Recent geologic mapping in the northern Plomosa Mountains (west-central Arizona, USA) documented a newly recognized exposure of the Laramide Orocopia Schist subduction complex in the footwall of the Miocene Plomosa detachment fault. Zircon U-Pb ages from this schist strongly resemble those from other Orocopia Schist exposures, and record dominantly Mesozoic age peaks at 85–75 Ma, ca. 100 Ma, ca. 120 Ma, ca. 165 Ma, and 245–210 Ma, and minor Proterozoic peaks at 1.2 Ga, 1.4 Ga, and 1.7 Ga. Thin (<10 μm) zircon overgrowths on detrital cores, resolved using laser ablation–inductively coupled plasma–mass spectrometry depth profiling of unpolished grains, range from 75 to 47 Ma and have low Th/U (<0.1) compared to the cores. These zircon grains record the transition from deposition in the paleotrench to subduction-related metamorphism between 77 and 68 Ma for individual samples, and an average depositional age of ca. 73 Ma. This well-defined depositional age of the Orocopia Schist protolith is older than previously thought but similar to reported ages for the outboard Pelona Schist. The timing of sediment subduction appears to be contemporaneous with extinction of magmatism in the Mojave region, suggesting the two events are linked. Zircon xenocrysts within Miocene plutonic rocks yield age spectra similar to those for the Orocopia Schist, indicating assimilation of the schist during core complex magmatism. Together, these data show the Orocopia Schist of the northern Plomosa Mountains was deposited off the California coast, subducted at ca. 73 Ma, and metamorphosed during Laramide subduction underplating and Paleogene exhumation from 73 to 47 Ma, before assimilation into Miocene plutons and final exhumation during core complex extension.

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