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.

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The Topsails igneous complex of western Newfoundland is a large (>6000 km2) intrusive complex of two contrasting geochemical suites and at least four distinct granitoid facies: (,i) a fineto medium-grained biotite-bearing peraluminous granite; (ii) a medium-to coarse-gained metaluminous biotite-hastingsite granite; (iii) a coarse-grained peralkaline aegi~e-arfvedsonite granite; and (iv) fine-to medium-grained biotite-hornblende granites and syenites. Pecalkaline microgranite dykes cut the other granitoid phases and the country rocks. Peralkaline granites occur within the central parts of the complex, the other granitoid phases being marginal. Volcanic and subvolcanic roof pendants are divisible into peralkaline and non-peralkaline groujps. The former are clearly related to the peralkaline granite, and the latter appear to represent the other intrusive phases of the complex. Contemporaneous basalt magmatism is indicated by dykes which cut the biotite granites and are in turn cut by the peralkaline granite. Available Rb--Sr datashow ages of 386 f 9-and 419 f 5 Ma for the peralumin6us and peralkaline granites respectively.The main difference in maior elements between the mralkaline and non-peralkaline grimites is in their agpaiitic indices, alrhough there is some overlap between them.-here are,however, marked dissimilarities in their respective trace element characteristics, e.g., KIRb, K/Zr, and Rb/Sr, with high Zn concentrations being especially characteristic of the peralkaline roclrs.The origin of these rocks is interpreted in terms of a model involving crustal melting due lo the interaction of crustal extension, basaltic intrusion, and volatile fluxing, with subsequent highlevel intrusion and fractionation, along with metasomatism by magmatically-derived fluitis.Le complexe igne de Topsails dans l'ouest de Terre-Neuve est un complexe intrusif de grandes dimensions (> 6000 kmz) compose de deux suites geochimiques contrastees et d'au moinls quatre faciks granitoi'des distincts: ( i ) un granite peralumineux a biotite et de granulometri~e fine moyenne, (ill un granite metalumineux a biotite-hastingsite de grain moyen a grossier., (iii) un granite peralcalin a aegirine-arfvedsonite de grain grossier, et (iv) des granites et sy16nites a biotite-hornblende de granulometrie fine a moyenne. Les dykes de microgranite peralcalin coupent les autres phases granitoides et les roches encaissantes avoisinantes. Les ;granites peralcalins se retrouvent dans les parties centrales du complexe alors que les autres phases granitoides se situent en bordure. Des lambeaux de toit de nature volcanique asub-volcsuuque se divisent en groupes peralcalins et non peralcalins. Les premiers s'apparentent clairernent au granite peralcalin et les derniers semblent representer les autres phases intrusives du co~rtlplexe. Le magmatisme basaltique contemporain se manifeste par les dykes qui recoupent les granites a biotite et ils sont a leur tour recoup& par le granite peralcalin. Les donnees Rb-Sr disponibles donnent des kges de 386 + 9 et de 419...

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