Textural and Geochemical Investigation of the Peach Spring Tuff CA-NV-AZ: Evidence for Dynamic Pre-Supereruption Processes

Abstract

The ~1000 km3 Peach Spring Tuff (PST) is the product of an 18.8 Ma supereruption from Silver Creek caldera, southern Black Mountains, AZ (Ferguson et al 2013). Five zones are recognized in thick outflow sections of the ignimbrite near Kingman, AZ, based on welding characteristics and phenocryst, pumice, and lithics abundances. The lower four zones are uniformly high-silica rhyolite (HSR) (74-76 wt% SiO2; Pamukcu et al 2013; Barry et al 2015). Phenocryst content increases gradually upward from ~2% in the lowest zone to ~20% modally in the fourth zone (Ferguson & Cook, 2015), and then rapidly upward into the capping fifth zone, a densely welded, phenocryst-rich (~35%) trachyte (65-69 wt% SiO2) whose fiamme match those in intracaldera PST in texture and composition (Pamukcu et al 2013). Zone 5 fiamme are characterized by high Ba, Sr and Zr concentrations, compared to the highly evolved chemical signature of the HSR outflow. Phenocrysts in intracaldera and outflow trachytic pumice are large and exhibit evidence of resorption and reaction. Glasses in the trachyte are extremely enriched in elements consistent with the partial dissolution of the phenocryst assemblage. Previous elemental and isotopic data, plus new elemental data, support a model of the PST reservoir as a single, relatively simple, vertically stratified chamber with a crystal-rich base and massive, high-silica, crystal-poor upper zone (Frazier, 2013; McDowell et al 2016). Outflow and intracaldera trachyte represent the crystal-rich cumulate base proposed to have been partially remelted and remobilized, via injection of a hot mafic magma.