THE MARS HILL TERRANE: EXTENT, AGE, AND ORIGIN OF THE OLDEST ROCKS IN THE SOUTHEASTERN USA

The Mars Hill Terrane (MHT) of eastern TN and western NC is a distinctive basement terrane in the Blue Ridge Province that includes the oldest known rocks within the southeastern USA. It is characterized by a wide diversity of rock types (mafic to felsic orthogneisses, pelitic to psammitic paragneisses) interspersed on cm to regional scale and strong Grenvillian granulite facies metamorphism (e.g. Ownby et al., 2002). The MHT has been documented from Roan Mountain, NC-TN, southwest through Mars Hill, NC (e.g. Gulley, 1985; Bartholomew & Lewis, 1988; Merschat & Weiner, 1990; Raymond & Johnson, 1994; Stewart et al., 1997). Based on antiquity of Nd T_DM (1.7-2.2 Ga) and magmatic ages (>1.20 Ga, up to 1.8 Ga), well-preserved granulite facies assemblages, and characteristic amphibolite-granitic gneiss associations, we have documented occurrences of MHT-like lithologies in North Carolina as far to the south and southwest as Sylva, milepost 418 on the Blue Ridge Parkway, and Fines Creek, all 30-50 km beyond the established limits of the MHT. These exposures within what has been considered to be both the Eastern and Western Blue Ridge (EBR and WBR) suggest that either the ancient basement is imbricated as tectonic slivers within both the overlying and underlying sheets or the MHT is far more extensive and intervening areas are strongly retrograded.

Magmatic and T_DM ages distinguish the MHT from EBR and WBR basement (mag 1.0-1.2 Ga, T_DM 1.5 –1.8 Ga) and all Blue Ridge basement from adjacent Proterozoic rocks of southeastern North America. Within the Blue Ridge juvenile Grenville (~1.3 Ga) and mid-continent-like (1.4-1.5 Ga) crust appear to be absent. Lead isotope ratios also have been interpreted to suggest an exotic, possibly non-Laurentian origin for this basement (Sinha & McLelland, 1998; Loewy et al., 2002). Early Proterozoic magmatic ages and T_DM and Pb isotopes may link the MHT with distant terranes in Laurentia (Yavapai/Mojave/Penokean?) or Gondwana (South America: Amazonia/Arequipa?; West Africa?).