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Volcanic groups of the Central Mobile Belt of the Newfoundland Appalachians have previously been subdivided into "early" and "late" arc sequences, separated in time by a quiescent Caradocian stage defined in some areas by fossil-bearing black shales.New U–Pb zircon ages of [Formula: see text] and 473 ± 2 Ma for rhyolites of the Buchans and Roberts Arm groups, respectively, show them to be correlative early Ordovician sequences. These ages serve to refute both the previous Rb–Sr whole-rock isochron ages of 447 Ma and the idea that these groups were "late arc" sequences. These new ages corroborate evidence from late Arenig – early Llanvirn conodonts in the Buchans Group and calibrate this fossil occurrence.A new U–Pb zircon age of 479 ± 3 Ma from plagiogranite of the Mansfield Cove Complex immediately west of the Roberts Arm Group shows that this plutonic body is only slightly older than the adjacent volcanic rocks and not Hadrynian as previously supposed. Local field relationships suggest this body may represent part of a disrupted ophiolite. It is coeval with the ophiolitic Annieopsquotch Complex along a strike to the south and may form part of a belt of rocks derived from early Ordovician ocean floor that is discontinuously exposed along the western boundary of the Buchans – Roberts Arm Belt.Zircons from rhyolite at the northeast termination of the Tulks Hill volcanics, part of the extensive Victoria Lake Group, give an age of [Formula: see text]. This dated sequence contains limestone previously dated as Llanvirn–Llandeilo by conodonts. This part of the group is therefore younger than the Buchans Group, and the designations "early" and "late" arc are not appropriate. The thrusting that juxtaposed these groups is no longer constrained to be of Silurian age but could have been middle to late Ordovician. Precambrian zircons included in the Victoria Lake Group rhyolite could have been incorporated from associated sedimentary rocks and suggest that the group may have formed in a tectonic setting transitional between oceanic and continental and received detritus from several sources.

The Lake Ambrose volcanic belt (LAVB) outcrops as a 45 km long northeast-trending belt of mafic and felsic volcanic rocks along the eastern side of the Victoria Lake Group in southcentral Newfoundland. It comprises roughly equal proportions of mafic pillow basalt and high silica rhyolite, locally interbedded with epiclastic turbidites. Volcanic rocks have been metamorphosed in the greenschist facies and are extensively carbonatized.U-Pb (zircon) dates from rhyolite at two, widely separated localities give identical ages of 513 + 2 Ma (Upper Cambrian), and this is interpreted as the eruptive age of the volcanic sequence. Primitive arc and low-K tholeiites can be recognized on the basis of major and trace element geochemistry, ranging from LREE-depleted to LREE-enriched. Geochemical variation between mafic volcanic types is interpreted predominantly to reflect contrasts in source characteristics and degree of partial melting; some variation within each geochemical type attributable to fractional crystallization can be recognized. Detailed examination of some samples indicates that the heavy REE and related elements have locally been mobile, probably as a result of carbonate complexing.The LAVB is the oldest well-dated island arc sequence in Newfoundland, and perhaps in the Appalachian-Caledonian Orogen. Its age requires modification of widely held models for the tectonic history of central Newfoundland. It is older than the oldest known ophiolite, demonstrating that arc volcanism was extant before the generation of the oldest known oceanic crust in this part of Iapetus. It further demonstrates that there was a maximum of approximately 30 Ma between the rift-drift transition which initiated Iapetus, and the initiation^ of subduction. This suggests that the oceanic sequences preserved in Newfoundland represent a series of arcs and back arc basins marginal to the main Iapetus Ocean, and brings into question whether the Appalachian accreted terranes contain any remnants of normal mid-ocean ridge type Iapetan crust.

In the Through Hill area of central Newfoundland, mafic to ultramafic complexes, which preserve varying amounts of ophiolite stratigraphy, mark the trace of a major fault zone that outcrops in a roughly elliptical pattern. The major, northeasttrending axis is about 60 km and the shorter axis is about 30 km in length. The most complete ophiolite stratigraphy is preserved in the Coy Pond and Pipestone Pond complexes, which have steep dips and face east and west, respectively, outwards from the centre of the ellipse.The ophiolitic rocks are bounded on the outside by rocks typical of the Dunnage Zone of central Newfoundland, principally Lower to Middle Ordovician volcanic and volcaniclastic rocks interpreted to be of island-arc affinity (Davidsville, Victoria Lake, and Baie d'Espoir groups, Cold Spring Pond Formation) and Upper Ordovician to Silurian clastic sediments (Botwood Group). The volcano-sedimentary sequences are interpreted to have been deposited on the ophiolite conformably, but the contacts are not exposed. Both ophiolite and volcano-sedimentary sequences have similar structural and metamorphic histories, exhibiting one principal deformation and the formation of folds with subhorizontal axes, local development of secondgeneration folds and associated cleavage, and an intervening period of metamorphism in the greenschist facies.The elliptical area enclosed by the ophiolite belt is referred to as the Mount Cormack Terrane, and is underlain by variably metamorphosed shale, quartz-rich sandstone, quartzo-feldspathic to amphibolitic gneisses, and granite. A limestone occurrence contains shelly fossils of Early to Middle Ordovician age. An early deformation formed folds with steep axes, and subsequent metamorphism resulted in a progression from greenschist facies to upper amphibolite facies, with the production of migmatite and granodiorite.The preferred interpretation of the geology is that the elliptical Mount Cormack Terrane is exposed as a window through an overlying allochthon composed of ophiolitic and volcano-sedimentary rocks of the Dunnage Zone. The emplacement of the allochthon probably postdated deposition of the Silurian Botwood Group. Paleontologic, lithologic, and structural considerations suggest that the sediments of the Mount Cormack Terrane were deposited at the eastern margin of Iapetus and are perhaps correlatives of rocks exposed in the Gander Zone. This implies that the Dunnage Zone has been thrust, probably in an eastwards direction, on a scale comparable with the allochthons mapped in the Scandinavian Caledonides and proposed for the Appalachians of Quebec and the United States. Dans la region de Through Hill du centre de Terre-Neuve, les complexes mafiques et ultramafiques, dans lesquels sont consemees des sequences ophiolitiques en diverses proportions, marquent la trace d'une zone de faible majeure qui affleure avec un motif plus ou moins elliptique. Le grand axe est orient6 nord-est et s'allonge sur 60 km et le petit axe a une longueur d'environ 30 km. La coupe stratigraphique la plu...

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Late Cambrian to Early Ordovician volcanic and hypabyssal rocks in the NotreDame subzone, Newfoundland Appalachians, are generally considered to have formed in a series of island arcs and back arc basins on the Laurentian side of Iapetus. Our compilation of new geochemical and Nd isotopic data provides a more detailed look at the nature of magmatism in these environments than has heretofore been possible. Nd isotopic data constrain the nature of the magma sources for the various volcanic episodes and, in particular, allow us to track the early Paleozoic interaction of mantle and continental lithosphere at the Laurentian margin. New and published geochronological data permit us to assign a time sequence to the various events that we recognize.Geochemical data show that volcanic rocks in the Notre Dame subzone can be divided into several groups, most of which have well-documented analogs in modern plate environments. In nonarc environments, tholeiitic rocks with characteristics of N-MORB (normal mid-ocean basalt ridge) and E-MORB (enriched-MORB) are present, as are tholeiitic and alkalic basalts with characteristics of oceanic island basalts. The geochemical and isotopic data indicate derivation from time-integrated depleted mantle sources. We interpret these to have formed in ensimatic back arc basins.Arc volcanic rocks include island arc tholeiites, transitional island arc tholeiites, calc-alkalic basalt and/or andesite, boninites, and felsic volcanics. Geochemical and Nd isotopic data indicate that the tholeiitic rocks were derived from time integrated LREE-(light rare earth element-) depleted mantle sources contaminated by subducted, hydrous oceanic lithospheric and continentally derived sediments. Calc-alkalic rocks and associated felsic volcanic rocks have Nd isotopic compositions that indicate the direct involvement of continental lithosphere in their petrogenesis.Integration of geochemical, geological, and geochronological evidence suggests the following sequence of events: (1

The Newfoundland Central mobile belt records the Early Paleozoic birth, development, and destruction of the Iapetus ocean. Following opening of the ocean in the Cambrian, a compressional plate margin developed in the Early Ordovician and an island arc was formed above an east-dipping subduction zone. Volcanism was active at this plate margin from late Tremadocian to early Caradocian, at which time subduction ceased and the products of the volcanism were blanketed by argillite and flysch.The pre-Caradocian island-arc volcanic sequences of the Central mobile belt are host to numerous volcanogenic massive sulfide occurrences. Both the volcanic rocks and the associated massive sulfide occurrences show distinct contrasts across the volcanic belt. In the northern part of the belt (Notre Dame Bay area), submarine, dominantly mafic, volcanic rocks host pyritic Cu-Zn deposits associated with local felsic domes. These sequences pass southward into the Victoria River area, where volcanic rocks are characterized by laterally extensive felsic pyroelastic sheets which host polymetallic Zn-Pb-Cu deposits. In southern Newfoundland (the Hermitage flexure area), partially emergent, preponderantly felsic volcanic rocks host Pb-Zn-Ag massive sulfide mineralization formed during the waning stages of volcanism.Lead isotope ratios in massive sulfide deposits in the Hermitage flexure area are consistently more radiogenic than in deposits hosted by more mafic volcanic sequences to the north. This, coupled with geologic evidence, suggests that these sequences formed under the influence of sialic crustal material. It is inferred that the Early-Middle Ordovician island-arc volcanism in the Newfoundland Central mobile belt occurred in both oceanic and continental margin settings. The compositions and geologic environments of the associated massive sulfide deposits reflect these variations in tectonostratigraphic settings.Time and space relationships within the volcanic arc during the interval of active volcanism are not entirely clear. All Early to Middle Ordovician volcanic rocks in central Newfoundland may have formed from contemporaneous volcanic activity. If this is the case, simultaneous volcanism must have occurred both in oceanic environments and at or near the continental margin. Alternatively, the volcanism may have been slightly diachronous with an early episode of ensimatic mafic volcanism followed by migration of the volcanic centers toward the continental margin.The massive sulfide deposits of the pre-Caradocian island arc e. xhibit lead isotope ratios which are distinctly more radiogenic than those in the Buchans, Roberts Arm, and postCaradocian Cutwell Groups. This appears to support geologically based inferences that the latter sequences are not part of the main phase of island-arc activity in central Newfoundland.