MONAZITE CHEMICAL AGES REVEAL MORE THAN ONE “GREAT SMOKY GROUP” IN SOUTHERN BLUE RIDGE, APPALACHIAN OROGEN

The common view for the southern Blue Ridge of the Appalachian Orogen that the entire Ocoee Supergroup has a late Proterozoic depositional age, and that regional metamorphism is everywhere Taconian, requires reassessment in view of recent Silurian-Miss.(?) fossil discoveries in units above and in apparent stratigraphic contact with Ocoee units (Unrug et al., ’00), including the Great Smoky Group (GSG), and Devonian U-Th-Pb monazite chemical ages (MCA) in GSG metasediments (Kohn, ’01). However, not all metasediments mapped as GSG in the Great Smoky Mountains (GSM) yield Devonian MCA. Mnz in Grt zone and Grt-Ky-Str zone samples of “GSG” from near the Western Blue Ridge (WBR)-Eastern Blue Ridge (EBR) boundary (the “Hayesville Fault”), yield MCA that are solely Ordovician or older (Mnz trace-element analyses and compositional images made on a Cameca SX50 at Virginia Tech using technique modified after Williams et al. ’99). Complexly zoned (in Th, U, Y), matrix Mnz in Grt grade rocks near Soco Gap ~10 km NW of the fault yield ages for various zones of ca. 480, 530, and 590 Ma. Younger ages are interpreted to be metamorphic and the oldest ages are probably detrital relics; the intermediate ages are uncertain but similar MCA have been obtained for other metasedimentary packages in the western Piedmont Zone in NC and VA. Equant, rounded, simply-zoned matrix Mnz and Mnz included in Grt in Grt-St-Ky-Ms-Bt(-Sil) gneisses yield cores and rims with ages of ca. 480 and 440 Ma, respectively; 480 Ma is accepted here as the approx. time of regional Taconian metamorphism (although recent U-Pb TIMS studies have show that Taconian meta. is as young as 460 Ma). A few Alleghanian (~300 Ma) ages are obtained for minor rim zones in both samples, but no Acadian ages (400-380 Ma) were detected in the samples analyzed here. Implications of these ages are: (1) metasediments mapped as GSG in the GSM are not all equivalent map units; (2) isograds in the GSM are polychronic with highest grade isograds being Taconian, as is metamorphism of the WBR-EBR boundary; (3) at least one major depositional and/or structural break must occur between the crest of the GSM (TN-NC state line) and the WBR-EBR boundary.