MICROSTRUCTURAL AND GEOCHEMICAL CONTROLS ON THE CRYSTALLIZATION OF GARNET AND STAUROLITE: AN EXAMPLE FROM THE MEGUMA TERRANE, NOVA SCOTIA

Despite the success of equilibrium thermodynamic methods for calculating the progressive stabilities of minerals during crystallization, the processes that control the shape, size, and distribution of crystallizing phases are still incompletely understood. Here, we integrate petrographic, geochemical, and microstructural data to elucidate some of the controls on metamorphic re-crystallization in a garnet and staurolite bearing pelitic rock from the Meguma Terrane, Nova Scotia. The Meguma Terrane is the most outboard terrane of the Appalachian Orogen. It underwent two stages of metamorphism during the Acadian Orogeny: a terrane-wide greenschist facies event and, in the southwest of its Nova Scotian exposure, amphibolite facies metamorphism. We study one such amphibolite facies sample, which contains primary muscovite, chlorite, plagioclase, quartz, garnet, staurolite, and ilmenite. This sample presents an excellent opportunity to study crystallization processes in part because the reactant minerals involved in the formation of garnet and staurolite are still present. Chemical zoning in garnet is broadly typical of prograde metamorphism, with a rim-wards decrease in manganese content and increases in iron and magnesium contents. Calcium zoning indicates two stages of garnet growth: a distinctly higher calcium garnet core/mantle and a lower calcium garnet rim. Phase equilibria suggest that garnet started to crystallize at ~475ºC and 3.8 kbar, with peak P-T conditions of ~550ºC and 4.1 kbar. The spatial organization and crystallographic orientation of garnet and staurolite porphyroblasts, obtained by High-Resolution X-Ray Computed Tomography (HRXRCT) and Electron Back Scatter Diffraction (EBSD) respectively, will be used to better constrain the microstructural controls on their growth, highlighting the importance of integrating textural analysis with chemical zoning in garnet and thermodynamic approaches, as eloquently advocated by Bob Tracy throughout his career.