A. Zagorevski

Upper cambrian to upper ordovician peri-gondwanan island ARC activity in the victoria lake supergroup, central newfoundland : Tectonic development of the Northern Ganderian Margin

The Exploits Subzone of the Newfoundland Appalachians comprises remnants of Cambro-Ordovician peri-Gondwanan arc and back-arc complexes that formed within the Iapetus Ocean. The Exploits Subzone experienced at least two accretionary events as a result of the rapid closure of the main portion of the Iapetus tract: the Penobscot orogeny (c. 480 Ma), which juxtaposed the Penobscot Arc (c. 513 –486 Ma) with the Gander margin, and c. 450 Ma collision of the Victoria Arc (c. 473 –454 Ma) with the Annieopsquotch Accretionary Tract that juxtaposed the peri-Laurentian and peri-Gondwanan elements along the Red Indian Line. The newly recognized Pats Pond Group forms a temporal equivalent to other Lower Ordovician intra-oceanic complexes of the Penobscot Arc. The Pats Pond Group (c. 487 Ma) has a geochemical stratigraphy that is consistent with rifting of a volcanic arc. An ensialic setting is indicated by low εNd values (εNd 0.3 to -0.5) near the stratigraphic base and its abundant zircon inheritance (c. 560 Ma and 0.9 –1.2 Ga). The spatial distribution of Tremadocian arc –back-arc complexes indicates that the Penobscot arc is best explained in terms of a single east-dipping subduction zone. This model is favored over west dipping models, in that it explains the distribution of the Penobscot arc elements, continental arc magmatism, and the obduction of back-arc Penobscot ophiolites without requiring subduction of the Gander margin or subduction reversal. The newly recognized Wigwam Brook Group (c. 454 Ma) disconformably overlies the Pats Pond Group and records the youngest known phase of ensialic arc volcanism (εNd –4.1) in the Victoria Arc, which is also related to east-dipping subduction. Thus the Penobscot and the overlying Victoria Arc are reinterpreted in terms of a single, relatively long-lived east-dipping subduction zone beneath the peri-Gondwanan microcontinent of Ganderia. The cessation of arc volcanism towards the top of the Wigwam Brook Group and the subsequent syn-tectonic sedimentation in the Badger Group constrain the arrival of the leading edge of Ganderia with the ensialic arc complexes to the Laurentian margin to c. 454 Ma.

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.

Middle Ordovician disorganized arc rifting in the peri-Laurentian Newfoundland Appalachians: implications for evolution of intra-oceanic arc systems

The Annieopsquotch Accretionary Tract comprises a tectonic collage of Ordovician peri-Laurentian terranes that formed during closure of Iapetus Ocean and imbricated beneath the composite margin of Laurentia. New zircon U–Pb geochronological data (n = 8) for felsic volcanic rocks representing three distinct tectonostratigraphic units (the Mary March Brook group, the Buchans Group and the Red Indian Lake Group) indicate deposition over a short interval between 465 and 462 Ma. These tectonostratigraphic units are characterized by submarine mafic and felsic magmatism, and sedimentation typical of intra-oceanic arc systems. New and compiled whole-rock geochemical (n = 315) and Sm–Nd isotopic (n = 33) data for mafic and felsic volcanic rocks indicate that these tectonostratigraphic units formed in tholeiitic island arc to calc-alkaline arc settings. Isotopic (εNd −11.2 to +6; TDM 1.0–2.4 Ga) and zircon inheritance data (0.5–3.4 Ga) are consistent with presence of Laurentian continental crust in the basement. Relationships within and between these and adjacent tectonostratigraphic units in the Annieopsquotch Accretionary Tract suggest that they represent imbricated slices of a submarine arc that experienced disorganized arc rifting, similar to sectors of the modern Tonga–Kermadec and Izu–Bonin–Mariana arcs. The recognition of disorganized spreading in the Annieopsquotch Accretionary Tract has significant implications for along-strike correlations within the Appalachian orogen, as the nature of the arc systems is expected to differ significantly along-strike. The presence of a clear isotopic and inheritance signature of peri-Laurentian continental crust within the Annieopsquotch Accretionary Tract submarine arc system is consistent with the discoveries of older arc and continental basement in modern intra-oceanic arc systems. Supplementary materials: Geochronological analytical procedures and data, summary of geochemical data and isotopic data are available at http://www.geolsoc.org.uk/SUP18860.

Discussion on ‘The sole of an ophiolite: the Ordovician Bay of Islands Complex, Newfoundland’ Journal, 170, 2013, pp. 715–722

Dewey & Casey (2013) present a model for generation of the Bay of Islands Complex and its metamorphic sole that is anchored on intra-oceanic subduction initiation at a ridge–transform boundary. Several aspects of this model are inconsistent with a wealth of geological relationships and cannot readily accommodate the geological complexity of associated units in Newfoundland. Their model presents an alternative to the models of Waldron & van Staal (2001) and van Staal et al . (2007, 2013), which propose formation of the Baie Verte oceanic tract, including the Bay of Island Complex, after a Late Cambrian collision between an oceanic arc (Lushs Bight oceanic tract) and the Dashwoods microcontinent. In these models, the Baie Verte oceanic tract formed following subduction initiation in the seaway trapped between the Laurentian margin and the Dashwoods microcontinent. This subduction generated the continental Notre Dame Arc. Herein, we address some aspects of the Dewey & Casey (2013) model and discuss its tectonic implications. The model of intra-oceanic subduction initiation conflicts with the observation that the c . 489–485 Ma Baie Verte oceanic tract formed coevally with formation of the first phase ( c . 490–476 Ma) of the continental Notre Dame Arc (Dube et al . 1996; van Staal et al . 2007; Table 1). In addition, the Baie Verte oceanic tract formation was preceded by a slightly older arc ( c . 510–500 Ma Lushs Bight oceanic tract) that collided with the Dashwoods microcontinent (≥493 Ma; see below). The relationships to the older arc and its accretion to Dashwoods are crucial to understanding the formation of the Baie Verte oceanic tract, because the Bay of Islands Complex, at least in part, spread within rocks generally assigned to the Lushs Bight oceanic tract (e.g. Kurth et al . 1998), indicating that ophiolite formation …