Location of a caldera source for the Soldier Meadow Tuff, northwestern Nevada, indicated by gravity and aeromagnetic data: Summary

Abstract

The Soldier Meadow Tuff in north-western Nevada is part of a sequence of peralkaline and subalkaline rhyolitic rocks of Miocene age that occupies an area of about 1,600 km 2 and has a volume of about 50 km 3 . The formation is mostly welded tuff containing abundant alkali feldspar and quartz phenocrysts in devitrified groundmass containing minute needles of blue-green alkali amphibole. The rhyolite of Badger Mountain is similar in mineralogy and chemistry, but textures suggest that it is a flow rhyolite. The underlying rhyolite of Catnip Mountain and tuff of Trough Mountain are phenocryst-poor but apparently also peralkaline. Gravity and aeromagnetic anomaly maps show 16- by 24-km anomalies near Badger Mountain. An analysis of these maps suggests that observed anomalies could be caused by a concealed caldera of 2,700-m thickness filled with rocks 0.2 g/cm 3 less dense and 0***002 emu/cm 3 lower magnetic susceptibility than the surrounding rocks. Distribution of the Soldier Meadow Tuff and the occurrence of numerous vents in the rhyolite of Badger Mountain suggest that the source for both units is associated with the indicated caldera. Possible alternative sources are other calderas whose locations are suggested by a 7- by 11-km gravity minimum near Rock Spring Table and a 12- by 24-km magnetic minimum west of Soldier Meadow.