Abstract

Detailed mapping, U-Pb ages of igneous and detrital zircons, and isotopic and geochemical characterization of a bimodal volcanic sequence from the Appalachian Blue Ridge province provide constraints for the timing and tectonic setting of volcanism along the Early Paleozoic southeastern Laurentian margin. These units, the Pumpkinvine Creek Formation and related rocks, are interpreted as having been deposited along an extended Laurentian margin in a backarc setting during the mid-Ordovician, and tectonically emplaced as late as the late Paleozoic Alleghanian orogeny.

Two felsite layers from the Pumpkinvine Creek Formation, a bimodal metavolcanic sequence, have ages of 466 ± 5 Ma and 461 ± 3 Ma. Geochemical analyses of metabasalts indicate compositions derived from a suprasubduction zone, and more specifically a backarc. Furthermore, εNd(460 Ma) = (+)3.3–(+)7.7 and 87Sr/86Sr(initial) = 0.7044–0.7069 indicate a juvenile source. The protoliths of the felsic lithofacies were low-potassium rhyodacite, and have εNd(460 Ma) = (−)3.2–(+)4.65 and 87Sr/86Sr(initial) = 0.709–0.722, suggesting incorporation of an evolved crustal component, which we interpret as Laurentian continental lithosphere. Furthermore, detrital zircons derived from within the Pumpkinvine Creek Formation yielded a majority of ages between 1017 and 1190 Ma, indicating that the source is likely a component of the rifted eastern Laurentian margin.

A backarc origin for the Pumpkinvine Creek Formation indicates that the geodynamic setting of the southeastern margin of Laurentia included a retreating subduction zone resulting in extensional accretionary tectonism. These results have implications for the evolution of the southeastern Laurentian margin during a time traditionally linked to collision of an exotic Taconic arc with Laurentia. Additionally, we suggest that there are significant along-orogen disparities that may result from fundamental tectonic boundaries.

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