Intermediate composition magma production in an intracontinental setting: Unusual andesites and dacites of the mid-Miocene Santa Rosa–Calico volcanic field, Northern Nevada

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 calc-alkaline and tholeiitic andesite–dacite magmas in an intracontinental setting. SC volcanism commenced at ∼ 16.7 Ma and is associated with the initial manifestations of the Yellowstone hotspot, 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 myr duration. Within this package are the products of at least four distinct intermediate composition 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 include lava flows with transitional to calc-alkaline affinities. Relative to locally erupted Steens Basalt, SC intermediate lava flows have similar elemental enrichments and depletions, but dissimilar Sr and Nd 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 Steens flood basalt magmas and the more evolved crustal melts ± assimilation of local upper crust provides a general template for the generation of SC tholeiitic and calc-alkaline 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 Steens Basalt, over at least a 2 myr duration. These results demonstrate that at least some of the mid-Miocene and younger calc-alkaline magmas that are found across the Pacific Northwest and are unrelated to subduction processes, were formed as a direct result of open-system processes affecting regional flood basalt magmas, rather than just melting of previously enriched (via subduction) lithospheric mantle as suggested by previous workers.