Collaborative Research: Evaluating the Tempo, Size, and Chemical Connectivity of Magma Batches in a Tilted Plutonic Complex

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Intellectual Merit. In the realm of magmatic arc studies, fundamental gaps exist in our understanding of how individual pulses of magma may physically and chemically interact with extant magma/rock in a growing pluton, how emplacement depth affects the crystallization history and longevity of distinct compositional zones within a vertically-extensive pluton; and (c) whether large, vertically-extensive plutons may form as single, well-mixed open-system reservoirs. The tilted, 15-km thick Wooley Creek batholith-Slinkard pluton system will be examined to test two central hypotheses: (1) pluton construction was via multiple increments of magma with little to no physical and chemical communication at the site of emplacement (i.e., isolated batches); and (2) the upper portions of the system represent a large volume of well-mixed magma at the site of emplacement (i.e., ?big tank?). Field study, high-precision U/Pb analyses, petrography, in situ oxygen isotopic analysis, and LA-ICPMS trace element microanalysis will be utilized to test these hypotheses. The objectives of this research are to (i) delineate the timescale of intrusion, crystallization and solidification; (ii) evaluate the possibility that recharge of mafic magmas remobilized existing crystal mushes; (iii) test the hypothesis that open system processes (e.g., including magma mixing and assimilation) occurred in a large volume, vertically-extensive magmatic system; and (iv) test the hypothesis that magmatic fabrics in plutons form diachronously and reflect the regional tectonic strain field during crystallization. The focus site is a near-ideal system to attain these objectives because (a) the intrusion displays vertical compositional zonation from structurally lower gabbro/diorite upward through quartz diorite, tonalite, granodiorite and granite; (b) co-magmatic compositional links between various phases have been previously established; (c) distinctive lithology, isotopic composition, and ages for the country rocks make it possible to investigate spatial variations in physical & chemical contamination within the magmatic system; (d) magmatic fabrics cross-cut compositional zones; (e) fine-grained dikes in the ?roof zone? represent magmas tapped during construction of the underlying batholith; and (f) the pluton contains minerals (augite, zircon, quartz) whose oxygen isotope and trace element compositions will track upward and lateral variations in magma composition, hence, open-system behavior. This project will test the possible coexistence of discrete large, vertically-extensive magmatic systems having identical crystallization ages and have open-system chemical links from one intrusive phase to another Ie.g., the ?big tank? model). In contrast, an emerging paradigm favors large batholith formation by incremental emplacement of small magma batches, over millions of years, and with distinct pulses with distinct chemical histories. Either outcome will have long-ranging, process-oriented implications for interpreting how large batholiths are constructed in arc settings. Broader Impacts. This project will support undergraduate & graduate student involvement in state-of-the-art analytical and field projects. Results from this work will also be integrated into teaching activities, and because the study area lies in a heavily traveled wilderness area, geologic summaries will be produced for distribution by the U.S. Forest Service to the general public.