Establishing the Time and Length Scales of Initial Arc Construction: The Mantle-crust Connection

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Establishing the time and length scales of initial arc construction: The mantle-crust connection(project jointly supported by EAR/Petrology & Geochemistry and OISE/Americas)This project aims to characterize exposed plutonic roots and compositional cycles in a 100 km long, continuous exposure of a continental arc section, the Sierra Valle Fértil - La Huerta (Argentina), with paleo-depths of 25 km to 8 km or less. A tilted section exposes identifiable end-member components of the continental arc: large bodies of granodiorite and tonalites in the upper crust, and a mafic- and ultramafic dominated lower crust that is interlayered with a lithologically homogenous sequence of metapelites and other supracrustal compositions. The "circulatory system" of the arc is continuous and decipherable: all units described above are physically linked via a complex of 'granitic' veins and dikes that connect source rocks to plutons. The preservation is exceptional because arc magmatism was largely arrested by a soft collision during the prograde peak, before long-lasting magmatism had obliterated the architecture of a voluminous mafic complex and a granodiorite batholith, formed over a span of 4-6 Ma. Hence much as volcanic systems are a 'snapshot' of a magmatic system, the Sierra Valle Fértil provides a 'snapshot' of a crustal-scale plutonic system, without significant overprint of high temperature structures and very long-lived repeated magmatic processing.Intellectual merit. The study will combine extensive field mapping with detailed geochronology, geochemistry and Sr, Nd and O isotopic fingerprinting to resolve composition-volume-time relationships on the scale of an arc section. This will allow us to make comparisons to volcanic systems, where source region processes can only be inferred and to combine 'hot zone' and mushy/MASH zone models for arc petrogenesis in a unified numerical model constrained by this natural example. This work will provide independent constraints on the compositional effects of mafic magmatic inputs to the crust, and the processes of homogenization and migration of mixed magmas into the upper crust. Major questions addressed are: (1) How does a large mafic complex assemble itself and on what time and space scales is internal differentiation important?; (2) How many mechanisms of assimilation can be documented and under what conditions are they most efficient and provide the most compositional leverage?; (3) Is mantle input simultaneously active along the strike of the arc, or does it vary in intensity alon strike and with time?; (4) Amphibole is widespread throughout the section - can its importance as a fractionating and hydrous phase be indentified in a geological example?; (5) Can a geological example of arc crust in high-flux mode be reconciled with both geophysical data from regions like the Altiplano-Puna or active island arcs and xenoliths studies from other batholithic belts?Broader impacts. This project will provide support for graduate students at both the University of Arizona and the University of Washington, as well as funding for undergraduate hands-on research measuring the radiogenic heat production in an intact arc section. We are working with the village of San Agustin del Valle Fértil to provide outreach materials. Closely involving Argentinean colleagues Prof. Juan Otamendi and Dr. Alina Tibaldi and their students in the project will foster international scientific collaboration and exchange.