METAMORPHIC EVOLUTION OF THE GREAT SMOKY MOUNTAINS SURROUNDING THE ELA AND BRYSON CITY DOMES, NORTH CAROLINA

Field mapping, microstructural analysis and electron microprobe analysis were performed on the Great Smoky Group (GSG) rocks surrounding the Bryson City and Ela domes, North Carolina, to help constrain the tectonic history of the region. The domes are en echelon northeast-trending antiformal structures that are cored by Grenvillian basement and bounded by the Greenbrier fault, which places GSG rocks directly over Grenvillian basement. A Taconic (~450 and 459 Ma) Barrovian metamorphic field gradient occurs across the Great Smoky Mountains that increases from greenschist facies rocks in the northwest to amphibolite facies rocks in the southeast (Hadley and Goldsmith, 1963). The GSG rocks surrounding the Bryson City and Ela domes are within the kyanite zone. Kyanite and staurolite in cover rocks are parallel to the main Taconic foliation (S₂) but lack a lineation. These porphyroblasts grew during M₂ and are correlative to the originally mapped isograds. Petrographic analysis of GSG rocks adjacent to both domes indicates that the rocks underwent multiple stages of metamorphism and deformation. Inclusions are concentrated in the core and outer rim of garnet porphyroblasts. Compositional maps of garnet yield Ca and Mn zoning patterns that are consistent with multiple stages of garnet growth. Preliminary pressure-temperature estimates were calculated using THERMOCALC and yielded 641°C ±64 and 9.7 kbars ±1.1 for a staurolite-kyanite schist. Randomly oriented sillimanite needles are arranged in bundles and occur along feldspar grain boundaries. Sillimanite occurs throughout the kyanite zone in the Great Smoky Mountains and has been correlated with an early M₃ metamorphic event (Massey and Moecher, 2005). Kyanite alteration to sericite or muscovite is interpreted as a post-kinematic M₄ retrograde metamorphism. Replacement of biotite and garnet with chlorite are further evidence for a post-kinematic M₄ event.