THE WEDOWEE-EMUCKFAW-DAHLONEGA BACK-ARC BASIN AND DADEVILLE COMPLEX ARC: A LAURENTIAN MARGIN ORDOVICIAN-SILURIAN SUPRASUBDUCTION COMPLEX IN THE SOUTHERNMOST APPALACHIANS

The southern Appalachian Wedowee-Emuckfaw-Dahlonega basin (WEDB) and Dadeville Complex of Alabama (AL) and Georgia (GA), USA, represent a paired back-arc and arc terrane, respectively. The oldest rocks of the WEDB in the Blue Ridge (Sally Free mafic complex) of GA are Early Ordovician in age and post-date initiation of a subduction zone dipping beneath the Laurentian margin (B-type subduction). Bimodal metavolcanic rocks such as the Hillabee Greenstone and Pumpkinvine Creek Formation in the Talladega belt (AL) and Blue Ridge of GA (e.g., Dahlonega gold belt) mark continued opening of this Sea of Japan-style back-arc basin from the Early to Middle Ordovician, and are similar in age to metamorphosed arc-affinity igneous rocks in the western Inner Piedmont Dadeville Complex (AL-GA). The magmatic roots of both the back-arc and arc terranes include a host of Middle Ordovician to Silurian metagranitoids in AL (e.g., Zana and Kowaliga plutons, Camp Hill Gneiss) and GA (e.g., Villa Rica Gneiss, Franklin Gneiss). Metamorphosed igneous rocks of the paired WEDB and Dadeville Complex include abundant evidence for interaction between suprasubduction magma and Grenville crust (e.g., xenocrystic zircon). Coupled with the presence of abundant Grenville-aged detrital zircon in intercalated metasedimentary rocks, this suprasubduction system must have formed atop extended Mesoproterozoic continental crust (i.e., Laurentia). The Marble Hill Hornblende Schist (western Blue Ridge, GA) has recently been identified as the youngest volcanic sequence in the WEDB. These metamorphosed alkaline mafic volcanic rocks are Silurian in age (ca. 430 Ma) and formed at the continental edge of the WEDB basin, unconformably above metamorphosed carbonate platform rocks of the Murphy Marble. Age, geochemical, lithologic, and stratigraphic constraints from these suprasubduction igneous rocks and their associated sedimentary rocks in the Blue Ridge and western Inner Piedmont of AL and GA indicate the presence of an extensive back-arc and arc system that formed no later than the earliest Ordovician and persisted well into the Silurian (>50 myr). The presence of this paired back-arc/arc terrane in the southernmost Appalachians provides important constraints for Ordovician-Silurian tectonic models in this part of the orogen.