PETROLOGY AND GEOCHEMISTRY OF MINARET CONE, MAMMOTH MOUNTAIN, CA

Approximately three dozen mafic vents surround Mammoth Mountain, a volcanic complex of 25-30 overlapping lava domes located in eastern California, including Minaret Cone on the northern flank. Mahood et al. (2010) determined an age of 94±9 ka based on ⁴⁰Ar/³⁹Ar analysis of Minaret Cone groundmass, which predates the 68-58 ka time period when the bulk of Mammoth Mountain lavas were erupted (Mahood et al., 2010). This study examines the mineralogy and chemical composition of basaltic material erupted from Minaret Cone. Minaret Cone lavas and scoria are plagioclase and clinopyroxene phyric with sparse olivine phenocrysts enclosed in an acicular groundmass composed mostly of plagioclase. Minaret Cone lavas and scoria yield bulk-rock trachybasaltic compositions that plot within the high-K calc-alkaline differentiation series, with 50.1-51.2 SiO₂, 1.58-1.69 TiO₂, 17.75-17.93 Al₂O₃, 8.20-8.49 Fe₂O₃, 5.42-5.81 MgO, 8.14-8.15 MnO, 8.77-8.99 CaO, 1.56-1.63 K₂O, 3.67-3.83 Na₂O, and 0.59-0.64 P₂O₅ (values in weight %). Modest modal olivine along with low concentrations of Ni (32-45 ppm) and Cr (42-73 ppm) require that Minaret Cone magmas are not primary mantle melts, but rather result from differentiation and assimilation in the lower crust. This interpretation is supported by trace element patterns in MORB-normalized spider diagrams, where Minaret Cone samples develop trends comparable to previously reported samples of Quaternary mafic lavas from the north moat region of Long Valley caldera (e.g., Bailey, 2004; Cousens, 1996). Interestingly, Minaret Cone samples share MORB-normalized trace element profiles to mafic enclaves contained in Mammoth Mountain trachyandesites and trachydacites in terms of immobile elements (e.g., Nb, Ce, Hf, and Ti), but are depleted in more mobile elements (e.g., K, Rb) compared to enclaves, suggesting that mafic magmas that intruded the Mammoth Mountain magma reservoir to form undercooled enclaves were more differentiated than those erupted from Minaret Cone.