A SYNTHESIS OF RECENT CONTRIBUTIONS IN SOUTHERN APPALACHIAN MAPPING, GEOCHEMISTRY AND GEOCHRONOLOGY OF LAURENTIAN MARGIN ROCKS: PART I – LATEST NEOPROTEROZOIC THROUGH ORDOVICIAN
Neoproterozoic-early Cambrian breakup of the Rodinian supercontinent is represented by rocks of the Ashland Supergroup (EBR) and includes metasedimentary units with typical rift sequence lithologies and Grenville Nd model ages, as well as orthoamphibolites with intraplate geochemical characteristics typical of rifted margin mafic rocks. These rocks, which formed outboard of the Alabama promontory continental hinge zone, served as basement to an Early-Middle Ordovician back-arc (Wedowee-Emuckfaw-Dahlonega basin) that includes metamorphosed sedimentary and bimodal volcanic sequences of the WBR (Hillabee Greenstone), EBR (Wedowee-Emuckfaw-New Georgia-Jacksons Gap Groups), and WIP (Opelika Complex). Like the underlying Rodinian rift units, most of these younger Ordovician back-arc rocks contain a significant Grenville-age detrital zircon population and Grenville-age xenocrystic components in magmatic rocks. Ordovician mafic rocks are typically subalkaline, tholeiitic metabasalt (e.g., Hillabee Greenstone, Pumpkinvine Creek Formation) moderately enriched in large ion lithophile elements. They have negative Nb anomalies and flat rare earth element patterns with geochemical trends intermediate between volcanic arc and MORB, typical of back-arc volcanic sequences. Development of this Ordovician accretionary orogen is also associated with the Blount (Taconic) clastic wedge and explains the presence of Middle-Upper Ordovician k-bentonites in the foreland (e.g., Deicke and Millbrig), all of which developed as a result of B-type subduction of Iapetus oceanic crust beneath the Laurentian plate. Northern Appalachian models suggesting initial subduction of the Laurentian continental margin in the early Paleozoic requires a major transform boundary separating the two ends of the orogen prior to ~480 Ma.