SHOSHONITIC VOLCANISM IN THE WALKER LANE, WESTERN NEVADA

Relative plate motion between the Pacific and North American plates is taken up largely along the San Andreas Fault of California and the Walker Lane of western Nevada. Unlike the San Andreas Fault, the Walker Lane is a transtensional feature characterized by significant volcanism over the last ~12 myr. 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, provides an excellent example of the relationship between tectonics and volcanism in the region. It consists of a subcircular volcanic complex about 15 km in diameter, with a central caldera. Volcanic activity spanned a period of ~1-2 myr 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 that reset the fractionation process. 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 0.21-0.85. 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), 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.