Deciphering the tectonometamorphic evolution and localization of deformation within the Wyoming Province of southwest Montana

The Archean Wyoming Province is a classic example of cratonic lithosphere within the North American continental core and is fundamental to understanding of processes controlling the stability and longevity of continental crust. We present details concerning the evolution of the Absaroka-Beartooth mountains in Montana. Exposures document five fabric generations. S ₁ is defined by rootless isoclinal veins and microlithon domains preserved as inclusion trails. S ₂ is characterized by a well-developed foliation that is axial planar to F ₁ . S ₃ and S ₄ are defined by two distinct steeply dipping cleavage orientations associated with a dome and basin interference pattern in S ₂ . S ₅ is locally developed as a moderately northwest dipping penetrative fabric associated with the Snowy shear zone (SSZ). The SSZ is traditionally interpreted as a Proterozoic normal sense mylonite zone, however, temporal and kinematic constraints are limited. Hanging wall exposures of the SSZ preserve a peak metamorphic assemblage of garnet+staurolite+biotite, which formed syn-to-post S ₂ . Thermodynamic modelling of the peak metamorphic assemblage indicates temperatures between 600°C and 700°C, with pressures between 500 MPa and 1000 MPa. THERMOCALC AvPT estimates of peak metamorphic conditions indicate temperatures of 630 ± 91°C and pressures of 830 ± 220 MPa. Low-strain rocks of the SZZ locally contain chlorite+muscovite pseudomorphs after staurolite, which suggests static hydration during retrograde metamorphism. Hydration is likely associated with deformation along the SSZ and development of S ₅ . Pseudomorphs of chlorite+muscovite have been sampled by microdrill and pending ⁴⁰ Ar/ ³⁹ Ar ages should record the timing of retrograde hydration. Although published ages from nearby higher-grade fabric forming micas are Proterozoic, we hypothesize that the replacement of staurolite occurred in the Cenozoic based on paleogeographic evidence for syn-depositional rotation of Eocene volcaniclastic rocks bounded by the SSZ. Kinematic constraints from electron backscatter diffraction (EBSD) data suggest deformation at moderate temperatures (~450°C) with top to the west directed shear. Our results help clarify the temporal, spatial, and kinematic significance of deformation and metamorphism within the Wyoming Province.