Notre Dame Subzone Research Articles

Volcanogenic massive sulfide (VMS) deposits of the Dunnage Zone of the Newfoundland Appalachians: setting, styles, key advances, and future research

The Dunnage Zone of the Newfoundland Appalachians hosts diverse Cambrian–Ordovician volcanogenic massive sulfide (VMS) deposits. The peri-Laurentian Notre Dame Subzone contains Cu–Zn–Au mafic and bimodal mafic deposits in ∼501–485 Ma ophiolitic rocks and Zn–Pb–Cu–(Au–Ag) deposits in ∼471–465 Ma bimodal rifted continental arc sequences (e.g., Buchans). The peri-Gondwanan rocks of the Exploits Subzone host Zn–Pb–Cu–(Au–Ag) bimodal felsic, felsic siliciclastic, and Zn–Ag–Au hybrid bimodal felsic deposits in the ∼513–486 Ma Victoria Lake supergroup; Cu–Zn bimodal felsic to bimodal mafic deposits of the ∼486 Ma Wild Bight Group; and Cu–(Au) mafic siliciclastic deposits of the ∼466 Ma Great Burnt Lake/South Pond belt. Regardless of age or stratigraphic hosts, all VMS deposits are associated with specific magmatic assemblages and extensional tectonism (i.e., rifting). Gold-enriched deposits of the Rambler-Ming district are associated with felsic rocks that formed via slab melting and subsequent melt-mantle wedge interaction, which likely enhanced precious metal enrichment in these deposits. Whereas many deposits exhaled on the seafloor, some deposits formed via subseafloor replacement of host units or as re-sedimented sulfides generated in sediment-gravity flows. Metals in the deposits were derived from leaching of underlying footwall rocks; however, Au–Ag- and epithermal suite element-enriched deposits show evidence for metal contributions from magmatic hydrothermal fluids. Sulfur in deposits was derived predominantly from leaching of H2S from underlying footwall rocks and from thermochemical sulfate reduction of seawater sulfate, with lesser input from bacteria-derived H2S and magmatic-hydrothermal-derived H2S. Despite recent research advances and historic mining, numerous questions remain unresolved and provide opportunities for future study.

Overview of age constraints for gold mineralization in central and western Newfoundland and new 40Ar/39Ar ages for muscovite from selected auriferous zones

This contribution presents new 40Ar/39Ar laser step-heating data for muscovite associated with four significant orogenic gold-mineralized zones in central and western Newfoundland to build upon existing geochronological constraints for orogenic gold mineralization across much of the Newfoundland Appalachians. Additionally, we provide the first 40Ar/39Ar laser step-heating data for detrital muscovite from gold-mineralized sandstone of the Wigwam Formation (Botwood Group) at the Moosehead gold prospect. Most known gold zones on the isIand occur in proximity to reactivated crustal-scale faults and related structures, and are commonly localized within competent, rigid lithological units (e.g., granitoid rocks); although, some vein systems cut less competent, clastic sedimentary rock sequences. Host rocks range in age from Neoproterozoic to Devonian and may extend into the Carboniferous. Robust Pragian to Emsian (ca. 410–375 Ma) rutile, muscovite and zircon ages constrain the timing of gold mineralization in parts of the Exploits Subzone, whereas less precise age estimates for gold mineralization in the Notre Dame Subzone and Laurentian margin range from Wenlock to Emsian (ca. 433–375 Ma). Collectively, the geochronological data suggest that fluid-pressure cycling associated with gold mineralization in central and western Newfoundland occurred during progressive Siluro-Devonian tectonics associated with the waning stages of the Salinic orogenic cycle and spanning the Acadian and Neoacadian orogenic cycles. Multiple, polyphase, overlapping orogenic events allowed for repeated, gold mineralizing fluid flow events, particularly in proximity to long-lived, reactivated crustal-scale fault zones.

New U-Pb zircon ages of plagiogranites from the Coastal Complex ophiolite and Twillingate batholith, Newfoundland: Evidence for the oldest and overlapping silicic magmatism in the nascent Cambrian peri-Laurentia forearc and arc terranes

The Coastal Complex (CC) and Bay of Islands Complex (BOIC) represent two supra-subduction zone (SSZ) forearc ophiolitic assemblages of different ages within the Taconic Humber Arm Allochthon of the Western Newfoundland Appalachians. To further constrain age relationships between the CC and BOIC and earliest initiation of Iapetus subduction, we acquired LA-ICP-MS U-Pb zircon ages for eleven plagiogranites sampled along strike of the CC belt. These ages combined with three prior legacy ages establish an age range of 514.3–502.7 Ma for the six CC plagiogranite plutons. The oldest precise ca. 514.3 Ma CC pluton age represents the oldest Taconic arc-related plutonic activity yet documented in Western Newfoundland and provides a minimum age for the intruded CC ophiolite basement and initiation of westerly vergent subduction within the peri-Laurentian Iapetus Ocean. This confirms the Early Cambrian CC mafic–ultramafic basement is significantly older than the juxtaposed SSZ BOIC, which formed by later rifting of the CC forearc along younger forearc spreading center in the Late Cambrian by ca. 488.3 Ma. After subduction initiation a prolonged early period CC forearc silicic magmatism lasted at least ∼ 11.6 Ma until ca. 502.7 Ma, a range consistent with age spans of magmatism in modern juvenile forearcs. We also reinvestigated the Central Newfoundland Twillingate Granite batholith thought to be one of the earliest major felsic batholiths formed within the adjacent juvenile arc massif of the Notre Dame Subzone, part of the same peri-Laurentian arc. We determined a precise 3-sample composite age of 504.3 ± 1.8 Ma that signals the formation of earliest Notre Dame arc edifice developed by Middle Cambrian. It overlaps the end-stage CC forearc plagiogranite magmatism and places a minimum mid-Cambrian age constraint on intruded ophiolitic arc tholeiitic and boninitic pillow lavas of the Sleepy Cove and correlated Lushs Bight volcanics, suggesting basement continuity with the CC.

A hafnium isotopic record of magmatic arcs and continental growth in the Iapetus Ocean: The contrasting evolution of Ganderia and the peri-Laurentian margin

We test the sensitivity of combined U-Pb-Hf isotopic analyses of detrital and magmatic zircon to document complex processes in accretionary orogens, using Ganderia from the Canadian Appalachian orogen as an example. Ganderia hosted a long-lived magmatic arc that began at least by 650Ma and continued until 450Ma, during which time it drifted northward from the Amazonian margin. Its isotopic record suggests arc activity began earlier, possibly at ~750Ma (proto-Ganderia) within an oceanic domain. The U-Pb-Hf zircon isotopic array suggests that Ganderia formed on a sliver of Paleoproterozoic-Mesoproterozoic basement (TDMc=2.2–1.1Ga). A transition towards more evolved Hf isotopic compositions between 650 and 600Ma is interpreted to coincide with its accretion to the Amazonian margin of Gondwana, consistent with other geological evidence. Accretion was followed by increasing amounts of juvenile crustal inputs between ~550 and 500Ma, which is interpreted to reflect subduction roll-back and the onset of drift from the Gondwanan margin. Trench retreat culminated in the separation of the Ganderia arc and formation of the exclusively oceanic ~510–450Ma Penobscot-Popelogan-Victoria arcs on Ganderia. Accretion of Ganderia to the active Notre Dame Subzone along the Laurentian margin occurred in the late Ordovician and is marked by the abrupt termination of Ganderian arc magmatism.The Notre Dame arc evolved along the Laurentian margin between ~515 and 425Ma and was built on Paleoproterozoic-Archean (T(DM)c=2.9–2.5Ga) crust. Supra-subduction zone magmatism occurred at ~490–460Ma during the closure of the Taconic Seaway and was dominated by the recycling of Laurentian crust, with progressive mixing of evolved and juvenile arc magmas over time. The preservation of very evolved Notre Dame arc zircon in Ganderian overstep sequences confirms the arrival of the leading edge of Ganderia to Laurentia by ~450Ma. The hafnium isotopic method is proven here to be a powerful tool when used in conjunction with geological constraints, and is able to identify the changing nature of magmatism during the evolution of complex accretionary systems.

Lower and Middle Ordovician conodonts of Laurentian affinity from blocks of limestone in the Rosroe Formation, South Mayo Trough, western Ireland and their palaeogeographic implication

The Middle Ordovician Rosroe Formation consists of some 1350 m of coarse, mainly siliciclastic to volcaniclastic sedimentary rocks, deposited in a submarine fan environment, and is restricted to the southern limb of the South Mayo Trough, western Ireland. Discrete allochthonous blocks, reaching 5 m in size, are present in the formation at several localities. Conodonts recovered from these blocks, collected from two separate locations, are of late Early and mid Mid Ordovician age. The conodonts have high conodont‐alteration indices (CAI 5) indicative of temperatures as high as 300o to max. 480 °C; some found in the Lough Nafooey area have abnormally high indices (CAI 6), which correspond to temperatures of about 360o to max. 550 °C. The oldest fauna is dominated by Periodon aff. aculeatus and characterized by Oepikodus evae typical of the Oepikodus evae Zone (Floian Stage; Stage Slices Fl2–3, Lower Ordovician). The younger conodont assemblage, characterized by Periodon macrodentatus associated with Oistodella pulchra, is referred to the P. macrodentatus conodont Biozone (lower Darriwilian; Stage Slices Dw1–2). The Rosroe conodont assemblages are of Laurentian affinity; comparable faunas are well known from several locations along the east to south‐eastern platform margin of Laurentia and the Notre Dame subzone of central Newfoundland, Canada. The faunal composition from the limestone blocks suggests a shelf edge to slope (or fringing carbonate) setting. The faunal assemblages are coeval with, respectively, the Tourmakeady Formation (Floian–Dapingian) and Srah Formation (Darriwilian) in the Tourmakeady Volcanic Group in the eastern part of the South Mayo Trough and probably are derived from the same or similar laterally equivalent short‐lived carbonate successions that accumulated at offshore ‘peri‐Laurentian’ islands, close to and along the Laurentian margin. During collapse of the carbonate system in the late Mid Ordovician, the blocks were transported down a steep slope and into deep‐water by debris flows, mixing with other rock types now found in the coarse polymict clastics of the Rosroe Formation. The faunas fill the stratigraphical ‘gap’ between the Lower Ordovician Lough Nafooey Volcanic Group and the upper Middle Ordovician Rosroe Formation in the South Mayo Trough and represent a brief interval conducive to carbonate accumulation in an otherwise siliciclastic‐ and volcaniclastic‐dominated sedimentary environment. Copyright © 2015 John Wiley & Sons, Ltd.

Laurentia-derived detritus in the Badger Group of central Newfoundland: deposition during closing of the Iapetus Ocean1This article is one of a series of papers published in CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.2Geological Survey of Canada Contribution 20110273.

The Dunnage zone of central Newfoundland records the closing of an oceanic tract — the Iapetus Ocean. Within this tract, the Red Indian line separates arc terranes developed close to Laurentia (Notre Dame subzone) from those associated with the peri-Gondwanan terrane Ganderia (Exploits subzone). Sandstones and conglomerates of the Badger Group were deposited on the Exploits subzone, southeast of the Red Indian line, from Late Ordovician (Katian) to early Silurian (Llandovery) time. Two samples were collected from the base and top of the Badger Group for detrital zircon U/Pb dating. The lower sample yielded detrital zircon populations with a large statistical peak at ca. 1.0 Ga, and other features characteristic of derivation from Laurentian sources. Paleozoic zircons with ages from Late Cambrian to Late Ordovician were probably derived from the Notre Dame arc on the margin of Laurentia. The upper sample yielded a comparable age distribution, but with a much smaller proportion of Mesoproterozoic relative to Paleozoic detrital zircon. These results date the earliest arrival of Laurentia-derived detritus on the peri-Gondwanan Exploits subzone crust in the early Katian (ca. 455 Ma). However, the absence, in both samples, of Neoproterozoic grains suggests that there was still a seaway separating the Gander margin from arc terranes accreted to Laurentia until at least the middle Llandovery (ca. 436 Ma).

U-Pb detrital zircon geochronological constraints on the Early Silurian collision of Ganderia and Laurentia along the Dog Bay Line: The terminal Iapetan suture in the Newfoundland Appalachians

The Dog Bay Line is a major Silurian terrane boundary in the Exploits Subzone of the Newfoundland Appalachians. Late Ordovician-Early Silurian rocks northwest of the Dog Bay Line, the Badger and Botwood groups, contain detritus sourced exclusively from Laurentia. These rocks were deposited upon peri-Gondwanan volcanic arc terranes that were accreted to Laurentia in the Middle Ordovician. The Davidsville and Indian Islands groups southeast of the Dog Bay Line have stratigraphic links to peri-Gondwanan terranes and were deposited during the Late Ordovician-Early Silurian upon the peri-Gondwanan margin of Iapetus and were then accreted to Laurentia in the Early Silurian. A change from Paleozoic-dominated (Badger Group) to Meso- and Neoproterozoic-dominated (Botwood Group) detritus in sequences northwest of the Dog Bay Line is attributed to the Middle Ordovician collision and exhumation of peri-Laurentian arc terranes of the Notre Dame Subzone with Laurentia. Unroofing and erosion of these accreted arc terranes re-exposed Laurentian basement and deposited detritus from the latter into the Botwood Group. Salinic orogenesis resulting from the collision of Ganderia with Laurentia resulted in obduction and erosion of the accreted Victoria and Exploits arcs and deposition of the detritus into a forearc basin on Laurentia. The absence of zircons in the 510 to 550 Ma and 1520 to 1600 Ma age range northwest of the Dog Bay Line and no 1600 to 1700 Ma zircons southeast of the Dog Bay Line suggests the presence of a Silurian arm of the Iapetus Ocean (Tetagouche-Exploits basin) that separated Laurentia from peri-Gondwanan microcontinents of Ganderia and Avalonia. The change in Late Ordovician deep marine turbidites to Early Silurian stable-shelf rocks and non-marine, subaerial sediments east of the Dog Bay Line parallels that on the Laurentian margin and indicates the destruction and subsequent closure of Iapetus. The upper sequences of the Botwood Group (Rogerson Lake Formation) contain 700 to 800 Ma zircons that are atypical of Laurentia, but are common in peri-Gondwanan terranes, and suggest that the youngest rocks of the Botwood Group may post-date closure of the Dog Bay Line and transgress the suture as an overlap sequence. The presence of Silurian orogenesis on both the Laurentian and peri-Gondwanan margins of Iapetus is consistent with the closure of the Tetagouche-Exploits basin and Iapetus Ocean by the Late Silurian. Laurentia and the Ganderian microcontinent were involved in a continent-continent collision suggesting that rocks along the Dog Bay Line represent the last known occurrence of Iapetus in the Newfoundland Appalachians.

A lost arc-back-arc terrane of the Dunnage oceanic tract recorded in clasts from the Garin Formation and McCrea mélange in the Gaspé Appalachians of Québec

Upper Ordovician Garin conglomerates (base of the Gaspe belt, Quebec Appalachians) contain three igneous clast populations. (1) Calc-alkaline intermediat-felsic rocks resemble Exploits (New Brunswick, Maine) and Notre Dame (Quebec, Newfoundland) subzone lavas. Clasts (monzonite-rhyodacite) give U-Pb zircon ages between 465 and 466 Ma, precluding correlation with Exploits rocks. We suggest that the suite represents peri-Laurentian continental arc magmas coeval with the Red Indian Lake Group of Newfoundland. Correlation of lavas from a Gaspe well with the Popelogan arc implies that the Red Indian Line passes through southern Gaspe. (2) Mafic tholeiitic-to-alkaline clasts resemble New Brunswick and Maine (Exploits subzone) alkaline lavas. We interpret them as magmas associated with initiation of a back-arc basin, possibly on the Laurentian margin. (3) Mafic-intermediate tholeiitic to calc-alkaline clasts have an oceanic subduction component and correlate with Notre Dame subzone lavas (Quebec, Newfoundland). They are interpreted as products of spreading and off-axis magmatism of a peri-Laurentian back-arc basin. We correlate McCrea melange lavas to the calc-alkaline intermediat-felsic and mafic-intermediate suites and interpret them as samples of a Notre Dame subzone terrane coeval with the Red Indian Lake Group of Newfoundland during initiation of a successor arc–back-arc, a terrane which is no longer preserved in New Brunswick and Maine but that provides some evidence for possible extensions of Newfoundland9s geology into the Gaspe Appalachians of Quebec. A mid-ocean-ridge basalt–like intrusion in the McCrea melange (Ar-Ar age: 471.2 ± 11.2 Ma) is coeval with peri-Laurentian ophiolites in Newfoundland and may record a back-arc opening event.

Geochemistry and tectonic setting of metabasic rocks of the Gneiss Dome Belt, SW New England Appalachians

Acadian structural domes in the Connecticut Valley Zone in western Connecticut expose polydeformed, amphibolite and upper amphibolite facies pre-Silurian rocks correlative with the Notre Dame subzone (western Dunnage zone) of Williams et al. [Geologic Survey of Canada Paper 88-1B, 1988, pp. 91–98] of the Canadian Appalachians. We have analyzed the geochemistry of 99 metabasic rocks from the Cambrian-Lower Ordovician Rowe Schist (RS), the Arenig or older Taine Mountain Formation (TMF), and the overlying Arenig Collinsville Formation (CF) to identify the tectonic setting of mafic volcanism in the Gneiss Dome Belt (GDB) in western Connecticut and southwestern Massachusetts. The metabasites record epidote-amphibolite to lower amphibolite facies metamorphism and exhibit equilibrium microstructures and only limited retrogression. We used our data to test whether the GDB represents part of a Taconic volcanic arc in southwestern New England. RS and TMF metabasites that can be interpreted as dikes range in composition from subalkaline basalt to basaltic andesite and exhibit overall arc-like signatures. Their REE patterns at 10–25× chondrite are mildly LREE enriched with (La/Yb) N=1.50–1.94. Enriched HFSE from Th to Ce and lack of negative Ta–Nb anomalies indicate that the RS and TMF metabasites experienced some degree of crustal contamination. The majority of CF metabasites range from basaltic andesite to subalkaline basalt. The CF metabasites are geochemically diverse and have a wide range of Ti/V ratios characteristic of boninites and low-Ti arc tholeiites, arc tholeiites, ocean floor basalts, and ocean island and alkali basalts. Based on their geochemical characteristics, CF metabasites have been subdivided into three major types: (1) boninitic and low-Ti IAT metabasites (20% of the analyzed samples), (2) arc-like metabasites (27%), and (3) MORB-like and MORB/WPB transitional metabasites (53%). Boninitic CF metabasites are characterized by extremely depleted HFSE, enriched LILE, and distinctive U-shaped REE patterns below the 10× chondrite level. Arc-like CF metabasites have moderately depleted HFSE from P to Yb and relatively enriched LILE; REE patterns at 10–25× chondrite are mostly mildly LREE enriched with (La/Yb) N=1.68–2.12. HFSE from Th to Ce are relatively enriched indicating a small degree of crustal contamination in these metabasites. MORB-like CF metabasites display signatures characteristic of BABB; they have flat, weakly enriched HFSE and moderately enriched LILE typical of evolved MORBs. REE patterns at 15–35× chondrite are flat to slightly LREE enriched with (La/Yb) N=1.08–1.49. On tectonic discriminant diagrams, the majority of MORB-like CF metabasites define MORB-WPB trend. Four samples with the most elevated REE patterns at 20–100× chondrite and moderately LREE enriched have a transitional MORB/WPB character. The tectonic position and geochemical signatures of the RS and TMF dikes suggest that they represent extension-related magmatism, either in an arc or forearc setting, prior to the establishment of Arenig- to Caradoc-aged magmatism in this part of western New England. The coexistence of boninitic, arc-like, and MORB-like compositions is consistent with a backarc setting as the eruptive environment of the CF metabasites in the GDB in western Connecticut and southwestern Massachusetts as predicted by Crawford et al.'s [Earth, and Planetary Science Letters 54, 1981, 346–356] model of boninite generation. Whether the initial rifting and opening of this backarc basin occurred in the forearc or as intra-arc splitting remains unclear. MORB-like compositions among the CF metabasites are well represented and denote a more evolved stage of this backarc basin development. Boninitic and arc-like CF metabasites conclusively prove a supra-subduction zone setting, but the polarity of this subduction remains equivocal. Our results indicate that Taconic accretion in western New England Appalachians was more complex than previously proposed and likely involved forearc, arc, and backarc complexes.

Taconian orogeny and the accretion of the Dashwoods block: A peri-Laurentian microcontinent in the Iapetus Ocean

The stratigraphy of the external Humber zone of the western Newfoundland Appalachians records protracted Neoproterozoic–Early Cambrian rifting, followed by development of a passive margin that persisted until late Early Ordovician (Arenigian) time (ca. 475 Ma). However, adjacent metamorphic rocks, derived from the Laurentian margin and preserved in the Dashwoods subzone, were deformed, overthrust by ophiolites, and intruded by arc plutons by 488 Ma. The adjacent Notre Dame subzone also records isotopic evidence of interaction with the margin by 488 Ma. We propose that a microcontinent (Dashwoods microcontinent) was rifted from Laurentia during the Early Cambrian after an earlier, Neoproterozoic opening of the Iapetus Ocean, and was separated from the margin by a narrow oceanic tract (Humber seaway). Attempted subduction of the Dashwoods microcontinent prior to 488 Ma was followed by closure of the Humber seaway in the Taconian orogeny.

Petrology of the Cormacks Lake Complex, Newfoundland: decompressional reaction relations in cordierite+orthoamphibole-bearing gneisses and associated rocks

AbstractCordierite+orthoamphibole (Crd+Oam)-bearing gneisses in the Cormacks Lake complex are regionally associated with metapelites containing prismatic sillimanite and K-feldspar, metabasites that locally contain metamorphic orthopyroxene, and other high-grade rocks in the Central Gneiss (Dashwoods) subzone, in the southwestern Newfoundland Appalachians. Retrograde features formed at the expense of the granulite-facies assemblages are ubiquitous. For example, in some migmatitic rocks, garnet is resorbed by Crd+Oam, and in metapelites, cordierite separates corroded garnet and sillimanite. Mineral thermobarometry suggests that, following granulite-facies metamorphism (T<785°C, P<7.5 kbar), retrogression occurred as the Cormacks Lake gneisses cooled through Mg-Fe diffusional blocking temperatures as they decompressed to a pressure of ∼3–4 kbar. Given the absence of Barrovian (or higher pressure) mineral assemblages in the metapelites, regional tectonic reconstructions involving the thrusting of a neighbouring terrane (Notre Dame subzone) over the Central Gneiss subzone appear to be supported only by the moderate pressure determined for the granulite facies event. Although scarcely discernible given re-equilibration effects and the imprecision of thermobarometers, subsequent decompression nonetheless had a marked impact on the mineralogy of the gneisses.

Lithostratigraphy and geochemistry of the Cottrells Cove Group, Buchans – Roberts Arm volcanic belt: new constraints for the paleotectonic setting of the Notre Dame Subzone, Newfoundland Appalachians

New sedimentological, geochemical, and geochronological data from the Cottrells Cove Group in central Newfoundland provide important constraints on the nature of the Notre Dame Subzone, its tectonic setting, and the history of the Laurentian margin during the Early Ordovician. The Cottrells Cove Group forms the eastern extension of the Roberts Arm Group and correlates with the Chanceport Group on New World Island. It is represented by two volcano-sedimentary formations that occur in a complex thrust stack. The Fortune Harbour Formation consists of calc-alkalic, island-arc lavas, followed by a 1250 m thick succession of volcaniclastic deposits, radiolarian cherts, and calc-alkalic, mafic flows, which were deposited in a back-arc, basin-plain setting. The volcaniclastic deposits include felsic tuff, which has a U–Pb zircon age of 484 ± 2 Ma and an inheritance component of 2517 ± 26 Ma. These new U–Pb and Nd-isotope data suggest that the island-arc–back-arc volcanism and sedimentation in the Notre Dame Subzone developed in the vicinity of continental margin and approximately 10 Ma earlier than has previously been established. The Moores Cove Formation is undated but contains boulders of calc-alkalic basalt and is presumed to be at least in part younger than the Fortune Harbour Formation. Tholeiitic lavas, together with associated radiolarian cherts and volcaniclastic deposits, constitute the basal part of the Moores Cove Formation and may have been deposited in a back-arc environment synchronously with some parts of the Fortune Harbour Formation. They are conformably followed by an over 1200 m thick, coarsening-upward succession of lower-slope and submarine-fan deposits. The polymictic flysch, containing clasts of island-arc basalt, accompanied by other volcanic, plutonic, ultramafic, and sedimentary detritus, may record Middle or Upper Ordovician uplift and erosion of obducted arc–back-arc, volcano-sedimentary sequences and their ophiolitic substrate.

Tectonic implications of early silurian thrust imbrication of the northern exploits subzone, Central Newfoundland

Central Newfoundland's Dunnage Zone contains a composite assemblage of island arc and oceanic rocks formed in the Paleozoic Iapetus Ocean. Two different and perhaps unrelated Late Cambrian-Middle Ordovician volcanic belts (Lush's Bight and Robert's Arm belts, and their correlatives) are preserved in the Notre Dame Subzone, and are interpreted to represent arc and back-arc basin sequences developed; (1) adjacent to the Laurentian margin of Iapetus and (2) in the intraoceanic realm of the 4000 km wide Paleozoic ocean. Paleozoic paleogeographic reconstructions suggest that the Exploits Subzone contains volcanic and sedimentary rocks originally deposited in a third arc located between the intraoceanic arc (Robert's Arm Belt) and the Peruvian promontory of Gondwana. The New Bay Pond area is located within the northern Exploits Subzone, and preserves an imbricate thrust stack formed during Late Ordovician/Early Silurian southeastward-directed thrusting. Thrust-load related subsidence led to deposition of cherts and argillites overlain by a southeastward-prograding wedge of orogenic flysch and molasse in a foreland trough. Late Ordovician-Early Silurian structural disruption of the flysch/molasse sequence and older strata of the northern Exploits Subzone, by a series of thrust faults, place Middle Ordovician rocks over the younger flysch, and formed a new, southeast-vergent accretionary wedge. This event records the emplacement of the Exploits Subzone over Gondwanan continental margin rocks of the Gander Zone, soon after the Notre Dame Subzone collided with the Appalachian margin of Laurentia. Continued closure of Iapetus brought together the collisionally modified margins of Gondwana and Laurentia. The Exploits Subzone is cut by numerous Silurian-mid Devonian predominantly dextral strike-slip faults, suggesting that this convergence involved a significant component of dextral transpression. Several pull-apart basins are located along these strike-slip faults, and are filled by Silurian volcano-sedimentary sequences. These faults formed in response to the counter-clockwise rotation of Gondwana, juxtaposing the Avolonian margin of Gondwana with the Appalachian margin of Laurentia by the Carboniferous.

Discussion on a reassessment of the early Ordovician 'Celtic' brachiopod province

R. B. Neuman, D. A. T. Harper & C. R. van Staal write: The results of several recent geological studies relevant to early Ordovician palaeogeography are not mentioned in this recent paper by Cocks & McKerrow (1993). Contributions of particular significance include the recognition that the Dunnage Zone of Newfoundland consists of two distinct parts, the Notre Dame and Exploits subzones of contrasting Ordovician stratigraphy, lithology, structure, metamorphism, plutonism, lead isotope signatures, mineralization, geophysics, and faunas ( Williams et al. 1988 ). The boundary between these subzones, named the Red Indian Line in Newfoundland, is probably a suture. Fossiliferous sedimentary rocks are rare in the Notre Dame Subzone, but Arenig-Llanvirn graptolites occur in shales at three places and at one place conodonts have been extracted from carbonate clasts in volcanic breccias. All these fossils are considered to have Laurentian affinities (Nowlan & Thurlow 1984; Williams 1992 ; Williams et al. 1992 ). Fossiliferous Lower Ordovician rocks are more abundant in the Exploits subzone. In addition to the several occurrences of Celtic assemblage brachiopods (Cl-C6 of Neuman & Harper 1992 ; Harper 1992 ; Cocks & McKerrow 1993), Gondwanan trilobites and graptolites (Cyclopyge and Undulograptus) are known in the Exploits subzone of central Newfoundland ( Williams et al. 1992 ) from shale that overlies a Lower Ordovician ophiolite fragment near an Occurrence of Celtic brachiopods (C5 of Neuman & Harper 1992 ). Palaeomagnetic studies of Newfoundland rocks near the Red Indian Line showed that these two subzones were separated by about 30° of latitude (van der Pluijm et al. 1990;

Silurian deformation in eastern Notre Dame Bay, Newfoundland: Discussion

I southeast thrusting and ocean closure across Exploits Subzone resembles that of Pickering et al. (1988), but Lafrance and Williams assume deposition in a single marine basin on Gondwanan side of Iapetus and postdepositional thrusting I rather than deposition in several basins on Laurentian side I of Iapetus and syndepositional thrusting (Arnott et al. 1985; Pickering et al. 1988). We wish to comment on terrane analysis offered by them and to point out a number of errors I 1 in their presentation. Of five defined in paper, two (Twillingate and Chanceport) are known Ordovician suspect terranes. The New World Island and Port Albert terranes, while suspect to North America during Early and Middle Ordovician, share same Late Ordovician to Early Silurian rocks and stratigraphy. Virtually all previous workers have also recognized Late Ordovician Early Silurian sedimentologic links between New World Island Terrane (Exploits Subzone) and Twillingate Chanceport (Notre Dame Subzone), indicating Middle Ordovician accretion to North America (Kay 1967; 1969; Helwig and Sarpi 1969; Nelson and Casey 1979; Nelson 1981; Arnott 1983a; 19836: Arnott et al. 1985; Pickering et al. 1988; Williams et al. 1988; and others). These data imply that any Silurian basin(s) in this region must have been on Laurentian margin of Iapetus rather than Gondwanan margin. The authors' Dildo Run Terrane, coextensive with Dunnage MClange, is interpreted as being faulted against New World Island Terrane, following Kay (1970) and van der Pluijm (1986). However, this contact has also been interpreted as stratigraphic (Home 1969; Hibbard and Williams 1979). The authors present no evidence that any of their terranes were distinctive in Silurian time period of chief interest to their paper. According to authors, Twillingate and Dunnage were affected by the earlier event. However, they conclude that these evolved on opposite sides of an Ordovician ocean. Hence there is no reason to suppose that pre-Silurian events can be correlated in any meaningful sense

The Pipestone Pond Complex, central Newfoundland: complex magmatism in an eastern Dunnage Zone ophiolite

Ophiolitic rocks are preserved in both the Notre Dame and Exploits subzones in the Dunnage Zone of the Newfoundland Appalachians. Ophiolites in the Exploits Subzone are generally less well preserved and exposed than their Notre Dame Subzone counterparts and, consequently, have received less attention in the literature.The Pipestone Pond Complex is an Exploits Subzone ophiolitic sequence, which outcrops on the western side of a structural window through the Exploits Subzone into the underlying Gander Zone. It includes a basal harzburgite, which passes upwards into a cumulate pyroxenite and gabbro sequence, and thence into isotropic gabbro intruded by pegmatitic gabbro, diabase, and plagiogranite. There is no sheeted dyke unit. Pillow lava occurs at the top of the sequence but is not observed to be in stratigraphic contact with the intrusive rocks. The ophiolitic rocks are structurally disrupted and no single cross section traverses the complete ophiolitic stratigraphy.Although the stratigraphic sequence of the Pipestone Pond Complex is relatively straightforward, whole-rock geochemical and Nd/Sm isotopic data provide evidence for a complex magmatic history. The intrusive rocks have εNd(t) ranging from −1.1 to + 4 and geochemical signatures indicating derivation from depleted and refractory mantle sources that were clearly influenced by subduction. Within the intrusive rocks, there are no simple petrogenetic relationships among the gabbros and dykes and trondhjemites. The extrusive rocks, in contrast, have εNd(t) of + 7.3 and geochemical signatures similar to those of normal mid-ocean-ridge basalts. They represent magmatism derived from depleted oceanic mantle, not affected by the subducted slab.The tectonic interpretation of the Pipestone Pond Complex is hampered by a lack of definitive evidence for the relative age of the arc-related plutonic rocks and the non-arc-related extrusive rocks. Two possible interpretations are (i) the initiation of subduction under oceanic crust and (ii) arc rifting. The Pipestone Pond Complex is geologically and geochemically analogous to the Boil Mountain Complex in Maine, and together they may form part of an extensive ophiolitic terrane that was emplaced upon the Gondwanan continental margin prior to its collision with Laurentia.

Early Silurian paleolatitude for central Newfoundland from paleomagnetism of the Wigwam Formation: Reply

Redbeds and minor volcanic rocks of the Early Silurian Wigwam Formation of the Botwood Group were sampled in the Exploits Subzone of the Dunnage Zone of central Newfoundland. At 18 sedimentary and 3 volcanic sites thermal demagnetization isolated a shallow northward (or southward) magnetization (D = 356.0°, I = −16.8°; k = 12.9; α95 = 9.2°; n = 21 site means) after correction for the tilt of bedding. A positive fold test demonstrates that this remanence predates regional Siluro–Devonian folding. A primary age for the remanence is indicated by positive conglomerate tests on volcanic clasts, and reversals correlated with sedimentary stratigraphy. Antiparallel remanence directions at sites of opposite polarity indicate that the primary remanence is not partially overprinted by a secondary component. The Exploits Subzone of the Dunnage Zone is interpreted to have been at low paleolatitudes (8.5°S ± 5°) during the Early Silurian. We reached a similar conclusion for the Notre Dame Subzone of the Dunnage Zone in...

A new Lower Ordovician (Arenig) faunule from the Coy Pond Complex, central Newfoundland, and a refined understanding of the closure of the Iapetus Ocean

A newly discovered fossiliferous horizon within sediments belonging to the Coy Pond Complex of the Exploits Subzone in central Newfoundland yields the graptolite Undulograptus austrodentatus s.l. and cyclopygid trilobite Cyclopyge grandis brevirhachis. This late Arenig faunule constrains the upper age limit of the ophiolite complex and is the first record of an Early Ordovician cyclopygid trilobite in North America. This is consistent with a paleogeographic affinity for south-central Newfoundland with the northern oceanic margin of Avalonia in a peri-Gondwanan position during the Early Ordovician and contrasts with coeval shelly and graptolitic faunas from the Notre Dame Subzone of central Newfoundland, which show marked Laurentian affinities. The Exploits Subzone is generally considered equivalent to the region of Scotland lying south of the Southern Upland Fault. The Newfoundland discovery, which is supported by faunal data from elsewhere in Newfoundland and in Ireland, suggests that the region around the Southern Upland Fault, rather than the Solway Firth, represents the location of the "Iapetus suture" in Britain in Lower Ordovician rocks.

Lower Ordovician (Arenig–Llanvirn) graptolites from the Notre Dame Subzone, central Newfoundland

Many lower Paleozoic rocks in the Notre Dame Subzone of central Newfoundland are of unknown or imprecise age. Several new Lower Ordovician graptolite occurrences are here reported and earlier records revised. New graptolite localities in the Balsam Bud Cove Formation at Snooks Arm on the Baie Verte Peninsula, previously recorded as "early Ordovician (Arenig)" have yielded an assemblage identical to that found in the middle part of Bed 11 of the Cow Head Group, western Newfoundland, indicating a probable lower Didymograptus bifidus Zone age. At Corner Pond, southeast of Corner Brook, an abundant, diverse fauna from the Corner Pond formation indicates a marginally older age for the black shales than those at Snooks Arm (Pendeograptus fruticosus Zone, equivalent to lower Bed 11). Black shales associated with felsic volcanics and limestone breccia belonging to the Cutwell Group at Lushes Bight, on Long Island, western Notre Dame Bay, which were previously assigned to the widespread black shales of the Lawrence Harbour Formation and equivalents in the Exploits Subzone, contain a rich lower Llanvirn (Paraglossograptus tentaculatus Zone) graptolite assemblage. This agrees with ages established using other macrofossils and conodonts from the associated limestones. In contrast, a lower shale unit from older strata at Southern Head on the eastern end of the island yields a late Arenig Isograptus victoriae maximus Zone assemblage. These newly discovered graptolite faunas provide precise ages for the upper and lower parts of the volcano-sedimentary sequence on Long Island. Interestingly, all four graptolite assemblages discussed here are of open-ocean affinity, permitting accurate correlation with localities not only in western Newfoundland but also elsewhere in the world.

Early Silurian paleolatitude for central Newfoundland from paleomagnetism of the Wigwam Formation

Redbeds and minor volcanic rocks of the Early Silurian Wigwam Formation of the Botwood Group were sampled in the Exploits Subzone of the Dunnage Zone of central Newfoundland. At 18 sedimentary and 3 volcanic sites thermal demagnetization isolated a shallow northward (or southward) magnetization (D = 356.0°, I = −16.8°; k = 12.9; α95 = 9.2°; n = 21 site means) after correction for the tilt of bedding. A positive fold test demonstrates that this remanence predates regional Siluro–Devonian folding. A primary age for the remanence is indicated by positive conglomerate tests on volcanic clasts, and reversals correlated with sedimentary stratigraphy. Antiparallel remanence directions at sites of opposite polarity indicate that the primary remanence is not partially overprinted by a secondary component. The Exploits Subzone of the Dunnage Zone is interpreted to have been at low paleolatitudes (8.5°S ± 5°) during the Early Silurian. We reached a similar conclusion for the Notre Dame Subzone of the Dunnage Zone in a previous paleomagnetic study of Early Silurian redbeds and volcanic rocks of the King George IV Lake area. The low paleolatitudes obtained for the Wigwam and King George IV Lake rocks do not differ significantly and, hence, provide no evidence for a large ocean between the two subzones in the Early Silurian. However, the azimuthal orientation of the subzones at that time is still uncertain, and an ocean with its axis oriented in a north–south direction would not be detected by the paleomagnetic method.