Explosive and effusive volcanism in the Salton Trough, Salton Buttes, CA

Five mid-late Holocene rhyolitic obsidian domes lie along the southern margin of the Salton Sea, California. The domes are aligned parallel to the axis of spreading along the boundary between the Pacific and North American plates in the Salton Trough pull apart basin in an area of rapid subsidence and high sedimentation (0.2-2.0 cm/year; Schmitt and Hulen 2008; Brothers et al. 2009). These volcanic domes are spatially associated with a broad area of high heat flow (tens of kilometers wide) and active geothermal energy production in the Salton Sea Geothermal Field. Here, we present new petrologic, paleomagnetic, and isotopic data that yield insight into the origin and eruption styles of the Salton Buttes rhyolites. Magmatic rocks in the Salton Trough are bimodal in composition, including buried rhyolites and subsurface diabase dikes (Schmitt and Hulen 2008). Previous work concluded that rhyolites were generated via partial melting of hydrothermally altered basalt (based on heterogeneous and low δ18O of zircon crystals; Schmitt and Vazquez 2006). Newly determined δ18O compositions for anorthoclase (6.1-6.3 ‰) and rhyolite glass (6.4-6.6 ‰) for all five domes are consistent with rhyolite evolution by fractional crystallization of basalt. The five domes differ slightly in composition, forming compositional trends on Harker-type diagrams, (e.g., FeO [2.7 to 1.9 wt%] or Ba [500 to 350 ppm] vs. SiO2 73.6-76.4 wt%). The most evolved dome, Mullet Island, is aphyric except for accessory zircon and magnetite. Other domes contain 1-2 vol% of phenocrystic anorthoclase, Fe-Ti oxides, and accessory zircon and likely antecrystic corroded pyroxene, fayalite, and amphibole. Our work reveals that temperatures of pre-eruptive rhyolites based on several geothermometers are: 770-790 °C (Zr in melt), 725 - 780 °C (Ti in zircon), and 700-800 °C (Fe-Ti oxides). Volcanism at the Salton Buttes is characterized by explosive and effusive eruptions, where a pyroclastic fall deposit underlies an obsidian flow at South Red Hill and Obsidian Butte. Dispersal of pyroclastic material was directed largely to the NE, based on the presence of pumiceous material in shallow drillcores NE of the domes and the absence of pumice to the SE. At least one of the domes experienced phreatomagmatic explosions, producing high temperature volcaniclastic deposits with a largely sedimentary (quartz sand and mud) matrix, minor sedimentary clasts, and abundant volcanic (pumice and obsidian) clasts. The paleomagnetic direction of this deposit is identical to that of the coeval rhyolite dome, indicating temperatures of emplacement in excess of 580 °C. These new data suggest an origin of the Salton Buttes by fractional crystallization of basalt in an area of active extension, where assimilation is minor. Only buoyant rhyolite was capable of reaching the surface, where eruptions consisted of both magmatic and phreatomagmatic explosive phases and an effusive magmatic phase.