| Pliocene basalts of the Greenwater Range, California erupted from 24 volcanic vents now represented by volcanic plugs, craters and scoria mounds. Basaltic magmas originated in the asthenospheric mantle, but show evidence of a lithospheric component. Depths and temperatures of melting calculated using a silica activity geobarometer are 54.3--89.6 km and 1367-1435°C, placing melting in the asthenosphere. The preferred petrogenetic model involves melting of lithospheric mantle thermally and mechanically, but not chemically, converted to asthenospheric mantle. Melting depths correspond to low velocity zones in the mantle as revealed in seismic profiles. Chemical and lithologic similarities between basalt in the Greenwater Range and basalt in Crater Flat, Nevada near the proposed nuclear waste repository at Yucca Mountain suggest that both are part of the same volcanic field. These factors result in an increase in the area and number of vents used to calculate volcanic hazard and accordingly an increased risk to the proposed repository. |
