FELDSPAR AS A PROVENANCE INDICATOR IN METASANDSTONE OF THE LOWER MOUNT ROGERS FORMATION, SW VIRGINIA

The Neoproterozoic Mount Rogers Formation (MRF) in the Blue Ridge province of SW Virginia formed during an initial, aborted rifting of the eastern Laurentian margin of Rodinia, prior to successful rifting ~200 million years later. The lower MRF rests unconformably on the 1.1-1.2 Ga Grenville basement (Cranberry Gneiss), and is overlain by 758 (+/- 12) Ma rhyolites of the upper MRF. The MRF was metamorphosed to lower greenschist facies during Paleozoic compressional tectonism. The lower MRF contains bimodal volcanic rocks overlain by alluvial fan sedimentary deposits. Sedimentary protoliths in the lower MRF are predominantly conglomerate, representing the proximal fan facies, and sandstone representing the more distal facies. The sandstone of the lower MRF is poorly sorted and contains quartz, feldspar, and abundant lithic clasts. The lithic clasts predominantly derive from the Cranberry Gneiss or the lower MRF rhyolites and basalts; however, some are of uncertain origin. Our study investigates the use of feldspars as provenance indicators, in order to assess the relative contributions of the different source rocks. We used thin section petrography to identify the types of K-feldspar and plagioclase present in the rhyolite, basalt, and basement granitoids, to compare with feldspars in the sandstone. The Cranberry Gneiss (CG) in the field area comprises both granitic and granodioritic varieties. Based on the Michel-Lévy method, the granitic samples of the CG contain albite, whereas the granodiorite contains andesine. Basalt of the lower MRF also contains andesine. Microcline is the dominant K-feldspar in the CG samples, as compared to altered perthitic K-feldspar in the lower MRF rhyolite. The sandstone contains both albite and andesine plagioclase , abundant microcline K-feldspar, and lesser amounts of perthitic K-feldspar. Despite the abundance of rhyolite lithic clasts in conglomerate of the proximal fan facies, it appears that the CG is a dominant source of feldspar in the distal sandstone, perhaps reflecting greater chemical weathering of the rhyolite. Feldspar compositions will be analyzed using the electron microprobe in order to further test our hypothesis.