U-PB ZIRCON AGE ESTIMATES FOR METAMORPHISM OF THE ASHLAND SUPERGROUP, EASTERN BLUE RIDGE OF ALABAMA RECORDS DIACHRONOUS METAMORPHISM IN THE SOUTHERN APPALACHIAN PIEDMONT

Metamorphism and deformation in the southern Appalachians has been shown to be increasingly complex and may reflect diachronous partitioning of deformation and metamorphism throughout the Appalachian Piedmont. To complement geologic mapping, LA-ICPMS U-Pb zircon ages from six samples in the Ashland Supergroup of the eastern Blue Ridge in Alabama were analyzed to investigate the timing of metamorphism in the southern Appalachians. Kernel density estimate diagrams of whole zircon populations illustrate Grenville orogeny recycling and inheritance. Ordovician high Th/U zircon suggest a genetic relationship with the nearby Dadeville back arc basin. Zircon rims are distinctly enriched in U compared to Th, with U/Th ratios greater than 10 and yield mean ages between ~345 and 370 Ma [MSWD ranging from 0.64 to 10.5]. High U/Th zircon are compatible with mid-crustal metamorphic conditions and Devonian-Mississippian ages correlate to the ongoing Neoacadian orogeny, temporally linking rocks of the Ashland Supergroup with rocks of the overlying Wedowee Group. The overlying Wedowee Group experienced higher grade metamorphism synchronous with mid-crustal metamorphism of the Ashland Supergroup, suggesting structural juxtaposition of the Ashland and Wedowee rocks along the Goodwater-Enitachopco fault occurred well after peak metamorphism. Two samples contained a noticeably younger population of 6 grains that yielded mean ages of ~275 and ~295 Ma, which, may reflect overprinting of Alleghenian metamorphism or Pb loss. These ages are within error of estimated age of metamorphism for the structurally underlying Talladega Group, suggesting that rocks of the Ashland Supergroup may have been juxtaposed above Talladega Group rocks along the Hollins Line fault prior to early Permian tectonism, requiring exhumation of Ashland Supergroup rocks in <40 Ma to lowest greenschist facies metamorphism prior to experiencing Alleghenian metamorphism.