MESOPROTEROZOIC GEOLOGY OF THE BUENA VISTA, VA 1:100,000-SCALE QUADRANGLE: LAYING THE FOUNDATION FOR USGS MAPPING ALONG THE BLUE RIDGE PARKWAY IN VIRGINIA

Detailed geologic mapping along the Blue Ridge Parkway in Virginia, in cooperation with the National Park Service, will compare lithologies, metamorphism and structural features to those elsewhere in the Blue Ridge of Virginia and North Carolina. Current geologic mapping is along the northern section of the parkway from Afton south to Roanoke, within the Buena Vista 30 by 60 minute quadrangle (BVQ). Four years of detailed mapping, mostly through USGS STATEMAP funding to Virginia Division of Geology and Mineral Resources, provides a firm knowledge base of significant lithologies, cross-cutting relationships, and geochemical and petrologic signatures in this region. Field relationships indicate that the oldest rocks are strongly foliated pyroxene-bearing orthogneisses, which range from thinly laminated to coarsely layered. In the Arnold Valley area of the BVQ, thinly laminated gneiss occurs as mappable bodies as well as xenoliths and screens within younger megaporphyroclastic granitoid; in the Buena Vista area, similar rocks are correlated with the Lady Slipper Granulite Gneiss. Younger rocks include fine- to coarse-grained orthopyroxene-bearing granitoids (Peaks of Otter and Piney River areas), megacrystic granitoid (Amherst area) and megaporphyry (Vesuvius area).

Oldest rocks exhibit upper amphibolite- to granulite-facies Elzeviran to Grenvillian foliations, whereas many of the younger granitoids exhibit ductile amphibolite-facies fabrics, which may be Neoproterozoic. Paleozoic foliation defined by greenschist-facies mineral assemblages locally penetrates all lithologies. Within the easternmost part of the BVQ, riebeckite occurs as a primary mineral phase in Neoproterozoic granite and as an alteration product of pyroxene and amphibole in sheared Mesoproterozoic orthogneisses, suggesting a significant thermal and deformational event during regional Neoproterozoic rifting in this region. Recent SHRIMP U-Pb zircon studies of ~50 samples of orthogneisses and metagranitoids from Virginia and North Carolina established strongly deformed 1200-1144 Ma (Group 1) and 1112 Ma (Group 2), and mostly undeformed 1078-1022 Ma (Group 3) Mesoproterozoic units. U-Pb zircon analyses of the rocks mapped in the BVQ will test these groupings.