Isotopic Age Constraints and Metamorphic History of the Talladega Belt: New Evidence for Timing of Arc Magmatism and Terrane Emplacement along the Southern Laurentian Margin

U-Pb and Ar/Ar dates from the Talladega belt, southernmost Appalachians, provide insight into the timing and nature of pre-Alleghanian tectonism in this region. Low-grade metasedimentary rocks of the Talladega belt represent the outermost preserved portions of the southern Laurentian margin, thus recording the earliest orogenic events that affected the margin, in addition to later overprinting events. These rocks are structurally overlain by the Hillabee Greenstone metavolcanic sequence, from which metadacite yields an age of 470 4 Ma (ion microprobe U-Pb zircon age). Hillabee geochemistry indicates formation in an arc or back-arc setting. We suggest correlation of the Hillabee Greenstone with 460–470-Ma arc-related rocks in the Dahlonega gold belt, which extends along strike with the Hillabee through Georgia and into North Carolina, and interpret the Hillabee as the southernmost volcanic expression of an Early-Middle Ordovician arc that formed outboard of Laurentia. Rocks of the Talladega belt, including the Hillabee, preserve a record of only one Paleozoic dynamothermal metamorphic event. Ar/Ar dates are consistent with metamorphism following deposition of the youngest biostratigraphically dated unit, the Erin Slate, which contains early Mississippian Periastron plant fossils (360–350 Ma). Muscovite (closure temperature 350 –400 C) yields internally consistent ages between 334 and 320 Ma. Metamorphism of the Talladega belt, therefore, is constrained to the interval between 360 and 320 Ma. The preto synmetamorphic thrust contact between the Hillabee Greenstone and Talladega belt metasedimentary rocks must have formed after deposition of the Erin Slate at 360–350 Ma but before metamorphism of the Talladega belt (no later than ∼320 Ma). We propose that the Ordovician HillabeeDahlonega arc terrane first collided with the Laurentian continental margin between 360 and 320 Ma, with variations in timing of deformation and metamorphic character along the collision zone. Online enhancements: appendix tables.