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).
