NEOARCHEAN POLYMETAMORPHISM IN THE WYOMING PROVINCE RECORDED BY ZONED GARNET PETROCHRONOLOGY

High temperature (HT) metamorphic rocks can record the dynamic processes that lead to crustal heating and departures from normal crustal geotherms. High temperatures in Archean crust led to significant degrees of melt generation and cratonic stabilization. Thus, understanding the depths, temperatures and rates of Archean metamorphism may reflect our clearest window into possible Archean tectonic styles. A complicating factor is that most Archean terranes record polymetamorphism. Unraveling the P-T-t histories of such terranes has proven difficult, with complexity inherent in both chronologic and petrologic data.

Here we present results from granulites of the Beartooth Mountains in the Wyoming Province. A multi-analytical approach involving garnet geochronology, thermodynamic and diffusion modeling has been applied to lithologically distinct Archean granulites from the same outctop that occur broadly as m- to km-scale xenoliths in a ~2.83-2.79 Ga granitoid batholith, and shows that they preserve different records of a shared metamorphic history. Field relations coupled with geochronology suggest that a phase of syn-batholith-emplacement metamorphism at ~2.79 Ga (M1) was followed by a distinct, second phase of HT metamorphism at ~2.70-2.68 Ga (M2).

Metapelitic garnet records both events. ~2.76 Ga cores, enriched in HREE, constrain the timing of growth initiation. ~2.71 Ga rims enriched in Ca, Sm, and Eu constrain growth during M2 biotite breakdown and melt generation at ~750˚C. Diffusion models imply that this later stage was brief: near-peak temperatures were maintained for < 1 Myrs. In contrast, garnet dates from a meta-granitoid record only M1 growth, most likely due to a lack of grain boundary fluids inhibiting further crystallization.

Evidence for the second event is cryptic in other granitoids, such that this period of heating, ~50-100 Myrs after initial batholith emplacement, is poorly recorded in the broader Beartooth rock record. However, a growing record is available from supracrustals. ~2.7 Ga metamorphism is well known in other regions of the Province, and here we propose a link that implies a regional-scale thermal event at that time. Good evidence for this extreme heat flux comes from widespread suites of mafic magmas and granulites from the Stillwater Complex, Teton, and Wind River Ranges.