EXTRACTING CHANGES IN METAMORPHIC CONDITIONS FROM COMPOSITIONAL ZONING, INCLUSION DISTRIBUTION PATTERNS, AND TEXTURAL CLUES IN GARNET PORPHYROBLASTS FROM GARNET HILL, NORTHWESTERN GEORGIA

The spatial distribution of mineral assemblages, compositional zoning, and porphyroblast growth textures are strong indicators of changing conditions during progressive metamorphism. These characteristics have been studied for large porphyroblastic garnets from the Garnet Hill locality in Paulding County, northwestern Georgia, in order to better constrain P-T-D-X evolution in the area during orogenesis of the Southern Appalachians. The rocks at Garnet Hill were produced by metamorphism of hydrothermally-altered basalts, and are adjacent to massive sulfide deposits and banded iron formation. The dominant rock-forming mineral assemblage includes extremely coarse-grained (>1cm) garnets in a very fine-grained matrix of chlorite, magnetite, ilmenite and quartz. Thin-section petrographic, back-scattered electron (BSE) image and semi-quantitative SEM-EDS analysis of serial central and non-central sections through garnet crystals were performed to document core-to-rim changes in mineral assemblages, major element compositional zoning, and growth textures. The spatial distribution of phases included within garnet, particularly sulfide- and phosphate-bearing accessory minerals, allows identification of changes in mineral stability via metamorphic reactions due to changes in pressure, temperature and/or fluid composition. The garnets are very iron-rich and the concentrations of iron and manganese tend to increase towards the core and decrease toward their rims, while the concentration of calcium tends to increase towards the garnet’s rim. Core-to-rim changes in inclusion density are also observed: garnet cores contain high concentrations of quarts inclusions that define an internal fabric; towards the rim, the garnets contain far fewer inclusions (mainly ilmenite); the outermost garnet rims are extremely skeletal and appear to be replacing chlorite in the matrix. These changes are indicative of either changes in growth rate, changes in nutrient supply, multiple episodes of garnet growth, or a combination of these factors.