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 has 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 meta-pelitic rocks of the Fork Mountain Formation. This study aims to collect data and create a detailed metamorphic timeline of the Oakville Volcanic Suite.

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 data indicates that the Oakville Volcanic Suite is bimodal. Tectonic discriminantion diagrams (Yb vs. Ta & Y+Nb vs. Rb) show the Oakville Volcanic Suite as the product of 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 outlying detrital zircons with crystallization ages of 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 overgrowths. Based on this geochemical and geochronological data, we interpret the Oakville Volcanic Suite as the product of Ediacaran rift-related magmatism during the opening of the Iapetus Basin, 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. This information calls into question existing tectonic models for the SRA.