TIMING OF ARC ACCRETION IN THE SOUTHERN APPALACHIANS: PERSPECTIVES FROM THE LAURENTIAN MARGIN

The Great Smoky Mountains occupy the southeastern margin of Laurentia, and metamorphism there has long been viewed as a response to Taconic accretion of an arc-terrane. However, new age determinations on metamorphic monazite from the Smokies now indicate an Acadian age, and require major revision of southern Appalachian tectonics.

Monazite is extremely abundant in amphibolite-facies rocks of the Great Smoky Group, and occurs in a variety of sizes (25-200 µm diameter) and habits (matrix grains, inclusions in Ky, St, Qtz, Pl). Most monazite forms during the St-in reaction (Grt + Chl + Ms=St + Bt + H₂O), and so principally occurs in St-, Ky-, and Sil-grade rocks. Porphyroblasts are growth zoned in Th (~7 to ~2 wt %) and REE's. Over 50 age determinations were made by measuring bulk U-Th-Pb contents of individual spots via the electron microprobe. Single spot uncertainties were ~±30 Ma. Within uncertainty, no difference in age was found for any texture of monazite, including large vs small grains or matrix vs inclusion grains. No correlation was found between age and Th content. Ages show a major peak at ~370 Ma, with a broad tail to younger ages, including a minor hump at ~320 Ma. No ages are older than 425 Ma or younger than 280 Ma. Calibration uncertainties are estimated to be <20 Ma. The compositional and age distributions are interpreted to reflect prograde growth of metamorphic monazite at ~370 Ma, with minor resetting at ~320 Ma. These ages indicate that major arc terrane accretion first occurred along the southeastern Laurentian margin during the Acadian orogeny, not the Taconian, as traditionally thought, with either a later thermal event or continued moderate temperatures during early stages of the Alleghanian. Whereas Taconian-age and earlier events may have affected outboard terranes, southeastern Laurentia apparently remained unaffected.