PETROLOGY AND MICROSTRUCTURE OF ROCKS FROM THE CHATTAHOOCHEE TUNNEL PROJECT, BREVARD FAULT ZONE, ATLANTA GA

Drill core retrieved during engineering studies for the Chattahoochee Tunnel Project have allowed us to take a fresh look at intensely deformed rocks from within the Brevard fault zone near Atlanta. Most of our samples are from 8 to 60 m below surface outcrops of the button schists of the Sandy Springs Group. One drill hole penetrated the Long Island Creek Gneiss, but here we discuss only samples we attribute to the Sandy Springs Group. Two lithologies occur in the core samples from the Sandy Springs Group: a relatively muscovite-rich, strongly foliated, medium to coarse grained mylonitic schist, and a finer grained, quartz-rich and muscovite-poor ultramylonite. The more aluminous schist preserves a peak amphibolite grade metamorphic assemblage of staurolite - garnet - biotite - plagioclase (An20-30) - muscovite. Garnet-biotite geothermometry suggest that this assemblage equilibrated at about 500-600oC. The garnets are almandine-rich (alm74-83) and exhibit growth zoning characterized by Mn-rich cores (to 6 wt.% MnO) and Mn-poor rims (to 0.2 wt.%). FeO and MgO vary antipathetically with MnO and beautiful S-shaped inclusion trails are preserved in the centers of large garnets. Inclusion-free rims and a host of small euhedral garnets suggest a second distinct period of garnet growth. The S1 foliation defined by muscovite is folded and we correlate this second deformational event with the period of later garnet growth. Other textures and mineral assemblages attest to an early period of B-metasomatism indicated by folded tourmaline-rich lenses, retrograde development of epidote and chlorite, and relatively late brittle deformation accompanied by the development of psuedotachylyte. We currently have no constraints on the ages of the events that affected these rocks but geochronologic work on the nearby Austell Gneiss suggests that the S1 foliation and peak metamorphism are no older than the Early Silurian.