PETROLOGICAL, GEOCHRONOLOGICAL, AND GEOCHEMICAL ANALYSES OF PRECAMBRIAN METASEDIMENTARY ROCKS IN THE CENTRAL GRAVELLY RANGE, SOUTHWEST MONTANA

Precambrian pelitic schists, quartzites, and phyllites in the central Gravelly Range of southwest Montana are conducive to geochemical, geochronological, and petrological analysis to determine the pressure-temperature path and time of metamorphism, and to constrain the tectonic history of the area. The study area contains both high- and low-grade metamorphic rocks within the span of a few kilometers; aluminosilicate-bearing schists and medium-to-fine-grained quartzites are located in the southern part of the study area, while both quartzites with remnant cross-bedding and very fine-grained phyllites are present in the structurally higher northern part. Geochemical and petrological analyses address whether these rocks were juxtaposed by a fault, an abnormal pressure-temperature gradient, or represent a metamorphosed depositional unconformity. Pelitic schists near the south end of the study area contain evidence of multiple aluminosilicate polymorphs, which constrain the P-T path of this part of the study area.

40Ar/39Ar dating of muscovite and biotite in a pelitic schist from the southern part of the study area demonstrates the extent to which the Big Sky orogeny affected the study area. The Big Sky orogeny was a 1.78 to 1.71 Ga upper amphibolite facies metamorphic and tectonic event preserved within the northwest Wyoming province. Big Sky metamorphism in the Tobacco Root Mountains, just to the north of the Gravelly Range, is characterized by a counterclockwise P-T path including a phase of isobaric cooling and rapid unroofing at the end of the orogeny. Juxtaposition of rocks in the study area with the core of the Big Sky orogen may have thermally reset the rocks in the study area and may also have influenced cooling in the Big Sky orogen after the peak metamorphism in its core at 1.78 Ga.