Abstract

Tectonically significant postrift metaigneous rocks occur within three genetically distinct cover-rock sequences along a 400 km strike segment of the southeastern margin of Laurentia, in the southern Appalachian western Blue Ridge. These include (1) Neoproterozoic rift facies rocks, (2) lower Paleozoic drift facies rocks, and (3) unconformably overlying postdrift Paleozoic successor basin sequences. This region is divisible into two along-strike, ensialic, continental margin volcanic belts, separated by a probable Neoproterozoic transfer fault. To the northeast, igneous rocks are intrusive into sequences 1 and 2 above, whereas in the overlying successor basins on both sides of the transfer boundary, they occur predominantly as eruptive rocks. These igneous rocks can be separated into several suites that are distinct from the Neoproterozoic rift-related igneous rocks, based on stratigraphic position, geographic location, and composition. Mafic dikes and sills intrusive into the rift facies and mafic metavolcanic rocks of the Hillabee Greenstone represent low-K tholeiitic magmatism associated with derivation from a “depleted” mantle source, whereas most igneous rocks in the northeastern belt exhibit an alkaline basalt affinity. The successor basin sequences associated with the volcanic activity formed above extended and thinned continental crust, near and parallel to the southeastern Laurentian margin. This igneous activity can be constrained broadly between stratigraphic position and age of metamorphism (Middle Ordovician to earliest Mississippian time). Most likely this magmatic activity was associated with a destructive plate boundary during Paleozoic A-type subduction, but was largely decoupled from slab-derived magmatism, being instead more likely associated with backarc or pull-apart basin evolution.

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