USING ISOTOPIC COMPOSITIONS OF THE MID-CENOZOIC FLARE UP TO PROBE FOR THE PELONA-OROCOPIA-RAND SCHIST AT DEPTH

The Orocopia Schist and related schists are sediments subducted during the Laramide orogeny and are thought to have been underplated as a laterally extensive layer at the base of the crust. This concept is hard to reconcile with studies indicating the preservation of continental mantle lithosphere in southeastern California and western Arizona. Another possibility is that the Orocopia Schist could have ascended through the mantle lithosphere as sediment diapirs or subsolidus crustal plumes to become emplaced in the middle to lower crust, consistent with time-temperature cooling histories. The presence of Orocopia Schist in the subsurface can be investigated using xenocrystic zircon age populations and the isotopic composition of igneous rocks erupted or emplaced during the mid-Cenozoic flare up. The Orocopia Schist is characterized by juvenile radiogenic isotopes and heavy oxygen isotopes, which is distinct from relatively radiogenically evolved North American basement and mantle-derived magmas with relatively light oxygen isotope ratios. As a result, assimilation and/or partial melting of Orocopia Schist imparts a characteristic isotopic signature. New zircon U-Pb, zircon εHf, and zircon and quartz δ¹⁸O data from the Orocopia Schist and from mid-Cenozoic flare up igneous rocks helps to map out the possible distribution and extent of the Orocopia Schist in the southwestern U.S. Cordillera.