ASSESSMENT OF EQUILIBRIUM ASSEMBLAGES, MINERAL CHEMISTRIES, AND DEFORMATIONAL FABRICS ASSOCIATED WITH THE GROWTH OF CENTIMETER-SCALE GARNET PORPHYROBLASTS AT GARNET HILL, PAULDING COUNTY, WEST-CENTRAL GEORGIA

Samples collected from the Garnet Hill locality contain idiomorphic garnet porphyroblasts up to two centimeters in diameter within a fine-grained phyllitic matrix. These unusually large garnets are associated with alteration zones formed at the margins of sulfide deposits and iron formations within the Ropes Creek Metabasalt (formerly Pumpkinvine Creek), which have been interpreted to result from hydrothermal alteration of the basaltic protoliths. This study explores how changes in the conditions prevalent during metamorphism (e.g. pressure, temperature, fluid composition, deformation) are preserved as progressive changes in mineral chemistries and microstructures during core-to-rim garnet growth. Thin-sections were cut orthogonally to the dominant matrix foliation defined by laths of chlorite, and approximately through the centers of the large garnet porphyroblasts. Detailed thin-section petrographic and SEM-EDS analysis have revealed consistent variations in chemical zoning, inclusion assemblage, and microstructures in core-to-rim profiles across individual garnet porphyroblasts. Garnet cores are characterized by fine-grained inclusions of magnetite + ilmenite + quartz + apatite + zircon + xenotime. The outer edge of the garnet core is marked by quartz inclusions which preserve the idiomorphic crystal form of the earliest garnet growth. This interface is marked by a sharp drop in Mn content in the garnet and a corresponding increase in Ca content. Inclusion assemblages observed within garnet rims include magnetite + ilmenite + quartz + monazite + chlorite ± apatite; these inclusions are aligned to form an internal foliation that wraps garnet cores. The matrix assemblage includes chlorite, coarse-grained magnetite, ilmenite, and quartz. Matrix fabrics wrap around the garnet porphyroblasts, are not continuous with the internal fabrics within the garnets, and are locally crenulated. Based on these observations, an inferred metamorphic history for Garnet Hill suggests a complex interplay between multiple stages of garnet growth, phases of deformation, and changes in the effective bulk composition of the sample.