LAURENTIAN MAGMATISM OF THE SOUTHERN APPALACHIAN BLUE-RIDGE - POST-IAPETAN RIFTING

Tectonically significant postrift metaigneous rocks occur within three genetically distinct cover-rock sequences along a 400 km strike segme nt of the southeastern margin of Laurentia, in the southern Appalachia n western Blue Ridge. These include (1) Neoproterozoic rift facies roc ks, (2) lower Paleozoic drift facies rocks, and (3) unconformably over lying postdrift Paleozoic successor basin sequences. This region is di visible into two along-strike, ensialic, continental margin volcanic b elts, separated by a probable Neoproterozoic transfer fault. To the no rtheast, igneous rocks are intrusive into sequences 1 and 2 above, whe reas in the overlying successor basins on both sides of the transfer b oundary, they occur predominantly as eruptive rocks. These igneous roc ks can be separated into several suites that are distinct from the Neo proterozoic rift-related igneous rocks, based on stratigraphic positio n, geographic location, and composition, Mafic dikes and sills intrusi ve into the rift facies and mafic metavolcanic rocks of the Hillabee G reenstone represent low-K tholeiitic magmatism associated with derivat ion from a ''depleted'' mantle source, whereas most igneous rocks in t he northeastern belt exhibit an alkaline basalt affinity. The successo r basin sequences associated with the volcanic activity formed above e xtended and thinned continental crust, near and parallel to the southe astern Laurentian margin. This igneous activity can be constrained bro adly between stratigraphic position and age of metamorphism (Middle Or dovician to earliest Mississippian time), Most likely this magmatic ac tivity was associated with a destructive plate boundary during Paleozo ic A-type subduction, but was largely decoupled from slab-derived magm atism, being instead more likely associated with backarc or pull-apart basin evolution.