72-8 Zircon Lu-Hf and δ¹⁸O Isotopic Evidence for the Subsurface Distribution of the Orocopia Schist in the southern U.S. Cordillera

Session: Toe to Toe: Cordilleran Systems from Trench to Retroarc Domains (Posters)

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The Orocopia Schist is a metamorphosed package of sedimentary rocks that were subducted and underplated during low-angle (flat-slab) subduction in the Laramide orogeny. Today, the Orocopia Schist is exposed at the surface in parts of southeastern California and western Arizona due to later faulting and uplift. However, its full extent beneath the surface remains uncertain. Understanding how widespread the Orocopia Schist is at depth can help us better understand the strength of the lower crust and how it may affect things like mineral deposits and regional tectonics. In this study, we use oxygen isotope data from zircons in Eocene to Oligocene igneous rocks across southeastern California and southern Arizona to investigate whether these magmas interacted with the Orocopia Schist. When magmas move through or assimilate metasedimentary rocks like the Orocopia Schist, they often show elevated δ¹⁸O values. Our results show that samples located up to ~200 km east of known Orocopia Schist exposures (near Phoenix, AZ) have high δ¹⁸O values, suggesting that the schist may be present much farther east in the deep crust than previously thought. All samples from southwestern Arizona showed signs of assimilation. These findings support the idea that the Orocopia Schist may be widespread beneath the region, even where it is not exposed at the surface. This work helps us better understand how subduction-related rocks are distributed in the lower crust and the role they play in shaping the southwestern U.S.

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

doi: 10.1130/abs/2025AM-8921

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