Inferences on late-stage evolution of the Russell Lake Allochthon and the Soapstone Ridge Complex in Georgia, Southern Appalachians, based on chlorite geothermometry

Abstract

Chlorite is a common mineral in most metamorphosed mafic and ultramafic rocks and the use of chlorite as a geothermometer was applied to rocks from the southern Appalachian Piedmont for this study. In particular, chlorite-bearing rocks from the Russell Lake Allochthon (RLA) and the Soapstone Ridge Complex (SSR) are the focus of this investigation. The RLA is composed of numerous metamorphosed mafic and ultramafic bodies exposed within rocks of the Carolina superterrane which forms part of the peri-Gondwana realm of the Southern Appalachians. The SSR is one of the largest metamorphosed mafic and ultramafic bodies in the southern Appalachians; 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 that comprise the RLA are generally small, with outcrop areas generally not exceeding a few square kilometers. In the RLA and SSR, chlorite occurs together with zoned amphiboles. In the RLA, chlorite exhibits low total iron atoms per formula unit and dominantly plots in the ripidolite field. In contrast, SSR chlorites are poorer in total iron and almost all are clinochlores. Results of chlorite geothermometry from rocks of the RLA yield temperatures which range from ~ 280 to ~ 320 °C, whereas chlorites from the SSR yield temperatures which range from ~ 201 to ~ 250 °C. These calculated temperatures indicate that chlorites in the RLA were formed at slightly higher temperatures in comparison to those from the SSR, which may suggest that these bodies were subjected to different late-stage metamorphic conditions and may also have originated under different tectonic settings. The RLA and SSR are approximately 100 km apart and a terrane boundary occurs between them; these bodies were each metamorphosed, and likely evolved, in different regions and each was later thrusted onto its country rocks during the final stages of the assembly of Pangea.