The stratigraphic and petrologic record of subduction initiation in the Permian metasedimentary succession of the El Paso Mountains, Kern County, California

An apparently continuous, ~950 m-thick stratigraphic section of Early Permian (~280- ~260 Ma) metasediments was measured, described, and sampled southwest of Mormon Flat in the El Paso Mountains of California. Representative samples were thin sectioned and petrographically analyzed for texture and composition. Although metamorphosed up to greenschist grade, sedimentary and primary mineralogical details are still discernible. Above a disconformable contact with underlying Ordovician-Upper Cambrian ( eugeoclinal) deep-marine passive margin rocks, the Permian section begins with a local basal sandy conglomerate (~33 m), followed by metacarbonate turbidites (62 m), then mostly covered noncalcareous metashales and metasiltstones (~393 m). Upsection, the mudstones become more calcareous (~ 152 m); several beds of metaarkose interrupt the upper ~64 m. An abrupt change to slightly tuffaceous metacalcarenite ( ~ 10 m) is followed by tu:ffaceous metasandstones with distinctly zoned plagioclase crystals that dominate the rest of the section (~297 m). Rare interbeds of quartz arenite occur ~ 15 m above the base of the tuffaceous metasandstones and interbeds of bioclastic metalimestone are present near the top below the andesite flows. Overall, exposures of Permian metasedimentary rocks in the measured section consist of 28% argillite and siltstone, 5% non-tuffaceous sandstone, 25% tuffaceous sandstone, 5% conglomerate, 21 % non-tuffaceous limestone, 3% tuffaceous limestone, 3% silicified limestone and partially recrystallized chert, 4% hydrothermally altered rocks, and 6% younger, less altered igneous intrusions of indeterminate age. Sandstone modal analysis suggests that the basin evolved from a tectonic setting involving compressive uplift (fold-thrust belt, collision suture, or subduction complex) or a transform-margin setting to an arc basin setting. The highly tuffaceous nature of the El Paso Mountains volcaniclastic rocks, their phenocryst assemblage (zoned plagioclase and amphibole), and common lathwork textures are most consistent with highly crystalline, intermediate (andesitic) volcanism.