Petrochemical Constraints on Generation of the Peach Spring Tuff Supereruption Magma, Arizona, Nevada, and California

Abstract

The 18.78 Ma Peach Spring Tuff (PST), the lone supereruption from the northern Colorado River extensional corridor (NCREC), exhibits diverse elemental characteristics, from crystal-rich intracaldera trachyte to crystal-poor high-silica rhyolite. By contrast, it is remarkably uniform isotopically, which suggests unusually thorough mixing during magma chamber assembly. Its isotopic composition requires that the PST magma was composed of ancient Proterozoic crustal and enriched mantle-derived materials, a hybrid origin typical of felsic NCREC magmas. Closed and open system trace element modeling cannot distinguish between derivation by open-system processes involving juvenile mafic magma and Proterozoic crust or by anatexis of igneous crust formed by similar processes during the Mesozoic, but the elemental data provide constrains on the petrogenetic conditions leading to the PST magma. Concentrations of Ba and Sr much lower and concentrations of Zr much higher than other NCREC magmas indicate that the PST underwent extensive feldspar fractionation prior to accessory mineral fractionation, potentially a consequence of relatively high temperatures and low water content. Most NCREC felsic magmas exhibit elemental characteristics indicative of cooler temperatures and higher water contents, similar to extension-related magmas in the Basin and Range province to the north, while PST magma is more akin to the hot, dry magmas erupted on the Snake River Plain. It is likely that the relatively hot and dry conditions inferred for the PST allowed the magma body to convect vigorously, homogenizing itself isotopically, while smaller batches of cooler, wetter magmas were unable to do so, leading to the NCREC’s regional isotopic heterogeneity.