DISTRIBUTIONS OF 40AR/39AR MINERAL AGES AND METAMORPHIC ISOGRADS IN REGIONALLY METAMORPHOSED LITHOLOGIES OF THE WESTERN BLUE RIDGE AND TALLADEGA BELT, SOUTHERN APPALACHIANS

The southern Appalachians developed from and record the Taconic, Acadian and Alleghanian stages of Appalachian orogeny. Widespread Alleghanian deformation and metamorphism in the southern Appalachians, resulting from culminating Laurentia-Gondwana collision, obscure the Taconic and Acadian record. In the Talladega belt and western Blue Ridge, from Alabama through North Carolina, recently obtained ⁴⁰Ar/³⁹Ar ages of muscovite are typically 335-320 Ma for sub-garnet zone metamorphic rocks with single-generation fabrics. These muscovite ages are interpreted to approximate the timing of muscovite growth. These Mississippian ages have a distribution in the western Blue Ridge that encompasses the Murphy synform and other, related structures (extending 180 km in north Georgia and North Carolina, and possibly to southwestern Virginia). The Murphy synform is an early Alleghanian, F₁-stage fold, and the regional age distribution indicates that it formed in a Visean stage of the Alleghanian orogeny. The mapped distributions of regional, Barrovian metamorphic isograds (Carpenter, 1970; Tull et al., 2012) are not folded by the Murphy synform, indicating that porphyroblast development occurred in a Visean or younger stage of Alleghanian orogeny (although these isograds are deflected by later Alleghanian folds). New ⁴⁰Ar/³⁹Ar metamorphic ages for muscovite from the region of the French Broad River, North Carolina, are ca. 320 Ma, and similar to ages reported for the Talledega belt and western Blue Ridge of the Cartersville, Georgia region. These results indicate a similar timing of Alleghanian deformation and isograd development throughout these regions and along these major F₁ Alleghanian structures. This interpretation of Alleghanian isograd development is difficult to reconcile with the region of the Great Smoky Mountains National Park, where Corrie and Kohn (2007) interpreted the isograds as Taconian on the basis of ~445 Ma U/Pb TIMS ages for monazite inclusions in garnet porphyroblasts. To encourage further discussion and study, a geologic map of the southern Appalachians has been compiled to represent the reported metamorphic conditions (index minerals, mapped isograds, P-T estimates) and thermochronometry data (principally ⁴⁰Ar/³⁹Ar ages for hornblende, muscovite and biotite).