EVALUATION OF MICROSTRUCTURES, MINERAL CHEMISTRY, AND PRELIMINARY THERMOBAROMETRY FOR THE KINNARD CREEK FAULT, CENTRAL GEORGIA

The Kinnard Creek fault in the Inner Piedmont Cat Square terrane near Jackson, Georgia, contains porphyroclastic garnet, amphibole, plagioclase, and biotite in a sheared gneiss. Petrographic data shows that large (up to one centimeter diameter) garnet and amphibole crystals were affected by shearing, producing recrystallized sigma tails, strain shadows, and brittle fracturing within crystals. Biotite, plagioclase, K-feldspar, and quartz are the dominant matrix minerals and show signs of high-temperature recrystallization. Plagioclase and biotite are also present as inclusions within the garnet and amphibole. In this study, quantitative chemical data obtained with the electron microprobe were used to determine compositional variations within these minerals and to determine equilibrium mineral assemblages and compositions. These were then applied to a suite of thermobarometers in an attempt to better constrain the pressures and temperatures through which the rock evolved; different P-T conditions for inclusion and matrix assemblages helps to quantify the pressure-temperature-time (PTt) path.

Analysis of chemical data established that the composition of the earliest growth of garnet, as located by peak spessartine content, was dominantly almandine-grossular; almandine component increases in garnet rims to > 60%. Elevated spessartine content in garnet rims may result from partial resorption. Biotite and amphibole are both iron-rich. The compositions of the plagioclase inclusions within garnet are slightly less albitic than the matrix plagioclase (An₂₈ vs An₂₆). Mineral assemblages and compositions that were evaluated to be at equilibrium based on textural relationships were used as input for several thermobarometers; preliminary results suggest peak temperatures in the upper-amphibolite facies at ~700 °C. These temperatures are consistent with observed assemblages and microfabrics. Additional samples and thin-sections will be evaluated to further constrain the PTDt evolution of the Kinnard Creek fault.