Silurian volcanism along the eastern margin of Laurentia: New constraints on suprasubduction zone tectonics, age of metamorphism, and successor basin formation in the southern Appalachian western Blue Ridge (U.S.A.)

Tectonic models of the Appalachian orogen (USA) often attribute Ordovician tectonism to underthrusting of the Laurentian continental margin beneath exotic or peri-Laurentian arc terranes during the Taconic orogeny. However, rocks of the southern Appalachian Blue Ridge and Inner Piedmont terranes suggest this segment of the margin developed in a Laurentian plate back-arc setting that was isolated from Ordovician, collisional arc obduction events affecting the central-northern Appalachians. Here, we present Silurian igneous isotopic ages for pre-metamorphic, back-arc alkaline mafic rocks that intruded through and erupted above the Lower Paleozoic, southern Appalachian, western Blue Ridge Laurentian platform. Metabasalt of the Marble Hill Hornblende Schist (423.50 + 12.50/−4.50 Ma) and an underlying feeder intrusion (424.8 ± 1.9 Ma), both of which are overprinted by the regional Barrovian metamorphic isograds, indicate unequivocal Silurian igneous activity on the southern Appalachian continental margin prior to peak metamorphism. Silurian volcanism in the western Blue Ridge/Talladega belt allochthon is consistent with other Laurentian affinity, Silurian, southern Appalachian successor basins. The age, geochemical character, and stratigraphic architecture of the Marble Hill Hornblende Schist indicate it formed in the most distal (cratonal) back-arc region of a Laurentian marginal basin and suggests a genetic relationship to similar rocks preserved in the structurally higher eastern Blue Ridge and Inner Piedmont terranes. We present a tectonic model involving an aborted subduction initiation attempt in the Early Paleozoic, Wedowee-Emuckfaw-Dahlonega back-arc basin that integrates all of the seemingly disparate data from the southern Appalachians and explains the differences in Taconic orogenesis from the southern, central, and northern Appalachians.