Origin of an Early Paleozoic arc-related sedimentary basin in the northern Vermont Appalachians: a detrital zircon study

New U-Pb ages of detrital zircons from Early Paleozoic metamorphosed sediments in the northern Appalachians of Vermont are at odds with previous tectonic interpretations. Metasedimentary rocks of the Moretown Formation of Vermont were originally interpreted as fore-arc basin deposits derived from a Cambrian-Ordovician volcanic arc (Shelburne Falls or composite Bronson Hill), adjacent to the Proterozoic Laurentian continental margin or rifted silver thereof on the western side of the Iapetus Ocean. LA-ICP-MS U-Pb ages of zircons from the Moretown Formation show strong peaks in kernel density estimates in the 550 - 700 Ma age range with few ages in the 1000 - 1200 Ma range. A few zircons from the Moretown metasedimentary rocks have ages in the 475 - 500 Ma range, similar to ages determined for the Shelburne Falls volcanic arc in southern Vermont and Massachusetts. In contrast, zircon ages from meta-sedimentary rocks from Late Proterozoic-Cambrian, rift-related formations are dominated, as expected, by strong peaks in the 1000-1200 Ma range, typical of Grenville sources in the Laurentian continent. The zircon age distributions in the Moretown Formation are unlike signatures from Laurentian (Grenville) sources; instead, they match most closely peri-Gondwanan (Ganderian?) sources from the eastern side of the Iapetus Ocean. In previous tectonic models in which the Shelburne Falls arc is on the western side of the Iapetus Ocean, the Laurentian continent should be a major source of zircons to sedimentary basins near the arc. However, our new data instead indicate an origin for the Moretown Formation of northern Vermont on the eastern (Gondwanan) side of Iapetus and either late or post-Taconian accretion.