AN EVALUATION OF THE USE OF BULK-ROCK GEOCHEMISTRY IN DETERMINING IGNEOUS PROTOLITHS OF ROCKS FROM THE SOAPSTONE RIDGE COMPLEX, ATLANTA, GEORGIA

The petrotectonic significance of basic and ultrabasic rocks in the reconstruction of the geologic history of an orogen is well known. An inherent assumption in such studies is that the bulk-rock geochemistry represents original igneous compositions and can therefore be used to infer the original protolith of these rocks. To test this idea we examined rocks of the Soapstone Ridge Complex in SE Atlanta, Georgia using the extensive geochemical data base of Higgins et al. (1988) supplemented with our own new data.

The Soapstone Ridge Complex (SSR) is one of the largest occurrences of basic rocks in the southern Appalachian Orogen. The rocks are mostly chlorite amphibole schists with minor amounts of talc and CrFeTi oxides. Epidote amphibolites form a small part of the SSR. The SSR represents structurally the highest thrust sheet in this part of the Georgia Piedmont. Despite the metamorphic mineral assemblages, workers continue to apply igneous terminology to describe these rocks.

Bulk compositions of SSR rocks correspond to mafic and intermediate igneous compositions (45 to 58 anhydrous wt. % SiO₂). No systematic variation trend is found among the major components. Normative mineralogy is rich in pyroxene, with small to moderate amounts of plagioclase. Quartz and olivine are present in low, variable amounts. Alkali content is low, resulting in normative corundum in over half of the analyses. Rare-earth element (REE) plots from the Soapstone Ridge show enrichment in light REE, but display scattered, crossing patterns.

If one uses igneous terminology, SSR rocks are either gabbros and pyroxenites or basalts and andesites, based on normative mineralogy. Ultramafic rocks are not part of the SSR. Comparison to a variety of settings does not produce a match for the SSR. Normative corundum and crossing patterns of REE are related to alteration of the SSR rocks and such altered rocks do not represent their igneous protoliths.