Pliocene potassic magmas from the Kings River region, Sierra Nevada, California: Evidence for melting of a subduction-modified mantle

During the Late Pliocene, absarokite and minette magmas (43-57 wt % SiO2) erupted along the western slope of the Sierra Nevada, California, within the Kings River drainage. The absarokites contain phenocrysts of olivine +/- augite, whereas the minettes contain phlogopite + augite +/- olivine; both are distinguished by a lack of feldspar phenocrysts. Pre-eruptive magmatic temperatures and pressures for a felsic and mafic minette are 1138 and 1144 (+/- 50)degrees C, and 12 and 16 (+/- 4) kbar, respectively. These magmas are characterized by extreme enrichments in the large ion lithophile elements (e.g. 1.9-8.1 wt % K2O, 1380-3719 ppm Ba), depletions in high field strength elements (Ba/Nb-PM of 7-33), and high oxygen fugacities (1-3 log units above the Ni-NiO buffer). Trace element ratios (e.g. Ba/Rb 20-100) are distinct from those observed for mid-ocean ridge basalt and ocean island basalt. Variations in K and Ba with respect to other incompatible elements require that phologopite +/- potassic amphibole was an important residual phase during magma generation. The buoyant ascent of the Kings River magmas through similar to 40 km of sialic crust requires pre-eruptive volatile concentrations (H2O and F) >2 wt %. Volcanism probably was triggered as part of the regional response to Basin and Range extension, which resulted in asthenospheric upwelling and therefore higher heat flow to the subduction-modified lithosphere.