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.

The evolution of many modern intra-oceanic and continental arc systems is exemplified by cycles of arc construction, rifting, and separation of remnant and active arcs by a backarc basin floored by oceanic crust. Rifted arc complexes and backarc basins are inherently subductable, and hence only a fragmentary record of rifting and arc construction is preserved in the ancient record. In this contribution, we synthesize available geochronological, geochemical, isotopic, and stratigraphic data in order to discuss the evolution of the Cambrian to Ordovician Penobscot-Victoria Arc, which developed on the leading edge of Ganderia, a peri-Gondwanan microcontinent. Although the Penobscot and Victoria stages of arc-backarc development occurred in a predominantly extensional suprasubduction-zone setting, they each display a distinctly different character of magmatism and sedimentation. These stages are separated by an orogenic episode marked by the obduction of backarc ophiolites onto the Ganderian passive margin. The Cambrian to Lower Ordovician Penobscot Arc is characterized by the continuous migration of the magmatic front and development of multiple volcanically active rift zones. The rift basins display a variety of characteristics, including bimodal calc-alkaline magmatism, felsic-dominated incipient rift magmatism, and formation of rifts floored by tholeiitic to boninitic suprasubduction-zone ophiolite. Comparison to modern analogues suggests that part of the Penobscot Arc area developed in a similar setting to the volcanically active Havre Trough and Taupo volcanic zone. In contrast, the Victoria Arc phase was dominated by multiple epiclastic rich, volcano-sedimentary basins overlying tectonically modified Penobscot basement. Igneous rocks are sparse, typified by calc-alkaline felsic volcanic and tholeiitic to alkaline backarc basin basalts. The change in character of the backarc volcanic rocks over time is interpreted to reflect multiple tectonic factors, including the variation of slab retreat rate, degree of extension in the arc (Cambrian Penobscot Arc) versus the backarc basin (Ordovician Exploits-Tetagouche backarc), reactivation of inverted Penobscot extensional faults during Middle Ordovician rifting, and/or depletion of fertile components by the Middle Ordovician.

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.