THE CASE FOR AN ACADIAN ARC IN THE EASTERN BLUE RIDGE BELT (TOE TERRANE), SOUTHERN APPALACHIAN OROGEN, USA

The Southern Appalachian Orogen has a Mesoproterozoic to late Paleozoic history of orogenesis. Proposed tectonic models involve rifting, subduction and arc development, collision of micro- continents, translational faulting, and continent-continent collision. Models that address the nature of the Eastern Blue Ridge Belt (EBRB=Toe Terrane=Jefferson Terrane=w. Tugaloo Terrane) at the core of the orogen have assigned rock units within the Belt to continental margin, rift basin (ocean) floor, arc/microcontinent margin, and accretionary complex tectonic settings.

There is some evidence of an Acadian arc in the EBRB including spatial, petrochemical, and structural evidence. Acadian plutons define a linear array in the EBRB that disappears beneath the Smith River Allochthon in Virginia and the Coastal Plain rocks in Alabama. EBRB Acadian plutons are separated by 46 to 210 km and have an average spacing along this array of 102 km, a value that falls near a modern average of 116 km derived from selected contemporary oceanic and continental arcs. In modern arcs, volcanoes are 5 to 645 km apart. The major element chemistry and modes of EBRB Acadian plutonic rocks shows them to be peraluminous, trondhjemitic to granitic rocks. Available trace element chemistry indicates that the rocks are low in HFSE and have isotopic signatures that range from those of primitive arc rocks to continental margin intrusions. The volume of individual plutons is low, however, and the apparent temporal span of plutonism is longer than would be expected (355-390 my) in a short-lived arc, leading some workers to conclude that the plutonic rock is too sparsely distributed and temporally disparate to represent a magmatic arc. Albeit the magmatic flux rate is lower than is typical of arcs, the small volumes of plutonic rock and metamorphic P,T estimates for country rocks are compatible with the plutons representing the tapered, crustal roots of sub-arc plutons in a short-lived arc.