Abstract

Red Cones are a pair of basaltic cinder cones located 5 km SSW of Mammoth Mountain at the southern end of the Mono-Inyo volcanic chain, in eastern California. Charcoal recovered at two separate locations beneath the Red Cones scoria-fall deposits indicates that the eruption most likely occurred shortly after 8490 ± 90 14C yr B.P. and no later than 9325 ± 83 14C yr B.P., which implicates Red Cones as the most recent eruption of basalt in the Mono-Inyo volcanic chain. Results from geologic field mapping combined with geochemical and petrologic analysis suggest that the ca. 8500 yr B.P. eruption produced 10.1 × 106 m3 of magma, possibly beginning from south Red Cone and later from north Red Cone via Hawaiian, Strombolian, and violent Strombolian eruptions over a minimum of 28 days. All deposits contain plagioclase, olivine, clinopyroxene, chrome-spinel, and titanomagnetite. Material erupted from each cone can be classified as high-aluminum basalts that exhibit calc-alkaline differentiation trends and belong to the medium-K series. Red Cone basalt samples are generally similar in terms of many major and trace element concentrations, but south Red Cone samples typically contain more SiO2, Sr, Zr, Rb, and Ba, and less MgO, FeO, CaO, Ni, and Cr than north Red Cone samples. Clinopyroxene-liquid thermobarometry calculations indicate that the majority of Red Cones clinopyroxene crystal cores crystallized at temperatures of 1160–1210 °C and pressures equivalent to 10–25 km depth, which supports the possibility of a basaltic dike and sill plexus located 10–25 km beneath the west and southwest flanks of Mammoth Mountain.

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