CONSTRAINING PROGRADE METAMORPHIC CONDITIONS IN ARCHEAN HIGH-GRADE GARNET-BEARING LITHOLOGIES FROM THE EASTERN BEARTOOTH MOUNTAINS, MONTANA, USA

High-grade garnet- and orthopyroxene- bearing lithologies from the eastern Beartooth Mountains of Montana provide valuable insights into the prograde evolution of this 2.8 Ga Archean terrane. Based on a variety of geothermobarometers, peak metamorphic conditions of these rocks have been constrained to 795 +/- 42°C and 7.0 +/- 0.9 kbar. However, conditions along the prograde path of metamorphism are equivocal. Garnet textural and compositional zoning patterns and compositions of garnet and orthopyroxene mineral inclusions (e.g. biotite, plagioclase, quartz and amphibole) provide a means to investigate pressure, temperature, and fluid conditions of entrapment during host mineral growth. Lithologies of interest include peraluminous migmatites and gneisses, mafic granulites and iron formations. Some garnets exhibit textures indicative of multistage growth with abundant quartz inclusion-rich cores that are consistent with uniform prograde growth in addition to distinct post deformational garnet rims. In most garnets compositional zoning is minor with slight Mg-Fe exchange near the rims. The Ti-in-biotite (TIB) thermometer is particularly useful to investigate relative changes in temperature of included vs. matrix biotites because the thermometer is only weakly related to Mg-Fe exchange with garnet. Biotites included in the cores of multistage garnets typically yield TIB temperatures 30-40°C higher than matrix biotite suggestive of an earlier higher temperature stage of metamorphism. Garnets with uniform prograde growth textures trap biotites with TIB temperatures 20-30°C lower than matrix biotite. Post-deformational garnets have TIB temperatures essentially the same between inclusion and matrix biotites. Within the high-grade iron formations, biotite and amphibole inclusions within garnet and orthopyroxene exhibit Cl-rich compositions, but matrix biotite and amphibole have significantly higher Cl contents suggestive of a progressively more saline environment with progressive metamorphism. With these textural and chemical frameworks in place, quartz-in-garnet mineral inclusion laser-Raman elastic geobarometry will further constrain prograde conditions and the early tectonic history of this Archean terrane.