Named for exposures at Mackay Peak, SW1/4 sec 35, T7N, R23E, Custer Co, ID in Idaho Mountains province. Is one of the largest intrusive bodies in study area. Was called "Mackay granite" as an informal name in earlier reports. Is exposed over an area of about 11.5 sq mi. Geologic map. Consists of weathered pinkish-gray granite porphyry made up of orthoclase and quartz phenocrysts in a fine-grained groundmass. Orthoclase phenocrysts make up 25 to 35 percent of the rock; they are as much as 10 mm in diameter, subhedral to euhedral. Quartz phenocrysts make up 5 to 15 percent of the rock; they are as large as 5 mm, and rounded. The groundmass is a mosaic of quartz, orthoclase, and plagioclase. Locally some of the groundmass is finer grained and a greener color probably due to fact that it is a chilled phase and because it has more hornblende and the biotite altered to chlorite. Two specimens--by the lead alpha using zircons--have ages of 40 +/-10 m.y. Assigned an early Tertiary age.

Source: GNU records (USGS DDS-6; Denver GNULEX).