DETAILED GEOLOGIC MAPPING OF A PLUTONIC COMPLEX IN THE SOUTHERN APPALACHIAN INNER PIEDMONT: GEOCHEMICAL AND GEOCHRONOLOGIC ANALYSES AND THEIR TECTONIC IMPLICATIONS
Detailed geologic mapping in the Georgia Inner Piedmont has revealed a plutonic complex in the Cat Square terrane (CST) between the Brindle Creek and Towaliga faults ~80 km SE of Atlanta. The complex has an area ~235 km2 and is composed of three distinct mappable granitoids. The High Falls granite (HFG) is a Siluro-Devonian (424-380 Ma) porphyritic granite with characteristic blocky microcline megacrysts. Acadian/Neoacadian emplacement and deformation of the HFG were followed by Alleghanian deformation and plutonism. Foliation in the HFG and map patterns are concordant with regional structural trends. HFG zircons have inherited cores with ages corresponding to detrital zircons in CST metasedimentary rocks, however, inheritance is generally rare in the southern Appalachian Inner Piedmont. Other components in the complex, the Dows Pulpit (DPG, 321 Ma) and Indian Springs (ISG, 313-299 Ma) granodiorites both have an equigranular, fine- to medium-grained texture. These Alleghanian granodiorites crosscut both HFG and metasedimentary units. Foliation within the DPG and ISG is relatively weak and discordant with regional foliation. The Walker Top granite (WTG, 407-357 Ma) in the North Carolina CST is roughly the same age, and is mineralogically, texturally, and geochemically similar to the HFG. These similarities indicate that the HFG and WTG may be equivalent plutons. WTG zircons lack inherited cores, and geochemical data suggest zirconium saturation is not responsible for lack of inheritance in the WTG. If zircon inheritance is primarily affected by temperature, this implies that the HFG formed at a lower temperature than the WTG. Anatectic melting of sedimentary rocks at relatively low temperatures could be achieved by the dewatering of phyllosilicates during metamorphism. The Acadian/Neoacadian plutons appear chemically to have formed in a transitional arc to collisional setting whereas the Alleghanian plutons probably formed in an arc setting, likely above a W-dipping subduction zone.