ORDOVICIAN EVENTS IN THE SOUTHERN APPALACHIANS: SYNTHESIZING THE IGNEOUS, SEDIMENTARY AND METAMORPHIC RECORD OF THE TACONIC OROGENY

Identification of a southern Appalachian, Ordovician back-arc basin on the Iapetus-facing Laurentian margin (Wedowee-Emuckfaw-Dahlonega basin: WEDB) provides a tectonic framework for igneous, sedimentary and metamorphic events in the southern Appalachians during the Taconic orogeny. Back-arc extension during the Early-Middle Ordovician (480-460 Ma) was associated with formation of volcanic-sedimentary sequences in the eastern Blue Ridge and Inner Piedmont of Georgia-Alabama. These WEDB sequences include the Wedowee and Emuckfaw Groups of the Ashland-Wedowee-Emuckfaw belt, the Auburn and Loachapoka Formations of the Opelika Complex, the Jacksons Gap Group of the Brevard fault zone, the Hillabee Greenstone of the Talladega belt, and the Otto Formation-Sally Free Mafic Complex-Helen Group-New Georgia Group of the Dahlonega gold belt. During waning stages of back-arc extension in the latest Middle Ordovician, sedimentation within the Blount basin developed on continental crust proximal to the WEDB. In contrast to models of thrust loading for the Taconic clastic wedge, we interpret it as having formed in a retroarc setting, consistent with the nature of sedimentation within the Blount basin and its palinspastic position immediately inboard of the WEDB. During the latest Middle-Late Ordovician, cessation of back-arc volcanism was coeval with formation of high pressure rocks in the central Blue Ridge and southern Appalachian eclogites. The onset of an apparent contractional phase of the Taconic is also marked by granitic plutonism in the WEDB (e.g. Villa Rica gneiss). Continuation of arc activity into the latest Ordovician is marked by k-bentonites in the Blount basin (e.g. Deicke and Millbrig) and indicates intermittent eruptions from silicic volcanic centers proximal to the WEDB. A latest Ordovician age for the Poor Mountain Formation of the eastern Tugaloo terrane may indicate resumption of back-arc extension outboard of the earlier-formed WEDB, typical of back-arc extension-contraction cycles in other accretionary orogens. Silicic magmatism in the eastern Blue Ridge (e.g. Zana and Kowaliga plutons) and Inner Piedmont (Farmville pluton) continued into the Silurian. Rocks of the Dadeville complex and central Blue Ridge may represent, at least in part, portions of the Taconic arc.