GEOLOGIC MAP OF THE LAWRENCEVILLE 7.5-MINUTE QUADRANGLE, GEORGIA

The Lawrenceville quadrangle lies in the western inner Piedmont. The City of Lawrenceville and surrounding area are suburbs of the City of Atlanta located approximately 40 km to the southwest. The northwestern corner of the Lawrenceville quadrangle is underlain by graphitic and slightly graphitic SC mylonites in the Dacula shear zone that splays from the Brevard fault zone northeast of Atlanta. Brevard zone mylonites, identical to the Dacula zone mylonites, dip beneath the northwestern edge of the Atlanta nappe and presumably underlie it. Dacula zone mylonites underlie the Atlanta nappe on the northeast and nappes in the Lawrenceville quadrangle on the northwest and north. Cores of the nappes in the Lawrenceville quadrangle are Lithonia Gneiss and rocks of the Allatoona allochthon, including distinctive Ropes Creek Metabasalt, with its characteristic magnetite quartzites (“banded iron formations”), and Sandy Springs Group (aluminous schist and Chattahoochee Palisades Quartzite) form part of the cover units folded around the cores of the nappes.

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.