AN IGNIMBRITE SANDWICH: UNDERSTANDING THE EVOLUTION OF THE PEACH SPRING TUFF SUPERERUPTION SYSTEM THROUGH PETROLOGIC EXAMINATION OF PRECEDING AND POSTDATING EXPLOSIVE ERUPTIONS – AN NSF-REU STUDY

Ten REU students focused on an ‘ignimbrite sandwich’ represented by the ~18.8 Ma supereruptive Peach Spring Tuff (PST) and tuffs and sediments below and above it to probe the nature and evolution of supervolcanoes. Three students used remote sensing to explore unit distribution (cf. Lang et al 2015); 7 others combined field observations with petrography and elemental geochemistry (SEM-EDS, XRF, LA-ICPMS) to characterize and correlate units and magmatic processes; two supplemented with paleomagnetic work.

The ~18.9 Ma trachytic Cook Canyon Tuff (CCT) (Buesch & Valentine 1986; Lidzbarski 2014) underlies PST and may share a source region (Varga unpub). CCT type locality characterization (Perry et al 2015) allowed correlation of distal deposits, elucidating its distribution (Scheland et al 2015). Variable pumice and glass compositions suggest magma mingling shortly before eruption. Comparison showed no petrogenetic relationship to PST, suggesting the PST magma accumulated in <100 kyr.

Tuffs, volcanogenic sediments, and carbonates mark a hiatus within a thick trachytic lava sequence adjacent to the caldera (Beard et al 2015; McCosby et al 2015), documenting a dynamic pre-PST environment where explosive silicic eruptions partially filled ephemeral lakes.

PST outflow deposits are divided into 5 zones based on morphology and pumice, lithic, and phenocryst content (Ferguson & Cook 2015), but four lower zones comprise rhyolite that is geochemically uniform; the top zone is crystal-rich trachyte (Barry et al 2015). This suggests a PST magma chamber with a basal, trachytic crystal mush, reheated by mafic input before eruption. Caldera megabreccia clasts are composed of the upper PST zone, suggesting caldera collapse occurred late in the eruptive sequence (Winslow et al 2015).

A pair of ignimbrites that directly overlie the PST, recently described as the 17.7 Ma Tuff of Bonelli House (TB) at Kingman, AZ (Ferguson & Cook 2015), were petrologically characterized and correlated to the southernmost Black Mtns (Regula et al 2015). Trace elements and modeling suggest that rhyolitic TB and PST were produced by similar high-T processes (hinting at the thermal flux responsible for rapid accumulation of the giant magma body), but composition and a 10⁶ yr age difference oppose a direct petrogenetic relationship (Hess et al 2015).