Granulite Facies and Related Xenoliths from Colorado-Wyoming Kimberlite

Abstract

Abstract Granulite fades xenoliths recovered from kimberlites in the Colorado-Wyoming State Line and Iron Mountain, Wyoming Districts are primarily two pyroxene-granulite, two pyroxene garnet granulite, and clinopyroxene garnet granulite. An igneous xenolith suite consisting primarily of gabbronorite appears to be gradational with the granulites. No known granulite facies rocks are exposed in this area and the entire nodule population is interpreted as lower crustal in origin. Garnet-clinopyroxene equilibrium temperatures of 580-700°C were obtained for the garnet-bearing granulites using the method of Raheim and Green (1974a). Based on the experimental work of Green and Ringwood (1972), Irving (1974b) and seismic determinations of crustal thickness (50 km) in northern Colorado (Pakiser and Zeitz, 1965), equilibration pressures for the garnet granulite suites are estimated to fall in a range from 10-15 kb. Major element and REE chemistry of analyzed granulite xenoliths show characteristics of continental basalts. Normative compositions of the mafic granulites are mostly equivalent to quartz tholeiite and olivine tholeiite. Relict allotriomorphic granular textures are observed in many samples, and sub-solidus reaction textures present in the garnet-bearing granulites indicate formation of garnet±clinopyroxene at the expense of orthopyroxene, plagioclase, and clinopyroxene. This mafic granulite xenolith suite is believed to represent basic intrusive rocks “equilibrated” under granulite facies conditions at various levels in the lower crust. The mineralogy of the nodule suite suggests that the lower crust in the Colorado-Wyoming State Line region is a predominantly mafic igneous-metamorphic complex.