SUPERERUPTION OF PEACH SPRING TUFF: NEW INSIGHTS FROM CALDERA-FILL AND OUTFLOW PUMICE AND ACCESSORY MINERALS

The Peach Spring Tuff is an 18.5 Ma ignimbrite that serves as a stratigraphic marker over much of NW Arizona, SE California, and S Nevada. It is the only widespread welded tuff of the region, with estimated volume of >600 km³ d.r.e. for the outflow alone. The tuff is recognizable by its distinctive phenocryst assemblage (abundant sanidine, minor to absent quartz, prominent sphene). Understanding eruptive dynamics and magma chamber evolution has been limited by the absence of a recognized caldera and paucity of petrologic data. Recent identification of the partially exposed, dissected Silver Creek caldera (Black Mountains, near Oatman, AZ) as the probable source of the tuff provides fresh opportunities for investigating this eruption (caldera identification based on age and phenocryst assemblage of intracaldera and outflow tuff and consistency with previously described regional patterns of thickness and flow directions; Ferguson, GSA abst 2008). Analyses of pumice, fiamme, and glass from intracaldera, proximal outflow, and distal outflow exposures reveal heterogeneity in the underlying chamber at the time of eruption. All pumice, fiamme, and glass analyzed to date are characterized by high to very high K₂O (typically ~6 wt. %), but in other respects samples range widely in composition (trachyte to high-Si rhyolite whole-rock, low- to high-Si rhyolite glasses). Fiamme in the intracaldera fill – possibly the last material to erupt – are trachyte (66-69% SiO₂); pumice and fiamme in proximal outflow have 69-76% SiO₂, and distal outflow pumice has 72-77% SiO₂. Whole-rock compositions are compatible with differentiation in a shallow magma reservoir at ~200-300 MPa. Sparse magmatic enclaves indicate that mafic to intermediate magma was present in the chamber during eruption. Accessory minerals also record heterogeneity within the evolving chamber prior to eruption. Sphene and zircon from intracaldera tuff show strong 'reverse' zoning, with less evolved rim compositions and higher temperatures (e.g. much higher MREE in sphene, very low U in zircon; higher temperature rims based on Zr-in-sphene and Ti-in-zircon). Sphene and zircon from the distal outflow display weaker ‘normal’ zoning with less-evolved rim compositions and considerably lower temperatures.