PEACH SPRING TUFF, AZ-CA-NV: PETROCHEMICAL CONSTRAINTS ON GENERATION OF SUPERERUPTION MAGMA

The 18.78 Ma Peach Spring Tuff (PST) documents the lone supereruption (DRE >700 km³) during protracted Miocene magmatism in the Colorado River Extensional Corridor (CREC). CREC magmatism is otherwise characterized by copious lavas, small explosive eruptions, and large upper crustal plutons, with compositions ranging from andesite to rhyolite with minor basalt. Analyses of pumice illuminate distinctiveness of PST magma and constrain its origin.

Pumice ranges from crystal-rich trachyte (65-70 wt% SiO₂; caldera fill, some proximal outflow) to crystal-poor rhyolite (70-76 wt% SiO₂; dominates outflow). Assuming a typical outflow:intracaldera mass ratio of ~2:1, the mean composition of PST magma was: (~wt%) SiO₂ 72, FeO 2.0, MgO 0.3, CaO 1.0, Na₂O 3.5, K₂O 6.1; (~ppm) Ba 400, Sr 80, Zr 350, La 90, Lu 0.5. This is broadly within the CREC range, but it differs in having much lower Ba and Sr and higher K, Zr, and REE than comparably silicic rocks. PST isotopic compositions also fall within the wide span of CREC rocks, but are more evolved and far more uniform than most (ε_Nd -11.5 to -11.7, ε_Hf -13.8 to -14.3, ⁸⁷Sr/⁸⁶Sr_i 0.7108-0.7121 [one outlier 0.723]; Pb 206/204 18.20-18.29, 207/204 15.60-15.62, 208/204 39.10-39.21). An andesite magmatic enclave is comparable to enriched lithospheric mantle-derived CREC basalts (ε_Nd -8.3, ε_Hf -8.7, ⁸⁷Sr/⁸⁶Sr_i 0.7090).

Mixing models indicate that PST magma was a ~1:1 mixture of evolved regional Proterozoic crust and enriched mantle-derived magma like the enclave. Such an origin is typical of felsic CREC magmas, but the isotopic homogeneity of PST suggests unusually thorough mixing (the lone high-⁸⁷Sr/⁸⁶Sr sample can be explained by minor late stage contamination of Sr-poor PST magma by Proterozoic crust). The wide range in elemental compositions and narrow range in isotopic compositions (excluding the high Sr_i sample) suggest that hybridization preceded fractionation. Production of the distinctive PST elemental signature transpired under different conditions from other CREC magmas, though final high-Si rhyolite melt required upper crustal fractionation as seen elsewhere in the CREC. We speculate that generation of a large volume of thoroughly blended magma and subsequent storage and initial fractionation under atypical conditions were important factors leading to the supereruption.