GEOLOGY AND GEOCHEMISTRY OF EARLY MIOCENE INTERMEDIATE COMPOSITION VOLCANISM IN NORTHERN NEVADA AND ITS RELATIONSHIP TO REGIONAL TECTONOMAGMATIC PROCESSES

Stratigraphically juxtaposed beneath mid-Miocene igneous rocks and above Cretaceous granitoid in the Santa Rosa Mountains and Bloody Run Hills of NV (SRBR) is a package of ~35-20 Ma dominantly trachyandesitic to trachydacitic calc-alkaline lava flows. Individual flows are thick (up to 150 m) and field relationships suggest that they were locally sourced. Also, SRBR flows are dominated by an anhydrous assemblage of plagioclase (An_30-58), orthopyroxene, clinopyroxene, Mg-rich olivine, and Fe-Ti oxides. As a group, they are characterized by Mg# = 22-58, wt.% TiO₂ = 0.67-2.15, Sr = 540-885 ppm, Ba = 770-1484 ppm, Y = 15-41 ppm, Nb = 9-22 ppm, Sr/Y = 16-45, Ba/Nb = 35-128, and La/Yb = 21-30. These physical and chemical characteristics have been used by previous workers to infer that their ultimate derivation was via subduction-related processes. However, their location is ~150-200 km east of the inferred Oligocene to early Miocene arc front (e.g. Dickinson, 2006; Cousens et al., 2008). Regionally, SRBR lava flows appear to be part of a broad zone of temporally similar extrusive products and mapped volcanoes, which includes the 30.9-26 Ma Salmon Creek volcanics in ID, the 25-23 Ma Steens Mountain volcanics in OR, 30-23 Ma lava flows in the Pine Forest Range of NV, 38-26 Ma units in the vicinity of the Warner Range in CA, and 26-20 Ma extrusive products and eroded stratovolcanoes that are exposed in the vicinity of the Coleman Hills and Hart Mountain in OR. Like SRBR lava flows, eruptive products from these other locations are primarily calc-alkaline, however many of these locations are characterized by silicic products and hydrous assemblages. When considered together, SRBR lavas, low-Si lavas from the Steens Mountain volcanics, and the Salmon Creek volcanics define a subalkaline to transitional array and have lower TiO₂, K₂O, FeO*, Na₂O and higher CaO, Sr, Ni, La/Yb, and Sr/Y than Coleman Hills-Hart Mountain area units, which are more alkaline. Overall, these variations, coupled with previous observations help define geographically distinct magmatic systems, likely reflect differences in lithospheric thickness and depth to melting across the region, and suggest that the regional late Oligocene - early Miocene volcanic arc was up to 200 km wide and characterized by intra-arc extension and transtension.