Paper No. 4
Presentation Time: 2:20 PM


HOLM-DENOMA, Christopher S., Central Mineral and Environmental Resources Science Center, United States Geological Survey, Box 25046, MS 973, Denver, CO 80225-0046, SOUTHWORTH, Scott, U.S. Geological Survey, MS 926A National Center, Reston, VA 20192-0001 and MERSCHAT, Arthur, US Geological Survey, MS 926A National Center, Reston, VA 20191,

In the Mount Rogers area of VA, NC, and TN, Mesoproterozoic basement rocks are nonconformably overlain by Neoproterozoic volcanic and volcaniclastic rocks of the Mount Rogers Formation (MRF). The MRF is unconformably overlain by glaciogenic sedimentary rocks of the Konnarock Formation (KF) which in turn is disconformably overlain by the Neoproterozoic-Cambrian Unicoi Formation (UF) of the Chilhowee Group.

SHRIMP U-Pb ages of detrital zircon from 15 samples of strata collected between the base of the MRF and lower UF have peaks at 1176 Ma, 1040 Ma, and 760 Ma. These zircon-forming events are the early and late stages of Grenville magmatism and the time of bimodal volcanism of the MRF during failed rifting of Rodinia, respectively. The lower MRF consists of arkose, conglomerate, metabasalt, and its oldest preserved rhyolite unit, the ~760 Ma Fees Rhyolite Member. Metasedimentary rocks beneath the Fees Rhyolite Member contain ~760 Ma zircons, supporting the concept that volcanic sources pre-date the Fees or structural deformation has displaced strata. A boulder bed above the Fees separates younger rhyolite flows and contains ~760 Ma detrital zircon. The KF contains detrital zircon with age peaks at 1170 Ma and 1040 Ma. Clasts of granitoid dominate KF diamictite which apparently contains no ~760 Ma detrital zircon in its lower part. Progressive erosion/uplift of the MRF sources are reflected in ~760 Ma detrital zircon in the upper KF, but it was likely covered prior to deposition of the lower Unicoi Formation pebble conglomerate alluvial fan which apparently contains no 760 Ma zircon. The ratio of 1170 Ma and 1040 Ma detrital zircon decreases up section. Most of the detrital zircon in these rocks are euhedral to slightly rounded, and reflect a proximal sediment source. The 1170 Ma zircon is dominant in the MRF, whereas the 1040 Ma zircon is more abundant in the upper KF and UF, providing insight into the unroofing history and basement sources exposed in the hinterland.