SITGREAVES TUFF: INSIGHTS INTO THE EVOLUTION OF A POST SUPERERUPTION HIGH SILICA RHYOLITE, SOUTHERN BLACK MOUNTAINS, NW AZ

The Sitgreaves Tuff (SGT; Thorson, 1971) is a widespread unit in the southern Black Mountains, AZ, in the vicinity of the caldera of the Peach Spring Tuff (PST, 18.8 Ma; Ferguson et al. 2013). The objectives of this study are to use geochemical and petrographic data in conjunction with pressure and temperature modeling to constrain the pre-eruptive conditions within the SGT magma chamber and illuminate the evolution of the regional magmatic system following a supereruption. The SGT consists of a lower fall unit (SGT1), ~18 m thick, an upper incrementally emplaced pyroclastic flow unit (SGT2), ~140 m thick, and a ~30 m cap of reworked volcanic sediments (SGT3) (Schwat et al. 2016; Thompson et al. 2016 GSA abs). This study focused on primary volcanic deposits (SGT1, SGT2). Pumice within SGT1 is relatively crystal rich and contains a phase assemblage of Pl>Bt>Am ± Opx ± Qtz (rare) + Fe-Ti oxides; + Ap. SiO₂ in whole rocks averages 67 wt%. and Sr 400 ppm (XRF); SiO₂ in glass averages 75 wt% SiO₂ (SEM-EDS). Pre-eruptive pressures estimated by Al-in-amphibole barometry (Schmidt, 1992) on rims average ~200 MPa (Al^T: 1.1 apfu); Al rich cores (Al^T: 1.5 apfu) indicate a distinct history, possibly at higher pressure.

SGT2 contains abundant crystal poor (~1% phenocrysts), highly vesiculated pumice characterized by more evolved compositions (whole-pumice 75 wt% SiO₂, 40 ppm Sr; glass composition 77 wt% SiO₂, 5 ppm Sr, Eu/Eu* ~0.06) (SEM-EDS +LA-ICPMS). SGT2 pumices contain phenocrysts of Afs>Qtz>Am + Ttn, Zrn, Ap, chevkinite and Fe-Ti oxides. Pre-eruptive pressures range from 100-150 MPa (Avg. Al^T: 0.8 apfu; ~190-210MPa by Gualda & Ghiorso, 2014 SiO₂ in glass calibration). Rhyolite-MELTS (Gualda et al 2012) and zircon saturation thermometry (Watson & Harrison, 1983) using glass compositions from SGT2 indicate pre-eruptive temperatures of ~760ºC (~700ºC by Boehnke et al., 2013, calibration).

These data suggest that SGT1 erupted from a deeper, less evolved magma, followed by eruption of the highly evolved SGT2 from a shallower source. SGT2 may represent fractionated melt extracted from mush that remained after eruption of SGT1.