DELIMITING THE SPATIAL EXTENT OF SUPERPOSED METAMORPHIC EVENTS OF THE BLUE RIDGE AND INNER PIEDMONT WITH SHRIMP U-PB AGES OF ZIRCON RIMS

The polydeformed crystalline southern Appalachians consists of dominantly amphibolite facies rocks with several sillimanite grade metamorphic highs, yet ambiguity exists regarding the areal extent and timing of peak metamorphism. This study presents SHRIMP U-Pb zircon rim ages from twenty-one samples across the Blue Ridge (BR) and Inner Piedmont (IP) to help delimit the timing and spatial extent of the superposed metamorphisms. Samples included both para- and orthogneisses lithologies. Standard mineral separation procedures were followed to obtain zircons for analysis. Zircons are round, and subhedral to anhedral, but soccer ball-shaped morphologies are not common. Zircon rims vary from 10-40 µm and were bright white, or dark gray to black in CL. Rims are unzoned, and truncate and/or embay interior oscillatory zoning. Beam placement was located near the grain edge to avoid analysis of older cores. Results of ²⁰⁶Pb/²³⁸U ages from 85 analyses range from 700-300 Ma and define a bimodal distribution with 95% of the data between 500-300 Ma. Ages greater than 500 Ma represent significant overlap with older zircon interiors. Th/U ratios range from 0.01-1.2, but the majority of the ratios are less than 0.1. Relative probability plots of the data define peaks at 460, 450, 425, 389, 362, and 344 Ma. The following can be recognized by comparing the spatial occurrence of zircon rim ages from separate samples with metamorphic isograds and other published reliable ages. (1) All 460-450 Ma ages occur in the western and central BR, define a continuous section from greenschist to granulite facies that represents the intact Taconic core. (2) The IP contains 420-325 Ma zircon rim ages with peaks at 389, 362, and 344 Ma and represents the Neoacadian metamorphic core. (3) The eastern BR contains evidence of three possible tectonothermal events, 460 Ma, 360-350 Ma and 320 Ma. Implications for terrane accretion suggest that the central BR terranes were accreted to the western BR and metamorphosed during the Taconian orogeny. The lack of a pervasive overprinting 360-350 Ma high-temperature metamorphism in the Carolina superterrane indicates it could not have been accreted prior to the Devonian. Instead, our data indicate that the Carolina superterrane was accreted during the Late Devonian to early Mississippian producing high-temperature metamorphism and migmatization of the IP and eastern BR.