CONTACT METAMORPHISM ADJACENT TO THE PALEOPROTEROZOIC WOLF CREEK ULTRAMAFICS, SOUTHERN RUBY RANGE, SW MONTANA, EVIDENCE FOR MAGMATIC EMPLACEMENT

The Wolf Creek ultramafics in the southern Ruby Range, southwest Montana, consist mostly of underformed orthopyroxenite and hartzburgite, in places serpentinized, that are exposed as numerous small bodies ranging in surface area from several square meters to ≈ 1 km² within in a narrow NE-trending belt. Post-metamorphic hydrothermal alteration of one of the larger bodies exposed along the Elk Gulch fault has produced a blackwall-type vermiculite deposit. Both tectonic and magmatic emplacement mechanisms into surrounding, anonomously low-Ca metasedimentary rocks have been proposed for the Wolf Creek bodies.

We present field and petrologic evidence of high-temperature metamorphism and deformation that uniquely affected anatectic metapelitic gneiss adjacent to the largest ultramafic bodies. Leucosomes consist of mesoperthite and quartz with garnet. Restite contains sillimanite, garnet, quartz, microcline and biotite that commonly appears to have been partially resorbed by the melt producing reaction. Petrogenic grids for the NCKFMASH system indicate conditions during anatexis were ≈ 800°C, 700-900 MPa. Garnet-biotite and ternary feldspar geothermometry are consistent with peak temperature being approximately 800 °C adjacent to the ultramafic body. These temperatures are more than 100 °C higher than previously reported peak T for the southern Rubies. Lower regional peak T is supported by the presence of plagioclase and the absence of ternary feldspars in metapelites farther from the Wolf Creek bodies that indicate peak metamorphic temperature away from the ultramafics did not exceed 750 °C. Local deflection of fabrics, minor shears zones, areas of anatectic flow, and small folds in migmatitic metapelites and associated rocks adjacent to the larger ultramafic bodies are consistent with melting during deformation associated with emplacement of the ultramafic. From these data we conclude that the Wolf Creek ultramafics were emplaced as hot, probably magmatic, bodies in the late stages of the Early Proterozoic Big Sky orogeny.