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The Petrogenesis of Andesites Produced During Regional Extension: Examples from the Northern Mccullough Range, NV and Xitle Volcano, Mexico

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The Petrogenesis of Andesites Produced During Regional Extension: Examples from the Northern Mccullough Range, NV and Xitle Volcano, Mexico UNLV Theses, Dissertations, Professional Papers, and Capstones 8-1996 The petrogenesis of andesites produced during regional extension: examples from the northern McCullough Range, NV and Xitle Volcano, Mexico Kelly Ann Boland 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 Volcanology Commons Repository Citation Boland, Kelly Ann, "The petrogenesis of andesites produced during regional extension: examples from the northern McCullough Range, NV and Xitle Volcano, Mexico" (1996). UNLV Theses, Dissertations, Professional Papers, and Capstones. 1440. http://dx.doi.org/10.34917/3432223 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]. CENTER mA ~~ c• ~~c •('Qfj itC.'~'IJ:tt~C ~n.&OtfS The Petrogenesis of Andesites Produced During Regional Extension: Examples from the Northern McCullough Range, NV and Xitle Volcano, Mexico by Kelly Ann Boland A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology Department of Geoscience University of Nevada, Las Vegas August, 1996 .~, ~~~·'n'-, ~\ ,: ' '' ', . .~i " This thesis of Kelly Ann Boland for the degree of Master of Science in Geology is Chairperson, Eugene L Examining Committee Me;tt er, Rodney V. Metcalf, Ph.D. Examining Committee Member, Wanda J. Taylor, Ph.D. Graduate Facultv Representative, Peter L Starkweather, Ph.D. I t Graduate Dean, Ronald W. Smith, Ph.D. ' University of Nevada, Las Vegas August, 1996 11 Abstract Andesite dominated volcanic systems produced during regional crustal extension remain a relatively unexplored aspect of andesite petrogenesis. In the southern Basin and Range province, thick sections of andesite and basaltic andesite have been documented in areas including the Piute Range, California, the Eldorado Range, Nevada, and the McCullough Range, Nevada. The northern McCullough Range, located 20 miles south-southeast of Las Vegas, is an ideal place to study the petrogenesis of an andesite dominated system. This area II contains a mid-Miocene stratocone composed mainly of a 1200 m thick section of 1 andesite flows that formed synchronous with regional extension. The northern McCullough Range andesites are calc-alkaline and contain phenocrysts of olivine, clinopyroxene, and plagioclase. Although the andesites are similar in major element chemistry, they exhibit wide variations in trace element chemistry. Resulting trace element Harker diagrams show considerable scatter and no discernible petrogenetic trends. On 87Srj86Sr and 8Nct isotope plots, however, andesites fall into 3 distinct groups: group A (87Sr j86Sr=0.70758 to 0.7077; C:Nct~-6.2 to -6.9), group B (87Srf86Sr~0.7081 to 0.7083; C:Nct=-6.7 to- 7.4), and group C (87Srf86Sr=0.70897 to 0.70904; BNct=-8.8 to -9.8). Chemical variations and petrogenesis were assessed using fractional crystallization, assimilation and fractional crystallization, magma recharge, and magma mixing models. Geochemical variations, isotopic data and petrogenetic modeling suggest that the andesites in the northern McCullough Range formed primarily iii from mixing together several different andesitic sources (represented by isotope groups A, B, and C) that are similar in chemistry but isotopically distinct. This proposed model involving mixing of different andesitic end members explains the andesite dominated section observed in the McCullough Range and provides an alternative explanation for andesite petrogenesis in extensional terranes. A preliminary study of Xitle volcano in central Mexico shows similarities to the McCullough Range results. It is possible, therefore, that the petrogenetic model developed in the northern McCullough Range may be applied to extensional environments in other areas of the world. CErm::R ~·on . WlCA"41C AND'\_ ll:CTOtflC STUif!S: \ iv I Table of Contents Abstract............................................................................................................ iii List of Figures................................................................................................. vii Acknowledgments........................................................................................ ix CHAPTER 1 INTRODUCTION................................................................. 1 CHAPTER 2 GEOLOGIC BACKGROUND AND REGIONAL............. 8 GEOLOGY The McCullough Range..................................................................... 8 Other Nearby Volcanoes and Plutons............................................. 11 CHAPTER 3 STRATIGRAPHY AND STRUCTURAL GEOLOGY....... 14 OF THE NORTHERN MCCULLOUGH RANGE Stratigraphy............................................................................. 14 Introduction...................................................................... 14 Precambrian basement. .................................................... 16 Ivan Canyon andesite...................................................... 16 Colonv dacite.................................................................... 16 Lower member of the Farmer Canyon andesite.......... 18 Upper member of the Farmer Canyon andesite .......... 19 Dikes 20 Fog Ridge andesite..... ........................................................... 20 Discussion-Stratocone facres and vent locations............... 21 Structural Geology................................................................. 23 CHAF'fER 4 GEOCHEMISTRY OF THE VOLCANIC ROCKS OF....... 28 THE NORTHERN MCCULLOUGH RANGE Analytical Techniques........................................................... 28 Major and trace element analysis................................... 29 Trace and rare-earth element analysis.......................... 29 Isotope analysis................................................................. 30 Major Elements....................................................................... 30 v Alteration and contamination........................................ 31 Metasomatism................................................................... 31 Incompatible Trace Elements ..................... :.......................... 32 Large Ion Lithophile (LIL) and High Field Strength 32 Elements (HFSE) REE..................................................................................... 33 Compatible Trace Elements.................................................. 33 Isotopes.................................................................................... 33 CHAPTER 5 GEOCHEMICAL MODELS 45 Introduction............................................................................ 45 Petrogenetic Models (Stage 3).............................................. 47 Fractional crystallization................................................. 47 Table 1. Distribution coefficients.............................. 49 Assimilation and fractional crystallization (AFC) ....... 50 Magma mixing/commingling ....................................... 52 Table 2. Summary of models.................................. 56 Discussion................................................................................ 57 Conclusions of modeling................................................ 57 Mixing styles in volcanic systems in the southern..... 58 Basin and Range Source and Evolution of Andesitic Magma Types............ 60 (Stages 1 and 2) Stage1 60 Stage 2 62 CHAPTER 6 XITLE VOLCANO, MEXICO.............................................. 75 Geologic Background ............................................................ 75 Geochemical Descriptions and Petrography ...................... 77 Major elements................................................................. 77 Trace elements.................................................................. 77 Petrography....................................................................... 78 Geochemical Modeling......................................................... 79 Recharge............................................................................ 79 Fractional crystallization................................................. 79 Group 1........................................................................ 80 Group 2... .... ...... ... .... .. ... ..... .. .... ....... .. .. .. ... ... .. .... .. ... .. .... 80 Mixing................................................................................ 81 Discussion................................................................................ 82 Stratigraphic implications............................................. 82 Comparisons to the northern McCullough Range 82 Tectonic setting........................................................... 82 vi Petrogenesis................................................................ 83 CHAPTER 7 Conclusions............................................................................. 93 Appendix I Point
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