PETROGRAPHY OF BIOTITE GNEISSES FROM THE WALHALLA NAPPE, INNER PIEDMONT, NORTHWESTERN SC

Mapping by Furman faculty and students in a nine-quadrangle area in northwestern South Carolina reveals several varieties of Paleozoic biotite gneiss occurring in the Walhalla thrust sheet and Chauga belt. Gneisses exhibit a limited range of textural and mineralogical characteristics, and the purpose of this study is to distinguish among them as mappable units. Thin section petrography of eleven biotite gneiss samples shows that the mineralogy in each rock is dominated by the assemblage microcline, plagioclase, and quartz. Some gneisses contain microcline augen with myrmekite rims. All of the samples have significant amounts of biotite (4-16%) and myrmekite (6-11%); accessories include minor muscovite (1-5%), titanite, and epidote; and trace amounts of zircon, allanite, garnet, iron oxides, and apatite.

There are at least three distinct groups of biotite gneiss, based on texture, mineral proportions from point counts, and An content of plagioclase from microprobe studies. Henderson Gneiss from Sassafras Mountain is porphyroclastic with 1-2 cm augen and a fine matrix (0.2 mm). It contains about 28% plagioclase (An24-26), 21% microcline, 19% quartz, and 13% biotite. It is mylonitic and distinguished by: 1) large, well developed augen that show distinctive tails; and, 2) its higher mafic content relative to the other gneisses in the area. A second group of porphyroclastic gneiss, with 0.5-1 cm augen and a fine-crystalline matrix (0.2 mm), has about 30% plagioclase (An18), 31% microcline, 28% quartz, and 11% biotite. It can be distinguished by its fine foliation, lower mafic content, and smaller augen. A third gneiss group is medium crystalline (1 mm) with about 40% plagioclase (An20-22), 20-25% microcline, 19% quartz, and 4-9% biotite. Greater plagioclase content and the presence of poorly developed augen generally lacking tails are the distinguishing features of this gneiss. Other biotite gneisses to be investigated contain no augen. Future studies will focus on microfabrics and gather more point count data, in order to better characterize the differences among similar biotite gneisses.