Abstract

The Soldier Meadow Tuff of late Miocene age was erupted from a linear vent area in northwestern Nevada. Numerous ash flows representing about 50 cu km of magma were erupted, followed by less energetic eruption of a small volume of air-fall tuff, which is welded near the vents. The vents then served as feeders for lava flows unquestionably comagmatic with the tuffs. Flow foliation in the lava flows defines four vents aligned in a north-south direction, and part of the internal structure of one of the vents has been exposed by faulting and erosion. The distribution pattern of the ash-flow sheet, the distribution and size variation of xenoliths, and the absence of a cooling break between tuff and lava all support this interpretation. The tuff and lava consist of peralkaline rhyolite (comendite) and are virtually identical in mineralogy and chemistry. Neither caldera collapse nor local subsidence occurred after eruption of the Soldier Meadow Tuff. The arrangement of vents parallel to a range-front fault trend implies control of alignment by extensional Basin-Range faults.

The tuff of Trough Mountain, an earlier unit of phenocryst-poor, comendite ash-flow tuff of more limited volume and extent, appears to have erupted from a vent area which is elongate northward and nearly coincident with that of the Soldier Meadow Tuff, as indicated by thickness, areal distribution, welding of air-fall tuff, and distribution of xenoliths.

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