METAMORPHISM OF KYANITE SCHISTS IN THE NORTHERN GRAVELLY RANGE OF SW MONTANA DURING THE PALEOPROTEROZOIC BIG SKY OROGENY

Paleoproterozoic kyanite schists found in the northern Gravelly Range of southwest Montana can help constrain the regional pressure and temperature conditions of the ~1.8-1.7 Ga Big Sky orogeny. These kyanite schists contrast with coeval sillimanite-bearing metapelites found in the adjacent Tobacco Root and Highland Mountains and in the Ruby Range. Fourteen kyanite-bearing, biotite-quartz schists from a 9-km long, N to S, across-strike transect of the study area were analyzed in hand-specimen, in petrographic thin section, and with a SEM-EDS instrument to identify the main metamorphic phases present. Two mineral parageneses were identified. In the southern part of the study area, the schists have a biotite-quartz-plagioclase-muscovite+chlorite matrix with kyanite, staurolite and garnet occurring as porphyroblasts in an equilibrium assemblage. Schists in the northern part of the study area, closest to the Tobacco Root Mountains, have the same matrix make-up and also include kyanite and garnet porphyroblasts but they lack staurolite. These schists preserve evidence for a transition from kyanite to sillimanite stability. Kyanite is rimmed with white mica, and mats of fibrolitic sillimanite with white mica occur. These observations demonstrate a gradient in metamorphic conditions during the Big Sky orogeny, where metamorphism in the southern part of the Gravelly Range study area took place at either higher pressure, or lower temperature, or both, relative to the northern study area and relative to adjacent ranges to the north and west.