FURTHER INVESTIGATIONS OF THE UNION MILL GNEISS IN THE RALEIGH TERRANE, PIEDMONT PROVINCE, VIRGINIA

The Raleigh terrane forms a portion of the Carolinia Domain, an amalgamation of exotic, peri-Gondwanan terranes in the southern Appalachians. Located in the eastern Piedmont region of Virginia and North Carolina, it is made up primarily of high-grade gneisses and schists. A distinctive variety of the terrane, known informally as the Union Mill gneiss, occurs within a few kilometers to the east and northeast of South Hill, Virginia. At all locations, the unit is a slabby-weathering, strongly foliated, fine- to medium-grained, gray gneiss. It has also been extensively intruded by pegmatite throughout, and in all outcrops, the pegmatites are parallel to foliation. Cheek and Owens (2007 GSA abstract) investigated in a preliminary way the origin of this gneiss, noting that it consists predominantly of quartz, plagioclase, and biotite, with variable amounts of pale green amphibole. Whole-rock compositions (8 samples) are consistent with dacitic protoliths. In support of an igneous protolith, Owens and Buchwaldt (2009 GSA abstract) obtained a U-Pb zircon date of 560 ± 2 Ma (MSWD=0.61) for one sample, interpreted as a crystallization age. The purpose of our study was to expand on the work of Cheek and Owens (2007), which was limited in geographic extent and lacked a full suite of trace element compositions. Our ultimate goal was to gain a better understanding of this unit’s areal extent, protolith rock type, and tectonic setting. Our new results (7 samples) extend the known occurrences to the NE and all rocks are mineralogically similar. The new results confirm the broadly dacitic character of the rocks (60-76 wt% SiO₂). Although a sedimentary protolith (graywacke?) is possible, the current mineralogy (e.g., rare chlorite, muscovite) and textures are less consistent with this interpretation. All samples are LREE-enriched (La_N/Yb_N = 3-47), and most have modest negative Eu-anomalies. On standard tectonic discriminant diagrams, samples fall uniformly in the volcanic arc granite field. In addition, on primitive mantle-normalized plots, they show negative Ta, Nb and Ti anomalies, typical of subduction-related igneous rocks. Collectively, our results suggest that the Union Mill gneiss protoliths originated in a peri-Gondwanan convergent margin setting in the late Neoproterozoic.