| Text Description: |
St. George Island, Group 2 type rocks (From Chang and others, 2009): "Group 2 rocks are generally porphyritic, with fine-grained, holocrystalline groundmasses of olivine, clinopyroxene, plagioclase, and opaque minerals. Olivine is the most common phenocryst phase, although a few plagioclase phenocrysts are commonly observed, and some rocks also contain clinopyroxene phenocrysts. Olivine phenocrysts in two rocks range from Fo91 to Fo69 and average Fo80. As in the group 1 rocks, the olivine phenocrysts may be skeletal to euhedral. Clinopyroxene compositions in one group 2 rock have XMg=85-79, with an average value of 83, with average core and rim compositions of XMg=84 and 81, respectively. Some clinopyroxene grains exhibit sector-zoning." |