REFINED GEOCHEMISTRY OF THE PAINTBRUSH GROUP TUFFS AT YUCCA MOUNTAIN, NEVADA

The Miocene Paintbrush Group (Tp) at Yucca Mountain, Nevada, includes two dominantly devitrified, densely welded, regionally extensive pyroclastic flow deposits, the upper Tiva Canyon Tuff (Tpc) and the lower Topopah Spring Tuff (Tpt), each composed of an upper quartz latite member and an underlying, thicker rhyolite member. These densely welded units are enclosed within nonwelded vitric units. Systematic chemical and mineralogical zonation in the Tpc and Tpt has been attributed to an eruptive inversion of a stratified magma chamber or to mixing of separate magmas prior to eruption. Progressive changes in rare-earth element (REE) patterns stratigraphically upward in the quartz latites and large negative anomalies for Eu/Eu* in the rhyolites are consistent with fractional crystallization in the magma chamber. Strontium and neodymium isotopic variations have been used by other workers to support the hypothesis of concomitant assimilation of country rock. Concentrations of halogens, bound water, and ferrous iron, typically not available in earlier analyses, differ substantially between welded and vitric units. Samples from the nonwelded vitric units between Tpc and Tpt average 499±78 (SE_MEAN) ppm Cl, 982±91 ppm F, and 3.94±0.14 weight percent H₂O+ (bound water in contrast to moisture or H₂O-). Samples of the welded Tpt contain only 85±12 ppm Cl, 71±19 ppm F, and 0.81±0.05 weight percent H₂O+. Iron is strongly oxidized in the welded tuffs with Fe₂O₃/Total Fe as Fe₂O₃ ranging from 0.96 in the rhyolite to 0.98 in the quartz latite. This ratio is highly variable (0.75 to 0.98) in the nonwelded vitric units, and the smaller values may be primary magmatic ratios. Substantial differences in the halogen and H₂O+ contents between the densely welded and nonwelded vitric units of Tp are attributed to degassing that occurred during welding and devitrification, as shown by extensive vapor-phase corrosion along cooling fractures and in lithophysae. These data demonstrate that complete chemical analyses of rocks add considerable value to petrogenetic studies, especially in determining the effects of post-depositional processes such as degassing associated with devitrification, crystallization, and welding.