MAGMA CHAMBER ZONATION AND ERUPTIVE INVERSION IN THE PEACH SPRINGS TUFF: GEOCHEMICAL AND PALEOMAGNETIC EVIDENCE

The 18.8 Ma Peach Springs Tuff (PST) is a voluminous (>640 km³ DRE) welded ash flow tuff in SE CA, NW AZ, and S NV (Glazner et al 1986, Buesch & Valentine 1986, Ferguson et al 2013). Exposures of outflow PST near Kingman, AZ, reveal four zones based on morphology and pumice, lithic, and phenocryst abundance (Ferguson & Cook 2015). Samples of these zones in Box Canyon, S of Kingman, and a sample from a fifth, uppermost zone in Golden Valley, AZ, were analyzed for bulk-rock, mineral, and glass compositions by XRF and SEM-EDS to test for variation up-section, a potential indicator of erupting chamber zonation. From base to top of the Box Canyon section we observe: 1) increase of phenocryst content from ~5 to ~20%; 2) increase of mafic magmatic enclaves, <<1%, max. 2 cm to prominent, max. 20 cm at top; 3) increase in size and abundance of pumice. Major phenocrysts include sanidine>plagioclase and biotite, with accessory hornblende, apatite, zircon, sphene, Fe-Ti oxides, and chevkinite. Bulk rock chemistry is uniform (excluding a basal surge deposit) up section: SiO₂ 73.8±0.4, TiO₂ 0.30±0.02, Fe₂O₃(t) 1.49±0.12, MgO 0.46±.21 wt %, Rb 192±5, Ce 147±7 ppm. Biotite and feldspar compositions are similar throughout, supporting evidence for minimal chemical zonation within the portion of the magma chamber represented by the lower four zones. Zircon saturation thermometry (Boehnke et al 2013) suggests a temperature of 793±22˚C (no temperature gradient), further suggesting a homogeneous magma. The Golden Valley sample is markedly different (35% phenocrysts; SiO₂ 68.0 wt %, TiO₂ 0.49, Fe₂O₃(t) 2.26, MgO 0.62, Rb 97 ppm, Ce 255), similar to uppermost outflow exposed in the southern Black Mountains (Foley et al 2014) and intracaldera tuff at Silver Creek (Ferguson et al 2013; Pamukcu et al 2013). Our data imply an increase in phenocryst content deeper in the chamber with a distinct increase in the deepest portion that was erupted. Elemental composition was essentially uniform but shows an abrupt change at the crystal-rich base. This is consistent with a single eruptive event that ejected the contents of the magma chamber, tapping down to a small portion of crystal mush. Analysis of paleomagnetic remanence and AMS of PST zones collected near Box Canyon is ongoing, but preliminary results similarly suggest a single eruptive event.