Ordovician volcanic-arc terrane in the Central Appalachian Piedmont of Maryland and Virginia; SHRIMP U-Pb geochronology, field relations, and tectonic significance

U-Pb zircon geochronology and field relations provide insights into metavolcanic and associated rocks in the Central Appalachian Piedmont of Maryland and northern Virginia. Ordovician ages were determined for volcanic-arc rocks of the James Run Formation (Churchville Gneiss Member, 458 + or - 4 Ma; Carroll Gneiss Member, 462 + or - 4 Ma), Relay Felsite (458 + or - 4 Ma), Chopawamsic Formation (453 + or - 4 Ma), and a Quantico Formation volcaniclastic layer (448 + or - 4 Ma). A previously dated first phase of volcanism in the Chopawamsic Formation was followed by the second phase dated here. The latter suggests a possible source for contemporaneous volcanic-ash beds throughout eastern North America. Dates from the Chopawamsic and Quantico Formations constrain the transition from arc volcanism to successor-basin sedimentation. Ordovician metatonalites of the Franklinville (462 + or - 5 Ma) and Perry Hall (461 + or - 5 Ma) plutons are contemporaneous with the James Run Formation, whereas granitoids of the Bynum Run (434 + or - 4 Ma) and Prince William Forest (434 + or - 8 Ma) plutons indicate an Early Silurian plutonic event. The Popes Head Formation yielded Mesoproterozoic (1.0-1.25 Ga, 1.5-1.8 Ga) detrital zircons, and metamorphosed sedimentary melange of the Sykesville Formation yielded Mesoproterozoic (1.0-1.8 Ga) detrital zircons plus a minor Archean (2.6 Ga) component. A few euhedral zircons (ca. 479 Ma) in the Sykesville Formation may be from granitic seams related to the Dalecarlia Intrusive Suite. A Potomac Orogeny in the Central Appalachian Piedmont is not required, but the earliest Taconic orogenesis remains poorly constrained.