PERMIAN ARC MAGMATISM AND ITS DETRITAL RECORD IN SOUTHWEST LAURENTIA

Most models of Mesozoic Cordilleran magmatism suggest that arc activity was not fully established until Late Triassic to Jurassic time. New U-Pb results from Permian metasedimentary units in the Inyo Mountains (IM) of eastern California and plutons in the El Paso Mountains (EPM) in the central Mojave Desert suggest a genetic link between the two and contribute to a growing body of evidence that demonstrates widespread arc magmatism by Late Permian time.

Our new data indicate crystallization of a foliated granite in the western EPM at ~270 Ma, the Fremont Peak orthogneiss at ~261 Ma, and the Mesquite Canyon and Weiss Mountain plutons at ~257 Ma, overlapping zircon ages in the IM succession. These ages document the initiation of arc magmatism by Middle Permian time, consistent with pluton ages from Sonora, Mexico.

Permian stratigraphy in the IM comprises the Lone Pine Fm, which consists of deep- to shallow-water carbonate to siliciclastic rocks and is overlain by the Permo-Triassic Conglomerate Mesa Fm. The Reward Conglomerate Mbr at the top of the Lone Pine Fm yielded zircon ages that suggest a maximum depositional age of ~260 Ma; several grains are 265-267 Ma. The Conglomerate Mesa Fm shows a major influx of ~257 Ma zircons into the upper sandstone/conglomerate member. Although the continental contribution to these units was substantial, as shown by the Paleozoic - Archaean range of ages in the detrital zircon spectra, the magmatic arc was actively contributing material by Late Permian time. These new data only provide maximum depositional ages, but are consistent with Late Permian fossil ages in the Conglomerate Mesa Fm. Additionally, the data show the proximity of this sub-basin to the magmatic arc. The proposed correlation between sedimentary and igneous units is supported by Th/U ratios that consistently range between ~0.4 and 0.9.

Magmatism initiated off western Laurentia by ~270 Ma and became widespread by 260 Ma. Sedimentary successions in the proximal retro-arc region received increasing amounts of detritus that likely originated, in part, either from subaqueous volcanic eruptions or erosion of plutons exhumed during Late Permian deformation.