Both Taconian (ca. 450 Ma) and Acadian (ca. 380 Ma) ages of high-grade deformation and metamorphism have been advocated for the Western Blue Ridge, southern Appalachians, United States. To resolve this debate, metamorphic monazite from the Great Smoky Mountains was dated via in situ microsampling plus isotope dilution–thermal ionization mass spectrometry (ID-TIMS). U-Pb ages are 450 ± 5 Ma (±2σ error), and mean 207Pb/206Pb ages are 453 ± 10 Ma. These data indicate that peak metamorphism and high-grade deformation occurred during the Taconian orogeny, although they do allow for a low-grade (sub-amphibolite-facies) Acadian overprint. Unresolved issues remain regarding the comparability of U-Pb TIMS ages with those obtained via other methods.

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