COMPARATIVE PETROGRAPHIC AND CHEMICAL ANALYSIS OF THREE ULTRAMAFIC BODIES IN THE WESTERN INNER PIEDMONT OF SOUTH CAROLINA

Three ultramafic bodies, in the Walhalla and Six Mile thrust sheets of the western Inner Piedmont of South Carolina, were studied to determine protoliths, deformational histories, and mutual genetic links. The Nine Times body (Sunset quadrangle) is < 10 km from the Salem quadrangle body, and the Landrum quadrangle body is 60 km to the northeast. The bodies are wholly or partially surrounded by amphibolite.

Petrographically, all three rock bodies are well foliated, lepidoblastic chlorite-amphibole schist. The Landrum body is cut by a NW trending fault, along which fluids were introduced, producing a secondary equigranular texture and epidote crystallization. Modally the rocks are roughly 50-60% chlorite, 40-50% amphibole, and ~10% accessory minerals, including relict olivine, epidote, pyroxene and magnetite. The highly magnesian chlorite has an average Mg# of 0.788. In thin section, the chlorite is colorless in plane light and abnormal gray-brown in cross-polarized light. The different varieties of amphiboles are optically indistinguishable, and are tabular with corroded edges and intergrown with other minerals. In thin section they are green to colorless in plane light, with maximum upper second order interference colors.

Microprobe analyses indicate the amphiboles are chemically complex. Individual samples contain up to four coexisting varieties of amphibole. The Sunset and Salem bodies are predominately tremolite, magnesio-hornblende, and magnesio-cummingtonite. The Landrum body contains similar amphibole varieties, but with actinolite rather than tremolite.

Whole rock chemistry indicates an aluminum-rich protolith. Much of the aluminum now resides in the chlorite. Tectonic discrimination plots show the whole rock chemistry of the bodies plot closely together, suggesting genetic relation. There is little agreement of protolith among the plots. However, the bodies all appear to have an oceanic affinity. Our whole rock analyses compare favorably to published lherzolite analyses, although they contained 1.8-9.6% more alumina. We suggest that the ultramafic rocks originated as a lherzolite from a tectonically-emplaced mantle fragment from the sub-oceanic lithosphere.