Detrital Mineral Chemistry and Petrographic Constraints on Provenance of Snowbird Group Siltstones, Eastern Great Smoky Mountains

Ocoee Supergroup strata are considered a classic continental, basement-sourced clastic sequence developed in rifted Laurentian crust. This provenance model was tested by assessing modes of framework minerals and mineral chemistry using electron microprobe analysis/back scatter imaging of minerals in selected siltstone-dominated Snowbird Group formations. Framework mineral abundances of micaceous (15-20%) Pigeon and Roaring Fork siltstones as plotted on ternary tectonic discrimination diagrams confirm a basement uplift source. Roaring Fork siltstone contains a higher abundance of alkali feldspar and less plagioclase compared to Pigeon siltstone suggesting different sources for the stratigraphically conformable but gradational units. Detrital feldspar compositions (feldspar components Or-Ab-An) of Pigeon and Roaring Fork siltstone are remarkably homogeneous [Afs: Or₉₄ Ab₆; 1σ_Or = 1 mol%; Plag: Ab₉₃ An₇; 1σ_Ab = 2 mol%] compared to feldspars in underlying basement granitic rocks [Afs: Or₉₄ Ab₆; 1σ_Or = 1 mol%; Plag: Ab₈₃ An₁₇; 1σ_Ab = 6 mol%]. Although K-feldspar compositions are similar, plagioclase compositions in siltstones are distinctly more sodic than those in basement rocks. The high alkali content of the detrital feldspars in both units is indicative of an A-type peralkaline granite source terrane. The siltstones of the Snowbird Group 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 titanite compositions are consistent with an igneous source for the Roaring Fork and metamorphic or igneous sources for the Pigeon. Although the framework mineralogy indicates a continental basement source, the detrital mineral abundances and compositions are not consistent with that source being contiguous basement of the Eastern Great Smoky Mountains. Heavy mineral ratios and detrital zircon geochronology will provide additional constraints on provenance.