TECTONIC IMPLICATIONS OF ORDOVICIAN-SILURIAN MAGMATIC-VOLCANIC SEQUENCES IN THE EASTERN BLUE RIDGE AND INNER PIEDMONT, SOUTHERN APPALACHIAN OROGEN

The Ashland-Wedowee-Emuckfaw belt in the eastern Blue Ridge of Alabama and Georgia consists of Neoproterozoic-Cambrian, Laurentian rift and passive margin sequences stratigraphically below rocks of the younger Wedowee-Emuckfaw-Dahlonega back-arc basin. Metamorphosed sedimentary-volcanic sequences in this younger, Laurentian-plate back-arc basin range into at least the Middle Ordovician. Intruding these Neoproterozoic-Early Paleozoic units are a number of pre-, syn-, and post-metamorphic granitoids. The oldest of these, the Acworth Gneiss (ca. 463 Ma), Villa Rica Gneiss (ca. 458 Ma) and Zana Granite (460-450 Ma) are coeval with the end of bimodal, back-arc volcanism in the Wedowee-Emuckfaw-Dahlonega basin. Younger plutons in the Wedowee-Emuckfaw-Dahlonega back-arc basin range from Late Ordovician to the Silurian and include the Sand Hill Gneiss (ca. 443 Ma), Farmville metagranite (ca. 440 Ma), Mulberry Rock Gneiss (ca. 432 Ma), and Kowaliga Gneiss (430-425 Ma), indicating that magmatism continued unabated for >20 m.y. following the end of Early-Middle Ordovician back-arc volcanism. Many of these plutons, like the encompassing country rocks, show evidence for involvement of Grenville-aged crust in their genesis. During this Late Ordovician-Silurian interval of silicic magmatism in the Wedowee-Emuckfaw-Dahlonega back-arc basin of Alabama and Georgia, bimodal volcanic rocks of the Poor Mountain Formation in the eastern Tugaloo terrane (western Inner Piedmont) formed atop the predominantly sedimentary units of the Tallulah Falls Formation. The 459-445 Ma Poor Mountain Formation, also attributed to back-arc volcanism, may represent the migration of back-arc extension outboard (east) of the older Wedowee-Emuckfaw-Dahlonega back-arc basin and underlying Laurentian margin deposits. A jump in back-arc spreading toward the arc, concomitant with roll-back of the subducting plate, is consistent with back-arc dynamics and accretionary orogens along other continental margins (e.g., Lachlan orogen). Significantly, rocks of the western Tugaloo terrane (eastern Blue Ridge) Tallulah Falls Formation must lie palinspastically between the Wedowee-Emuckfaw-Dahlonega back-arc basin and a recently identified Taconic arc terrane in the Inner Piedmont’s Dadeville Complex.