PALEOZOIC ASSEMBLY OF THE BLUE RIDGE AND INNER PIEDMONT RECORDED BY METAMORPHIC ZIRCON GEOCHRONOLOGY

Amalgamation of Blue Ridge and Inner Piedmont terranes can be bracketed by the age of regional metamorphism and shared magmatic events. Twenty-five samples collected across the Blue Ridge and Inner Piedmont of NC–GA–SC yield SHRIMP U-Pb zircon rim ages that range from 490-300 Ma. The data can be separated into three events: 465-450 Ma Taconian, 390-340 Ma Acadian/Neoacadian, and 330-300 Ma early Alleghanian events. Combined with metamorphic isograds and other published reliable ages of metamorphism, the following spatial patterns can be distinguished. (1) All Ordovician zircon rim ages occur in the Blue Ridge and define a continuous section from greenschist to granulite facies that represents the Taconic core. Peak metamorphism in the Taconic core occurred at 465-450 Ma at which time the Cartoogechaye, Cowrock, Dahlonega gold belt and Tugaloo terranes docked with the Laurentian margin (western Blue Ridge). Undeformed ~ 450 Ma trondhjemite dikes occur throughout these terranes in SW NC–NE GA and represent a shared magmatic event. (2) Devonian-Mississippian ages (405-325 Ma) dominate throughout the Inner Piedmont and easternmost parts of the Blue Ridge (peaks at 389, 362, and 344 Ma) and represent the Acadian/Neoacadian metamorphic core. The lack of a pervasive overprinting high-temperature deformation at 365–340 Ma in the Carolina superterrane indicates that it had not accreted prior to the Devonian. Instead, our data suggest that the Carolina superterrane was accreted during the Devonian to early Mississippian, producing high-temperature metamorphism and migmatization in the Inner Piedmont and eastern Blue Ridge. (3) Mississippian and younger zircon rim and Ar/Ar cooling ages occur in the Blue Ridge and Inner Piedmont, the results of either Acadian/Neoacadian cooling or the onset of the Alleghanian collision of Africa with Laurentia. This event is more pronounced in the eastern and southeastern part of the Inner Piedmont in GA and the Pine Mountain window, GA–AL.