SPARTA GRANITE VARIATION AMONG THREE QUARRIES IN SOUTHEAST GEORGIA

In the eastern Appalachian Piedmont of Georgia, the Sparta granite intrudes metamorphic rocks of the Savannah River and Milledgeville Terranes on the southeast flank of the Modoc fault zone. Sparta granite yields Alleghanian crystallization ages of 287-312 Ma from differing isotopic systems (Fullager and Butler, 1979; Pray, 1997), overlapping the 285-295 Ma deformation of the mylonitic rocks of the Modoc fault (Sacks and Secor, 1990). This report is based on field observations and petrographic study of rocks in aggregate quarries currently operated by Vulcan (VQ) and Heidelberg Materials (HQ) near Sparta, and by Martin Marietta near Warrenton. The VQ quarry reveals magmatic flow banding disrupted locally by healed breccia and faults. Brittle deformation dominates at the HQ, 4.6 km to the SE, with cross-cutting thin (< 5 cm) biotite-aplite fractures and shears, veins of smoky quartz, and smoky quartz-biotite +/- K-feldspar veins. Pegmatite-aplite dikes striking NE and NW are common at the HQ as the youngest intrusive phase. Pegmatitic phases at the VQ are more prevalent within flow bands. The texture and mineralogy of the host granite are uniform at HQ but more variable in the flow banded rocks at VQ. Investigations with Massey (2019) and Fischer (2020) of Sparta granite at the Warrenton Quarry (WQ) 29.2 km to the northeast, reveal a well-defined intrusive sequence. The oldest, a coarse-grained granodiorite to granite is intruded, with sharp contacts, by a medium grained granite. Biotite defines the foliation in both units. A pink leucocratic coarse-grained to pegmatitic granite occurs as pods and layers with gradational to sharp contacts with the host granites. Pegmatite dikes intrude each of the major granite units and thin aplite layers occur as a border phase around most units. Biotite schist and gneiss are exposed in the quarry margins and intrusion often parallels metamorphic foliation. The abundance of metamorphic rock in the upper levels of the WQ suggests this part of the Sparta granite is near the roof of the pluton.