Metamorphic Evolution of Precambrian Rocks in the Southern Highland Mountains, Montana, and Implications for Proterozoic Tectonics

The southern Highland Mountains, located in southwest Montana, contain a diverse assemblage of high-grade metamorphic rocks cored by migmatized leucocratic orthogneiss, with garnet-sillimanite paragneiss and schist at structurally higher levels. Based on the structure of the metamorphic rocks, O’Neill et al. (1988) hypothesized that the southern Highland Mountains is a gneiss dome. This study evaluates the gneiss dome hypothesis by conducting a petrologic study of the metamorphic rocks in the southern Highland Mountains using thin section analysis and pseudosections to determine if spatial trends in metamorphic conditions consistent with those of gneiss domes exist. In total, six samples were selected for P-T analysis. Pseudosections were constructed using the program Theriak/Domino with the internally consistent thermodynamic database of Holland and Powell (1998) in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe¬2O3 (NCKFMASHTO). Of the six pseudosections, four are of metapelitic rocks with the assemblage garnet + sillimanite + biotite + plagioclase + potassium feldspar + ilmenite + quartz, one is an orthoamphibolite with the assemblage garnet + orthopyroxene + biotite + anthophyllite + plagioclase + ilmenite + rutile + quartz, and one is an amphibolite with an assemblage of garnet + biotite + hornblende + plagioclase + ilmenite + quartz. Petrographic analysis in conjunction with pseudosections reveals a prograde clockwise pressure-temperature (P-T) path with peak metamorphic conditions ranging from 8.6 to 9.6 kbar and 750 to 840 °C. The P-T path for the metapelitic rocks went through the kyanite stability field before stabilizing within the sillimanite stability field. A lack of a significant change in P-T conditions from core to flanks suggests that the metamorphic evolution of these rocks is not consistent with the gneiss dome hypothesis. When the metamorphic conditions of the southern Highland Mountains are compared to those of the surrounding mountain ranges (the Tobacco Root Mountains, Ruby Range, and Gravelly Range), a succession of progressively decreasing pressures and temperatures is observed from north to south. This succession suggests that the metamorphic rocks in these mountains ranges represent a nearly continuous stratigraphic exposure of rocks from the metamorphic core of a regional scale thermotectonic event called the Big Sky orogeny