Temporal and spatial trends of Late Cretaceous-early Tertiary underplating of Pelona and related schist beneath southern California and southwestern Arizona

The Pelona, Orocopia, and Rand Schists and the schists of Portal Ridge and Sierra de Salinas constitute a high–pressure-temperature terrane that was accreted beneath North American basement in Late Cretaceous–earliest Tertiary time. The schists crop out in a belt extending from the southern Coast Ranges through the Mojave Desert, central Transverse Ranges, southeastern California, and southwestern Arizona. Ion microprobe U-Pb results from 850 detrital zircons from 40 metagraywackes demonstrates a Late Cretaceous to earliest Tertiary depositional age for the sedimentary part of the schist’s protolith. About 40% of the Pb/U spot ages are Late Cretaceous. The youngest detrital zircon ages and post-metamorphic mica Ar/Ar cooling ages bracket when the schist’s graywacke protolith was eroded from its source region, deposited, underthrust, accreted, and metamorphosed. This interval averages 13 ± 10 m.y. but locally is too short (<~3 m.y.) to be resolved with our methods. The timing of accretion decreases systematically (in palinspastically restored coordinates) from about 91 ± 1 Ma in the southwesternmost Sierra Nevada (San Emigdio Mountains) to 48 ± 5 Ma in southwest Arizona (Neversweat Ridge). Our results indicate two distinct source regions: (1) The Rand Schist and schists of Portal Ridge and Sierra de Salinas were derived from material eroded from Early to early Late Cretaceous basement (like the Sierra Nevada batholith); and (2) The Orocopia Schist was derived from a heterogeneous assemblage of Proterozoic, Triassic, Jurassic, and latest Cretaceous to earliest Tertiary crystalline rocks (such as basement in the Mojave/Transverse Ranges/southwest Arizona/northern Sonora). The Pelona Schist is transitional between the two.