C.R. van Staal

The margins of Avalonia

During Cambrian and earliest Ordovician times, Avalonia was an area forming an integral part of the huge Gondwanan continent, probably along the northern margin of Amazonia, until in early Ordovician (late Arenig or Llanvirn) time it split off from Gondwana, leaving a widening Rheic Ocean to its south. Today, its southern margin with Gondwana extends northeast from east of Cape Cod, Massachusetts, through Nova Scotia north of the Meguma terrane, and thence below sea level to the south of Newfoundland. On the eastern side of the present Atlantic, the southern margin may separate southwest Portugal from the rest of the Iberian Peninsula; it can be traced eastwards with more certainty from the south Cornwall nappes to a line separating the Northern Phyllite Belt (on the southern margin of the Rhenohercynian terrane) and the Mid-German Crystalline High. There is no certain evidence of Avalonian crust to the northeast of the Elbe Line. The northern margin of Avalonia extends westwards from south of Denmark to the British Isles, where it merges with the Iapetus Ocean suture between Scotland and England. Traced westwards, it crosses Ireland and reappears in northern Newfoundland to the east of New World Island, where it may follow the trace of the Dog Bay Line and the Cape Ray Fault. Recent work suggests that the northern margin of Avalonia may clip the northern tip of Cape Breton Island in Nova Scotia, and then enter the North American mainland at the Bay of Chaleur; it may then be traced from north and west of the Popelogan and Bronson Hill arcs to Long Island Sound near Newhaven, Connecticut. The Cambrian to Devonian faunas reflect the history of Avalonia: initially they were purely Gondwanan but, as Ordovician time proceeded, more genera crossed firstly the Tornquist Ocean as it narrowed between Avalonia and Baltica to close in latest Ordovician and early Silurian times, and secondly the Iapetus Ocean, so that by the early Silurian most of the benthic shelly faunas, apart from the ostracods, were the same round the adjacent margins of all three palaeocontinents.

Lower to Middle Ordovician evolution of peri-Laurentian arc and backarc complexes in Iapetus: Constraints from the Annieopsquotch accretionary tract, central Newfoundland

The Annieopsquotch accretionary tract in Newfoundland is composed of a series of west-dipping structural panels, each containing remnants of ophiolitic and arc-backarc complexes of Laurentian affinity formed during the Ordovician closure of Iapetus. Panels were transferred from an upper-plate to a lower-plate setting during their Middle to Late Ordovician accretion to the Laurentian margin and become progressively younger eastward. Geochronological data indicate a complex and rapid history of generation and accretion of peri-Laurentian suprasubduction zone rocks. The rapid changes in tectonic environments and the complexity of the relationships are analogous to the complex arc-backarc relationships observed in the western Pacific today. The recognition of the peri-Laurentian provenance of these units based on stratigraphy, geochronology, isotopes, and geochemistry defines the position of the Red Indian Line, the fundamental suture zone in the northern Appalachians, but more importantly enables the development of a realistic tectonic model for the Annieopsquotch accretionary tract involving both thrust and sinistral transcurrent displacements. The oldest and most inboard unit in the Annieopsquotch accretionary tract is the Annieopsquotch ophiolite belt (ca. 480 Ma), which marks the initiation of subduction outboard of the Laurentian margin. The Lloyds River ophiolite complex (ca. 473 Ma) preserves a fragment of younger, more mid-ocean-ridge–like backarc-oceanic crust than the adjacent, structurally overlying Annieopsquotch ophiolite belt. The Lloyds River ophiolite complex originated as a backarc to the Buchans Group (ca. 473 Ma) ensialic bimodal calc-alkaline arc. The panels containing the Annieopsquotch ophiolite belt and Lloyds River ophiolite complex were stitched and overlain by ensialic arc rocks of the Otter Pond Complex (ca. 468 Ma) immediately after their accretion to composite Laurentia together with the structurally underlying Buchans Group. The youngest, structurally lowest two panels comprise the elements of the Red Indian Lake group (465–460 Ma), which record the opening of a backarc basin and the subsequent establishment of a bimodal ensialic calc-alkaline arc sequence. The observed relationships indicate that the Annieopsquotch accretionary tract was generated above a single west-dipping subduction zone outboard of the Laurentian margin over ∼20 m.y. Accretion mainly took place in two stages at ca. 468 and 450 Ma, which correspond with the collision between Laurentia and the Dashwoods ribbon continent and the collision with the peri-Gondwanan Victoria Arc along the Red Indian Line, respectively. Both collisions form part of the Taconic orogeny. The latter, Late Ordovician collision terminated the relatively rapid closure of the main Iapetan tract. The proposed model is similar to the correlative tracts in the British and Irish Caledonides, and may encourage a new look at the New England Appalachians.

Pressure-temperature paths and exhumation of Late Ordovician–Early Silurian blueschists and associated metamorphic nappes of the Salinic Brunswick subduction complex, northern Appalachians

The Late Ordovician–Early Silurian Brunswick subduction complex in northern New Brunswick preserves a nearly intact southeast-facing forearc terrane with clear links among Silurian subduction, under-plating, exhumation, and forearc sedimentation. The forearc terrane grew over time due to successive accretion and underplating of seamounts and isolated ribbons made up of continental and transitional crust concomitant with foreland migration of the trench. A sliver of strongly deformed epidote-blueschists derived from an accreted seamount was stacked together with slightly lower-pressure winchite-bearing tectonites and medium- to high-pressure greenschists of the Fournier, California Lake, and Tetagouche blocks into a series of D 1 metamorphic nappes during Salinic closure of the Tetagouche-Exploits backarc basin. The blueschists (~375 °C, 7.2 kbar) locally preserve mineralogical (zoning from actinolite or barroisite core to a rim of glaucophane) and microstructural evidence for a counterclockwise pressure-temperature ( P-T ) path ascribed to underthrusting beneath the young oceanic lithosphere of the Fournier block shortly after the inception of subduction at ca. 450 Ma. The next phase of accretion is represented by underplating of the California Lake block at ca. 442 Ma. This may have caused emplacement of the blueschists above the slightly lower metamorphic grade Canoe Landing Lake winchite-bearing nappe (350 °C, 6.5 kbar). Underplating of the Tetagouche block at ca. 435 Ma extruded the blueschist sliver and underlying nappes of the California Lake block immediately above greenschist-facies nappes (~5.8 kbar) and beneath lower-pressure (4–5 kbar) greenschist-facies basalt and gabbro of the ophiolitic Fournier block, which was deformed and imbricated earlier, probably during accretion of the California Lake block. Shear sense indicators confirm that the hanging-wall shear zone of the blue-schist sliver accommodated Early Silurian normal motion. D 1 is generally time-trans-gressive due to down-stepping of the under-plating-related deformation. Metamorphic relicts, as well as zoning in amphibole and epidote, indicate that the Canoe Landing Lake and other nappes underwent clockwise P-T paths related to subduction of the cool oceanic lithosphere and/or accretion of the California Lake and Tetagouche blocks.

The Record of Ordovician Arc–Arc and Arc–Continent Collisions in the Canadian Appalachians During the Closure of Iapetus

The Northern Appalachian orogen records complex Late Cambrian to Late Silurian closure of the Iapetus Ocean, which led to significant outboard growth of the Laurentian margin over time. The outboard growth during the Ordovician was primarily achieved by progressive accretion of peri-Laurentian and peri-Gondwanan arc, rifted arc and supra-subduction zone ophiolite terranes. The most important Ordovician accretionary episodes involved three successive arc–continent collisions: the Early-Middle Ordovician Laurentian Humber margin – Notre Dame Arc, Middle Ordovician composite Laurentian margin – Annieopsquotch accretionary tract, and Middle Ordovician composite Laurentian margin – peri-Gondwanan Victoria Arc collisions. The three collisions exhibited different tectonic configurations that were akin to Taiwan, Izu, and Molucca Sea collision zones respectively. These configurations exhibit different tectonic setting of the arc with respect to the collision zone and result in significant differences in style of deformation, metamorphism, preservation of arc and ophiolite terranes, and syn- to post-collisional stratigraphic record.

Does the Birchy Complex of Newfoundland extend into Ireland

Metabasalts from the Birchy Complex in Newfoundland are chemically similar to the Kill-Callow suite of metabasalts in Co. Mayo, Ireland. The latter, which are also chemically akin to Cambrian amphibolites of the Highland Border Series of Scotland, lie along the shear zone associated with the Fair Head-Clew Bay line, which has been equated with the Baie Verte-Brompton line in Newfoundland. The Birchy Complex occupies an analogous tectonic position along the Baie Verte-Brompton line and hence we postulate that chemical similarities suggest a correlation between the Birchy Complex of Newfoundland and the Kill-Callow suite of Ireland.

Tectonic setting and regional significance of the ‘Port aux Basques Gneiss’, SW Newfoundland

The ‘Port aux Basques Gneiss’ of southwest Newfoundland comprises three paragneiss and schist assemblages: the Grand Bay and Port aux Basques complexes, and the Harbour le Cou Group. They are intruded by large volumes of basic sheets and coeval Middle Ordovician granitoids: the Margaree and Kelby Cove orthogneiss, which also cut a tectonic slice of earlier ultrabasic to basic rocks within the Grand Bay Complex: the Big Barachois assemblage. The Grand Bay and Port aux Basques complexes are juxtaposed by the Late Silurian Isle aux Morts Fault Zone against the Harbour le Cou Group to the east, which dominantly comprises sillimanite and garnet-bearing, pyritiferous metasediments with amphibolitic intrusions and meta-pillow basalts in its lower stratigraphic units. Geochemical studies have shown that metabasites of the Grand Bay Complex, Port aux Basques Complex, Margaree and Kelby Cove orthogneiss comprise fractionated and crustally contaminated MORB-like and within plate tholeiites, suggestive of an ensialic Okinawa type back-arc basin. These have comparable characteristics to metabasites in the Harbour le Cou Group and suggest that these units comprise remnants of an extensive Mid-Ordovician marginal basin developed within a promontory on the peri-Gondwanan continental margin. In contrast, the Big Barachois assemblage in part comprises a distinct suite of pre-Mid-Ordovician Island Arc Tholeiites

Evolution of the Early to Middle Ordovician Popelogan arc in New Brunswick, Canada, and adjacent Maine, USA: Record of arc-trench migration and multiple phases of rifting

A detailed tectonic analysis of the magmatic evolution of the Early to Middle Ordovician west-facing Popelogan arc in New Brunswick and adjacent Maine is presented based on combining new U-Pb zircon radiometric age dates (thermal ionization mass spectrometry and sensitive high-resolution ion microprobe) with existing age constraints on the various magmatic events recognized in this arc system. The Popelogan arc had a life span of nearly 20 m.y. (ca. 476–457 Ma) and becomes progressively younger toward the northwest. Existing lithogeochemical and isotope data combined with field relationships and new zircon inheritance data indicate that the Popelogan arc was built on the leading edge of Ganderia and represents a predominantly continental calc-alkaline arc. Incursions of arc to within-plate–like tholeiitic magmatism correspond to rifting events temporally and spatially linked to trenchward migration of the arc magmatic axis, isolation of arc ribbons, and formation of mafic oceanic and transitional crust in the associated Tetagouche backarc basin. Arc-trench migration and backarc basin opening were caused by a retreating subduction zone. Slab retreat prior to 467 Ma was at least partially accommodated by arc migration, but thereafter, it was mainly accommodated by extension and spreading in the associated Tetagouche backarc basin. New U-Pb zircon radiometric ages and lithogeochemistry of calc-alkaline felsic tuff beds interlayered with the oceanic backarc rocks of the Fournier Supergroup provide a direct link between the Tetagouche backarc rocks and coeval calc-alkaline arc activity in the Popelogan arc. The oldest incursion of arc tholeiite (470–467 Ma) corresponds to cessation of the ca. 476–470 Ma calc-alkaline Meductic phase of the Popelogan arc, the onset of the calc-alkaline Balmoral phase (467–457 Ma) farther to the west, and formation of oceanic and mafic-dominated transitional crust in the Canoe Landing Lake Formation of the Tetagouche backarc basin. A less-well-constrained second incursion of tholeiitic magmatism between 465 and 459 Ma corresponds to rifting of the Tetagouche arc ribbon and formation of the ophiolitic Devereaux complex in the Fournier Supergroup. Cessation of calc-alkaline arc magmatism and eruption of transitional and alkalic mafic volcanic rocks between 459 and 455 Ma may represent slab breakoff following accretion of the Popelogan arc to composite Laurentia, and/or ridge subduction immediately prior to arc-continent collision.