Geochemistry, Petrogenesis, and Tectonic Setting of Amphibolites from the Southernmost Exposure of the Appalachian Piedmont

Abstract

The Uchee belt of the southern Appalachian Piedmont consists of a sequence of amphibolite, amphibolitic gneiss, and gneissic calc-silicate surrounded by highly strained mylonitic rocks associated with motion along the Bartletts Ferry-Goat Rock fault system. Amphibolites from three major localities within the belt, the Hudson Mill Rapids, Davis Mill, and Lindsey Creek localities, have been analyzed for their major-and trace element chemistry. The mafic amphibolites are concluded to have been derived from intrinsically basaltic protoliths, as determined by the presence of patchy zoning in relict igneous plagioclase, normative mineralogy, Niggli trends, and a variety of diagnostic geochemical criteria. Davis Mill amphibolites are characterized by high REE abundances and flat REE pattern [], similar to volcanic rocks from Pacific-type island arcs. Geochemical parameters such as high La/Nb and V/Ni ratios, low Ti/V and Ti/Zr ratios, and discriminant trace element diagrams based on elements generally considered immobile during metamorphism, indicate an island-arc and back-arc basin affinity for the amphibolites. Hudson Mill Rapids amphibolites, characterized by a wide range of REE concentrations and strongly negative Ce anomaly are interpreted as having been derived from parental basalts produced in a back-arc basin by melting of a descending slab plus mantle wedge. The chemical diversity of the amphibolites and the presence of thrusts, strike-slip faults, and highly strained rocks are consistent with the premise that the western Uchee belt is a deformed composite part of an extensive arc system.