RELICT AND RE-SET OXYGEN ISOTOPE RATIOS FROM THE RUSSELL LAKE ALLOCHTHON, SOUTHERN APPALACHIANS

The Russell Lake Allochthon crops out as small, numerous, metamorphosed mafic/ultramafic bodies in the southern Appalachians Piedmont in Georgia, USA. Oxygen isotope compositions obtained by laser-fluorination mass spectrometry of samples from one of these bodies, at Bakers Ferry, show average plagioclase and olivine δ 18 O values of 6.6‰ and 5.5‰, respectively, with an average Δ 18 O plagioclase-olivine value of 1.1‰, and a closure temperature of 962 °C using the fractionations of Chiba et al. (1989). Samples from two other bodies, the River Road and the Nancy Hart complex, show average plagioclase δ 18 O values of 7.2‰ and 7.5‰, olivine δ 18 O values of 3.4‰ and 4.0‰, and Δ 18 O plagioclase-olivine values of 3.8‰ and 4.5‰, respectively. Similarly, the River Road body and the Nancy Hart complex yield much lower oxygen isotope thermometry temperatures of 409 °C and 419 °C, respectively, consistent with subsolidus 18 O exchange. These three bodies are all within 6 km of each other. Such a wide range in both mineral δ 18 O values and isotope thermometry temperatures in spatially closely located bodies is consistent with values encountered in ophiolite complexes, rather than layered intrusions. The occurrence of numerous bodies with different oxygen isotope signatures within relatively short distances from one another may suggest their origin as an ophiolite, where fracturing associated with their formation in extensional environments provided numerous pathways for fluids to interact with these bodies and reset their δ 18 O values.