GEOCHEMICAL INSIGHTS INTO THE EVOLUTION OF A SUPERERUPTIVE VOLCANIC CENTER: MAGMATIC PRECURSORS AND SUCCESSORS OF THE MIOCENE PEACH SPRING TUFF, SOUTHERN BLACK MOUNTAINS, WESTERN AZ

Volcanism in the southern Black Mountains region (Colorado River Extensional Corridor [CREC], western AZ) was active for several hundred thousand years before it generated the >700 km³ 18.78 Ma Peach Spring Tuff (PST- the only supereruption in the CREC), and remained active for ~1.5 m.y. afterward. Volcanic and intrusive rocks from this period offer a singular record of supereruption antecedence and aftermath. New whole rock elemental and Nd-Sr-Pb-Hf isotopic data constrain pre- and post-PST magmatic sources and processes and map the broad magmatic evolution of this eruptive center.

Pre-PST units (~55-68 wt% SiO₂) comprise a ~19.5-18.8 Ma, >1 km-thick pile of homogeneous phenocryst-rich trachyte lavas capped by a thinner and more variable sequence including trachyte and basaltic trachyandesite lavas and the ≤100 km³, trachytic Cook Canyon Tuff. Post-PST volcanics include rhyolite to trachyte lavas and small-volume pyroclastic units as young as ~17 Ma. The PST’s source, the Silver Creek caldera, exposes a 30 km² suite of texturally diverse, enclave rich 18.8-18.6 Ma epizonal stocks and crosscutting dikes (~55-77 wt% SiO₂). Both pre- and post-PST units have variable but coherent isotopic ranges (ε_Nd -11.6 to -8.3, ε_Hf -14 to -8.8, ⁸⁷Sr/⁸⁶Sr_i 0.7093-0.7124; Pb 206/204 18.19-18.45, 207/204 15.60-15.62, 208/204 39.05-39.28) that fall within the spectrum of Miocene CREC rocks. Magmatic enclaves in the intrusive suite and the lowest SiO₂volcanics approach local mantle isotope values.

The isotope data for pre- and post-PST units are consistent with mixing of substantial fractions of ancient enriched mantle lithosphere and regional Proterozoic (Mojave) crust. Unlike the PST, which has relatively uniform isotopic ratios (Frazier et al., this meeting), smaller-volume pre- and post-PST units are less evolved and more diverse. Pre-PST magmatism may have thermally “primed” the system, permitting thorough mixing of materials that contributed to PST magma, along with greater incorporation of crust. Post-PST thermal “resetting” of the magmatic center apparently restored the system to its small-volume, isotopically diverse mode.