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Volcanic rocks of the Ediacaran Musgravetown Group on Bonavista Peninsula, Avalon terrane, Newfoundland, include basal ca. 600 Ma calc-alkaline basalt succeeded by continental tholeiite and alkaline rhyolite of the ca. 592 Ma Plate Cove volcanic belt (Bull Arm Formation), indicating a change from subduction-related to extensionrelated tectonic regimes during that interval. Alkalic basalts on northeastern (Dam Pond area) and southwestern (British Harbour area) Bonavista Peninsula occur below and above, respectively, the ca. 580 Ma glacial Trinity facies. Dam Pond basalt occurs in a structural dome intercalated with and flanked by fine-grained, siliciclastic deposits (Big Head Formation) overlain by Trinity facies. The British Harbour basalt occurs above the Trinity facies, in an upward- coarsening sandstone sequence (Rocky Harbour Formation) overlain by red beds of the Crown Hill Formation (uppermost Musgravetown Group). The Rocky Harbour and Big Head formations are likely stratigraphically interfingered proximal and distal deposits, respectively, derived from erosion of the Bull Arm Formation and older Avalonian assemblages.The Big Head basalts have lower SiO2, Zr, FeOT, P2O5, TiO2 and higher Mg#, Cr, V, Co and Ni contents, and are therefore more primitive than the more FeOT-, TiO2-, and P2O5-rich British Harbour basalts. Large-ionlithophile and rare-earth-element concentrations and ratios indicate that both suites originated from low degree partial melts of deep, weakly garnet-bearing, undepleted asthenospheric peridotite sources, with magma conduits likely focused along regional extensional faults. The protracted and episodic extension-related volcanic activity is consistent with a geodynamic setting that evolved from a mature arc into extensional basins with slowly waning magmatism, possibly involving slab rollback and delamination followed by magmatic underplating. The duration and variation of both volcanism and sedimentation indicate that the Musgravetown Group should be elevated to a Supergroup in order to facilitate future correlation of its constituent parts with other Avalonian basins., Les roches volcaniques du groupe d’Ediacara de Musgravetown sur la péninsule de Bonavista, terrane d’Avalon, à Terre-Neuve, comprennent des basaltes calcoalcalins basaux d’environ 600 Ma auxquels ont succédé de la tholéiite continentale et de la rhyolite alcaline de la ceinture volcanique d’environ 592 Ma de Plate Cove (Formation de Bull Arm), ce qui révèle un passage d’un régime tectonique de subduction à un régime d’extension durant cet intervalle. Les basaltes alcalins dans le nord-est (secteur de Dam Pond) et le sud-ouest (secteur de British Harbour) de la péninsule de Bonavista se manifestent au- dessous et au-dessus, respectivement, du faciès glaciaire d’environ 580 Ma de Trinity. Le basalte de Dam Pond apparaît dans un dôme interstratifié et flanqué de dépôts silicoclastiques à grains fins (Formation de Big Head) recouverts du faciès de Trinity. Les basaltes de British Harbour se trouvent au-dessus du faciès de Trinity, dans une séquence de grès à granulométrie croissante vers le haut (Formation de Rocky Harbour) recouverte de couches rouges de la Formation de Crown Hill (partie sommitale du groupe de Musgravetown). Les Formations de Rocky Harbour et de Big Head sont vraisemblablement stratigraphiquement interdigitées de dépôts proximaux et distaux, respectivement, provenant de l’érosion de la Formation de Bull Arm et d’assemblages avaloniens plus anciens.Les basaltes de Big Head ont des teneurs inférieures de SiO2, de Zr, de FeOT, de P2O5 et de TiO2, et des teneurs supérieures de Mg#, de Cr, de V, de Co et de Ni; ils sont en conséquence plus primitifs que les basaltes plus riches en FeOT, en TiO2 et en P2O5 de British Harbour. Les ratios et les concentrations d’éléments lithophiles à grand rayon ionique et de métaux des terres rares signalent que les deux suites proviennent de fontes partielles de faible degré de sources de péridotite asthénosphérique non appauvrie, faiblement grenatifère, profondes, avec des conduits magmatiques probablement concentrés le long de failles d’extension régionales. L’activité volcanique apparentée à l’extension, prolongée et épisodique, correspond à un milieu géodynamique ayant évolué d’un arc à maturité en bassins d’extension au magmatisme s’affaiblissant lentement, comportant possiblement un repositionnement de dalles et une délamination suivis d’un sous-placage magmatique. La durée et la variation du volcanisme et de la sédimentation révèlent que le groupe de Musgravetown devrait être reclassé en un supergroupe pour faciliter la corrélation future de ses composantes avec d’autres bassins avaloniens.

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 ± 2 Ma for the calc-alkaline Broad Island Group (former Love Cove Group); 605 ± 1.2 Ma for rhyolite from near Bull Arm (type area, Bull Arm Formation, lower Musgravetown Group); 589 ± 2.0 Ma for a schist from the Love Cove type locality; 568.7 ± 1.4 Ma for rhyolite of the Rocky Harbour Formation, upper Musgravetown Group; and 572 ± 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. 605–589 Ma, culminating in alkaline magmatism by ca. 572 Ma. εNdtvalues (t = 620–569 Ma) for felsic rocks range from 3.7 to 5.6 and yield TDMages consistent with derivation from a juvenile Neoproterozoic (878–730 Ma) basement. Mafic rocks exhibit a time-progressive increase in εNdt, 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.

The snowball Earth hypothesis predicts that low-latitude glaciation lasted millions of years while CO2 built up to critical levels to culminate in catastrophic deglaciation in a supergreenhouse climate. The Gaskiers Formation of eastern Newfoundland (Canada) has been attributed to a snowball glaciation event, but the lack of robust paleomagnetic data and precise geochronological constraints has precluded tests of the hypothesis. Here we present high-precision U-Pb zircon geochronology (chemical abrasion–isotope dilution–thermal ionization mass spectrometry) from eight tuffs from multiple distant stratigraphic sections that bracket glacial diamictites and the first appearance of large Ediacaran fossils. Including internal error, deposition of the Gaskiers diamictite on the Avalon Peninsula is constrained to have been between 580.90 ± 0.40 and 579.88 ± 0.44 Ma, and the Trinity diamictite on Bonavista Peninsula was deposited between 579.63 ± 0.15 and 579.24 ± 0.17 Ma. Assuming approximately synchronous deglaciation, these results imply a maximum duration for deposition of the Trinity diamictite of ≤340 k.y.; this is inconsistent with the multimillion year duration predicted by the snowball Earth hypothesis. Our geochronologic data also constrain the first appearance datum of Ediacaran fossils to