PROVENANCE ANALYSIS OF THE NEOPROTEROZOIC OCOEE SUPERGROUP, EASTERN GREAT SMOKY MOUNTAINS, NC/TN

The Ocoee Supergroup is a sequence of immature continental rift facies clastic metasedimentary rock; potential source terranes are Grenville basement orthogneisses and Neoproterozoic rift-related granitoids. Potential sources were tested by assessing modes of framework minerals, mineral chemistry, whole rock geochemistry and detrital zircon U/Pb geochronology by LA-ICP-MS for Ocoee siltstone-sandstone dominated formations. Ocoee units are arkosic to subarkosic siltstones/sandstones and ternary tectonic discrimination diagrams confirm a continental basement uplift source; alkali feldspar predominates over plagioclase feldspar. Detrital feldspar compositions of most Ocoee sediments (except Pigeon siltstone) are remarkably homogeneous [Afs: Or_99-90 Ab_1-10; 1σ_Or = 1 mol%; Plag: Ab_99-90 An_1-10; 1σ_Ab = 2 mol%] compared to feldspars in inferred basement granitic rocks [Afs: Or₉₄ Ab₆; 1σ_Or = 1 mol%; Plag: Ab₈₃ An₁₇; 1 σ_Ab = 6 mol%]. The Pigeon siltstone is the youngest and thickest Ocoee unit. It contains a bimodal plagioclase distribution, which is not observed in other Ocoee formations. This suggests a common source for these older formations with a change in source once Pigeon deposition commenced. Although Kfs compositions are similar, plagioclase compositions are distinctly more sodic and more homogeneous compositionally than plagioclase in basement rocks, indicative of a more restricted origin for these sediments. The high alkali content of the detrital feldspars in the Ocoee Supergroup is consistent with derivation from an A-type granite source terrane. Normative Q-A-P values calculated from whole-rock chemistry and trace element diagrams are also supportive of an A-type granite source for these rocks. The siltstones and sandstones of the Neoproterozoic Ocoee Supergroup contain high abundances of heavy minerals (zircon, titanite, ilmenite, epidote and apatite), which occur dispersed among other detrital grains and as concentrations of heavy minerals in discrete laminae. Detrital zircon geochronology in Ocoee sediments indicates a dominantly Grenville (1000 ± 50 Ma - 1300±48 Ma) source for these sediments. The youngest zircon ages in the basal Ocoee Wading Branch Formation (639±8 Ma -720±25 Ma) are related to rift magmatism and provide a minimum depositional age for the Ocoee Supergroup.