Geologic Implications of a Geo-Chemical Study of Three Two-Mica Granites in Southern Arizona Item Type text; Thesis-Reproduction (electronic) Authors Arnold, Andrew Herbert Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the Antevs Library, Department of Geosciences, and the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author or the department. Download date 07/10/2021 11:51:56 Link to Item http://hdl.handle.net/10150/231236 GEOLOGIC IMPLICATIONS OF A GEOCHEMICAL STUDY OF THREE TWO -MICA GRANITES IN SOUTHERN ARIZONA by Andrew Herbert Arnold A Thesis Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCES In the Graduate College THE UNIVERSITY OF ARIZONA 1 9 8 6 STATEMENT BY AUTHOR This thesis has been submitted in partial fulfill- ment of requirements for an advanced degree at The Univer- sity of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknow- ledgement of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgement the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. SIGNED: aAiLd,,Q.(,,r- .6L,,,-ed APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: P.E. DAMON Professor of Geology GEOLOGIC IMPLICATIONS OF A GEOCHEMICAL STUDY OF THREE TWO -MICA GRANITES IN SOUTHERN ARIZONA Andrew Herbert Arnold, M.S. The University of Arizona, 1986 Director: Paul E. Damon The biotite + muscovite±garnet- bearing Texas Canyon (TC) , Presumido Peak (PP) , and Gunnery Range (GR) granites are members of an enigmatic suite of Eocene age granites in southern Arizona. The late orogenic granites intrude Precambrian through Jurassic metasediments and metavolcanics. The major and minor element geochemistry of the high silica, weakly peraluminous granites is rather uniform. However, trace element concentrations, REE patterns, and isotopic compositions imply gross similarities between the TC and GR granites when both are compared to the PP granite. The TC and GR granites were derived from a depleted Precambrian lower crustal source area with low Rb /Sr, while the PP granite was the result of anatectic melting of an enriched Precambrian mid -to -upper crustal source. The PP granite is an integral part of a metamorphic core complex, and this tectonic setting accounts for the geochemical differences between it and both the TC and GR granites. ACKNOWLEDGEMENTS Interaction with many people was crucial in the completion of this thesis. People from several institutions have all provided support of one sort or another. I would like to thank my major advisor Dr. Paul Damon for his belief in me and for many encouraging and stimulating conversations about this thesis and the geology of Arizona. Dr. John Guilbert also served as an advisor and a stimulus in every sense of the word during my time at the University. Thanks also go to my third committee member Dr. Joaquin Ruiz who provided crucial reviews of many of the geochemical arguments in the thesis. Other members of the University of Arizona who provided support were: Dr. George Nelson of the Nuclear Engineering department who donated nuclear reactor time for INAA ;Dr. Bill Boynton of the Planetary Sciences department who allowed me to use his INAA counting facilities; Dolores Hill of the Planetary Sciences department who taught me how to reduce, and assisted in reducing, raw INAA data; and Drs. Mohammed Shafiqullah and Dan Lynch of the Laboratory of Isotope Geochemistry who both taught me all there was to know about the analytical side of Rb -Sr isotope geochemistry and counseled me in laboratory practices. iii iv XRF analyses were done at U.C. Berkeley under the tutelage of Joachim Hampel and Al Deino. I would like to thank Vicki Bennett of U. C. L. A. for providing important Sm and Nd isotopic analyses and for taking an interest in this project. Gordon Haxel of the U.S. Geological Survey acted as a guide during the field investigation of the Presumido Peak pluton and provided many stimulating conversations about two -mica granites in general. Dick Tosdal, also of the U.S. Geological Survey, provided a crucial sample (GR -20) from a remote and inaccessible part of the Gunnery Range area. Partial funding of the thesis was provided by several groups who are all gratefully acknowledged. The Graduate Student Program Development Fund of the Graduate College at the University of Arizona funded the INAA analyses. The Laboratory of Isotope Geochemistry funded the isotopic analyses and, in part, the XRF analyses and field vehicle costs. The Department of Geosciences provided some funding for field vehicle costs. Magmachem and Stan Keith also provided financial support for late stage costs of the thesis. Field assistance was provided by Annie Fischer, Paul Ryberg, Dan Lynch, and my dog Dip. Special thanks go to Dan Lynch who introduced me to the magic of the Gunnery Range area and steaks cooked on an open mesquite fire in the Sonora desert. v Last, but definitely not least, I would like to thank my parents, Thomas and Ilze Arnold, for encouraging me, prodding me, and providing me with emotional support during the course of the thesis and also during the many pre- thesis formative years. TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS . ix LIST OF TABLES . xii ABSTRACT . xiii 1. INTRODUCTION . 1 Purpose. 3 PreviousWork . 4 Scope of the Investigation 6 Organization. 7 2. GEOLOGIC SETTING 8 TexasCanyon . 8 Location and Accessibility 8 General Geology . 9 Presumido Peak . 12 Location and Accessibility 12 General Geology . 12 Gunnery Range . 14 Location and Accessibility 14 General Geology . 15 3. PETROGRAPHY 18 TexasCanyon . 20 PresumidoPeak 24 GunneryRange . 26 OverallComparison ... 29 4. GEOCHEMISTRY . 30 Major and Minor Element Geochemistry 30 Summary of Major and Minor Element Geochemistry . 41 Trace Element Geochemistry . 42 Inter -pluton Trace Element Variation 43 Trace Element Variation in Granite /Aplite Pairs 49 Rare Earth Element Geochemistry . 53 Qualitative View of REE Patterns 54 vi vii TABLE OF CONTENTS -- Continued Page Anomalous Nd Depletion in Aplite /Pegmatite . 60 Samples. Quantitative Modeling of REE Geochemistry 61 5. ISOTOPE STUDIES . 69 Rb and Sr Isotopes 73 Rb -Sr Isochrons . 73 SrValues 81 Sm and Nd Isotopes . 82 NdValues 83 OxygenIsotopes . 87 Summary of Isotope Studies 88 6. DISCUSSION . 90 Source Areas as Inferred From Geochemical Data 90 Isotopic Evidence . 91 REE Evidence . 93 Trace Element Evidence . 94 Source Area Summary . 99 Magma Genesis . 100 Tectonic Regime . 100 Conditions During Magma Genesis . 108 Significance of Muscovite . 110 Tectonic Implications . 113 Relationship of GR Batholith to the Mojave- Sonora Megashear . 113 Crustal Age and Compositional Variations . 117 Two -Mica Granites and Metamorphic Core Complexes 118 Magmato- Tectonic Model for Two -Mica Granite Formation . 118 Granite Classification Systems . 121 7. CONCLUSIONS . 126 Suggestions for Further Work 128 APPENDIX A: SAMPLE LOCATIONS . 131 APPENDIX B: XRF TECHNIQUES . 133 APPENDIX C: INAA TECHNIQUES 136 APPENDIX D: REE NORMALIZATION INFORMATION . 138 APPENDIX E: ISOTOPE DILUTION TECHNIQUES . 140 APPENDIX F: ISOTOPIC DATA REPRESENTATION . 144 viii TABLE OF CONTENTS-- Continued Page LITERATURE CITED . 147 LIST OF ILLUSTRATIONS Page Figure 1. Distribution of Phanerozoic Muscovite -Bearing Plutonic Rocks in Western North America 2 2. Location Map of the Three Granites in this Study 5 3. General Geology of the Texas Canyon Area Pocket 4. Crustal Age Provinces in the Southwestern United States . 11 5. General Geology of the Presumido Peak Area . Pocket 6. General Geology of the Gunnery Range Area . Pocket 7. Ternary Diagram Showing Modal Composition of the Granites Studied . 21 8. CaO Versus Si02 Variation Diagram for TC, PP, and GR samples 34 9. K20 -Ca0 Versus Si02 Variation Diagram for TC,PP, and GR Samples . 34 10. AFM (A =K20 + Na20; F =Total Fe; M =MgO) Diagram for Rock Samples From Each Pluton . 37 11. ACF Diagram for TC, PP, and GR Granite Samples 38 12. Plot of Normative Quartz (Qz), Albite (Ab), and Orthoclase (Or)(Normalized to 100 %) for the Three Granites . 40 13. K /Rb Versus Rb /Sr Diagram for Granite Samples . 46 14. Enrichment Factors for 15 Trace Elementsfor the TC, PP, and GR Granite /Aplite Pairs 51 15. Chondrite -Normalized REE Patternsfor Texas Canyon Samples . 55 ix X LIST OF ILLUSTRATIONS Continued Page Figure 16. Chondrite -Normalized REE Patterns for Gunnery Range Samples . 56 17. Chondrite -Normalized REE Patterns for Presumido Peak Samples . 57 18. Physical Mixing of Two Magmas to Produce a Third "Hybrid" Magma . 62 19. Chondrite -Normalized REE Patterns of Proposed SourceRocks 65 20. Comparison of TC and GR REE Patterns With Model REE Patterns . 67 21. Texas Canyon Whole Rock and Mica Isochron . 75 22. Gunnery Range Whole Rock Isochron . 78 23. Gunnery Range Whole Rock and Mica Isochron 79 24. Presumido Peak Whole Rock and Mica Isochron 81 25. Nd Isotopic Provinces for Granites in the Southwestern United States 85 26. £Nd VersusESrfor Southern Arizona Granites 87 27. Distribution of Radiometric Ages Plotted as a Function of Distance from the Paleotrench of Southwestern North America . 102 28. Various Dip -Angles of Benioff Zone Below Southwestern North America 103 29. Benioff Zone Dip Angle as a Function of Time Beneath Southwestern North America 103 30.