GEOLOGIC MAP OF THE LAWRENCEVILLE 7.5-MINUTE QUADRANGLE, GEORGIA
The Lithonia Gneiss, which underlies large areas in the quadrangle, has been mapped to the southwest into Alabama and into the Ordovician Farmville Granite (~477 Ma). The granitoid phase of the Lithonia is an Ordovician batholith that intruded the Stonewall Gneiss producing the Mount Arabia Migmatite. The Stonewall crops out around the edges of the Lithonia granitoid phase and in local roof pendants. The migmatite appears to occur only around the edges of the Lithonia and around roof pendants of Stonewall Gneiss, whereas the Lithonia away from any Stonewall is mostly metamorphosed granitoid. The aluminous schist and Chattahoochee Palisades Quartzite of the Sandy Springs Group structurally overlie the Lithonia, the Stonewall, and the Allatoona allochthon as well, on the Sandy Springs fault. The Lithonia, its country rock, the Stonewall Gneiss, and overlying Sandy Springs rocks constitute a mantled gneiss dome assemblage. The aluminous schist and quartzite are interpreted to have been deposited unconformably upon the Lithonia and the other units and were metamorphosed with the Lithonia and thrust across it. Sandy Springs Group rocks also border the Dacula shear zone on the northwest and probably dip beneath the zone and the Lithonia Gneiss.