Phenocryst abundances and glass and phenocryst compositions as indicators of magmatic environments of large-volume ash flow sheets in southwestern Nevada (original) (raw)
1989, Journal of Geophysical Research
The Topopah Spring, Tiva Canyon, Rainier Mesa, and Ammonia Tanks tuffs are large-volume, silicic ash flow sheets that provide samples of four magmatic systems in southwestern Nevada. Successively erupted within a span of 2 m.y. from the same source area, they allow comparison of the sequential evolution of large, mature Cordilleran magmatic systems. Each large-volume sheet has a rhyolitic lower zone and quartz latitic upper zone. Coeval basaltic andesite and basalt show petrochemical continuity with these sheets and may represent mantle contributions that triggered eruptions of the midcrustal silicic portions. Abundances of phenocrysts and accessory phases increase upward with whole rock Fe (FeOt) from the base of all four sheets to maximum values unique for each system. Although maximum abundances of each mineral are unique for each sheet, each maximum occupies the same relative position within each sheet. High-temperature minerals such as plagioclase increase in abundance continuously with FeOt in each system, showing a decrease with FeOt only within basaltic andesite at the base of the Rainier Mesa system. Late crystallizing minerals such as quartz and sphene show maximum abundances at much lower FeOt, at or near the top of the rhyolitic zone.
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