An Ordovician Accretionary Orogen in the Southern Appalachians-Part 1: Geochemistry of Coeval Igneous Rocks

Traditional evolutionary models of the early Paleozoic eastern Laurentian margin involve the transition of a rifted passive margin to arc-continent collision. In the northern Appalachians, the Ordovician Taconic orogeny was caused by collision of Laurentia and possibly exotic arc terranes. From the foreland to hinterland there is evidence of a deformed margin, preserved arc(s), and a relatively thick accretionary prism. Existence of high-pressure metamorphism and geochemical analyses of concomitant igneous rocks, support an arc accretion model involving partial subduction of Laurentia.

The southern Appalachians lack criteria typical of Taconian arc collision. For example, in the southernmost Appalachians of Alabama, the Hillabee Greenstone (HG) is an isolated fragment of an Ordovician bimodal volcanic sequence that lies structurally above the outermost Laurentian Devonian-earliest Mississippian(?) shelf sequence, and is not associated with a preserved subduction complex, high-pressure metamorphism, accretionary wedge, arc rocks proper, ophiolitic fragments, or plutonic rocks or dikes. Further, there is no evidence that the HG or related rocks experienced significant Ordovician deformation. Structural relationships indicate that the HG remained relatively undeformed for >90 m.y. There is compelling evidence that the HG and similar rocks of the Pumpkinvine Creek Formation (PCF), located further to the northeast in Georgia, represent backarc volcanism emplaced along a highly attenuated continental margin. Geochemical studies of coeval plutonic rocks in the adjacent to the southeast Ashland Wedowee Belt (AWB) have geochemical characteristics of volcanic arcs. A relatively simple tectonic model involves west-dipping subduction beneath attenuated crust of the Laurentian margin where accretion involved at least one period of extension and possibly discrete periods of both extension and contraction. Geochemistry of Ordovician igneous rocks in terranes of similar time-tectonic positions to the HG and PCF and AWB suggests that the accretionary model could be applicable to the southern, and potentially, central Appalachians.