METAMORPHISM OF MYLONITIC SILLIMANITE-GARNET-BIOTITE-K-SPAR GNEISS IN THE HIGHLAND MOUNTAINS OF SOUTHWEST MONTANA

The Highland Mountains are immediately west of the Tobacco Root Mountains (TRM). The Precambrian basement rocks in the Highland Mountains consists of a doubly plunging antiform cored by quartzofeldspathic gneiss mantled by a characteristically mylonitic sillimanite-garnet-biotite-k-spar gneiss unit (O’Neill 1996). The base of the mantling unit is typically migmatitic, extremely coarse grained and contains additional lithologies including various types of amphibolite and marble. Locally garnetiferous clinoamphibolite occurs as layers throughout the mantling unit. The quartzofeldspathic gneiss in the Highlands is very similar to rocks found in the TRM, whereas the sillimanite-garnet-biotite-k-spar-gneiss unit is lithologically distinct from TRM units. Previous work in the TRM identified a major orogeny between 1.78-1.81 Ga, which, in the TRM, is characterized by a clockwise P-T path with peak pressures of 0.8-1.0 kb followed by peak temperatures of 700-850 °C and then by isobaric cooling and finally by an isothermal decompression event. Previous geochronology studies in the mantling unit of the Highland Mountains yielded ages between 1.84 and 1.75 Ga, suggesting that this same orogenic event or a closely related event also affected the sillimanite-garnet-biotite-k-spar gneiss.

The mantling unit and intercalated garnet-bearing amphibolites were sampled in the summer of 2009. The samples from the mantling unit contain the equilibrium mineral assemblage of biotite + sillimanite +garnet + k-feldpsar + plagioclase + quartz. This mineral assemblage allows for the calculation of the pressure and temperature conditions under which the rocks formed and can also be used to constrain the P-T evolution of the rocks. Geothermobarometry calculations from the garnet amphibolite samples provide an independent test on these P-T paths. Monazite from these samples provide the age of metamorphism. Studying this unit better constrains the Paleoproterozoic metamorphic history of the Highland Mountains and its relationship to the orogenic event documented in the Tobacco Root Mountains.