Event Details

Stanford Postdoc Nikki Seymour speaking (via Zoom) about her research on "Discovery of the Orocopia Schist in the northern Plomosa Mountains and ongoing investigations into Laramide subduction & metamorphism in west-central Arizona." Abstract: 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. The schist of the northern Plomosa Mountains locally contains graphitic plagioclase poikiloblasts and scattered coarse-grained actinolitite pods, both of which are characteristic of the Orocopia and related schists. Actinolitite pods are high in Mg, Ni, and Cr, and are interpreted as metasomatized peridotite-an association observed in Orocopia Schist at nearby Cemetery Ridge. A 3.5-km-long unit of amphibolite with minor interlayered ferromanganiferous quartzite is localized along a SE-dipping contact between the Orocopia Schist and gneiss. Based on their lithologic and geochemical characteristics, we interpret the amphibolite and quartzite as metabasalt and metachert, respectively. U-Pb ages from this schist strongly resemble those from other Orocopia Schist exposures. Thin (<10 μm) zircon overgrowths on detrital cores record the transition from deposition in the paleotrench to subduction-related metamorphism at ca. 73 Ma. The timing of sediment subduction is 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.