PETROLOGY AND GEOCHEMISTRY OF METAMORPHOSED PROTEROZOIC IGNEOUS ROCKS IN THE BLUE RIDGE AND WESTERN PIEDMONT TERRANES NEAR ROANOKE, VIRGINIA

Detailed petrographic analyses of more than 100 thin sections and 27 new whole rock chemical analyses constrained by careful 7.5 minute scale geologic mapping may provide new perspectives on the origins and interrelationships among Middle Proterozoic basement rocks, late Proterozoic plutonic rocks and multiphase rift related metavolcanic rocks in the late Proterozoic Lynchburg Group southwest of the Gossan Lead–Red Valley Fault zone. The work has been continuously supported by USGS /VDMR COGEOMAP and STATEMAP programs since 1990. Basement units in the Blue Ridge near Roanoke have been divided into separate Middle Proterozoic layered gneiss terranes by the Fries fault. Each of the Middle Proterozoic layered gneiss terranes is host to a distinctive Middle Proterozoic plutonic suite containing coarse charnokitic orthogneiss units. Northwest of the Fries, Nellysford layered granulite is host to the Bottom Creek Plutonic Suite whereas southeast of the Fries Fault the Sandy Creek layered granulite contains large plutons of Horsepen Mountain charnokitic orthogneiss. Most samples are hypersthene and diopside normative. Chemical data delineate a trend from tonalite to granite on the ab-an-or diagram. Rb/Yb +Ta cluster along the boundary between the “within plate granites” and “volcanic arc granites” fields. Major differences in lithology within the charnokitic plutonic suites southeastwards from Roanoke may be related to increasingly severe deformation, hydration and retrograde metamorphism in the western Piedmont thrust sheets. Late Proterozoic metavolcanic and plutonic rocks from the White Oak Creek and Stewartsville-Crowell Gap Plutons , SE of Roanoke are correlated with A-type granites and rhyolites of the Cryogenian Magmatic Province of Eastern North America (666-745 Ma). These plutons are alkali granites that have trace element characteristics of within plate granites. Late Proterozoic volcanic rocks in the Blue Ridge, dikes and volcanic xenoliths in Cryogenian plutons east of the Fries Fault, mafic to intermediate volcanics in the basal Lynchburg (Wills Ridge and Moneta Fms) and Upper Lynchburg (Alligator Back-Catoctin) range from greenschists to barely-altered olivine basalts. Thin rhyolites, chemically similar to felsic layers in the Moneta Fm. were discovered in the Wills Ridge Fm. near Boones Mill.