EVALUATING THE TIMING, CONDITIONS, AND REGIONAL EXTENT OF PALEOPROTEROZOIC METAMORPHISM IN SOUTHWESTERN MONTANA

Metapelite samples containing the stable mineral assemblage biotite-garnet-sillimanite-k-feldspar ± muscovite were collected from the Highland Mountains, the Ruby Range and the Hebgen Lake area (no k-feldspar) in southwestern Montana and northeastern Idaho. Lower-grade biotite-garnet-kyanite schists were also collected from the Henrys Lake Mountains. Samples from the Highland Mountains and the Ruby Range have coarse-grained garnet porphyroblasts and sillimanite laths, with foliation defined by biotite. The presence of k-feldspar and muscovite in local back-reaction textures indicate that the samples likely underwent partial melting. Field relationships of interlayered schist and migmatite support this conclusion. Metapelites from the Hebgen Lake area are noted for their fine-grained texture, thin threads of fibrolite, relict kyanite and staurolite, and lack of k-feldspar. Henrys Lake Mountains metapelite, however, contains large, poikiloblastic garnet with some kyanite. Equilibrium muscovite is rare in samples from the Henrys Lake Mountains. SIMS-based ²⁰⁷Pb/²⁰⁶Pb spot ages on in situ monazite grains constrain the timing of peak metamorphism in the different pelites of these four areas. The garnet-biotite exchange geothermometer and the GASP geobarometer constrain their equilibrium pressures and temperatures. Together, geochronology, geothermobarometry, petrography, and petrology constrain P-T-t paths of metapelites from all areas sampled. Comparison of the metamorphic timing and conditions experienced between each of the areas studied provides new insight into the regional extent and variation of metamorphism associated with the 1.78-1.72 Big Sky orogeny, which has been characterized in the neighboring Tobacco Root Mountains.