St. George Island, Group 3 type rocks (from Chang and others, 2009): "Most group 3 rocks are distinguished petrographically by their coarse-grained and plagioclase-rich nature. Group 3 rocks also contain abundant olivine phenocrysts and a few clinopyroxene and opaque phenocrysts. Olivine compositions for seven rocks range from Fo85 to Fo47 and average Fo75. Clinopyroxene phenocrysts have XMg compositions between 85 and 78, with an average of 81. Plagioclase compositions for five rocks range from An69 to An21 and average An59. Plagioclase commonly contains apatite inclusions and may exhibit deformation twinning. Although plagioclase grains were counted as phenocrysts based on their size, grains in several rocks have acicular habits, suggesting that they grew to a large size from relatively slow and prolonged cooling following eruption in the interiors of thick flows. Group 3 rocks sometimes exhibit a trachytic texture and have few, if any, vesicles."
Petrogenesis of basaltic volcanic rocks from the Pribilof Islands, Alaska, by melting of metasomatically enriched depleted lithosphere, crystallization differentiation, and magma mixing