EVIDENCE FOR OROCOPIA SCHIST IN THE FOOTWALL OF THE PLOMOSA MOUNTAINS METAMORPHIC CORE COMPLEX, WEST-CENTRAL ARIZONA

New geologic mapping, geochemical data, and detrital zircon geochronology indicate that the mylonitic footwall of the Plomosa Mountains metamorphic core complex is dominated by the Orocopia Schist, which was subducted during the Laramide Orogeny. U-Pb detrital zircon ages for three schist samples display a bimodal age distribution consisting of a Proterozoic population with a peak centered around 1.7 Ga, and a more prevalent Mesozoic population ranging from Early Triassic to Late Cretaceous. The youngest zircons in each sample indicate a maximum depositional age of ~70-80 Ma, consistent with previous detrital zircon studies of the Orocopia Schist.

The schist contains abundant graphitic albite and locally monomineralic pods of green tremolite, both of which are hallmarks of the Orocopia Schist. Whole-rock geochemistry of two tremolite samples indicate up to 1290 ppm Ni and 2330 ppm Cr, suggesting a mantle source for the tremolite pods.

Mylonitic foliation in the schist dips moderately to the SW, and a well-developed S-C-C’ fabric consistently records top-to-the-NE shear. Early Miocene mylonitic leucogranite dikes cut the schist. Microstructures recorded in several samples and SIMS Ti-in-quartz analysis of one sample indicate that mylonitization of the schist occurred primarily in the upper greenschist to lower amphibolite facies.

Our 1:10,000-scale geologic mapping indicates that this schist is the dominant unit in the northern Plomosa Mountains, covering ~12 km². This area is located ~70 km NW of the recently identified Orocopia Schist locality at Cemetery Ridge, which is the northeastern most known exposure of schists underplated during shallow subduction of the Farallon slab. The presence of Orocopia Schist within the Maria fold and thrust belt, a zone of mid- to Late Cretaceous crustal thickening, suggests that this region locally experienced major early Tertiary exhumation prior to unroofing of mid-crustal rocks during Miocene detachment faulting.