NEW AGE CONTROLS ON THE PETERSBURG TERRANE, CENTRAL-EASTERN VIRGINIA PIEDMONT

New SHRIMP-RG geochronology confirms the crystallization age of foliated granitoid gneiss that makes up much of the exposed Petersburg terrane (Pt) in the central-eastern VA Piedmont, and the timing of late Paleozoic intrusion and metamorphism. A sample of strongly foliated biotite granitoid gneiss from near Hunton, VA on the Chickahominy River (Glen Allen 7.5’ Quadrangle) yielded a U-Pb zircon age of 425 ± 4 Ma, with metamorphic rim ages of 325 ± 6 Ma. Another sample of layered granitoid gneiss along Pocoshock Creek south of the James River (Chesterfield 7.5’ Quadrangle) yielded a U-Pb zircon age of 418 ± 3 Ma; rim ages were mostly discordant but two concordant ages averaged 315 Ma. Foliated metagranitoid from the James River near Belle Isle in Richmond (Richmond 7.5’ Quadrangle) yielded 404 ± 5 Ma zircon cores, with several concordant rims at ~300 Ma.

LA-ICP-MS detrital zircon analysis from a mica schist xenolith within foliated and layered granite gneiss along Powhite Creek (Bon Air 7.5’ Quadrangle) yielded zircons as young as Cambrian. Zircon from a sample of weakly layered granitoid gneiss from near French Hay, VA (Yellow Tavern 7.5’ Quadrangle) yielded mostly discordant ages but preserved an inherited core of ~650 Ma.

A sample of massive granite from the Vulcan Materials Jack Quarry (Sutherland 7.5’ Quadrangle) yielded a SHRIMP-RG U-Pb zircon age of 318 ± 4 Ma, which compares well with the Pennsylvanian date (~314 Ma) of Lee and Williams (1993).

Coupled with published data, the new data confirm that most rocks previously mapped as late Paleozoic Petersburg Granite are part of an infrastructural terrane of Devonian age. Devonian metagranitoid was intruded by at least two late Paleozoic plutons: the southern DeWitt-Sutherland pluton (~320 Ma) and the northern Richmond pluton (~300 Ma, first reported by Buchwaldt and Owens, 2012). Field evidence does not support intrusion of these Devonian and younger granitoids into the Roanoke Rapids terrane (RRt). Additionally, new mapping at the southernmost end of the Pt near Jarratt, VA shows that the RRt-Pt contact is not intrusive, rather foliated metagranitoid and mica schist of the Pt are juxtaposed against the RRt along a 1.5 km-wide high-strain phyllonitic zone locally overprinted by silicified breccia. The Pt is apparently its own terrane, separate from the RRt.