EVIDENCE FOR BASALT-BASALT MAGMA MIXING PRESERVED IN LAVAS OF GROUNDHOG CONE, SOUTH CENTRAL SIERRA NEVADA, CA

Groundhog Cone (2738 m) is a Holocene-aged basaltic cinder cone located approximately 30 km south of Mount Whitney in the south central Sierra Nevada of California. The cone is comprised of a 140-m-high pile of scoria with and a 0.03 km3 lava flow that extends more than 5 km west where it spills over the rim of Kern River Canyon and flows partially down its steep walls. Whereas lava flows contain approximately 10 vol.% phenocrysts, with subequal proportions of plagioclase, orthopyroxene, and clinopyroxene, and slightly less olivine and resorbed quartz and orthoclase feldspar, scoria fragments contain only 5 vol.% phenocrysts and are characterized by an assemblage composed almost entirely of plagioclase and orthopyroxene with virtually no clinopyroxene or olivine. Field investigations of portions of the Groundhog lava flow located immediately adjacent to the breached cinder cone reveal the presence of flow banding and small (1 to 3 cm), rounded, basaltic inclusions hosted within the basaltic lava. Similar to the scoria fragments, these basaltic inclusions are composed almost entirely of plagioclase and orthopyroxene with rare clinopyroxene and olivine. The texture and crystal morphologies of the inclusions, such as radiating and acicular plagioclase habit, vesicular groundmass, and fine-grained margins, imply a magma-mixing origin for the inclusions, where a high-temperature basaltic magma intruded into a lower-temperature basaltic reservoir and rapidly cooled (quenched) within the host immediately prior to or during the eruption of Groundhog. No known examples of such inclusions are known to exist at cinder cones. Previous and concurrent investigations of quenched basaltic inclusions strongly link their occurrence with volcanic eruptions triggered by magma mixing events.