CHARACTERIZATION OF PALEOZOIC TERRANES AND TERRANE ACCRETION AT THE SOUTHEASTERN LAURENTIAN MARGIN: GEORGIA AND ALABAMA APPALACHIANS

The Paleozoic growth of the eastern margin of the North American continent is exemplified by the amalgamation of a series of terranes due to the closure of intervening ocean(s) and the obduction of fragments of oceanic and continental crust. The Appalachian orogen has traditionally been described as a culmination of three distinct events, including the Taconic, Acadian, and Alleghenian orogenies. While evidence of the aforementioned discrete events has been well documented in the Appalachians in general, substantiation of the effects and timing of each orogeny appears to be more ambiguous regionally, likely requiring differing tectonic models along strike of the orogenic belt. The response of Laurentia to orogenesis is important in determining the timing and extent of Paleozoic accretionary events as well as characterizing the accreted terranes themselves. In the southern Appalachians, the Hollins Line- Allatoona fault is the terrane boundary between native Laurentian sequences in the footwall and the accreted exotic/suspect terranes of the eastern Blue Ridge (EBR) in the hanging wall. Immediately adjacent to the fault system are a series of correlative (?) bimodal meta-volcanic sequences termed the Hillabee Greenstone (HG) and the Pumpkinvine Creek Formation (PCF). These are unique in that they are fragments of an Ordovician arc system. Nd and Sr isotope models of the PCF and HG place them outboard of the Laurentian continent. New LA-ICP-MS U-Pb zircon ages of the PCF help constrain crystallization to ~460 Ma. Timing of accretion is constrained to emplacement of the HG above the Devonian-earliest Mississippian Jemison Chert/Erin Slate and prior to or during metamorphism of these units. Above the HG and PCF is a composite terrane that may contain parts of the Laurentian slope-rise as well as oceanic fragments. Constraints on age of these units is not well known though they contain suites of intrusive rocks, including the ~490 Ma Elkahatchee Quartz Diorite, ~366 Ma granitoids, as well as ~430-460 Ma granitoids. The latter two are S-type granitoids and contain inherited Grenville zircon components, suggesting a Grenville source. Along with palinspastic restoration of the Talladega belt, there is evidence that the EBR allochthon remained outboard of Laurentia until accretion during the late Devonian-Mississippian.