ABSTRACT The eastern Great Smoky Mountains basement complex consists of the following components: (1) ca. 1350–1325 Ma orthogneiss and mafic xenoliths that represent some of the oldest crust in Appalachian Grenville massifs (similar to “pre-Grenville” basement components in the Adirondack, Green Mountain, Hudson Highland, and Shenandoah massifs); (2) ca. 1150 Ma augen orthogneisses and granitic orthogneisses correlating with the Shawinigan phase of Grenville magmatism; and (3) paragneisses (cover rocks) that have either pre- or syn-Grenville (i.e., Mesoproterozoic) versus post-Grenville (Neoproterozoic) depositional ages, and that experienced Taconian metamorphism and migmatization. Mesoproterozoic paragneisses contain major zircon age modes that require a component of Proterozoic crust in the source region. The Neoproterozoic paragneisses exhibit the archetypical “Grenville doublet” in detrital zircon age distributions that matches the age distribution of Ottawan and Shawinigan magmatic/metamorphic events in eastern Laurentia. Most zircon U-Pb age systematics exhibit variable lead loss interpreted to result from high-grade Taconian (ca. 450 Ma) regional metamorphism and migmatization. Neodymium mantle model ages (T DM) for ortho- and paragneisses range from 1.8 to 1.6 Ga, indicating that all rocks were derived from recycling of Proterozoic crust (i.e., they are not juvenile), which is consistent with Proterozoic detrital zircon ages in pre- to syn-Grenville paragneisses. Lead isotope compositions confirm the presence of an exotic (Amazonian) crustal component in the source region for the protoliths of the pre-Grenville orthogneisses and xenoliths, and that this exotic component was incorporated to varying degrees in the evolution of the basement complex. The oldest age component may represent an Amazonian pre-Grenville analog to the ca. 1.35 Ga native Laurentian crust present in Adirondack and northern Appalachian basement massifs.

ABSTRACT The southern Appalachian orogen is a Paleozoic accretionary-collisional orogen that formed as the result of three Paleozoic orogenies, Taconic, Acadian and Neoacadian, and Alleghanian orogenies. The Blue Ridge–Piedmont megathrust sheet exposes various crystalline terranes of the Blue Ridge and Inner Piedmont that record the different effects of these orogenies. The western Blue Ridge is the Neoproterozoic to Ordovician Laurentian margin. Constructed on Mesoproterozoic basement, 1.2–1.0 Ga, the western Blue Ridge transitions from two rifting events at ca. 750 Ma and ca. 565 Ma to an Early Cambrian passive margin and then carbonate bank. The Hayesville fault marks the Taconic suture and separates the western Blue Ridge from distal peri-Laurentian terranes of the central and eastern Blue Ridge, which are the Cartoogechaye, Cowrock, Dahlonega gold belt, and Tugaloo terranes. The central and eastern Blue Ridge terranes are dominantly clastic in composition, intruded by Ordovician to Mississippian granitoids, and contain ultramafic and mafic rocks, suggesting deposition on oceanic crust. These terranes accreted to the western Blue Ridge during the Taconic orogeny at 462–448 Ma, resulting in metamorphism dated with SHRIMP (sensitive high-resolution ion microprobe) U-Pb ages of metamorphic zircon. The Inner Piedmont, which is separated from the Blue Ridge by the Brevard fault zone, experienced upper amphibolite, sillimanite I and higher-grade ­metamorphism during the Acadian and Neoacadian orogenies, 395–345 Ma. These events also affected the eastern Blue Ridge, and parts of the western Blue Ridge. The Acadian and Neoacadian orogeny is the result of the oblique collision and accretion of the peri-Gondwanan Carolina superterrane overriding the Inner Piedmont. During this collision, the Inner Piedmont was a forced mid-crustal orogenic channel that flowed NW-, W-, and SW-directed from underneath the Carolina superterrane. The Alleghanian orogeny thrust these terranes northwestward as part of the Blue Ridge–Piedmont megathrust sheet during the collision of Gondwana (Africa) and the formation of Pangea.