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The Bras d'Or terrane of central Cape Breton Island, Nova Scotia, Canada, contains a well-preserved record of the Ediacaran to early Cambrian evolution of Ganderia, a Gondwana-derived terrane in the northern Appalachian orogen. A complex assemblage of low- to high-grade metasedimentary rocks has varied detrital zircon signatures from laser ablation inductively coupled plasma mass spectrometry U-Pb zircon dating, but combining three or more samples yielded representative age spectra that support correlation of the low- and high-grade metasedimentary rocks throughout the Bras d'Or terrane and the corresponding Ganderian Brookville terrane of southern New Brunswick. In quartzite samples from the McMillan Flowage Formation in the northwestern Bras d'Or terrane, the youngest detrital zircons have ages >900 Ma, in contrast to previously studied psammitic and semipelitic samples from correlative units in the eastern Bras d'Or terrane in which youngest detrital ages are 620-600 Ma. Both quartzite and semipelitic samples from the McMillan Flowage Formation contain Neoproterozoic dates from zircon rims, which reflect metamorphic overgrowths during peak metamorphism at ca. 550 Ma, providing a robust age for peak metamorphism in the Bras d'Or terrane that supports similar, albeit sparse, ages reported previously from monazite and titanite samples. This metamorphism is coeval with the emplacement of voluminous dioritic to granitic plutons that occur throughout the Bras d'Or terrane and form in an Andean-type continental margin subduction zone. New U-Pb zircon ages presented here from plutons in the northern Bras d'Or terrane, combined with previously published ages, are consistent with subduction-related magmatism and associated metamorphism between ca. 575 and 540 Ma. Key words: U-Pb geochronology, Appalachian metamorphism, Bras d'Or terrane, Cape Breton Island, McMillan Flowage Formation, detrital zircon, plutonic ages, Introduction The Bras d'Or terrane of central Cape Breton Island, Nova Scotia (Fig. 1), consists of mainly Ediacaran and Cambrian metamorphic and plutonic rocks overlain unconformably by Carboniferous sedimentary rocks [...]

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 [+ or -] 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. Key words: acritarch, trace fossils, Cambrian, U-Pb zircon, Avalonia, correlation, Introduction Cambrian rocks in southeastern Cape Breton Island, Nova Scotia (Fig. 1), have been visited and described by geologists since the 1800s (e.g., Fletcher 1878; Matthew 1903; Hutchinson 1952; Landing [...]

Devonian granitoid plulons comprise a major part of the bedrock of northwestern Maine representing the magmatic expression of the Acadian orogeny in this part of the northern Appalachian orogen. They are petrographically diverse with minerals characteristic of both I- and S-type granites, in some cases within the same intrusion, and some are compositionally zoned. New LA-ICP-MS ages presented here elucidate the timing and duration of this magmatism. The earliest phase of granitoid magmatism began around 410-405 Ma with the emplacement of the Flagstaff Lake Igneous Complex, and the presence of contemporaneous mafic rocks suggests that mantle-derived magmas were also produced at this time. Late Devonian ages, ca. 365 Ma, for many intrusions, such as the Chain of Ponds and Songo plutons, reveal that magmatism continued for 45 million years during which compositionally diverse I- and S-type magmas were produced. In addition, there is evidence that intrusive activity was prolonged within some plutons, for example the Rome-Norridgewock pluton and the Mooselookmeguntic Igneous Complex, with 10-15 myr between intrusive units. The new ages suggest a break in magmatism between 400 Ma and 390 Ma apparently separating Acadian magmatism into early and late pulses. The production of lower crustal I-type magmas appears to have been concentrated later, ca. 380-365 Ma, although several S-type granitoids were also emplaced during this period. These Late Devonian plulons display abundant zircon inheritance with ages around 385 Ma, which suggests that the crust was experiencing enhanced thermal perturbations during this extended timeframe. The new data for granitoid plutons in northwestern Maine are consistent with tectonic models for other parts of Ganderia which propose initial flat slab subduction followed by slab breakoff and delamination. Les plutons granitoides du Devonien constituent une tranche importante du substrat rocheux du nord-ouest du Maine representant l'expression magmatique de l'orogenese acadienne dans cette partie de l'orogene appalachien septentrional. Ils constituent une succession petrographiquement diversifiee, composee de mineraux caracteristiques tant des granites de type I que de type S, dans certains cas a l'interieur de la meme intrusion, alors que la composition des aulres plutons regionaux est zonee. Les nouvelles datations par ablation laser et speclrometrie de masse a plasma induclif presentees ici elucidenl le moment ou est survenu ce magmatisme et sa duree. L'episode le plus ancien du magmatisme granitoide a debute vers 410-405 Ma avec la mise en place du complexe de roches eruptives du lac Flagstaff, meme si la presence de roches mafiques contemporaines laisse supposer que des magmas d'origine mantellique ont egalement ete produits a ce moment. Les datations du Devonien tardif faisant remonter a environ 365 Ma de nombreuses intrusions, comme les plutons Chain of Ponds et Songo, revelent que ce magmatisme s'est dans l'ensemble poursuivi durant 45 millions d'annees au cours desquelles des magmas de type I et S de compositions diverses ont ete produits. II existe de plus des preuves que l'activite intrusive s'est prolongee a l'interieur de certains plutons, par exemple le pluton Rome-Norridgewock et le complexe de roches eruptives de Mooselookmeguntic, qui accusent un ecart de 10 a 15 Ma entre les unites intrusives. Les nouvelles datations permettent de supposer une interruption du magmatisme entre 400 et 390 Ma., qui semble separer le magmatisme acadien en impulsions precoce et tardive. La production de magmas de type I crustaux inferieurs semble s'etre concentree a un moment ulterieur, vers 380 a 365 Ma, bien que plusieurs roches granitoides de type S se soient egalement mises en place durant celte periode. Ces plutons du Devonien tardil presentent un heritage abondanl de zircons remontant a peu pres a 385 Ma, ce qui laisse entendre que la croute terrestre a subi des perturbations thermiques de plus en plus prononcees au cours de cet intervalle prolonge. Les nouvelles donnees sur les plutons granitoides du nord-ouest du Maine correspondent aux modeles tectoniques d'autres parties du terrane de Gander, qui supposent une subduction initiale de plaques plates suivie par une rupture des plaques et une delamination. [Traduit par la redaction], INTRODUCTION Granitic magmas produced during collisional tectonic events are important repositories of information regarding (i) the duration of orogenesis, (ii) the role of mantle inputs, both physical and thermal, and [...]

The Margaree pluton extends for >40 km along the axis of the Ganderian Aspy terrane of northern Cape Breton Island, Nova Scotia. The pluton consists mainly of coarse-grained megacrystic syenogranite, intruded by small bodies of medium-grained equigranular syenogranite and microgranite porphyry, all locally displaying rapakivi texture. The three rock types have similar U-Pb (zircon) ages of 363 [+ or -] 1.6, 364.8 [+ or -] 1.6, and 365.5 [+ or -] 3.3 Ma, respectively, consistent with field and penological evidence that they are coeval and comagmatic. The rare earth elements display parallel trends characterized by enrichment in the light rare earth elements, flat heavy rare earth elements, moderate negative Eu anomalies, and, in some cases, positive Ce anomalies. The megacrystic and rapakivi textures are attributed to thermal perturbation in the magma chamber caused by the mixing of mafic and felsic magma, even though direct evidence of the mafic magma is mainly lacking at the current level of exposure. Magma evolution was controlled by fractionation of quartz, K-feldspar, and Na-rich plagioclase in molar proportions of 0.75:0.12:0.13. The chemical and isotopic (Sm-Nd) signature of the Margaree pluton is consistent with the melting of preexisting continental crust that was enriched in heat-producing elements, likely assisted by intrusion of mantle-derived mafic magma during Late Devonian regional extension. The proposed model involving magma mixing at shallow crustal levels in a cryptic silicic-mafic magma chamber during post-Acadian extension is consistent with models for other, better exposed occurrences of rapakivi granite in the northern Appalachian orogen. Key words: rapakivi, megacrysts, U-Pb zircon, Devonian, crystal fractionation, PER diagrams. Le pluton de Margaree s'etale sur plus de 40 km le long de l'axe du terrane ganderien d'Aspy du nord de l'île du Cap-Breton (Nouvelle-Ecosse). Le pluton est principalement constitue de syenogranite megacristique à grains grossiers, recoupe par de petits corps de syenogranite equigranulaire à grains moyens et de porphyre microgranitique, qui presentent tous une texture rapakivi. Les trois types de roche donnent des âges U-Pb sur zircons semblables de 363 [+ or -] 1,6, 364,8 [+ or -]1,6 et 365,5 [+ or -] 3,3 Ma, respectivement, ce qui concorde avec des observations petrologiques et de terrain indiquant qu'ils sont contemporains et comagmatiques. Les terres rares presentent des spectres paralleles caracterises par un enrichissement des terres rares legeres, des tronçons des terres rares lourdes plats, des anomalies negatives moderees d'Eu et, dans certains cas, des anomalies de Ce positives. Les textures megacristique et rapakivi sont attribuees à une perturbation thermique dans la chambre magmatique causee par le melange de magmas mafique et felsique, bien que des preuves directes de la presence d'un magma mafique soient generalement absentes au niveau d'exposition actuel. L'evolution du magma etait controlee par la cristallisation fractionnee du quartz, du feldspath potassique et du plagioclase riche en Na dans des proportions molaires de 0,75:0,12:0,13. Les signatures chimique et isotopique (Sm-Nd) du pluton de Margaree concordent avec la fusion de croûte continentale preexistante qui etait enrichie en elements thermogenes, vraisemblablement aidee par l'intrusion de magma mafique derive du manteau durant l'extension regionale au Devonien tardif. Le modele propose qui comprend le melange de magmas à des niveaux crustaux peu profonds dans une chambre magmatique siliceuse-mafique cryptique durant l'extension post-acadienne concorde avec les modeles proposes pour d'autres cas mieux exposes de granite rapakivi dans le nord de l'orogene des Appalaches. [Traduit par la Redaction] Mots-cles: rapakivi, megacristaux, U-Pb sur zircons, Devonien, cristallisation fractionnee, diagramme de Pearce., Introduction Rapakivi granite sensu stricto is characterized by the presence of plagioclase-mantled K-feldspar, independent of K-feldspar morphology or origin (Ramo and Haapala 1995). The debate over the formation of rapakivi [...]

Geological mapping, petrological studies, and U-Pb (zircon) dating were done to better characterize metasedimentary and metaigneous rock units in the northeastern Aspy terrane, Cape Breton Highlands, Nova Scotia. Foliated biotite [...]

Remote predictive mapping is increasingly used to identify and investigate potential exploration targets. This project applies principal component (PC) analyses and K-means clustering algorithms to geochemical data from the New Quebec Orogen to investigate spatial distribution of possible exploration vectors, and to link those exploration vectors to large-scale crustal structures. The dataset of11 260 lake sediment samples containing the chemical results using inductively coupled plasma mass spectrometry was obtained from the Système d'information géominière of Québec and ioGAS-64™ was used to apply the multivariate techniques. The metals Ag, As, Au, Bi, Co, Cr, Cu, Fe, Mo, Pb, Se, Th, U, V, W, and Zn were selected because were analyzed in all the samples and are economically significant. The results show that four principal components (PC1-PC4) yielded eigenvalues >1.0 and account for 58.0% of the variability. PC1 and PC2 account for 38.7% of the variability between Se, U, Mo, and W (PC1) and Cu, Ag, Au, Se, Zn, and Cr (PC2). PC3 accounts for 11.6% with an enhanced separation between As, Zn, Bi, Pb, Fe, Ag, and Mo. PC4 accounts for 7.8% with positive eigenvectors for Pb and Bi. A K-means clustering equals to 6 was obtained using these PCs. The association yielded by PC1 was interpreted as metals transported by sedimentary processes and might highlight hazard areas. The high scores of PC2 and the K-means results correlated with Ni-Cu-PGE occurrences reported within the Retty and Hurst zones. The results of these analyses show that the methods can pinpoint the location of known metal occurrences with excellent spatial accuracy, and in several tested areas accurately outline known geological units. These types of statistical techniques can therefore be used to predict similar occurrences in underexplored areas that can then be selected for exploration. [ABSTRACT FROM AUTHOR]

In the Wolfville and Bear River areas pre-Carboniferous rocks of the Goldenville, Halifax, and Rockville Notch groups are intruded by mafic sills (Type I and Type II) that have similar chemistry but different textures, alteration, and mineralogy. Type I sills appear to be penecontemporaneous with the country rock deposition based on field relations but there are no conclusive geochronological data for the sills. The relationship between Type I and Type II sills is not clearly understood and their respective effects on the country rock have not been investigated. Field mapping of sills in the Wolfville area shows that sills range from 1 to 65 m in thickness and are concordant with bedding. All the sills have sharp bottom contacts with the country rock but only larger sills have sharp top contacts. The larger sills are generally homogeneous from bottom to top and show poorly developed cleavage along their bottom contacts with the country rocks. Some of the smaller sills have vesicular textures and convolute margins along their top contacts. Contact aureoles at the margins of the sills are variable in thickness and some appear to be silicified. Further field mapping and petrographic work will focus on mineralogical changes from bottom to top in the sills and comparisons among different sills, as well as the extent and mineralogy of the contact aureoles in contrast to the country rock unaffected by the sills. Laser Ablation ICP-MS U-Pb analysis will be used to determine the zircon crystallization and apatite cooling ages for the sills. [ABSTRACT FROM AUTHOR]

The Meguma terrane of mainland Nova Scotia underwent regional greenschist to amphibolite facies metamorphism between ca. 410 and 380 Ma. The relationships between deformation and metamorphism in the northeastern Meguma terrane are not well understood, a problem further complicated by the presence of voluminous granitic intrusions (some of which are deformed). The plutons produced contact metamorphic aureoles that overprinted regional metamorphic mineral assemblages in some areas. This study is focused on the area between Trafalgar and Country Harbour with the objective of documenting and interpreting the mineralogy, deformation history, and pressure-temperature conditions recorded in the rocks to help contribute to the understanding of the relative timing and conditions of deformation and metamorphism in the northeastern Meguma terrane. The highest grade regional metamorphic rocks in the study area are garnet-staurolite-biotite-muscovite schist, in which fabrics indicate that the metamorphism was pre- and syn-deformational in most areas but may be post-deformational in others. Additionally, andalusite schist, andalusite-cordierite schist, and gneiss occur in contact aureoles around plutons. Forward and inverse thermodynamic modeling suggest temperatures from ~500°C to ~550°C and pressures ranging between ~3.5 kbar and ~4.5 kbar in the main metasedimentary assemblages of the study area. Temperatures are up to ~600°C in contact aureoles at similar pressure conditions. However, the fabrics and overprinting relationships in the contact aureole rocks show that contact metamorphism predated the deformation, and therefore potentially predated regional metamorphism. This result is surprising because the sequence of events has previously been interpreted to be the opposite, suggesting that the events are diachronous across the area. The implication is that the Chedabucto fault system may have had a major influence on the sequence of metamorphism and deformation in this part of the Meguma terrane, related to juxtaposition with Avalonia. [ABSTRACT FROM AUTHOR]

The article critiques the comments made by Landing et al. on their paper, emphasizing that their interpretation of Cambrian strata in the Caledonian Highlands is flawed and lacks scientific support. It also discusses the issues with forced lithostratigraphic frameworks, the inadequate correlation methods used by Landing et al., and the inaccuracies in their data and interpretations regarding the Cambrian geological record.

The Cambrian syn-rift strata preserved in western Avalonia provide a distinctive example of how unconformity-bounded sequences are diachronous throughout proximal to marginal rift branches. Terreneuvian--Miaolingian third-order sequences of the Caledonian Highlands, New Brunswick, Canada, reflect a complex interplay among syn-rift tectonic events, denudation pulses, and sea-level fluctuations. Unconformably overlying the early, rift-related volcanosedimentary Coldbrook Group (ca. 560-550 Ma), the Ratcliffe Brook, Glen Falls, Hanford Brook, and Forest Hills Formations can be subdivided into two transgressive systems tract (TST)--highstand systems tract (HST) sequences (each ~10 m.y.) and an incomplete TST sequence that are separated by stratigraphic gaps. They reflect uplift and tilting events affecting the basement, transgressive and drowning surfaces, and condensed sections. Arid to semi-arid climatic episodes are supported by the excellent preservation of mafic to felsic volcanic clasts in non-marine breccias and conglomerates, which are derived from the Ediacaran basement, and the local precipitation of marine gypsum through the evaporation of pore fluids. Early Miaolingian episodes of microbial/ shelly carbonate production preserved precipitates of coeval evaporite (gypsum pseudomorphs after drusy mosaics of calcite) and ikaite (glendonitic, star-shaped aggregates and crusts). Both minerals, traditionally considered to be indicators of contrasting climate conditions, potentially co-occur in temperate-water substrates recording high rates of microbial activity. The early rift phases preserved in the western Avalonian rift transect comprise stepwise uplift and unroofing of rift shoulders, which are related to diachronous, angular discordances and paraconformities bounded by syntectonic slope-apron deposits. Facies homogenization was attained during Miaolingian times as a result of generalized flooding, sealing of paleotopographies, and blanketing with monotonous offshore-dominant shales. [ABSTRACT FROM AUTHOR]

Field, petrological, dating, and structural studies in the central Cape Breton Highlands have led to new understanding of the complex history related to convergence between the Aspy and Bras d'Or terranes along the Eastern Highlands shear zone in the late Devonian. The Park Spur pluton (PSP) was emplaced in the Aspy terrane in the early Late Devonian (ca. 375 Ma, U-Pb zircon). Its age combined with S-type petrological characteristics and abundant pegmatite and aplite show that it is co-magmatic with the Black Brook Granitic Suite and satellite plutons that characterize the northern Aspy terrane. These widespread plutons may have formed by crustal melting caused by subducted slab failure. On its northern margin the PSP intruded high-grade Ordovician-Silurian metamorphic rocks of the Cape North Group at mesozonal depth (>10 km), consistent with its petrological features and abundant pegmatite and aplite dykes. On its southern margin the PSP was synchronously deformed in the east-west-trending Park Spur Road shear zone and juxtaposed in a southerly direction over the low-grade metasedimentary and metavolcanic rocks of the Ordovician-Silurian Calumruadh Brook Formation (CBF). Ongoing dextral transpression between Aspy and Bras d'Or terranes reactivated the north-south-trending Central Highlands shear zone, on which the higher- grade Middle River metamorphic suite and Silurian Taylors Barren pluton to the west were transported upward relative to the adjacent CBF and PSP. By about 365 Ma, convergence had mainly ended, as evidenced by shallow emplacement of the ca. 363 Ma Margaree pluton in an extensional setting. [ABSTRACT FROM AUTHOR]