BEFORE AND AFTER A SUPERVOLCANO: REU INVESTIGATIONS OF THE EVOLUTION OF MAGMATISM IN THE SOUTHERN BLACK MOUNTAINS VOLCANIC CENTER, AZ, SURROUNDING THE PEACH SPRING TUFF SUPERERUPTION

Volcanic rocks preceding and postdating the Peach Spring Tuff (PST; ~18.8 Ma, Ferguson et al 2013) provide evidence of how magmatic systems build up to and evolve following supereruptions. 18 undergraduate projects over 3 years of an REU program addressed these questions with field, geochemical (XRF, SEM-EDS, LA-ICPMS), and petrographic (SEM, optical microscopy) analysis of units spanning ~2 myr of volcanism (~19-17 Ma) in the Southern Black Mountains Volcanic Center (SBMVC; McDowell et al 2016). Voluminous, effusive, intermediate volcanism dominated the SBMVC before the PST eruption, with trachyte lavas interbedded with rare thin tuffs and some mafic lavas (Rice et al 2014, Rentz et al 2014, Flansburg et al 2014, Lee et al 2014, Williams et al 2014). Biotite was the dominant mafic mineral, and most feldspars were relatively unzoned. A large explosive trachyte eruption (Cook Canyon Tuff) barely preceding the PST (~18.9 Ma; Lidzbarski 2014) shows evidence of incomplete magma mixing, the first hint of open system processes (Pratt et al 2014, Perry et al 2015, Scheland et al 2015). Heating by mafic calc-alkaline lavas preceded the PST (Flansburg et al 2014) and may have triggered the PST eruption (Pamukcu et al 2013, Foley et al 2014). Mineralogy and petrochemistry of the post-PST section in Meadow Creek Basin near Silver Creek caldera reflect post-PST changes: (1) rhyolite dominates (Sitgreaves Tuff is the most evolved volcanic rock known in the SMBVC; Wallrich et al 2016); (2) Hbl, rare or absent in PST/pre-PST rocks, is the dominant mafic mineral; higher Al content suggests magma storage at deeper levels than in PST/pre-PST time; (3) Zr/Sr is lower than in PST/pre-PST at similar SiO₂, suggesting cooler, wetter conditions (Miller et al 2014); (4) late mafic diabase and lavas are tholeiitic, unlike pre-PST mafics (Smith et al 2016). Open system processes that began just before the PST continue following the super eruption, including magma mixing and mingling in Antelope Lava (Schlaereth et al 2016) and entrainment of fspar megacrysts from a genetically diverse crystal mush by ascending magmas (Collins et al 2016). In the case of the SBMVC, the supereruption precipitates or coincides with changes toward more evolved, cooler, wetter, deeper magmas, more open system magmatic processes and a significant decrease in magmatic flux.