AGE, GEOCHEMISTRY AND TECTONIC SETTING OF THE MOST INBOARD ACCRETED PALEOZOIC ARC RELATED TERRANE IN THE SOUTHERN APPALACHIANS

The Pumpkinvine Creek Formation (PCF) represents a bimodal (at the map and outcrop scale) back arc volcanism that was later accreted onto Laurentia during the Paleozoic and is now in the most inboard position of any accreted terranes in the Georgia southern Appalachian. The PCF is composed of amphibolite (metabasalt), felsic gneiss (metarhyolite), relatively minor proportions of metapelitic components and very local thin discontinuous bands of quartzite. Major and trace element chemistry shows that the PCF amphibolite is tholeiitic, displays modest LILE enrichment making it most similar to the back arc basin basalt. The PCF felsic lithofacies informally named as Galts Ferry Gneiss (GFG) is calc alkaline, peraluminous and trondhjemitic in composition and shows moderate LREE enrichment. LA-MC-ICPMS data of zircons from GFG yielded 461.5±6.8 Ma age. The analysis of 100 detrital zircon from the quartzite shows that 80% of the grains range in age between (1017-1190) Ma. The Rb-Sr systematics in the GFG is complicated, as the whole rock measurements do not conform to a unique isochron. Assuming that the source of the felsic magma was from multiple or mixed sources, simplification of the data yielded two separate isochrons of similar age (420-450 Ma with significant errors) but differing initial ⁸⁷Sr/⁸⁶Sr of 0.7082 and 0.7059. Nd and Sr isotopic modeling has been employed as a tracer for the source of both mafic and felsic components of the PCF. Epsilon (T₄₆₀ Ma) Nd values of -3.21 to 4.65 for GFG and +3.3 to +7.7 for PCF amphibolites are indicative of a very primitive mantle source likely containing little or no contamination from the Laurentian margin or similar aged crust. This would suggest that this arc/back-arc system formed in a Benioff-type subduction setting (on oceanic crust) not too far off the Mesoproterozoic (Grenville aged) continental margin of Laurentia which served as the source of detritus.