Neoarchean polymetamorphism and crustal melting due to magmatic heat advection in the northern Wyoming Province, U.S.A: insights from petrochronology and thermal modeling

Constraining the depths, temperatures and rates of Archean metamorphism may provide a 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 (Sm-Nd) garnet and (U-Pb) monazite petrochronology, thermodynamic, diffusion, and thermal modeling were applied to Archean granulites from the Beartooth Mountains in the northern Wyoming Province. The data reveal two phases of garnet growth and high-temperature metamorphism likely driven by magmatic heat advection. 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.

We propose that emplacement of the Stillwater Complex was responsible for high-grade metamorphism at ~2.71 Ga. 1-D thermal models suggest that the P-T-t path determined from our pelitic samples can be reproduced by emplacement of a large mafic sheet with the geometry of the Stillwater ~10 km above the current exposure of the Beartooth mountains. Our work serves as a case study in which: 1) field and petrologic evidence for polymetamorphism is cryptic, but can be revealed through detailed petrochronology, and 2) rapid granulite-facies metamorphism of mid-crustal rocks was coeval with, and likely driven by, high magmatic flux during upper crustal emplacement of a potential large igneous province. Lastly, we highlight the potential challenges associated with the dating of high metamorphic grade, Archean lithologies, which include the effects of deleterious mineral inclusions, polymetamorphism and multi-stage melting episodes.