NEW ZIRCON GEOCHRONOLOGY AND ND-PB ISOTOPE GEOCHEMISTRY FOR GRENVILLE BASEMENT ROCKS OF THE SOUTHERN APPALACHIANS AND EASTERN KENTUCKY: PROBING FOR THE BOUNDARY BETWEEN AMAZONIA AND LAURENTIA DURING RODINIAN ASSEMBLY (Invited Presentation)

Lead isotope compositions for eastern Laurentia (Grenville of Adirondacks/Texas, Appalachian Inliers; Granite Rhyolite provinces) and Amazonian Grenville-aged crust demonstrated that southern Appalachian Blue Ridge basement is largely exotic and was transferred to Laurentia from Amazonia during Rodinian-forming collision. Paleomagnetic studies supported this inference and further demonstrated that the collision involved sinistral displacement. It is now recognized that the pre-Grenvillian Laurentian margin was the Southern and Eastern Granite Rhyolite (EGR) province crust, which was reworked during the Grenville orogeny. Questions remain regarding the location of the Laurentia-Amazonia suture (how far SE does reworked EGR crust extend?); how to account for pre-collisional (ca. 1.35 Ga) magmatic rocks found in the Adirondacks and in Appalachian inliers outboard of most Grenville rocks; and whether crustal mixing occurred (there exists some overlap among Laurentian and Amazonian Pb isotope compositions). Paleozoic deformation and high-grade metamorphism overprint almost all original lithologic features in the Great Smoky Mountains Basement Complex (GSMBC) but key exposures reveal that older crustal components may be more widespread. Old crustal components in the GSMBC include ~1.33 Ga granite containing ~1.35 Ga xenoliths; ~1.6-1.8 Ga detrital zircons in pre- or syn-Grenvillian paragneisses; and T_DMages of 1.5-1.8 Ga in all lithologies. Typical Grenville ~1.15 Ga granites intrude the older complex. Although the GSMBC occurs within the region of exotic Amazonian Grenville outlined by previous workers, GSMBC lithologies span the range of Amazonian to Laurentian crust in Pb isotope space, consistent with mixing of crustal components. Drill core/cuttings in KY define the Grenville Front and prove that Grenville ~1.0 Ga overprinting of 1.45 Ga EGR crust extends that far south. The oldest crustal components in the GSMBC fall within the range of Southern Granite Rhyolite rocks (SGR: ~1.34-1.40 Ga) with similarly native Laurentian Pb isotope compositions (but not juvenile Nd isotope compositions). This permits tentative extension of the SGR into the Grenville province, similar to the EGR, with the GSMBC being a zone of crustal mixing and the margin of Laurentia during Rodinian collision.