AGE AND GEOCHEMISTRY OF SOUTHERN APPALACHIAN BASEMENT, NC-SC-GA, WITH IMPLICATIONS FOR PROTEROZOIC AND PALEOZOIC RECONSTRUCTIONS

Ion microprobe U-Pb analyses of zircons from basement units in the southern Appalachians, combined with supporting isotopic compositions and major and trace element geochemistry, have delineated a granitic magmatic pulse ~1170-1150 Ma. Lithologic units included are the Watauga River Gneiss (western Blue Ridge-WBR), Toxaway, Wiley, and Sutton Creek gneisses (eastern Blue Ridge-EBR), Pilot Mtn. and Grassy Creek gneisses (Sauratown Mtns. Window), and possibly the Forbush gneiss (~1140 Ma, Inner Piedmont) and Cranberry-Mine Layered Gneiss (~1190 Ma, WBR). Additional samples analyzed include the Blowing Rock Gneiss (~1085 Ma, Grandfather Mtn. Window), and the Carvers Gap Granulite Gneiss (~1.8 Ga) and Cloudland Granulite Gneiss (detrital cores ~1.0-1.8 Ga) from the Mars Hill terrane. Age data were evaluated by calculating concordia ages and concordia probability plots (sums of the Gaussian distribution of analyses along concordia in million year intervals) using ²⁰⁶Pb*/²³⁸U and ²⁰⁷Pb*/²⁰⁶Pb* data simultaneously. Rocks in the main magmatic pulse are granitic (63-72 wt. % SiO₂), but elevated in K and incompatible trace elements compared to typical subduction-related magmas, and initial Nd ratios cluster tightly near CHUR. Mars Hill terrane samples are distinct in age, geochemistry (poorer in K and incompatible elements), and isotopic compositions (initial e_Nd -7.6 and -5 at 1.0 Ga). Almost all samples have metamorphic rims that yield ages ~1030 Ma, with the exception of the Blowing Rock Gneiss. Ages of magmatism and metamorphism are similar to reported ages from the Blue Ridge of Virginia, suggesting a common history between these two areas. Surprisingly few Paleozoic metamorphic rims have been identified, but the few analyses obtained (from samples in the EBR) yield late Acadian Ages (~350 Ma). Although consistent with known ages in Laurentia, the presence of 1.8 Ga rocks and T_DM ages uniformly >1.5 Ga is inconsistent with the inferred 1.6 Ga margin of Laurentia defined by Van Schmus et al. This suggests either that the 1.4-1.5 Ga midcontinent terrane separates older portions of Laurentia, that this region was exotic, or that it was a rifted fragment of Laurentia reattached during Grenville or Paleozoic orogeny. The similarity of basement units across the Blue Ridge and Inner Piedmont suggests a relationship between these terranes.