The Silver Peak Volcanic Center: Shoshonitic Volcanism in the Walker Lane, Western Nevada

Shoshonitic and high-K volcanism is widespread in the Sierra Nevada and western Nevada. The Silver Peak center, located in the Walker Lane, just east of the White Mountains, lies within the Silver Peak detachment. It consists of a subcircular volcanic complex about 15 km in diameter, with a central caldera. Volcanic activity spanned a period of 1myr from 6.0±0.5 to 4.8±0.6 Ma. The lavas range from trachybasalt to rhyolite and plot in the shoshonite field on the SiO2-K2O diagram. Oxide trends indicate relatively continuous fractionation of a parental basaltic magma. Fractionation took place in a subvolcanic magma chamber, which was periodically injected with new batches of basalt, leading to two major explosive events and resetting the fractionation processes. The first event, which erupted voluminous rhyolite ashflows, resulted in an elliptical caldera 12 by 9 km; the second produced highly feldspar-phyric trachyandesite ashflows and lavas. The most mafic lava is a trachybasalt with about 49 wt% SiO2, 7.5 wt% MgO and 2.5 wt% K2O, which contains olivine phenocrysts of Fo85. Chondrite-normalized REE patterns of the lavas show flat HREE, strong enrichment in LREE (LaN/SmN =3.5-7.7; aver. = 4.7) and variable negative Eu anomalies ranging from .21-.85. In-situ 87Sr/86Sr compositions of plagioclase grains are quite uniform, mostly between 0.7060 and 0.7070, similar to the whole-rock values (0.7061) and indicating little if any crustal contamination. We suggest that the parental magma was derived from a metasomatised mantle source by small degrees of partial melting. It was mixed periodically with new pulses of mafic magma and underwent fractional crystallization to produce the observed range of compositions. . Melting was probably triggered by transtensional deformation in the Walker Lane.