AN UNUSUAL ARCHEAN CORDIERITE ORTHOPYROXENE ROCK (COR) FROM THE EASTERN BEARTOOTH MOUNTAINS (MT, USA): INSIGHTS INTO A COMPLEX METAMORPHIC HISTORY

Metamorphic xenoliths from the eastern Beartooth Mountains (MT, USA) of the Archean Wyoming Province preserve evidence of a complex metamorphic history. Metamorphic conditions are not uniformly constrained among individual xenolithic lithologies due to regional polymetamorphism associated with emplacement of enveloping ~2.8 Ga granitic continental arc rocks. An unusual cordierite-orthopyroxene rock (COR) xenolith from the Quad Creek plateau provides mineralogical, chemical, and textural data that defines a granulite facies assemblage overprinted by an upper amphibolite facies hydration event. Detailed petrography paired with Ti-in-quartz (TitaniQ) and Zr-in-rutile (Zrt) thermobarometry, as well as pseudosection modeling in the NCKFMASHTO system, constrain the high-grade conditions preserved in the Quad Creek COR. Rutile (or ilmenite)-quartz-biotite-orthopyroxene-cordierite-plagioclase (trace) +/- sillimanite constitutes the observed granulite facies assemblage. Plagioclase grains are rare and are restricted to biotite and quartz grain boundaries. Anthophyllite occurs in all samples solely along orthopyroxene grain boundaries and fractures and is interpreted to reflect an upper amphibolite facies overprint. TitaniQ and Zrt isopleths from matrix grains intersect at approximately 5-6 kbar and 775-800 °C, likely representative of granulite facies peak conditions. The inferred peak assemblage: rutile-quartz-biotite-orthopyroxene-cordierite-melt occurs along the solidus (approximately 800-830 °C) in pseudosection models. Onset of melting is interpreted from the presence of plagioclase as an apparent wetting phase along quartz and biotite grain boundaries. Anthophyllite after orthopyroxene overprinting is constrained to 600-750 °C. Overprint pressures are more ambiguous, but were likely lower than granulite pressures, possibly ~4-5 kbar near the modeled rutile-ilmenite reaction boundary in pseudosection diagrams. These data indicate that the Quad Creek COR experienced granulite facies peak conditions of ~5-6 kbar and 800 °C; later overprinting occurred under amphibolite facies conditions. These findings are consistent with an overall clockwise pressure-temperature path experienced by neighboring xenolithic lithologies in this important Archean terrane.