VALUE OF MODERN GEOCHRONOLOGY IN UNDERSTANDING SOUTHERN APPALACHIAN ACCRETIONARY HISTORY

State-of-the-art geochronologic data have revolutionized our understanding of southern Appalachian accretionary history. Grenvillian basement rocks were emplaced ~1.15 Ga, and a pre-Grenvillian 1.6-1.9 Ga basement terrane has also been confirmed in the western Blue Ridge. Geochemistry and SHRIMP dating of plutons, volcanic rocks, and detrital zircons indicate Middle Ordovician (MO, 480-460 Ma) arc/MORB volcanism dominated the Tugaloo-Milton terrane (TMt), with possible recognition of a MO unconformity. The Penobscot orogeny is not represented here. Early Mississippian ~350 Ma zircon rims (SHRIMP) and ~360 Ma monazite (TIMS; Dennis and Wright), Devonian to Carboniferous plutonic suites, and deformation confirm Late Devonian-early Mississippian Neoacadian (Quaboagian) orogeny in the TMt and Cat Square terrane (CSt). The Cst is defined by: the Brindle Creek fault to the W and the Central Piedmont suture to the E; a mixed Laurentian and Gondwanan provenance; mid-Paleozoic deposition delimited by Early Silurian detrital zircons deposited in a remnant ocean; 380 and 366 Ma and younger plutons; and its location in the Neoacadian Inner Piedmont imbricate structural channel. A 400-380 Ma plutonic suite in the Carolina superterrane (CS; Avalonian), a 360 Ma thermal overprint (⁴⁰Ar/³⁹Ar ages, Dallmeyer et al.) in the western CS, and 600 and 500 Ma peri-Gondwanan detrital zircons in the CSt, provide the earliest real connection between the CS and Laurentian terranes to the W. These data preclude docking of CS earlier than the mid-Paleozoic. The most consistent plate tectonic scenario includes obduction of more westerly Ordovician terranes to the Laurentian margin during the Mid- to Late Ordovician, and subduction of Cst, then Tmt, during the Devonian and early Mississippian with docking of the CS. TMt obduction occurred during the late Neoacadian or early Alleghanian orogeny (zircon rim and monazite ages). e_Nd data clearly separate the juvenile CS from other accreted terranes to the W. The Alleghanian event involved SW escape of blocks southward from the initial collision of the assembled terranes with Gondwana, and formation of a metamorphic core and plutons from 300-270 Ma. This was followed by beheading of all earlier terranes as the Blue Ridge-Piedmont megathrust sheet propagated westward as a result of head-on collision and transported the orogen onto the Laurentian platform.