EXPLOSIVE SILICIC VOLCANISM PRECEDING THE PEACH SPRING TUFF SUPERERUPTION, PART 2: MAGMATIC PROCESSES RECORDED BY PETROCHEMISTRY

Most volcanism in the N Colorado River extensional corridor was effusive and intermediate in composition until supereruption of the 18.8 Ma rhyolitic Peach Spring Tuff (PST) from the Black Mountains, AZ (Ferguson et al 2013). The Black Mtns were also the likely source of the only large pre-PST ignimbrite in the region: the ~18.9 Ma trachytic Cook Canyon Tuff (CCT) (Buesch & Valentine 1986; Lidzbarski 2014). Fallout tuffs are exposed immediately below and above CCT near Kingman, AZ, with a third, upper fall deposit beneath PST (Buesch & Valentine 1986). Based on feldspar compositions, Buesch (1993) correlated the lower two deposits to the CCT and the upper tuff with the Fort Rock Creek Rhyodacite on the Colorado Plateau (Fuis 1996). Ferguson & Cook (2015) mapped the three fall deposits and associated sediments together as Unit of Fort Rock. Other prominent, undescribed fall deposits are present southernmost Black Mtns [SBM]. Wood et al (GSA 2016) present whole-rock, pumice, glass, and mineral compositions from these deposits and evaluate correlations. We present petrogenetic implications of this growing data set.

Zircon saturation thermometry (Boehnke et al 2013; SEM-EDS & LA-ICPMS glass analyses) indicates melt T of ~680-700°C for Fort Rock tuff on the Plateau, 740° for the upper, possibly correlated fall deposit at Kingman, and 690° for fallout tuffs in the SBM. In contrast, the tuffs enveloping the CCT yield 860°, matching values for CCT (Perry et al, 2015). Tuff glasses from Fort Rock, SBM, and upper Kingman deposit also have low REE (e.g. La_n ~100 ppm) suggesting accessory fractionation, but CCT and its enveloping tuff has high REE (La_n ~400). Likewise, whole-rock tuffs from Fort Rock, SBM, and upper Kingman have relatively low Zr/Sr (<1 @ 70 wt% SiO₂), in contrast to CCT and enveloping deposits (>2 @ 70 wt% SiO₂), suggesting that the former evolved under cooler, wetter conditions (Miller et al 2014). Like CCT, PST has a hot, dry signature, with high zircon saturation T and very high Zr/Sr. These distinctions hint at different modes of silicic magma generation and evolution in the region before and including PST. We are expanding the dataset to further evaluate conditions and evolution of the tuffs, with amphibole barometry (Schmidt 1992), Rhyolite-MELTS (Gualda et al. 2012), and trace element modeling.