INVESTIGATING AN INVERTED METAMORPHIC GRADIENT IN THE CHEYENNE BELT, SOUTHEASTERN WYOMING, USING KINEMATIC VORTICITY, THERMOBAROMETRY, AND GEOCHRONOLOGY

The Cheyenne Belt of southern Wyoming represents ca. 1750 Ma accretion of Paleoproterozoic rocks of the southwestern United States to the Archean Wyoming Craton. Previous studies describe an inverted metamorphic gradient within the upper metasedimentary Snowy Pass Supergroup in the footwall of the northern shear zone. Structurally upward across the steep, south dipping shear fabric, key assemblages in the French Slate range from Chl + Bt + Ms to Grt + Bt + Ms + St ± And/Ky, suggesting an increase in temperature of at least 100 °C over a 1 km horizontal distance. Although thrusting of a hot hanging wall onto a cold footwall has been interpreted to have initially induced the gradient, several characteristics of the gradient and aspects of the thrust-related interpretation remain uncertain. For example, the relative amounts of pure versus simple shear deformation have not been previously determined. Relative and absolute timing constraints of metamorphism and deformation within the metamorphic gradient are not well established. Reactivation of the shear zone appears to be a distinct possibility, both from previously published data and the recent discovery of ca. 1650-1630 Ma high-grade deformation and magmatism in northern Colorado.

We are employing new thermobarometry, quantitative petrologic modeling, kinematic analysis of microfabrics, and in situ monazite geochronological analyses to further evaluate this inverted metamorphic gradient. Previous peak P-T estimates for the French Slate are a minimum of 480 °C and ~0.4 GPa. Our initial estimates indicate conditions closer to 600 °C and 0.6 GPa. In the hanging wall Barber Lake block, key metamorphic assemblages are Grt + Sil + Kfs, with textures indicating incipient partial melting and estimated metamorphic conditions of ~670 °C and ~0.7 GPa. Initial kinematic vorticity estimates along a 1.5 km transect across the French Slate suggest a significant component of pure shear (40% to 65%) and a potential increase in the ratio of pure to simple shear towards the “core” of the shear zone. This suggests that post-metamorphic shortening may have significantly steepened the temperature gradient. These new data will allow further testing of models for the development and preservation of the inverted metamorphic gradient and for the evolution of the Cheyenne Belt in general.