Multiple phases of high-temperature metamorphism recorded in Archean continental crust: A record from polyphase garnet growth

High-grade metamorphic rocks can record the dynamic processes that lead to crustal heating and result in a departure from normal crustal geothermal gradients. High temperatures in the Archean crust led to particularly significant melt generation and cratonic stabilization, and understanding the depths, temperatures and rates of Archean metamorphism may reflect our clearest window into possible tectonic styles at this time. However, several Archean metamorphic terranes record polymetamorphism and unravelling the pressure-temperature-time (P-T-t) histories of such terranes has proven difficult, with complexity inherent in both chronologic and petrologic data.

Here we synthesize recent results from Archean granulites from the Beartooth Mountains in the northern Wyoming Province. A multi-analytical approach involving geochronology, thermodynamic and diffusion modeling has been applied to metasedimentary samples that occur as m- to km-scale xenoliths in a 2.83-2.79 Ga granitoid batholith. Results based on field relations and monazite and whole-crystal garnet geochronology suggest that a phase of syn-batholith-emplacement metamorphism at 2.79 Ga was followed by a distinct, second phase of granulite-facies metamorphism at 2.70-2.68 Ga. Zoned garnet geochronology from pelitic meta-sediments further constrains core Sm-Nd ages of 2.76 Ga and rims of 2.71 Ga. Rims have elevated Ca, Sm, and Eu concentrations and are interpreted to record biotite breakdown and melt generation during the second metamorphic event, at peak temperatures of 750˚C. Diffusion models imply that this later stage was brief: near-peak temperatures were maintained for < 1 Myrs. In contrast, core and rim dates of garnet from a meta-granitoid from the same outcrop record only the initial phase of growth, most likely because a lack of grain boundary fluids inhibited further crystallization in these rocks. Evidence for this second event is cryptic in other granitoid samples, such that this period of heating to at least 750˚C some 50-100 Myrs after initial batholith emplacement is poorly recorded in the broader rock record of the Beartooths. Its extent and significance for cratonic assembly and stabilization thus remain unclear, despite its synchronicity with metamorphism and magmatism in other portions of the Wyoming Province.