LINKING METAMORPHIC ASSEMBLAGES TO FABRICS PRESERVED AS INCLUSIONS IN PORPHYROBLASTS: IMPLICATIONS FOR THE TECTONIC SIGNIFICANCE OF THE SNOWY SHEAR ZONE, SOUTHWEST MONTANA

The construction of P-T-D-t paths from field-based observations, petrographic studies, and quantitative compositional analyses is a fundamental component of tectonic investigations of middle to lower crustal rocks. Metamorphic and structural discontinuities across shear zones are especially informative in regions dominated by complex deformation styles and in areas that exhibit polyphase or protracted tectonic histories. The Snowy shear zone located along the western flank of the Beartooth uplift in southwestern Montana is an excellent location to investigate disparities in peak metamorphic grade and deformation style because hanging-wall exposures record peak metamorphic conditions of approximately 470ºC and 0.3 GPa, whereas foot-wall exposures record conditions of approximately 780ºC and 0.5 Gpa. The Snowy shear zone is characterized by a wide region (> 2 km) of mylonitic metasedimentary and metaigneous rocks that host a zone of retrograde greenschist facies assemblages and an extreme reduction in grain size. Lower strain domains record at least two oblique generations of folds developed on a dominate schistose foliation, both with an incipient axial planar cleavage. Mineral lineations display a range of trends from northeast to northwest with shallow and moderate plunges, respectively. Despite evidence for polyphase deformation, previous workers have attributed the Snowy shear to a single Proterozoic extensional event. In order to better constrain the tectonic significance of the Snowy shear zone we present a detailed description and analysis of biotite-staurolite schists from the hanging-wall of the Snowy shear zone that preserve quartz inclusion trails in staurolite and garnet interpreted to record at least one previously developed crenulation cleavage. Thermodynamic modeling and quantitative compositional analysis will aid in defining the P-T evolution of the hanging-wall. These results are compared with a structural analysis of the Snowy shear zone to indicate the relative timing of features with an additional focus on geochronologic targets to better constrain the absolute timing of deformation phases. This approach provides an outline for additional studies concerned with the reconciliation of P-T-D-t paths across important lithotectonic boundaries in other regions.