TACONIAN METAMORPHISM IN EASTERN GREAT SMOKY MOUNTAINS INFERRED FROM U-TH-PB MONAZITE CHEMICAL AGES

Ocoee Supergroup rocks in the Great Smoky Mountains (GSM) have been traditionally interpreted as having experienced primarily Taconian metamorphism. However, recent paleontologic interpretations, scattered muscovite ⁴⁰Ar-³⁹Ar plateau ages, and U-Th-Pb monazite chemical ages (MCA) suggest that regional isograds may be Acadian. The age of metamorphism has important implications for tectonic reconstructions of the southern Appalachians. We are examining the petrotectonic history of the Great Smoky Group (GSG) and its equivalents in the easternmost Western Blue Ridge (WBR), including collecting preliminary MCA data from samples of GSG along the Blue Ridge Parkway. We previously analyzed monazite (Mnz) in migmatitic Grt-Ky-St-Pl-Qtz-Bt-Ms (+ Sil) gneiss at the WBR-Eastern Blue Ridge contact, and in Grt-Bt-Ms-Pl-Qtz metapsammites near Soco Gap, NC. These samples are overprinted by a late penetrative regional F3 folding event. The overwhelming majority of ages obtained for these samples were 440 to 480 Ma, with a second mode at 510-540 Ma. We have analyzed a third sample of Grt-St-Ky-Ms-Bt-Qtz-Pl-Ru schist in GSG proper between Soco Gap and GSM Nat. Park. This sample shows no late F3 effects, with Ky and St poikiloblasts overgrowing the matrix foliation. The Mnz analysis protocol at VA Tech involved: (1) Y, Ca, Th, and U X-ray mapping to assess compositional zoning that could arise from multiple growth events; (2) at least two microprobe traverses across each grain in matrix and included in garnet (20 to 75 points/grain). The third sample corroborates our previous results. The greatest number of ages obtained for all monazites (~ an estimate of the volume fraction of monazite with that age) are in the Taconian age range of 440-510 Ma, with weak modes at 440 and 480 Ma; the weighted mean age=480(±10) Ma. Only 15 of ~350 ages fall between 415 and 430 Ma; five ages from schist are 360 to 410 Ma (~Acadian); four rim ages in schist are £ 330 Ma (~Alleghanian); 18 are much older (520 to 540 Ma), similar to ages in the Smith R. Allochthon. Although the possibility exists that significant monazite growth did not occur during each phase of mineral reaction and deformation, it must be concluded that a dominant phase of Mnz growth and metamorphism in the eastern WBR was indeed Taconian, with possible weak Acadian and Alleghanian overprints.