Collaborative Research: Deciphering Sierran Magma Sources and Modes of Diversification Using Trace Element, O, and Hf Isotopic Analyses of Zircon

学术价值：美国大的中生代到新生代科迪勒拉岩基是地球上体积最大、最令人印象深刻的火成岩活动记录之一，代表了复杂的构造和火成岩过程的产物，导致了一个主要的地壳生长时期。对岩基的现代跨学科研究改变了我们对大型大陆弧岩浆系统如何以及在什么时间尺度上发育的理解。关于弧环境下地壳生长速率和预算的重要新约束，以及弧岩浆活动期间地幔和地壳的耦合和解耦，从根本上改变了我们对弧地球动力学的认识。然而，关于大型弧岩浆系统的构造模式和多样化，弧内大型岩浆房是常见的还是罕见的（或短暂的还是长期存在的），弧壳幔柱岩浆在哪里获得其基本的地球化学/同位素特征，以及这些特征在多大程度上反映了岩浆来源和/或被随后的开放系统岩浆过程所掩盖，这些问题仍然存在关键的未解决的问题。作为回答这些问题的第一步，该项目试图通过对包括内华达山脉中部岩基在内的几个大体积侵入体的锆石中的微量元素和O和Hf同位素的原位测量来评估弧岩浆过程。本项目解决的具体问题包括：(1)地壳和地幔组分对大陆弧岩浆的相对贡献是什么？(2)在什么深度（弧下地幔、下地壳、中地壳）花岗岩类熔体获得其基本的地球化学和同位素特征，以及形成岩浆的独特成分是什么？(3)有多少浅层地壳输入的地球化学和同位素特征叠加在原生岩浆上？(4)看似均匀的大岩体是混合得很好，还是许多不相互作用的离散注入物的混合物？最近对火山岩和深成岩的研究表明，它们是由具有不同历史和来源的晶体组成的。这种异质性的程度和意义是什么？锆石是这项研究的重点，因为它是火成岩的卓越地壳地质计时器，它保留了丰富的其他信息档案，可以用来评估岩浆的来源。锆石晶体内的地球化学和同位素变化以及锆石种群内的元素和同位素变化为岩浆演化提供了非常详细的记录。此外，由于Hf同位素是火成岩地幔提取年龄的敏感指标，而O同位素可以清晰地追踪地幔与表壳岩石对岩浆的贡献，因此锆石中Hf和O的耦合分析已被证明是确定新地壳生长与地壳再循环体积的有力工具。这项工作将利用内华达山脉中部岩基中岩石学特征良好的岩体和侵入层的锆石同位素和微量元素记录来检验上述问题。与研究Tuolumne和John Muir侵入套件的工作人员（Scott Patterson， Allen Glazner及其同事）的特别合作旨在帮助解决关于深部岩和岩基如何组装的争论。更广泛的影响：该项目将为圣何塞州立大学（SJSU）的研究生和本科生以及波莫纳学院和其他四所克莱蒙特学院的本科生提供研究培训。学生将参与项目的所有阶段，最终在国内或可能的国际会议上展示他们的成果，并在评审期刊上发表他们的成果。波莫纳大学和上海州立大学都是本科院校（上海州立大学也是一所少数民族院校）。该项目还与一名职业中期的科学家合作，他在内华达山脉中部的基岩中工作了十多年，还有一名初级教员，他已经对基岩的理解做出了重要贡献。该项目还将促进国际和机构间的合作，包括加州大学洛杉矶分校、加州大学圣克鲁斯分校美国地质调查局、斯坦福大学和阿姆斯特丹自由大学的领先实验室。

Intellectual Merit: The large Mesozoic to Cenozoic US Cordilleran batholiths are among the most voluminous and impressive records of igneous activity on Earth and represent the product of complex tectonic and igneous processes that led to a major period of crustal growth. Modern interdisciplinary studies of the batholiths have transformed our understanding of how and on what time scale large continental arc magma systems develop. Important new constraints on the rates and budgets of crustal growth in arc settings and the coupling and decoupling between mantle and crust during arc magmatism have fundamentally changed our perception of arc geodynamics. Nevertheless, there remain critical unresolved issues concerning the mode of construction and diversification of large arc magma systems, whether large magma chambers in arcs are common or rare (or ephemeral or long-lived), where in the arc crust-mantle column magmas acquire their fundamental geochemical/isotopic signatures, and to what extent this signature reflects magmatic sources and/or is obscured by subsequent open system magmatic processes. As a step towards answering these questions, this project seeks to evaluate arc magmatic processes using in situ measurements of trace elements and O and Hf isotopes in zircon from several large-volume intrusions comprising the central Sierra Nevada batholith. Specific questions that this project addresses include: (1) what are relative contributions of crustal and mantle components to continental arc magmas? (2) At what depths (sub-arc mantle, lower crust, middle crust?) do granitoid melts acquire their fundamental geochemical and isotopic signatures, and what are the distinct components that contribute to the magmas? (3) How much of the geochemical and isotopic signature of shallow crustal input is superimposed on primary magmas? (4) Are large and seemingly homogeneous plutons well mixed, or are they amalgams of many discrete injections that do not interact? (5) Recent studies of volcanic and plutonic rocks suggest they consist of crystals having disparate histories and sources. What is the extent and significance of such heterogeneities? Zircon is the focus of this study because it is the preeminent crustal geochronometer for igneous rocks, and it retains a rich archive of other information that can be exploited to assess magma parentage. Intracrystalline geochemical and isotope variations in zircons and elemental and isotopic variation within populations of zircon have provided remarkably detailed records of magmatic evolution. In addition, because Hf isotopes are sensitive indicators of the mantle extraction age of igneous rocks, and O isotopes cleanly trace mantle versus supracrustal rock contributions to magmas, coupled analysis of Hf and O in zircons has proven to be a powerful tool for determining the volumes of new crustal growth versus recycling of crust. The proposed work will use zircon isotopic and trace element records from petrologically well-characterized plutons and intrusive suites in the central Sierra Nevada batholith to examine the questions posed above. Special collaborations with workers studying the Tuolumne and John Muir intrusive suites (Scott Patterson, Allen Glazner, and colleagues) are designed to help resolve debates on how plutons and batholiths are assembled. Broader Impacts: This project will provide research training for graduate and undergraduate students at San Jose State University (SJSU) and undergraduates at Pomona College and the other four Claremont Colleges. Students will be engaged in all stages of the project, ultimately presenting their results at national and possibly international conferences, and publishing their results in refereed journals. Pomona and SJSU are both primarily undergraduate institutions (SJSU is also a minority institution). The project also partners a mid-career scientist who has worked for over a decade in the central Sierra Nevada batholith with a junior faculty member who has already made important contributions to understanding of the batholith. This project will also foster international and inter-institutional collaborations involving leading labs at UCLA, UC-Santa Cruz USGS, Stanford, and Vrije University (Amsterdam).