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The northwestern Avalon Terrane, Newfoundland, is underlain by Neoproterozoic rocks traditionally divided into older Love Cove Group, medial Connecting Point Group, and the unconformably overlying Musgravetown Group. New lithogeochemical, isotopic, and updated U-Pb (zircon) age data demand changes to stratigraphic nomenclature and maps and help constrain the tectonomagmatic evolution. U-Pb age constraints include 620 [+ or -] 2 Ma for the calc-alkaline Broad Island Group (former Love Cove Group); 605 [+ or -] 1.2 Ma for rhyolite from near Bull Arm (type area, Bull Arm Formation, lower Musgravetown Group); 589 [+ or -] 2.0 Ma for a schist from the Love Cove type locality; 568.7 [+ or -] 1.4 Ma for rhyolite of the Rocky Harbour Formation, upper Musgravetown Group; and 572 [+ or -] 2 Ma for Louil Hills granite. In light of these results, remnants of the main Avalonian arc are re-designated 'Broad Island Group', for the site of the dated 620 Ma tuff. The ca. 589 Ma schist from Love Cove is included in the Musgravetown Group and may be a tectonized equivalent of Bull Arm Formation. Our data outline a tectonomagmatic change from arc-dominated magmatism at ca. 620 Ma, to an extensional regime ca. 605589 Ma, culminating in alkaline magmatism by ca. 572 Ma. eNdt values (t = 620-569 Ma) for felsic rocks range from 3.7 to 5.6 and yield [T.sub.DM] ages consistent with derivation from a juvenile Neoproterozoic (878-730 Ma) basement. Maficrocks exhibit a time-progressive increase in [[epsilon]Nd.sub.t], indicating more juvenile mantle sources through time. Further delimitation of map units of the area await integrated lithostratigraphy, precise modern U-Pb geochronology, and lithogeochemistry. Key words: Avalonia, Neoproterozoic, geochronology, Sm-Nd isotope chemistry, lithogeochemistry, felsic and maficmagmatism. Le nord-ouest du terrane d'Avalon (Terre-Neuve) est sous-tendu par des roches neoproterozoiques traditionnellement divisees en trois groupes, soit leGroupe de LoveCove, plus ancien, leGroupe intermediaire de Connecting Point et, reposant en discordance, le groupe de Musgravetown au-dessus. De nouvelles donnees lithogeochimiques et isotopiques et des ages U-Pb sur zircons a jour appellent des modifications a la nomenclature stratigraphique et aux cartes et permettent de mieux definir l'evolution tectonomagmatique. Les ages U-Pb comprennent des ages de 620 [+ or -] 2 Ma pour le Groupecalcoalcalin de Broad Island (anciennement le Groupe de Love Cove), de 605 [+ or -] 1, 2 Ma pour une rhyolite des environs de Bull Arm (region type, Formation de Bull Arm, Groupe de Musgravetown inferieur), de 589 [+ or -] 2, 0 Ma pour un schiste de la localitetype de Love Cove, de 568, 7 [+ or -] 1, 4 Ma pour une rhyolite de la Formation de Rocky Harbour, du Groupe de Musgravetown superieur, et de 572 [+ or -] 2 Ma pour le granite de Louil Hills. A la lumiere de ces resultats, les restes de l'arc avalonien principal sont renommes[much less than] Groupe de Broad Island [much greater than], pour la localite du tuf de 620 Ma. Le schiste d'environ 589 Ma de Love Cove est inclus dans le Groupe de Musgravetown et pourrait etre un equivalent tectonise de la Formation de Bull Arm. Nos donnees font ressortir une transition tectonomagmatique qui passe d'un magmatisme a dominance d'arc vers 620 Ma a un regime d'extension vers 605-589 Ma, culminant en un magmatisme alcalin vers 572 Ma. Les valeurs de [[epsilon]Nd.sub.t] (t = 620-569 Ma) pour les roches felsiques vont de 3, 7 a 5, 6 et donnent des ages TDM qui concordent avec l'interpretation que ces roches seraient derivees d'un socle neoproterozoique juvenile (878-730 Ma). Les roches mafiques presentent une augmentation au fil du temps des valeurs de [[epsilon]Nd.sub.t], ce qui indique des sources mantelliques de plus en plus juveniles. Une meilleure delimitation des unites cartographiques de la region necessitera l'integration de leur lithostratigraphie, de leur geochronologie U-Pb precise et moderne et de leur lithogeochimie. [Traduit par la Redaction] Mots-cles : Avalonie, Neoproterozoique, geochronologie, chimie des isotopes de Sm-Nd, lithogeochimie, magmatisme felsique et mafique., Introduction The Avalon Terrane in Newfoundland is the type locality for the microcontinent Avalonia, one of the peri-Gondwanan terranes accreted to the Laurentian margin by the latest Silurian as the [...]

New U-Pb zircon ages from volcanic, plutonic, and sedimentary units in the Avalonian Caledonia terrane of southern New Brunswick provide better timing constraints in this geologically complex area. Previous ca. 620 Ma ages from the Broad River Group are now corroborated by additional dates from felsic tuff in the Gordon Falls Formation and rhyolite in the former Fairfield (now East Branch Black River) Formation of 620 [+ or -] 5 Ma and 622 [+ or -] 1.9 Ma, respectively. Combined with ages ranging from ca. 625 Ma to 615 Ma from cross-cutting plutons, the data suggest that the minimum age of the Broad River Group is about 615 Ma. A quartz-feldspar porphyry dyke in mafic volcanic rocks of the previously undated Long Beach Formation yielded an igneous crystallization age of 685 [+ or -] 10 Ma, the oldest unit yet dated in the Caledonia terrane but similar in age to porphyry in the Stirling belt in the Avalonian Mira terrane of Nova Scotia. The age of the Coldbrook Group was constrained previously by U-Pb (zircon) ages of volcanic rocks between 560 and 550 Ma as well as by similar ages from comagmatic plutons. Five additional samples from both volcanic and plutonic units lie in the same range of 560-550 Ma, including errors, demonstrating that the Coldbrook Group and related plutons formed in less than 10 million years. Such a large volume of mainly felsic magma erupted and emplaced in a short time span suggests a 'supereruption/supervolcano' environment such as the late Cenozoic southwestern USA but not yet recognized at ca. 560-550 Ma elsewhere in Avalonia. Two units yielded Paleozoic ages: felsite of the Bloomsbury Mountain Formation with a zircon population at 427 [+ or -] 9 Ma, indicating a Silurian maximum emplacement age, and dacite of the Grassy Lake Formation with several zircon grains at 382.8 [+ or -] 8.3 Ma, indicating a maximum age of middle Devonian, the first rocks of this age to be identified in the Caledonia terrane. De nouveaux ages U - Pb sur zircon provenant d'unites volcaniques, plutoniques et sedimentaires dans le terrane avalonien Caledonie du sud du Nouveau-Brunswick fournissent de meilleures contraintes temporelles dans cette region geologiquement complexe. Les ages d'environ 620 Ma precedemment obtenus du Groupe de Broad River sont maintenant appuyes par des dates supplementaires provenant du tuf felsique dans la Formation de Gordon Falls et de la rhyolite dans l'ancienne Formation de Fairfield (maintenant East Branch Black River) de 620 [+ or -] 5 Ma et 622 [+ or -] 1,9 Ma, respectivement. Combine avec des ages allant de 625 Ma a 615 Ma a partir de plutons encaisses, les donnees suggerent que l'age minimum du Broad River Group est d'environ 615 Ma. Un dyke de porphyre de quartz-feldspath dans des roches volcaniques mafiques de la Formation de Long Beach non datee a donne un age de cristallisation ignee de 685 [+ or -] 10 Ma, la plus ancienne unite encore datee dans le terrane de Caledonie mais similaire en age au porphyre dans la ceinture de Stirling dans le terrane avalonien Mira de la Nouvelle-Ecosse. L'age du Groupe de Coldbrook etait auparavant limite par les ages U - Pb (zircon) des roches volcaniques entre 560 et 550 Ma, ainsi que par les ages similaires des plutons co-magmatiques. Cinq echantillons supplementaires provenant des unites volcaniques et plutoniques se situent dans la meme periode de 560 a 550 Ma, y compris les erreurs, demontrant que le Groupe de Coldbrook et les plutons associes se sont formes en moins de 10 millions d'annees. Un volume aussi important de magma principalement felsique ayant entre en eruption et mis en place dans un court laps de temps suggere un environnement de 'super-eruption / supervolcan' tel que le sud-ouest du Cenozoique superieur aux etats-Unis, mais pas encore reconnu entre 560-550 Ma ailleurs dans la zone d'Avalonia. Deux unites ont donne des ages paleozoiques: la felsite de la Formation de Bloomsbury Mountain avec une population de zircon a 427 [+ or -] 9 Ma, indiquant un age de mise en place maximal du Silurien, et la dacite de la Formation de Grassy Lake avec plusieurs grains de zircon a 382,8 [+ or -] 8,3 Ma, indiquant un age maximal du Devonien moyen, premieres roches de cet age identifiees dans le terrane de Caledonie. [Traduit par la redaction], INTRODUCTION The Caledonia terrane is a large area of mainly Neoproterozoic rocks that forms the Avalonian part of southern New Brunswick, extending from the Saint John area to the northeast [...]

The Horse Cove Complex (HCC) is a recently recognized late Ediacaran swarm of mafic-to-felsic dykes that occurs in the Newfoundland Avalon Zone. It is focused along the extrapolated trace of the Topsail Fault along the east coast of Conception Bay and is hosted by mafic submarine volcanic rocks, diorite, and, locally, granodiorite. The HCC has been divided into rock units that represent dykes of mafic-to-felsic composition, hosted by feldspar porphyry, gabbro, and diorite. The age of magmatism in the complex has been bracketed by U-Pb zircon ages, determined by chemical abrasion--thermal ionization mass spectrometry (CA-TIMS), of feldspar porphyry (581 ±2.0 Ma) and an andesitic dyke (578 ±2.3 Ma). Magmatism occurred over a period of 6.5 Ma (or less) and overlaps in age with felsic and mafic volcanic centres and related intrusions on the northeastern Avalon Peninsula. Feldspar porphyry and rhyolitic dykes show arc-related chemistry and may be co-magmatic. The mafic-to-intermediate rocks in the HCC are calc-alkaline to tholeiitic and exhibit a range of compositions, from those that have the characteristics of enriched mid-ocean ridge basalt (E-MORB-like) to those that show subduction-related light rare-earth element (LREE) enrichment and negative Nb anomalies. This range of compositions has ζNd values (at 580 Ma) of +6.4 to +4.1. The HCC likely formed in a back-arc basin environment, in which rocks with LREE-enriched mantle sources and subduction-contaminated sources were emplaced side by side and closely in time. The complex may represent the last phase of subduction-related magmatism in the eastern Avalon Zone in Newfoundland prior to deep marine, deltaic, and alluvial fan sedimentation during the late Neoproterozoic. Le complexe de Horse Cove a ete recemment reconnu comme un essaim de dykes, mafiques a felsiques, datant de l'Ediacarien, situe dans la zone Avalon de Terre-Neuve. Il est localise le long de la trace extrapolee de la faille Topsail le long de la cote est de la baie Conception et il est encaisse dans des roches meres volcaniques sous-marines mafiques, des diorites et, par endroits, des granodiorites. Le complexe de Horse Cove a ete subdivise en unites rocheuses representant des dykes de composition mafique a felsique, encaisses dans des feldspaths porphyriques, des gabbros et des diorites. L'age du magmatisme dans le complexe a ete delimite par des ages determines sur des zircons par U-Pb, CA-TIMS [abrasion chimique--spectrometrie demasse a thermoionisation] dans des feldspaths porphyriques (581 ± 2,0Ma) et un dyke d'andesite (578 ± 2,3Ma). Lemagmatisme a eu lieu sur une periode de 6,5 Ma (ou moins) et chevauche, dans le temps, des centres volcaniques felsiques et mafiques ainsi que des intrusions connexes du nord- est de la peninsule d'Avalon. Les dykes de feldspath porphyrique et de rhyolite presentent une chimie reliee aux arcs; ils pourraient etre comagmatiques. Les roches, mafiques a intermediaires, dans le complexe de Horse Cove sont calcoalcalines a tholeiitiques et presentent une gamme de compositions, depuis celles du type E-MORB (basaltes de rides medio-oceaniques enrichis) a celles qui presentent un enrichissement en elements de terres rares legeres reliees a une subduction ainsi que des anomalies Nb negatives. Cette gamme de compositions a des valeurs 3Nd (a 580 Ma) de +6,4 a +4,1. Le complexe de Horse Cove s'est probablement forme dans un environnement de bassin d'arriere-arc, dans lequel des roches ayant des sourcesmanteliques enrichies en elements de terres rares legeres et de sources contaminees par la subduction ont etemises en place cote a cote presque en meme temps. Le complexe pourrait representer la derniere phase d'un magmatisme relie a la subduction dans l'est de la zone d'Avalon a Terre-Neuve avant la sedimentation enmer profonde, deltaique et de cone alluvial au cours du Neoproterozoique tardif. [Traduit par la Redaction], Introduction The newly recognized and informally named Horse Cove Complex (HCC; Sparkes 2006) is a map unit in the Avalon Zone of Newfoundland. It occurs over a distance of approximately [...]

Fieldwork coupled with U--Pb dating has clarified the relationships between the Exploits and Gander-derived rocks in central Newfoundland during the Mid-Ordovician opening of the Exploits back-arc basin, reinforcing correlation across the peri-Gondwanan segments of the Appalachian-Caledonian orogen. We have separated a new back-arc-related unit, the Red Cross Group, from the arc-related Victoria Lake Supergroup. This group includes 466 [+ or -] 3 Ma felsic volcanic rock interlayered with alkali basalt-enriched mid-ocean ridge basalt, 457 [+ or -] 6 Ma gabbro, Llanvirn-Caradoc normal mid-ocean ridge basalt-island arc tholeite, limestone, black shale and melange. Locally the Red Cross Group is in parautochthonous contact with low-grade Gander psammite. This contact was overrun by the Meelpaeg Metamorphic Nappe during the Devonian. The nappe contains Gander metapsammite (Meelpaeg Subzone) and the coticule-bearing Howley Waters Complex (Exploits), which hosts 467 [+ or -] 3 Ma felsic porphyry. The Gander and Exploits metasediments in the nappe are stitched by Arenig-Llanvirn granite (467 [+ or -] 6 Ma and 468 +3.5/-3 Ma) and Caradoc intrusions (458 [+ or -] 3 Ma and 454 [+ or -] 2 Ma). Metamorphic monazite (462 [+ or -] 1 Ma) and titanite (460 [+ or -] 3 Ma) from the nearby Mount Cormack Subzone (Gander) indicate the coeval formation of a low-pressure-high-temperature metamorphic dome. The widespread presence of Caradoc magmatism suggests that the width of the back-arc basin remained relatively narrow.

The Ordovician Exploits Group in the Central Mobile Belt of Newfoundland were composed of rocks from the late Tremadoc to early Caradoc. The volcanic and sedimentary rocks of the Exploits Group are also composed of Arenig and Llanvirn igneous rocks that are located in the northwesternmost part of the Exploits Subzone. Furthermore, the rocks were generated in ensimatic environments and they did not interact with the continental lithosphere during their formation.

U/Pb and Ar/Ar isotopic age data from the Corner Brook Lake region of the eastern Appalachian Humber zone in western Newfoundland indicate that regional deformation and peak amphibolite-facies metamorphism are Early Silurian. A lower limit on deformation is provided by a U/Pb zircon age of 434 +2/-3 Ma for a pegmatite that is affected by the regional foliation and is interpreted to be syntectonic. Monazite and rutile from a garnet-kyanite-staurolite schist, which records peak-metamorphic conditions and in which porphyroblasts have overgrown the regional foliation, gave U/Pb ages of 430 + or - 2 Ma and 437 + or - 6 Ma, respectively. Ar/Ar cooling ages for hornblende from amphibolites and muscovite from psammitic and pelitic schists range from 430 to 420 Ma. A Silurian age for deformation and metamorphism of the Laurentian margin is coincident with the timing of similar events along the Newfoundland Gondwana margin and suggests that the Silurian was a period of major continent-continent collision.

U/Pb zircon ages of five felsic intrusions in eastern Notre Dame Bay bracket the absolute ages of a number of key units and events in the Newfoundland Appalachians. The age of 507 +3/-2 Ma obtained for the Twillingate pluton confirms an age obtained by earlier workers. It strengthens evidence of widespread Cambrian arc volcanism in Newfoundland and places a lower limit on the initiation of deformation in central Newfoundland. The Loon Bay suite, which includes the Loon Bay batholith and a swarm of quartz- and feldspar-phyric tonalite dikes, has an age of 408 [+ or -] 2 Ma. This age places a Late Silurian upper limit on the timing of thrust fauting and three generations of folding in the New World Island area of Notre Dame Bay. This age also constrains the timing of sinistral ductile movement on the Reach fault and of initiation of high-angle brittle faulting in the region. An age of 422 [+ or -] 2 Ma for a composite mafic and felsic dike on the Port Albert Peninsula places an upper limit on the age of the Botwood Group in that area. This age also supports the correlation between the Botwood Group and mixed felsic and mafic igneous rocks of the Springdale Group in western Notre Dame Bay. Placed in context with the timing of other units and events in the Newfoundland Appalachians, our ages show that the formation and deformation of tectonic terranes making up the orogen did not take place in an orderly sequential fashion. Instead, at any one time, more than one tectonic element was being generated, subducted, or deformed. Although the locus of strain varied within the orogen, deformation occurred continuously from the Late Cambrian Epoch until the end of the Paleozoic Era.

Late Neoproterozoic to Early Cambrian geologic, geochronologic, and paleomagnetic data from along the Iapetus margin of Laurentia may be reconciled within a multistage rift history that involved an initial separation of Laurentia from the west Gondwana cratons ca. 570 Ma, followed by rifting of a further block or blocks from Laurentia ca. 540-535 Ma into an already open Iapetus Ocean to establish the main passive-margin sequence in the Appalachians. Paleomagnetic data suggest that Laurentia rifted from Amazonia--Rio de la Plata cratons and began its northward movement ca. 570 Ma to produce a wide Iapetus Ocean by 550 Ma. Geologic data from the Newfoundland segment of the Laurentian margin provide evidence for a rift-drift transition ca. 540-535 Ma, as constrained by the youngest rift-related magmatism at 550.5 +3/-2 Ma (U/Pb zircon) for the Skinner Cove Formation and 555 +3/-5 Ma for the Lady Slipper pluton, and a late Early Cambrian age of ca. 525-520 Ma for the oldest drift-related sedimentation. Rifting between the Laurentia and the west Gondwana cratons was probably distributed among multiple rift systems that fostered the production of a number of terranes (such as the Argentine Precordillera, Oaxacan) as well as the Iapetus Ocean. Development of Laurentian-derived Iapetan terranes during the final breakout of Laurentia from Rodinia may have been facilitated by preexisting 760-700 Ma rift weaknesses and apparently rapidly changing plate vectors during latest Neoproterozoic time. Keywords: Appalachian-Caledonian, paleogeography, paleomagnetism, Precordillera, tectonics, U-Pb.