METAMORPHIC EVOLUTION AND P-T PATH OF CORDIERITE-BEARING BIOTITE-SILLIMANITE-GARNET GNEISSES DURING THE BIG SKY OROGENY, LITTLE BELT MOUNTAINS, MONTANA

Precambrian rocks of the Little Belt Mountains (LBM) of central Montana mark the northern-most exposure of the Wyoming Province. Previous work in the LBM has shown that the rocks of this area are dominantly intermediate metaintrusive rocks that can be associated with an original arc environment (Vogl. et al. 2004). The paleoarc environment is thought to be associated with an orogenic event known as the Big Sky Orogeny (BSO) (Harms, et al. 2004). In this study, samples were taken from a twenty-meter thick metapelitic layer known as the Aspen Gneiss (Vogl, et al. 2004). Situated between larger bodies of metadiorite and amphibolite, this layer of rock underwent the same P-T conditions as the metaigneous rocks around it; thus, it is analyzed in order to shed light on the metamorphism of the arc rocks. The Aspen Gneiss contains sillimanite + garnet + biotite + quartz + K-spar + plagioclase ± cordierite. Equilibrium mineral assemblages include sillimanite + biotite + garnet + quartz + K-spar + plagioclase and sillimanite + biotite + garnet + cordierite + quartz + K-spar + plagioclase. No kyanite was observed. The equilibrium mineral assemblages observed allow for constraint of P-T conditions experienced at the time of metamorphism. Monazites in the samples provide the basis for the first direct date of the timing of metamorphism. Work in the Tobacco Root Mountains (TRM) to the southwest of the LBM has left unanswered questions about the process by which the Wyoming Province joined the North American craton. The TRM, which comprise the core of the Big Sky orogen, underwent high pressure and temperature metamorphism during the BSO; however, this is only a small portion of the regional Big Sky orogen. The P-T path and dates of metamorphism in the LBM provide important new details on the regional scale structure of the Big Sky orogen.