LASER 40AR/39AR AGES OF MUSCOVITE AND EVIDENCE FOR MISSISSIPPIAN (VISEAN) DEFORMATION NEAR THE THRUST FRONT OF THE SOUTHWESTERN BLUE RIDGE PROVINCE

The age of metamorphism in phyllites along the thrust front of the southwestern Blue Ridge is difficult to decipher due to their low grade, the relict detrital minerals they contain, and the effects of superimposed tectonic events. However, the low metamorphic grade of these rocks (near the closure conditions of muscovite) renders them useful for estimating the timing of metamorphism. We have used laser 40Ar/39Ar methods in the Auburn Noble Isotope Mass Analysis Laboratory (ANIMAL) to date single crystals of muscovite from about thirty samples of phyllite collected near the Talladega-Cartersville and Great Smoky thrusts in Georgia and from the central western Blue Ridge in North Carolina. Mississippian ages for muscovite (ca. 335-330 Ma) are typical of our study samples, from locales as far south as Cartersville and as far north as U.S. Hwy 64 in North Carolina (between Ducktown and Murphy). A few ages as young as ca. 320-315 Ma are present among our samples, and we suggest these reflect localized recrystallization in ongoing stages of Alleghanian deformation. (Our attempts to date deformation in the Cartersville/Great Smoky thrust directly, in exposures at the Carter Lake dam, were unsuccessful, because of excess argon in the brittlely deformed muscovite.) We suggest that an important component of tectonic evolution along the frontal Blue Ridge occurred in Visean time (although later deformation along the thrusts certainly affected Pennsylvanian strata in the Valley and Ridge province). This interpretation is supported by the coeval re-appearance of 1.7-2.0 and 2.6-3.0 Ga detrital zircons in Mississippian strata of northern Alabama after an orogen-wide absence of such age populations since the Ordovician. This provenance data suggests Visean exhumation of Neoproterozoic-early Ordovician rift and passive margin strata in the Valley and Ridge or their metamorphosed equivalents in the Blue Ridge, and signals a Mississippian onset of Alleghanian orogenesis in the southern Appalachians.