OROCOPIA SCHIST AT CEMETERY RIDGE, ARIZONA: A TYPE EXPOSURE OF SUBUCTED ROCKS ACCRETED DURING LARAMIDE FLAT SUBDUCTION, THEN INCORPORATED INTO A MIOCENE CORDILLERAN CORE COMPLEX

The Late Cretaceous-early Paleogene Pelona-Orocopia-Rand Schists (PORS) comprise an accretionary complex emplaced beneath craton-arc crust of southwest North America during low-angle subduction. In 2012, we discovered a previously unknown body of Orocopia Schist at Cemetery Ridge, Arizona, 90 km west of greater Phoenix. This is 50 km farther inboard along the direction of subduction than other PORS exposures. Orocopia Schist at Cemetery Ridge (CR-OS) is lithologically and texturally similar to other PORS, consisting of quartzofeldspathic schist with minor metabasalt, metachert, marble, and peridotite. Metamorphic grade is upper amphibolite. The schist is bounded on the south and east by stocks of undeformed earliest Miocene diorite to granite and intruded by numerous postkinematic dikes. Similar undeformed mid-Cenozoic intrusives cut many bodies of PORS. However, CR-OS is unique in that it also includes synkinematic dikes and sills of diorite, granite, and aplite. The most deformed of these have been transposed parallel to foliation in the schist and are themselves strongly foliated.

We determined U-Pb LA-ICP-MS zircon ages for 8 CR-OS metasandstones and 8 syn- to postkinematic igneous rocks. Metasandstones include Proterozoic, Jurassic, and Late Cretaceous zircon, similar to age spectra for inferred detrital zircon from other bodies of Orocopia Schist. Some metasandstones also include abundant 60–40 Ma ages, as individual grains and as overgrowths on older zircon. All dated intrusive rocks, including strongly foliated ones, are 22–21 Ma. We tentatively interpret the 60–40 Ma zircon as having grown during metamorphism associated with initial underplating of the schist and/or mantle upwelling during Eocene foundering of the flat slab. Presence of mid-Cenozoic synkinematic intrusions within CR-OS indicates affinities with classic Cordilleran core complexes. We attribute both the 60–40 Ma metamorphic zircon growth and incorporation of CR-OS within the core complex belt to the extreme inboard setting beneath the craton. As such, Cemetery Ridge provides a rare glimpse into a region in which deeply subducted trench sediment and oceanic igneous rocks have been assimilated into continental crust. We also suggest that strongly reworked Orocopia Schist could be present within other Arizona core complexes.