DELAMINATION OF OPHIOLITE-INTERSPERSED CRUST OF THE VALLEY AND RIDGE PROVINCE

Igneous rocks from a small volcanic region in the Valley and Ridge province in Virginia that were probably emplaced around the time of continent rupture provide insight into Jurassic tectonics of the region. The igneous rocks are bimodal in character (high Si and low Si) and have the typical high- and low-Si adakitic geochemical characteristics such as high K2O (up to 9.88 wt%) abundances, steep rare earth element patterns, and significantly high Sr (2473 ppm) and relatively low Rb (35 ppm) contents for high-Si adakitic rocks. The petrogenetic relation of these melts to partial melting of metagabbroic rocks (high-Si adakites) and interaction of these melts with ambient peridotite (low-Si adakites) suggests that the geodynamic process for the formation of the Jurassic central Virginia igneous rock succession is delamination of mantle lithosphere and lower crust below the Atlantic rifted margin.

In our model for the formation of these rocks, oceanic lithosphere that formed by slow-spreading was emplaced in the continental crust and the lithospheric mantle during formation of the Valley and Ridge province. Oceanic lithospheres were deformed, fragmented, and metamorphosed during the Taconic orogenesis and hydrated during late Taconic, Acadian, and Alleghenian metamorphism. Such a lithosphere fulfills the prerequisites to form hydrous eclogitic gravitational instabilities in an extensional setting. Geodynamic models show that the lower crust and mantle lithosphere destabilized and delaminated during rifting and breakup.