CORRELATION OF EASTERN BLUE RIDGE LITHOSTRATIGRAPHY ACROSS THE NORTH CAROLINA­–VIRGINIA BORDER: ASHE METAMORPHIC SUITE AND LYNCHBURG GROUP

New mapping in the Wytheville and Galax 30 x 60-minute quadrangles for the USGS Piedmont and Blue Ridge Project aims to address several edge-match issues along the NC–VA border, including correlations of eastern Blue Ridge (eBR) lithostratigraphy. Decades of research have proposed many different ideas about the lithostratigraphy and tectonic setting of the eBR, however, little mapping across state boundaries has occurred to help resolve these issues. We discuss the preliminary results of partnered FEDMAP and EDMAP geologic mapping in northwestern NC and southwestern VA that integrates various datasets — geochronology, geochemistry, geophysics — to resolve fundamental relationships between eBR lithostratigraphy of the Lynchburg Group (LG) and Ashe Metamorphic Suite (AMS).

Our mapping identifies a package of interlayered gritty metagraywacke, minor conglomerate, graphitic schist, and amphibolite that is thrust over Mesoproterozoic basement rocks along the Gossan Lead fault. The rocks are penetratively deformed with rare F₁ intrafolial folds transposed into a regional amphibolite facies foliation. Regionally, this package is continuous along the western part of the eBR and strikes into the defined LG, and a correlation thereof is proposed. A SHRIMP U-Pb zircon date of 455 ± 12 Ma was obtained from a discordant amphibolite in the LG. Southeast of the LG, polydeformed amphibolite-facies metagraywacke, mica schist, and minor metaconglomerate occur with many mappable elongate bodies, lenses, and pods of amphibolite and meta-ultramafic rocks. This ultramafic-bearing package is correlated with the AMS. The contact between the LG and AMS is interpreted as a cryptic syn-metamorphic fault. Monazite U-Th-Pb geochronology yields metamorphic dates from the LG of 365–340 Ma, whereas AMS monazite records populations dated to 465–448, 370–350, and 350–­340 Ma. Detrital zircons from the LG and AMS show a dominance of 1.4–1.0 Ga ages with peaks at 1.16, and 1.04 Ga and minor Neoproterozoic ages. This suggests a Laurentian source for both the LG and AMS, but distal enough to have limited input from Neoproterozoic volcanics in the western Blue Ridge. The unnamed fault contact between the LG and AMS may represent an internal suture in the eBR and may explain similar lithostratigraphic problems in other parts of the orogen.