EXTENT AND STRUCTURAL SETTING OF THE ORDOVICIAN DADEVILLE VOLCANIC ARC COMPLEX IN GEORGIA

The Dadeville Complex (DC), part of a Laurentia-fringing volcanic arc sequence of Ordovician age in the Inner Piedmont (IP) of Alabama and Georgia, resides as a large (>9,000 km²), far-traveled (>50 km) klippe structurally above Ordovician back-arc basin rocks of the eastern Blue Ridge (EBR)/Brevard zone (BZ) on the NW, and equivalent units of the Opelika Group (OG) in the IP on the SE. The DC is bounded below by the Stonewall Line/Katy Creek thrust system (SKCT) and is folded into the regional, open, doubly-plunging, NE-trending Tallassee synform (TS). The NE-plunging hinge of the synform lies just south of the Gulf Coastal Plain onlap; the SW-plunging hinge occurs NE of Atlanta. Details of the arc/back arc system were established in Alabama, but its presence and extent in Georgia have not been previously resolved because of limited geologic mapping. However, recent mapping in the Georgia IP indicates that the arc complex is much more extensive than previously recognized. DC country rocks are predominantly metabasalt, interlayered with lesser felsic metavolcanic and turbiditic metasedimentary rocks, but the major volume of the DC consists of Ordovician granitic and tonolitic intrusives, and lessor mafic and ultramafic intrusives. The SKCT is regionally concordant to footwall units within the BZ and equivalent units in the OG, including distinctive vitreous micaceous quartzites (Chattahoochee Palisades, Devil’s Backbone, Tallassee) interlayered with aluminous schist in the Jacksons Gap Group (BZ), and the equivalent Loachapoka Fm. (Saugahatchee Qtz.) in the IP. Rocks of the EBR and OG below the SKCT were intruded by an abundant series of Middle Ordovician to Silurian S-type, sill-like granite to granodiorite gneissic plutons that rim the DC but are not found above the thrust. These include the Kowaliga (467-455 Ma), Zana (477-457 Ma), Villa Rica (458 Ma), Mulberry Rock (450-430 Ma) and Austell (460-430 Ma) in the EBR and the Farmville (440 Ma) and Lithonia (450-443 Ma) in the OG. These plutons likely formed from the melting of Grenville-aged continental crust beneath an evolving back-arc basin. The distinctive sequence of rocks in the EBR/BZ can be traced S. and then NE around the SW hinge of the TS into the IP of Alabama and Georgia, and are found to continue on the SE limb of that structure northeastward to 25 km E. of Atlanta.