AGE, ORIGIN, AND DISTRIBUTION OF MESOPROTEROZOIC LITHOSPHERE IN THE SOUTHERN APPALACHIANS

Mesoproterozoic crust in the southern Appalachian orogen (SAO) crops out along the backbone of the Blue Ridge from Virginia to Alabama. Most, if not all, of this lithosphere is attributed to tectonic and magmatic processes during the formation of supercontinent Rodinia, which attained its maximum extent ~1000-1100 Ma. Some workers have proposed that the Mesoproterozoic crust and lithosphere in the SAO formed along the current southwestern margin of Amazonia (Sunsas orogeny), rather than the eastern margin of Laurentia (Grenville orogeny). This lithosphere was then left attached to Laurentia as Rodinia dispersed. The combined effects of the amalgamation and dispersal of Rodinia and Pangea over ~800 Ma appear to have significantly altered the structure, extent, and composition of the original Mesoproterozoic lithosphere of eastern Laurentia, making it difficult to test the terrane transfer hypothesis. One way to see through the Rodinia-Pangea Wilson cycle's disruption and asses the current configuration of Mesoproterozoic lithosphere is to use the secondary isotope systematics of Paleozoic and Mesozoic igneous rocks. The original basis of the terrane transfer hypothesis for southern Appalachian Grenville inliers was based on whole-rock Pb isotopic compositions of Andean and Laurentian Mesoproterozoic rocks. Re-evaluating this hypothesis in light of new data, principally Lu-Hf isotopic systematics of Mesoproterozoic zircons from Grenvillian and younger magmatic rocks of the SAO, leads to an alternative proposal for the origin of the Grenville inliers. The advantage of the Lu-Hf system in zircons is that these mineral grains provide more insight into the petrogenesis of magmatic rocks, particularly in regions of older continental lithosphere, than averaging tracers such as whole-rock Pb-Pb or Sm-Nd. A compilation of several thousand U-Pb age and Lu-Hf isotopic determinations of magmatic zircons in the SAO shows that Mesoproterozoic magmatism associated with the assembly of Rodinia likely formed as the result of subduction beneath the Granite-Rhyolite terrane along the current eastern margin of Laurentia. This original Mesoproterozoic magmatic province extends throughout the SAO at least as far as the western Blue Ridge.