Collaborative Research: Testing the Existence of Magma Mush Zones and Potential Processes of Magma Differentiation in the Mid-crust with In situ Mineral Geochemistry

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Detailed knowledge of magma plumbing systems beneath continent-margin volcanoes is crucial for understanding the evolution and growth of continental crust and the associated effects on crustal deformation and ore deposit formation. Understanding the time and length scales of processes in the magma plumbing system sheds light on the spatial distribution, type and frequency of volcanic eruptions and associated hazards. Although our understanding of continent-margin magmatism at depth is vastly improving, debate continues as to the dimensions, and even existence, of large magma volumes in sub-volcanic magma reservoirs, the interconnectivity between magma bodies through the thickness of the continental crust, and the importance and extent of processes that modify the chemistry of such magmas. The aim of this study is to investigate these missing, but critical pieces of the puzzle through study of plutons - ancient magmatic rocks formed at depth - as exposed in Yosemite National Park, California. Traditional bulk-rock element and isotope analyses have left too many fundamental questions about the architecture of crustal magmatic systems, resolution of which is a goal of this study through a new approach of utilizing micro-scale geochemical analysis of individual crystals in plutons. This research will also provide better data for numerical modeling efforts and has great potential to guide future studies of plutonic rocks.This study will use texturally-constrained, element and isotope geochemical analysis of compositionally heterogeneous rock-forming and accessory minerals from strategically selected sites of the Tuolumne Intrusive Complex ? which was once a composite, mid-crustal arc magma reservoir - to test the following hypotheses. (1) Mineral zoning and inclusion relationships in plutonic rocks record distinct, decipherable magmatic histories. (2) Interconnected magma mush zones exist in mid-crustal reservoirs. (3) Mixing of crystal-rich magmas occurs between magmas injected from deeper levels and coexisting mush zones. (4) Crystal-liquid separation is an important process in the middle crust; consequently, many plutonic rocks are cumulate and do not represent liquid compositions. Testing these hypotheses will allow assessment of two models for mid-crustal arc plutons. A dike model predicts that magmas attain their chemical signatures in deep-crustal ?hot zones? and rise rapidly to sub-volcanic reservoirs without producing large magma bodies. A magma mush column model predicts that magmas form vertically-extensive reservoirs and undergo compositional modification at many crustal levels. Mineral analysis will allow identification of distinct mineral populations, interpretation of prevalent magmatic processes, assessment of the presence or absence of coeval magmas and their dimensions, and the ability of such magmas to chemically ?communicate?.