Pre-Ordovician, probable Early Cambrian metavolcanic and somewhat younger meta-intrusive rocks form a linear belt in the central Virginia Piedmont. On the basis of the geologic and geochemical features of these rocks in the Fredericksburg area, and insofar as this area is representative of the entire belt, it is concluded that this belt constitutes an ancient island-arc sequence. The westernmost Chopawamsic Formation consists of a tholeiitic island-arc suite and associated calcalkaline metavolcanic rocks. Prior to Upper Ordovician time, low-potassic tonalitic plutons of plagiogranitic (pretectonic) and trondhjemitic (late-tectonic) affinities were intruded into the Chopawamsic terrane. The tholeiitic amphibolites of the Ta River Metamorphic Suite are considered to be an eastern, oceanward facies of the Chopawamsic Formation.
The James Run Formation of Maryland is probably a northern lateral equivalent of the central Virginia volcanic-plutonic belt. It may have formed as part of the ancient Virginia belt or along a penecontemporaneous but independent volcanic arc. The metavolcanic rocks and associated plutons of the Carolina slate belt in the southeast Piedmont are considered to have formed independently of the central Virginia volcanic-plutonic belt and perhaps in a different tectonic environment.
