NEOPROTEROZOIC UNROOFING HISTORY AND CRUSTAL STRUCTURE OF SOUTHERN APPALACHIAN GRENVILLE BASEMENT FROM DETRITAL ZIRCON AGES AND MORPHOLOGY

The southern Appalachian Blue Ridge consists of Mesoproterozoic basement rocks of the French Broad massif that are nonconformably overlain by Neoproterozoic metasedimentary and metavolcanic rocks of the Mount Rogers (Zmr), glaciogenic rocks of the Konnarock (Zk), and metasedimentary rocks of the Unicoi (Zu) formations. Pooled SHRIMP U-Pb ages of detrital zircon from ~20 Neoproterozoic metasedimentary rocks in a vertical section in southwest Virginia yield population peaks at 1176 and 1040 Ma (Grenvillian), and 760 Ma, the time of Zmr bimodal volcanism. The morphology and age of detrital zircon help constrain the protolith(s) and provenance(s) for metasedimentary rocks beneath and above the Zmr rhyolites. The prevalence of euhedral and relatively unabraded zircon support a proximal source during deposition. Moving up stratigraphic section, Grenville zircon populations change from dominantly 1176 Ma in Zmr, to sub-equal amounts of 1040 Ma and 1176 Ma zircon in the lower Zk, to a progressively higher proportion of 1040 Ma zircon in the upper Zk diamictites, and a dominant 1040 Ma in the unconformably overlying fluvial rocks of the base of the Zu. 760 Ma detrital zircon is the domaninant age population in the upper Zmr (where Zmr rhyolite is interbedded with sedimentary units) and persists in almost equal proportions to 1040 Ma and 1176 Ma populations through the diamictite and arkose of the lower Zk, after which it is notably absent up section. The change in detrital zircon age population proportions is related to progressive unroofing of crust, normal faults, and sedimentary environments during long-term extension. Significantly, the variations in detrital zircon populations provide insight into the crust exposed in the Neoproterozoic early magmatic rocks comprised the upper crust and later (1040 Ma) magmatic rocks were at deeper crustal levels. Other studies are hindered because the polymetamorphic and deformed Grenvillian crust was beveled by Zu time and structurally modified and transported during multiple Paleozoic orogenies. This study thus provides a Neoproterozoic crustal cross-section of Grenville basement. It records both phases of Rodinian amalgamation, early extension, and formation of the global fluvial system that developed after rifting to form the eastern margin of Laurentia prior to drift phase.