Deciphering Neoarchean polymetamorphism and crustal melting in the northern Wyoming Province using garnet petrochronology

High-grade metamorphic rocks can record the dynamic processes that lead to crustal heating and 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 results of a multi-analytical study in which garnet and monazite petrochronology, coupled with thermodynamic and diffusion modeling, were applied to Archean granulites from the Beartooth Mountains in the northern Wyoming Province, U.S.A. The data reveal two phases of garnet growth and high-temperature metamorphism. Garnet cores grew coeval with emplacement of a granitoid batholith at ~2.78-2.76 Ga. This was followed by a distinct, second phase of peritectic garnet rim growth at ~2.71 Ga, during biotite breakdown melting at peak temperatures of ~750˚C. Diffusion modeling of chemical zoning in garnet rims shows that this second event 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, ~50-100 Myrs after initial batholith emplacement, is poorly recorded in the broader rock record of the Beartooths.

The results of our study show that different parts of the metamorphic history of a rock may be recorded differently between garnet and accessory phases. Lastly, while field and petrologic evidence for polymetamorphism may be cryptic, direct dating of distinct garnet growth zones with preserved major and trace element zonation allows for a clear interpretation between isotopic dates and the metamorphic history of the rock.