EOCENE BASALT VOLCANISM IN CENTRAL VIRGINIA: IMPLICATIONS FOR CENOZOIC TECTONISM

Middle Eocene volcanics from Virginia are the youngest igneous rocks in the eastern United States; they are not correlated with any known tectonic event. We examined their geochemistry to constrain the source region mineralogy as well as the thermal structure of the underlying Appalachian lithosphere in the Cenozoic. Olivine-phyric (Fo90) Eocene basalts contain 8-12 wt.% MgO, 235-630 ppm Cr and up to 160 ppm Ni, and have CaO/Al2O3 values between 0.7-0.9, suggesting they underwent minor differentiation prior to eruption (Southworth et al. 1993; this study). They are enriched in incompatible trace elements relative to MORB, and are enriched in LREE relative to HREE (La/Ybn <20). The geochemical features indicate an enriched mantle source, presumably the subcontinental lithosphere, and are consistent with melting in the presence of garnet.

The Eocene basalts are similar in many ways to the 570 Ma Catoctin basalts, but are quite different from the Mesozoic Appalachian tholeiites (MAT) that erupted near the continental margin. The Eocene and Catoctin lavas have higher overall incompatible trace element abundances and lower Sr-isotope values than the MAT. In addition, the MAT have anomalous relative depletions in Nb and TiO2 that suggest involvement of an arc-like mantle (Pegram 1990); this feature is absent from the Cambrian and Eocene suites. These geochemical differences indicate that the Cenozoic thermal event does not sample the same source region recorded by the MAT.

The cause of lithospheric melting and the resulting basalt magmatism is not known. Two plausible scenarios will be discussed. One possibility is the channeling of asthenospheric heat and/or material associated with formation of the Bermuda rise, perhaps along lithospheric fractures created during Mesozoic extension. A second model involves edge-driven convection (King & Ritsema 2000) along the continent-ocean lithospheric transition. A geologically reasonable model of Cenozoic Appalachian tectonism requires more than reactivation of extant faults, but must incorporate the thermal history indicated by primitive Eocene basalts.