A Newly Identified Gondwanan Terrane in the Northern Appalachian Mountains: Implications for the Taconic Orogeny and Closure of the Iapetus Ocean

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The Taconic and Salinic orogenies in the northern Appalachian Mountains record the closure of the Iapetus Ocean, which separated peri-Laurentian and peri-Gondwanan terranes in the early Paleozoic. The Taconic orogeny in New England is commonly depicted as an Ordovician collision between the peri-Laurentian Shelburne Falls arc and the Laurentian margin, followed by Silurian accretion of peri-Gondwanan terranes during the Salinic orogeny. New U-Pb zircon geochronology demonstrates that the Shelburne Falls arc was instead constructed on a Gondwanan-derived terrane preserved in the Moretown Formation, which we refer to here as the Moretown terrane. Metasedimentary rocks of the Moretown Formation were deposited after 514 Ma and contain abundant ca. 535–650 Ma detrital zircon that suggest a Gondwanan source. The Moretown Formation is bound to the west by the peri-Laurentian Rowe belt, which contains detrital zircon in early Paleozoic metasedimentary rocks that is indistinguishable in age from zircon in Laurentian margin rift-drift successions. These data reveal that the principal Iapetan suture in New England is between the Rowe belt and Moretown terrane, more than 50 km farther west than previously suspected. The Moretown terrane is structurally below and west of volcanic and metasedimentary rocks of the Hawley Formation, which contains Laurentian-derived detrital zircon, providing a link between peri-Laurentian and peri-Gondwanan terranes. The Moretown terrane and Hawley Formation were intruded by 475 Ma plutons during peak activity in the Shelburne Falls arc. We propose that the peri-Laurentian Rowe belt was subducted under the Moretown terrane just prior to 475 Ma, when the trench gap was narrow enough to deliver Laurentian detritus to the Hawley Formation. Interaction between peri-Laurentian and peri-Gondwanan terranes by 475 Ma is 20 m.y. earlier than documented elsewhere and accounts for structural relationships, Early Ordovician metamorphism and deformation, and the subsequent closure of the peri-Laurentian Taconic seaway. In this scenario, a rifted-arc system on the Gondwanan margin resulted in the formation of multiple terranes, including the Moretown, that independently crossed and closed the Iapetus Ocean in piecemeal fashion.