redefinition of the wild bight group, newfoundland: implications for models of island-arc evolution...

Redefinition of the Wild Bight Group,Newfoundland: implications for models ofisland-arc evolution in the Exploits Subzone

Kate MacLachlan, Brian H. O’Brien and Greg R. Dunning

Abstract: The Wild Bight Group and correlative plutonic rocks of the South Lake Igneous Complex comprise one ofthe accreted, Ordovician, peri-Gondwanan, oceanic terranes of the Newfoundland Appalachians. Recent field work andisotopic ages from the eastern Wild Bight Group require that the stratigraphic sequence be redefined. A package of bi-modal volcanic rocks, which forms the oldest part of the group and contains all of its volcanogenic massive sulphidedeposits, is redefined as the Glovers Harbour Formation. This formation is correlative with intra-oceanic ophiolitic se-quences elsewhere in the Exploits Subzone. Previous stratigraphic nomenclature for the upper Wild Bight Group islargely retained, although the lithological variation within and spatial distribution of the Omega Point, Seal Bay Brook,and Pennys Brook formations are revised, and the Side Harbour Formation is included as part of the Seal Bay BrookFormation. The upper Wild Bight Group is interpreted to represent a second and distinct arc sequence that formed onthe Gondwanan continental margin. There is a ca. 10 million-year hiatus in volcanic activity between the Glovers Har-bour Formation and upper Wild Bight Group, although marine sedimentation was likely continuous during this time.This hiatus corresponds with Penobscot deformation and obduction of Exploits Subzone ophiolites onto the GanderZone farther to the east and south. The Glovers Harbour Formation is correlated with the Tea Arm and Saunders Coveformations of the Exploits Group, whereas the upper Wild Bight Group can be correlated in some detail with the NewBay and Lawrence Head formations. The upper Wild Bight Group and correlative rocks of the Exploits Group are in-terpreted to represent the arc and back arc, respectively, of the same Middle Ordovician arc system.

Résumé: Le Groupe de Wild Bight et les roches plutoniques corrélatives du Complexe ignée de South Lake compren-nent l’un des terranes océaniques accrétés des Appalaches de Terre-Neuve (Ordovicien, péri-Gondwana). De récents tra-vaux de terrain et des âges isotopiques du Groupe est de Wild Bight nécessitent une redéfinition de la séquencestratigraphique. Un ensemble de roches volcaniques bimodales formant la partie la plus ancienne du Groupe et conte-nant tous ses dépôts de sulfures volcanogènes massifs est redéfini en tant que Formation de Glovers Habour. Cette for-mation est reliée à des séquences ophiolitiques intra-océaniques situées ailleurs dans la sous-zone de Exploits.L’ancienne nomenclature stratigraphique pour le Groupe de Wild Bight supérieur est gardée en grande partie, bien quenous ayons effectué une révision de la variation lithologique et de la distribution spatiale des formations d’OmegaPoint, de Seal Bay Brook et de Pennys Brook; de plus, la Formation de Side Harbour forme une partie de la Forma-tion de Seal Bay Brook. On interprète le Groupe de Wild Bight supérieur comme la représentation d’une deuxième sé-quence d’arc distincte qui s’est formée en bordure de la marge continentale du Gondwana. L’activité volcanique a cessédurant 10 Ma entre la Formation de Glovers Harbour et le Groupe de Wild Bight supérieur, bien que la sédimentationmarine s’est probablement poursuivie durant ce temps. Cet hiatus correspond à la déformation Penobscot et àl’obduction des ophiolites de la sous-zone de Exploits sur la zone de Gander plus à l’est et au sud. La Formation deGlovers Harbour est reliée aux formations de Tea Arm et de Saunders Cove du Groupe de Exploits, alors que leGroupe de Wild Bight supérieur peut être relié avec assez de détails aux formations de New Bay et de Lawrence Head.Le Groupe de Wild Bight supérieur et les roches reliées du Groupe de Exploits sont interprétées respectivement commedes représentations de l’arc et de l’arrière arc d’un même système d’arc datant de l’Ordovicien moyen.

[Traduit par la Rédaction]

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Can. J. Earth Sci.38: 889–907 (2001) © 2001 NRC Canada

889

DOI: 10.1139/cjes-38-6-889

Received April 19, 2000. Accepted December 7, 2000. Published on the NRC Research Press Web site at http://cjes.nrc.ca onJune 11, 2001.

Paper handled by Associate Editor L. Corriveau.

K. MacLachlan1, 2 and G.R. Dunning. Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, NF A1B3X5, Canada.B.H. O’Brien. Geological Survey, Newfoundland Department of Mines and Energy, PO Box 8700, St. John’s, NF A1B 4J6,Canada.

1Corresponding author (e-mail: [email protected]).2Current address: Geological Survey of Canada, 601 Booth St, Ottawa, ON K1A 0E8, Canada.

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MacLachlan et al.Introduction

Ordovician oceanic rocks of the central NewfoundlandDunnage Zone (Williams 1978) have played a critical role inunderstanding the Iapetus Ocean and the tectonic evolutionof the northern Appalachians. Williams et al. (1988) dividedthe Dunnage zone into the northwestern Notre Dame andsoutheastern Exploits subzones (Fig. 1); with Laurentian andGondwanan early Paleozoic faunal affinities, respectively(Newman 1988). The Early to Middle Ordovician WildBight Group (WBG) and genetically related plutonic rocksof the South Lake Igneous Complex (SLIC) occur in the Ex-ploits Subzone (Fig. 1). The WBG is one of the geochemicallybest studied volcanic groups of the Appalachian Orogen andhas been used as a template for interpreting other, lesswell-documented successions.

A regional geochemical study of volcanic rocks of theWBG, by Swinden et al. (1990), led to the recognition of a

geochemically distinct package of bimodal tholeiitic rocks.Based on the stratigraphic sequence defined by Dean (1978),these rocks were believed to occur in the middle of theWBG. The resulting chemostratigraphy of the WBG lead tothe interpretation that the bimodal tholeiitic rocksrepresented the initiation of arc rifting. In this scenario,calc-alkaline rocks lower in the stratigraphy represented thearc, and rocks with intraplate geochemical signatures at thetop of the WBG represented the main rift sequence. Basedon this model of the WBG, bimodal volcanic sequences andophiolitic rocks with similar geochemical signatures, else-where in the Exploits Subzone, were subsequently inter-preted to represent a period of arc rifting and extensiveback-arc basin volcanism across the Exploits Subzone.

Remapping and U–Pb geochronology in the eastern WBG(MacLachlan and Dunning 1998a, 1998b) have shown thatthe bimodal volcanic rocks are in fact the oldest part andthat the WBG is a fault-imbricated package. Extrapolation of

Fig. 1. Tectonostratigraphic zones of the Newfoundland Appalachians. Exploits Subzone ophiolites are shown in black (GRC, GanderRiver Complex). AZ, Avalon Zone; HZ, Humber Zone. The Exploits Subzone and Gander Zone are subdivided: SLIC, South Lake Ig-neous Complex, ExG, Exploits Group; VLG, Victoria Lake Group; BNG, Bay du Nord Group; BEG, Bay d’Espoir Group; DvG,Davidsville Group; IB, Indian Bay Subzone; MP, Meelpaeg Subzone; MC, Mount Cormack Subzone; GL, Gander Lake Subzone.Dashed lines are major faults. Simplified from Colman-Sadd et al. 1992. The Humber Zone represents the Paleozoic Laurentian marginof Iapetus; the Dunnage Zone is a composite of Iapetus oceanic terranes; and the Gander and Avalon zones are Peri-Gondwanan sus-pect terranes (Williams 1979).



these relationships to the rest of the WBG requires redefini-tion of the stratigraphic nomenclature. The redefinition pro-posed here, has implications for volcanogenic massivesulphide (VMS) exploration in, and tectonomagmatic evolu-tion of, the WBG and correlative rocks of the ExploitsGroup. The proposed revised stratigraphy of the WBG facil-itates correlation with the Exploits Group and, together witha study of detrital components in the volcaniclastic rocks,elucidates paleogeographic relationships between the twogroups. Furthermore, this redefinition permits the recogni-tion of a sequence of Ordovician tectonomagmatic events,which we suggest is common to the entire Exploits Subzone.

Regional geology and correlations

The Wild Bight Group underlies a significant area of thenorthern Exploits Subzone in Notre Dame Bay and has pre-viously been correlated with the Exploits Group (e.g., Horneand Helwig 1969; O’Brien et al. 1997). Both groups com-prise predominantly volcanic and volcaniclastic rocks withsubsidiary amounts of argillite and chert, and they rangefrom Early to Middle Ordovician in age. Both groups gradeconformably up into grey chert and then into Caradocianblack shale of the Shoal Arm Formation, which forms aregional stratigraphic marker in the northern ExploitsSubzone. Both the Wild Bight and Exploits groups comprisea conformable marine sequence on a regional scale, throughthe Early to Middle Ordovician. However, in the southernand eastern parts of the Exploits Subzone, Middle Ordovi-cian rocks form a regionally unconformable overlapsequence above structurally juxtaposed Early Ordovicianoceanic rocks of the Exploits Subzone and psammites of thecontinental Gander Zone. On its western margin the WBG isseparated from oceanic rocks of the Notre Dame Subzone bythe Red Indian Line (Williams et al. 1988), a regional Silurianstructure.

The Victoria Lake Group occurs adjacent to the Red In-dian Line in the southern Exploits Subzone. It comprisespredominantly submarine rocks, including a variety of geo-chemical types of mafic and felsic volcanic rocks similar tothe Wild Bight Group (Swinden et al. 1989). However, thearea is structurally complex (Evans et al. 1990) and relation-ships between many of the units are not well understood.The complexity is compounded by a wide range in ages ofdifferent volcanic sequences (see Fig. 7), including the TallyPond volcanics (513 ± 2 Ma; Dunning et al. 1991), whichare the oldest dated volcanic rocks in the Exploits Subzone.Because of strong temporal and geochemical similarities be-tween the two, the Wild Bight Group has previously servedas a direct analogue for the tectonomagmatic history of theVictoria Lake Group (e.g., Swinden and Thorpe 1984). Thetectonic evolution of several other, more spatially limited se-quences of mafic volcanic and marine greywacke-dominatedsequences in the northern Exploits Subzone (e.g., NewWorld Island, Elliot et al. 1989; Hamilton Sound Group,Johnston et al. 1994) can also be interpreted in terms of a di-rect analogue with the WBG (Fig. 7).

Sparse geochronological constraints and abundant geo-chemical similarities with the WBG have led to directcorrelation of volcanic and ophiolitic rocks of the PipestonePond, Great Bend, and Coy Pond complexes in the central

Exploits Subzone with various parts of the WBG (e.g., Jennerand Swinden 1992). However, these ophiolitic rocks sitstructurally above a window of Gander Zone rocks (MountCormack Subzone) and are disconformably overlain by lateLlanvirn to Llandeilo (Middle Ordovician) limestone con-glomerates and calcarenites (Boyce 1987), which contrastwith the conformable Ordovician sequence in the WBG. Theonly other ophiolitic rocks in the Exploits Subzone are thoseof the Gander River Complex (GRC) which mark the bound-ary between the Exploits Subzone and the Gander Zone(Fig. 1). The GRC comprises a narrow, discontinuous, linearbelt of deformed, predominantly ultramafic bodies (O’Neilland Blackwood 1989) that have been interpreted to have astructural relationship with psammitic sedimentary rocks ofthe Gander Zone (Williams et al. 1991). Middle Ordovicianrocks of the Davidsville Group unconformably overlie theGRC (O’Neill and Blackwood 1989). The Indian Bay BigPond Formation (Indian Bay Subzone of the Dunnage Zone,Fig. 1) has been correlated with the lowermost DavidsvilleGroup (O’Neill and Blackwood 1989), although the formeris conformable with underlying Gander Zone rocks (Williams1995). Together the Davidsville Group and the Indian BayBig Pond Formation form an overlap sequence that links theeastern Exploits and Gander zones in the Middle Ordovician(Williams 1995). Although there are no age constraints onthese ophiolitic rocks they occupy a similar structural andstratigraphic position to those in the central Exploits Subzone.

In the southern Exploits Subzone, both the Baie D’Espoirand Bay du Nord groups comprise predominantly pelitic andpsammitic rocks intercalated with lesser amounts of felsicvolcanic rocks, including welded tuffs (e.g., Blackwood1985; Cooper 1954), likely of subaerial origin. Felsic volca-nic units in the Bay du Nord and Baie D’Espoir groups weredated at 466 ± 2 Ma (Dunning et al. 1990) and 468 ± 2 Ma(O’Brien et al. 1986), respectively. These ages are correla-tive with the younger parts of the Wild Bight, Exploits andVictoria Lake groups (Fig. 7), although they arelithologically quite different, in that they lack abundant sub-marine mafic volcanic rocks. There are no stratigraphic rela-tionships preserved between either of these groups andGander Zone or ophiolitic rocks. It is not obvious how thetectonic setting of these rocks relates to the moremafic-dominated sequences in the northern ExploitsSubzone.

Redefining the Wild Bight Group

Previous workThe first stratigraphic subdivision of what would become

the upper Wild Bight Group was made by Espenshade(1937) in the area west of Badger Bay (Fig. 2). His workdemonstrated lithological similarity and a tectonstratigraphiccorrelation between rocks of this area and those mapped byHeyl (1936) around New Bay in the Bay of Exploits (nowthe Exploits Group). Based on his regional synthesis,Williams (1962, 1972) elevated Middle Ordovician and olderrocks of the Wild Bight Formation (Wild Bight Volcanics ofEspenshade 1937 and Beaver Bight member of Hayes 1951)to group status (Wild Bight Group). Dean (1977) compiledthe regional geology of central Notre Dame Bay at a scale of1 : 50 000 for the first time and formally divided the Wild

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Bight Group into five conformable formations (Fig. 3). Dean(1977) also revised the boundaries of the Shoal Arm Forma-tion and excluded Caradocian black shale of this formationfrom the underlying WBG. Upper Ordovician marinegreywacke and conglomerate successions above the ShoalArm Formation (and equivalent units) were originally givenvarious local names, including Point Leamington Greywacke(Helwig 1969) and Gull Island Formation (Espenshade1937), but were subsequently informally included in the Up-per Ordovician to Lower Silurian Badger group (Williams etal. 1995).

Swinden et al. (1990) did a regional geochemical study ofvolcanic rocks of the Wild Bight Group and showed that vol-canic rocks with both arc and non-arc signatures occur inclose spatial proximity in the eastern WBG. However, therewas a notable lack of correlation between geochemical types

and stratigraphic position, implying contemporaneity of allvolcanic rock types throughout the evolution of the WBG(Fig. 3).

Based on geochemical similarity, Swinden et al. (1990)corre-lated several different occurrences of bimodal volcanic rockswithin the WBG. However, based on the stratigraphic se-quence defined by Dean (1978), these units occur at quitedifferent stratigraphic levels (Fig. 3). Furthermore, a U/Pbisotopic age (>486 ± 4 Ma; MacLachlan and Dunning 1998a)for one of these bimodal volcanic packages in the easternWild Bight Group is inconsistent with its apparent strati-graphic position above felsic tuffs dated at 472 ± 3 Ma(MacLachlan and Dunning 1998b). This suggests that thesequence is structurally disrupted and that there is a need torevise the stratigraphic nomenclature.

In previous studies, geographically separate volcanic units

Fig. 2. (a) Regional geology of the Wild Bight Group (WBG), based on MacLachlan and O’Brien 1998 (compiled from Dean 1977;Swinden et al. 1990; MacLachlan and Dunning 1998a and b; O’Brien and MacDonald 1997; O’Brien 1992, 1998; Williams andO’Brien 1994). Geographic volcanic units: a, Glovers Harbour; b, Nanny Bag Lake; c, Long Pond; d, Indian Cove; e, New Bay Pond;f, Sparrow Cove; g, Squid Cove; h, Northern Arm; i, Kerry Lake; j, Side Harbour; k, Big Lewis Lake; l, New Bay; m, Badger Bay; n,Seal Bay Head; o, Osmonton Arm. Named faults are: LPF, Long Pond Fault; SHF, Side Harbour Fault; WHCF, Winter House CoveFault; WABT, Western Arm Brook Thrust.PB, Pennys Brook;SBB, Seal Bay Brook. The numbers designate different lithologic units(see legend Fig. 2c). The area of study in the eastern WBG is outlined with a solid line, and the area of Fig. 4 is outlined with adashed line. Type sections for the formations of the WBG are shown with double-headed arrows. (b) Schematic diagrams showing aninterpretation of structural relationships in the third dimension (see Fig. 2a for locations). (c) Legend for Figs 2a, 2b.



have been named (e.g., Glovers Harbour unit and IndianCove unit; Swinden et al. 1990), and for ease of discussion,these names have been retained (see Fig. 2), although theyare not part of the formal stratigraphic nomenclature.

Proposed subdivision of the Wild Bight GroupThe stratigraphic subdivisions defined by Dean (1978;

Fig. 3) are based primarily on lithological variation withinthe clastic rocks, which constitute greater than 60% of thegroup, although he did give formation status to several later-ally continuous volcanic units. The Omega Point Formation,which defined the base of Dean’s stratigraphic succession,remains largely unchanged. However, the newly proposedGlovers Harbour Formation forms the base of the WBG inthe new scheme (Fig. 3). In Dean’s (1978) definition, boththe Seal Bay Brook and Pennys Brook formations includedat least a minor component of all lithologies recognizedwithin the WBG, making it difficult to distinguish between

the two on any given outcrop. However, the type section ofthe Seal Bay Brook Formation is considerably different fromall other packages included in this formation by Dean(1977). In fact, other than the type section of the Seal BayBrook Formation, the rocks he mapped as such are indistin-guishable from the lower part of the Pennys Brook Forma-tion. In the new scheme presented here, much of the SealBay Brook Formation of Dean (1977) is included in thePennys Brook Formation. Rocks in the type section of theSeal Bay Brook Formation are lithologically similar to felsicvolcanic rocks in the Indian Cove unit of the Glovers Har-bour Formation, and, in the absence of geochemical or geo-chronological data, we tentatively suggest a correlation. Asthese felsic rocks are interbedded with and overlain by maficvolcanic rocks (Side Harbour Formation of Dean 1978), wepropose to include these mafic volcanic rocks as part of theSeal Bay Brook Formation. As such, the Seal Bay BrookFormation would now comprise a bimodal volcanic succes-

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Fig. 3. A comparison of previous stratigraphic divisions of the Wild Bight Group with those proposed in this study. Geographic volca-nic units (lower case letters) are as in Fig. 2. cgr, coarse-grained; fgr, fine-grained; Fm, Formation. Only those geographic volcanicunits that were distinguished in each study are shown. Units that were grouped together in previous studies are linked with a “+.”



sion, a lithological association that would strengthen the ar-gument for correlation with the Glovers Harbour Formation.In the original definition (Dean 1978), volcanic rocks withinthe Seal Bay Brook and Pennys Brook formations were as-signed to a particular formation based on the nature of theclastic rocks surrounding them. However, isotopic ages haveshown that the bimodal volcanic packages are significantlyolder than the surrounding clastic successions (MacLachlanand Dunning 1998a, 1998b) and must be separated fromthem by faults. These older volcanic rocks have been rede-fined as the Glovers Harbour Formation, which we proposeconstitutes the base of the WBG. Volcanic rocks that aretruly interbedded with volcaniclastic rocks of the PennysBrook Formation occur at different stratigraphic levels andare not lithologically distinct from each other. Thus, abun-dant faults within the formation make it very difficult to re-gionally correlate these volcanic units. Therefore, we feelthat the volcanic rocks should be considered as facies ratherthan as distinct formations. The lower and upper parts of theWBG contain volcanic rocks interpreted to have formed indifferent tectonic settings, separated by ca. 10 million years(MacLachlan and Dunning 1998a, 1998b). Furthermore, evi-dence to be presented in the section on detrital componentsin the upper WBG will suggest that there is a local uncon-formity between the Glovers Harbour Formation and the restof the WBG. Thus, we propose that the WBG be officiallyseparated into lower and upper parts.

Lower Wild Bight Group

Glovers Harbour Formation (new):The informal nameGlovers Harbour volcanic unit was given to the volcanicrocks on both sides of Glovers Harbour (Fig. 4) by Swindenet al. (1990). Rocks on the west side of Glovers Harbourcomprise a bimodal sequence of volcanic rocks with incom-patible element-depleted island-arc tholeiite geochemicalcompositions that are > 486 ± 4 Ma in age(MacLachlan andDunning 1998a). Pillowed volcanic rocks on the east side ofGlovers Harbour are about 10 million years younger (472 ±3 Ma) and have incompatible element-enriched,calc-alkaline basalt compositions (MacLachlan and Dunning1998b). We propose that rocks on the peninsula on the westside of Glovers Harbour, extending south to the SLIC(Fig. 4), be the type locality of the Glovers Harbour Forma-tion and that rocks on the east side of Glovers Harbour beincluded in the Pennys Brook Formation. As defined here,the Glovers Harbour Formation comprises predominantlyaphyric, commonly vesicular and hematized, pillowed flows,pillow breccias, and white to pink, quartz–feldspar porphy-ritic rhyolite flows and domes that are interbedded with mi-nor felsic to intermediate breccias, tuffs, lapilli tuffs andagglomerates, red and green argillite and red chert. Althoughthere are no rhyolite flows or domes exposed at surface inthe type area, they have been observed in drill core (H.S.Swinden, personal communication, 1996; Altius ResourcesInc., personal communication, 1997), and are characteristicof this sequence elsewhere in the WBG. Because of foldingand fault imbrication, the stratigraphic sequence and thick-ness of the Glovers Harbour Formation is difficult to define,but the above rock types are interbedded and interfinger lat-erally. Several fault-bounded sections of the Glovers Har-

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Fig. 4. Detailed geology for the type locality of the Glovers Har-bour Formation. No stratigraphic positions are implied in legendorder. See legend of Fig. 2 for additional units. Numbers corre-spond to lithological units (Fig. 2).



bour Formation occur in the eastern WBG, but the formationhas not been recognized west of Seal Bay. The basal contactof the Glovers Harbour Formation is everywhere a fault. Theoccurrence of volcanic blocks of the Glovers Harbour For-mation in olistostromes of theoverlying Omega Point Forma-tion suggests that the upper contact is locally unconformable.Though in most cases the upper contact of the Glovers HarbourFormation is now a fault.

Seal Bay Brook Formation (revised):The type locality ofthe Seal Bay Brook Formation is along Seal Bay Brook atthe bottom of Seal Bay (Dean 1978; Fig. 2). As defined byDean (1978), the Seal Bay Brook Formation comprises mas-sive to poorly bedded agglomerate and tuff, commonly con-taining clasts of green chert, and minor felsic flows andpyroclastic rocks, bedded chert, greywacke and mafic volca-nic rocks. However, the type locality of the Seal Bay BrookFormation comprises felsic volcanic rocks that arelithologically similar to those of the Indian Cove unit (seeFig. 2) of the Glovers Harbour Formation. Furthermore,these felsic rocks are interbedded with and grade up intooverlying pillowed volcanic rocks (Side Harbour Formationof Dean 1978). Thus they comprise a bimodal mafic–felsicsuccession that is characteristic of the Glovers Harbour For-mation. In the absence of geochronological and geochemicaldata for these felsic rocks, we tentatively suggest that theSeal Bay Brook Formation, as defined here, is afault-bounded package correlative with the Glovers HarbourFormation. Mafic volcanic rocks of the Side Harbour volca-nic unit are known to have an enriched tholeiitic to alkalinegeochemical signature, which is not typical of the GloversHarbour Formation. However the gabbro dyke used to datethe type locality of the Glovers Harbour Formation also hasan enriched tholeiitic geochemical signature, indicating thatmagmatism of that kind was occurring at that time. Thus, therevised Seal Bay Brook Formation comprises mixed felsicand mafic volcanic breccias, flow-folded rhyolite, pillowlava and pillow breccia, quartz–feldspar crystal tuff, andmafic lithic tuff. Many of the vesicular basalt horizons in theSeal Bay Brook Formation are strongly hematized (e.g., SideHarbour basalt), which is another characteristic of theGlovers Harbour Formation. Cross-laminated sandstoneforms rare intervals a few decimetres thick within thepyroclastic rocks. However, sedimentary rocks are rare inthe Seal Bay Brook Formation as defined here.Coarse-grained clastic rocks originally considered part ofthis formation are actually separated from it by faults andare redesignated as part of the lower Pennys Brook Forma-tion. At its type locality, the Seal Bay Brook Formation isabout 2.5 km thick, and both its lower and upper contactsare faults.

Upper Wild Bight Group

Omega Point Formation (revised):The type locality of theOmega Point Formation occurs in the core of the Seal Bayanticline (Dean 1978), although it also outcrops in two otherareas in the eastern WBG (Fig. 2). As defined by Dean(1978), this formation comprises thinly bedded to laminatedred and green argillite and tuffaceous sandstone. In thisstudy, non-fossiliferous, dark grey to black, argillaceousshale, which locally forms the matrix to an olistostrome, as

well as block in matrix deposits with gritty and pebbly ma-trices, were also found to be characteristic of the OmegaPoint Formation. The occurrence of dark grey to blackargillite serves to distinguish the Omega Point Formationfrom other argillite-rich parts of the upper WBG. At the typelocality of the Omega Point Formation, the first volcanic ho-rizon above it was named the Sparrow Cove Formation byDean (1978). Because lithologically and geochemically sim-ilar volcanic horizons occur at various stratigraphic levelswithin the overlying Pennys Brook Formation, and becauseintervening faults preclude regional correlation of these vol-canic units, we feel that this thin (<500 m) volcanic horizondoes not warrant formation status. We propose to include itas a volcanic facies of the Pennys Brook Formation. At itstype locality, the Omega Point Formation is roughly one kmthick, but the base is not exposed. The top of the Formationis defined as the base of the first laterally continuous pil-lowed volcanic unit, which we include in the Pennys BrookFormation.

Pennys Brook Formation (revised):The type locality of thePennys Brook Formation is at the mouth of Pennys Brook inWild Bight Badger Bay (Fig. 2). As redefined here, thePennys Brook Formation underlies the entire area west ofSeal Bay and the southeastern and northeastern parts of theWBG. As defined by Dean (1978), the Pennys Brook For-mation comprises predominantly green, thinly and thicklybedded tuff with thick units of mafic pillow lava, felsiclavas, tuffs and agglomerate, and thin units of chert andargillite. Mafic–felsic bimodal volcanic packages included inthe Pennys Brook Formation by Dean (1978) have geochem-ical and lithological characteristics that suggest they are partof the Glovers Harbour Formation (MacLachlan andDunning 1998a). We have interpreted them as such and sug-gest that they are separated from the Pennys Brook Forma-tion by faults. Thus, the revised Pennys Brook Formationcomprises thinly and thickly bedded green tuffaceousgreywacke and agglomerate, and massive to graded, thicklybedded heterolithic pebbly gritstone and conglomerate.Thesethick successions are interbedded with thinner successionsof thinly bedded to laminated greywacke, chert, and chertyargillite. These clastic rocks are interbedded at many strati-graphic levels with discrete lenses of volcanic rocks com-posed of pillowed basalt, pillow breccia, mafic agglomerate,and olistostrome. Because of intervening faults and discon-tinuous stratigraphy, volcanic units within the Pennys BrookFormation cannot be easily correlated. At its type locality,the Pennys Brook Formation is about 2 km thick (Dean1978), although regionally its thickness varies considerably.The Pennys Brook Formation fines upward, and the upperpart is dominated by thinly bedded red and green chertyargillite and red chert. The Pennys Brook Formation gradesup into Caradocian black shale of the Shoal Arm Formation,which is not part of the WBG.

Relationships within the Wild Bight Group

By combining the regional geochemical study of Swindenet al. (1990) with U/Pb ages in the eastern WBG and therevised stratigraphic sequence proposed above a betterunderstanding of regional structural and paleogeographic

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relationships has emerged. The structural complexity recog-nized is particularly important for mineral exploration, as allthe VMS deposits in the WBG occur within the GloversHarbour Formation, which is almost exclusively fault-bounded.

Deformation historySeveral previously unrecognized faults that are responsi-

ble for interleaving the upper and lower WBG have been in-ferred. They include the Side Harbour Fault and severalsplays, similar to the West Arm Brook Thrust (WABT,Kusky 1985; also referred to as the New Bay Pond Thrust bySwinden and Jenner 1992); the bounding faults of the SLICand several splays; and the eastern boundary fault of theWBG, which emplaces various stratigraphic levels of theWBG and the SLIC over Caradocian and younger rocks ofthe Shoal Arm Formation and Badger group (Figs. 2a, 2b).Based on the style of fold and thrust deformation docu-mented in this area by other workers (e.g., Kusky 1985; vander Pluijm et al. 1987; and Williams and O’Brien 1994),these faults are inferred to be thrusts. However, most of theinferred faults are not well exposed and fabrics close to themare generally steep, making interpretation of the dip direc-tion difficult.

Evidence from detrital components in the upper WBGsuggests that the lower WBG was locally unroofed duringthe Middle Ordovician. Considering the abundant geochemicalevidence for an extensional setting(MacLachlan and Dunning1998b), we have speculated that this occurred by normalfaulting. Strictly speaking this would be the earliest phase ofdeformation. However, these structures cannot be identifiedbecause of subsequent deformation. That being said, thethrusts that have been inferred based on interleaving of dif-ferent stratigraphic sequences, are commonly localized alongboundaries of the Glovers Harbour Formation and SLIC.This suggests that faults formed during regional compressionmay have reactivated earlier normal faults (see followingsection on detrital components). No structures related toobduction of the older arc have been documented, althoughthere are several rafts of deformed felsic volcanic rockswithin a tonalite phase of the SLIC, indicating that there wasdeformation of the volcanic rocks prior to 486 Ma. There isalso abundant deformation within rocks of the SLIC, butthese were interpreted to be largely synvolcanic with respectto the Glovers Harbour Formation (MacLachlan andDunning 1998a; O’Brien 1992).

In terms of regional compressional deformation in theWBG, the first phase (D1) fault imbrication, and relatedfolding, created northwest-striking, fault-bounded domesand basins. Faulting during D1 is interpreted to have beenboth northeast- and southwest-directed, exposing the olderrocks in the cores of fault-bounded domes (Figs. 2a, 2b).The second (D2) and main phase of compressional deforma-tion (e.g., Karlstrom et al. 1982; van der Pluijm et al. 1987;Elliot and Williams 1988; Elliot et al. 1989; Blewett 1991;Lafrance and Williams 1992; and O’Brien 1993) creatednortheast-trending regional and mesoscopic asymmetricfolds and minor faults, and locally reoriented D1 structuresinto parallelism with D2. There is also a regional, axial pla-nar, slaty to spaced cleavage associated with D2 deforma-tion. D2 faults also show both northwest- andsoutheast-directed reverse displacement (Figs. 2a, 2b). Re-

gional D3 and D4 fabrics occur in localized domains andhave been recognized in some parts of the WBG (O’Brien1993).

Because of the steepness of most D1 structures now, it isdifficult to determine the overall vergence of D1 deforma-tion. However, regional structural studies (e.g., Lafrance andWilliams 1992; O’Brien 1993; Karlstrom et al. 1982; andBlewett 1991) indicate that D2 faults were predominantlywest-directed.

Detrital components in volcaniclastic rocks of the upperWBG

In light of the fault-bounded nature of the Glovers Har-bour Formation, there is a possibility that it was only juxta-posed with the upper WBG during Silurian deformation andthat these packages had no common Ordovician history. Thishypothesis was addressed by a study of detrital componentsin the upper WBG.

A shale olistostrome in the Omega Point Formation con-tains blocks of pillowed volcanic rocks up to several metresin diameter. At least some of the volcanic blocks haveboninitic geochemical compositions (Fig. 5a) and could bederived from the adjacent Glovers Harbour Formation.Debris flows in the lower Pennys Brook Formation containlarge (>2m) and small blocks of rhyolite, and geochemicalanalyses confirm that these blocks have compositions similarto rhyolite flows of the Glovers Harbour Formation(Fig. 5a). The slight difference in the degree of depletion ofNb and Ti, may be due to slightly different degrees of partialmelting of different source compositions (MacLachlan andDunning 1998a). One debris flow in the Pennys Brook For-mation contains rhyolite blocks that have been derived fromboth the Glovers Harbour Formation and from calc-alkalinerocks within the Pennys Brook Formation (Fig. 5b).Although boninitic rocks are common elsewhere in theDunnage Zone, there is a preserved stratigraphic relationshipin the adjacent Exploits Group, between equivalents of thelower and upper WBG (see correlation section later in thepaper). This suggests that the most likely source of boniniticpillowed volcanic rocks and trace-element-depleted felsicrocks is the immediate substrate represented by the presentlyadjacent Glovers Harbour Formation. The present spatialassociation between boninite-bearing olistostromes and anolder sequence of boninitic volcanic rocks is interpreted toreflect the predeformation architecture of the arc (Fig. 6).

A quartz diorite cobble from a heterolithic conglomeratenear the base of the Pennys Brook Formation has a lightrare-earth element (LREE)-enriched geochemical signaturesimilar to calc-alkaline volcanic rocks in the Pennys BrookFormation (Fig. 5c). This cobble was dated by U–Pb zircongeochronology: four fractions ranging from 11 to 1% discor-dant define a discordia line, with a 14% probability of fit andan upper intercept age of 473 ± 3 Ma (Fig. 5c, Table 1). Thisis the same age (472 ± 3 Ma) as felsic tuffs that occur withinthe Pennys Brook Formation (MacLachlan and Dunning 1998b).

There is also evidence of an old, continentally derivedcomponent within the Upper WBG. A single large zirconprism tip from one of the dated tuffs in the eastern WBG(MacLachlan and Dunning 1998b) is highly discordant(70%) and has a207Pb/206Pb age of 1419 Ma (Table 1).

Detrital components in the upper WBG are derived both



from within the sequence and from its substrate, which com-prises predominantly island-arc tholeiitic to boninitic mate-rial and plutonic rocks related to calc-alkaline volcanism inthe upper WBG. However, there is also some evidence foran older component, possibly continental crust and (or)sedimentary rocks derived from it (e.g., Gander Zone siliciclasticsedimentary rocks). The presence of detritusfrom the GloversHarbour Formation in the upper WBG suggests that theolder rocks were unroofed during development of the upperWBG. The occurrence of cobbles of synvolcaniccalc-alkaline plutonic rocks in the upper WBG suggests thatstructural uplift continued within the arc throughout its evo-lution. A detrital zircon study of clastic rocks in the upperWBG that were deposited adjacent to these structural upliftsshould provide evidence to support or refute the idea of asignificant continentally derived component in the substrate.

Facies changesDetrital components in the upper WBG suggest that expo-

sures of the Glovers Harbour Formation and SLIC in theeastern WBG represent intra-arc structural uplifts. This ideais supported by rapid changes in thickness of units adjacentto the Glovers Harbour Formation and SLIC. For example,northwest of the north end of the Winter House Cove Fault(WHCF; Figs. 2 and 4), the lower Pennys Brook Formation,which is exposed on all the islands south of Leading Tickles,thins to about 40 m toward the southwest. Although theselarge-scale relationships are enhanced by faults, similar rela-tionships can be observed on the scale of individual debrisflow units, which change thickness abruptly over short dis-tances. A similar relationship is interpreted to occur aroundthe Indian Cove volcanic unit (Fig. 2) in Seal Bay, althoughit is complicated by numerous small faults.

Despite significant Silurian deformation, the structuraland stratigraphic architecture of the WBG in the Ordoviciancan be pieced together. The age and geochemical composi-tion of detrital components and facies changes in the upperWBG provide evidence of intra-arc tectonism that unroofeddeeper parts of the arc. Furthermore, normal faults thatdetermined the Ordovician architecture of the arc seem to

have been reactivated as reverse faults that imbricated theWBG in the Silurian.

Correlation with the Exploits Group

Detailed correlation of the stratigraphic sequences of theWild Bight and Exploits groups provides important con-straints on the tectonomagmatic history and paleogeographyof the northern Exploits Subzone. Based on similarities in

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Fig. 5. (a) Primitive mantle (PM)-normalized, ex-tended-REE plots of two rhyolite blocks (open squares) froma debris flow in the Pennys Brook Formation and a boniniticolistolith (filled circle) from a shale olistostrome in theOmega Point Formation. Field shown is for felsic volcanicrocks from the Glovers Harbour Formation, and the linewithout symbols is a boninitic volcanic rock from theGlovers Harbour Formation. (b) A comparison oftrace-element compositions of rhyolite blocks in a debrisflow in the Pennys Brook Formation, with fields for felsic tointermediate tholeiitic and calc-alkaline volcanic rocks fromthe Glovers Harbour (GHF) and Pennys Brook (PBF) forma-tions, respectively. (c) U–Pb age and trace-element composi-tion of a quartz diorite cobble from the Pennys BrookFormation. All trace-element concentrations were analysedby inductively coupled plasma mass spectrometry at Memo-rial University of Newfoundland. Primitive mantle normaliz-ing values are from Swinden et al. 1990.



age, lithology, stratigraphic position, and geochemical com-position of volcanic rocks (O’Brien et al. 1997), the TeaArm and Saunders Cove formations of the Exploits Groupare interpreted to be correlative with the Glovers HarbourFormation. The Omega Point formation may in part be cor-relative with the Saunders Cove Formation, although theolistostromes in the Omega Point Formation suggest thatthis unit formed in a distinct setting, probably related tointra-arc uplifts that seem to be unique to the eastern WBG.The fining-upward nature of the two basal members of theNew Bay Formation (O’Brien et al. 1997), which overlie theSaunders Cove Formation (Fig. 7), suggest a deepening ofthe basin with time. Although generally finer grained, thevolcaniclastic rocks of the Brooks Harbour and CharlesBrook members of the New Bay Formation (O’Brien et al.1997) are interpreted to be correlative with, and probablylateral facies equivalents of, the Pennys Brook Formation ofthe Wild Bight Group, which is also a fining-upwardsequence. The olistostromal Saltwater Pond member at thetop of the New Bay Formation, and overlying, enriched,within-plate volcanic rocks of Lawrence Head Formation(Exploits Group) are interpreted to be time equivalent withthe upper Pennys Brook Formation, although they may rep-resent a depositional setting unique to the Exploits Group.

Paleogeography of the Middle OrdovicianWild Bight – Exploits Arc

Detailed correlation of the Exploits and Wild Bightgroups provides new constraints on paleogeographic rela-tionships within the northern Exploits Subzone. Detrital ma-terial and wedging of sedimentary facies in the upper WBGadjacent to occurrences of the Glovers Harbour Formationand SLIC suggest that these older rocks originally repre-sented intra-arc horsts of the older substrate. These horstsoccur primarily in the eastern WBG, which also containsabundant calc-alkaline volcanic rocks, related coarse-grainedvolcaniclastic units and mafic agglomerates, suggestive of anintra-arc setting. In the eastern WBG, the Pennys BrookFormation comprises predominantly calc-alkaline volcanicrocks in the lower part and within-plate tholeiitic to alkalinerocks in the upper part (MacLachlan and Dunning 1998b).This suggests that there was a change with time from an arcto a rift-dominated setting. West of Seal Bay (Fig. 2) thereare no occurrences of the Glovers Harbour Formation; thisarea comprises fine-grained sedimentary rocks of the upperPennys Brook Formation and interbedded volcanic unitswith within-plate geochemical signatures (e.g., Swinden etal. 1990; MacLachlan and Dunning 1998b). The westernWBG contains the thickest and most extensive sequence ofPennys Brook Formation, although it thins toward the eastand west, suggesting that this was a major depositional cen-tre during the evolution of the upper WBG (O’Brien 1998).Although within-plate geochemical signatures predominatein the western Pennys Brook Formation, not all of the volca-nic units have been analysed, and some may turn out to havecalc-alkaline geochemical signatures typical of the lowerPennys Brook Formation in the eastern WBG. Consideringthe more distal nature of the eastern Pennys Brook Forma-tion, the chemostratigraphy observed in the eastern WBGmay not apply. However, it would not be unusual for some

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Fig. 6. Schematic representation of the paleogeographic relationships proposed for the late Arenig to Llanvirn Wild Bight – Exploits arc system: 1, active arc; 2, rift zone; 3,intra-arc rift basin; 4, remnant arc; 5, pre-rift back-arc basin. E-thol, enriched tholeiitic; WP, within-plate; vclastic, volcaniclastic; Cgr, coarse-grained; CA, calc-alkaline; mfc,mafic; int, intermediate; tss, tuffaceous sandstone; arg, argillite; IAT, island-arc tholeiite; WBG, Wild Bight Group. Dashed line is assumed detachment surface betweenallochthonous oceanic rocks of the tholeiitic arc and autochthonous rocks of the continental margin. Units in brackets correspond to map units in Fig. 2.

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calc-alkaline magmas to be generated in a rifting-arc envi-ronment, as some subduction-affected mantle would be ex-pected to survive at least the initial stages of rifting.

The above regional variations in the distribution andthickness of different parts of the WBG are consistent withthe western WBG, representing the main rift basin, and theeastern WBG, representing the remnant arc following rifting.Although the active arc is not preserved at surface in theWBG, thinning of the Pennys Brook Formation toward thewest suggests that it may be buried (O’Brien 1998). Alterna-tively, O’Brien and MacDonald (1997) interpreted rocks ofthe Notre Dame Subzone to have been thrust over the west-ern edge of the WBG along the Red Indian Line (Fig. 2),which would imply that the active arc was tectonically buried.

The Tea Arm Formation and overlying marine red-bedsuccession of the Saunders Cove Formation at the base ofthe Exploits Group (O’Brien et al. 1997) are correlative withthe Glovers Harbour Formation and represent the immediatesubstrate, upon which the Middle Ordovician volcaniclasticsequence of the New Bay Formation was deposited (O’Brienet al. 1997). In contrast to the eastern WBG, where the con-tact is locally disconformable, the New Bay Formation sitsconformably upon the older arc substrate, suggesting thatthis area did not experience intra-arc tectonism. Detailedsedimentological studies (Helwig 1967) indicate thatvolcaniclastic turbidites and shale of the two lower membersof the New Bay Formation (Fig. 7) were derived from alandmass to the southwest. This fining-upward sequence wasinterpreted to have been deposited on a prograding, deep-seasubmarine fan and (or) oceanic abyssal plain (Helwig 1967;Dec et al. 1993). As subduction along the Gondwanan mar-gin during the Middle Ordovician is believed to have beentoward the east (present coordinates; MacLachlan andDunning 1998b and references therein), we interpret thissequence to represent a distal back-arc environment withvolcanic detritus provided to the basin as turbidity flowsdriven by arc volcanism in the eastern WBG. The more dis-tal nature of the New Bay Formation with respect to the arcis supported by the generally finer grain size of clastic unitsand lack of extrusive calc-alkaline volcanic rocks in thesequence.

The uppermost member of the New Bay Formation comprisesmuddy olistostromal rocks with abundant volcanic blocks,that grade up into a fining-upward sequence of volcaniclasticgreywacke and cherty argillite. This formation represents achange in source (O’Brien et al. 1997) and transport direc-tion (from the southeast; Helwig 1967). The New Bay For-mation is overlain by enriched tholeiitic to alkalinewithin-plate volcanic rocks of the Lawrence Head Formation(O’Brien et al. 1997). The olistostromal nature of the rocksbelow the Lawrence Head volcanic rocks and the riftlike sig-nature of the volcanic rocks themselves suggest that thissequence represents the onset of rifting in this area anduplift of a landmass toward the continental margin. Thisuplift could explain the occurrence of the Hummock IslandLimestone that locally occurs above the Lawrence HeadVolcanics in the Exploits Group. This limestone unit has nocorrelative in the WBG.

Thus, the Wild Bight and Exploits groups are interpretedto represent different parts of the same late Arenig toLlanvirn arc system. They were probably also contiguous

during Early Ordovician development of the tholeiitic oce-anic arc (MacLachlan and Dunning 1998a) represented bythe Glovers Harbour and Tea Arm formations, which consti-tute the base of each group.

Tectonic implications of Wild Bight Groupredefinition

The redefinition proposed here requires modification oftectonomagmatic models for the evolution of the WBG.Incompatible element-depleted, island-arc tholeiites andhigh-Si, low-K rhyolites of the Glovers Harbour Formationwere previously considered to postdate calc-alkaline arc vol-canism and were interpreted to be related to rifting of anisland arc (Swinden et al. 1990). However, as the GloversHarbour Formation has been shown to be the oldest part ofthe WBG, it was reinterpreted to have been formed by theinitiation and stabilization of an intra-oceanic arc outboardof the Gondwanan margin during northward subduction(MacLachlan and Dunning 1998a).

The hiatus of about 10 million years (~485–475 Ma) be-tween basaltic volcanism in the lower and upper WBG cor-responds with the period of Penobscot deformation(Newman 1967), and obduction of Exploits Subzoneophiolites onto Gander Zone rocks in central and southernNewfoundland (Colman-Sadd et al. 1992; Tucker et al.1994). Thus, the volcanic hiatus is interpreted to record thisevent within the Wild Bight Group. The submarine nature ofthe upper WBG suggests that the arc remained submergedduring obduction, and that only minor disruption of the se-quence occurred near intra-arc structural uplifts. Attemptedsubduction of the continental margin is interpreted to havecaused termination of northward subduction and cessation ofensimatic volcanism (MacLachlan and Dunning 1998b).

Renewed arc-related volcanism at ~ 473 Ma (MacLachlanand Dunning 1998b) suggests that continued convergencefollowing obduction of the tholeiitic arc resulted in, and wasaccommodated by, the initiation of a new subduction zonedipping to the south beneath the composite Gondwanan mar-gin (MacLachlan and Dunning 1998b). Enriched tholeiitic toalkaline within-plate volcanic rocks of the Pennys BrookFormation were interpreted to represent arc rifting (Swindenet al. 1990) and formation of an intra-arc marginal basin(MacLachlan and Dunning 1998b).

Variations in arc–continent interaction alongthe Gondwanan margin

In general, there are similarities in the ages and geochemi-cal affinities (where known) of many of the volcanicsequences within the Exploits Subzone (Fig. 7). Thus, themodel proposed for the evolution of the Wild Bight andExploits groups can be extrapolated to infer possible envi-ronments of formation for these other, less well-documentedsuccessions. However, in detail, there are regional differ-ences in lithologies and structural and stratigraphic relation-ships (Fig. 7), which we suggest reflect different degrees ofinteraction between the arc(s) and the Gondwanan margin.

In the central and eastern parts of the Exploits Subzone,there is abundant evidence for Early Ordovician obductionof ophiolitic rocks onto the Gondwanan margin prior to the

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deposition of late Arenig to Llanvirn sequences. Based onavailable U–Pb ages (Dunning and Krogh 1985) and geo-chemical affinities (Jenner and Swinden 1993), ophioliticrocks of the Pipestone Pond, Coy Pond, and Great Bendcomplexes are interpreted to be correlative with the GloversHarbour Formation and SLIC (Fig. 8). These ophiolites rimthe Mount Cormack Subzone of the Gander Zone (Fig. 1),which is a structural window within the Exploits Subzone(Colman-Sadd and Swinden 1984). This relationship providesevidence that in this area the ophiolites are allochthonousand sit above a continentally derived substrate. ThePartridgeberry Hills Pluton (474−

+36 Ma) intrudes both the

Gander Zone and Exploits Subzone in this area (Colman-Saddet al. 1992), indicating that these two sequences were juxta-posed by the late Arenig, during the time the younger WBGarc was active. Detrital components in late Llanvirn toLlandeilo (Boyce 1987) limestone conglomerate andcalcarenite beds, which locally overlie the Mount CormackSubzone, suggest that these rocks represent an unconform-able overlap sequence on the structurally juxtaposed Ex-ploits Subzone ophiolites and Gander Zone rocks(Colman-Sadd et al. 1992). Similar relationships have beendocumented between ophiolites of the Gander River Com-plex (O’Neill and Blackwood 1989) and overlying limestone

of the Middle Ordovician Davidsville Group at Weirs Pondalong the eastern margin of the Exploits Subzone (Stouge1979; O’Neill 1987). In contrast, on a regional scale there isa conformable stratigraphic sequence throughout the Ordovi-cian and Silurian in the northern Exploits Subzone (e.g.,Wild Bight and Exploits groups).

In the calc-alkaline arc and rift sequence of the upperWild Bight and Exploits groups, the volcanic rocks are pil-lowed mafic flows; in contrast most of the late Arenig toLlanvirn volcanic sequences of the central and southern Ex-ploits Subzone have a significant component of felsic volca-nic and pyroclastic rocks (e.g., Evans et al. 1990; Fig. 7).This suggests that there may have been significant subaerial,felsic volcanism in the central and southern areas. One pos-sible explanation for this variation is that there was a moreextensive continental substrate to the felsic-dominatedsequences, as previously suggested by Swinden and Thorpe(1984).

Regional variations in the Exploits Subzone can be ex-plained if the late Arenig to Llanvirn peri-Gondwanan arc(s)interacted to a varying degree with the continent such that,in the south (e.g., Victoria Mine sequence, Bay du NordGroup, Bay d’Espoir Group), the arc was truly continental,whereas farther north it was on thinned or discontinuous

Fig. 8. Schematic diagram showing the variable degree to which the peri-Gondwanan arc (or arcs) might have interacted with the con-tinental margin as the result of a promontory (Cabot Promontory of Lin et al. 1994) on the Gondwanan margin. Heavy solid line is thewestern extent of thick continental crust; dashed line is the extent of thinned continental crust; circles represent a calc-alkaline arcformed on the composite Gondwanan margin; and the ornamented lines are subduction zones with the symbols on the overriding plate.



crust of the continental margin (Wild Bight and Exploitsgroups), and perhaps in places predominantly intra-oceanic(Fig. 8c). Such a situation could occur if a promontory onthe Gondwanan margin in southern Newfoundland (e.g.,Williams 1979; Lin et al. 1994) exerted its influence in theEarly to Middle Ordovician (Fig. 8). In the area of the prom-ontory, where no oceanic crust remained to accommodatecontinued convergence, collision between the early Ordovi-cian tholeiitic arc and the continental margin would haveproduced a collisional event resulting in complete obductionof the arc (Fig. 8). In the reentrant, convergence continuedto be accommodated by subduction of oceanic crust. Whenthe oceanic arc reached the continental margin, attemptedsubduction of the continental crust caused termination of thewest-dipping (present coordinates) subduction zone. Thus, inthe reentrant, the tholeiitic arc was only partially obducted.The subsequent east-dipping (present coordinates)subduction zone would have formed on a composite marginvarying from dominantly continental in the south, wherethere was a promontory, to dominantly oceanic toward thenorth in the embayment (Fig. 8).

Conclusions

This redefinition of the WBG has significant implicationsfor ongoing mineral exploration in the area. All VMS depos-its in the WBG occur in association with bimodal volcanicsequences of the Glovers Harbour Formation, which is nowrecognized as the oldest part of the group. As such, it isclear that most occurrences of the Glovers Harbour Forma-tion and associated VMS deposits are fault-bounded and arelikely further disrupted by internal structural complexity.Furthermore, this redefinition of the WBG facilitates de-tailed correlation with the Exploits Group and may lead toenhanced mineral exploration potential in this group. TheWild Bight and Exploits groups are genetically related, andare interpreted to represent the arc and back arc, respec-tively, of a Middle Ordovician continental margin arc. Thisredefinition of the WBG has also elucidated a sequence ofEarly to Middle Ordovician tectonomagmatic events that wesuggest occurred throughout the Exploits Subzone. One pos-sible explanation for lithological, structural, and stratigraphicvariations between the northwestern and southeastern partsof the Exploits Subzone is the presence of a promontory onthe Gondwanan margin in southern Newfoundland.

Acknowledgments

We would like to thank Hank Williams for reviewing anearlier version of this paper; and together with SteveColman-Sadd, for encouraging the first author to officiallyredefine the stratigraphy of the Wild Bight Group. The firstauthor would also like to thank Scott Swinden and BrianO’Brien for invaluable discussions on the Wild Bight Groupand Dunnage Zone geology in general, and for field trips inthe Wild Bight Group. The fifth floor geology buildingtechnical staff at Memorial University, St. John’s, Newfound-land, helped to produce the excellent geochemical data inthis and two earlier papers, upon which much of this re-gional synthesis is based. We would also like to thank JeanBedard, Louise Corriveau, and Bruno LaFrance for careful

reviews that significantly improved this manuscript. Lastly,but certainly not least, the first author would like to thankGreg Dunning for his patience in teaching the ins and outsof U–Pb geochronology and providing general guidancethroughout her Ph.D.

References

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redefinition of the wild bight group, newfoundland: implications for models of island-arc evolution...

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