………………………………. 7 3-1: Terminology ……………………………………………………………… 7 3-2: Syn-Kinematic Structures and Morphologies ………………………. 7 Chapter 4 Sample Descriptions ………………………………………………………… 9 4-1: Pre-Kinematic Megacrystic Granite …………………………………… 9 4-2: Post-Kinematic Hornblende Biotite Tonalite …………………………. 9 4-3: Aspen Basin Migmatite …………………………………………………. 10 Chapter 5 Analytical Methods …………………………………………………………… 12 Chapter 6 Zircon Occurrence and Morphology ………………………………………… 14 6-1: Plutonic Zircon ………………………………………………………… 14 6-2: Migmatite Zircon ……………………………………………………… 14 Chapter 7 Results ………………………………………………………………………… 17 7-1: SIMS Analysis of Zircon ……………………………………………… 17 7-2: EPMA Analysis of Migmatite Zircon ………………………………… 18 Chapter 8 Interpretations of Ages ……………………………………………………… 20 8-1: Plutonic Ages …………………………………………………………… 20 8-2: Migmatite Ages ………………………………………………………… 20 Chapter 9 Evidence of Zircon Recrystallization ………………………………………… 22 Chapter 10 Discussion ……………………………………………………………………… 26 10-1: Zircon Behavior during Anatexis …………………………………… 26 10-2: Implications for Precambrian Geology ………………………………. 27 Chapter 11 Summary and Conclusions …………………………………………………… 30 Tables …………………………………………………………………………………… 32 Figures …………………………………………………………………………………… 36 Bibliography …………………………………………………………………………………… 52 Curriculum Vita …………………………………………………………………………………… 55 vi LIST OF TABLES Table 1 Analytical data for pre-kinematic megacrystic biotite granite zircon by SIMS … 32 Table 2 Analytical data for post-kinematic hornblende biotite tonalite zircon by SIMS .. 32 Table 3a Analytical data for Aspen Basin migmatite leucosome zircon by SIMS ………. 33 Table 3b Analytical data for Aspen Basin migmatite melanosome zircon by SIMS …….. 34 Table 3c Analytical data for Aspen Basin migmatite paleosome zircon by SIMS ……….. 34 Table 4 Chemistry of Aspen Basin migmatite zircon by EPMA …………………………. 35 vii LIST OF FIGURES Figure 1 Proterozoic crustal provinces in southwest Laurentia …………..…………. 36 Figure 2 Uranium-Pb probability plot of dates in New Mexican
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