Antigonish Highlands Research Articles

Reply: Upper Lower Cambrian depositional sequence in Avalonian New Brunswick

I thank the discusser (L.R. Fyffe). His comments illustrate an approach to Avalonian stratigraphy that has had important consequences for understanding the history of the cover sequences deposited on the Late Proterozoic Avalonian orogen. In particular, this approach has led to local stratigraphic nomenclatures that are arbitrarily limited by political boundaries, obscure the lateral continuity of formationand member-level units and bounding surfaces, and confound an understanding of Avalonian epeirogeny . The reason for these problems is that lithostratigraphic units were not considered to extend beyond single provinces, states, or even counties (as in Massachusetts). I refer the discusser to an areally more comprehensive study of Avalonian stratigraphy (Landing 1996a), which demonstrates that most uppermost Precambrian lower Ordovician units extend on the Avalonian marginal and inner platforms from Rhode Island to England. Condensed limestone intervals, macroscale shoaling cycles, and genetically related unconformities (i .e., sequence boundaries) and onlap deposits can be traced for over 2500 km. Landing (1996a) showed how a unified nomenclature for the Avalonian lower Paleozoic developed, with recognition of lithostratigraphic synonymies in the Avalon and Burin peninsulas, east Newfoundland, and later comparison with the sequences of Cape Breton Island, the northern Antigonish Highlands of mainland Nova Scotia, and east Massachusetts. New Brunswick was not included in this detailed synthesis until now. Standard procedures of the North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature, 1983) were followed to develop a unified stratigraphic nomenclature in Landing (1996a) in the paper under discussion. This has allowed establishment of appropriate formation and group names for lithostratigraphic units on the basis of shared lithic characteristics, stratigraphic position, and genetically related upper and lower boundaries. These features allow the demonstration of synonymy for identical lithosomes that had earlier received different names in separate geographic areas. Biostratigraphy serves to test lateral continuity of rock bodies, but is only part of the synthesis. Abandonment of local stratigraphic names (such as Glen Falls based on synonymy of its lower part with the Random Formation, lateral continuity of its upper part

Geochemical and isotopic characteristics of Early Silurian clastic sequences in Antigonish Highlands, Nova Scotia, Canada: constraints on the accretion of Avalonia in the Appalachian – Caledonlde Orogen

Avalonia is a terrane that accreted to Laurentia–Baltica during the development of the Appalachian–Caledonide Orogen. Interpretations of the timing of accretion have been constrained by comparing faunal affinities, overstep sequences, age and kinematics of inferred accretionary deformational events, and controversial paleomagnetic data. We show that the time of accretion of Avalonia may also be constrained by contrasts in the geochemical and isotopic signatures of its igneous rocks (which reflect the characteristics of the underlying continental basement and mantle) and sedimentary rocks (which reflect provenance). Early Silurian clastic sedimentary rocks of the Beechill Cove Formation, Antigonish Highlands, Nova Scotia, were deposited on Avalonian crust. The formation predominantly consists of approximately 80 m of siltstones and shales deposited in a nearshore environment and derived from the northeast. Their age is constrained by paleontological data and by directly underlying Late Ordovician – Early Silurian bimodal volcanic rocks that have typically Avalonian geochemical signatures. In comparison with typical Avalonian rocks, the Beechill Cove sediments are characterized by high SiO2, Ce/Yb, and initial 87Sr/86Sr, low Fe2O3, MgO, and TiO2, and strongly negative εNd(ur). These characteristics cannot be attributed to erosion of underlying Avalonian basement or coeval volcanic rocks and are consistent with derivation via significant transport from radiogenically enriched continental crust. εNd data are typical of Grenvillian basement compositions and suggest that the Beechill Cove sedimentary rocks were derived from an adjacent landmass with Grenvillian crust. The data, in conjunction with paleocontinental reconstructions and recent geochronological and structural data from the northern Appalachians, suggest that the Caledonide orogenic belt is the most likely source. Deposition of the Beechill Cove Formation is inferred to have occurred in an intracontinental basin associated with strike-slip tectonics during the oblique collision of the Avalon with Laurentia–Baltica.

550–580 Ma magmatism in Cape Breton Island (Nova Scotia, Canada): the product of NW-dipping subduction during the final stage of amalgamation of Gondwana

Cape Breton Island is host to widespread magmatic activity dated between ∼ 550 and 580 Ma. In the south, its products include the Capelin Cove pluton and host Coastal Volcanic Belt; both are composed of bimodal, tholeiitic-calc-alkaline suites that were emplaced on a thin sialic crust in a volcanic arc system. Their trace element and Nd isotope variations are consistent with fractional crystallization accompanied by lower crustal contamination in a compositionally zoned magma chamber. In contrast, the Indian Brook and Kerrs Brook plutons and Price Point volcanic rocks in central Cape Breton Island have higher contents of strongly incompatible trace elements and resemble modern arc-related calc-alkaline rocks emplaced on a moderately thick continental crust. Comparison with available data for other 550–580 Ma igneous rocks across Cape Breton Island indicates that they were produced above a NW-dipping subduction zone. This convergence was synchronous with the final stage of globalscale orogenic activity that culminated with the final amalgamation of Gondwana. Available detrital zircon ages in late Neoproterozoic (∼ 650-545 Ma) sediments in the Antigonish Highlands (Avalon Composite Terrane) suggest derivation from the Amazon craton. It is proposed that the Avalon Composite Terrane formed off northwestern South America during the late Neoproterozoic.

Upper Placentian—Branchian series of mainland Nova Scotia (middle-upper Lower Cambrian): Faunas, paleoenvironments, and stratigraphic revision

Lithostratigraphy and depositional and epeirogenic history of the upper Placentian Series (Cuslett-Fosters Point Formations of the Bonavista Group) and Branchian Series (Brigus Formation) are identical in the northern Antigonish Highlands; Cape Breton Island; and eastern Placentia Bay, southeastern Newfoundland. Preliminary evidence suggests that the lower Middle Cambrian is present in the field area. A unified, uppermost Precambrian–Lower Cambrian, formation- and member-level nomenclature is appropriate to Avalonian North America, and the stratigraphic nomenclature of southeastern Newfoundland is applied in northern mainland Nova Scotia.Latest Placentian shoaling and deposition of a peritidal carbonate lithosome and unconformable onlap of the trilobite-bearing Branchian Series occurred in shallow Avalonian shale basins from eastern Massachusetts to central England.Uppermost Placentian Series faunas are very diverse in the Fosters Point Formation. Limited similarities with the South Australian Lower Cambrian are indicated by the presence ofCamenellasp. cf.C. reticulosa, Conotheca australiensis, andHyptiothecasp., but these forms do not contribute to highly resolved correlation.Twenty-eight taxa are illustrated from the upper Placentian and Branchian Series.Caveacus rectusn. gen. and sp., a phosphatic problematicum, is limited to the upper Placentian Series. The oldest, skeletalized, macrophagous predators are the Pseudoconodontida and the later appearing Protoconodontida (n. orders). The Pseudoconodontida includes the Protohertzinacea n. superfamily and Strictocorniculacea n. superfamily (with the Rhombocorniculidae and Strictocorniculidae n. families).Strictocorniculum vanallerumn. gen. and sp. is described. The tommotiid family Sunnaginiidae emend. includesEccentrotheca, Sunnaginia, Kulparina, andJayceia deltiformisn. gen. and sp.

Geochemistry and tectonic discrimination of Late Proterozoic arc-related volcaniclastic turbidite sequences, Antigonish Highlands, Nova Scotia

The Late Proterozoic (ca. 618–610 Ma) Georgeville Group of northern mainland Nova Scotia lies within the Avalon Composite Terrane and consists of subgreenschist- to greenschist-facies mafic and felsic volcanic rocks overlain by volcaniclastic turbidites that were deposited in an ensialic basin within a rifted volcanic arc. Geochronological data indicate that the volcanic and sedimentary rocks are coeval. The geochemical and isotopic signatures of the sedimentary rocks are attributed to erosion of the coeval Avalonian volcanic rocks that flank the basin and are consistent with synorogenic deposition. There is no evidence of significant chemical contribution from Avalonian basement.Knowledge of the tectonic setting facilitates the testing of published geochemical discriminant diagrams for clastic sedimentary rocks. Discrimination diagrams using ratios such as K2O/Na2O and Al2O3/(CaO + Na2O) give inconclusive results, probably due to elemental mobility during secondary processes. Plots involving MgO, TiO2, and Fe2O3 detect the chemical contribution of mafic detritus, give much tighter clusters of data, and plot between Aleutian- and Cascade-type arc-derived sediments, suggesting a moderate thickness of continental crust beneath the arc.The arc-related signature of the Georgeville sedimentary rocks is clearly recognizable on ternary plots involving inter-element ratios of high field strength elements (e.g., Ti–Y–Zr, Nb–Y–Zr, and Hf–Ta–Th) in which the samples plot as mixing trends between mafic and felsic end members. Diagrams of this type may have widespread application to tectonic discrimination of sedimentary rocks because in most suites these ratios are relatively insensitive to sedimentary and metamorphic processes.

Tectonic significance of Late Paleozoic deformation in the Cape George Peninsula, Antigonish Highlands, Nova Scotia

Late Paleozoic deformation of the Cape George Peninsula, Antigonish Highlands, Nova Scotia, provides information on post-accretionary fault movements associated with waning stages of Appalachian orogenic activity. Anomalously intense brittle to ductile deformation of the low-grade Late Paleozoic rocks of the peninsula occurred along east-west shear zones in a ca. 4 km-wide belt bounded by the NE-trending Hollow and Greendale faults. Deformation adjacent to, and between, these two faults resulted in brecciation, folding and thrusting, the development of slickensides on major dislocation surfaces, the local development of S-C fabrics and stretching lineations defined by elongate pebbles, and/or the production of extensional fractures and veins. The data suggest dextral and subordinate thrust components of movement along the east-west shear zones. Deformation is attributed to dextrally oblique compression between the bounding Hollow and Greendale faults along which significant reverse displacements are proposed on the basis of fault geometry and kinematics. The Cape George Peninsula is interpreted as a "popup" structure between these back-to-back oblique-slip reverse faults and is considered to occupy a strongly transpressive step-over zone between them. The east-west shear zones, which record dextral transpressive motion and steepen towards the north in a positive half-flower structure configuration, are parallel to Reidel R-shears of the shear fracture array and are interpreted to be transfer faults within the step-over zone along which oblique slip with dextral and reverse components of motion was transferred from the Hollow Fault to the Greendale Fault. Development of the regional stress regime required by these fault kinematics is consistent with coeval post-accretionary dextral motion between the Meguma and Avalon composite terranes along the east-west Cobcquid-Chedabucto fault system.
 
 RÉSUMÉ
 La déformation du Paléozoique tandif de la péninsule dc Cap George, dans les hautes terres d'Antigonish en Nouvelle-Écosse, fournit des informations sur les mouvements de failles post-accrétionaires associés aux stades terminaux de l'activité orogénique appalachienne. Une déformation fragile à ductile anormalement intense des roches du Paléozoique supérieur de faible grade s'est produite le long de zones de cisaillement est-ouest dans une ceinture d'environ 4 km de largeur limited par les failles de Hollow et de Greendale d'orientation nord-est. La déformation adjacente et entre ces failles a résulté en de la bélchification, du plissement et du chevauchement, le développement de slickensides sur des surfaces de dislocation majeures, le développement local de fabriques C-S et de linéations d'étirement définies par des cailloux allongés, et/ou la production de fractures et de veines d'extension. Les données suggèrent des composantes de mouvement dextres et, dans une moindre mesure, de chevauchement le long des zones de cisaillement est-ouest. La déformation est attributée à une compression dextre oblique entre les failles limitrophes de Hollow et de Greendale le long desquelles des deplacements inverses importants sont proposés sur la base de la géométrie et de la cinémalique des failles. La péninsule de Cap George est interprét6e comme une structure d'extrusion verticale entre ces deux failles obliques a mouvement inverse, dos à dos, et est considérée comme occupant une zone de recouvrement fortement transpressive entre elles. Les zones de cisaillement est-ouest, qui montrent un mouvement de transpression dextre et deviennent plus abruples vers le nord en une configuration de demie "flower structure" positive, sont parallèles aux riedels synlhétiques du réseau de fractures de cisaillement et sont interprétées comme étant des failles de transfert à l'intérieur de la zone de transfert suivant lesquelles un mouvement oblique avec des composantes de mouvement dextre et inverse furent transferées de la faille de Hollow à la faille de Greendale. Le développement du régime de contrainte régional requis par ces cindmatiques de failles est en accord avec un mouvement post-accrétionaire dextre entre les terrains de Meguma et d'Avalon composite le long du systeme de faille est-ouest de Cobcquid-Chedabucto.
 
 [Traduit par la rédaction]

Uppermost Precambrian(?)–Lower Cambrian of mainland Nova Scotia: faunas, depositional environments, and stratigraphic revision

The uppermost Precambrian(?)–Lower Cambrian of the Avalon Zone in the northern Antigonish Highlands is composed of two dissimilar sequences in thrust contact. These include the sandstones and slates of the Doctor's Brook allochthon and the volcanoclastic-rich Malignant Cove authochthon.Lithostratigraphy of the “Black John Formation” (designation abandoned) in the Doctor's Brook allochthon is comparable to the uppermost Precambrian–Lower Cambrian in eastern Placentia Bay, southeastern Newfoundland, and Cape Breton Island. A unified stratigraphic nomenclature is appropriate in these Avalonian areas. The lower part of the “Black John” is an unconformity-bounded depositional sequence with subaerial rift facies (Rencontre Formation, 178+ m), overlying marine siliciclastic mudstones and fine sandstones (Chapel Island Formation, 59 m), and a quartzite cap (Random Formation, 2.05 m). The Chapel Island Formation has the oldest faunas from mainland Nova Scotia (Watsonella crosbyiZone, lower Placentian Series). A post-Random unconformity known in Newfoundland and Cape Breton Island lies at the Random-Bonavista Group (Cuslett Formation) contact in the upper “Black John Formation.”Fossiliferous Lower Cambrian limestones and siliciclastic mudstones previously reported from the Malignant Cove autochthon are actually clasts in basalt pebble-dominated slope deposits of the Arbuckle Brook Formation. These clasts were eroded from shallow-marine facies comparable to those in the Doctor's Brook allochthon during local uplift associated with Middle Cambrian(?) extension and mafic volcanism.Eight species are illustrated from the Placentian and Branchian Series.AnabaritellusMissarzhevsky, 1974, emend. (=SelindeochreaValkov, 1982) is a Lower Cambrian calcareous tube-dwelling metazoan(?) known from tri- through multisulcate conchs that are morphologically intermediate betweenAnabaritesandColeoloides.

Interpretation of till geochemical data in Nova Scotia, Canada, using mapped till units, multi-element anomaly patterns, and the relationship of till clast geology to matrix geochemistry

The significance of multi-element (Cu, Pb, Zn, Cd, Ag, Sn and W) till anomalies in selected areas of northern mainland Nova Scotia was examined by: (1) comparing local element levels for mapped till types with regional element levels calculated from a combination of all till types; (2) examining the spatial pattern of multi-element anomalies in selected study areas; and, (3) assessing the inter-element associations and element -clast geology associations for each major till type. Large areas of single-element anomalies were outlined using the 75th and 95th percentile contours from the regional data set. Local case studies (Wentworth Station, Scotsburn and Ohio) were selected within these regionally anomalous areas. At Wentworth Station, till matrix geochemistry and clast correlations identify Cobequid Highlands granitic rocks as the host for Sn-W minerals producing till anomalies. Cassiterite has been identified in the Cobequid Highlands Massif. The Su-W till anomalies overlying Carboniferous sedimentary rocks north of the Cobequid Highlands are related to northward glacial dispersal and/or Carboniferous paleoplacer deposition. At Scotsburn the proximity of multi-element base-metal till anomalies to known Pb-Zn and Cu-Ag mineral occurrences and primary metal associations like Pb and Zn suggest that the till anomalies are significant. Within the Ohio study area one group of base-metal till anomalies is related to a known copper belt. A second group of tungsten-base-metal till anomalies may reflect an undiscovered polymetallic mineral occurrence hosted by volcanic rocks of the Antigonish Highlands. Elevated tungsten levels are correlated with felsic volcanic clasts in stony tills of the area.

Tectonic control on the origin and orientation of igneous layering: An example from the Greendale Complex,Antigonish Highlands, Nova Scotia, Canada

Layering in appinites (hornblende-rich gabbros) of the ca. 610 Ma Greendale Complex, Antigonish Highlands, Nova Scotia, is defined by variations in texture and modal abundances of hornblede and plagioclase, and ranges in width from 5 to 50 cm. Regional studies indicate that the complex was probably emplaced during late Precambrian dextral shear on northeast-trending faults by brittle failure at the roof of the magma chameber. The origin and orientation of the layers may have been controlled by the regional tectonic setting. The layering is steep to vertically dipping and varies in strike from 080° to 180° (clockwise) with concentrations at 090° and 160°. The 090° layers are dilational and are thought to have developed perpendicular to north-south extension along the extensional plane of the instantaneous strain ellipsoid associated with progressive dextral shear. The 160° layers commonly display boudinage. They may have rotated clockwise from an original 090° attitude toward the plane of flattening (and hence into a stretching field) during progressive dextral shear. Late-stage cogenetic felsic veins fill conjugate shear fractures consistent with a dextral shear regime. This study emphasizes the potential importance of considering regional tectonic setting when analyzing the origin and orientation of igneous fabrics.

The geochemistry and petrology of the Late Precambrian Georgeville Group: a volcanic arc-rift succession in the Avalon terrane of Nova Scotia

Abstract The Antigonish Highlands of northern mainland Nova Scotia lie within the Avalon terrane of the Canadian Appalachians. They are predominantly underlain by late Precambrian interbedded metasedimentary and metavolcanic rocks of the Georgeville Group. In the southern highlands, subaerial volcanic rocks of the Keppoch Formation consist of bimodal, porphyritic and aphyric mafic and felsic flows. Pophyritic mafic flows display volcanic arc calc-alkalic affinities and are extensively fractionated. The aphyric mafic flows are characterized by strong Fe and Ti enrichment and are probably continental tholeiites derived from a heterogeneous metasomatized upper mantle source. Felsic lavas display volcanic arc affinities and were probably derived by anatexis of the crust. In the northern highlands, mafic flows of the Chisholm Brook Formation are dominated by calc-alkalic basalts and andesites and minor interlayered continental tholeiites. In the central highlands, submarine volcanic rocks of the Clydesdale Formation consist of continental tholeiites and are broadly comparable with those of the Keppoch Formation. The close spatial and temporal association of calc-alkalic and tholeiitic mafic rocks, the felsic rocks with volcanic arc chemistry and the stratigraphy which documents the infilling of a narrow sedimentary basin suggest a rift setting within an ensialic volcanic arc environment. The style of volcanism exhibited in the Antigonish Highlands compares favourably with other late Precambrian successions in Nova Scotia. In general late Precambrian volcanism in northern Nova Scotia is consistent with an ensialic volcanic arc environment with local development of volcanic arc rifts and provides constraints on the evolution of the Avalon terrane as a whole.

Anatomy of a telescoped pull-apart basin: an example from the Cambro-Ordovician rocks in the Antigonish Highlands of Nova Scotia

Cambrian-Lower Ordovician rocks outcrop in the northernmost Antigonish Highlands where they unconformably overlie the Precambrian Georgeville Group. They are divided into two groups that are interpreted to be lateral fades equivalents. The Iron Brook Group is predominantly sedimentary and consists of red clastic sedimentary rocks, pink limestones containing late Early Cambrian fossils, quartzites, calcareous tuffs and ironstones containing Late Cambrian-Ordovician inarticulate brachi-opods. The McDonalds Brook Group consists of red clastic sedimentary rocks at the base overlain by bimodal, alkalic, continental rift volcanic rocks with minor interlayered red clastic rocks and late Early Cambrian fossiliferous pink limestones. These rocks are inferred to have been deposited in a pull-apart basin produced in the overlap zone between two dextral NE-trending faults. Connection between the pull-apart basin and a larger ocean is indicated by cosmopolitan, open-ocean. Early Cambrian trilobites.
 
 Cambro-Ordovician rocks in the central part of the basin were subsequently thrusted (?)northeastwards and then back thrusted. Associated NW-SE folds verge away from a central zone. This complex structure dies out towards the NE and S W ends of the basin, where the lowest Cambrian units are relatively mildly deformed and the basal unconformity with Precambrian rocks is preserved. These structures are inferred to have been produced by sinistral motion on the NE-SW bounding faults with compression concentrated locally in the pull-apart basin. This movement expelled Cambro-Ordovician rocks lying on attenuated continental lithosphere from the basin centre onto die basin margins thereby accounting for the limited extent of the deformation. These structures are truncated by die sub-Silurian unconformity, thereby constraining the time of deformation to the Ordovician. Both the Cambro-Ordovician and Siluro-Devonian were deformed during the Devonian by upright NE-SW folds that are preferentially developed adjacent to the major NE-SW faults. The folds are consistently oriented slightly anticlockwise of the faults thereby indicating renewed dextral movements on the NE-SW faults.
 
 RÉSUMÉ
 Des roches du Cambrien à l'Ordovicien inférieur affleurent dans la région la plus au nord des Monts Antigonish et recouvrent en discordance le Groupe précambrien de Georgeville. Elles constituent deux groupes interprèts comme des équivalents latéraux de faciès. Le Groupe d'Iron Brook est surtout de nature sédimentaire et se compose de roches sédimentaires clastiques rouges, de calcaires roses renfermant des fossiles du début de l'Éocambrien, des quartzites, des tufs calcaires et des roches ferrugineuses contenant des brachiopodes inarticulés tardicambriens à ordoviciens. Le Groupe de McDonalds Brook comprend des roches sédimentaires clastiques rouges en sa base, surmontées par des volcanites bimodales alcalines, de type rift continental et englobant quelquesniveaux à roches clastiques rouges et calcaires roses à fossiles de la finde l'Éocambrien. Ces dépôts se seraient accumulés à l’inférieur d’un basin rhomboédrique sur décrochement installé dans un ilot en distension formé par l’intersection de deux failes dextres orientée NE. La présence de trilobites cosmopolites et franchement marins du début du Cambrien prouve que ce bassin était en large communication avec la haute mer.
 
 Dans la partie centrale du bassin, les roches cambro-ordoviciermes furent par la suite chevauchées (?vers le nord-est) puis rétrochevauchées. S'y associent des plis NO-SE dont la vergence s'éioigne d'une zone centrale. Cette structure complexe s'éstompe vers les extremilés NE et SO du bassin, la ou la déformation des roches cambriennes initiales est rélativement modeste et la discordance basale avec les roches précambriennes est préservée. Ces structures seraient le produit d'un jeu senestre des accidents bordiers NE-SO, conjugué à une concentration locale du serage dans le basin de décrochement. Ce movement expulsa des roches cambro-ordoviciennes, sises sur une lithosphere continentale amincie, du centre vers les abords de la cuvette, ce qui explique leur faible déformation. Le recoupement de ces mêmes structures par la discordance angulaire antésilurienne cantonne la déformation à l’Ordovicien. Tant le Cambro-Ordovicien que le Siluro-Dévonien furent déformes lors du Dévonien par des plis droits NE-SO développés de façon préterentielle au voisinage des failles majeures NE-SO. L'orientation de ces plis s'inscrit, conforménient, selon une faible rotation dans le sens inverse des aiguilles d'une montre par rapport aux failles, ce qui suggère un rejeu dextre des failles NE-SO.
 
 [Traduit par le journal]

Late Precambrian to Late Devonian mafic magmatism in the Antigonish Highlands of Nova Scotia: multistage melting of a hydrated mantle

Five suites of alkalic basalt ranging in age from Late Precambrian to Late Devonian are found in the Antigonish Highlands of Nova Scotia. In contrast, on neighbouring Cape Breton Island, alkalic basalts are rare even in suites that are contemporaneous with those in the Antigonish Highlands. Late Precambrian alkalic basalts in the Antigonish Highlands are genetically associated with calc-alkalic rocks and are probably subduction related, whereas the younger suites are continental, rift related, and within plate. Major and compatible trace-element abundances can be explained by crystal fractionation of olivine ± clinopyroxene ± orthopyroxene ± spinel ± garnet. However, incompatible trace-element concentrations are strongly influenced by mantle metasomatism that occurred prior to, or synchronously with, the oldest alkalic rocks. The metasomatic event enriched the mantle in Fe, Ti, P, Zr, and light rare-earth elements. The trace-element composition of the younger suites is similar to that of the oldest alkalic rocks and may have been strongly influenced by the Late Precambrian metasomatic event. The anomalously low Nb/Y ratio (generally less than 1 in all suites) and application of phase-equilibria studies indicate that the metasomatic fluid was probably rich in H2O. This fluid may have been derived from dehydration of the subducting slab in Late Precambrian time, resulting in metasomatism of the overlying mantle wedge in the Late Precambrian. It is proposed that the younger suites obtained their fluids by dehydration of the previously metasomatized mantle associated with the generation of local pull-apart basins. Thus, the metasomatic fluid was exotic with respect to the oldest basalts but indigenous with respect to the younger basalts. In the younger basalts, the indigenous fluid was probably focussed at the site of melting by structural events (i.e., rifting). In situations in which the chemistry of mafic magmas is predetermined by earlier metasomatic events, caution is advised in using trace-element criteria to evaluate the tectonic setting.

Paleomagnetism and rock magnetism of Early Silurian Dunn Point volcanics, Avalon Zone, Nova Scotia

Paleomagnetic results have been obtained from the subaerial volcanics (andesites, rhyolites, ignimbrites) from the Dunn Point Formation exposed in the Antigonish Highlands of the Avalon Zone, Nova Scotia. An Early Silurian age of 430 ± 15 Ma is assigned to these volcanic rocks. The age is in agreement with the fossil evidence from conformably overlying sedimentary rocks. The volcanic units were tightly folded and metamorphosed to sub-greenschist facies during one major and one secondary phase of deformation probably associated with the Mid-Late Devonian Acadian orogeny. This paleomagnetic study is based on 45 samples collected from nine sites. The site-mean directions of the natural remanent magnetization (NRM) make a large deviation from the present Earth's field with NRM intensities ranging from 10 −2 to 10 Am −1. The most commonly observed remanence direction, A( D=153°, I=−28°), obtained from both alternating fields (30–75 mT) and thermal (500–560°C) treatments yields a corresponding direction A′( D=0°, I=−63°) after correction for geological tilt. A large scatter in direction (up to = 60°) is observed in A′ and the fold test is not significantly positive. A second and less common remanence component, B ( D=146°, I=+54°), which presumably postdates folding, is also observed; this component is mainly isolated by thermal treatment in the 540–630°C range. Room-temperature isothermal experiments and optical examination of polished sections indicate the presence of magnetite and hematite as the carriers of both A and B directions. The corresponding reversed paleomagnetic poles fall at A (163°E, 52°N) and B (146°E, 4°N) in close agreement with cratonic North American Upper Jurassic and Late Devonian poles, respectively. In the Mid-Late Devonian time interval, the Avalon Zone was situated slightly to the south but close to the North American eastern margin.

Petrology of Upper Ordovician – Lower Silurian rocks of the Antigonish Highlands, Nova Scotia

Upper Ordovician to Lower Silurian rocks in the Antigonish Highlands consist of interlayered basalts, rhyodacites, arkoses, and conglomerates overlain by a thick sequence of marine clastic rocks and minor rhyolites. The stratigraphy documents a marine transgression. The volcanic rocks were deposited in a within-plate, continental, extensional environment. The basalts display alkalic and tholeiitic affinities, and the rhyodacites were formed by anatexis of the crust. The origin of the younger rhyolites is not clear: they are compositionally distinct from the rhyodacites but may be related to them as late-stage differentiates. At present, it is not possible to evaluate whether the tectonic setting and magmatic affinities are regionally or locally controlled.The geological history is very similar to that of Lower Silurian rocks immediately north of the Antigonish Highlands at Arisaig. In the simplest sense, this indicates these areas may have been juxtaposed prior to the Late Ordovician and limits cumulative post-Silurian movement on the boundary (Hollow) fault to about 40 km.

The stratigraphy and depositional environment of Upper Ordovician to Lower Devonian rocks in the Antigonish Highlands, Nova Scotia

Upper Ordovician to lower Devonian rocks in the Antigonish Highlands, Nova Scotia, consist of biinodal volcanics and redbeds at the base overlain by a thick sequence of fossiliferous marine slliciclastic rocks. These rocks belong to the Arisaig Group and correlate well with coeval rocks to the north of the Antigonish Highlands. The definition of the Arisaig Group is revised to Include the uppermost Ordovician-lowermost Silurian Bears Brook Formation which occurs at the base, and the lower Devonian Knoydart Formation which occurs at the top of the succession.
 
 The sequence as a whnle is probably related to a local rifting event. The Bears Brook Formation consists of continental redbeds and biinodal rift-related volcanic rocks. A marine transgression at. the base of the Silurian is indicated by the first appearance of marine fossiliferous rocks and by the overstep relations exhibited by Silurian strata onto Precambrian basement. The marine transgression may have been associated with a world-wide eustatic rise in sea level and with local rifting. The Knoydart Formation is dominated by fluviatile redbeds indicating a return to subaerial conditions. The distribution of the Arisaig Group rocks around the southern and western margins of the Highlands together with the overstep relationships indicate that the Antigonish Highlands became progressively submerged during the Silurian. This was followed by a return to subaerial conditions In the Early Devonian.
 
 RÉSUMÉ
 Dans les Monts Antigonish, les roches de l'Ordovicien supérieur au Dévonien inférieur comprennent un ensemble inférieur de volcanites bimodales et de lits rouges sur lequel repose une puissante assise de roches fosslliferes marines silicoclastiques. Ces roches appartiennent au Groupe d'Arisaig et peuvent aiseraent être corrélées avec leurs bomologues au nord des Monts Antigonish. On redèfinit le Groupe d'Arisaig pour y inclure la Formation de Bears Brook. présente en sa base et s'étageant du sommet de l'Ordovicien au Silurien basal, ainsi que la Formation de Knoydart, du Dévonien inférieur et répartie au sommet de la succession.
 
 Dans son ensemble, la séquence correspond probablement à un épisode local de rifting. La Formation de Bears Brook comprend des lits rouges contiuentaux et des volcanites bimodales spparentées au rift. La première apparition de roches marines fossillferes et la discordance fondaraentale entre les strates sliuriennes et le bati précambrien teraoignent d'une transgression marine à l’amorce du Silurien. Cette incursion marine apparalt reliée à une remontée du niveau marin global ainsi qu'a un rifting local. L'importance des lits rouges fluviatlles dans la de Knoydart y atteste d'un rétour à des conditions subaerlennes. La distribution des roches du Groupe en bordures méridionale et occidentale des Monts ainsi que leur contact en discordance indiquent une progressive des Monts Antigonish au cours du Silurlen. L'Eodévonien marque un rétour aux conditions eustatique Formation d'Arisaig submersion subaériennes
 
 [Traduit par le journal]

Cambrian volcanism in Nova Scotia, Canada

Cambrian volcanic rocks in Nova Scotia occur in small grabens or half grabens in the Avalon Zone (Composite Terrane) as part of a thin sequence of continental to shallow-marine Cambro-Ordovician rocks. In the northern Antigonish Highlands, the volcanic rocks occur mainly in the Lower Cambrian McDonalds Brook Group. In southern Cape Breton Island, they occur predominantly in the Middle Cambrian Bourinot Group. The chemistry of these volcanic rocks indicates that they are bimodal (basalts–rhyolites) and within plate. The basalts are alkalic in the Antigonish Highlands and tholeiitic in Cape Breton Island. The rising basaltic magma is postulated to have produced the felsic magma by anatexis of the crust. It is proposed that the Antigonish Highlands volcanic rocks erupted in a small pull-apart basin. A similar structural setting is probable in southern Cape Breton Island, but there the bounding faults are poorly exposed. These basins probably formed during a period of transpression in the last stages of the late Hadrynian Cadomian deformation.

A microfauna associated with Early Cambrian trilobites of the Callavia Zone, norther Antigonish Highlands, Nova Scotia

Trilobites of the Callavia Zone s.1. occurring together with phosphatic fossils in the Little Hollow Formation are the first known Early Cambrian faunas from mainland Nova Scotia. The tectono-stratigraphic affinity of the northern Antigonish Highlands with less intensely deformed portions of the Avalonian Platform is indicated by: (1) the Avalonian character of the trilobites and Early Ordovician brachiopods, (2) the nature of the lithologic sequence of the Iron Brook Group, and (3) local evidence for a Hadrynian orogeny.Rhombocorniculum cancellatum (Cobbold) is here informally designated a "pseudoconodont" based on its unique histology and is regarded as a conodontophorid-like organism. Eccentrotheca kanesia n. gen., n. sp. is a multiplated organism with apparent affinities to groups regarded as mitrosagophorans. Sunnaginia imbricata Missarzhevsky and possibly Turcutheca are demonstrated to occur in strata younger than Tommotian (lowest Lower Cambrian).

Early Silurian volcanic rocks at Arisaig, Nova Scotia

Petrochemical data for the Early Llandovery subaerial volcanic rocks at Arisaig, Antigonish Highlands, Nova Scotia, show that they are a bimodal suite of altered rhyolites and basalts of tholeiitic affinity with some alkaline tendencies extruded in a nonorogenic extensional environment within a continental plate. These data do not support the island arc origin frequently inferred for these rocks but are consistent with recent petrological data from the Silurian – Lower Devonian Coastal Volcanic Belt in southern Maine, which may be a strike continuation of the Arisaig volcanics. The close spatial and temporal association of basalts and rhyolites is attributed to anatexis of the crust upon contact with the rising basaltic magma.

Radiometric Age of the Keppoch Formation, Browns Mountain Group, Northern Mainland of Nova Scotia

The lower Paleozoic Browns Mountain Group of volcanic and sedimentary rocks underlies much of the Antigonish Highlands on the northern mainland of Nova Scotia. The rocks are apparently unfossiliferous and pre-Lower Silurian in age. Volcanic rocks belonging to the Keppoch Formation give a Rb–Sr whole-rock isochron age of 528 ± 40 m.y.; the indicated value for the initial ratio 87Sr/86Sr is 0.7032 ± 0.0020. The apparent stratigraphic age of the lower part of the Browns Mountain Group then is Cambrian with a middle Cambrian age favored.