DYNAMICS OF DESTABILIZATION: PUMICEOUS EVIDENCE OF REHEATED, REMELTED, AND REMOBILIZED CUMULATE FROM THE BASE OF A SUPERVOLCANO MAGMA CHAMBER

The 18.8 Ma Peach Spring Tuff (PST) supereruption was the largest eruption within the northern Colorado River Extensional Corridor (AZ-CA-NV).The PST outflow is dominated by high-silica rhyolite, which is well studied (Pamukcu et al 2013 J Pet). We focused on less-studied, crystal-rich trachyte pumice at the top of the PST section, in a proximal outflow deposit in the southern Black Mountains and distal outflow in the northern Hualapai Mountains, AZ; this pumice is very similar to that found in the PST caldera (Silver Creek, Ferguson et al 2013 Geology). The fact that, despite pronounced textural and elemental differences, PST trachyte and rhyolite are isotopically indistinguishable is consistent with a common origin (Frazier et al 2012, Overton et al 2013, GSA absts). To evaluate the genesis of the trachyte, we studied samples by optical and SEM microscopy and XRF to assess phenocryst assemblages and proportions, textures, and major element compositions of phenocrysts, glass, and bulk rock. Major phenocrysts include sanidine>plagioclase and biotite, accompanied by accessory magnetite, sphene, apatite, zircon, and chevkinite. Phenocrysts exhibit evidence of resorption and reaction, including heavily embayed and sieve textures. Glass compositions averaged 70-71% SiO₂, considerably lower than the 77 wt% typical of most outflow pumice. Zircon and apatite saturation thermometry using bulk pumice (glassy fiamme) composition suggests temperature of ~880-900°C, consistent with MELTS modeling also based on bulk compositions and 1-3wt% H₂O (we determined the T range constrained by best match of observed phenocryst assemblage and melt fraction and composition.) This high temperature, together with textural evidence, suggests that trachyte represents material from the base of the PST chamber that was heated after initial growth of crystals (cf. Pamukcu et al 2013). The less-silicic melt in trachyte fiamme may reflect the resorption of accumulated crystals as a result of reheating. A trace element model is consistent with the interpretation that trachyte composition is that of a cumulate formed during fractional crystallization of a parental PST magma, with the high SiO₂ rhyolite as segregated liquid. The cumulate was reactivated by a late-stage heating event potentially associated with the onset of the eruption.