MINERALOGY AND MICRO-FABRIC ANALYSIS OF GARNET-SILLIMANITE GNEISS AND SILLIMANITE-TOURMALINE SCHIST FROM THE WESTERN INNER PIEDMONT OF SOUTH CAROLINA

Sillimanite-bearing schist and gneiss from the Western Inner Piedmont of South Carolina are widespread in the Middle Tallulah Falls Formation of the Six Mile thrust sheet. Sillimanite schist is abundant in the Taylors 7.5-minute quadrangle near Greenville, SC where MacLean and Blackwell (2001) mapped a broad zone of sillimanite-mica schist. Less abundant garnet-sillimanite-muscovite gneisses are found in Sunset 7.5-minute quadrangle ~50 km west of Taylors quadrangle. Sunset aluminous gneisses consist of microcline, plagioclase, quartz, muscovite, sillimanite, garnet, and biotite (in order of abundance), with accessory myrmekite, epidote, zircon, and opaques. Quartz ribbons (1-3 mm) resulting from ductile deformation occur in most samples. Micas and sillimanite wrap around large plagioclase and quartz crystals. Sillimanite (5-15%) exhibits chiefly an acicular habit, but fibrolite also is present. Microcline porphyroclasts (2-4 mm) are embayed by myrmekite at their margins, and plagioclase porphyroclasts (2-6 mm) exhibit irregular fractures, reacting at their margins to muscovite. All samples exhibit a dominant foliation defined by micas and quartz-feldspar ribbons, with one sample showing a weaker secondary foliation defined by biotite.

Unusual schists from Taylors contain thin compositional layers (mm to cm scale) of recrystallized tourmaline (75-90%) and sillimanite developed along a later discrete crenulation cleavage. In thin section tourmaline (0.5-1 mm) exhibits olive green to tan pleochroism and is zoned with blue-green cores. Sillimanite needles in lenses within the tourmaline matrix were deformed by an earlier zonal crenulation cleavage. Deformation took place after peak metamorphism, and at least two cleavage-forming episodes have affected these rocks.