THE HAMMETT GROVE META-IGNEOUS SUITE, SOUTH CAROLINA: BULK-ROCK, MINERAL COMPOSITION, HYDROGEN, AND OXYGEN ISOTOPE DATA FROM METAGABBROS

The Hammett Grove meta-igneous suite (HGMS) (Mittwede, 1986), is a metamorphosed mafic-ultramafic body located in the Cat Square terrane of the southern Appalachians (e.g., Hatcher et al., 2007) in northwestern South Carolina. Rock types range from altered ultramafites, metapyroxenites, metagabbros, metabasalts, and metacherts (Mittwede et al., 1987, 1990). Bulk-rock compositions of the HGMS are described in Mittwede et al. (1987, 1990) and they are dominantly pyroxenitic and gabbroic. HGMS metagabbros studied have bulk-rock compositions ranging from 47.8 – 49.2 wt. %, within the 45.7 – 51.9 wt. % range of other metagabbros from Mittwede et al. (1987, 1990). Mineral composition data from HGMS are very scarce. Mineral compositions were determined on the electron mocroprobe on amphibole, plagioclase, and clinopyroxene. Neither orthopyroxene nor olivine were observed in the metagabbros studied. Amphiboles are comprised of magnesiohornblende cores rimmed by actinolite. Plagioclase compositions range from andesine and labradorite (An_49.1-53.0) which partially rim anorthite (An_89.6-94.0) cores. Clinopyroxenes are highly calcic, with a rather limited range in composition: Mg_41.0-42.6Fe_4.9-8.7Ca_49.1-53.0. The highly calcic nature of the clinopyroxenes is probably either due to partitioning of the Fe into Fe²⁺ and Fe³⁺, or that the clinopyroxenes are metamorphic, or both. One sample has plagioclase and amphibole δ¹⁸O values of 7.2 and 5.8 ‰, respectively, and a δD_amphibole value of -49 ‰. The limited number of ultrabasic rocks from HGMS (only 3 of the 29 samples plot in the ultrabasic range) and the predominance of metapyroxenites and metagabbros support the interpretation that these rocks were formed in an island arc setting. Zoned amphiboles probably represent metamorphism of the HGMS under amphibolites facies conditions, followed by greenschist facies conditions, probably during thrusting of the HGMS. Highly calcic plagioclase cores may represent either a primary feature (similar to anorthites from island arc cumulates, e.g., Beard, 1986), or a metamorphic feature. The less calcic plagioclases were probably formed during thrusting of the HGMS. The isotope values of the HGMS are comparable to those from ophiolite complexes (e.g. Bosch et al., 2004; Demeny et al., 2008).