THE SALLY FREE MAFIC COMPLEX AND ASSOCIATED MAFIC ROCKS OF THE DAHLONEGA GOLD BELT: EVIDENCE OF ORDOVICIAN ARC VOLCANISM AND INTRUSION NORTHWEST OF DAHLONEGA, GEORGIA

Recent detailed geologic investigation within the central Blue Ridge northwest of Dahlonega, Georgia, provides convincing evidence of Ordovician, arc-related igneous activity. The Dahlonega area contains three distinct stratigraphic assemblages: the Dahlonega gold belt, Coweeta Group, and the Great Smoky Group, bounded by three crustal-scale faults (Hayesville-Soque River, Chattahoochee, and Allatoona faults). Mafic to felsic orthogneisses occur within rocks of the Dahlonega gold belt, which includes mixed mafic rocks within paragneisses of the Otto Formation and mafic to felsic rocks within the Sally Free mafic complex. The fault-emplaced Sally Free mafic complex contains amphibolite, metagabbro, hornblende gneiss, Cane Creek felsic gneiss, and locally abundant, migmatitic, intermediate metatuff. Geochemical analyses of mafic and felsic samples from the Sally Free mafic complex and mafic samples from within the Otto Formation reveals a complex assemblage of gabbro/diorite (hornblende gneiss) cumulates, tholeitic arc basalt/andesite (amphibolite), and arc felsic tuff/rhyodacite (felsic gneiss). The age of the Sally Free mafic complex is Early Ordovician delimited by the ²⁰⁶Pb/²³⁸U SHRIMP RG age of 481 ± 7 Ma (2s, MSWD=0.7, n=10) for the Cane Creek felsic gneiss. Chemical similarities between amphibolite within the mafic complex and amphibolite from within the Otto Formation indicate probable synchronous igneous activity within the Dahlonega gold belt. The interlayered relationship of the amphibolite (arc basalt) and paragneisses of the Otto Formation is indicative of extrusive basalt flows, shallow sills, and/or mafic tuff emplacement coeval with Otto Formation deposition (Cambrian or Ordovician?). Observed stratigraphic relationships within the Dahlonega gold belt here, combined with chemical characterization of mafic and felsic lithologies within the Sally Free mafic complex and mafic lithologies within the Otto Formation have provided new crucial information concerning the tectonic evolution of a syndepositional volcanic arc within the north Georgia central Blue Ridge. The Lake Burton, Kimsey Bald, and Laura Lake mafic-ultramafic complexes, along with the Sally Free mafic complex, may be part of a more extensive Ordovician arc system in southeastern Laurentia.