Tectonic implications of para- and orthogneiss geochronology and geochemistry from the southern Appalachian crystalline core

Sedimentary and metasedimentary rocks the southern Appalachian Blue Ridge and Inner Piedmont contain a valuable record of Late Proterozoic Laurentian margin evolution following the breakup of Rodinia. These paragneisses contain rift-and-drift facies cover rocks deposited unconformably on Grenville basement in the western and eastern Blue Ridge. Paleogeographic reconstructions and increasing amounts of geochronologic and isotopic data limit the derivation of most southern Appalachian crystalline core cover sequences to the Laurentian and/or west Gondwanan craton(s). Southern Appalachian crystalline core paragneiss samples have &egr;Nd values between -8.5 and -2.0 at the time of deposition and contain abundant 1.1--1.25 Ga zircon cores with Grenville 1.0--1.1 Ga metamorphic rims. Less abundant detrital zircon core ages include pre-Grenvillian Middle Proterozoic 1.25--1.6 Ga, Early Proterozoic 1.6--2.1 Ga, and Late Archean 2.7--2.9 Ga sources. Blue Ridge Grenvillian basement has almost identical &egr;Nd values and displays the same dominant magmatic core and metamorphic rim zircon ages. Based on these data, nonconformable basement-cover relationships, and crustal ages in eastern North America, we suggest that the extensive sedimentary packages in the southern Appalachian Blue Ridge and western Inner Piedmont are derived from Laurentia. Relatively high initial 87Sr/86Sr values between 0.707 and 0.723, Middle to Upper Proterozoic TDM ages, high SiO2 content, LREE enrichment, low chemical index of alteration values mostly between 41 and 66, and restricted Th/Sc and Zr/Sc values for Laurentian sedimentary and metasedimentary samples further support derivation from Grenvillian rocks like those exposed in the western Blue Ridge. &egr; Nd values from Carolina terrane volcanic, plutonic, and volcaniclastic rocks are isotopically less evolved than southern Appalachian paragneisses and Blue Ridge Grenvillian basement easily separating this composite terrane from the mostly Laurentian terranes to the west. Neoproterozoic (∼590 Ma), Ordovician (peaks at ∼500 and ∼485 Ma), and Silurian (peak at ∼435 Ma), as well as Grenvillian and older, zircons in eastern Inner Piedmont paragneisses indicate that these rocks were deposited much later (late Silurian to Devonian) and could be entirely derived from a Panafrican source or possibly a mixture of Panafrican and recycled Laurentian margin assemblages. (Abstract shortened by UMI.)...