SIGNIFICANCE OF SOUTHERN APPALACHIAN BLUE RIDGE AND WESTERN INNER PIEDMONT ORDOVICIAN PLUTONIC AND VOLCANIC ASSEMBLAGES

Ordovician (Ord) metaplutonic and metavolcanic rocks are widely distributed throughout the southern Appalachian Blue Ridge (BR) and Inner Piedmont (IP) in the Cowrock and Cartoogechaye terranes, Dahlonega gold belt (DGB), and western and eastern Tugaloo (Tt) terranes in the Carolinas and NE GA, and their equivalents to the SW and NE. Enclosing rocks consist of deep-water clastics (Ashe-Tallulah Falls Fm., and similar units) with a Laurentian source. All have been subjected to amphibolite facies metamorphism and polyphase ductile deformation, which makes their lineage more difficult to ascertain. Today metavolcanic units occur as mafic and mafic-ultramafic complexes, and as interlayers and boudins in medium to high-grade rocks. Ord granitoids are more abundant in the eastern Tt than to the W, and metavolcanic rocks also become more abundant in the eastern Tt. Numerous studies of major and trace element geochemistry of the mafic rocks have revealed that almost all of these bodies have a non-continental origin, and most frequently plot in the arc to MORB fields of tectonic discrimination diagrams. Although the DGB was recently characterized as an arc terrane, metasedimentary rocks make up >85% of the northern two thirds of the belt, and it contains several isolated arc complexes (Lake Burton, Sally Free), with other BR terranes containing a similar distribution. The eastern Tt, however, contains voluminous arc-to-MORB volcanic rocks, many of which belong to the Mid-Late Ord (452 Ma SHRIMP) Poor Mountain Fm. Four prominent Ord BR-IP granitoids (Persimmon Creek, 468 Ma; Whiteside, 466 Ma; Henderson, 470 Ma, and Elkahatchee, 490 Ma) were likely derived from mafic sources. Eastern BR granitoids from NC to AL may have formed over a W-dipping subduction zone, whereas the younger Ord volcanic and plutonic rocks of the eastern Tt may have formed in arcs generated over an E-dipping subduction zone. Ironically, the Middle Ord clastic wedge contains no volcanic detritus.