EVOLUTION OF THE RUSSELL LAKE ALLOCHTHON AND THE SOAPSTONE RIDGE COMPLEX IN GEORGIA, SOUTHERN APPALACHIANS: INFERENCES FROM CHLORITE GEOTHERMOMETRY

Chlorite, a common mineral in most metamorphosed mafic and ultramafic rocks which occur in the Southern Appalachians Piedmont in the state of Georgia, USA, was the focus of this investigation. The Russell Lake Allochthon (RLA) is composed of numerous metamorphosed mafic and ultramafic bodies exposed within rocks of the Carolina terrane which forms part of the peri-Gondwana realm of the Southern Appalachians. The Soapstone Ridge complex (SSR) is the largest metamorphosed mafic and ultramafic body in the southern Appalachians where it is in thrust contact with underlying rocks of the Piedmont domain which forms part of the Iapetan realm of the Southern Appalachians. The metamorphosed bodies constituting both the RLA are generally small, with areas generally not exceeding a few square kilometers. Although the contacts of the RLA and their country rocks are poorly exposed, these metamorphosed mafic and ultramafic bodies generally overlie their country rocks, often in tectonic contacts.

Chlorite occurs together with zoned amphiboles, and chlorite crystals in the RLA have rather low total iron atoms per formula unit and they dominantly plots in the ripidolite field whereas SSR chlorites are even poorer in total iron and almost all are clinochores. Application of chlorite thermometry to rocks of the RLA yields temperatures which range from ~280^oC to ~320^oC, whereas chlorites from the SSR yield temperatures which range from ~200^oC to ~250^oC. These calculated temperatures indicate that chlorites in the RLA were formed at higher temperatures in comparison to those from the SSR, which may suggest that these bodies underwent different metamorphic conditions and may also have originated under different tectonic settings. Since these two geographically distinct locations where metamorphosed mafic/ultramafic bodies crop out are at least 100km apart, and a terrane boundary occurs between them, it is interpreted here that they were each metamorphosed, and perhaps formed, in different regions and each was later thrusted onto its country rocks during the final stages of the assembly of Pangea.