Rapid High-T Decompression Recorded by Archean Granulites in the Northern Wyoming Province: Insights from Petrological Modelling

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This study places new constraints on the pressure–temperature (P–T) path and duration of high-temperature (HT) metamorphism recorded by Archean granulite facies metasedimentary rocks from the northern Wyoming Province in the eastern Beartooth Mountains, MT and WY, USA. These rocks exist as m- to km-scale xenoliths within a c. 2.8 Ga calc-alkaline granitoid batholith. Different interpretations of the timing of HT metamorphism relative to batholith intrusion in previous works have led to ambiguity over the mechanism by which these rocks were heated (i.e. batholith intrusion v. a later, cryptic event). The P–T path recorded by these rocks and the duration of this path may be indicative of the heating mechanism but are not currently well constrained. Here, we combine phase equilibria thermobarometry and diffusion modelling of major element zonation in garnet in order to constrain the P–T path of HT metamorphism and the durations of different parts of this path. It is shown that these rocks record a tight, clockwise P–T path characterized by near-isobaric heating at ~6.5–7 kbar to ˜770–800°C, HT decompression to ~6 kbar, 780–800°C, followed by limited decompression while cooling. Diffusion modelling of major element zonation in garnet suggests that HT decompression was brief (likely <1 >Ma), and that cooling rates following this decompression were on the order of 10–100°C/Ma. Substantial changes in apparent thermal gradient along this P–T path indicate that the rocks record a significant but short-lived thermal anomaly that occurred in the Wyoming mid-crust in the Late Archean.