A pulse of cryptic granulite-facies metamorphism in the Archean Wyoming Craton revealed by Sm–Nd garnet and U–Pb monazite geochronology

Abstract

Granulite-facies metamorphism of large tracts of continental crust is a characteristic feature of Archean cratons, where metamorphism ultimately led to crustal stabilization and chemical differentiation. Constraining the timing and duration of this metamorphism can provide insight into the processes behind Archean cratonic evolution, and here we present new Sm–Nd garnet ages from granulite-facies metasediments from the eastern Beartooth Mountains of the Wyoming Craton. The eastern Beartooth Mountains are dominantly comprised of a ∼2.8Ga calc-alkaline batholith known as the Long Lake Magmatic Complex (LLMC). Within this batholith, numerous cm- to km-scale granulite-facies metasedimentary xenoliths and roof pendants have previously been interpreted to represent metamorphism due to contact heating with the LLMC or prior to LLMC emplacement. Garnet ages from five granulite-facies metasediments range from 2669±48Ma to 2681±58Ma: significantly younger than the LLMC and consistent with newly described field relations. Preserved major and trace element garnet zoning suggest that these dates are biased towards late stage garnet growth during granulite-facies metamorphism, and that this metamorphism post-dates LLMC emplacement. U–Pb monazite petrochronology of a metasediment and an tonalite reveals distinct periods of monazite (re)crystallization at 2.79–2.78Ga (consistent with LLMC emplacement) and at 2.69–2.67Ga (coincident with the garnet ages). The Sm–Nd bulk garnet ages exhibit larger errors and high MSWDs, which may result from sampling a polymetamorphic population, as suggested by REE zoning in garnet and supported by the U–Pb monazite data. Diffusion modeling of preserved major element garnet zoning suggests that the timescale at near-peak temperatures during the 2.69–2.67Ga event was <1Ma. Phase equilibria modeling suggests that the LLMC would have experienced minimal partial melting during this cryptic granulite-facies metamorphism, implying that large volumes of crust may have experienced a significant thermal event, but that only some parts of the crust record strong evidence for it. The ages presented here are similar to known granulite-facies metamorphic and mafic magmatic events in other parts of the Wyoming Craton, though no clear evidence of mafic magmas is present locally.