GEOCHRONOLOGY OF PALEOZOIC ROCKS OF THE DINWIDDIE TERRANE, CENTRAL-EASTERN VIRGINIA PIEDMONT, USA

The Dinwiddie terrane is a newly recognized 1500 km² crustal block in the central-eastern Virginia Piedmont that was previously interpreted to be wholly comprised of late Paleozoic granite of the Petersburg batholith. It is separated to the northwest from the Mesoproterozoic to middle Paleozoic Goochland terrane by the Hylas fault zone and to the southwest from the exotic Neoproterozoic Roanoke Rapids terrane by the Nottoway River fault zone. Its eastern boundary is covered by Coastal Plain sediments, but geophysical and deep borehole data suggest an orogen-scale suture separates it from the Neoproterozoic Chesapeake block.

Geologic mapping coupled with U-Pb SHRIMP and LA-ICP-MS zircon geochronology demonstrates that the Dinwiddie terrane consists of Mississippian to Permian massive equigranular and porphyritic granite (Petersburg granite, sensu stricto), Silurian to Devonian foliated metagranite and layered granite gneiss (Pocoshock gneiss), and Cambrian paragneiss (mica schist). Undated amphibolite xenoliths are also present. Strongly foliated Pocoshock gneiss yields U-Pb zircon igneous crystallization ages of ca. 423 Ma to ca. 402 Ma. Zircon metamorphic rim ages (low Th/U ratios) from Pocoshock gneiss indicate diachronous recrystallization at ca. 375 Ma and during intrusion of Petersburg granite (sensu stricto), ca. 320 to 299 Ma. Deformation in Pocoshock gneiss may have occurred during the Acadian orogeny but was then strongly overprinted by Alleghanian deformation.

Zircons from both Pocoshock gneiss and Petersburg granite also preserve inherited cores that range from ca. 631 Ma to 375 Ma; most are 554 Ma to 502 Ma. These data suggest the terrane is part of the peri-Gondwanan realm; there may be correlations with the Goochland terrane across the Hylas fault zone, but none exist with the Roanoke Rapids terrane across the Nottoway River fault zone. Raleigh terrane rocks in North Carolina share similar ages to Dinwiddie terrane rocks but correlation requires structural connections that aren’t apparent in the field or recognizable in geophysical datasets. The Dinwiddie terrane may have been part of Ganderia in the northern Appalachians at least through the Early Devonian. If this model is correct, orogen-scale dextral transpression may have translated the rocks southward during the Alleghanian orogeny.