CORRELATING DEFORMATION AND METAMORPHISM IN MYLONITIC GNEISS FROM THE KINNARD CREEK FAULT, CENTRAL GEORGIA

The Kinnard Creek fault within the Inner Piedmont Cat Square terrane of Central Georgia has been interpreted as a southwest-directed, dextral-oblique thrust fault active during Paleozoic orogenesis. Samples collected along the trace of the fault contain porphyroblastic garnet and amphibole up to one centimeter in diameter in a sheared matrix of plagioclase, quartz, biotite, potassium feldspar, epidote, and apatite. This study examines the timing relationships between deformation and metamorphic mineral growth and constrains changes in metamorphic conditions as determined by petrographic, microstructural, and chemical analyses.

The dominant foliation in these gneissic rocks is defined by aligned biotite and ribboned quartz crystals. Many feldspars show evidence for subgrain development and dynamic recrystallization. Pressure shadows, in some cases forming σ-type tails, wrap the amphibole and garnet crystals; most amphiboles exhibit signs of shear strain and recovery including undulose extinction, partial recrystallization, and extensional deformation; most shear-sense indicators are consistent with dextral shearing. These textures indicate that high-temperature deformation and recrystallization persisted after the growth of the porphyroblasts. Garnet compositions are dominantly almandine-grossular with minor spessartine component; garnet rims are slightly more iron-rich. The amphiboles are generally iron-rich, calcic hornblendes and are only weakly zoned. Plagioclase compositions consistently plot as oligoclase (~An₂₅) for both matrix feldspars, and for crystals present as inclusions within garnet. PT estimates were produced for two equilibrium assemblages/compositions: (1) Grt core + Hbl core + Bt inclusion + Plag inclusion + Qtz; and (2) Grt rim + Hbl rim + Plag matrix + Bt matrix + Qtz. Both give results consistent with upper-amphibolite facies conditions (650-700 °C, 0.7-0.9 GPa), in line with the high-temperature deformational fabrics preserved in these rocks.