SUBDUCTION AND METAMORPHISM OF THE OROCOPIA SCHIST, NORTHERN PLOMOSA MOUNTAINS, WEST-CENTRAL ARIZONA: INSIGHTS FROM ZIRCON U-PB GEOCHRONOLOGY

Recent detailed geologic mapping in the northern Plomosa Mountains of west-central Arizona documented the presence of the Orocopia Schist in the footwall of a Miocene detachment fault (Strickland et al., this meeting). Here we present zircon U-Pb ages from the schist (OS) and a mylonitic igneous complex (IC) that intrudes the OS. Polished grain mounts (3 OS, 2 IC) were imaged using cathodoluminescence. Detrital OS grains have a variety of internal patterns including oscillatory, convoluted, and sector zoning. A subset of OS grains have thin (<10 μm), bright rims with irregular internal boundaries. Similar zonation and overgrowth patterns are seen in IC samples.

LA-ICP-MS U-Pb analyses (n = 120, 50 per OS, IC sample) of the polished mounts record a dominantly Mesozoic signature in OS samples with peaks at 75–85 Ma, ~100 Ma, ~120 Ma, ~165 Ma, and 210–245 Ma. Some OS samples also include small Proterozoic populations at ~1200 Ma, ~1400 Ma, and ~1700 Ma. These Mesozoic and Proterozoic peaks are also recorded in xenocrystic IC zircons. To resolve the ages of the thin overgrowths imaged on the polished mounts, we analyzed an additional 4 OS and 4 IC samples by LA-ICP-MS depth profiling on unpolished grains. Overgrowth ages of both OS and xenocrystic IC grains ranged from ~45–75 Ma and yielded low Th/U ratios of 0.01–0.1, whereas Th/U of cores ranged from 0.1–2.8. For OS samples, we defined the maximum depositional age as the transition from ages with scattered Th/U ≥0.1 to ages with uniformly low Th/U, which varies from ~70–75 Ma between samples. LA-ICP-MS split-stream analyses on one OS sample recorded a correlation between low Ce* (<60) and low Th/U (<0.1) in 57-75 Ma zircons, suggesting the overgrowths developed during metamorphism in the presence of seawater. Crystallization ages for the mylonitic IC ranged from 22.6 ± 0.6 Ma to 22.9 ± 0.4 Ma.

Together with recent work at Cemetery Ridge (~70 km to the SE), these data indicate the OS samples are very similar in age spectra to the Orocopia Schist of southern California but more similar to the Pelona Schist in depositional age. The ~45–75 Ma low Th/U and low Ce* overgrowths in the OS record metamorphism during subduction underplating. Xenocrystic zircons in the IC with the same age spectra as the OS indicate the schist was assimilated into the IC during early Miocene plutonism.