Character of the Alleghanian orogeny in the southern Appalachians: Part II. Geochronological constraints on the tectonothermal evolution of the eastern Piedmont in South Carolina

Abstract

A nearly concordant U-Pb zircon age of 550 ± 4 Ma is interpreted to closely date crystallization of the epizonal Little Mountain metatonalite in the southeastern part of the Charlotte belt in South Carolina. This confirms field studies which indicate that the Charlotte belt contains a plutonic metaigneous complex that developed as a sub-volcanic-arc infrastructure, contemporaneous with vulcanism manifested in the Carolina slate belt. Both paleontological and geochronological controls indicate that the South Carolina slate belt is mostly younger than 570 Ma (Cambrian?), whereas the slate belt in North Carolina and Virginia is mostly of late Proterozoic age. A regionally significant mid-Paleozoic (ca. 340–360 Ma) thermal event is suggested by discordant 40Ar/39Ar whole-rock age spectra of slate/phyllite in the northwestern Carolina slate belt and from hornblende in the southeastern Charlotte belt. It is uncertain if this event was associated with deformation in the eastern Piedmont; however, mid-Paleozoic deformation has been previously documented elsewhere in the western Piedmont. A slightly disconcordant U-Pb zircon age of 317 ± 4 Ma is interpreted to closely date initial crystallization of the deformed Edge-field granite and confirms a record of late Paleozoic penetrative deformation in the Kiokee belt of South Carolina. U-Pb isotope data for the Lake Murray orthogneiss and Clouds Creek granite are discordant and suggest that the magmas of these plutons were derived by partial melting of a sialic Precam-brian source and then emplaced in the eastern Piedmont at ca. 315 Ma (prior to or during the early stages of the Alleghanian orogeny). The thermal maximum of Alleghanian regional metamorphism (amphibolite facies in the Kiokee belt; greenschist facies in the southeastern part of the Carolina slate belt) occurred during ca. 295–315 Ma. During the late Carboniferous and Early Permian, the eastern Piedmont experienced differential uplift, erosion, and relatively rapid postmeta-morphic cooling. Isothermal surfaces were folded into an antiform-synform-antiform configuration corresponding to the Kiokee, Carolina slate, and Charlotte belts, respectively. The geochronological data provide the following calibration for the late Paleozoic deformational chronology recorded in the Kiokee and Carolina slate belts: D2 (Lake Murray deformation), ca. 295–315 Ma; D3 (Clarks Hill deformation), ca. 285–295 Ma; and D4 (Irmo deformation), ca. 268–290 Ma.