TIMING OF ARC MAGMATISM AND TERRANE EMPLACEMENT ALONG THE SOUTHEASTERN LAURENTIAN MARGIN: EVIDENCE FROM THE TALLADEGA BELT, SOUTHERNMOST APPALACHIANS

U-Pb and 40Ar/39Ar dates from the Talladega belt (TB), southernmost Appalachians, provide insight into the timing and nature of pre-Alleghanian tectonism in this region. Lower greenschist facies clastic and carbonate rocks of the TB represent the outermost preserved portions of the southern Laurentian margin, thus recording the earliest and subsequent orogenic events that affected the region. TB metasedimentary rocks are structurally overlain by metavolcanic rocks of the Hillabee Greenstone (HG). Subordinate metadacitic rocks in the HG yield an age of 470±4 Ma (ion microprobe U-Pb zircon age). HG geochemistry indicates formation in an arc or back-arc setting, possibly along an active continental margin. Geochemistry, age, and tectonic setting suggest correlation of the HG with ~460-470 Ma arc-related rocks in the Dahlonega terrane (DT), which extends nearly along strike with the HG through GA and into NC. We interpret the HG as the southernmost volcanic expression of an Early-Middle Ordovician arc that formed outboard of Laurentia. Rocks of the TB, including the HG, appear to have experienced one Paleozoic, prograde, dynamothermal metamorphic event. 40Ar/39Ar dates from the TB are consistent with metamorphism following Early Mississippian deposition of the youngest biostratigraphically dated unit, the Erin Slate, which contains Early Mississippian Periastron plant fossils (~360 – 350 Ma). Muscovite (closure temperature ~350 to 400ºC) yields internally consistent ages that range from 334 to 320 Ma. Metamorphism of the TB, therefore, is constrained to the time interval between ~360 Ma and 320 Ma. The pre- to synmetamorphic thrust contact between the HG (Middle Ordovician) and TB metasedimentary rocks (as young as Early Mississippian) therefore formed after deposition of the Erin Slate at ~360 – 350 Ma, but before metamorphism of the TB (no later than ~320 Ma). This timing is consistent with data from the southern Appalachians to the northeast that indicate a metamorphic peak between ~350 and 320 Ma; however, evidence of Taconian tectonism, documented farther to the northeast, is lacking in this area. We propose that the Ordovician HG-DT arc terrane first collided with the Laurentian continental margin between 365-320 Ma, with variations in timing of deformation and metamorphic character along the collision zone.