THE MULBERRY ROCK GNEISS STRUCTURAL RECESS OF NORTHWESTERN GEORGIA: GRENVILLE BASEMENT OR PALEOZOIC INTRUSION?

The southeastern boundary of the southern western Blue Ridge and northern Talladega belt in Georgia is the Allatoona fault. This terrane-bounding fault formed very late in the kinematic sequence and maintains a relatively straight trace for >110 km, and remains undeflected past the Cartersville recess. However, on the Yorkville, GA 7.5’ quadrangle, a large structural recess occurs along the southeastern border of the Talladega belt in west Georgia in which Talladega belt rocks are indented ~10 km, trending north-south, into metavolcanic and metaplutonic rocks of the eastern Blue Ridge. Large culminations of plutonic rocks, termed Mulberry Rock Gneiss (MRG) occur within this structural recess. Previous work suggested that these yet undated rocks are Grenvillian basement overlain nonconformably(?). This would be the only known exposure of basement rocks in the Talladega belt. The Mulberry Rock Gneiss is a medium-grained two-mica orthogneiss unlike the megacrystic Corbin Metagranite of well documented Grenville age, which cores the large basement culmination east and northeast of Cartersville, GA. Lithologically, the MRG is similar to Paleozoic granitic gneisses of the immediately adjacent eastern Blue Ridge. The cover sequence surrounding the MRG consists of graphitic and nongraphitic sericite phyllite. These rocks are quite different than the coarse metaclastic sequence nonconformably overlying the Corbin basement rocks 30 km to the northeast in the southern Blue Ridge. Three possible working hypotheses for the tectono-stratigraphic setting of the MRG are currently being tested: 1) the MRG is Grenville basement lithologically dissimilar to other basement culminations to the northeast, surrounded by cover rocks of the Talladega belt; 2) the MRG occurs within the eastern Blue Ridge terrane and represents a Paleozoic intrusion, implying that an unrecognized tectonic break to the northwest bounds the MRG and its cover sequence with rocks of the Talladega belt; and 3) the structural indentation is a half window or eyelid window where the eastern Blue Ridge hanging wall rocks as well as the terrane bounding faults, are sharply folded into the Mulberry Rock structural recess.