Temporal and Spatial Trends of Late Cretaceous-Early Tertiary Underplating of Pelona and Related Schist Beneath Southern California and Southwestern Arizona
spe374-14 page 1 of 26 Geological Society of America Special Paper 374 2003 Temporal and spatial trends of Late Cretaceous-early Tertiary underplating of Pelona and related schist beneath southern California and southwestern Arizona M. Grove Department of Earth and Space Sciences, University of California, 595 Charles Young Drive E, Los Angeles, California 90095-1567, USA Carl E. Jacobson Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa 50011-3212, USA Andrew P. Barth Department of Geology, Indiana University–Purdue University, Indianapolis, Indiana 46202-5132, USA Ana Vucic Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa 50011-3212, USA ABSTRACT 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 southwest- ern Arizona. Ion microprobe U-Pb results from 850 detrital zircons from 40 meta- graywackes demonstrates a Late Cretaceous to earliest Tertiary depositional age for the sedimentary part of the schist’s protolith. About 40% of the 206Pb/238U spot ages are Late Cretaceous. The youngest detrital zircon ages and post-metamorphic mica 40Ar/39Ar 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.
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