DEFORMATION AND METAMORPHISM IN THE FINGERVILLE EAST 7.5-MINUTE QUADRANGLE, SPARTANBURG COUNTY, SOUTH CAROLINA

New 1:24,000-scale geologic mapping and petrography of samples from Fingerville East quadrangle, Spartanburg County, SC indicates a complex metamorphic and deformational history. Three map units, stacked structurally bottom to top are: 1) Tallulah Falls paragneiss, 2) a mixed unit of schist and amphibolite, and 3) the Fingerville-Chesnee schist (informal name), separated from the lower units by a west-vergent thrust. The latter includes garnet-muscovite schist, sillimanite-muscovite schist, and muscovite schist. Tallulah Falls paragneisses include muscovite-biotite-quartz gneiss, sillimanite-biotite-quartz gneiss, and muscovite-biotite-quartz-plagioclase gneiss. The gneisses crop out along major stream drainages; Fingerville-Chesnee schist caps the higher interfluves. Brittle faults of northeast and north strike disrupt the metamorphic units. Cataclastic rock bodies strike northeast.

Fingerville-Chesnee schist samples are fine- to medium-crystalline and locally migmatitic. Schistosity locally shows evidence for strong ductile shearing and consists of the parallel alignment of muscovite ± sillimanite, quartz-feldspar layers alternating with muscovite layers, and/or polycrystalline quartz ribbons. Isoclinal folds and chevron folds (a zonal crenulation cleavage) are visible in schist hand specimen and thin section. The zonal crenulation cleavage affecting schistosity and earlier isoclines consists of muscovite ± sillimanite alternating with quartz-rich layers, representing two episodes of folding. Gneisses are dominantly quartz and feldspars, and show weak to moderate compositional layering of thin micas and/or quartz ribbons. Garnet is an accessory.

Based on thin section petrography, mineral assemblages indicate that metamorphism peaked in the first sillimanite zone of the upper amphibolite facies. However, in the garnet-bearing schists, the garnets locally are embayed and surrounded by chlorite and are breaking down by the reaction garnet + biotite + H₂0 → chlorite + muscovite + quartz. The textures indicate retrograde metamorphism associated with fluid introduction.