MIOCENE TUFF OF BONELLI HOUSE, NW ARIZONA: PETROCHEMICAL CONSTRAINTS ON MAGMATIC PROCESSES AND COMPARISON TO UNDERLYING PEACH SPRING TUFF

The 17.72 + 0.01 Ma Tuff of Bonelli House (TB) (Ferguson & Cook, 2015; Regula et al 2015) is a pair of ignimbrites that overlie the 18.8 Ma Peach Spring Tuff (PST) in its well-known exposures around Kingman, AZ. We conducted a geochemical and petrographic investigation of TB aimed at elucidating its petrogenesis and evaluating a possible relationship to the PST, the product of a supereruption (Ferguson et al, 2013; Pamukcu et al 2013). The TB contains ~5% phenocrysts (alkali feldspar>>plag; trace biotite, quartz, apatite, Fe-Ti oxides, zircon, chevkinite); PST has a very similar assemblage, but a much higher phenocryst abundance (most outflow ~5-20%, up to 35%, mafics more prominent; Ferguson et al 2013, Ferguson & Cook, 2015, Barry et al 2015). Alkali feldspar in the TB is more sodic than that in PST (mostly anorthoclase, Ab_58-69, vs sanidine, Ab_40-45, SEM-EDS analysis). Whole-rock elemental chemistry of TB and typical PST outflow (XRF) documents distinctive similarities: SiO₂ of TB 72-74 wt%, PST 73-76 wt%, low Sr (TB 30-90 ppm vs. PST 10-100 ppm) and Ba (<50 ppm vs 10-200 ppm). Zr is high in both, yet higher in TB: 500-900 ppm vs. ~200-300 ppm. TB Zr/Sr ranges from 5-27, high compared to ratios in other regional Miocene volcanics in the region (most < 5; Metcalf 2004) and in typical silicic igneous rocks (most < 1; Miller et al 2014), but consistent with the ratios in PST outflow (3-22; Frazier 2013; Barry et al 2015). The high Zr/Sr suggests unusually high temperatures for a highly silicic magma. Zircon saturation thermometry (Boehnke et al 2013) based on glass compositions ( SEM-EDS, LA-ICPMS) indicate a TB temperature prior to eruption of 820-850°C compared to pre-eruption T indicated by PST outflow of ~750°C (Pamukcu et al 2013). Rhyolite-MELTS further constrained the TB pre-eruption temperature to ~830-875°C, whereas MELTS suggests PST outflow T ~740°C (Pamukcu et al 2013). The PST marks a shift from trachytic to rhyolitic magmatism, apparently coinciding with an increase in magmatic temperatures (McDowell et al 2014. Data for TB reveal distinctions between TB and PST that appear to rule out a simple petrogenetic connection between these two ignimbrites, but petrochemical fingerprints suggest important similarities in process corresponding to the onset and aftermath of supervolcanism.