EVIDENCE FOR MAGMA-ICE INTERACTIONS AT LITTLE WHITNEY CONE, GOLDEN TROUT WILDERNESS, SIERRA NEVADA, CA

Little Whitney Cone (LWC, 2682 m) is a glaciated basaltic cinder cone located in the south central Sierra Nevada of California. Previous work by Lanphere indicates an eruption age of approximately 743,000 years based on K-Ar dating. Geologic mapping performed during the 2007 summer field season reveals that LWC is a 120-m-high pile of loose scoria characterized by 5 vol% phenocrysts with two lava flows that extend away from its eastern and southern flanks and flow towards the south. LWC does not possess an amphitheater resulting from the breach of a flank from lava flow generation as is observed in other cinder cones. Instead, LWC displays semi-symmetrical cone morphology, suggesting that scoria cone formation occurred after lava flow generation. The cone represents approximately 6.4x105 m3 (DRE) of magma. Lava flows possess columnar joints 3 meters in diameter and up to 10 m in height along the eastern lava flow indicating that it likely encountered thick glacial ice during emplacement. Additional field evidence in the form of columnar splaying and jointing as well as their confinement around LWC's base suggests that glacial ice obstructed the flow while cooling as inflation occurred within the flow. Lava flows account for 4.0x106 m3 (DRE) of magma. Point-counting of thin sections indicate that the phenocryst assemblage of LWC scoria is orthopyroxene (80 vol.%) and clinopyroxene (18 vol.%), with trace proportions of olivine and plagioclase. Scoria groundmass is composed almost entirely of plagioclase, with lesser orthopyroxene, clinopyroxene, and olivine groundmass. The two lava flows extending away from the LWC base are mineralogically distinct. Whereas the southern lava flow is characterized by an assemblage of orthopyroxene (57 vol.%), clinopyroxene (34 vol.%), and subequal proportions of olivine and plagioclase (~5 vol.% ea), the eastern lava flow is dominated by orthopyroxene (75 vol.%), with lesser amounts of clinopyroxene and olivine (15 vol.% and 10 vol.%, respectively) and contains only trace amounts of plagioclase.