Orocopia Schist in the northern Plomosa Mountains, west-central Arizona: A Laramide subduction complex exhumed in a Miocene metamorphic core complex

Abstract

We document field relationships, petrography, and geochemistry of a newly identified exposure of Orocopia Schist, a Laramide subduction complex, in the northern Plomosa Mountains metamorphic core complex of west-central Arizona (USA). This core complex is characterized by pervasive mylonitic fabrics associated with early Miocene intrusions. The quartzofeldspathic Orocopia Schist records top-to-the-NE mylonitization throughout its entire ~2-3 km structural thickness and 10 km2 of exposure in the footwall of the top-to-the-NE Plomosa detachment fault. The schist of the northern Plomosa Mountains locally contains graphitic plagioclase poikiloblasts and scattered coarsegrained 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. The top of the Orocopia Schist is only ~3-4 km below a ca. 21 Ma tuff in the footwall of the Plomosa detachment fault, suggesting that a major Paleogene exhumation event brought the schist to upper-crustal depths after it was subducted in the latest Cretaceous but before most Miocene core complex exhumation. The Orocopia Schist in the northern Plomosa Mountains is located near the center of the Maria fold-and-thrust belt, which likely represented a crustal welt in the Late Cretaceous. The keel of this crustal welt may have been sheared off by the shallowly dipping Farallon slab prior to underplating of rheologically weak Orocopia Schist. Paleogene exhumation of the Orocopia Schist in the northern Plomosa Mountains is consistent with extensional exhumation recorded in Orocopia Schist in the Gavilan Hills of southeasternmost California, which shortly postdated schist underplating, suggesting that subduction of schist may have triggered Paleogene extension in the region.