Rapid high‐T decompression recorded by Archean granulites in the northern Wyoming Province: Insights from petrological modelling

Abstract

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.

The versatility of petrological modeling: Thermobarometry of high‐pressure metabasites from the Renge and Sanbagawa belts and phase evolution during warm subduction at Nankai

“…By working backward, the P-T conditions can be refined by; for example, comparing measured versus predicted mineral compositions and proportions within a calculated field of interest (Powell & Holland, 2008. Examples in the literature applying phase equilibrium modeling to do thermobarometrical calculations are vast (Baziotis et al, 2017;Cao et al, 2019;Dragovic et al, 2020;Groppo et al, 2015;Guevara et al, 2017;Gutiérrez-Aguilar et al, 2021;Hern andez-Uribe et al, 2019;Klonowska et al, 2017;Li et al, 2020;Massonne, 2015;Palin et al, 2012;Štípsk a & Powell, 2005;St-Onge et al, 2013;Wei et al, 2013;Weller et al, 2015; among many others). In the case of forward modeling, petrological calculations can be performed along a sequence of P-T points to determine paragenesis along a given P-T-X path.…”

Multi‐mineral petrochronology on a high‐pressure mafic granulite reveals short‐lived high‐temperature metamorphism in the North China Craton

“…Accordingly, the duration of the high‐temperature metamorphic conditions must have been relatively short (less than ~4 Myr), with subsequent exhumation (and cooling) being very rapid. A local and transient heat source rather than conductive heating is likely required to produce such short‐lived metamorphism (Guevara et al, 2017), indicating a temporary thermal pulse or punctuation of a longer‐lived metamorphic event.…”

“…The possibility that mafic intrusions derived from relatively shallow crustal levels, unrelated to the large‐scale orogenic evolution of the Khondalite Belt, could not be ruled out. Nevertheless, a temporary heat input may result from the emplacement of smaller magmatic bodies on a local scale, associated with larger scale underplating in the lower crust (Ague & Baxter, 2007; Guevara et al, 2017). We thus conservatively suggest that the random distribution of mafic intrusions within the Khondalite Belt, and the rare occurrence of mafic intrusions in the Yinshan Block, reflect localized magmatic underplating or intrusion during the extension following collision between the Ordos and Yinshan Blocks.…”

The metamorphic PT history of Precambrian Belomorian eclogites (Shirokaya Salma), Russia

“…Meanwhile, considering possible ion exchange reactions, amphibole decomposition, and/or partial melting in the matrix, the increase of Grs at the garnet rim may alternatively implicate a temperature increase with pressure decrease (e.g. Faryad et al., 2018; Guevara et al., 2017; Lang & Gilotti, 2007).…”

“…Guevara et al. (2017) conclude an exhumation rate of ~1–10 mm/year from modelling garnet zonal diffusion and argue for <1 Ma high‐ T decompression for the Archean granulite in the Wyoming Province. Similarly, Baldwin et al.…”