COMPLEX MAGMA SYSTEM DEVELOPMENT IN NORTHERN NEVADA AND THE ROLE OF EPISODIC BASALT INJECTION INTO THE CRUST

The ~17-13 Ma Santa Rosa-Calico volcanic field (SC) of north-central Nevada lies within the northern extension of the northern Nevada rift and was formed during a period of increased regional magma production concurrent with the initial manifestation of the Yellowstone-Newberry mantle upwelling. Development of the SC is contemporaneous with other mid-Miocene Oregon Plateau volcanic fields (e.g. McDermitt, Lake Owyhee). Together, these older volcanic fields are differentiated from younger Yellowstone-Snake River Plain eruptive centers by four main features: 1) their location in an area of both continental and oceanic lithosphere, 2) their diverse eruptive styles, 3) the diversity of magma compositions (basalt through rhyolite) present throughout their entire eruptive history, and 4) their temporal and spatial relationship with flood basalt volcanism and extensional structures.

The dominantly subalkaline SC volcanic products (basaltic through high-Si rhyolitic lava flows, domes, and shallow intrusive bodies) overlie ~19-22 Ma calc-alkaline andesite flows and Mesozoic metasediments and granitoid bodies. Present within this 500-1000m package of Miocene eruptive material are the products of at least three major pulses of the Oregon Plateau-wide Steens Basalt. Preliminary geochemical evidence from the western SC suggests that Steens Basalt magmas acted as both parental compositions and mixing end members during the generation and evolution of spatially and temporally associated intermediate and silicic magmas. Additionally, the local Cretaceous granitoids also appear to have played a role in the petrogenesis of these evolved magmas. Based primarily on field relations and these preliminary geochemical data, it appears that the formation and generation of SC evolved magmas was triggered by episodic fluxes of Steens Basalt into the crust. Accompanying these mafic inputs, ongoing lithospheric extension helped concentrate and establish numerous, small magmatic systems throughout the SC and may have inhibited the development of a single, long-lived magma system.