PEACH SPRING TUFF (AZ, USA): IMPLICATIONS OF ZONATION FOR THE SUPERERUPTION AND RECONSTRUCTION OF CHAMBER STRATIGRAPHY

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 upward increasing pyroclastic (phenocryst, pumice, and lithics) abundances. In the lower four, the ignimbrite is uniformly high-silica rhyolite (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 phenocryst-rich (~35%) trachyte (65-69 wt% SiO₂) whose pumice match intracaldera PST fiamme in texture and composition (Pamukcu et al 2013; Foley et al 2014). This uppermost outflow (zone 5) is characterized by high Ba, Sr and Zr concentrations in pumices and fiamme (avg 1000, 230, and 570 ppm respectively), compared to the typical highly evolved chemical signature of the high silica outflow. Phenocrysts in intracaldera and outflow trachytic pumice are large (sanidine ≤6 mm) and exhibit evidence of resorption and reaction, including heavily embayed and sieve textures. The outflow rhyolite, intracaldera trachyte, and outflow trachyte have the same phenocryst assemblage consisting of sanidine>plagioclase and biotite, accompanied by accessory magnetite, sphene, apatite, zircon, and chevkinite. Amphibole and very sparse quartz are present in the outflow rhyolite, but rare or absent within the outflow and intracaldera trachyte. The contact between the outflow trachyte and underlying rhyolite ignimbrite is also marked by an abrupt decrease in lithics and increase in welding culminating in a vitrophyre horizon just above the contact. 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. Field evidence suggests zone 5 marks a change in eruption style upon chamber evacuation.