METAMORPHIC EVOLUTION OF PRECAMBRIAN BASEMENT IN THE SOUTHERN RUBY RANGE, SW MONTANA

Field mapping, phase equilibria modeling, and U-Pb geochronology provide insight into the P-T-t evolution of the southern Ruby Range, a Laramide basement-cored uplift in SW Montana. Mapping was conducted at a 1:24,000 scale within the Sweetwater Creek and Elk Gulch areas of the southern Ruby Range. Basement exposures within the Sweetwater Creek area can be divided into four lithologic units. The first is dominated by quartzofeldspathic gneiss, with minor garnet bearing layers. The second is defined by the presence of significant metapelitic lithologies, including garnet-sillimanite-cordierite gneisses. Pelitic layers range in thickness from less than 1 m to 3 m, with individual sillimanite porphyroblasts up to 5 cm long. The third unit is dominated by migmatitic metapelitic rocks, including a banded garnet-sillimanite bearing migmatite and minor layers of orthoamphibolite. The fourth unit is defined by the presence of a strongly lineated quartzofeldspathic gneiss, with minor migmatitic rocks. Amphibolite occurs throughout all of the units, as well as ultramafics, leucogneiss, and garnet bearing leucogneiss. In the Elk Gulch area, similar lithologies are present. Pelitic migmatites and garnet-sillimanite migmatites are more abundant in the Elk Gulch area. Quartzofeldspathic gneiss and lineated quartzofeldspathic gneiss are present, though in the Elk Gulch area migmatites are absent in this unit. Amphibolite and garnet amphibolite occur in greater abundance throughout the Elk Gulch area, with some layers traceable for several miles along strike. The largest pluton of ultramafic rocks present in the Ruby Range occurs in the Elk Gulch area and numerous small ultramafic bodies occur throughout the migmatitic gneiss unit. Samples of garnet-sillimanite-bearing migmatites and metapelites from both areas in the Ruby Range have been targeted for P-T analysis using pseudosections. P-T paths will be constructed for each locality and integrated with monazite geochronology to establish the Archean to Proterozoic metamorphic evolution of this region.