NEW U/PB AGE DATA ON SILICIC METAVOLCANIC ROCKS IN THE PINE GROVE HILLS, WESTERN NEVADA (38.5° N): IMPLICATIONS FOR REGIONAL CORRELATIONS OF MID-TRIASSIC THROUGH LOWER JURASSIC CONTINENTAL ARC STRATA

Roof pendants in the Sweetwater Mtns (SWM) and Pine Grove Hills (PGH) near the CA – NV state line may provide links between the Pine Nut terrane of Nevada and eastern Sierran pendants. The SWM-PGH pendants are ~20 km south of the Pine Nut terrane (with Middle–Upper Triassic continental arc rocks and Upper Triassic--Lower Jurassic volcanic and carbonate cover). The SWM-PGH pendants lie ~20 km northeast of the Saddlebag Lake and Snow Lake pendants in the eastern Sierra, where complexly faulted Paleozoic basement and Mesozoic arc rocks have been juxtaposed along the Mojave-Snow Lake fault.

The SWM-PGH pendants expose complexly faulted subaerial to shallow marine Mesozoic volcanic, clastic, and minor carbonate rocks, generally similar to units in the other pendants. Units generally strike ENE and dip steeply NW. An extensive, fault-bound felsite breccia and rhyolitic, pumice-bearing ignimbrite unit at the south end of the PGH was sampled for U/Pb analysis. This sample yielded two igneous zircon populations. The older population (n=22), with an age of 241.04±0.69 Ma (2σ), is interpreted to represent antecrysts from a slightly earlier magmatic event, while the younger population (n=12), with an age of 236.5±0.85 Ma (2σ), likely represents zircon crystallized during the eruption of the tuff. Therefore, the fault-bound rhyolitic tuffs are early Carnian age and predate a Lower Jurassic marine section to the north. The marine metasedimentary units, which include thinly bedded siltstone, lithic sandstone, and minor conglomerate and thin limy interbeds, contain Sinemurian fossils and are overlain discordantly by younger intermediate to silicic metavolcanic units.

The ~ 236 Ma rhyolitic metatuff unit in the PGH now appears to be one of the oldest volcanic units within the Triassic continental arc near 38.5⁰N. It slightly predates 232 Ma and possibly slightly older intermediate and silicic metavolcanic units within the Pine Nut terrane) and 232-224 Ma metarhyolite ash-flow tuffs within Saddlebag Lake pendant. We are exploring possible correlations of the SWM-PGH pendants and their tectonic implications.