AGE AND ORIGIN OF THE OAKVILLE VOLCANIC SUITE, NORTHERN SMITH RIVER ALLOCHTHON, VIRGINIA PIEDMONT

The Smith River Allochthon (SRA), in the southern Virginia Piedmont, is an enigmatic terrane that’s long been interpreted as a regional scale thrust sheet of metamorphic rocks emplaced over lower grade rocks of Laurentian affinity. The northern SRA, in Appomattox and Buckingham counties, includes a distinctive but poorly studied suite of meta-volcanic rocks (Oakville Volcanic Suite) that occurs in association with pelitic rocks of the Fork Mountain Formation.

The Oakville Volcanic Suite includes hornblende gneiss, fine-grained epidote gneiss, and fine-grained quartzofeldspathic schist. Some of these rocks preserve vestiges of primary igneous structures including phenocrysts in felsic rocks as well as epidote amygdules in mafic rocks. Whole-rock geochemistry indicates that the Oakville Volcanic Suite is bimodal. Tectonic discriminant data (Yb vs. Ta & Y+Nb vs. Rb) are consistent with the rocks of the Oakville Volcanic Suite being sourced as within-plate rift-generated magmatism. U-Pb zircon geochronology (LA-ICP MS) on four samples of schistose meta-rhyolite yield crystallization ages between 545 to 570 Ma. A volcaniclastic mafic schist near Appomattox yielded a few detrital zircons with 1.0 to 1.2 Ga (Grenvillian ages). Another mafic schist in Buckingham County yielded significant populations of Grenvillian and Ediacaran detrital zircons as well as some Paleozoic rims. Based on these geochemical and geochronological data we interpret the Oakville Volcanic Suite as the product of Ediacaran rift-related magmatism during the opening of Iapetus, likely on a highly extended block of Laurentian crust. The northern end of the SRA records a significantly different geological history than the type-location for the SRA in southern Virginia which calls into question existing tectonic models for the SRA.