PROGRADE P-T PATHS OF ARCHEAN HIGH-GRADE GARNET-BEARING LITHOLOGIES FROM THE EASTERN BEARTOOTH MOUNTAINS, MONTANA, USA: INSIGHTS FROM QUIG RAMAN GEOBAROMETRY AND MINERAL CHEMICAL THERMOMETRY

Prograde metamorphic pressures and temperatures of high-grade garnet-bearing lithologies from the Eastern Beartooth Mountains of Montana are only broadly constrained relative to the well-defined peak metamorphic conditions of 795 +/- 42 °C and 7.0 +/- 0.9 kbar. Accurately determining the prograde pressure-temperature (P-T) paths of these lithologies constrains tectonometamorphic models for the evolution of the Archean (2.8 Ga) northern Wyoming Province terrane and has general implications for early Earth processes. The abundant and diverse types of mineral inclusions within garnet hosts can be used to determine estimates of entrapment conditions during prograde growth of garnet porphyroblasts. Four garnet-bearing lithologies were examined: peraluminous migmatites, garnet-biotite quartzo-feldspathic gneisses, granulite-facies iron formations, and mafic granulites. Inclusion chemical thermometry of primarily biotite inclusions coupled with quartz-in-garnet (QuiG) Raman barometry establishes prograde P-T inclusion entrapment conditions of approximately 675-775 °C and 9-11 kbar for all lithologies. These conditions generally are consistent with the upper limits of calculated peak metamorphic pressures but are marginally lower than calculated peak metamorphic temperatures. Garnet hosts generally show no apparent compositional zoning except for minor retrogressive Fe-Mg exchange along rims. Further, garnets typically display inclusion-rich cores and inclusion-poor rims. Differences between calculated prograde and peak temperatures are interpreted to represent nearly isobaric growth of garnet hosts almost at peak metamorphic pressures as temperature increased. Inclusion-deficient rims of garnet hosts indicate that near-isobaric garnet growth likely occurred in multiple stages during heating to peak temperatures. The resulting P-T paths follow a flattened clockwise path through time and are consistent with burial to mid-crustal depths (~25-30 km) before heating to peak temperatures due to widespread 2.8 Ga plutonism likely in a subduction setting comparable to modern plate tectonics.