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

Abstract

During the Late Pliocene, absarokite and minette magmas (43–57 INTRODUCTION wt % SiO2) erupted along the western slope of the Sierra Nevada, A genetic relationship between potassic magmas and California, within the Kings River drainage. The absarokites contain subduction zones has long been recognized (e.g. Johnson phenocrysts of olivine ± augite, whereas the minettes contain et al., 1978; Peccerillo, 1985; Rogers & Hawkesworth, phlogopite + augite ± olivine; both are distinguished by a lack 1985). Trace element enrichments in large ion lithophile of feldspar phenocrysts. Pre-eruptive magmatic temperatures and elements (LILE) and light rare earth elements (LREE) pressures for a felsic and mafic minette are 1138 and 1144 combined with relative depletions in the high field (±50)°C, and 12 and 16 (±4) kbar, respectively. These magmas strength elements (HFSE) are common to arc lavas (e.g. are characterized by extreme enrichments in the large ion lithophile Gill, 1981). The eruption of potassic and ultrapotassic elements (e.g. 1·9–8·1 wt % K2O, 1380–3719 ppm Ba), magmas with an extreme arc trace element signature depletions in high field strength elements (Ba/NbPM of 7–33), and often post-dates active subduction and occurs synhigh oxygen fugacities (1–3 log units above the Ni–NiO buffer). chronously with uplift, extension or strike-slip motion (e.g. Trace element ratios (e.g. Ba/Rb 20–100) are distinct from those Sloman, 1989). This relationship indicates that chemical observed for mid-ocean ridge basalt and ocean island basalt. heterogeneities, produced in the mantle during subVariations in K and Ba with respect to other incompatible elements duction via modal (the formation of new minerals) and/ require that phlogopite ± potassic amphibole was an important or cryptic (enrichment of pre-existing minerals in inresidual phase during magma generation. The buoyant ascent of compatible elements) metasomatism, can exist for subthe Kings River magmas through ~40 km of sialic crust requires stantial periods of time after subduction has ceased; pre-eruptive volatile concentrations (H2O and F) >2 wt %. subsequent tectonic or thermal events may then trigger Volcanism probably was triggered as part of the regional response further magma generation ( Johnson et al., 1978; Roden, to Basin and Range extension, which resulted in asthenospheric 1981; Rogers et al., 1987; Mauger, 1988; Sloman, 1989). upwelling and therefore higher heat flow to the subduction-modified On the western slope of the Sierra Nevada, small lithosphere. volume eruptions of potassic and ultrapotassic lavas have