NEW PETROLOGIC AND SINGLE-CRYSTAL 40AR/39AR DATA FOR THE WESTERN BLUE RIDGE AND IMPLICATIONS FOR UNDERSTANDING THE EXTENT AND CONDITIONS OF PEAK ALLEGHANIAN METAMORPHISM

In common with many other portions of the Appalachians and Caledonides, the Western Blue Ridge province (WBR) is a polymetamorphic terrane formed through Ordovician, Devonian and Carboniferous orogenic stages. U/Pb ages for zircon and monazite of the WBR commonly are Middle to Late Ordovician; ⁴⁰Ar/³⁹Ar ages for amphiboles and micas are commonly Silurian to Carboniferous, interpreted to recording cooling following Ordovician metamorphism or Acadian and/or Alleghanian metamorphism. The Murphy Synform contains the structurally and stratigraphically highest metamorphic sequences of the WBR in Georgia, correlated with the Talladega Belt that contains fossils as young as Early Mississippian. We find that an amphibolite from the Marble Hill Hornblende Schist (overlying the Murphy Marble) contains pargasitic hornblende with the formula (Na_.35K_.31)Ca_1.89(Mg_1.97Fe_2.44Ti_0.08Al_0.64)(­Al_1.53Si_6.47)O₂₂(OH)₂ and a slight chemical zoning from core to rim reflecting increasing Tschermak substitution. The petrology of this amphibolite and also gar+sta+kya schists of the Mineral Bluff Formation (from the same drill cores), along with their apparent lack of post-crystallization chemical changes (from diffusion or alteration), are interpreted to reflect prograde crystal growth during one brief amphibolite facies metamorphic event. ⁴⁰Ar/³⁹Ar incremental heating analysis of a single pargasitic hornblende crystal from the amphibolite yields a plateau age of 317.74+0.88 Ma (2σ) defined by 100% of the ³⁹Ar_K released; spectra for other hornblende crystals are variably discordant with minimum ages of ~320 Ma. Single crystal ⁴⁰Ar/³⁹Ar analyses of muscovite from Mineral Bluff schists yield mean ages of ca. 314 Ma. Collectively, the new ⁴⁰Ar/³⁹Ar ages and petrologic data are interpreted to record Alleghanian kyanite-grade metamorphism and subsequent rapid cooling through the ⁴⁰Ar retention temperatures of hornblende and micas in the interval from ca. 318-314 Ma. The new single crystal ⁴⁰Ar/³⁹Ar and petrologic data support regional interpretations that the Murphy Synform — and the WBR from the correlated Talladega Belt to Great Smoky Mountain National Park — experienced more widespread Alleghanian metamorphism, isograd development, faulting and folding than generally recognized. We suggest many of the previously published, bulk-sample ⁴⁰Ar/³⁹Ar ages for this region are biased to indicate older events due to unresolved excess or inherited ⁴⁰Ar, a phenomenon that is also widespread in the broader Appalachian-Caledonian orogen.