Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)

Paper No. 2
Presentation Time: 8:00 AM-5:00 PM


HART, William K. and BRUESEKE, Matthew E., Geology Dept, Miami Univ, 114 Shideler Hall, Oxford, OH 45056-2473,

Mid-Miocene volcanism in the Santa Rosa-Calico volcanic field (SC) of northern Nevada initiated at ~16.4 Ma, coeval with regional flood basalt volcanism. Throughout its duration, basalt through high-Si rhyolite magma were erupted and emplaced as lava flows, ignimbrites, domes, and shallow intrusive bodies. As a result, SC volcanic units are chemically and physically diverse. We divide the SC into three zones of intermediate through silicic eruptive loci, each broadly NW trending and characterized by specific eruptive styles and chemical units. The western (WSC) and eastern (ESC) zones overlap with the northern Santa Rosa Range and the Calico Mts., while the central zone (CSC) helps divide the centrally located Goosey Lake depression into two distinct basins. The WSC extends from the Spring City region north along the western SC margin, to the vicinity of the National Mining District. WSC silicic activity is illustrated by abundant dikes, necks, large hypabyssal bodies, and coulées. WSC silicic units typically have hydrous (biotite-amphibole) mineral assemblages and in some cases, flowed into the actively subsiding eastern Goosy Lake depression. The CSC is characterized by eroded silicic domes and coulées that are present at its northern and southern margins. Much of the CSC is floored by two ignimbrites: 1) a welded peralkaline unit consisting of a simple cooling unit (likely sourced in the McDermitt volcanic field), and 2) exposures of a thicker unit (the Cold Springs Tuff) that may be chemically zoned and could represent portions of a compound cooling unit. At the southeast end of the CSC, pyroclastic activity associated with a collapsed rhyolite dome characterizes Coyote Mt. and on its flanks, block and ash deposits underlie a highly welded rheomorphic ignimbrite. ESC silicic volcanism is best exposed in and around the Calico Mts., where compositionally distinct rhyolite lava flows/eroded domes and rheomorphic ignimbrites are typically characterized by anhydrous mineral assemblages. These resemble silicic units often associated with the ~14.5 - 12.8 Ma Owyhee Humboldt eruptive center, located to the east/north. In summary, the styles and diversity of silicic volcanism in the SC is a direct result of the close spatial/temporal association with flood basalt volcanism and lithospheric extension.