Eocene basalt volcanism in Central Virginia: Implications for Cenozoic tectonism

Recent geomorphological and structural studies indicate that the North American margin has undergone regional uplift since the Cenozoic. Eocene volcanics from Virginia, the youngest igneous rocks in the eastern United States, are spatially and temporally correlated with this uplift. Their location is in close proximity to two earlier eruptive sequences, the Precambrian Catoctin basalts and the Mesozoic Appalachian tholeiites (MAT). The geochemical signatures of these basalt suites help constrain evolution of eastern North America. Primitive Eocene basalts contain 8-12 wt.% MgO, 235-630 ppm Cr and up to 160 ppm Ni, so they provide insight into the geochemical and thermal structure of the mantle source region in Cenozoic time. The Eocene basalts are geochemically distinct from both the Catoctin lavas and the MAT, requiring derivation from different source regions. The Eocene and Catoctin lavas have higher incompatible trace element abundances and lower initial Sr-isotope values than the MAT, but lack the anomalous relative depletions in Nb and TiO₂ that are observed in the Mesozoic suite. The apparent depth of melting is greatest for the Eocene lavas (ca. 50-70 km), and the lack of an arc-like signature suggests melting occurred at depths greater than the lithospheric source region of the MAT. The cause of Cenozoic basalt magmatism and regional uplift is not known, and may result from far-field stresses associated with convergence of the North American and Caribbean Plates or from small-scale convection along the craton edge as reflected also by development of the Bermuda Rise.