STRUCTURAL RELATIONSHIPS, PETROGRAPHY, AND BULK-ROCK GEOCHEMISTRY OF THE RUSSELL LAKE ALLOCHTHON, SOUTHERN APPALACHIANS

Small and isolated exposures of metamorphosed mafic and ultramafic bodies are scattered in the Piedmont Province of the southern Appalachians in east central Georgia. A compilation of a map of the bodies, termed the Russell Lake Allochthon (RLA), was made by Allard and Whitney (1994) who hypothesized that they form an allochthonous sheet that was thrusted onto country rocks of the Carolina superterrane, probably during Alleghanian times. Since this work, there has been no further published work on the RLA. Rocks types of the RLA are metatroctolites, metapyroxenites, metagabbronorites and olivine metagabbronorites. Rocks such as metatroctolites display relict cumulate textures, with relict psuedomorphs after cumulate minerals such as olivine and plagioclase.

Some RLA rocks have high concentrations of oxides like MgO (concentrations up to 25 wt. %) and high SiO₂ contents (> 45 wt. % SiO₂). On plots of MgO versus incompatible oxides like potassium, low K₂O concentration of less than 0.2 wt. % occur in rocks with MgO contents of over 15 wt. % MgO, but K₂O of over 0.8 wt. % occur in rocks with <15 wt. % MgO, consistent with igneous compositions. Compatible trace elements such as Ni and Cr show high concentrations of over 800 ppm and 2000 ppm, respectively, in rocks with >25 wt. % MgO contents, whereas in rocks with MgO contents of <10 wt. % MgO, these concentrations drop to below 200 ppm. Incompatible and relatively immobile trace elements such as Y are characterized by concentrations of over 17 ppm in rocks with <15 wt. % MgO, which drop to <5 ppm in samples with >30 wt. % MgO. The predominance of rocks such as metapyroxenites, metatroctolites and metagabbroic rocks, coupled with the general lack of rocks highly enriched in MgO, such as metaperidotites, points to protoliths similar to island arc intrusives and cumulates. We propose that the RLA probably represent fragments of mostly pyroxenite and gabbroic intrusive and cumulate rocks of island arc complexes, where upper volcanic sections have now been eroded away.