Petrology of the contact metamorphic rocks beneath the Stillwater Complex, Montana : conditions and assemblages of metamorphism

The Archaean metasedimentary rocks beneath the Stillwater Igneous Complex, southwestern Montana, preserve a nearly continuous record of low-pressure fades types ranging from pyroxene hornfels through amphibole hornfels to regionally metamorphosed schists with decreasing proximity to the Stillwater contact. The pyroxene hornfels fades is characterized by the assemblage hypersthene + cordierite + garnet whereas the amphibole hornfels fades assemblages are gedrite + garnet + staurolite + cordierite, garnet + cordierite + staurolite, staurolite + anthophyllite + cordierite, and sillimanite + staurolite + cordierite. The schistose rocks contain lowgrade assemblages of garnet + chlorite and andalusite + chlorite + staurolite. Due to the lack of formational names, the term Boulder River Complex is introduced to describe the metasedimentary rocks beneath the Stillwater Complex. The Boulder River Complex is composed of four mappable units: the Stillwater iron-formation, Stillwater "Blue" Quartzite, Stillwater hornfels, and the Boulder River schist. The occurrence of four-phase assemblages in the amphibole hornfels zone suggests that the chemical potential of H20 was buffered by the rock. Isograds were mapped on the basis of the coexistence of garnet + chlorite, amphibole + staurolite + garnet + cordierite, cordierite + cummingtonite, and hypersthene. The reactions involving chlorite are 1) garnet + chlorite = anthophyllite + staurolite + H20 and 2) chlorite = staurolite + anthophyllite + cordierite + H20. The cummingtonite + cordierite isograd was mapped based on the following reactions: 1) staurolite + anthophyllite = garnet + cordierite + H20 and 2) garnet + anthophyllite = cummingtonite + cordierite + H20. The hypersthene isograd involves the breakdown of cummingtonite by the following reaction: cummingtonite = hyperthene + quartz + H20. Based on mineral equilibria, the pressure of metamorphism is bracketed between 2.0 and < 3.75 kilobars. The presence of pigeonite in the iron-formation at the base of the Stillwater Complex indicates temperatures near the contact were a minimum of 825°C; whereas, temperatures from garnet/biotite geothermometry at a distance of 9 km range from 430°C to 550°C . Temperature estimates, as well as textural evidence in both the hornfels and the schists, indicate that the Stillwater Complex was emplaced at a depth of 7 to 12 km where the ambient temperature was relatively high.