PETROGRAPHY AND PRELIMINARY MAJOR AND TRACE ELEMENT GEOCHEMISTRY OF AMPHIBOLITES FROM THE INNER PIEDMONT OF NORTHWESTERN SOUTH CAROLINA

The Inner Piedmont, located in northwestern South Carolina, is a highly metamorphosed region made up of a series of stacked thrust sheets separated from the Eastern Blue Ridge by the Brevard Fault Zone. The three westernmost thrust sheets are the Six Mile thrust sheet, the Walhalla nappe, and the Chauga belt. Amphibolite is interlayered with biotite gneiss that is widespread throughout the area.

Petrographic analysis of amphibolites yields mineralogy of amphibole, epidote, quartz and plagioclase, with minor titanite, zircon, chlorite, garnet, and opaque minerals. Amphibolites are well foliated with some degree of separation of hornblende-type amphibole and epidote into horizons. A few contain veins or lenses of coarse-grained quartz/plagioclase or calcite. Although most of the amphibole is hornblende, two samples, one from the Chauga belt and one from the Six Mile thrust sheet, contain actinolite. The actinolite in the Chauga belt sample supports the theory that this belt experienced lower grades of metamorphism. The actinolite in the Six Mile thrust sheet appears to be breaking down and may represent a transition to a higher metamorphic grade.

Major element geochemistry from this and a previous study (Prince and Ranson, 2004) reveals remarkably similar bulk compositions, indicating similar protoliths for amphibolites from the three thrust sheets. A Niggli diagram shows that amphibolites originated from an igneous rather than a sedimentary protolith. Preliminary trace element geochemistry plotted on tectonic discrimination diagrams suggests Walhalla nappe samples originated from tholeiitic magma and Chauga belt and Six Mile thrust sheet samples came from an alkali basaltic magma. Additional trace element studies are planned to confirm the tectonic setting(s) of amphibolites from the three thrust sheets.