Published online by Cambridge University Press: 01 May 2009
Precambrian rocks throughout most of the southern part of the Bighorn Mountains, Wyoming, are dominated by a biotite-bearing plagioclase–quartz gneiss containing hornblende-rich or microcline-rich layers locally. Compositional and/or optical data are presented for plagioclase, biotite, and hornblende as well as optical and X-ray data for microcline and preliminary morphologic data for zircon in the gneiss. These data, supplemented by field and petrographic observations, suggest that the gneiss was formed under conditions of the low amphibolite facies, that late-occurring potash metasomatism accompanied the metamorphism, and that the gneiss was derived from a relatively chemically uniform parent rock.