NEW GEOCHRONOLOGY FROM THE EASTERNMOST PIEDMONT AND COASTAL-PLAIN BASEMENT IN VIRGINIA AND MARYLAND

New geologic mapping, U/Pb geochronology of zircon, and ⁴⁰Ar/³⁹Ar data from amphibole and muscovite constrain the tectonic history of rocks across the easternmost Piedmont and coastal-plain basement in VA-MD. Near the Fall Zone in central VA, Silurian-Devonian granitoid gneisses were intruded by Pennsylvanian to Permian granites and together comprise the Dinwiddie terrane (Dt, formerly “Petersburg batholith”). The gneissic character of Silurian-Devonian granitoids and presence of staurolite schist suggest the older components of the Dt experienced amphibolite-facies metamorphism. Ages of inherited cores in zircon and detrital zircon geochronology suggest the Dt is of peri-Gondwanan affinity. The Dt is separated from the Goochland terrane to the west by the Hylas fault zone and from the Roanoke Rapids terrane (RRt) to the southwest by the Nottaway River fault zone. The exposure of amphibolite-facies rocks in the Dt suggests deep exhumation in this restraining bend of the Eastern Piedmont fault system. All bedrock to the east is covered by coastal plain sediments.

At the eastern margin of the Dt, boreholes and geophysical data delineate the Triassic Studley subbasin of the Taylorsville basin, bounded to the east by the Sussex mafic complex. Zircon from Bt-Qz-Fsp gneiss beneath the subbasin yields a U/Pb crystallization age of ~570 Ma, and hornblende from well-foliated amphibolite in the same borehole yields a ⁴⁰Ar/³⁹Ar cooling age of ~326 Ma in these amphibolite-facies rocks. Zircon from a cataclastic felsic intrusive rock in the Sussex mafic complex yields a U/Pb crystallization age of ~580 Ma. Eastward, U/Pb zircon analyses of five samples from the Chesapeake zone (Cz) in MD and VA record Neoproterozoic igneous ages from 675 to 625 Ma. However, evidence for amphibolite-facies metamorphism is lacking in dated samples. Magmatic muscovite from one granitoid retains an Ediacaran cooling age, two tuffaceous metasiltstones have a single foliation dominated by chlorite, and evidence for ductile deformation of feldspars is absent. The Neoproterozoic crystallization ages and lack of evidence for Paleozoic metamorphism in the Cz are consistent with Gondwanan or peri-Gondwanan volcanic-arc rocks. East of the Cz, previous studies indicate Alleghanian amphibolite-facies metamorphism in the Hatteras terrane in VA.