TEXTURAL AND MINERALOGICAL OBSERVATIONS OF QUENCHED BASALTIC INCLUSIONS IN LOW-SILICA ANDESITE FROM THE 2006 ERUPTION OF AUGUSTINE VOLCANO, ALASKA

Quenched basaltic inclusions account for approximately 1 volume % of material emplaced during the 2006 eruption of Augustine Volcano (Alaska), which corroborates several proposed models from previous geological and geophysical studies calling for intrusion of a basaltic magma into the subvolcanic reservoir followed by limited mixing between basalt and resident andesite magma prior to, and possibly during, the 4-month-long eruption. Inclusions are observed exclusively in variably dense blocks of the low-silica andesite (‘LSA' from Larsen et al., In Press) lithology. In outcrop, inclusions are dark black in color and characterized by vesicular interiors (approximately 20 volume % vesicles), quenched and cuspate margins and porphyritic texture. Inclusion mineralogy is dominated by 1-4mm-diameter plagioclase crystals and acicular hornblende, with lesser clinopyroxene and magnetite, ilmenite, orthopyroxene, olivine and apatite. Plagioclase crystal textures are characterized as either: (1) larger crystals (>0.25 mm diameter) with oscillatory zoned interiors surrounded by a dusty sieved layer that is finally enclosed by clear and euhedral overgrowths at the crystal rims; (2) smaller plagioclase (<0.1 mm diameter) crystals that either display acicular or anhedral forms; and (3) larger crystals (0.20 mm diameter) with coarsely-sieved interiors characterized by 0.01 to 0.02 mm diameter melt inclusions and/or similarly sized inclusions of clinopyroxene, orthopyroxene or hornblende. Future work aims to investigate major and trace element compositions of common mineral phases from basaltic inclusions in order to better understand the petrogenesis of basalt in the subvolcanic magma plumbing system and its role in the generation of lava lithologies at Augustine Volcano.