Regional and local correlations of feldspar geochemistry of the Peach Spring Tuff, Alvord Mountain, California

The chemical composition of feldspar grains in an ignimbrite from the Spanish Canyon Formation in the Alvord Mountain area, California, have been used to confirm similarities in three measured sections locally, and they are similar to exposures of the Peach Spring Tuff (PST) regionally. Feldspar grains were identified on the basis of texture (zoning, as mantled feldspars, or in crystal clusters), whether the grains were attached to glass or were in pumice clasts, or were simply crystal fragments with no textural context. Chemistry was determined by electron microprobe analysis, and each analysis is calculated in terms of the percent endmember and plotted on orthoclase (Or) versus anorthite (An) plots. In general, the PST has sanidine and plagioclase compositions that are consistent with having formed in high-silica rhyolite and trachyte within a zoned magma chamber. Feldspars from the PST in Spanish Canyon area cluster along the rhyolitic trend with no grains along the trachytic trend. Similar clustering of feldspars along the rhyolitic trend with no grains along the trachytic trend also occur in the PST from Granite Spring and Providence Mountains to the east of the Alvord Mountain area, and the ranges in compositions are also similar in these locations. In contrast, the PST in the Kane Wash area of the Newberry Mountains has feldspars only from the rhyolitic trend in the basal deposits, but some grains from the trachytic trend are in the upper part of the deposit, and the range in compositions are greater than in the Spanish Canyon area. The variations in vertical compositional zoning and compositional range in these different deposits suggests there were probably different flow paths (or timing of the delivery) during the eruption and runout of the pyroclastic flow(s) generated from the climactic eruption of the PST magma chamber.