Island Arc Origin for the Gladesville and Associated Bodies in the Carolina Superterrane in Central Georgia, Southern Appalachians

Abstract

Terrestrial mafic and ultramafic bodies form in various tectonic settings and from a wide variety of magma types. Clarifying the origin of such rocks in orogenic belts such as the southern Appalachians is important to understanding the formation of mountain belts and associated mafic and ultramafic rocks. A petrographic and geochemical study was conducted on the Gladesville, Hillsboro, Juliette, Holly Grove, and Berry Creek bodies in the Carolina superterrane of the southern Appalachians in central Georgia to place some constrains on the origin and tectonic setting of these mafic and ultramafic rocks. Results from this study show that the rock types investigated in the Gladesville and associated bodies are mostly metabasalts and olivine metagabbronorites. The mineralogies of these bodies are plagioclase, hornblende, clinopyroxene, and opaques. MgO contents range from 3.5 to 7.5 wt% MgO for the Gladesville, Hillsboro, and Berry Creek bodies, with the Juliette and Holly Grove bodies having slightly higher MgO concentrations, ranging from 11.8 to 12.4 wt%. CaO contents of samples from all the bodies range from 6.9 to 18.1 wt%. Concentrations of trace elements, including Ni and Cr, in all the bodies range from 6 to 224 ppm and from 17 to 1932 ppm, respectively, generally akin to those of tholeiitic basalts. Plagioclase mineral compositions vary widely: the Gladesville body is characterized by plagioclase with An75.1 to An95.7, whereas the Hillsboro body has plagioclase with anorthite compositions of An46.2–69.8. Other bodies, such as the Juliette and Holly Grove bodies, are highly calcic, with plagioclase content ranges of An96.6–99.8 and An90.8–96.9, respectively. Amphiboles from the investigated bodies are tschermakite, magnesiohornblende, and pargasite, and they plot within the range of amphiboles from island arcs and ophiolite complexes. Calcic pyroxenes vary from body to body: Juliette (Ca38.5–50.9Mg54.2–44.9Fe7.2–4.2) through Holly Grove (Ca47.1–49.4Mg43.2–41.2Fe9.7–9.4) to Gladesville (Ca48.9–52.5Mg41.4–42.5Fe9.7–5.2). Both spider diagrams, which show Nb depletions, and tectonic discrimination diagrams are consistent with formation of these bodies in island arc settings. The highly calcic content of most of the plagioclase further supports an island arc origin for these bodies.