Journal of Petrology | Volume 40 | Number 8 | Pages 1301-1320 | 1999
© Oxford University Press 1999
Pliocene Potassic Magmas from the Kings River Region, Sierra Nevada, California: Evidence for Melting of a Subduction-Modified Mantle
Department of Geological Sciences, University of Michigan 2534 C. C. Little Building, Ann Arbor, MI 48109–1063, USA
Received June 22, 1998; Revised typescript accepted February 2, 1999
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Abstract |
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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.
KEY WORDS: absarokite; minette; metasomatism; phlogopite; potassic magmas
* Corresponding author. Telephone: 734-764-7421. Fax: 734-763-4690. e-mail: becky{at}umich.edu
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