Eruption chronology and petrologic reconstruction of the ca. 8500 yr B.P. eruption of Red Cones, southern Inyo chain, California

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, clino-pyroxene, 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 thermo barometry 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.