PETROGRAPHIC COMPARISON OF ULTRAMAFIC ROCKS IN THE BLUE RIDGE AND INNER PIEDMONT FOR THE UPSTATE OF SOUTH CAROLINA

Samples of ultramafic rocks were collected at almost every known site for Oconee and Pickens counties in the upstate of South Carolina. Sample locations were identified from existing geologic maps and publications as well as unmapped outcrops located by the Clemson Mapping Group over the last six years. Samples were made into thin sections for petrographic analysis.

Three samples were collected from the Blue Ridge and eight from the Inner Piedmont. The results indicate that relict olivine is most abundant in the Blue Ridge and is rare to absent in the Inner Piedmont. Hydrous mineral phases, including talc, serpentine, chlorite, and the tremolite-actinolite solid solution series, are minor components in the Blue Ridge but the Inner Piedmont ultramafics are dominated by the hydrous minerals. Degree of hydration increases eastwardly over the Inner Piedmont which is evident from the increase in chlorite and tremolite-actinolite from west to east. An indicator of this trend is the Walhalla ultramafic body which is located very close to the Blue Ridge front but is still located in the Inner Piedmont. It represents a transition between the two provinces by containing more hydrous mineral phases and less olivine than the Blue Ridge ultramafics but more olivine than Inner Piedmont ultramafics.

The abundance of serpentine and chlorite in several ultramafic bodies suggests late stage upper greenschist to lower amphibolite facies retrograde metamorphism. Anthophyllite is found in ultramafics from both regions, which suggests the rocks were subjected to pressures in excess of 11 kilobars and high grade amphibolite facies metamorphism. These results infer that ultramafic minerals in the Inner Piedmont have metasomatically recrystallized from the reaction of the protolith with hydrothermal metamorphic fluid.