Paper No. 2
Presentation Time: 8:00 AM-12:00 PM
AN ASSESSMENT OF MINERAL CHEMISTRY, EQUILIBRIUM ASSEMBLAGES, MICROSTRUCTURAL FABRICS AND THERMOBAROMETRY FOR SAMPLES OF PELITIC SCHIST FROM CARROLL COUNTY, GEORGIA
In this study we identify and interpret changes in mineral chemistry and microfabrics, and assess PT conditions for samples of pelitic schist collected in southwestern Carroll County, Georgia, that are part of the Piedmont province of the Southern Appalachians. This project focuses on samples collected from two localities: (1) a coarse-grained schist collected near Tyus, and (2) a fine-grained graphitic schist collected near Roopville. The mineral assemblage of the Tyus sample from most to least abundant is: muscovite-quartz-plagioclase-garnet-biotite-chlorite-ilmenite-pyrite, with accessory apatite, epidote, and zircon. This sample has a strong foliation defined by alternating mica-rich and quartz-rich layers. Garnets in this sample are up to approximately 5 mm in diameter; some garnets are irregularly shaped due to the mineralogic layering. The mineral assemblage in the finer-grained Roopville sample is similar, but garnets are less abundant. This sample is less aluminous, lacks mineralogic banding, and the garnet porphyroblasts are much smaller (1-2 mm). A suite of petrographic thin-sections were prepared for polarized light microscopic and Scanning Electron Microscope (SEM) analysis of microstructural and mineralogic relationships; quantitative analyses by Electron Probe Microanalyzer (EPMA) were performed at the University of Kentucky. Compositional mapping reveals zoned garnets characterized by a decreasing Mn, Fe and Ca from center to rim and a rimward increase of Mg in both Tyus and Roopville samples. Inclusion assemblages within garnet include ilmenite, plagioclase, quartz, apatite, and calcite; curvature of internal fabrics and continuity with external fabrics suggests that garnet growth may have been syn-kinematic. Chlorite partially rims some Tyus garnets; late chlorite-bearing fractures in garnet also result from post-growth retrograde deformation. An increase in intensity of matrix crenulations adjacent to Tyus garnets indicates that some deformation occurred after garnet growth. Precise chemical data from bulk chemical analyses and microprobe data will be used to model equilibrium assemblages and further constrain the evolution of pressure and temperature conditions during metamorphism in this portion of the Southern Appalachians.