T11 Proterozoic Tectonic Evolution of the Grenville Orogen in Eastern North America I

Monday, 5 November 2001
8:00 AM - 12:00 PM

PROVENANCE AND GEOCHEMISTRY OF LATE PROTEROZOIC SOUTHERN APPALACHIAN CRYSTALLINE CORE PARAGNEISSES, NC-SC-GA-TN

Paper number: T11-9
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

The breakup of Rodinia in the southern Appalachians is marked by an extensive array of rift basins containing the more proximal Ocoee Supergroup, Mt. Rogers Formation, and Grandfather Mountain Formation; and the more distal, widely distributed Tallulah Falls-Ashe Formation (and equivalents). The proximal units are linked to Laurentia by deposition on or near Grenvillian basement massifs. The more distal assemblages were deposited on oceanic crust and/or on continental fragments, making it more difficult to resolve the provenance of these rocks. They may also be time transgressive from west to east, Late Proterozoic in the western Blue Ridge to as young as Ordovician within the Inner Piedmont, reflecting opening of the Iapetus ocean and the eastward younging of the oceanic crust and thus deposition. Blue Ridge and Inner Piedmont paragneisses are dominantly metapsammites and metapelites with varying amounts of amphibole gneiss, amphibolite, quartzite, calc-silicate, and ultramafic rocks.

Despite agreement among most workers concerning the Late Proterozoic to early Paleozoic age of Blue Ridge and Inner Piedmont paragneisses, the provenance and geochemical nature of these areally extensive units has been largely ignored. Our geochemical and geochronologic work demonstrates that the paragneisses of the Blue Ridge and western Inner Piedmont were derived from similar Grenvillian felsic continental sources (LREE-enriched, relatively high Th/Sc ratios ~0.6-8.0, relatively high incompatible element concentrations, and initial eNd values between -5 and -2 [a single sample at -8]). Paragneiss zircon cores are dominated by ~1100-1250 Ma ages whereas rims are either late Grenville (~1.0-1.1 Ga), or, less commonly, Paleozoic (~350 Ma). Basement exposed within the region is strikingly similar, with comparable REE patterns, a nearly identical range of Nd and Sr isotope ratios at the time of deposition of the sediments, and the same dominant magmatic core and metamorphic rim ages. The presence of Neoproterozoic as well as Grenvillian zircons from the eastern Inner Piedmont suggests influx of sediment from the east, docking of the Carolina (Avalon) terrane, and closing of the Iapetus ocean in the Ordovician. This would have terminated development of the Iapetan rifted margin in eastern North America and initiated Appalachian orogenesis.