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 ± 4 Ma; Carroll Gneiss Member, 462 ± 4 Ma), Relay Felsite (458 ± 4 Ma), Chopawamsic Formation (453 ± 4 Ma), and a Quantico Formation volcaniclastic layer (448 ± 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 ± 5 Ma) and Perry Hall (461 ± 5 Ma) plutons are contemporaneous with the James Run Formation, whereas granitoids of the Bynum Run (434 ± 4 Ma) and Prince William Forest (434 ± 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 mélange 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.