Laurentian and Amazonian sediment sources to Neoproterozoic–lower Paleozoic Maryland Piedmont rocks
Several terranes of variable tectonic affinity and history underlie the central Appalachian Piedmont Province (eastern United States). These terranes mostly consist of widespread metasedimentary and lesser metavolcanic rocks. Intense and pervasive deformation and metamorphism have made the depositional ages and provenance of sediment in these rocks difficult to determine. The lack of tight constraints on such basic information led to a century-long debate about the tectonic significance of these rocks, particularly how they correlate to similar rocks along and across strike in the Appalachian orogen. We address these issues using U/Pb isotopic ages from single spots in 2433 zircon grains from 18 metasedimentary rock samples distributed across the Maryland Piedmont.

The resulting age signatures indicate that the Marburg Formation and Prettyboy Schist, heretofore assigned to the Westminster terrane, actually belong to the Potomac terrane, making the Hyattstown thrust the contact between the two terranes. Ediacaran Laurentia could have supplied all Potomac terrane sediment except for the detritus in one sample from the northern part of the terrane that likely came from Amazonia. This is one of the first recognitions of a Gondwana-derived terrane between Carolinia to the south and Ganderia to the north. Maximum depositional ages for Potomac terrane suprasubduction zone sedimentary rocks are latest Neoproterozoic or early Cambrian, and some may have been deposited ca. 510 Ma. Continental rifting ended ca. 560 Ma at the longitude of our study, so the transition from rifting to subduction at this location in eastern Laurentia may have lasted only 50 M.y. Lower Ordovician arc intrusions into these rocks demonstrate that the transition lasted no longer than 90 M.y. The Iapetan margin of central-eastern Laurentia was one of the shortest lived passive margins that formed in Neoproterozoic time.