CONTACT METAMORPHISM IN PELITIC GNEISS ADJACENT TO WOLF CREEK ULTRAMAFIC BODY, RUBY RANGE, SW MONTANA, EVIDENCE FOR HOT EMPLACEMENT

Field and petrologic evidence indicate high-temperature metamorphism and deformation uniquely affected pelitic gneiss adjacent to the Wolf Creek ultramafic body in the southern Ruby Range, SW Montana. The metapelites are anatectites. Leucosomes consist of mesoperthite and quartz with garnet. The restite contains sillimanite, garnet, quartz, microcline and biotite that commonly appears to have been partially resorbed by a 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 also indicate that peak temperature exceeded 800 °C adjacent to the ultramafic body. The presence of plagioclase and the absence of ternary feldspars in metapelites further from the Wolf Creek body indicate peak metamorphic temperature away from the ultramafic did not exceed 750 °C. Furthermore, foliation in the metapelites is deflected in the vicinity of the Wolf Creek body; and minor shears zones, areas of anatectic flow, and small folds are consistent with movement of material away from the center of the ultramafic body. These data indicate contact metamorphism and emplacement driven deformation. The Wolf Creek ultramafic is primarily coarse orthopyroxenite to harzburgite except where it has been serpentinized near the Elk Gulch fault. Although it has not been dated directly, it was emplaced after regional deformation and metamorphism. In the nearby Tobacco Root Mountains peak metamorphic conditions and deformation have been dated at approximately 1800 Ma, and cooling ages for hornblende from the Ruby Range are consistent with late Paleoproterozoic regional metamorphism. Past studies of the Wolf Creek body have inferred either magmatic or solid emplacement mechanisms. The data presented here do not resolve the issue, although they do require forceful emplacement of a high-temperature body, suggestive of a magmatic origin. However, coarse orthopyroxene grains (to 10 cm in length) imply slow cooling, and the presence of igneous spinel intergrown with orthopyroxene blades requires high-pressure crystallization. Neither seems consistent with magmatic emplacement at mid-crustal levels.