TACONIAN METAMORPHISM ALIVE AND WELL IN THE SOUTHERN BLUE RIDGE

Recent monazite U-Th-Pb chemical ages (MCA) for Ky-grade schists of the Great Smoky Group, eastern Great Smoky Mountains (Moecher et al. ’03) are overwhelmingly Early Ordovician (~480 Ma), with preservation of scattered older ages (530 Ma) in the cores of some grains, and younger ages along rims (300-270 Ma). Silurian to Devonian ages are conspicuously absent. If (1) the Ordovician ages are accurate (an extensive dataset of MCA cross-checked by conventional TIMS U-Pb ages supports this accuracy); and (2) if monazite grows by prograde reactions during regional Str-grade metamorphism (the widely held interpretation), then age distributions strongly support the traditional interpretation that the main period of metamorphism in this part of the Western Blue Ridge (Bt through Ky isograds) occurred during the Taconian orogeny. Many of the available Hbl and Mu 40Ar-39Ar ages are consistent with slow cooling from peak Taconian conditions to the T for Ar retention in the Devonian. Confirmation of Ordovician metamorphic ages awaits U-Pb TIMS analysis of the same monazites that yielded MCAs, and of other metamorphic minerals (Grt, Ms). Recent U-Pb TIMS analysis of zircon that grew during melt-forming metamorphic reactions in the area of the granulite facies high of the Eastern Blue Ridge indicate that regional metamorphism here is also Taconian; concordant zircon U-Pb ages are 458 ± 1.0 Ma (MSWD=0.9) (Moecher et al. ‘04). Thus, in two areas of classic regional Barrovian sequences in the southern Blue Ridge, metamorphism can be shown to be Taconian. Although many parts of the WBR and EBR were metamorphosed in the Taconian, scattered younger ages, and stratigraphic and structural interpretations, do not preclude younger tectonism. Alleghanian deformation and overprinting of Taconian assemblages, structures, and fabrics is likely to be present throughout the southern Blue Ridge. The distribution and intensity of this overprint has not been systematically evaluated in the area of Taconian metamorphism. Acadian tectonism and magmatism are demonstrated in the southern- and southeasternmost Blue Ridge, but how it extends into the area of demonstrable Taconian metamorphism remains one of the outstanding issues requiring careful mapping, structural analysis, petrology, and geochronology.