Granodiorite from the Captains pluton 1 mile southwest of the head of Captains Bay. The texture is hypidiomorphic-granular to porphyritic. Felsic minerals compose 75-85 percent of the rock: 10-15 percent quartz, 13-17 percent orthoclase partly altered to kaolin (2-4 percent), and 50-55 percent plagioclase partially altered to sericite (0.5-1.5 percent) and clinozoisite (<0.01 percent). The plagioclase composition, by extinction angle, is An48 [mean of 24 crystals, standard deviation is 14 percent An, average amount of zoning per individual plagioclase crystal is 16 percent An per crystal], by index oils, is An44. Mafic minerals compose 15-25 percent: 1 percent pyroxene (all as small inclusions in one large plagioclase crystal), 12-15 percent hornblende (50 percent of this is massive and poikilitic with magnetite, 50 percent of this is uralitic fibers), 3-5 percent biotite partially altered to penninite (1-2 percent) and ilmenite and sphene (0.5-1.5 percent) which in turn are altered to leucoxene (<1 percent), 2.4 percent magnetite, and <1 percent apatite (in feldspar).
Geology of Unalaska Island and adjacent insular shelf, Aleutian Islands, Alaska