Phenocryst abundances and glass and phenocryst compositions as indicators of magmatic environments of large‐volume ash flow sheets in southwestern Nevada
Authors: Warren R. G. , Byers F. M. , Broxton D. E. , Freeman S. H. , Hagan R. C. 
Published: February 2008

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. Minerals that normally form at intermediate stages of crystallization, such as sanidine, show maxima at intermediate FeOt for each sheet. A continuum of glass and phenocryst compositions occurs within the Topopah Spring and Rainier Mesa sheets. Variations in phenocryst compositions with FeOt are generally consistent with those expected for crystallization within magma reservoirs characterized by vertical thermal and compositional gradients. However, simple fractional crystallization does not adequately explain the close relationship in each sheet among the mineral chemistry, glass (magma) chemistry, and phase assemblages, which indicate a close approach to equilibrium within each magma system. Nor does fractional crystallization account for resorption textures and maximum abundances for phenocrysts, which demonstrate dissolution rather than crystal growth within the deepest parts of each magma system. We hypothesize that phenocrysts reequilibrate as they gravitationally settle into higher‐temperature magma at greater depth or dissolve if the phase becomes unstable. In either case, lower‐density liquids are produced that should buoyantly migrate upward. Their compositions suggest that glasses and minerals with the lowest FeOt, in the base of each sheet, equilibrated at 3.5 kbar water pressure and ∼660°C. Phase assemblages and mineral chemistries indicate progressively higher temperatures with increasing FeOt within each sheet. Maximum equilibrium temperatures, ∼900°C, are obtained for the most Fe‐rich assemblage, basaltic andesite, also found at the base of the Rainier Mesa sheet. The phase assemblages indicate water saturation or near saturation for the entire magmatic system and thus indicate depths of ∼10 km to the top of each magmatic system. Considering similarities in magmatic environments of the four sheets, the distinctive petrochemical differences among rhyolitic zones probably reflect chemical differences inherited from their source regions.