300-8 Using Zircon Trace Elements to Track the Magmatic Evolution of a Retreating Arc System in the Patagonian Andes

Session: Reconstructing Earth Surface Processes in Orogenic Systems (Posters)

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Trace elements in zircon record melt evolution as magmatic processes respond to tectonic setting, depth of melting, mineral fractionation, and magma chemistry. Combined U-Pb geochronology and trace elements in zircon have been applied to identify crustal thickening events through time to reconstruct orogenic histories. Specifically, empirical relationships between medium and heavy rare earth elements (MREE/HREE) and depth of melting have been observed and attributed to the effects of pressure on co-crystallization of MREE- and HREE-concentrating minerals with zircon. The zircon Eu anomaly (Eu/Eu*) has been proposed as a tool for quantifying crustal thickness due to the relationship between pressure-sensitive mineral fractionation and magma redox with depth of melting. While these proxies are well-developed for advancing accretionary margins, their ability to reconstruct margins with complex accretionary histories and significant extensional phases has yet to be tested. This study applies zircon crustal thickness proxies to the Patagonian Andes using arc-derived ash deposits from the Jurassic-Cretaceous Rocas Verdes Basin, a back-arc basin that underwent extension and subsequent compression during basin closure and arc-continent collision. Zircons from stratigraphically-constrained ash deposits were dated via LA-ICMS U-Pb geochronology and analyzed for trace elements to track magmatic conditions from the onset of extension through earliest compression (150-120 Ma). Results indicate that zircon Eu/Eu* models project > 20 kms of crustal thickening during basin extension, in contrast with well-established crustal attenuation in the Rocas Verdes Basin. Redox-sensitive pressure-independent Ce/Ce* values and modelled parent melt oxidation state (FMQ) were co-elevated with Eu/Eu*, suggesting that oxidation state, rather than pressure or crustal thickness, exerted a primary control on zircon Eu/Eu* in Patagonian arc magmatism during retreat. Progressively oxidized magmatic conditions were concurrent with slab rollback, arc migration, and back-arc extension. We propose that closer proximity of the arc front to the subducting slab and migration of the melt zone towards the former serpentinized mantle wedge via slab rollback-induced arc migration led to an increase in oxidation of the melt driving the elevation in Ce/Ce* and Eu/Eu*. Our results highlight the need for consideration of oxidation state to accurately interpret crustal thickness indicators in extensional arc settings, and demonstrate the potential for observing the magmatic response to slab rollback in zircon trace element data.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5831

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