LATE ORDOVICIAN ACCRETION OF THE CHOPAWAMSIC ARC IN THE WESTERN PIEDMONT OF VIRGINIA: U-PB ZIRCON GEOCHRONOLOGY OF META-IGNEOUS AND META-SEDIMENTARY ROCKS ACROSS THE CHOPAWAMSIC FAULT

The western Piedmont of Virginia is bisected by the Late Ordovician Chopawamsic fault. The metaclastic Potomac terrane (Pt) to the west of the fault has been interpreted to be derived of the Laurentian craton and the metavolcanic Chopawamsic terrane (Ct) to the east of the fault is of unknown crustal affinity. If the Ct is of peri-Gondwanan origin, then the Chopawamsic fault marks the suture of the early Paleozoic Iapetus Ocean. Preliminary U-Pb geochronological data presented here expand the current dataset with which to deduce tectonic development along southeastern Laurentia during the early Paleozoic.

Samples from the Pt include three granitoid bodies which yield ages between 473 – 469 Ma. Though previously interpreted to represent map-scale detrital blocks within a mélange matrix, we interpret these bodies to be either faulted or intrusive into the Pt. The lack of any Ordovician debris from detrital zircon samples (n=5) of metasediments in the Pt supports this interpretation. Geochemical data from the dated bodies and other intrusive rocks sampled from the Pt have the characteristics of forming in a suprasubduction zone setting.

Preliminary U-Pb zircon data from the Ct include the Chopawamsic Formation (n=3, ca. 474 – 465 Ma), the Richland Run composite pluton (n=2, ca. 474 – 468 Ma), and the Garrisonville complex (n=2, ca. 470 – 465 Ma). Geochemical data from these samples and other samples collected from the Ct have characteristics of volcanic arc magmatism. These new ages and complementary detrital zircon data from the Ct (n=4) appear to support a model for a single, continuous, Middle-Late Ordovician phase of Chopawamsic magmatism.

Prior to the addition of the Ct to Laurentia, it appears that subduction-related arc magmatism occurred simultaneously (ca. 475-455 Ma) in the Pt and Ct. Following the accretion of the Ct, the ca. 444 Ma Ellisville pluton, emplaced as a result of subduction of oceanic crust outboard of the Ct, stitched the Chopawamsic fault. Detrital zircon data from the Arvonia successor basin (n=2) suggest it received mixed Pt-Ct debris following regional uplift, and at least some part of it was deposited at or after ca. 430 Ma. Evidence for deposition of the Arvonia basin only after the accretion of the Ct to Laurentia indicates that it cannot be used directly to determine provenance for the underlying Ct.