40Ar/39Ar Ages of Muscovite from the Western Blue Ridge and Talladega Belt, Georgia and North Carolina

Abstract

Radiometric age constraints regarding the timing of metamorphism in the western Blue Ridge province of the southern Appalachians have been too limited or uncertain to enable broader correlation and understanding of Taconian, Acadian, and Alleghanian events in the region between western North Carolina and Alabama. Moreover, various fossil, stratigraphic and radiometric data for the timing of metamorphism in the Talladega belt of Alabama are difficult to reconcile with radiometric ages of comparable tectonic units in the western Blue Ridge of North Carolina. New 40Ar/39Ar radiometric data for single-muscovite crystals have been collected, utilizing the Auburn Noble Isotope Mass Analysis Laboratory (ANIMAL), to help constrain the timing of metamorphism in the western Blue Ridge of Georgia and North Carolina. The mean ages of muscovite in phyllites collected from the Cartersville transverse zone record crystallization and cooling in the interval from 321.4±0.3 - 335.5±0.8 Ma (corresponding to Serpukhovian and Visean stages of the Mississippian). The mean ages of muscovite in three phyllites from a sampling transect along U.S. Hwy. 64 (between Murphy, North Carolina and Ducktown, Tennessee) are similar to those of the Cartersville transverse zone. However, the age distributions of single-crystal muscovite in six other schists and phyllites from this Murphy-Ducktown transect are complex, with ages ranging up to ca. 465 Ma. The mean ages of muscovite in metasiltstones and phyllites collected within or near the Great Smoky fault (between the Murphy-Ducktown transect and the Cartersville transverse zone) also tend to be Visean. However, several samples from this tectonic setting also yield very complex age distributions. The complex age distributions determined in this study seem mainly due to effects of polymetamorphism in some rocks, but are also influenced by the presence of relict-detrital muscovite and incorporation of unsupported-extraneous (‘excess’) 40Ar in some samples. The single-crystal 40Ar/39Ar ages determined in this study are interpreted to indicate that Mississippian tectonic events dominated metamorphic and structural evolution in the Talladega belt and in the Mineral Bluff Group of the western Blue Ridge. For Ocoee Supergroup rocks, in the western Blue Ridge, the muscovite in phyllites and schists record a history of pre-Carboniferous (Acadian or Taconian) events partially overprinted by Mississippian deformation. The Mississippian ages are interpreted to represent Alleghanian events recorded in the muscovite of Talladega belt-western Blue Ridge rocks that pre-date final emplacement of the Blue Ridge thrust sheet, as the youngest deformed strata of the Valley and Ridge (Pottsville Formation; 316-303 Ma) indicates Pennsylvanian deposition. This combined with Tournaisian fossil control (Periastron plant fossil; 360-350 Ma) indicates Alleghanian metamorphism and deformation in the Talladega belt-western Blue Ridge is constrained to ~ 360-300 Ma, older and longer in duration than the classic view of the Alleghanian as Pennsylvanian to Permian.