Archean cordierite-orthopyroxene rocks from the eastern Beartooth Mountains, Montana, USA: insights into the local metamorphic pressure–temperature history

Abstract The metamorphic history of the eastern Beartooth Mountains of Montana in the northern Archean Wyoming Province is challenging to establish given the multi-staged nature of metamorphism in the area and the prevalence of metamorphic overprinting. A petrologically illustrative cordierite-orthopyroxene xenolith in ∼2.80 Ga calc-alkaline granitic igneous rocks exhibits a mineral assemblage responsive to multiple metamorphic overprints. Cordierite-orthopyroxene and/or -orthoamphibole rocks (CORs) have mineral assemblages that provide instructive information on the metamorphic development of the terranes and the processes that form their protoliths. This investigation combines detailed petrographic and mineral-chemical investigations with petrological modelling of this COR locality, to establish important aspects of the local pressure-temperature ( P-T ) history. The COR lithology, found in the Quad Creek locality of the study area, preserves textural evidence for three discrete metamorphic events consisting of a granulite-facies event (M1) that is overprinted by two, lower-grade hydration events: an upper amphibolite-facies event (M2) and a greenschist-facies event (M3). The M1 event is characterised by the mineral assemblage rutile-biotite-orthopyroxene-cordierite-quartz-melt ± sillimanite that equilibrated at ∼0.60 GPa and 775–815°C. M2 overprinting is marked by partial hydration of orthopyroxene to anthophyllite, crystallisation of plagioclase from a putative melt and ilmenite–rutile co-existence. The final metamorphic stage, M3, is marked by local hydration of orthorhombic inosilicates to talc and cordierite to pinite (chlorite). Estimates of M2 P-T conditions suggest development below ∼0.40 GPa at 615–725°C, whereas M3 probably developed below 0.40 GPa and 500–600°C. The resultant clockwise cordierite-orthopyroxene P-T path resembles modelled metamorphic P-T paths in proximal xenolithic lithologies. This interpretive P-T history is consistent with metamorphism in an Archean continental arc subduction zone with M1 metamorphism due to compression and tectonic burial, M2 due to hydration following, or coeval, with emplacement of voluminous ∼2.80 Ga plutons and M3 due to later low- T fluids. Although not conclusive, the Quad Creek COR was probably derived from a metasomatised basaltic protolith from the Archean seafloor, a comparable mode of origin to many COR occurrences worldwide.