PETROLOGY, STRUCTURE, AND GEOCHRONOLOGY OF THE OAKVILLE METAVOLCANIC SEQUENCE: IMPLICATIONS FOR THE PROVENANCE OF THE SMITH RIVER ALLOCHTHON, VIRGINIA

The Smith River Allochthon (SRA), a suspect terrane in the southwestern Piedmont of Virginia, is interpreted as a regional scale thrust sheet of metaclastic and meta-igneous rocks emplaced over lower-grade greenschist facies assemblages. Previous researchers have hypothesized the SRA to be either an accreted peri-Gondwanan terrane or a distal Laurentian terrane, based on U-Th-Pb monazite and U-Pb detrital zircon ages respectively. In the northern end of the SRA, the Oakville metavolcanic suite consists of bimodal meta-igneous rocks which remain enigmatic but are key to understanding the paleogeography of central Virginia. We aim to characterize the petrology and structural geometry of the Oakville metavolcanic suite and determine the timing of the SRA’s magmatism using U-Pb zircon geochronology.

Our mapping of the northern end of the SRA indicates that layers of felsic schist and gneiss are associated with amphibole schist/greenstone that occur over a broad area; these are subordinate to pelitic schists known as the Fork Mountain Formation. Regional-scale areas that have previously been mapped as felsic metavolcanics are dominated by muscovite schist derived from Al-rich sedimentary protoliths. Felsic metavolcanics with relict quartz phenocrysts occur, but are not common. Chemically, the felsic rocks range from 68-77% SiO₂ and the mafic rocks range from 45-48% SiO₂.

A mylonitic meta-rhyolite contained a population of euhedral zircons that yielded a U-Pb age of 580 ± 5 Ma which we interpret as magmatic. A greenstone sample yielded a multimodal U-Pb zircon age spectra, including xenocrystic cores featuring 1.0 to 1.2 Ga ages with the oldest grains between 1.7 – 1.8 Ga, possibly magmatic grains between 575 and 610 Ma, and abundant rim ages between 320 and 350 Ma. Other ‘felsic metavolcanic’ samples yielded preeminently Grenvillian zircon populations and are likely arkosic metasedimentary rocks.

Ediacaran magmatic ages and the prevalence of Grenvillian zircons suggest the Oakville metavolcanic suite to be the product of rift-related magmatism, that likely occurred during the rift-to-drift transition along the distal Laurentian margin as Iapetus sea-floor spreading commenced. These data require that existing structural models associating the SRA with Gondwanan affinity be reevaluated.