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.

ABSTRACT The Baie Verte Line in western Newfoundland marks a suture zone between (1) an upper plate represented by suprasubduction zone oceanic crust (Baie Verte oceanic tract) and the trailing continental Notre Dame arc, with related upper-plate rocks built upon the Dashwoods terrane; and (2) a lower plate of Laurentian margin metasedimentary rocks with an adjoining ocean-continent transition zone (Birchy Complex). The Baie Verte oceanic tract formed during closure of the Taconic seaway in a forearc position and started to be obducted onto the Laurentian margin between ca. 485 and 476 Ma (early Taconic event), whereas the Birchy Complex, at the leading edge of the Laurentian margin, was subducted to maximum depths as calculated by pseudosection techniques (6.7–11.2 kbar, 315–560 °C) by ca. 467–460 Ma, during the culmination of the Taconic collision between the trailing Notre Dame arc and Laurentia, and it cooled isobarically to 9.2–10.0 kbar and 360–450 °C by 454–449 Ma (M 1 ). This collisional wedge progressively incorporated upper-plate Baie Verte oceanic tract rocks, with remnants preserved in M 1 high-pressure, low-temperature greenschist-facies rocks (4.8–8.0 kbar, 270–340 °C) recording typical low metamorphic gradients (10–14 °C/km). Subsequently, the early Taconic collisional wedge was redeformed and metamorphosed during the final stages of the Taconic cycle. We relate existing and new 40 Ar/ 39 Ar ages between 454 and 439 Ma to a late Taconic reactivation of the structurally weak suture zone. The Taconic wedge on both sides of the Baie Verte suture zone was subsequently strongly shortened (D 2 ), metamorphosed (M 2 ), and intruded by a voluminous suite of plutons during the Salinic orogenic cycle. Calculated low- to medium-pressure, low-temperature M 2 conditions in the Baie Verte oceanic tract varied at 3.0–5.0 kbar and 275–340 °C, with increased metamorphic gradients of ~17–25 °C/km during activity of the Notre Dame arc, and correlate with M 2 assemblages in the Birchy Complex. These conditions are associated with existing Salinic S 2 white mica 40 Ar/ 39 Ar ages of ca. 432 Ma in a D 2 transpressional shear zone and synkinematic intrusions of comparable age. A third metamorphic event (M 3 ) was recorded during the Devonian with calculated low-pressure, low-temperature conditions of 3.2–3.8 kbar and 315–330 °C under the highest metamorphic gradients (23–30 °C/km) and associated with Devonian–early Carboniferous isotopic ages as young as 356 ± 5 Ma. The youngest ages are related to localized extension associated with a large-scale transtensional zone, which reused parts of the Baie Verte Line suture zone. Extension culminated in the formation of a Middle to Late Devonian Neoacadian metamorphic core complex in upper- and lower-plate rocks by reactivation of Baie Verte Line tectonites formed during the Taconic and Salinic cycles. The Baie Verte Line suture zone is a collisional complex subjected to repeated, episodic structural reactivation during the Late Ordovician Taconic 3, Silurian Salinic, and Early–Late Devonian Acadian/Neoacadian orogenic cycles. Deformation appears to have been progressively localized in major fault zones associated with earlier suturing. This emphasizes the importance of existing zones of structural weakness, where reactivation took place in the hinterland during successive collision events.

Abstract During the Early to Middle Palaeozoic, prior to formation of Pangaea, the Canadian and adjacent New England Appalachians evolved as an accretionary orogen. Episodic orogenesis mainly resulted from accretion of four microcontinents or crustal ribbons: Dashwoods, Ganderia, Avalonia and Meguma. Dashwoods is peri-Laurentian, whereas Ganderia, Avalonia and Meguma have Gondwanan provenance. Accretion led to a progressive eastwards (present co-ordinates) migration of the onset of collision-related deformation, metamorphism and magmatism. Voluminous, syn-collisional felsic granitoid-dominated pulses are explained as products of slab-breakoff rather than contemporaneous slab subduction. The four phases of orogenesis associated with accretion of these microcontinents are known as the Taconic, Salinic, Acadian and Neoacadian orogenies, respectively. The Ordovician Taconic orogeny was a composite event comprising three different phases, due to involvement of three peri-Laurentian oceanic and continental terranes. The Taconic orogeny was terminated with an arc–arc collision due to the docking of the active leading edge of Ganderia, the Popelogan–Victoria arc, to an active Laurentian margin (Red Indian Lake arc) during the Late Ordovician (460–450 Ma). The Salinic orogeny was due to Late Ordovician–Early Silurian (450–423 Ma) closure of the Tetagouche–Exploits backarc basin, which separated the active leading edge of Ganderia from its trailing passive edge, the Gander margin. Salinic closure was initiated following accretion of the active leading edge of Ganderia to Laurentia and stepping back of the west-directed subduction zone behind the accreted Popelogan–Victoria arc. The Salinic orogeny was immediately followed by Late Silurian–Early Devonian accretion of Avalonia (421–400 Ma) and Middle Devonian–Early Carboniferous accretion of Meguma (395–350 Ma), which led to the Acadian and Neoacadian orogenies, respectively. Each accretion took place after stepping-back of the west-dipping subduction zone behind an earlier accreted crustal ribbon, which led to progressive outboard growth of Laurentia. The Acadian orogeny was characterized by a flat-slab setting after the onset of collision, which coincided with rapid southerly palaeolatitudinal motion of Laurentia. Acadian orogenesis preferentially started in the hot and hence, weak backarc region. Subsequently it was characterized by a time-transgressive, hinterland migrating fold-and-thrust belt antithetic to the west-dipping A–subduction zone. The Acadian deformation front appears to have been closely tracked in space by migration of the Acadian magmatic front. Syn-orogenic, Acadian magmatism is interpreted to mainly represent partial melting of subducted fore-arc material and pockets of fluid-fluxed asthenosphere above the flat-slab, in areas where Ganderian's lithosphere was thinned by extension during Silurian subduction of the Acadian oceanic slab. Final Acadian magmatism from 395– c . 375 Ma is tentatively attributed to slab-breakoff. Neoacadian accretion of Meguma was accommodated by wedging of the leading edge of Laurentia, which at this time was represented by Avalonia. The Neoacadian was devoid of any accompanying arc magmatism, probably because it was characterized by a flat-slab setting throughout its history.

Abstract The Taconian–Grampian tract was characterized by a diachronous collision of a north-facing oceanic arc–forearc terrane and associated backarc basins with an irregular Laurentian margin with hyperextended segments. Hyperextension produced outboard continental terranes, separated by exhumed subcontinental mantle from the inboard margin. The exhumed mantle facilitated continued subduction of the extended margin after it had entered the trench. Enhanced slab-rollback resulted in spreading in lenticular backarc basins, which gradually transitioned along-strike into extensional arcs where rollback was less. Obduction of the oceanic elements onto the irregular Laurentian margin was followed by diachronous slab breakoff and a subduction polarity reversal, such that south- and north-dipping subduction zones locally were coeval along-strike. The polarity flip changed the convergence obliquity from dextral to sinistral and was accompanied by shallowing of the subducting slab near the end of the Middle Ordovician. Strike-slip movements locally juxtaposed segments where tectonic events occurred at different times, producing conflicting relationships. Slab breakoff produced punctuated magmatism, largely driven by mantle-derived melts, and drove and/or enhanced metamorphism in the overlying and enveloping crustal rocks. Boninite was generated episodically over a time span of 32 myr; the oldest Cambrian phase in the Lushs Bight Oceanic Tract (LBOT) and correlatives was associated with subduction initiation.