GEOLOGY OF THE CATOCTIN FORMATION IN THE EASTERN BLUE RIDGE, CENTRAL VIRGINIA: STRATIGRAPHY, GEOCHEMISTRY, AND PETROGENESIS

The Catoctin volcanic province in the central Appalachians consists of a sequence of mafic rocks extruded during Late Neoproterozoic continental rifting. Metavolcanic rocks of the Catoctin Formation occur on both sides of the Blue Ridge anticlinorium, but there has been debate regarding the origin of metabasalts in the eastern Blue Ridge. Typically correlated with Catoctin Formation rocks on the western limb, the metabasalts in the eastern belt have alternatively been hypothesized to be arc basalts associated with the Jefferson Terrane, a postulated exotic terrane sutured onto the Laurentian margin during closure of the Iapetus Ocean. This study represents a test of these competing models. Our study focuses on metabasalts south of Charlottesville in central Virginia. The proposed Jefferson Terrane suture bifurcates this study area. The Lynchburg Group, a Neoproterozoic metasedimentary unit, unconformably underlies metabasalts in this area. Metabasalts here are predominantly moderately-foliated blue-gray rocks, and include deformed pillow basalts at one field site. These rocks are capped by a series of intercalated feldspathic sandstones and thin basalt flows. The phyllitic Candler Formation unconformably overlies this greenstone sequence. The compositional congruence of the sedimentary rocks and the largely undeformed nature of all contacts are inconsistent with a tectonic suture in this area. In addition, major and trace element compositions of ten metabasalts samples show that most represent metamorphosed tholeiitic basalts, with low-Mg (MgO < 6.6 wt%) high-Ti (TiO₂ > 1.8 wt%), similar to other Catoctin rocks elsewhere interpreted to represent late stage volcanism. The samples show uniform rare-earth element (REE) patterns that are moderately light-REE enriched (La_N/Yb_N = 3-9), with only slight positive or negative Eu-anomalies. Trace element compositions consistently plot in the within-plate basalt field on several standard tectonic discriminant diagrams. Collectively, the geochemical compositions are similar to Catoctin Formation rocks in the western Blue Ridge. All of our observations and data suggest that the metabasalts of the eastern Blue Ridge were emplaced as rift related within-plate tholeiites, and inconsistent with the hypothesized Jefferson Terrane model.