A WORKING HYPOTHESIS FOR THE RECORD OF TACONIAN AND ALLEGHANIAN METAMORPHISMS AND ASSOCIATED RADIOMETRIC AGES IN THE SOUTHWESTERN BLUE RIDGE

Interpretations of stratigraphy and metamorphism from the Talladega slate belt to the western Blue Ridge of North Carolina form the basis for some of the most polemic debates in Appalachian geology. Evidence for Taconian, Barrovian metamorphism (Middle Ordovician) has been reported in the western Blue Ridge for the region of the Great Smoky Mountains with data including ⁴⁰Ar/³⁹Ar cooling ages of hornblende and micas, interpretations of ⁴⁰Ar/³⁹Ar age spectra for slates, and U/Pb ages of monazite inclusions in garnet. The biostratigraphic and geochronologic evidences for Alleghanian (Carboniferous) metamorphism in the Talladega belt and southernmost Blue Ridge (near Cartersville, GA) are also compelling. Stratigraphic correlations of the Lay Dam formation (Talladega belt) and its proposed stratigraphic equivalent (Mineral Bluff Group) northward into the Murphy syncline have been difficult to reconcile with southeastern extrapolation of the Taconian isograds and their associated lithotectonic stratigraphy. We are undertaking a sampling program and laser ⁴⁰Ar/³⁹Ar study of muscovite in this region in order to derive new data bearing on this problem. We find that muscovite from phyllites and schists in the Great Smoky Group and Snowbird Group yield ⁴⁰Ar/³⁹Ar age results that are generally complex (determined for populations of single crystals or small groups of crystals incrementally heated) and typically range from ca. 460-310 Ma. In contrast, we find that muscovite from the Lay Dam formation and from the Mineral Bluff group yield ages ranging from ca. 335-310 Ma, and most of these samples yield relatively precise ages that seem clustered near 335 and 325 Ma (Visean to Serpukhovian). Our favored hypothesis to explain these relationships is that the basal Lay Dam/Mineral Bluff unconformity is post-Ordovician and pre-Visean, and (at least in the western Blue Ridge) developed on rocks with a Taconian metamorphic and structural evolution. Subsequently, in Late Mississippian stages of Alleghanian tectonics, the western Blue Ridge lithologies, the unconformity, and overlying units including the Lay Dam and Mineral Bluff groups experienced folding, faulting and metamorphism to form the relationships now present in the Murphy syncline.