GEOCHEMISTRY, PETROGRAPHY, AND MINERAL CHEMISTRY OF AMPHIBOLITES FROM THE INNER PIEDMONT OF NORTHWESTERN SOUTH CAROLINA

The Inner Piedmont of northwestern South Carolina consists of a series of stacked thrust sheets, including the Six Mile thrust sheet, the Walhalla nappe, and the Chauga belt. Although biotite gneiss is the dominant lithology, amphibolite units are common in all three thrust sheets. A total of ten major element chemical analyses of amphibolites taken from all three thrust sheets revealed them to be mostly basaltic in composition, with two samples being more andesitic in nature. On the alkali/silica diagram of MacDonald and Katsura (1964) six samples plotted as tholeiites with the other three lying just in the alkali basalt field. There is no distinction among the amphibolites from the different thrusts based on whole-rock major element analysis.

Petrographic analysis of amphibolite samples in thin section showed the mineralogy to consist of amphibole, plagioclase, epidote, and quartz with minor biotite, myrmekite, and iron oxides. Samples show good foliation with a tendency towards amphibole-rich and plagioclase/epidote-rich layers. Thin quartz veins are common. Samples displayed a variation in the type of amphiboles present, with some samples showing two amphibole phases. Microprobe analysis of amphiboles permitted chemical classification of the various amphibole phases, which include actinolite, actinolitic hornblende, edenite, edenitic hornblende, and magnesio-hornblende. Magnesio-hornblende and edenite were the most abundant phases. Actinolite and actinolitic hornblende are restricted to the Chauga belt, which reflects the lower metamorphic grade of that belt. There are no other aspects of mineral chemistry that distinguish samples of the three thrust sheets. Plagioclase composition showed minimal variation within samples but varied widely among samples. Composition ranged from An 22 to An 82, with most plagioclases falling in the range An 22 to An 39. Analysis of additional samples in addition to trace element studies will be conducted in an attempt to more clearly define the geochemical differences among amphibolites of these thrust sheets.