We present here the first geochemical data from adakitic rocks from an extensional system—the U.S. East Coast rifted margin. Adakitic magmas are high-K melts that have been petrogenetically interpreted to be partial melts of subducting slab and/or lower crustal lithologies in delamination events. The adakitic rocks presented here are from a small volcanic region in the Valley and Ridge province in Virginia and were probably emplaced around the time of continent rupture and Central Atlantic magmatic province activity. They are bimodal in character (high Si and low Si) and have the typical high- and low-Si adakitic geochemical characteristics such as high K 2 O (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 studied Jurassic central Virginia igneous rock succession is delamination of mantle lithosphere and lower crust below the volcanic rifted margin. We present with geodynamic models that negatively buoyant mantle lithosphere instabilities developed below this passive margin during continent rupture. After foundering, warm asthenosphere welled up and heated the lower crust of the East Coast margin. This lithosphere was interspersed in our study area with fragmented hydrated metamorphic mafic to ultramafic lithologies. In situ and/or dripping melting of such meta-igneous rocks reproduces the observed geochemistry of the studied high-Si adakitic rocks. Further recycling processes within the convecting mantle of delaminated floating fertile meta-igneous rock packages could be responsible for Atlantic melting anomalies such as the Azores or Bermuda.

The ca. 570 Ma Catoctin volcanics, exposed in the Blue Ridge of northern Virginia, include metamorphosed rift-related basalts extruded during breakup of the supercontinent Rodinia. Field relationships, petrography, and geochemistry are used to decipher the stratigraphy for two areas of the volcanics, one at the base of the formation, and the other near its top. Geochemical characteristics of sequential flows can be explained by fractional crystallization of minerals commonly occurring in basalts. Intervening flows with slightly different geochemical features that cannot be explained by fractional crystallization from magma corresponding to an underlying flow, or by crustal contamination, most likely represent new pulses of magma. Positive Pb and negative Nb anomalies, coupled with ε Nd values of +1.5 to +4.5 and model ages that exceed crystallization ages by over 500 m.y., suggest that an enriched component was added to the mantle prior to melting. This component resulted from devolatilization of subducting sediments prior to assembly of Rodinia, likely during Grenville orogenic events. Stratigraphic control shows that lower lavas in the sequence contain more of the enriched component than upper flows.

The Blue Ridge and Great Valley of western Virginia are part of a detached master thrust sheet that extends through the central-southern Appalachian change of trend and has a root zone situated east of the Blue Ridge under the Piedmont. The mapped Pulaski fault and North Mountain fault crop out in the Great Valley as splay faults terminating in thrust tip anticlines and merge at depth with the buried master detachment floored primarily in Upper Ordovician Martinsburg Shales. Overall transposition of the thrust sheet from the root zone indicates that as much as 32 km (20 mi) of displacement may be translated westward into the Valley and Ridge proper and Allegheny Plateau as initial cover shortening above the Martinsburg Shale. Within the Great Valley and into the Roanoke recess at the juncture of the central and southern Appalachians, much of the cover shortening of this master sheet is accommodated by the Pulaski and North Mountain faults. From northeast to southwest, movement on the outcropping and buried master segment of the North Mountain fault decreases, and the surface fault terminates in southern Rockbridge County. Displacement on the Pulaski fault increases from northeast to southwest, and we infer that it assumes the decreasing displacement on the outcropping and buried segments of the North Mountain fault by displacement transfer.