Eruption styles of Quaternary basalt in the southern Sierra Nevada Kern Plateau recorded in outcrop and mineral-scale stratigraphies

Abstract:

The Kern River Plateau in the southern Sierra Nevada contains Quaternary basalt (~0.1 km³) and rhyolite (~2 km³) that ascended through ~30 km of Mesozoic granitic crust. Basaltic vents include from oldest to youngest: Little Whitney Cone, Tunnel and South Fork Cones, and unglaciated Groundhog Cone. Little Whitney Cone is a 120-m-high pile of olivine-CPX-phyric scoria overlying two columnar jointed lava flows extending to the south and east. Tunnel Cone formed through a Hawaiian style eruption along a 400-m-long north-south trending fissure that excavated at least three 25-65-m-wide craters. Crater walls up to 12 meters high are composed of plagioclase-olivine-phyric spatter-fed flows that dip radially away from the crater center and crumble to form steep unconsolidated flanks. South Fork Cone is a 170-m-tall pile of plagioclase-olivine-phyric scoria that formed as a result of Strombolian to violent Strombolian eruptions. It overlies the South Fork Cone lava, the largest lava flow of the Kern Plateau (~0.05 km³), which flowed 7.5 km west into the Kern River Canyon. Scoria and ash fall deposits originating from South Fork Cone are found up to 2 km from the vent. Groundhog Cone is a 140-m-tall cinder and spatter cone breached on the north flank by a 0.03 km³ lava flow that partially buried the South Fork Cone lava and extends 5 km west to Kern River Canyon. Trends in mineral assemblage, texture, composition, and xenocryst abundance exist as a function of eruption style. Scoria and spatter deposits typically have (1) elevated olivine/plagioclase ratios, (2) oscillatory zoned (An₆₃-An₇₂) plagioclase phenocrysts surrounded by unzoned rims and (3) abundant xenocrysts, where up to 20% of plagioclase >200 micron diameter in some samples are granitoid xenocrysts with resorbed and/or reacted textures overprinted by abrupt compositional changes. In contrast, lava flow samples have (1) reduced olivine/plagioclase ratios and (2) plagioclase aggregates with oscillatory zoned cores (An₅₅-An₆₇) surrounded by normally zoned rims (An_38-49). Kern Plateau basalts record a variety of open system magmatic processes that occurred during ascent and storage through thick granitic crust, and results from this study suggest that these processes may influence eruption style and the associated hazard.