THE ENIGMATIC GOOCHLAND TERRANE, VIRGINIA: A TALE OF ACCRETION, TRANSPRESSION, AND RAPID EXHUMATION IN THE APPALACHIAN PIEDMONT

The Appalachian Piedmont is a mosaic of terranes with both Laurentian and peri-Gondwanan affinities that were amalgamated during the closure of oceanic and marginal basins in the Paleozoic. In the eastern Virginia Piedmont, the Goochland Terrane is a distinctive, yet enigmatic, sequence of igneous and metamorphic rocks. Previous workers have long debated the provenance of the Goochland Terrane with various models purporting either a Laurentian or Gondwanan origin. The terrane is characterized by a suite of Mesoproterozoic granitoid gneiss and meta-anorthosite that are intruded by Neoproterozoic A-type granitic plutons and exposed in a series of elongate NE-SW trending domes. The domes are framed by an aerially extensive cover sequence that includes a heterogeneous sequence of high-grade paragneiss (traditionally known as the Maidens Gneiss) with discontinuous layers of amphibolite and pegmatitic gneiss. The contact between the gneiss domes and cover sequence has traditionally been interpreted as either unconformable or intrusive.

New U/Pb (zircon, rutile, and apatite) geochronology from the domes and the overlying cover sequence indicates that Mesoproterozoic rocks in the Goochland domes were not in contact with cover sequence gneisses until after 400 Ma. We propose that the high-grade cover sequence is peri-Gondwanan, and was emplaced over a Proterozoic basement sequence (possibly of Laurentian origin) during the Acadian orogeny. Accretion of these disparate crustal blocks likely occurred in the northern Appalachians. The domes formed after accretion as a consequence of transpressional deformation and orogen-parallel elongation during the 310-290 Ma Alleghanian orogeny. Strain and vorticity analysis requires that the Goochland Terrane was translated hundreds of kilometers southwest along the Laurentian margin during the Alleghanian. These data also indicate that high-grade rocks in Goochland terrane were rapidly exhumed from mid- to lower-crustal depths in the Triassic at the onset of pre-Atlantic rifting.