DEPTHS OF EMPLACEMENT OF THE PELONA-OROCOPIA-RAND SCHIST AS A TEST FOR TECTONIC MODELS FOR THE WESTERN US

The Pelona-Orocopia-Rand schist (PORS) of the southwestern USA is a key geologic unit in understanding the Mesozoic history of the western US. The PORS has long been inferred to represent underplated sediment accreted to the base of the upper plate during Laramide shallow-slab subduction. However, models for the emplacement of the schists remain open to reinterpretation because the pressures of metamorphism are poorly constrained. Clear determinations of the pressure (and therefore depth) conditions of PORS formation have been hampered by bulk compositions that are not compatible with chemical thermodynamic modeling and the inconsistent presence of mineral assemblages available for estimation of peak metamorphic pressure. Thus, sparse barometric data exist for the PORS. Quartz-in-garnet elastic geobarometry (QuiG) provides a new tool to determine metamorphic pressures within the suite. New QuiG isomekes on garnet from the Orocopia mountains are interpreted to record pressures of quartz entrapment in garnet during prograde metamorphism at 10-14 kbar at the relevant temperatures of 500-700°C, consistent with emplacement at a depth of 30-42 km. This depth range (inferred from pressure estimates) overlaps with previous estimates of 30–37 km for the Sierra Pelona and the Orocopia Mountains (Xia and Platt, 2017; Graham and R. Powell, 1984), 26–44 km for the Catalina schist (Platt, 1975; Sorensen, 1986), and 32–41 km for the San Emigdios (Chapman et al., 2011). Additional pressure data from other sample locations and further efforts to constrain temperatures using Raman spectroscopy on carbonaceous material and pseudosection modeling will allow for precise refinement of pressures across the PORS. New Raman-based thermometry and QuiG geobarometry on exposures of the schist from the Sierra Pelona, Portal Ridge, and Orocopia Mountains will reveal the depths of the slab across the E-W distance of the PORS exposures using consistent quantitative thermobarometry. QuiG, coupled with detailed geochronology and thermometry, will allow us to determine how depths varied with space and time across the exposure of the schist bodies, providing a crucial test for various tectonic models for forming the POR.