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)°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/NbPM 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 phlogopite ± potassic amphibole was an important residual phase during magma generation. The buoyant ascent of the Kings River magmas through ∼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.