CONSTRAINTS ON THE DEVELOPMENT AND EVOLUTION OF QUATERNARY SCORIA CONE PLUMBING SYSTEMS IN THE SIERRA NEVADA: INSIGHTS FROM CLINOPYROXENE-MELT GEOTHERMOBAROMETRY

Basaltic scoria cones situated in continental monogenetic volcanic fields are generally thought to erupt single magma batches over short periods of time. However, field mapping efforts combined with petrologic, geochemical, and thermobarometric analysis of erupted products from two unrelated volcanic fields in California (Red Cones, 5 km SW of Mammoth Mountain; and Golden Trout, 5 km SW of Mt Whitney) indicate pronounced differences in the eruption volumes and Pressure-Temperature crystallization histories of erupted basalts despite overall similarities in magma source. Geothermobarometry results from Red Cones clinopyroxene microphenocryst cores indicate nucleation and crystallization pressures ranging widely from 1.5 to 16.7 kbars and temperatures between 1155 and 1233°C, compared to 5.7-13.2 kbars and 1185-1230°C for the 4 cores of clinopyroxene phenocrysts analyzed. Interestingly, 87% of analyzed clinopyroxene cores from scoria deposits formed through explosive volcanic activity originated from 10-25 km depth compared to those from lava flows formed through effusive volcanic activity, which exhibit a broader range in crystallization depths extending from approximately 4 to 60 km depth. Similar results are observed from the basaltic scoria cones and lava flows in the Golden Trout Wilderness field, where clinopyroxene crystals in scoria tend to record shallow crystallization conditions compared to those from lava flows. These findings suggest that magma plumbing systems and the mechanisms for magma supply at scoria cones are actually quite complex, and therefore require us to modify our perspectives on how they form as well as the types of geophysical signals they potentially yield before, during, and after eruptions.