Cobequid Highlands Research Articles

Late Paleozoic igneous rocks at Clarke Head, Nova Scotia: magmatism at an arc to back-arc transition

Abstract This study re-examines a reported mylonitic ‘metabasic granulite’ block in a megabreccia that is the most outboard igneous rock outcrop in the Avalon terrane, near the Meguma–Avalon terrane boundary in the northern Appalachians. The block of foliated gabbro is one of several igneous rock blocks in a largely dissolved salt wall and in style of deformation, mineralogy, lithogeochemistry and Sm/Nd isotopes resembles foliated and locally mylonitized late Devonian–early Carboniferous gabbro plutons along the Cobequid Shear Zone to the north. Garnet porphyroclasts in the foliated gabbro are exceptional, with a distinctive composition of Alm 55 Pyr 25 Grs 13 And 4 Sps 3 . Inclusions of pyroxene, andesine and ilmenite, lack of zoning and corroded rims suggest the garnets are antecrysts. Elsewhere in the world, garnets of similar composition in arc-related andesites are interpreted to be from disintegration of comagmatic cumulate material at 0.8–1.0 GPa under hydrous conditions. The Clarke Head foliated gabbro has two mafic components, one resembling arc-related hydrous magma and the other with tholeiitic back-arc character, similar to coeval rocks in the Cobequid Highlands. The gabbro is a product of complex mixing in crustal magma chambers and rapid rise of magma containing lower crustal antecrysts along strike-slip faults.

Magmatic and Inherited Zircon Ages from a Diorite Xenolith of the Popes Harbour Dyke, Nova Scotia: Implications for Late Ediacaran Arc Magmatism in the Avalon Terrane of the Northern Appalachians

The Meguma terrane is a unique unit of the Northern Appalachians as it is only identified in Nova Scotia. It was thrust over the Avalon terrane during the Early Devonian Acadian Orogeny. The Avalon and Meguma terranes are exotic to North America and likely originated along the margin of Gondwana. The precise relationship between the terranes is uncertain and very little is known about the basement rocks of each terrane. Hosted within the Late Devonian lamprophyric Popes Harbour dyke of the Meguma terrane are xenoliths of meta-sedimentary and meta-igneous rocks that are from the basement of the Avalon terrane. The xenoliths offer a glimpse into the nature of the lower crust of the Northern Appalachians. In this study, we present in situ zircon U-Pb age dates from a rare dioritic xenolith in order to assess its origin. The results show that the majority of zircons ages are between ~580 Ma and ~616 Ma with smaller groups at 750–630 Ma, ~2100 Ma, and <570 Ma. The zircon 206Pb/238U weighted-mean age of the rock is 603 ± 5.3 Ma and contemporaneous, with granitic intrusions of the Avalon terrane located within the Antigonish and Cobequid highlands of Nova Scotia. The diorite is compositionally similar to granitoids from an active continental margin. The discovery of Early Paleoproterozoic (~2100 Ma) zircons and the absence of Late Paleoproterozoic (1900–1700 Ma) and Mesoproterozoic (1600–1000 Ma) zircons suggests that the parental magma either encounters only Early Paleoproterozoic and Late Neoproterozoic rocks during emplacement or is derived by the melting of Paleoproterozoic rocks and/or the melting and mixing of Paleoproterozoic and Late Neoproterozoic rocks. Therefore, it is possible that Paleoproterozoic rocks may exist within the basement of the Avalon terrane.

Zircon reveals multistage, magmatic and hydrothermal Rare Earth Element mineralization at Debert Lake, Nova Scotia, Canada

Rare Earth Element (REE) mineralization at Debert Lake (Nova Scotia, Canada) is hosted by the anorogenic Hart Lake-Byers Lake granites which are associated with the Late Devonian to Early Carboniferous Wentworth plutonic complex in the Cobequid Highlands. Mineralized zones contain up to 1.2 wt% REE2O3, 3.4 wt% ZrO2, and 0.4 wt% Nb2O5 produced by multiple, separate mineralizing events. We use a multi-faceted approach combining whole-rock and mineral geochemistry, together with U-Pb radiometric dates to uncover complex formative processes and resolve the timing for REE mineralization.Episodic mineralization yielded distinct REE assemblages including Nb-oxide (pyrochlore and fergusonite) phases, chevkinite, zircon, thorite, monazite, and titanite, formed during multiple magmatic-to-hydrothermal stages in the rare-metal bearing granites. Zircon, as a persistent phase of the REE assemblages, occurs with variable texture, morphology, and composition in all granites. Zircon grains, displaying intergranular to interstitial textures in REE rich granite, have spongy cores in which Y, Th, and Zr were redistributed by magmatic fluids. U-Pb dating of these zircons yields an age of 355 to 360 Ma, approximately coeval with igneous monazite crystallization, dated at 366 ± 5 Ma. These findings document a late-magmatic REE mineralization event. A subsequent pulse of post-magmatic hydrothermal mineralization associated with Zr-F-rich fluid complexes produced two other zircon populations, dated at ca. 309 and 318 Ma, with unusual habits and REE patterns that are interpreted to be of hydrothermal origin. Principal Component Analysis (PCA) separates zircon populations based on element concentrations and element associations. The LREE-rich hydrothermal zircon types with their distinct geochemical patterns revealed by PCA are characterized by the relative enrichment of either Na or Ca, reflecting differences in the composition of hydrothermal fluids driving sodic and calcic alteration processes, respectively

Controls on Barite Mineralization in a Major Intracontinental Shear Zone: Carboniferous of the Cobequid Highlands, Nova Scotia

Prominent veins of late Carboniferous barite, associated with fluorite and calcite, outcrop close to older granite plutons along an intracontinental shear zone that was active throughout the Carboniferous in southeastern Canada. Some barite is stratigraphically constrained to younger than 315 Ma and final mineralization is constrained by a published Rb–Sr isochron of 300 ± 6 Ma. Barite occurrences in the Carboniferous basins of central Nova Scotia, 50 km to the south, are synchronous with or post-date ankerite-siderite-magnetite-pyrolusite and Pb-Zn mineralization, which was facilitated by fluid interaction with thick evaporites. This study aims to determine controls on the distribution of barite in the shear zone, from field relationships, vein petrography and isotope geochemistry of minerals. The isotope chemistry of shear zone barite is similar to that occurring in Pb-Zn-Mn-Ba mineralization to the south, suggesting a common origin. Veins of barite, associated with fluorite, represent the youngest and regionally coolest phase of a 70 Ma history of Carboniferous mineralized veins along the Minas Fault Zone. Their prominence close to granite plutons reflects brittle deformation of the deeply-rooted granites in a complexly deforming fault zone, but the origin of abundant F remains uncertain.

Paleolatitude and tectonic rotations of the Early Carboniferous Fountain Lake Group, Cobequid Highlands, Nova Scotia, Canada

The ca. 355 Ma Fountain Lake Group in the Cobequid Highlands of Nova Scotia, is part of the transtensional basin fill that formed during dextral strike-slip motion between Avalonia and the Meguma terranes following the Acadian Orogeny. Paleomagnetic analysis of the Fountain Lake Group offers a paleolatitude estimate for the Laurentian accretionary margin in the Early Carboniferous as well as locality-specific paleomagnetic directions, which indicate clockwise-sense block rotations during dextral strike-slip motion along the Cobequid Fault zone. Stepwise demagnetization of 142 specimens from 20 sites in three Fountain Lake Group localities across the Cobequid Highlands (Squally Point, West Moose River, and Wentworth exposures) reveals remanence consisting of an easily removed component of probable recent origin, and more persistent components carried by magnetite and hematite, which in petrographic and electron beam analysis appear to be of primary igneous and volcanic oxidation origins, respectively. Sites from all three localities carry stable characteristic remanent magnetization (ChRM) directions that assume similar moderate downward inclinations when tilt-corrected. A Block Rotation Fisher analysis inclination-only fold test demonstrated best agreement at 90% unfolding, showing that remanence acquisition pre-dates Alleghenian deformation in the Late Carboniferous and is most likely of primary 355 Ma age. Paleomagnetic results for the Squally Point, West Moose River, and Wentworth localities show relative rotations between the blocks that are variously clockwise-rotated compared with a Laurentia cratonic reference frame. Inclinations at all three localities imply a subtropics paleolatitude for the margin (at Squally Point, 27.2° ± 9.4°; N = 7 sites), directly supporting the depicted location of Laurentia and its Appalachian accretionary margin in most Devonian to Early Carboniferous reconstructions.

Age and tectonic setting of Neoproterozoic granitoid rocks, Antigonish Highlands, Nova Scotia, Canada: implications for Avalonia in the northern Appalachian orogen

The oldest rocks in the Avalonian Antigonish Highlands of northern mainland Nova Scotia, Canada, are late Neoproterozoic (>618 Ma) volcanic and sedimentary rocks of the Georgeville Group intruded by gabbroic/dioritic to granitic plutons. New U–Pb zircon ages presented here for five samples from plutons intruded into the James River and (or) Keppoch formations of the Georgeville Group have ages ranging from ca. 615 to 604 Ma. They have petrological characteristics of expanded calc-alkalic “Andean-type” suites but are compositionally biased toward evolved high-silica compositions and are interpreted as evolved I-type granites. They were emplaced at shallow depths and some were likely comagmatic with felsic volcanic components of their host rocks. These plutons are younger and show less varied Sm–Nd isotopic compositions than most plutonic rocks formed in the early Ediacaran “main arc phase” elsewhere in Avalonian terranes in the northern Appalachian orogen, although they are similar in age to plutons in southeastern New England and in the Bass River and Jeffers blocks of the Cobequid Highlands, Nova Scotia. The Jeffers block of the Cobequid Highlands appears to be most similar to the Antigonish Highlands but both areas record a Neoproterozoic history less protracted than in other parts of Avalonia.

Evolution of late Paleozoic shearing in the Cobequid Highlands: constraints on the fragmentation of the Appalachian Orogen in Nova Scotia along intra-continental shear zones

Abstract The Cobequid Highlands of northern Nova Scotia lie at the intersection of two major dextral intra-continental shear zones which developed during closure of the Rheic Ocean and formation of Pangea. The Cobequid Shear Zone was an ENE–WSW transfer zone in a NE–SW-trending, orogen-parallel, shear system in the late Devonian–early Carboniferous (Neo-Acadian phase), in which syntectonic granite–gabbro plutons and volcanic strata <4 km thick were progressively deformed along multiple faults. In the Late Carboniferous–Permian, the Alleghanian collision of Africa with Laurentia formed the east–west-trending Minas Fault Zone, reactivating parts of the Cobequid Shear Zone. This deformation was mostly taken up on the master Cobequid–Chedabucto Fault. Deformation chronology is well constrained by the biostratigraphy of syntectonic sedimentary rocks, and by radiometrically dated igneous rocks and minerals in faults and veins. Early strike-slip faults were lubricated by magmatic heating, leading to dyke-to-pluton construction along multiple faults. During cooling, rooted gabbro was more resistant than ductile granite, thereby deflecting solid-state deformation. Neo-Acadian strike-slip displacement across the shear zone is estimated as >50 km. The location of Alleghanian deformation was influenced by the paucity of magmatism and the resistance offered by older stitching plutons.

Evolution of late Palaeozoic shearing in the Cobequid Highlands: constraints on the fragmentation of the Appalachian orogen in Nova Scotia along intra-continental shear zones

Evolution of late Palaeozoic shearing in the Cobequid Highlands: constraints on the fragmentation of the Appalachian orogen in Nova Scotia along intra-continental shear zones

Derivation of the Early Carboniferous Wedgeport pluton by crystal fractionation of a mafic parental magma: a rare case of an A-type granite within the Meguma terrane (Nova Scotia, Canada)

AbstractThe Cambrian–Ordovician metasedimentary rocks of the Meguma terrane (Canadian Appalachians) were extensively intruded by silicic plutons during Middle Devonian to Early Carboniferous times. Syn-plutonic but volumetrically minor mafic-ultramafic intrusions were also emplaced. In most localities, the silicic plutons and mafic-ultramafic intrusions do not appear to be petrogenetically related and are likely derived from different sources. The Attwoods Brook gabbronorite of SW Nova Scotia yielded an in situ zircon weighted-mean 206Pb–238U age of 357.9 ± 3.3 Ma that is within the uncertainty of the age of the neighbouring Wedgeport pluton (357 ± 1 Ma). The Wedgeport pluton is a rare example of a mantle-derived, peraluminous A-type granite within the Meguma terrane. The similar ages and Nd isotopes of the Attwoods Brook gabbronorite (εNd(t) = +1.1 to +4.0) and Wedgeport pluton (εNd(t) = +2.1 to +3.3) suggest the two intrusions are petrogenetically related. Fractional crystallization modelling demonstrates that a parental magma similar to the Attwoods Brook gabbronorite can produce residual silicic liquids that resemble the granites of the Wedgeport pluton, indicating that they could be members of the same intrusive complex. The emplacement of the gabbronorite and Wedgeport pluton occurred during a period of tensional plate stress that was contemporaneous with rifting of the Maritimes Basin that produced the Fountain Lake continental flood basalts and A-type granites of the Cobequid Highlands within the Avalon terrane. It is possible that the Early Carboniferous rocks of SW Nova Scotia are related to the rifted-related magmatism within the Maritimes Basin.

Scapolite and analcime: Monitors of magmatic fluid metasomatism in a major shear zone

Scapolite and analcime are uncommon alteration minerals in syenite. Here we describe a syenite body in a fault zone that hosts these minerals and we elucidate the unusual chemical conditions that led to their formation. The Clarke Head syenite is part of a regional late Devonian–early Carboniferous back-arc A-type granitoid suite in the Cobequid Highlands of Nova Scotia, Canadian Appalachians. The syenite contains magmatic rutile and analcime. Magmatic K-feldspar has been largely replaced by scapolite of marialite composition, which shows a temporal transition from more Cl rich to more carbonate rich varieties. Scapolite veins cut both the syenite and associated gabbro and diorite. Analcime I occurs as a late stage interstitial magmatic mineral. Analcime II forms by alteration of albite lamellae in perthite replaced by scapolite, analcime III vugs that cross-cut scapolite and analcime IV occurs in late veins cross-cutting earlier analcime phases and scapolite. The regional A-type granite plutonism was Na- and halogen rich, with widespread late-magmatic albitization and F-related mobility of REE minerals. However, only at Clarke Head is there extreme Na- and Cl-rich metasomatism, indicated by scapolite and Cl-rich hastingsite (with >1 wt% Cl content). Cl-rich fluids were derived from regional halite evaporites at faulted basin margins. The main circulating fluids were likely of magmatic origin, based on the δ18O of both replacement and vein scapolite. As the NaCl supply was reduced, scapolite became more Ca-rich. Halogen-rich fluids also mobilized Ti and Zr, which were reprecipitated in hydrothermal rutile and zircon as the halogen activity diminished. Scapolite replacement of K-feldspar released large amounts of K+ into the circulating fluids, which converted ferromagnesian silicates in fault zones to secondary biotite.

Fractured latest Devonian granites of the West Moose River pluton along the Cobequid Shear Zone, Nova Scotia: implications for regional mineralization

Latest Devonian (∼365–358 Ma) A-type granites in the Cobequid Highlands host complex sequences of rare-earth element (REE) and other hydrothermal minerals. The West Moose River pluton is the only pluton truncated and brittly deformed by the mid-Carboniferous (∼327 Ma) strike-slip Minas Fault Zone during the Alleghanian orogeny. Fractures in the granite provide a record of several deformational and hydrothermal events with distinct mineral assemblages. Early sodic alteration produced albitization of feldspar, and riebeckite and tourmaline veins. The δ18O of albite and albitized granite (5‰–6‰) is similar to other regional granites, suggesting a mantle source of albitizing fluids. Nearby halite deposits are younger and thus not a source of Na. Early chlorite veins were followed by potassic alteration and hydrothermal biotite, and by diabase and lamprophyre dyke emplacement. Euhedral magnetite occupies new cross-cutting fractures and vugs, correlated with regional iron oxide – carbonate – sulphide mineralization following initiation of the Minas Fault Zone. This change in stress field resulted in widespread fracturing of the granite, greatly increasing its permeability. Magnetite is postdated by titania minerals with hydrothermal REE minerals in dissolution voids. The spatial variation in REE mineral types indicates variable availability of F, Cl, and CO2 in mineralizing fluids derived from groundwater. REE mineralization is rare in veins in country rock, demonstrating local plutonic sources of REEs. The emplacement of REE minerals was complex in time and space and was a consequence of pervasive microfracturing of the granite.

Systematic mineralogical diversity in A-type granitic intrusions: Control of magmatic source and geological processes

The origin and causes of mineralogical diversity of A-type granites are debated. The series of A-type granite plutons, with distinct mineralogical differences, emplaced along an Upper Paleozoic crustal-scale shear zone in the Cobequid Highlands, Nova Scotia, provide an opportunity to examine the origin of different A-type plutons in a similar tectonic setting. Based on the ferromagnesian minerals present, the plutons are classified into sodic granites with sodic amphibole, calcic granites with calcic amphibole, and biotite granites. Sodic and calcic granites occur exclusively in complex intrusions with subequal amounts of gabbro in the eastern shear zone, whereas plutons in the western shear zone, with lesser gabbro, are solely biotite granites. Trace elements and radiogenic isotopes show that the three granite types have different sources. Intensive parameters including temperature, pressure, and water-in-melt contents were estimated from mineralogical and geochemical data. Modeling of these geochemical data suggests that the biotite and calcic granites were derived by 20%–40% partial melting of intracrustal feldspathic rocks, whereas the sodic granites are extreme fractionates (90%) of coeval mafic magma. We propose that supply of Upper Paleozoic mafic magma, probably related to regional extension and decompression melting beneath the Magdalen Basin, created a deep crustal hot zone in the eastern Cobequid Highlands, and extreme fractionation of underplated mafic sills produced the sodic granites. Heat transfer from crystallizing mafic magma induced partial melting of the surrounding crust, creating batches of biotite and calcic granitic melts in different depths. Fractionated and crustally derived melts segregated along crustal-scale faults, constructing the complex plutons in the east. Melting of the crust was further facilitated by the release of water from the crustal rocks upon heating. In the eastern shear zone, water was released predominantly by magmatic rocks and in lesser amounts compared to the west, where Neoproterozoic sedimentary and volcaniclastic rocks are more abundant. The volatile-rich granitic melts in the western part of the shear zone were crystallized rapidly, stabilizing only biotite. This study demonstrates that the mineralogical variations in A-type granites arise from rather similar magma compositions, but they are important petrogenetic indicators of varying sources, specific magmatic processes, and emplacement conditions.

VARIATION OF REE-HYDROTHERMAL CIRCULATION IN COMPLEX SHEAR ZONES: THE COBEQUID HIGHLANDS, NOVA SCOTIA

Abstract The Cobequid Highlands in Nova Scotia comprise a series of 365–360 Ma, REE-enriched, A-type granite plutons emplaced along an active shear zone. The purpose of this paper is to identify the sequence of REE minerals produced during long-lived, regional, hydrothermal circulation and its evolution through time. The fractures hosted in these granites were investigated using petrographic and scanning electron microscopes and the electron microprobe. Epidote-rich, biotite-rich, chlorite-rich, and Fe-oxide-rich fracture fillings have been observed. Mineral analyses revealed a variety of REE minerals in the fractures, including hydroxylbastnasite-(Ce), parisite-(Ce), synchysite-(Ce), cerianite, thorite, Nb-bearing minerals, hingganite-(Y), and chernovite-(Y). Cross-cutting relationships and mineral associations indicate that the REE minerals precipitated at different times and are associated with specific styles of hydrothermal alteration. Epidote was one of the earliest minerals to precipitate in the fractures, associated only with minor precipitation of thorite and hydroxylbastnasite-(Ce) and removal of As from sedimentary rocks, under low temperatures and reducing conditions. Biotite and chlorite precipitated shortly after epidote under more oxidizing conditions during which comparatively larger amounts of hydroxylbastnasite-(Ce), thorite, hingganite-(Y), and cerianite formed. The Fe-oxide-filled fractures formed in the latest stages of hydrothermal circulation and are associated with the largest variety and highest amounts of hydrothermal REE minerals, such as parisite-(Ce), synchysite-(Y), thorite, Nb-bearing minerals, and chernovite-(Y). The oxidizing conditions led to the precipitation of As and the formation of chernovite-(Y) after the alteration of hingganite-(Y). Thus the greatest remobilization and concentration of REE minerals took place during late, high-temperature, oxidizing hydrothermal circulation, apparently driven by minor late mafic intrusions.

Geochemical variation of amphiboles in A-type granites as an indicator of complex magmatic systems: Wentworth pluton, Nova Scotia, Canada

The Wentworth pluton is the most complex intrusion among a series of A-type granitic plutons, emplaced along an active shear zone in the Cobequid Highlands of Nova Scotia during the latest Devonian–earliest Carboniferous. This pluton consists of A-type granites of different generations and a large gabbroic body. Among all late Paleozoic plutons in the Cobequid Highlands, only the Wentworth granites contain both primary calcic (edenite, hornblende) and sodic amphiboles (arfvedsonite). Whole-rock, and mineral chemical data were examined along with estimated magmatic parameters, such as temperature, emplacement pressure, oxygen fugacity and volatile contents, in order to understand the geological factors responsible for this mineralogical variation. The granites with calcic amphiboles show systematically lower εNd values, magmatic temperatures, and F-in-melt contents and appear geochemically less evolved, compared to the granites with the sodic amphiboles. All granites, however, appear geochemically homogeneous with limited evidence of fractionation. Specific chemical differences between these two types of granites indicate the presence of two coeval but distinct granitic systems in the Wentworth pluton. The granites with the calcic amphiboles were formed from a geochemically less evolved, hydrous, relatively calcic melt, whereas the granites with sodic amphiboles were emplaced at structurally deeper levels and were derived from a relatively drier sodic melt, which had a larger mantle component and was enriched in fluorine. Coexistence of these two magmas is indicated by the presence of interstitial sodic–calcic amphiboles in a late granitic dyke. This study provides evidence that in A-type granites, where fractional crystallization is limited, extreme variation in the type of amphibole can be the result of a very complex magmatic history.

Structural setting and age of the Partridge Island block, southern New Brunswick, Canada: a link to the Cobequid Highlands of northern mainland Nova Scotia

The Partridge Island block is a newly identified tectonic element in the Saint John area of southern New Brunswick, located south of and in faulted contact with Proterozoic and Cambrian rocks of the Ganderian Brookville and Avalonian Caledonia terranes. It includes the Lorneville Group and Tiner Point complex. The Lorneville Group consists of interbedded volcanic and sedimentary rocks, subdivided into the Taylors Island Formation west of Saint John Harbour and West Beach Formation east of Saint John Harbour. A sample from thin rhyolite layers interbedded with basaltic flows of the Taylors Island Formation at Sheldon Point yielded a Late Devonian – Early Carboniferous U–Pb (zircon) age of 358.9 +6/–5 Ma. Petrological similarities indicate that all of the basaltic rocks of the Taylors Island and West Beach formations are of similar age and formed in a continental within-plate tectonic setting. West of Saint John Harbour, basaltic and sedimentary rocks of the Taylors Island Formation are increasingly deformed and mylonitic to the south, and in part tectonically interlayered with mylonitic granitoid rocks and minor metasedimentary rocks of the Tiner Point complex. Based on magnetic signatures, the deformed rocks of the Tiner Point complex can be traced through Partridge Island to the eastern side of Saint John Harbour, where together with the West Beach Formation, they occupy a thrust sheet above a redbed sequence of the mid-Carboniferous Balls Lake Formation. The Tiner Point complex includes leucotonalite and aegirine-bearing alkali-feldspar granite with A-type chemical affinity and Early Carboniferous U–Pb (zircon) ages of 353.6 ± 5.7 and 346.4 ± 0.7 Ma, respectively. Based on similarities in age, petrological characteristics, alteration, iron oxide – copper – gold (IOCG)-type mineralization, and deformation style, the Partridge Island block is correlated with Late Devonian – Early Carboniferous volcanic–sedimentary–plutonic rocks of the Cobequid Highlands in northern mainland Nova Scotia. Deformation was likely a result of dextral transpression along the Cobequid–Chedabucto fault zone during juxtaposition of the Meguma terrane.

The relationship between REE-Y-Nb-Th minerals and the evolution of an A-type granite, Wentworth Pluton, Nova Scotia

The Wentworth Pluton in the Eastern Cobequid Highlands consists principally of metaluminous to peralkaline A-type granite (~362 Ma), a large part of which was remelted by a major gabbro intrusion (~357 Ma). Magmatic minerals like allanite-(Ce), chevkinite-(Ce), zircon, and hingganite-(Y) and post-magmatic mineral phases, such as REE-bearing epidote, samarskite-(Y), aeschynite-(Y), fersmite, thorite-like phases, and hydroxylbastnäsite-(Ce), were identified. The presence of fluorine in the parental magma, indicated by whole-rock geochemical data and the presence of fluorite, increased the solubility of monazite and xenotime and thus facilitated retainment of rare metals in the magmatic system. Fractionation of allanite-(Ce) and chevkinite-(Ce) led to a melt enriched in heavy rare earth elements (HREE), from which hingganite-(Y) crystallized during late magmatic stages. The remelting of the early granite led to fluorine and sulfur release in volatile phases, which circulated with hydrothermal fluids, thus mobilizing the REE and other rare metals. Reduction of fluorine activity during the late to post-solidus crystallization resulted in the precipitation of HREE and rare metals in samarskite-(Y), thereby enriching the residual hydrothermal fluids in light rare earth elements (LREE). Post-magmatic LREE minerals, such as hydroxylbastnäsite-(Ce), either replaced earlier minerals or precipitated from these hydrothermal fluids. Carbonate fluids involved in a late regional hydrothermal circulation event along the Cobequid-Chedabucto fault (320-315 Ma) promoted Ti mobility and the formation of TiO 2 minerals and probably of aeschynite-(Y). This mineralogical diversity, in addition to the complex geological history of the pluton, provides a unique opportunity to correlate the formation of individual rare-metal minerals to different stages of pluton evolution and thus provide an insight to the formation conditions of these minerals.

The late Neoproterozoic Frog Lake hornblende gabbro pluton, Avalon Terrane of Nova Scotia: evidence for the origins of appinites

The late Neoproterozoic Frog Lake pluton, in the Avalon terrane of the Cobequid Highlands, Nova Scotia, consists predominantly of hornblende gabbro. It shows petrographic similarities to water-rich mafic intrusions known as appinites that are present in some collisional orogens. This study aims to further understanding of the origin of appinitic intrusions. In the field, the main hornblende gabbro was intruded between screens of metasedimentary country rock that is of upper greenschist metamorphic grade. The contacts appear to have been pathways for magma of gabbroic, tonalitic–granodioritic, and granitic composition that carried enclaves of gabbroic lithologies. Some of these magmas had a high volatile content, resulting in abundance of hydrous mineral phases, pegmatites, and diffuse felsic segregations. These varied rocks in the contact zones experienced progressive shear resulting in syn-magmatic deformation. Low-Ti hornblende gabbros have trace-element abundances similar to subduction-related low-K mafic rocks, including some enrichment in large-ion lithophile elements and marked relative depletion in Nb and Y. High-Ti hornblende gabbros and pyroxene–mica gabbro show more alkaline characteristics, with higher amounts of Nb, Y, P2O5, and high-field-strength elements. Tonalite and granite veins are geochemically similar to volcanic-arc granite. Comparison with appinites in the literature suggests that the Frog Lake pluton represents a deeper structural level than most appinites. The Frog Lake appinites were part of the feeder system to back-arc volcanic rocks of the Jeffers Group. Comparison with other appinites also leads to the conclusion that there is not a single type of “appinitic magma”: different appinitic plutons range in composition from low-K calc-alkaline to shoshonitic. The essential characteristic is a water-rich mafic magma. Appinites occur in settings undergoing crustal-scale strike-slip shear, where the faults allow rapid rise of mafic magma to shallow crustal levels.

The West Indian Road pit, central Nova Scotia: key to the Early Cretaceous Chaswood Formation

The West Indian Road pit is the only large outcrop in Nova Scotia of the Chaswood Formation, the terrestrial equivalent of the offshore Missisauga and Logan Canyon formations. It provides outcrop information on sedimentology, gravel petrology, and structures for a formation that is otherwise known from a few small overgrown pits and from boreholes. The Chaswood Formation in the pit is > 60 m thick and consists principally of sorted sand and gravel with three thinner clay units. Successions of sedimentary structures indicate deposition from a coarse-bedload river flowing to the east-southeast. Gravel consists principally of vein quartz, quartz arenite, and subarkose, together with minor igneous lithologies that can be matched to sources in the Cobequid Highlands to the north. Quartz arenite and subarkose appear derived from Carboniferous Horton Group. Single-crystal 40Ar/39Ar dates of detrital muscovite are a little older than the muscovite ages for the South Mountain batholith, interpreted to mean that the muscovite is second cycle from the Horton Group, which records the earliest unroofing of the batholith. The Chaswood Formation accumulated during progressive tectonic deformation along NNE-trending strike-slip faults in basement rocks, resulting in syn-sedimentary faulting and local unconformities. Sedimentation kept pace with the creation of accommodation. Unrelated younger deformation folded the Chaswood Formation at the pit into an east–west-trending syncline. The Early Cretaceous paleogeography of the Maritime Provinces is interpreted to have consisted of fault-bound horsts shedding coarse detritus surrounded by an interconnected series of basins that accumulated fluvial sands and gravels and overbank muds with well-developed paleosols.

Early Carboniferous deformation and mineralization in the Cobequid shear zone, Nova Scotia: an 40Ar/39Ar geochronology study

Latest Devonian to Early Carboniferous igneous rocks along the Cobequid shear zone of central Nova Scotia record a complex history of intrusion, volcanism, deformation, and hydrothermal alteration. Twenty new 40Ar/39Ar age determinations have been made on biotite and hornblende separates from rocks that constrain these events. The mafic plutons of the Cobequid shear zone are synchronous with the previously dated granitoid plutons (363–355 ± 4 Ma), with the exception of the younger Folly Lake gabbro (355–350 ± 4 Ma). High temperature shear deformation continued along the Cobequid shear zone during this entire period of pluton emplacement. Several samples from mylonitic and hydrothermally altered rocks with dates ca. 340 Ma indicate renewed movement along the Cobequid shear zone at this time, accompanied by minor magmatism. Younger ∼330–320 Ma ages reflect final movement along the Rockland Brook fault resulting in mid-Namurian uplift of the Cobequid Highlands. Following this regional Alleghenian event, there was no significant motion on the northern faults of the Cobequid shear zone and deformation moved southward to the Cobequid fault and its continuation in the Hollow and Chedabucto fault zones. Hydrothermal alteration, probably driven by a mid-crustal gabbro heat source, was widespread along the Cobequid and Rockland Brook faults in the early Carboniferous, resulting in albitization, potassic alteration, and iron mineralization.

A synopsis of the geology of the Cobequid Highlands, Nova Scotia

The Cobequid Highlands of Nova Scotia constitute a horst, originally developed in the latest Devonian, exposing Neoproterozoic Avalon terrane rocks on the northern side of the Cobequid Fault. The Neoproterozoic rocks consist of arc volcanic and plutonic rocks and associated sedimentary rocks in two major blocks. The geology of the Jeffers block is similar to that of the Antigonish Highlands, with volcanic rocks and turbidites intruded by granodioritic and granitic plutons. The Bass River block, in the southeast, shows evidence of shear-zone emplacement of a suite of plutonic rocks ranging from gabbro to granodiorite to granite into tectonically juxtaposed ocean floor volcanic and shelf sedimentary rocks. The highlands are dominated by E—W trending faults of the Late Paleozoic Cobequid Shear Zone, which developed during oblique convergence between the Avalon and Meguma terranes, and which facilitated the emplacement of gabbro and granite plutons and their extrusive equivalents (Fountain Lake Group) in the latest Devonian — earliest Carboniferous. Lower Paleozoic sedimentary rocks, including Silurian to early Devonian shelf sandstone and siltstone, were highly dismembered by faults associated with terrane convergence. The youngest stratified rocks in the Cobequid Highlands comprise fluvial and lacustrine sedimentary rocks of the latest Devonian — Tournaisian Horton Group, which accumulated in fault-bound basins. This paper reviews the regional geology of the Cobequid Highlands and provides a comprehensive bibliography of recent work.
 
 RESUMÉ
 Le massif de Cobequid en Nouvelle-Écosse constitue un horst, qui est initialement apparu à la toute fin du Dévonien et qui a mis à nu des roches du terrane néoprotérozoïque d'Avalon sur la lèvre septentrionale de la faille de Cobequid. Les roches du Néoprotérozoïque se composent de roches volcaniques et plutoniques d'arcs et de roches sédimentaires associées au sein de deux blocs d'importance. La composition géologique du bloc de Jeffers est semblable à celle du massif d'Antigonish : des roches volcaniques et de les turbidites sont pénétrées par des plutons granodioritiques et granitiques. Le bloc de Bass River, dans le sud-est, témoigne de l'intrusion d'une zone de cisaillement constituée d'un cortège de roches plutoniques variant du gabbro à la granodiorite puis au granite dans des roches sédimentaires du plateau continental et volcaniques tectoniquement juxtaposées du plancher océanique. Le massif présente une prédominance de failles orientées d'est en oust de la zone de cisaillement du Paléozoïque tardif de Cobequid, qui se sont constituées pendant la période de convergence oblique entre les terranes d'Avalon et de Meguma et qui ont facilité l'intrusion du gabbro et des plutons de granite ainsi que de leurs équivalents extrusifs (groupe Fountain Lake) à la toute fin du Dévonien et au tout début du Carbonifère. Les roches sédimentaires du Paléozoïque inférieur, y compris le siltstone et le grès continental du Silurien et du Dévonien précoce, ont été fortement démembrées par les failles associées à la convergence des terranes. Les roches stratifiées les plus récentes du massif de Cobequid comprennent des roches sédimentaires fluviales et lacustres du Dévonien le plus tardif – le groupe tournaisien de Horton – qui se sont accumulées dans des bassins délimités par des failles. Cet exposé examine la structure géologique régionale du massif de Cobequid et fournit une bibliographie détaillée des travaux récents.