Journal of Petrology, Volume 40, Issue 8: August 1999.

Pliocene Potassic Magmas from the Kings River Region, Sierra Nevada, California: Evidence for Melting of a Subduction-Modified Mantle

SHARON N. FELDSTEIN AND REBECCA A. LANGE

DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF MICHIGAN, 2534 C. C. LITTLE BUILDING, ANN ARBOR,MI 48109-1063, USA

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/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 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.

Keywords: absarokite;minette;metasomatism; phlogopite; potassic magmas

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