THE AGE AND ORIGIN OF CRYSTALLINE GNEISS IN THE PLOMOSA MOUNTAINS, WEST-CENTRAL ARIZONA

Recent discoveries of Laramide Pelona-Orocopia-Rand schist (PORS) ~300 km inboard of the paleo-Farallon trench highlight the extreme lateral scale of underplating during shallow-slab subduction and call into question which mechanisms exhumed the PORS through the North American upper plate. Gneissic units are found in association with the PORS throughout southern California and Arizona, but how these units relate to exhumation is unclear. Here we report preliminary zircon U-Pb geochronology and whole rock geochemistry on a mylonitized crystalline gneiss that structurally overlies the Orocopia Schist in the northern Plomosa Mountains to determine its age, geochemical affinity, and origin. Lithologically, the gneiss can be grouped into 3 units: a compositionally banded gneiss, homogeneous leucocratic intrusions, and a coarse-grained melanocratic unit. The compositionally banded unit, composed of alternating hornblende- and plagioclase-rich bands with well-developed L-S fabrics, is derived from a granitic protolith and has negative REE slope (LaN/LuN 11.9–55.6). Eu anomalies are slightly positive for 2 samples and moderately negative for 3 samples. Zircon U-Pb results yielded an upper intercept age of 1683 ± 35 Ma. A set of homogenous leucocratic intrusions that commonly exhibit L>S fabrics are derived from a tonalite/quartz monzonite protolith with negative REE slope (LaN/LuN 9.4) and a slightly negative Eu anomaly. Three samples returned overlapping Jurassic ages: 161.0 ± 1.2 Ma (intrusion), 164.1 ± 4.1 Ma (overgrowths on inherited zircon in a dike), and 160.2 ± 1.3 Ma (inherited cores in a dike). One dike had 69.6–82 Ma Cretaceous overgrowths. The coarse-grained melanocratic unit, which occurs as a stock-sized black and lumpy exposure with S>L fabric, is derived from a gabbroic diorite protolith with a negative REE slope (LaN/LuN 5.4) and a slightly negative Eu anomaly. Zircon U-Pb yielded a crystallization age of 21.5 ± 0.4 Ma, which overlaps the local ~22-20 Ma intrusive complex within error. Zircon U-Pb geochronology and whole rock geochemical data indicate the gneiss is largely derived from Proterozoic continental crust, best fitting with the geochemical characteristics of the Yavapai-Mojave basement terrane, and was intruded by the leucocratic Jurassic arc.