MONAZITE OCCURRENCE IN GARNET BEARING SCHIST AND GNEISS FROM THE RUBY RANGE, SOUTHWEST MONTANA

Formation of monazite compositional zones is tied to garnet forming and garnet consuming reactions as well as to fabric formation in garnet bearing migmatitic metapelites and mylonitic metagranites from the Ruby Range of southwest Montana. Thin section maps created on the scanning electron microcope and on the electron probe establish the sites of monazite grains and show grain size distributions of monazite populations. Maps are used to determine differences in orientation and size between monazite grains included in the matrix of neosome, the matrix of paleosome, and those included within key index minerals. Monazite grain maps created on the electron probe show trace element compositions and compositional zones in select monazite grains that are correlated to garnet forming or garnet consuming reactions during metamorphism. Further, based on their shape and orientation, some monazite domains are interpreted as coeval with fabric elements in the host rock. Tying monazite domains to coeval garnet-and fabric-forming events lays the foundation for geochronology of those grains. Associating ages with pressure and temperature points as well as with deformation history will aid in the construction of a P-T-t-d path for the Ruby Range which can be compared to the P-T-t-d path of the nearby Tobacco Root Mountains to constrain the tectonic development of the Proterozoic Big Sky orogen.