THE PRODUCTION OF INTERMEDIATE COMPOSITION MAGMAS IN AN INTRACONTINENTAL SETTING

The mid-Miocene Santa Rosa-Calico volcanic field (SC) of northern Nevada provides an outstanding example of the role open-system magmatic processes play in producing andesite-dacite magmas in an intracontinental setting. SC volcanism commenced at ~16.7 Ma and is associated with the initial manifestations of the Yellowstone-Newberry mantle upwelling, the Columbia River-Steens flood basalt event(s), and the formation of the Northern Nevada rift. Locally a diverse package of magmatic products ranging from tholeiitic basalt to high-Si rhyolite was produced during an ~2 Ma duration. Within this package are the products of at least four distinct andesite-dacite magmatic systems that may represent as much as 40% of the SC volcanic pile. These help differentiate the SC from contemporaneous Oregon Plateau volcanic fields (e.g. McDermitt, Lake Owyhee, Northwest Nevada) that are dominated by bimodal basalt-rhyolite assemblages. All SC intermediate units are characterized by textural and mineralogic complexities including xenoliths and xenocrysts of local crust and crystal clots of plagioclase ± clinopyroxene ± orthopyroxene ± oxide. SC intermediate units are dominantly tholeiitic, but span the tholeiitic/calc-alkaline boundary and relative to locally erupted mafic units (primarily Steens Basalt), have similar elemental enrichments and depletions, but dissimilar Sr, Nd, and Pb isotopic compositions. These isotopic differences, coupled with the abundant disequilibrium features and variable incompatible element ratios, indicate that open system magmatic processes played a major role in the genesis of the intermediate units. SC silicic magmas were produced primarily via upper crustal melting of chemically and isotopically heterogeneous Cretaceous granitoid. Interaction between fractionating mafic magmas and the more evolved crustal melts ± assimilation of local upper crust provides a general template for the generation of SC intermediate magmas. Between system heterogeneities primarily reflect magma- versus assimilation-dominated “mixing.” These complex processes and the resulting production and eruption of SC intermediate composition melts were stimulated by incipient, focused lithospheric extension and driven by the virtually continuous local availability of upwelling mafic magma.