Southeastern Cape Breton Island Research Articles

The Terreneuvian MacCodrum Brook section, Mira terrane, Cape Breton Island, Nova Scotia, Canada: age constraints from ash layers, organic-walled microfossils, and trace fossils

The MacCodrum Formation is a classical “lower” Cambrian unit in southeastern Cape Breton Island stratigraphy, described since the 1800s. The age of this formation and its correlation with other Avalonian Cambrian units in eastern Newfoundland and southern New Brunswick have remained uncertain through numerous revisions. Here we present U-Pb CA-TIMS ages from an ash bed in the basal part of the MacCodrum Formation in its type-section on MacCodrum Brook that fix the maximum time of deposition at 531.86 ± 0.34 Ma. Organic-walled microfossils sampled throughout the MacCodrum Formation type-section yield acritarch taxa identifying the Asteridium– Comasphaerdium Zone, whereas the first acritarchs of the Skiagia– Fimbriaglomerella Zone appear in the overlying Canoe Brook Formation in other sections. The radiometric age and acritarch zonation place the MacCodrum Formation in the upper Fortunian, Cambrian Stage 2. Among trace fossils in the MacCodrum Formation, the meandering trace fossil Didymaulichnus dailyi comb. nov. is of particular note and morphologically identical to the type material from the lower part of the Ratcliffe Brook Formation in New Brunswick. The new radiometric and biostratigraphic data presented here provide the first firm constraints on the age of the MacCodrum Formation and enable more precise correlation with sections in southern New Brunswick and eastern Newfoundland.

Comment on "A New Analysis of the Magnitude of the February 1663 Earthquake at Charlevoix, Quebec" by John E. Ebel

Modern seismologists perhaps too easily become enamored of their modern seismographs and instrumental arrays that can do wonders for the analysis of recent events. In the instrumental era, we tend to forget that such recordings are barely 110 years old and that we are trying to understand seismic‐source zones with an activity return period in the order of 300–600 years. Documenting and understanding preinstrumental historic events is still a necessary endeavor and requires the input of our allies in the field of history. I welcomed John E. Ebel’s re‐examination of the 5 February 1663 earthquake (Ebel, 2011). As someone who has worked on the historical seismicity of parts of Atlantic Canada, I appreciate the great difficulty the author had in digging out data from the seventeenth century. Early European settlement in eastern Canada was very sparse at the time, many settlers were not literate, and even fewer could afford the cost of a pen and ink and the paper to keep a journal or diary—or the time that a journal takes away from the task of subsisting in the harsh environment of their New World. The author kindly credited me (p. 1027, col. 1) with information with respect to the 5 February 1663 event being felt and to having rattled cooking utensils and tableware in the village of St. Peters in southeastern Cape Breton Island in Acadia (now the Province of Nova Scotia). The credit for these data rightly should go to Ronnie‐Gilles LeBlanc, a historian at Parcs Canada, Centre de services de l’Atlantique in Halifax, Nova Scotia; I was only a conduit. In fact it is not quite certain where Nicolas Denys was living on 5 February 1663. M. LeBlanc’s exact words of 9 September 2009 to me on this location were, “It is not clear whether Denys was …

Provenance variations in northern Appalachian Avalonia based on detrital zircon age patterns in Ediacaran and Cambrian sedimentary rocks, New Brunswick and Nova Scotia, Canada

Avalonia in the northern Appalachian orogen is generally assumed to have originated near the margin of Gondwana, although its position along that extensive margin and the timing of its separation remain disputed. U–Pb ages determined by laser ablation – inductively coupled plasma – mass spectrometry are reported here for detrital zircon from Avalonian clastic sedimentary units, three in southern New Brunswick, and one in southeastern Cape Breton Island, with depositional ages ranging from ca. 630 to ca. 505 Ma. Considered in combination with previously published detrital data sets from two other samples from southern New Brunswick deposited in a similar age range, the samples show a wide spectrum of ages from Ediacaran to Neoarchean (as old as 3.2 Ga). The dominant zircon populations in all six samples are Ediacaran–Cryogenian with ages between 770 and 540 Ma and can be matched with known magmatic events in Avalonia. Pre-Avalonian cycle detrital zircon dates in the samples are relatively sparse and do not reveal a consistent signature that might be used to place Avalonia in a unique paleogeographic position, although they are compatible with Amazonian provenance. An increase in ca. 2.2–1.9 Ga zircon dates in Early Cambrian through Early Ordovician samples suggests that sources for zircon of those ages became available in Avalonia by the end of the Ediacaran, possibly by motion of Avalonia to a different part of the Amazonian margin or to the West African craton.

Provenance of detrital muscovite in Cambrian Avalonia of Maritime Canada: 40Ar/39Ar ages and chemical compositions

We report single-grain ages for detrital muscovite separated from sandstone samples from five localities in southern New Brunswick and southeastern Cape Breton Island, Nova Scotia, and from two quartzite clasts from a quartzite-pebble conglomerate that underlies the sampled sandstone units in New Brunswick. The oldest detrital grains were found in one of the quartzite clasts; their age range, ca. 650−630 Ma, is defined not only by the single-grain analyses but also by spot dating (using a UV laser) within a single large grain, suggesting that these grains came from a single source. The second quartzite clast has a blastomylonitic fabric with muscovite “fish,” and most of the muscovite ages have been partially reset (at ca. 550 Ma) from the original ca. 650–630 Ma ages. The age distribution plots obtained for the sandstone samples suggest the presence of muscovite that still retains the original source age, but most of the grains have been partially reset by the same ca. 550 Ma event that reset muscovite ages in the second quartzite clast. We suggest that the quartzite source that produced the two clasts was also the source of muscovite in the Avalonian Cambrian rocks of Maritime Canada. The original source rock was likely a metamorphic or perhaps granitic rock unit situated relatively proximal to the site of deposition of the quartzite protolith, but the actual source is not known, and locally, no potential candidates are exposed. The resetting event at ca. 550 Ma may be linked to initial stages of regional transtension associated with rifting of Avalonia from Gondwana.

Acritarchs from the MacLean Brook Formation, southeastern Cape Breton Island, Nova Scotia, Canada: New data on Middle Cambrian–Lower Furongian acritarch zonation

We present here the results of a pilot study on Cambrian acritarchs from Cape Breton Island, Nova Scotia, based on a material from the MacLean Brook Formation in the Mira River Valley. An assemblage from the base of the formation contains 8 species including Eliasum llaniscum, Cristallinium dubium and Symplassosphaeridium cambriense, and correlates with a position close to the Paradoxides davidis– P. forchhammeri Zone boundary. An assemblage from the upper part of the formation contains 16 species, of which 3 are new, including Cristallinium aciculatum, Petaloferidium lacrimiferum n. sp., Pirea orbicularis, Stelliferidium pingiculum, S. magnum n. sp., S. albanii n.sp, and Timofeevia microretis, correlated to the Olenus Zone. The upper assemblage is particularly noteworthy as it provides the first direct evidence for the Olenus Zone in the MacLean Brook Formation. Furthermore, it contains stratigraphically significant taxa known from sections on Baltica, and northern Gondwana but which have not been reported previously on Avalonia. The distinction between Stelliferidium and Timofeevia is discussed in part based on new data on Timofeevia lancarae from its type area in northern Spain. It is suggested that many reports of Furongian Timofeevia phosphoritica and T. lancarae are better referred to Stelliferidium.

Geological correlations between Cape Breton Island and Newfoundland, northern Appalachian orogen

Geological correlations between Cape Breton Island and Newfoundland are apparent both in surface geology and at deeper crustal levels, based on similarities in Sm-Nd isotopic signatures. The Mira terrane of southeastern Cape Breton Island is part of the Avalon terrane sensu stricto and is composed of Neoproterozoic volcanic-sedimentary-plutonic belts and overlying Cambrian rocks directly comparable to those in the western part of the Newfoundland Avalon terrane. The Bras d'Or terrane is also mainly of Neoproterozoic age, but shows lithological and isotopic contrasts with the Mira terrane. Small areas of similar Neoproterozoic rocks occur in southern Newfoundland and to the north as inliers in the Exploits terrane. The Bras d'Or terrane and similar rocks in Newfoundland are interpreted to represent a peri-Gondwanan terrane where rocks of the Gander terrane were later formed. Hence this area is part of the Central Mobile Belt and distinct from Avalon terrane sensu stricto. The Aspy terrane is a complex area that may include fragments of Bras d'Or crust and components of the Gander, Exploits, and possibly Notre Dame terranes of Newfoundland. It formed by subduction and back-arc basin opening and closure during the Silurian to Early Devonian. The Blair River Inlier is a fragment of Grenvillian rocks, similar to those in the Grenvillian inliers in the Humber zone of western Newfoundland in terms of age, rock types, and isotopic composition. Silurian and Devonian promontory-promontory collision resulted in juxtaposition and stacking of these elements in Cape Breton Island, as in the Hermitage Flexure - Port aux Basques area of Newfoundland. Because the lower crust under Bras d'Or - Gander - Aspy terranes seems distinct from that under Avalon terrane sensu stricto, it is preferable to use the term peri-Gondwanan rather than Avalonian to refer to these areas.

Ice-flow history and glacial dispersal patterns, southeastern Cape Breton Island, Nova Scotia: implications for mineral exploration

Regional till sampling and stratigraphic studies were completed in southeastern Cape Breton Island, Nova Scotia, in 1990 and 1991 to document the directions and distances that successive ice-flow events have dispersed material and to document regional till geochemical patterns to aid mineral exploration in this heavily drift-covered area. Three major Wisconsinan ice-flow events affected the area: an early eastward flow, followed by a northeastward flow, and finally a southward flow. The shape and magnitude of dispersal trains in the area are the net result of the three ice-flow events, which are variable from north to south. In the north, the northeast ice flow was the dominant erosive and depositional event, producing a large northeast-trending drumlin field. Salmon River porphyry clasts and Pb- and Zn-rich till from the Yava Pb mine were dispersed more than 15 km to the northeast. In the south, eastward ice flow was the dominant erosive and depositional event, producing large east-trending till ridges. Dispersal in this area is difficult to detect because exotic red silty till in the giant till ridges masks underlying mineralized bedrock and dilutes the locally derived glacial debris. Many precious and base metal anomalies and dispersal trains were identified in the regional till samples, several of which are related to unknown mineralized sources.

Appalachian basement and its intrusion by Cretaceous dykes, offshore southeast Nova Scotia, Canada

Aeromagnetic data collected between eastern Nova Scotia and southern Newfoundland provide new information about the offshore extension of the Avalon and Meguma terranes. A zone of short-wavelength anomalies that delineates Scatarie Ridge extends westward to the Late Proterozoic Fourchu Group in southeastern Cape Breton Island and eastward towards the Burin Peninsula of Newfoundland, suggesting that both regions belong to the same tectono-stratigraphic province of the Avalon composite terrane. A different zone of short-wavelength, discontinuously lineated anomalies at the northern edge of the Canso Ridge correlates with amphibolite-facies metamorphic rocks of the Meguma terrane on the Canso Peninsula, interpreted as an exhumed deeper metamorphic level of the Meguma terrane at its boundary with the Avalon terrane. The S-shaped pattern of long linear magnetic trends, characteristic of lower grade Meguma rocks on the southern flank of the Canso Ridge, indicates plastic deformation of the Meguma terrane during the Acadian orogeny when emplaced against the rigid Cape Breton Island block indentor. Analogous patterns occur off western Nova Scotia, suggesting little strike-slip motion occurred between the Meguma and Avalon terranes since the Acadian orogeny.Late Proterozoic rocks on Scatarie Ridge are intruded by Cretaceous diabase dykes. The diabase is alkaline with a within-plate geochemical signature, similar in composition to basalt flows in the Orpheus half-graben. A depleted-mantle model age TDM (Nd) of 731 Ma, εNd = +6.5, suggests that the magma was sourced from a lithospheric mantle reservoir involved in Late Proterozoic magmatic activity. Aeromagnetic data interpretation confirms the distribution of Cretaceous basalt flows and sills within Mesozoic sedimentary strata of the Orpheus half-graben previously outlined by seismic methods but was unable to differentiate between Proterozoic and Mesozoic intrusive rocks where the Proterozoic rocks lay near to the ocean floor.

Multi-media geochemistry and surficial geology of the Yava Pb deposit, southeastern Cape Breton Island, Nova Scotia, Canada

A detailed multi-media geochemical study was conducted over the east zone of the Yava lead deposit, southeastern Cape Breton Island. A 2 × 2 km grid was centred over the projected subcrop of the east ore zone and sample sites were located at 250 m spacing. Samples collected at each site included: (1) Balsam fir ( Abies balsamea ); (2) A 1 -horizon (organic humus) soil; and (3) C-horizon till. In addition, stream sediment samples were collected at approximately 250 m spacing along first and second order streams that transect the grid. Overburden drilling of a drumlin located in the grid, to the northeast of the ore zone, and a till fabric study were conducted in conjunction with the geochemical survey. The drumlin is cored by a silty/sandy till and overlying moderately-compacted silty till. The entire study area, including the drumlins, is blanketed by a loosely-compacted sandy till. The silty upper drumlin till and surface sandy till were deposited by NE and NNE ice flows respectively. Analysis of mineralized sandstone clasts from the two uppermost till facies in the overburden holes returned values of ≤3.05% Pb and ≤2.65% Zn. Our data indicate the Yava deposit has a robust geochemical signature, mostly for Pb and Zn in the −230 (−62.5 μm) mesh fraction and heavy mineral separates (−60+230 (−250+62.5 μ m) mesh fraction) of tills and stream sediments and ash of Balsam fir twigs. These data clearly delineate the subcrop of the east ore zone. The distribution of Ag, Cd, As and Sb within all sample media varies substantially and generally are less reliable for outlining the location of the deposit. Till clast distribution and till geochemical data indicate that NNE-trending dispersal in the upper sandy till sheet was minimal. Large NE-trending ribbon-shaped Pb and Zn “anomalies” in the −230 (−62.5 μm) mesh fraction of this till reflect inheritance of mineralization from the underlying drumlin core till with NE dispersion and possibly additional base metal mineralization along the margin of the sedimentary basin. The geochemical data for heavy mineral separates from both till and stream sediments mirror the results from the −230 (−62.5 μm) mesh fraction but are not recommended for future studies because of high sample preparation costs. Balsam fir is the most cost-efficient, yet geochemically effective, sample medium. Humus geochemistry reflects hydromorphic processes and is not recommended for future surveys.

Geochemistry and tectonic setting of late Precambrian volcanic and plutonic rocks in southeastern Cape Breton Island, Nova Scotia

Late Precambrian volcanic–sedimentary belts in the Mira (Avalon) terrane of southeastern Cape Breton Island display differences in rock types, petrochemistry, and age, showing that they did not form contemporaneously above a single northwest-dipping subduction zone, as proposed in earlier models. The oldest rocks are 680 Ma mafic and felsic flows and tuffs, and abundant, mainly tuffaceous, sedimentary rocks in the Stirling belt. They are interpreted to have formed in a trough within or peripheral to a volcanic-arc complex. Northwest of the Stirling belt, the East Bay Hills, Coxheath Hills, and Sporting Mountain belts consist of ca. 620 Ma mafic to felsic subaerial pyroclastic rocks and flows and contemporaneous dioritic to granitic plutons. Both volcanic and plutonic rocks are calc-alkalic to high-K calc-alkalic suites, formed in a continental margin volcanic arc. A correlative 620 Ma plutonic suite intruded the western margin of the Stirling belt, suggesting that subduction may have been toward the present southeast. The ca. 575 Ma Coastal belt, located southeast of the Stirling belt, is significantly younger than the other belts and appears to represent a less evolved calc-alkalic to low-K continental margin volcanic-arc and intra-arc basin formed above a northwest-dipping subduction zone. These various volcanic–sedimentary belts were juxtaposed by lateral movements along major faults in the late Precambrian to form this part of the Avalon composite terrane. Subduction-related, calc-alkalic magmatism at ca. 620 Ma was apparently widespread throughout the Avalon terrane of the northern Appalachian Orogen. However, ca. 680 Ma magmatism like that in the Stirling belt has been documented elsewhere only in the Connaigre Bay Group of Newfoundland. Circa 575 Ma and younger subduction-generated igneous activity like that in the Coastal belt has been recognized in southern New Brunswick, but alkaline magmas were forming in extensional regimes in other areas of the Avalon terrane at that time.

Till geochemistry and its implications for mineral exploration: Southeastern Cape Breton Island, Nova Scotia, Canada

Till samples were collected across southeastern Cape Breton Island, Nova Scotia, Canada, at a reconnaissance scale (3–5 km spacing) to define regional background levels of base and precious metals in drift and to identify anomalous areas that may indicate mineralization in the underlying bedrock. Till samples were also collected from detailed sampling grids (100–500 m spacing) over three known mineral occurrences to document the nature of glacial dispersal from mineralized sources, i.e. the directions and distance that successive ice flow events have dispersed material and the variation in trace element concentration in till around the bedrock source. Several areas contain elevated levels of base and precious metals. Elevated Pb and Zn concentrations occur along the Salmon River valley and are related to glacial dispersal from PbZn mineralization in Carboniferous sandstone down (northeastward) the valley. Elevated Au values in the region, up to 78 ppb, overlie Fourchu Group felsic volcanic rocks. At a local scale, Mo and Cu have been dispersed at least 500 m to the east of the Deep Cove pluton by an older, eastward ice flow event. In the Blue Mountain area, dispersal of sandy, metal-rich till was mainly northward. At Stirling, exotic red till masks the underlying mineralized bedrock and dilutes the locally derived glacial debris so that there is little geochemical response in the till.

U–Pb geochronologic constraints on the volcanic evolution of the Mira (Avalon) terrane, southeastern Cape Breton Island, Nova Scotia

New U–Pb ages for late Precambrian volcanic and associated plutonic units in the Mira (Avalon) terrane of southeastern Cape Breton Island indicate that volcanic suites were erupted over a span of at least 100 Ma. The oldest dated rock is a quartz–feldspar rhyodacitic porphyry from the unit that hosts the Mindamar Zn–Pb–Cu–Ag–Au deposit in the Stirling belt, which has an age of [Formula: see text]. The most widespread volcanism and plutonism occurred at ca. 620 Ma in the East Bay Hills and Coxheath Hills belts, and probably the Sporting Mountain belt, as indicated by U–Pb ages and U–Pb maximum ages for rhyolite flows and U–Pb and Ar–Ar ages of crosscutting plutons, as well as stratigraphic constraints. Younger volcanic rocks occur in the Coastal belt, from which a rhyodacitic crystal tuff is dated at [Formula: see text] and a pluton is dated at 574 ± 3 Ma. A rhyolite flow from the contiguous Main-à-Dieu sequence yields a maximum age of ca. 563 Ma, and a minimum age for this sequence is indicated by overlying latest Precambrian to Cambrian fossiliferous sedimentary rocks. Middle Devonian plutonism in the Mira terrane is confirmed by an age of [Formula: see text] from the Lower St. Esprit granodiorite in the Coastal belt. The range of ages of volcanic and plutonic rocks in Mira terrane is similar to that in other parts of Avalon terrane in eastern Newfoundland and southern New Brunswick. Many of the dated rocks contain xenocrystic zircons of Middle Proterozoic ages which suggest a South American source.

Geochemistry of mineralized and barren, late proterozoic felsic volcanic rocks in southeastern Cape Breton Island, Nova Scotia (Canada)

Abstract Late Proterozoic volcanic rocks of southeastern Cape Breton Island, Nova Scotia (Canada), comprise part of the Avalon Composite Terrane of the Canadian Appalachians. They occur in several fault blocks (from southeast to northwest: Coastal, Stirling, East Bay Hills and Coxheath). The blocks are composed mainly of Late Proterozoic volcanic suites which are predominantly bimodal and were emplaced in a volcanic arc setting. The basaltic rocks range from tholeiitic (Coastal and Stirling) to calc-alkalic (East Bay Hills and Coxheath). The felsic volcanic rocks of the Stirling block host a massive sulphide deposit but no economic mineralization has been discovered in the volcanic rocks of the other blocks. The felsic volcanic rocks display significant differences in composition amongst the fault blocks. The Coastal and Stirling blocks contain low-K rhyolites with low-k abundances of strongly incompatible trace elements and relatively flat chondrite-normalized REE patterns. These rocks were probably derived by the melting of mafic igneous rocks compositionally similar to tholeiitic basalts of Archean greenstone belts or island arc tholeiites. Compared to the Coastal block rocks, the Stirling rhyolites are depleted in Eu, Sr and Al 2 O 3 indicating extensive feldspar fractionation. Trace element characteristics of the Stirling block rhyolites are closely comparable to those of the Archean felsic metavolcanic rocks from the Superior Province of the Canadian Shield which are preferentially associated with massive sulphide mineralizations. The similarities suggest that the low-K felsic volcanic rocks with flattish REE patterns and negative Eu and Sr anomalies are also indicative of a promising environment for massive base-metal sulphide mineralization in volcanic belts at least as young as Late Proterozoic. The rhyolites of the Coxheath block have calc-alkalic affinities, higher contents of strongly incompatible elements than the Coastal and Stirling rhyolites and their REE are patterns are enriched in light REE. The East Bay Hills felsic tuffs resemble the Coxheath rhyolites. They both could be derived by melting of a source with a composition of the average lower crust.

Nd isotopic compositions of felsic igneous rocks in Cape Breton Island, Nova Scotia

Nd isotopic data from 18 felsic plutonic and volcanic units in Cape Breton Island show variations consistent with other geological and geophysical evidence for at least three distinct terranes. A ca. 1.2 Ga syenite considered to be part of Grenvilleage basement exposed in the northwestern part of the island yields an initial εNd value of +0.4 and a depleted-mantle model age (TDM) of 1.66 Ga, suggesting substantial involvement of older (Archean or Early Proterozoic) crust in its petrogenesis. A TDM of 1.38 Ga indicated by Nd isotopic data for a Devonian granite spatially associated with the syenite is also consistent with the presence of older crust in that part of Cape Breton Island. In contrast, Silurian rhyolite and Devonian granites from the Aspy terrane have TDM ranging from 0.7 to 1.1 Ga and initial εNd between +2.8 and −1.2, and overlap in isotopic characteristics with late Precambrian and Early Ordovician plutons in the Bras d'Or terrane that yield TDM of 0.9–1.1 Ga and initial εNd of +1.4 to −1.8. The relatively small variation in εNd and TDM in these terranes suggests that old crust like that under the Blair River Complex may not be present. Granitic plutons and rhyolite in the Mira terrane of southeastern Cape Breton Island have a range in TDM (0.8–1.2 Ga) similar to that of the Aspy and Bras d'Or terranes, but initial εNd values ranging from +0.8 to +5.0 indicate more juvenile crust. This juvenile crust also appears to be present under southern New Brunswick and eastern Newfoundland and may be characteristic of the Avalon terrane.

Devonian plutons in southeastern Cape Breton Island, Nova Scotia

Four granitic plutons and a suite of granitic dykes of Devonian age occur in the Mira (Avalon) Terrane of southeastern Cape Breton Island. Granodiorite gradational to granite in the Lower St. Esprit pluton is chemically distinct from and unlikely to be genetically linked to comparable units in the other plutons. The Salmon River pluton consists of highly evolved rhyolite porphyry and is also not likely to be genetically linked to the other units. In contrast, the Gillis Mountain and Deep Cove plutons and.the Blue Mountain dykes are interpreted to be co-genetic and possibly co-magmatic on the basis of chemical continuity among their units, which range in composition from quartz monzodiorite to granite and rhyolite porphyry. They have I-type petrographic and chemical features, whereas the Lower St. Esprit has some M-type characteristics and the Salmon River rhyolite porphyry appears transitional between felsic I-type and A-type granites. Cu-Mo mineralization is associated with the Gillis Mountain pluton, and polymetallic Cu-Mo-Pb-Zn-Bi-Ag mineralization with greisen zones in the Deep Cove pluton and with skarn in the Blue Mountain contact metamorphic aureole. Minor Cu-Zn mineralization occurs adjacent to the Lower St. Esprit pluton, although a definite genetic relationship with thc pluton has not been established. Trace Pb mineralization has been reported from the Salmon River pluton. The origin of these Devonian plutons is uncertain, but is postulated to have been linked to regional strike-slip faulting and extension.
 
 RÉSUMÉ
 Quatre plutons granitiques et une suite de dykes granitiques d'âge Dévonien giseent dans le terrain de Mira (Avalon) du sud-est de l'ile-du-Cap-Breton. La granodiorite, en transition avec du granite, du pluton de Lower St-Esprit est chimiquement distincte et n'est probablement pas reliée génétiquement aux unités comparables des autres plutons. Le pluton de la rivière Salmon consiste en une rhyolite porphyrique très évoluée et n'est probablement pas relié génétiquement aux autres unités. En contraste, les plutons des monts Gillis et de l'anse Deep ainsi que les dykes du mont Blue sont interprétés comme étant cogénétiques et possiblement comagmatiques, sur la base de la continuité chimique entre leurs unités, qui vont des monzodiorites quartzifères aux granites et aux rhyolites porphyriques. Ces roches intrusives ont des caractéristiques pétrographiques et chimiques de granites de type I tandis que le pluton de Lower St-Esprii a quelqucs caractéristiques de type M et que la rhyolite porphyrique de la rivière Salmon semble être transitionnelle entre les granites felsiques de type I et ceux de type A. De la minéralisation en Cu-Mo est associée au pluton du mont Gillis, de la minéralisation polymétallique en Cu-Mo-Pb-Zn-Bi-Ag avec des zones de greisen se retrouvé dans le pluton de l'anse Deep et il y a des skarns dans l'aurdole métamorphique du mont Blue. Il y a des occurences mineures de minéralisation en Cu-Zn près du pluton de Lower St-Esprit bien qu'un lien génctique définitif avec le pluton n'ait pas été établi. Des traces de minéralisation en Pb sont mentionnées pour le pluton de la rivière Salmon. L'origine de ces plutons dévoniens est incertaine mais on suppose qu'elle est reliée aux failles de décrochement et d'extension régionales.
 
 [Traduit par le journal]

Upper Precambrian through Lower Cambrian of Cape Breton Island: faunas, paleoenvironments, and stratigraphic revision

Latest Precambrian through Early Cambrian tectonic history and stratigraphy are comparable in southeastern Cape Breton Island and the western Placentia–Bonavista axis, southeastern Newfoundland. The lithostratigraphic nomenclature of southeastern Newfoundland is used for this interval in Cape Breton Island. Upper Precambrian volcanic rocks of the Forchu Group (=“Giant Lake Complex,’ designation abandoned) are unconformably overlain by uppermost Precambrian through lowest Cambrian strata termed the “Morrison River Formation’ (designation abandoned). This depositional sequence consists of three formations: 1) red beds through tidalites of the Rencontre Formation (to 279+ m; =“Kelvin Lake Formation,’ designation abandoned); 2) prodeltaic clastics of the Chapel Island Formation (to 260 m); and 3) macrotidal quartzites of the Random Formation (to 71 + m). Post-Random block faulting and 300 m of local erosion took place prior to onlap of the “MacCodrum Formation’ (abandoned). Siliciclastic mudstones of the lower “MacCodrum’ are re-assigned to the middle Lower Cambrian Bonavista Group. Sub-trilobitic faunas from the Bonavista Group include“Ladatheca’ cylindricafrom the West Centre Cove Formation(?) and higher diversity faunas (23 species) in theCamenella balticaZone of the Cuslett and Fosters Point Formations. Trilobite-bearing, upper Lower Cambrian (Branchian Series) strata (Brigus Formation, =upper “MacCodrum’ and overlying “Canoe Brook’ Formations) unconformably overlie the Placentian Series in Cape Breton Island, southeastern Newfoundland, Shropshire, and, probably, eastern Massachusetts. Correlations based on small shelly fossils indicate an earlier appearance of trilobites in Avalon than on the South China Platform. Twenty-six species are illustrated.Halkieria fordin. sp., the conodont(?)“Rushtonites’ asiatican. sp., and the zhijinitid(?)Samsanoffoclavus matthewin. gen. and sp. are described.Ischyrinia? sp. may be the oldest ischyrinoid rostroconch.

Paleomagnetism of Late Hadrynian Fourchu gabbro, southern Cape Breton Island, Nova Scotia.

A paleomagnetic study was carried out on three gabbroic intrusives in the Avalonian terrane of southeastern Cape Breton Island. Thirty five samples (96 specimens) from seven sites were collected for this purpose. Thermal and/or alternating field treatments were performed. Four components of magnetization were detected. One component coincides with the present Earth's field direction. A second component (A) oriented south-southeast with shallow to intermediate inclination (165, +41) is interpreted as representing a Siluro-Devonian overprint. A third component (B) yields a mean direction to the southeast and an intermediate inclination (147, -54). This component is not observed in North American rock units of Devonian age or younger; it is thus bracketed in the Early Devonian-Late Hadrynian interval. Based on magnetic properties, mineralogical and geological grounds, an Ordovician-Cambrian age of acquisition is attributed to this component of magnetization. A fourth component characterized by a low unblocking temperature and median destructive field has a mean direction to the northeast and an intermediate inclination (54, +47). This uncommon direction is inferred to represent an incompletely isolated magnetization. Compaged to low southern paleolatitudes for the Late Hadrynian period of cratonic North America, the paleolatitudes of the Avalonian terrane are slightly higher but this is not really significant. A sea at the most 1000km wide may have separated North America and Avalon at that time. A comparison of the paleolatitude of Avalon and those of Late Hadrynian Atlantic Ocean borderlands indicates that Avalon could have belonged to North Africa at that time. The Avalonian apparent polar wander path in the Devonian-Late Hadrynian interval still remains uncharted. The collision of Avalon with North America occurred most probably at Middle Devonian time.

Geochemistry of Late Proterozoic basaltic rocks from southeastern Cape Breton Island, Nova Scotia

Late Proterozoic volcanic rocks of the Fourchu Group from the Avalon Zone in southeastern Cape Breton Island, Nova Scotia, are composed predominantly of mafic and felsic types with subordinate intermediate units that were all affected by subgreenschist- to greenschist-facies metamorphism. The rocks crop out in four fault blocks (Coastal, Stirling, East Bay Hills, and Coxheath) and have geochemical characteristics of ensialic orogenic volcanic suites. The basaltic rocks range from tholeiitic to calc-alkaline and show a distinct compositional zonation that resembles the across-arc variation observed in recent volcanic-arc systems. The variations include a progressive increase in abundances of light rare-earth elements, Th, Zr, Hf, Nb, and Ta and in the ratios of Zr/SiO2, Th/SiO2, Zr/Y, La/Yb, and Th/Hf from the Coastal block in the southeast to the Coxheath block in the northwest. The zonation may be explained in terms of a northwesterly-dipping subduction zone, with the trench lying to the southeast of Nova Scotia.

Tectonic implications of 40Ar/ 39Ar hornblende ages from late Proterozoic-Cambrian plutons in the Avalon Composite Terrane, Nova Scotia, Canada

The Avalon Composite Terrane in Nova Scotia mainly represents a late Proterozoic, rifted, cratonic, magmatic arc complex. The geo-chemistry of the volcanic rocks indicates that most of southeastern Cape Breton Island is underlain by a volcanic arc, whereas the Antigonish and Cobequid Highlands expose intra-arc rift sequences. In Cape Breton Island, late Proterozoic rocks are both volcanic and plutonic south of the Macintosh Brook fault (transecting the Boisdale Hills), whereas they are mainly plutonic rocks north of the fault. These igneous units are unconformably overlain by subaerial to shallow-marine Cambrian-Ordovician sedimentary sequences containing an Atlantic fauna that are interbedded with Early to Middle Cambrian, within-plate, continental-rift volcanic rocks. 4O Ar/ 39 Ar plateau ages recorded by hornblende from plutons spatially associated with these volcanic rocks correspond to three groupings: (1) ca. 600-635 Ma in Cape Breton Island south of the Macintosh Brook fault; (2) ca. 525-555 Ma in Cape Breton Island north of the Macintosh Brook fault (Boisdale Hills, North Mountain and Creignish Hills); (3) ca. 600-625 Ma in the Antigonish and Cobequid Highlands. Based upon the low metamorphic grade of the late Proterozoic and early Paleozoic rocks, and the low pressures recorded in contact aureoles and plutons, it is inferred that these plutons were emplaced at relatively shallow crustal levels, and therefore the 40 Ar/ 39 Ar plateau ages probably closely date their emplacement. Interpretation of the age data suggests that plutonism in Nova Scotia associated with the rifted magmatic arc lasted from ca. 635-600 Ma. Igneous activity continued between ca. 555-525 Ma and may have been related to the Cambrian, within-plate, rift volcanism.

Northeastern extension and crustal expression of terranes from Cape Breton Island, Nova Scotia, based on geophysical data

The narrowest section of the Appalachian Orogen in Atlantic Canada (from Grenville-age basement in the west to the Avalon Terrane in the east) is preserved in Cape Breton Island, where components of four terranes display distinctive stratigraphic, igneous, metamorphic, aeromagnetic, and gravity characteristics. The Blair River Complex of northwestern Cape Breton Island is a fragment of Grenvillian rocks that is similar to units in western Newfoundland. The Aspy Terrane is characterized by mid-Paleozoic granitic rocks and high-pressure paragneisses and appears to continue across the Cabot Strait as the Gander Terrane of the Hermitage Flexure in Newfoundland. The Bras d'Or Terrane is composed of Hadrynian to Early Cambrian low-pressure metasedimentary units and dioritic to granitic plutonic rocks. As indicated by aeromagnetic anomalies, this terrane continues across the Laurentian Channel to the south coast of Newfoundland, where it may be terminated by the Hermitage Fault. The Avalon Terrane of southeastern Cape Breton Island bears many similarities to the Avalon Terrane of Newfoundland.A seismic reflection profile obtained as part of the Lithoprobe-East project shows that the Blair River Complex is juxtaposed against the Aspy Terrane by steeply dipping fault systems that cut through the entire crust and that the Bras d'Or Terrane is similarly juxtaposed with the Avalon Terrane. The nature of the boundary between the Aspy and Bras d'Or terranes is less clear, although metamorphic conditions and the seismic reflection profile both suggest that the Aspy Terrane represents a deeper crustal level, over which the Bras d'Or Terrane has been emplaced.