Petrology of Pre-Cambrian Granites in the Northern Bighorn Mountains, Wyoming

Abstract

The pre-Cambrian rocks of the northern Bighorn Mountains comprise two series: an older one of variable gneisses, amphibolites, and schists, and a younger group of granitic rocks. The granitic rocks predominate at the north and south ends of the area. The granites are extremely variable in composition and range from gneissic to massive types which are thoroughly intermixed. Well-foliated rocks predominate near the margins of the range. Inclusions of biotite-rich rocks are common in a narrow zone along the margins of a large synclinal structure. The microscopic criteria for age sequence and mode of emplacement of minerals in sulfide ore bodies have been applied to the constituent minerals of the granites. Particular attention is given to the age relations of plagioclase, potash feldspar, muscovite, and quartz. The criteria suggest that potash feldspar replaced plagioclase and muscovite, and that quartz is later than these three minerals. Muscovite and epidote formed from plagioclase. Myrmekitic intergrowths, vein, string, and patch perthites, and inverse zoning of plagioclase suggest replacement of plagioclase by potash feldspar. Granites are cut by aplites, pegmatites, and diabase dikes of pre-Cambrian age. Diabase dikes have been subjected to palingenesis, indicating that the area underwent two stages of pre-Cambrian deformation. The granitic rocks contain a mineral assemblage critical of PT conditions between the amphibolite and epidote-amphibolite facies. Chemical analyses of rocks from the gneissic and the granitic series, when plotted in a A'CFK' tetrahedron, show that the formation of Bighorn granites involved a simple addition of , and , and a removal of FeO, MgO, and MnO; CaO and remained constant. The available evidence implies that the Bighorn granites, although rendered partially mobile, formed by replacement of older gneissic rocks at a PT level well below the field of magmatism.