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