| Earthquake Dome is a 2.2 km3 trachydacite dome located 3 km northwest of Mammoth Mountain in eastern California that formed ∼86 ka. Earthquake Dome lava consists of 9--13% phenocrysts, where plagioclase is the major phase with lesser hornblende, biotite, sanidine, quartz, and trace pyroxene and Fe-Ti oxides. Field mapping and comprehensive sampling efforts reveal the presence of quenched trachyandesite enclaves, which form as a result of mingling between two compositionally distinct end-member magmas. Although enclaves are evenly distributed throughout Earthquake Dome, they only account for ∼1 volume percent. Earthquake Dome lavas were derived through a combination of partial melts from the lower continental crust, magma mingling with a trachyandesite magma, and assimilation of a highly evolved incompatible-rich upper-crustal granitoid (or possibly metasediments) via assimilation. In addition, the enclave-forming trachyandesite magma itself is a product of the mingling between host trachydacite and a more mafic (N-MORB type) magma. Pre-erupted storage depth has been determined via amphibole geobarometry which indicate a depth of 19 to 21 km that agrees with previous geophysical studies data that depth also coincides with typical depths of long period earthquakes (indicative of magma movement through solid material) currently being detected in the Mammoth Mountain area. |
