Cobequid Highlands Research Articles

Chronology of Devonian to early Carboniferous rifting and igneous activity in southern Magdalen Basin based on U-Pb (zircon) dating

Fifteen U–Pb (zircon) radiometric age determinations have been made on igneous rocks of Middle Devonian to Early Carboniferous age from the southern margin of the Magdalen basin in Cape Breton Island and northern mainland Nova Scotia. Volcanic rocks interbed with early rift-basin sedimentary rocks with some palynological biostratigraphy; dated intrusive rocks cut these sedimentary units. Our biostratigraphically constrained ages are in close agreement with the current Devonian time scale. Combined with previously published data, the age determinations show that igneous activity occurred in four pulses: Middle Devonian (390–385 Ma), early Late Devonian (375–370 Ma), latest Devonian to early Tournaisian (365–354 Ma), and late Tournaisian to early Visean (ca. 339 Ma). Middle Devonian (385–389 Ma) volcanic rocks are confined to the Guysborough Group. The Fisset Brook Formation (basalt and minor rhyolite) in the type area and elsewhere in Cape Breton Island and northern mainland Nova Scotia is Late Devonian (ca. 373 Ma), whereas the biostratigraphically distinct succession at Lowland Cove is younger (365 Ma). These Late Devonian rocks are synchronous with plutonism in the Cape Breton Highlands and the Meguma terrane. In the Cobequid Highlands, rhyolite of the Fountain Lake Group was synchronous with Horton Group deposition and with widespread granite plutons (362–358 Ma) emplaced during shear on the Cobequid fault zone. The overlying Diamond Brook Formation basalts are slightly younger (355 Ma). Late Tournaisian – early Visean mafic intrusions and minor basalt occur along the Cobequid – Chedabucto fault zone and in a belt from southern New Brunswick through Prince Edward Island to southwestern Cape Breton Island.

The role of dextral transpressional faulting in the evolution of an early Carboniferous mafic–felsic plutonic and volcanic complex: Cobequid Highlands, Nova Scotia, Canada

The Wentworth plutonic complex, consisting of gabbro and granite, was emplaced in the earliest Carboniferous in the Cobequid shear zone of the northern Appalachians. The plutonic complex is coeval with a 5-km-thick pile of volcanic rocks. Early alkalic A-type granite correlates with thick felsic pyroclastics and minor basalt, which are overlain by 1.5-km-thick basalts that correlate with a large gabbro pluton that is intruded, in turn, by late granites. The basalt and gabbro are Fe-rich tholeiites. The geochemistry of the late granites suggests that they formed by differentiation of a granodioritic magma resulting from assimilation of early granite by the gabbroic magma. The Wentworth plutonic complex lies on the north side of the dextral Rockland Brook fault, near the western tip of wedge-shaped basement block of the Avalon terrane. Field observations of mesoscopic structures and map contacts show that the plutonic bodies at all structural levels are related to transpressive strike–slip faults. Dykes parallel to the mylonitic foliation in the Rockland Brook fault zone and at the contacts between igneous phases suggest that the plutons developed largely through dyke to pluton construction. The plutonism was initiated by dyking related to major faults under transpression that was partitioned into shear zone-bounded blocks, while the sinking of those blocks finally provided the space for mafic magma emplacement. Dyking was active over at least a 10-Ma time period. The overall location of plutonism in the Cobequid shear zone appears related to its position at the intersection of the shear zone bounding the southwestern margin of the Magdalen basin and the E–W transpressional contact of the Avalon and Meguma terranes. Magmatism enabled thermomechanical softening of the crust and the vertical and lateral extrusion of the wedge-shaped basement blocks, whose movement controlled the localisation of the voluminous magmatic activity.

Tectonic deformation and magmatism along the southern flank of the Maritimes Basin: the northeastern Cobequid Highlands, Nova Scotia

Distributed crustal-scale faulting in the Cobequid Highlands in the Middle Devonian to Carboniferous resulted from the oblique convergence of the Meguma and Avalon terranes. In the northeastern Cobequid Highlands, seismic reflection profiles show Neoproterozoic and lower Paleozoic rocks, together with enigmatic foliated rocks, overlying the Early Carboniferous Fountain Lake Group. The foliated rocks form the hanging wall of a north-vergent thrust fault. Their protolith is inferred from petrography and geochemistry to be principally Neoproterozoic rhyodacitic tuff and late Paleozoic hypabyssal intrusions. The age of thrusting is stratigraphically constrained to the late Tournaisian – mid-Viséan, and sericite from mylonite yielded a Tournaisian K–Ar age of 352 ± 8 Ma. The thrusting occurs at the base of a tectonic escape sheet and resulted from a restraining bend in the Rockland Brook master fault. Farther west, where the Rockland Brook fault trends almost east–west, Tournaisian extensional features include the Nuttby basin and widespread gabbro dykes, sills, and stocks. At deeper structural levels, granite plutons were intruded in a similar tectonic regime of thrusting and local extension by lateral movement of basement blocks. The emplacement process resulted from progressive widening of initial dykes, analogous to the dykes deformed in the thrust hanging wall. Regionally, in the Tournaisian of the southern Maritimes Basin half-graben formation was synchronous with pluton emplacement and thrusting in adjacent horsts.

Tectonic deformation and magmatism along the southern flank of the Maritimes Basin: the northeastern Cobequid Highlands, Nova Scotia

Tectonic deformation and magmatism along the southern flank of the Maritimes Basin: the northeastern Cobequid Highlands, Nova Scotia

Geochemistry of the Namurian Lismore Formation, northern mainland Nova Scotia: sedimentation and tectonic activity along the southern flank of the Maritimes Basin

Geochemical and isotopic data from the clastic rocks of the Namurian Lismore Formation in mainland Nova Scotia identify key episodes of tectonic activity during the development of the Maritimes Basin in Atlantic Canada. The Lismore Formation forms part of the Mabou Group and is an upward-coarsening 2500 m thick fluvial sequence deposited in the Merigomish sub-basin along the southern flank of the Maritimes Basin. Based on stratigraphic evidence, the Lismore Formation can be divided into upper and lower members which reflect variations in depositional environment and paleoclimate. The geochemical and isotopic data may also be subdivided into two groupings that primarily reflect varying contributions from accessory phases, clay minerals, or rock fragments. This subdivision occurs 115 m above the base of the upper member. The data from the lower grouping (group A) show an important contribution from underlying Silurian rocks, with a relatively minor contribution from Late Devonian granitoid rocks from the adjacent Cobequid Highlands and possibly metasedimentary rocks from the Meguma Terrane to the south. The data from the upper grouping (group B) reveal a more important contribution from the Cobequid Highlands granitoid rocks. This variation in geochemistry is thought to constrain the age of renewed motion and uplift along the faults along the southern flank of the Maritimes Basin and, more generally, suggests that geochemical and isotopic data of continental clastic rocks may help constrain the age of tectonic events that influence deposition of basin-fill rocks.

U-Pb Geochronology of Devonian Granites in the Meguma Terrane of Nova Scotia, Canada: Evidence for Hotspot Melting of a Neoproterozoic Source.

U-Pb isotopic analyses of monazite and zircon from six granitic plutons in the Meguma Terrane yield nearly concordant ages of 373+/-3 Ma, interpreted as the time of intrusion. U-Pb analyses of euhedral zircons with thick rims overgrowing cores, which were abraded to remove all or most of the rim, plot on chords between 370+/-3 and 628+/-33 Ma (Larrys River and Halfway Cove plutons), 372+/-3 and approximately 660 Ma (Shelburne pluton), and 373+/-2 and approximately 732 Ma (Barrington Passage pluton). The upper intercepts are interpreted as the age of magma source, correlatives of which are present in the Avalon Composite Terrane to the north. This basement may be either in depositional or tectonic contact with the overlying Cambro-Ordovician Meguma Group. Other zircons in the granites are generally irregular-euhedral with thin rims, and most U-Pb isotopic analyses fall between two chords from 373-2040 and 373-2300 Ma, with a few lying outside this field. These zircons are probably derived from the country rock (Goldenville Formation), which a previous study has shown contains detrital zircons with concordant U-Pb ages of 3000, 2000, and 600 Ma, and numerous intermediate discordant ages. These new ages, along with published data, document a relatively short (5-10 m.yr.) but voluminous period of magmatism. This age is approximately synchronous with intrusion of mafic rocks and lamprophyre dikes and regional low-pressure metamorphism and was followed by rapid denudation of 5-12 km. These observations may be interpreted in terms of shallowly dipping subduction and overriding of a mantle plume that eventually penetrates through the subducting plate to melt the overriding continental plate. Subsequent northward migration of the plume could explain both the approximately 360 Ma magmatism in the Cobequid Highlands (Avalon Composite Terrane) and the mid-Carboniferous plume-related intrusions around the Magdalen Basin.

Using whole rock geochemistry to locate the source of igneous erratics from drumlins on the Atlantic coast of Nova Scotia

Whole rock geochemistry has been determined for erratics from the surface Lawrencetown Till in two drumlins on the Atlantic coast of Nova Scotia. The geochemical data, particularly trace element composition, allow source rocks to be identified more precisely than by traditional petrographic microscopy. A drumlin near Lunenburg contains erratics of Neoproterozoic arc‐related plutonic rocks and basalts that outcrop only in a small area near Parrsboro in the Cobequid Highlands. Some associated erratics of granite precisely match a small late Devonian pluton in the same area and other granites and rhyolite could also be derived from late Devonian rocks in the same area. The type section of the Lawrencetown Till contains different Neoproterozoic plutonic rocks of a type found in the eastern Cobequid Highlands, together with a distinctive late Devonian granite with sodic amphiboles that is restricted to a small area near the Debert River. Both these source areas indicate a southward flow of ice (Escuminac Phase) depositing the Lawrencetown Till, in contrast to earlier southeastward‐flowing ice (Caledonia Phase) which deposited the Hartlen Till that forms the core of the drumlin. Drumlins in Nova Scotia are complex palimpsest landforms that were formed by till accretion, and evolved in surface form throughout multiple flow events.

Scapolite Pegmatite from the Minas Fault, Nova Scotia: Tangible Manifestation of Carboniferous, Evaporite-Derived Hydrothermal Fluids in the Western Cobequid Highlands?

Pegmatite cutting chlorite schist in the Minas fault at McKay Head, Nova Scotia, consists of Cl-rich (2.7–3.8 wt.% Cl) marialitic scapolite (EqAn21–32) with interstitial, apparently primary analcite, hematite and rutile, and later (including vug-lining) analcite, pyrite, chlorite, titanite and calcite, and cross-cutting epidote veins. Some of the latter phases might have crystallized from residual pegmatitic fluids. Unlike many other primary scapolite-bearing igneous rocks, the McKay Head occurrence has compositional affinities with mafic (rather than felsic) systems: it is enriched in transition metals (e.g. Cr≤53 ppm), and has very low LILE concentrations (e.g. Rb<10 ppm; U<1 ppm; Th<2 ppm; Ba<20 ppm) and Rb/Sr ratios (~0.05). The presence of interstitial rutile and hematite rather than ilmenite indicates that the pegmatitic fluid was oxygenated late (T~400°C) in its crystallization history. The pegmatite is interpreted to be related to highly sodic hydrothermal solutions derived from (or affected by) early Carboniferous evaporites of the Windsor or Horton groups. Compositionally-similar fluids, perhaps also related to an evaporite source, may be responsible for a regional, early Carboniferous Na-metasomatic event that altered a suite of alkaline granitoid intrusions shortly after their emplacement.

U-Pb geochronological constraints on Precambrian stratified units in the Avalon Composite Terrane of Nova Scotia, Canada: tectonic implications

Concordant U-Pb ages of detrital zircons in several Proterozoic units in the Avalon Composite Terrane of Nova Scotia, along with previously published data, indicate that deposition of low- and high-grade stratified units in the Cobequid Highlands (Gamble Brook Formation) and central Cape Breton Island (variously assigned to the George River Group or George River Metamorphic Suite and the Bras d'Or Gneiss) started after ~977 Ma, extended through a period of volcanic activity between ~694 and 637 Ma, and ended before the ~629-612-600 Ma volcanic activity associated with deposition of the Georgeville and Jeffers groups in the Antigonish and Cobequid highlands, and the East Bay Hills and Coxheath groups in southern Cape Breton Island. These age data, along with U-Pb analyses of metamorphic and igneous zircon and monazite, also constrain three Late Proterozoic tectonothermal events: (i) oblique sinistral ductile shear deformation and greenschist-amphibolite-facies metamorphism probably occurred between ~694 and 629 Ma; (ii) oblique dextral deformation and greenschist-facies metamorphism is bracketed between ~612 and 607 ± 2 Ma; and (iii) low-angle, normal shear deformation associated with low-pressure, high-temperature metamorphism took place at ~553-550 Ma. These three events may record (i) opening and (ii) closing of the Antigonish-Cobequid back-arc basin, followed by (iii) gravitational collapse of the central Cape Breton magmatic arc following cessation of subduction, respectively. The combination of ~700-550 Ma arc magmatism and ~1.3-1.0 Ga concordant detrital zircon ages in the Avalon Composite Terrane suggests provenance in the Amazon craton or Oaxaquia in Mexico.

U-Pb geochronology of Late Palaeozoic plutons, Cobequid Highlands, Nova Scotia, Canada: evidence for Late Devonian emplacement adjacent to the Meguma-Avalon terrane boundary in the Canadian Appalachians

The Cobequid Highlands in the Canadian Appalachian orogen lie within Avalonia adjacent to the Meguma Terrane. U-Pb (zircon) data show that the age range of voluminous bimodal plutonism in the highlands is from 358 to 363 Ma (late Devonian). This age range is much narrower than that previously suggested by Rb/Sr geochronology and confirms that the Cobequid Highlands preserve the youngest large-scale plutonic event in the Canadian Appalachians. Late Palaeozoic tectonic history of the Appalachian orogen is profoundly influenced by predominantly dextral motion on the Avalon-Meguma terrane boundary. This age of plutonism is coeval with previously published 40Ar/39Ar (muscovite) plateau ages derived from shear zones in the Meguma terrane adjacent to the terrane boundary, which is defined by the Minas fault zone. The NNE trending structural grain of the Appalachian orogen is disturbed in this area by the E-W Minas fault zone and pluton emplacement may have been associated with motion along this terrane boundary.

The stratigraphy and geochemistry of late Devonian to early Carboniferous volcanic rocks of the northern Chignecto peninsula, Cobequid Highlands, Nova Scotia

The volcanic succession of the Fountain Lake Group of the northern Chignecto peninsula consists of basalt and andesite flows, rhyolite shallow intrusions and minor flows, and rhyolitic pyroclastic rocks. Fault scarp breccia, fluvial conglomerate and sandstone, and lacustrine siltstone and minor limestone are interbedded with the volcanic rocks. The succession is cut by mafic dykes, some of which predate and others postdating brittle north-vergent thrusting. Detailed geochemical and petrographic study shows that the volcanic rocks geochemically resemble the plutonic rocks of the Cape Chignecto pluton that are thrust over the volcanic succession. Diabase dykes may record the contamination of gabbroic magma by residual basc-of-crust plagioclase-pyroxenc granulite from which felsic magma had been extracted. Fractionation of this mafic magma resulted in enrichment in incompatible elements in some dykes. Andesites evolved from basaltic parents cither by fractionation or by melting of hydrous gabbros. Much of the volcanic succession shows low-temperature alteration including silicification.
 
 RÉSUMÉ
 La succession volcaniquc du Groupe du lac Fountain dans la partie septcntrionale de la péninsule de Chignecto comprend des ccoulcments de basalte et d'andésite, des intrusions peu profondes de rhyolite et des écoulements mineurs, de même que des roches pyroclastiques rhyolitiques. Des breches d'escarpements de failles, du grès et un conglomerat fluvial, ainsi que du siltstone lacusfre et une faible quantité de catcaire sont interstratifiés-de roches volcaniques. La succession est entrecoupeé par des filons intrusifs mafiques, dont certains sont antérieurs et d'autres postérieurs à une poussée fragile orientée vers le nord. Une étude géochimique et pétrographique détaillée montre que les roches volcaniques ressemblent géochimiquement aux roches plutoniques de l'intrusion ignée de Cape Chignecto qui recouvrent la succession volcanique. Des filons intrusifs diabasiques peuvent enregistrer la contamination du magma gabbroique par la granulite résiduelle de ptagioclase-pyroxène de la base de la croûte de laquelle du magma felsique avait été extrait. Le fractionnement de ce magma maflquc a produit un enrichissement en éléments incompatibles de certains filons intrusifs. Les andésites ont évolué è partir de parents basaltiques, par fractionnement ou par fusion de gabbros aqueux. Une vaste part de la succession volcanique rcvele une altération, notamment une silicification, à basse température.
 
 [Traduit par la rédaction]

Kinematics of the Rockland Brook Fault, Nova Scotia: Implications for the interaction of the Meguma and Avalon terranes

The Cobequid-Chedabucto fault system of northern mainland Nova Scotia represents the surface expression of the Avalon-Meguma terrane boundary, but because it is exposed at high crustal levels in the Cobequid Highlands, the fault system provides little information as to the kinematic relationships of the two terranes in this area. In the eastern Cobequid Highlands, the Rockland Brook Fault (RBF) is exposed within the more deeply eroded highlands massif and juxtaposes units of widely varying ages and lithologies. Therefore, this fault is better suited to define the nature and timing of fault movement associated with Avalon-Meguma terrane interaction. In several large Carboniferous plutons along the length of the RBF, and in previously deformed Precambrian rocks, mylonitic foliation orientations are predominantly east-west trending and mineral lineations plunge southeast. Kinematic indicators such as minor fold vergence, porphyroclast systems, asymmetric boudins, shear-band fabrics, and preferred recrystallization orientations indicate dextral shear. These data are taken to infer that the central section of the RBF is dominated by dextral strike-slip motion. Transpression occurs locally where the RBF curves into restraining bends. Kinematic data in these bends indicate top to the northwest thrusting. At the easternmost extent of the RBF, high-level brittle normal faults predominate in the locally extensional environment. The timing of RBF movement is constrained only by the ca 360 Ma granite bodies which it deforms and by the Westphalian sedimentary rocks which are affected by only the latest stages of movement. These kinematic data are consistent with previously published kinematic models for the interaction of the southern margin of the Avalon Composite Terrane with the Meguma Terrane in mainland Nova Scotia. These models suggest that regional dextral shear was accompanied by localized components of transpressional thrusting, wrench tectonism, and small-scale sedimentary basin development during Devonian to Carboniferous terrane interaction.

The impact of climate change, tectonism and hydrology on the formation of Carboniferous tropical intermontane mires: the Springhill coalfield, Cumberland Basin, Nova Scotia

Both climate and tectonism are influential in determining basin and mire hydrology. In the rapidly subsiding intermontane basins of Euramerica which formed in response to the Variscan-Hercynian-Alleghenian orogeny, however, the record of climate change has been muted and may therefore go unrecognized. The complex interplay of climate and tectonism in a tropical continental setting is recorded in the > 4 km thick Westphalian A-B fill of the Cumberland Basin. Thick peat accumulation was favoured at the southern basin margin where rheotrophic forested mires 4–9 km wide flourished between basin-axis rivers and coalesced alluvial fans derived from the Cobequid highlands. As a result of their geomorphic setting the thicker coals (⩽ 4.3 m) exhibit distinct areal zones: piedmont, inner mire and riverine. Evidence that the forest swamps were nourished both by rainfall and by groundwater discharge from alluvial fans includes the stratigraphic affinity of thick coals and basin-margin conglomerates, interfingering of distal fan sheetflood and mire deposits, petrographic evidence of elevated pH (partial gelification of tissues), and predominance of the arboreous lycopsids Lepidodendron and Paralycopodites. Whereas such groundwater supply is crucial to maintenance of modern wetlands in drier periods, a subhumid climate is implied. Coals at Joggins, further removed from groundwater discharge at the basin margin, are less well developed. A 600-m thick “coal window” within the fining and reddening upward basin-fill sequence at Springhill is inferred to represent a coincidence of optimal groundwater supply and net subsidence. The ultimate decline of peat accumulation in the basin may reflect decrease in catchment area and fan size and loss of rainshadow as the Cobequid Massif was denuded and the basin filled. The coal window [ca. (6−9) × 10 5 yr] is punctuated by non-marine cyclothems of coal, grey sideritic mudrock and multistorey sandstone that share similar time frames with astronomic cycles of axial precession [ca. (1−4) × 10 4 yr]. The stress of related climate change is inferred to have caused peat accumulation to fall into disequilibrium with sediment flux and basin subsidence. The Late Quaternary history of certain African mires illustrates the dramatic impact of climate change (especially temperature and seasonality) and orographic effects on tropical continental peat accumulation. Aspects of the groundwater-recharge model may be applicable to the Westphalian-Stephanian intermontane mires of Spain and Mesozoic coal basins of northern China.

Devonian-Carboniferous igneous intrusions and their deformation, Cobequid Highlands, Nova Scotia

Field relationships and regional distribution patterns of igneous rocks in the Cobequid Highlands provide information on sequential styles of deformation during the later Devonian and Carboniferous formation of the Magdalen Basin. New mapping and aeromagnetic data show that large diorite plutons have orthogonal outlines, suggesting that they occupy pull-apart space. Diorites both cut and are cut by granite plutons, with magma intermingling relationships suggesting co-existing magmas. Limited dating suggests that most plutonism occurred at about the Devonian-Carboniferous boundary and was preceded by mid to late Devonian volcanism in the eastern Cobequids, younger ages reflect alteration and/or deformation events. Granite veins in diorite plutons show strike-slip syn-magmatic deformation. Ductile thrust deformation of the western granite plutons and Fountain Lake Group was followed by brittle faulting and emplacement of dykes along zones of weakness showing east-west pull-apart. Magnetic data reveal that rocks similar to the major diorite plutons underlie much of the central and western Cobequid Highlands and the northeastern part of the Minas sub-basin. The diorite plutons may be an exposed representative of mafic magma that underplated much of the extensional Magdalen Basin. The structure of the Cobequid Highlands is dominated by east-west faults parallel to the Cobequid-Chedabucto Fault that at times acted as transfer faults for major extensional structures in the Magdalen Basin.
 
 RÉSUMÉ
 Les relations de terrain et les types régionaux de distribution des roches ignées dans les hautes terres de Cobequid fournissent des informations sur les styles séquentiels de déformation pendant la formation du bassin de la Madeleine. Une cartographic et des données aéromagnétiques récentes montrent que de grands plutons de diorite ont des contours orthogonaux, suggèrant qu'ils occupent des espaces de transtension. Les diorites à la fois coupent des plutons de granite et sont recoupés par eux, avec des relations d'enchevétrement de magmas suggèrant leur coexistence. Quelques datations suggèrent que la plupart du plutonisme a eu lieu près de la limite Dévonien-Carbonifère et qu'il a été précedé par le volcanisme du Dévonien moyen à tardif dans les Cobequids orientales; les âges plus jeunes refletent des événements d'alteration ou de déformation. Des veines de granite dans les plutons de diorite montrent une déformation syn-magmatique en décrochement. La deformation ductile en chevauchement des plutons granitiques occidentaux et du Groupe de Fountain Lake a été suivie par un faillage cassant et la mise en place de dykes le long de zones de faiblesse montrant des rhombochasmes est-ouest. Les données magnétiques révèlent que des roches similairés aux plutons majeurs de diorite sont sous-jacentes à une grande partie des hautes terres de Cobequid centrales et occidentalcs et à la partie nord-est du sous-bassin de Minas Les plutons de diorite pourraient etre un échantillon affleurant du magma mafique qui s'est accumule sous une grande partie du bassin d'extension de la Madeleine. La structure des hautes terres de Cobequid est dominée par des failles est-ouest parallèles à la faille Cobequid-Chedabucto qui, par moments, a âge commc une faille de transfert pour les structures majeures d'extension dans le bassin de la Madeleine.
 
 [Traduit par la rédaction]

Tectonic significance of the Late Proterozoic Economy River gneiss, Cobequid Highlands, Avalon Composite Terrane, Nova Scotia

A 734 ± 2 Ma U–Pb (zircon) age for the Economy River orthogneiss, Coboquid Highlands, Nova Scotia, is interpreted as being representative of a regionally extensive ca. 820–660 Ma event that is recorded in many parts of the Late Proterozoic – Early Cambrian Avalon Composite Terrane and the Gondwanan margin. The geochemistry of the gneiss is consistent with an arc environment. Although the gneiss may represent part of the sialic basement to the terrane, field relationships indicate that some of the basement is significantly older. The date may provide a minimum age for the platformal sedimentary rocks (Gamble Brook Formation) that the orthogneiss intruded and thus help constrain the Late Proterozoic paleogeographic position of Avalon relative to Gondwanaland.

Late Proterozoic tectonic model for the Avalon Terrane in Maritime Canada

The Avalon Terrane in Maritime Canada extends from Cape Breton Island through northern Nova Scotia (Antigonish and Cobequid highlands) to southern New Brunswick. The terrane contains abundant late Proterozoic volcanic rocks (∼570–680 m.y. old). The geochemistry of the volcanic rocks of southern Cape Breton Island and southern New Brunswick indicates that they were erupted in a volcanic arc environment. In both regions the mafic rocks display a remarkably similar compositional zonation which resembles the across‐arc variations observed in modern volcanic arcs. The progressive compositional changes that include a transition from island arc tholeiites along the southeastern coasts to calc‐alkaline rocks inland to the northwest are probably due to a northwest dipping subduction zone with a trench located to the southeast. In the Antigonish and Cobequid highlands, continental tholeiites and calc‐alkaline volcanic rocks are interbedded and overlain by turbidites that have probably been deposited in a back arc or intra‐arc environment. The superimposed positive gravity and magnetic anomalies that may be traced discontinuously from the Grand Banks through Nova Scotia (Collector Anomaly) and along the northern side of the Gulf of Maine to near Boston, probably represent late Proterozoic ophiolitic remnants preserved by incomplete suturing along a late Proterozoic plate boundary. Northeast trending segments are inferred to mark subduction zones, and the E‐W portions could correspond to transform faults. Thus the present geographical distribution of the Late Proterozoic volcanic rocks is explained by a tectonic model involving a single subduction zone offset by two late Proterozoic transform faults located approximately along the Minas Fault and the Scatarie Ridge.

Persistent mafic igneous activity in an A-type granite pluton, Cobequid Highlands, Nova Scotia

The West Moose River pluton is one of a series of latest Devonian – Early Carboniferous plutons of granite and gabbro–diorite in the Cobequid Highlands (southern edge of Avalon zone). Intrusion of minor gabbro preceded emplacement of the granite, which is fractured and cut by later mafic dykes, of which some predate and others postdate a Namurian compressive deformation. Minor hybrid phases reflect mixing of mafic and felsic magmas, which are also represented by basalt and rhyolite in the synchronous Fountain Lake Group. The mafic rocks are continental tholeiites reflecting regional Early Carboniferous extension. Later dykes are more fractionated, with high P2O5, TiO2, and other high-field-strength elements. The dykes are pervasively altered, probably by residual fluids from the granite hydrothermally circulated by the hot mafic rocks. Higher Ga/Al, Hf, and Zn content in basalts than dykes suggests upward increase in halogen complexing in the mafic plumbing system. Late-stage fluids concentrated K, Rb, and Cs in the dykes but not the lavas. Alteration in granite includes mafic partings (biotite and Fe–Ti oxides) along fractures, particularly near dykes, and local albitization. Such alteration hinders determination of granite protolith from geochemistry, but mineral composition and the regional enrichment of the granite in Nb, Y, and Ga/Al is similar to that in A-type granites. Alteration associated with the mafic dykes continued for tens of millions of years after pluton emplacement.

U–Pb geochronology of Late Proterozoic rocks of the eastern Cobequid Highlands, Avalon Composite Terrane, Nova Scotia

In the Cobequid Highlands of Nova Scotia, low-grade late Precambrian arc-related volcano-sedimentry rocks typical of the Avalon Composite Terrane overlie platformal metasedimentry rocks and are spatially associated with gneisses previously considered to be basement to both these units. U–Pb zircon dates of 580–587 Ma from an orthogneiss and an amphibolite are similar to the U–Pb zircon dates of 580–610 Ma from both syntectonic granites in ductile shear zones and high-level posttectonic plutons that intruded the Avalonian successions. Hence, the gneisses do not represent basement but are an integral part of the Avalonian orogenic cycle. The geochronological data indicate that penetrative fabrics in the gneisses, syntectonic granites, and volcano-sedimentary successions are penecontemporaneous (ca. 580–620 Ma) and not sequential, as previously interpreted. The gneisses have a metamorphic fabric (S1a), crystallized under amphibolite-facies conditions, and may represent the deeper roots of a late Precambrian magmatic arc. Fabrics within the deformed granite gneisses (S1b) are interpreted as reflecting crystallization within active ductile shear zones associated with intra-arc transtension and basin development. Fabrics in the volcano-sedimentary successions (S1c) are associated with deformation of the basin.

Granite and associated mafic phases, North River pluton, Cobequid Highlands, Nova Scotia

The North River pluton is one of a series of upper Devonian - lower Carboniferous granite and gabbro/diorite plutons in the Cobequid Highlands. It lies within a regional zone of east-west strike-slip faulting associated with the Cobequid Fault. The pluton consists of a single main granite unit that intruded small marginal bodies of gabbro. The southern part of the pluton is locally foliated and pervasively fractured. Diabase and microgranite sheets and dykes cut the pluton and adjacent country rock. A relative sequence of intrusion can be determined from cross-cutting relationships with foliated or fractured granite. Hybrid rocks demonstrate that some of the mafic magma was intruded synchronously with granitic magma.
 
 The pluton is geochemically similar to other Devono-Carboniferous plutons in the Cobequid Highlands, although it represents a deeper structural level than most of the other plutons. The granite is a subalkalic A-type granite. The mafic rocks show "within-plate" geochemical characteristics; early intrusions resemble olivine-normative continental tholeiites, but later dykes are highly fractionated and show strong enrichment in incompatible elements. Enrichment in Rb and K in the northeastern part of the pluton reflects a late metasomatic event associated with the growth of secondary biotite.
 
 The Devono-Carboniferous plutons of the Cobequid Highlands differ from other published examples of shear zone plutonism in that magma generation appears to have been related to either local or more probably regional extension rather than the shear zone having tapped a magma that originally resulted from either subduction or continental collision.
 
 RÉSUMÉ
 Le pluton de North River est l'un des batis dévoniens supérieurs à carbonifères inférieurs constituent une suite de plutons granitiques et gabbrolques/dioritiques présente dans les Monts Cobequid. Il git au sein d'une zone régionale de décrochement est-ouest associée à la faille de Cobequid. Le pluton consiste en une unité principale unique de granite qui fit intrusion dans de petits batis gabbrolques marginaux. La portion méndionale du pluton montre une foliation locale et une fracturation penetrative. Des lames ainsi que des flkms de diabase et de microgranite recoupent le pluton et la roche encaissante adjacente. Les recoupements du granite folié ou fracturé permettent de déterminer une séquence relative d'intrusion. Des roches hybrides démontrent qu'une fraction du magma mafique fit intrusion au même moment que le magma granitique.
 
 La géochimie du pluton est semblable à celle des autres plutons dévono-carbonifères présents dans les Monts Cobequid, et ce bien qu'il représente un niveau structural plus profond que celui des autres plutons. Le granite est un granite subalcalin de type A. Les roches mafiques montrent des caractéristiques géochimiques "intraplaques"; les intrusions précoces ressemblent à des tholéiites continentales à olivine normative mais les filons plus tardifs ont subi un fractionnement prononcé et ils sont fortement enrichis en éléments incompatibles. Un enrichissement en Rb et en K dans la région nord-est du pluton reflete un événement métasomatique tardif associé à la croissance d'une biotite secondaire.
 
 Les plutons dévono-carbonifères des Monts Cobequid diffèrent des autres exemples publiés de plutonisme associé à une zone de cisaillement en ce que la venue du magma semble avoir été reliée à une extension locale ou, de façon plus vraisemblable, régionale plutôt qu'au captage par la zone de cisaillement d'un magma résultant à l'origine soil d'une subduction, so it d'une collision continentale.
 
 [Traduit par le journal]

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.