Voluminous mid-Miocene (8-13 Ma) rhyolitic ash-flows and lava flows (Idavada volcanics) are exposed continuously along the northern (Mount Bennett Hills or MBH) and southern margins of the west-central Snake River Plain (SRP). These rhyolites unconformably rest on Cretaceous granitic rocks of Idaho batholith and Eocene Challis volcanics and are locally overlain by (or intercalated with) minor Tertiary basalt flows and fluvial and lacustrine sediments. At both margins, the rhyolites dip gently and thicken toward the SRP. NW-to EW-trending normal faults drop the western SRP forming a graben-like structure where the inferred source vents of the rhyolites are buried by younger basalt flows (Pliocene to Recent) and sediments. The stratigraphy of Idavada volcanics in the MBH and evaluation of correlations with those exposed in the adjacent areas suggest that the rhyolites exposed in the northern margin of the west-central SRP are different from the southern margin counterparts and that the majority of west-central SRP rhyolites were erupted almost concurrently with the formations of the normal faults that formf"d the western SRP graben. Idavada volcanics range in composition from quartz latite to high-silica rhyolite and are characterized by anhydrous mineral assemblages (Plagioclase ±Sanidine ±Quartz + Augite

• Pigeonite ± Hypersthene + Fe-Ti Oxides ± Fayalite + accessory Zircon and Apatite), suggesting that the rhyolites were formed under dry conditions. Pyroxene temperatures of Davidson and Lindsley (1985) range from 800 (high-silica rhyolites) to lOOO°C (quartz latites). Major and trace element, and isotopic (87Sr/86Sr = 0.709-0.713; 143Nd/144Nd = 0.51219-0.51230) compositions of the rhyolites suggest that the rhyolites may have formed by partial melting (caused by intrusions of hotspot-related basaltic magmas into the lower crust) of sialic lower crust with significant Juvenile component under dry conditions. Major and trace element variations observed in the west-central SRP rhyolites can be explained by differences in residence times which resulted in variable degrees of fractional crystallization. Variations of isotopic compositions and estimated magmatic temperatures of the rhyolites suggest that higher temperature rhyolitic magmas assimilated more Archean upper crustal material than lower temperature ones.