THE GENERATION OF INTERMEDIATE COMPOSITION MAGMAS IN A BIMODAL SETTING: EVIDENCE FROM THE SANTA ROSA-CALICO VOLCANIC FIELD, NEVADA

Volcanic fields associated with the Yellowstone-Newberry mantle upwelling (e.g., McDermitt through Yellowstone) are characterized by dominantly bimodal basalt-rhyolite volcanism. In contrast, intermediate eruptive products are common throughout the volcanic assemblage of the mid-Miocene Santa Rosa-Calico volcanic field (SC) of north-central NV. Tholeiitic basalt and subalkaline rhyolitic lava flows, domes, and shallow intrusive bodies that characterize the other Yellowstone-Snake River Plain volcanic systems also crop out throughout the SC. Commencing at ~16.4 Ma, SC mafic volcanism was present over at least an ~2 Ma duration. The continuous availability of mafic magma, coupled with coeval lithospheric rifting, was important in the generation and eruption of SC intermediate lavas. Geochemical data from a representative suite of SC intermediate eruptive materials define three broad groups: high-Si andesite (AA), dacite (D), and rhyodacite (RD). AA and D lavas from the central and eastern SC exhibit geographically controlled major and trace element characteristics confirming field observations that indicate that these lavas were erupted from multiple loci throughout the SC. RD lavas from the northern and southern SC are chemically similar, but field information again indicates that these lavas were erupted from different locations. Initial Sr isotopic data from AA and D lavas range from 0.7042 to 0.7056, values overlapping with and more radiogenic than those of SC basalt and basaltic andesite lavas (0.7035 to 0.7045). These data, coupled with the extensive disequilibrium textures observed within the AA and D lavas, suggest that magma mixing/AFC has played an important role in their genesis. Regionally upwelling Steens Basalt magma may have interacted with local Mesozoic crust (initial Sr=0.7040 to >0.7065) and crustal melts (SC high-Si rhyolitic magmas) while undergoing differentiation, leading to the generation of intermediate SC magmas. The presence of contemporaneous extensional tectonism facilitates these open system processes and enables tapping of magma chambers at various stages of differentiation. This tectonic regime also provides for establishment of multiple crustal magma chambers and a concentration of SC eruptive loci along the margins of the volcanic field.