Celebration of Scholarship and Creative Activity 2021
The Penokean orogenic belt represents accretionary growth of North America in the Proterozoic ~1.8 billion years ago. Rock thin section petrography and geochemical analysis of metamorphic garnet-kyanite schists from the Watersmeet Terrane of northern Wisconsin was used to determine the pressure and temperature conditions for Penokean metamorphism. Accessory rutile grains are found in the matrix and as inclusions within kyanite, garnet, and staurolite. To determine peak temperatures of the rocks zirconium concentrations in rutile were analyzed using an electron probe. Zirconium concentrations between 186-385ppm yield temperatures that range between 577- 631℃. Rutile inclusions in minerals can constrain the formation temperature of that mineral. For example, in one rock, rutile grains in garnet averaged 604.9°C, the inferred temperature of formation for that mineral. Corona textures (rims) of plagioclase partly to wholly surrounding kyanite grains. This unique texture could be explained by a kyanite consuming reaction during decompression of the rocks. Likely, a pressure and temperature loop occurred where the rocks were heated and compressed through the staurolite zone into the kyanite zone, and then decompressed during cooling below the kyanite zone. The results indicate a metamorphic pressure and temperature path that is indicative of rapid exhumation.