ABSTRACT Synthesis of the Ordovician Taconic orogeny in the northern Appalachians has been hindered by along-strike variations in Laurentian, Gondwanan, and arc-generated tectonic elements. The Dashwoods terrane in Newfoundland has been interpreted as a peri-Laurentian arc terrane that collided with the Laurentian margin at the onset of the Taconic orogeny, whereas along strike in New England, the Moretown terrane marks the leading edge of peri-Gondwanan arcs. The peri-Laurentian affinity of the Dashwoods terrane hinges on the correlation of its oldest metasedimentary rocks with upper Ediacaran to Lower Ordovician rift-drift deposits of the Laurentian Humber margin in western Newfoundland. Here, we report U-Pb dates and trace-element geochemistry on detrital zircons from metasedimentary rocks in the southern Dashwoods terrane that challenge this correlation and provide new insights into the Taconic orogeny. Based on age and trace-element geochemistry of detrital zircons analyzed by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS), we identified ca. 462–445 Ma sedimentary packages with a mixed provenance consisting of Laurentian, Gondwanan, and arc-derived Cambrian–Ordovician sources. These deposits overlap in age with Upper Ordovician strata of the Badger Group of the Exploits subzone, which also contain Laurentian detritus. We infer dominantly east-directed transport of Laurentian detritus from the Taconic collision zone across a postcollisional arc–back-arc complex at ca. 462–455 Ma followed by dominantly west-directed transport of detritus from the Red Indian Lake arc at ca. 455–445 Ma. Our analysis of zircon inheritance from Dashwoods igneous rocks suggests that 1500–900 Ma Laurentian crystalline basement of the Humber margin is an unlikely source of Dashwoods inherited zircon. Instead, a more cosmopolitan Laurentian inheritance may be best explained as sourced from subducted Laurentian sediment. Our results demonstrate that the sampled metasedimentary units from the southern Dashwoods terrane do not correlate with rift-drift strata of the Humber margin as previously proposed, nor with the basement of the Moretown terrane; yet, these Middle to Upper Ordovician successions suggest the potential for an alternative plate-tectonic model in which the Taconic orogeny may have been initiated by collision of Gondwanan arc terranes that closed the main tract of the Iapetus Ocean along the Baie Verte–Brompton Line.

The Taconic orogeny in Newfoundland consisted of three accretionary events (Taconic 1, 2, and 3). Taconic 1 is represented by ca. 495 Ma, west-directed obduction of the infant-arc Lushs Bight oceanic tract (510–501 Ma) onto the peri-Laurentian Dashwoods microcontinent. Subduction is inferred to have initiated at a spreading center abandoned during an inboard ridge jump responsible for separation of Dash-woods from Laurentia and opening of the Humber seaway. Clogging of the subduction zone by Dashwoods forced subduction to step back into the Humber seaway. Inception of the new subduction zone led to formation of the ca. 490 Ma Baie Verte oceanic tract. Closure of the Humber seaway formed the Notre Dame arc (489–477 Ma) built on Dashwoods and the coeval Snooks Arm arc built on the Baie Verte oceanic tract. Sea-way closure led to collision (Taconic 2) between the arcs and Laurentia, which caused significant shortening of the Notre Dame arc. After a magmatic gap of 7–10 m.y., the Notre Dame arc records a voluminous flare-up of predominantly tonalite magmatism (464–459 Ma) during the waning stages of Taconic 2. Magmatism overlaps with deformation and includes both arc and non-arc-like tonalite. This flare-up was related to break-off of the oceanic lithosphere of the downgoing slab. The rapidly upwelling asthenosphere that replaced the broken-off slab induced melting in the subarc mantle and arc infrastructure. Taconic 3 is represented by 455–450 Ma accretion of a peri-Laurentian arc that had formed after the ca. 480 Ma initiation of west-directed subduction in the Iapetus Ocean outboard of the Dashwoods microcontinent.