Defining the Hayesville–Soque River and Allatoona Faults and an Ordovician Arc Assemblage Within the Central Blue Ridge Northwest of Dahlonega, Georgia

The goal of this research within the central Blue Ridge northwest of Dahlonega, Georgia, is to provide new information toward better understanding of the Allatoona(?)/ Hayesville/Gossan Lead fault and terrane boundary, and to provide convincing evidence of Ordovician arc-related igneous activity. The Hayesville-Soque River and Allatoona faults separate three distinct tectonostratigraphic assemblages in this area: the Coweeta Group, Great Smoky Group, and Dahlonega gold belt. The gold belt here contains metasandstone, pelitic schist, and mafic rocks of the Otto Formation and the Sally Free mafic complex, while the Great Smoky Group contains metasandstone and pelitic schist with abundant calc-silicate. The abundance of calc-silicate and apparent lack of mafic rocks in the Great Smoky Group versus the apparent lack of calc-silicate and presence of abundant mafic rocks in the gold belt support the delineation of the Allatoona fault. Inequigranular, biotite-dominant (over trace muscovite), often migmatitic gneiss and schist of the Coweeta Group, Coleman River Formation, versus two-mica, nonmigmatitic, often equigranular metasandstone and schist of the Great Smoky Group and Otto Formation constitute the principal criteria for the delineation of the Hayesville and Soque River faults. Delineation of the Hayesville and Soque River faults here confirms that they are equivalent, and that the area northwest of Dahlonega marks the southwestern terminus of the Hayesville-Soque River thrust sheet.

Fault geometries observed within the study area indicate that a pre- to synmetamorphic ancestral Allatoona fault exists, which was later truncated by the pre- to synmetamorphic Hayesville-Soque River fault approximately 10 km west of Dahlonega. Thus, the postmetamorphic Allatoona fault observed to the southwest of Dahlonega possibly represents reactivation along the ancestral Allatoona fault mapped here. This observation indicates that the Hayesville and Allatoona faults do not represent a continuous Blue Ridge terrane boundary, as previously hypothesized.

The fault-emplaced Sally Free mafic complex (Etowah River fault) within the Dahlonega gold belt contains amphibolite, metagabbro, hornblende gneiss, Cane Creek felsic gneiss, and locally abundant migmatitic intermediate gneiss. Geochemical analyses and comparison of mafic and felsic samples from the Sally Free mafic complex, maficsamples from within the Otto Formation, and a 206Pb/238U age of 482 ± 7 Ma for the Cane Creek felsic gneiss reveal a complex Early Ordovician volcanic arc assemblage of gabbro/diorite cumulates (hornblende gneiss), tholeiitic arc basalt/andesite (amphibolite), and arc felsite/rhyodacite (felsic gneiss). The interlayered relationship of the amphibolite(arc basalt) and metasedimentary rocks of the Otto Formation indicative extrusive basalt flows, shallow sills, or mafic tuff emplacement coeval with Otto Formation deposition related to a syndepositional Early Ordovician volcanic arc and/or back-arc basin. The interpreted Early Ordovician age for both igneous activity and sediment deposition for the Dahlonega gold belt implies that faulting (Hayesville-Soque River and Allatoona), metamorphism, and deformation within this portion of the southern Appalachian Blue Ridge is related to the Acadian rather than the Taconic orogeny.