WAS THE EARLY ORDOVICIAN SHELBURNE FALLS ARC BUILT ON GONDWANAN CRUST CLOSE TO LAURENTIA?

Following the Ediacaran breakup of Rodinia, Grenville-derived sediment dominated the rift to drift sediments of the Laurentian margin of western New England. A dramatic change in detrital zircon age distribution coincides with the Rowe Schist-Moretown Fm contact, and provides evidence that this is the Laurentian-Gondwanan suture. Ultramafic lenses are concentrated along this suture in VT and western MA, suggesting that it is a paleo-subduction zone. Structurally above and east of the Moretown Fm are forearc mafic volcanic rocks and sediments of the Hawley Fm.. Both the Moretown and Hawley Fm were intruded by 475 Ma arc-related plutons, the Hallockville Pond Gneiss and Dell Metatrondhjemite, respectively. These intrusive rocks are coeval with the plutonic core rocks of the Shelburne Falls dome; we include the Hallockville Pond Gneiss and the Hawley Fm in the Shelburne Falls arc, which we suggest was built on the Moretown terrane. However, detrital zircon from Hawley Fm metasediments are dramatically different from those in the underlying Moretown Fm, and constrain the timing of collision between Laurentia and the Shelburne Falls arc. Two samples contain Laurentian-derived detrital zircons and suggest that the Shelburne Falls arc was proximal to Laurentia at ca. 475 Ma. One contains Ordovician arc zircons and indicates that deposition was active during or after arc magmatism. The Shelburne Falls arc is dominated by felsic plutonic and volcanic rocks that formed between 475 to 470 Ma, when the Shelburne Falls arc was close to Laurentia. This burst of magmatic activity may have been triggered by the onset of subduction of water-rich Laurentian sediments of the Rowe Schist or slab breakoff. Collision between 480 and 470 Ma is consistent with the oldest Ar cooling ages from Laurentian margin rocks. A newly dated 466 Ma rhyolite from the Barnard Volcanic Member was erupted after collision of Laurentia and the Shelburne Falls arc; it may be coeval with slab breakoff of the older east-dipping subducting plate or initiation of younger west-dipping subduction beneath the newly accreted Moretown terrane. This subduction polarity reversal produced a younger Laramide-style phase of the Taconic orogeny responsible for the youngest Ordovician thrusts in the Taconic Range and the Late Ordovician Black River and Trenton Groups basin.