Churchill Province Research Articles

Taltson basement gneissic rocks: U–Pb and Nd isotopic constraints on the basement to the Paleoproterozoic Taltson magmatic zone, northeastern Alberta

The Taltson magmatic zone is a 300 km long, northerly trending belt of Archean to Paleoproterozoic granitic basement gneiss, amphibolite, supracrustal gneissic rocks, and voluminous Paleoproterozoic magmatic rocks underlying northeastern Alberta and the southwestern portion of the Northwest Territories. Taltson basement gneissic rocks exposed in the southern Taltson magmatic zone in northeastern Alberta are composed of a lithologically diverse suite of banded gneissic rocks, including amphibolitic gneiss, paragneiss, and metaplutonic gneiss. Metaplutonic gneissic rocks are predominant; six new U–Pb ages range from 2.4 to 2.1 Ga. Taltson basement also contains tectonic slices and layers of Archean gneissic rocks dated at 3.2 Ga, 3.1 Ga, and 2.6 Ga. e Nd (2.2 Ga) values of –1.6 to –14.3 and T DM ages of 2.59–3.73 Ga from Taltson basement rocks (27 new analyses) overlap with values for the western Churchill Province and are interpreted to represent Paleoproterozoic recycling of the latter. The similarity in U–Pb ages and partial overlap in Nd isotopic compositions of the Taltson basement gneissic rocks and the Buffalo Head terrane are consistent with the two domains representing a single Paleoproterozoic crustal entity, which evolved from variable contributions of depleted mantle and preexisting Churchill Province crust. Greater contributions from a depleted mantle component in the Buffalo Head terrane are suggested by the Nd data. Taltson basement gneissic rocks and Buffalo Head terrane may represent earliest Paleoproterozoic magmatism associated with breakup of the western Churchill Province prior to Thelon– Taltson orogenesis.

Geology and petrology of the Voisey's Bay intrusion: reaction of olivine with sulfide and silicate liquids

The Voisey's Bay Ni–Cu–Co sulfide deposit is located in Eastern Labrador, and transects the 1.85 Ga collisional boundary between interbanded garnet–sillimanite and quartzo-feldspathic paragneisses (Tasuiyak gneisses) of the Proterozoic Churchill Province to the west and granitic, intermediate and mafic orthogneisses of the Archean Nain Province to the east. The deposit is associated with an upper troctolitic chamber (the Eastern Deeps chamber) which is connected to a lower troctolitic chamber (the Reid Brook chamber) by a gabbroic/troctolitic feeder sheet approximately 1 km in vertical extent. Rock sequences that have been defined to date include Leuco-troctolite (LUT) in the Reid Brook chamber, Feeder Breccia (FB) in the Reid Brook Zone, Feeder Olivine Gabbro (FOG), Leopard Troctolite (LT) and Basal Breccia Sequence (BBS) in the feeder sheet, and Olivine Gabbro (OG), Normal Troctolite (NT) and Varied-textured Troctolite (VT) in the Eastern Deeps chamber. Mineralisation has been found in the feeder sheet, as massive stringers in gneisses flanking the feeder, in the `Ovoid', a 600×300×110 m 3 basin of massive sulfide at surface, and as a zone of massive sulfide that has developed close to the entry of the feeder to the Eastern Deeps chamber. The mineralisation comprises pyrrhotite (both hexagonal and troilite in varying proportions), pentlandite, chalcopyrite, magnetite and minor cubanite. An important component of all mineralized zones is a breccia of gneissic fragments in a troctolitic to noritic matrix referred to as BBS or FB. Considerable reaction has occurred between the troctolite magma and the gneiss inclusions. Olivines in the LUT, OG and FOG have low Ni contents relative to their forsterite (Fo) contents. Olivines in the NT and VT have higher Ni contents but show a great scatter in both their Ni and Fo contents. The Ni content of olivine in the LT increases systematically with decreasing Fo content. All of these variations are explained in terms of fractional crystallisation of magma, sulfide segregation, trapped liquid shift and re-equilibration between sulfide and olivine. Our current understanding of the development of the deposit is that an initial pulse of magma rose to a lower chamber, fractionated to form mafic-ultramafic cumulates and at the same time became sulfide-saturated as a result of interacting with the enclosing, sulfide- and graphite-bearing Tasiuyak gneisses. The magma lost much of its Ni and Cu as it fractionated olivine and segregated sulfides. A new wave of fresh magma entered the lower chamber, forcing the chalcophile-depleted magma, along with some of its contained sulfide, up the feeder to spread out in the upper chamber. Remnants of this chalcophile-depleted magma are preserved as olivine gabbro which forms upper layers in the upper chamber, and which in places has frozen to the walls of the feeder. Sulfides that were swept up in this event were forced out into fractures in the feeder walls, are preserved within thickened zones within the feeder, and were deposited as the magma flow decreased at the entry of the feeder to the upper chamber. Continued influx of the later, undepleted magma resulted in sulfides being picked up, transported farther up the system, upgraded during transportation and then redeposited.

Wide‐angle seismic transect across the Torngat Orogen, northern Labrador: Evidence for a Proterozoic crustal root

A refraction/wide‐angle reflection seismic transect across the Labrador peninsula covers the Core Zone of the SE Churchill Province, the Paleoproterozoic Torngat Orogen, and the Archean Nain Province including a portion of the Labrador continental margin. An airgun array was used as source, and 11 ocean‐bottom seismometers and 16 land stations recorded the shots. Forward modeling of travel times and amplitudes reveals a deep asymmetric crustal root beneath the Torngat Orogen, with a crustal thickness of >49 km and with P‐wave velocities of 6.9–7.0 km/s. The geometry of the velocity model and the root can be best explained by either westward dipping subduction or westward underthrusting of the Nain crust. Gravity modeling suggests a correlation of the crustal root with a gravity low that extends ∼100 km in an E‐W direction and ∼200 km from north to south. The preservation of the crustal root is attributed to the lack of postorogenic heating and ductile reworking consistent with the lack of abundant postcollisional magmatism in the SE Churchill Province. A discontinuity possibly cutting through the entire crust is interpreted as a zone of major transcurrent shearing associated with the main phase of deformation. West of the Torngat Orogen, the crustal thickness in the Core Zone of the Churchill Province varies between 35 and 38 km (P‐wave velocities of 5.8–7.0 km/s). East of the orogen, the crystalline crust in the Nain Province is ∼38 km thick (velocities from 5.8 to 6.9 km/s) but thins to 9 km in the shelf area of the Labrador margin, where it is covered with up to 8 km of sediments. No high‐velocity zone was found beneath the thinned continental crust at the margin.

Electrical conductivity in the precambrian lithosphere of western canada

The subcrustal lithosphere underlying the southern Archean Churchill Province (ACP) in western Canada is at least one order of magnitude more electrically conductive than the lithosphere beneath adjacent Paleoproterozoic crust. The measured electrical properties of the lithosphere underlying most of the Paleoproterozoic crust can be explained by the conductivity of olivine. Mantle xenolith and geological mapping evidence indicate that the lithosphere beneath the southern ACP was substantially modified as a result of being trapped between two nearly synchronous Paleoproterozoic subduction zones. Tectonically induced metasomatism thus may have enhanced the subcrustal lithosphere conductivity of the southern ACP.

Archaean quartz arenites in the Canadian Shield: examples from the Superior and Churchill Provinces

Units of remarkably pure Archaean quartz arenite occur in the northwestern part of the Superior Province and in the northern terrane of the Western Churchill Province (Rae Province) of the Canadian Shield. In the Superior Province, silica-cemented quartz arenites of Archaean age are well preserved in several greenstone belts. The example from the Keeyask Lake sedimentary assemblage displays tabular–planar and trough cross-beds, ripple marks, reactivation surfaces and pebble lag deposits. In spite of penetrative deformation and greenschist-grade metamorphism, primary textures are extremely well preserved, showing framework grains to be very well rounded and sorted. The succession of Keeyask Lake quartz-arenite beds is overlain by siltstones containing small-scale stratiform, domal and columnar stromatolites. A shallow-marine environment of deposition is inferred. Detrital heavy minerals include pyrite, magnetite, zircon, tourmaline, apatite, sphene and topaz. In the northern part of the Western Churchill Province (Rae Province), Archaean quartz arenites occur in northeasterly trending belts where intense structural deformation has in most places obscured or obliterated primary textures and structures. This has led to speculation that some of these units are metachert or recrystallized vein quartz, but local preservation of primary textures and structures provides clear evidence of epiclastic origin. In the example described herein, quartz arenites of the Woodburn Lake Group display sparse occurrences of trough and tabular–planar cross-beds, channels, ripple marks and pebble lag deposits. Probable environments of deposition for these quartz arenites include fluvial systems and shallow-marine shelf settings. The occurrence of unequivocal quartz-arenite clasts in beds of intercalated conglomerate provides direct evidence of at least two episodes of accumulation of almost pure quartz sand. Thin sections and polished slabs reveal frameworks of clastic quartz grains with little to no matrix (now mainly muscovite), and rare detrital grains of accessory heavy minerals, predominantly zircon and opaque iron oxides. Pyrite and other sulphides have been introduced along fractures, but some intergranular sulphide grains may be of detrital origin. The principal source for the quartz arenites in both areas must have been quartz-rich granitoid rocks. Conditions of intense chemical weathering are indicated. The widespread occurrence of extremely mature quartz arenites throughout Archaean terranes of the Canadian Shield, and in other shields of the world, are suggestive of crustal stability during early Earth history. The association of quartz arenites and ultramafic rocks, uncharacteristic of younger terranes, is now recognized in many Archaean greenstone belts of the Canadian Shield.

The Michael Gabbro and other Mesoproterozoic lithospheric probes in southern and central Labrador

The Michael Gabbro (1426 Ma) and the Shabogamo Gabbro (1459 Ma) represent two large diabase intrusive episodes that affected the northwestern margin of the northeastern Grenville Province. Both have sustained variable effects of subsequent Grenvillian metamorphism. Other broadly contemporaneous magmatic activity in the region includes the formation of Elsonian anorthosite–granitoid complexes (1.46–1.29 Ga) that intruded Churchill Province and Nain Province rocks to the north, the Harp dykes (1.27 Ga), and Mealy dykes (~1.25 Ga). Petrologic and geochemical data show that the Michael Gabbro and Shabogamo Gabbro are similar, with the latter displaying more cumulate rock characteristics, and the former having compositions closer to those of rapidly cooled magma. Both have compositions comparable to those of other continental diabases and to some continental flood basalts. Sr and Nd initial isotopic compositions of Michael Gabbro (εNd(1426 Ma) = −4.7 to −6.0, ISr(1426 Ma) = 0.7032–0.7044) and Shabogamo Gabbro (εNd(1459 Ma) = −4.0 to −5.5, ISr(1459 Ma) = 0.7020–0.7060) are alike and overlap, suggesting similar sources and processes of development. Comparisons with other nearly contemporary mafic suites in central and southern Labrador show that only the Mealy dykes have a distinctly more radiogenic isotopic signature. Paradoxically, other mafic suites that intrude older basement rocks north of the Grenville Front have less enriched Nd signatures than do those that intrude younger basement rocks south of the front. The argument is made that the subcontinental lithospheric mantle, and not crustal contamination, played the most influential role in evolution of the magmas.

Provenance of icebergs during Heinrich Event 3 and the contrast to their sources during other Heinrich episodes

The sources of ice‐rafted debris (IRD) from Heinrich layer 3 (H 3) in the North Atlantic were studied by means of Pb isotopic analyses of single‐ and multiple‐grain samples of ice‐rafted feldspars from core V28‐82 (49° 27′N, 22° 16′W). Pb ratios of H 3 grains point to multiple continental sources and are similar to the isotopic composition of the background sediment. This similarity, as well as previous observations about the age and the Sr and Nd isotopic composition of the fine fraction of H 3 in cores of the same area, are all consistent with the hypothesis that in the eastern North Atlantic along the latitude band of maximum IRD deposition (40°–50°N), H 3 is a product of foraminifera dissolution and/or low productivity. In contrast, analyses of composite samples of H 1, H 2, H 4, and H 5 plot in 206Pb/204Pb versus 206Pb/204Pb coordinates along the Churchill province line, supporting the premise that the Hudson Strait was the major pathway of iceberg drainage during these four events. Since an advance of the Laurentide ice sheet during H 3 is demonstrated by increases in detrital carbonate at the level of H 3 in cores from the western North Atlantic, the results obtained here suggest that if indeed that advance took place, the iceberg discharge melted mostly in the western basin.

Paleoproterozoic lithotectonic divisions of the southeastern Churchill Province, western Labrador

The southeastern Churchill Province (SECP) is a Paleoproterozoic system of orogens that formed during collision of the Nain and Superior cratons with a composite lithotectonic terrane that now forms the medial, metamorphic–plutonic core zone of the SECP. In western Labrador, the core zone consists of reworked Archean gneisses, Paleoproterozoic supracrustal rocks, and variably deformed 1.83–1.81 Ga granitic plutons. It is subdivided into three Paleoproterozoic lithotectonic domains (McKenzie River, Crossroads, and Orma), which are separated from each other by dextral transpressive high-strain zones. Crossroads and Orma domains are thought to be derived from Archean high-grade granite–greenstone terrane crust, whereas McKenzie River domain is inferred to have been part of an Archean orthogneiss terrane dominated by the ca. 2776 Ma Flat Point gneiss. U–Pb geochronological data indicate that the igneous precursor of the Flat Point gneiss is >80 Ma older than the oldest tonalité–granite intrusions in Crossroads and Orma domains. The three domains were variably reworked during dextral oblique convergence of the Superior and Rae cratons. Field and geochronological data demonstrate that in McKenzie River and Crossroads domains, 1.83–1.80 Ga tectono-thermal reworking included medium- to high-grade metamorphism and the formation of north-trending structures and regionally persistent high-strain zones. Crossroads domain also contains a significant amount of 1.83–1.81 Ga granitic intrusions, including the southern part of the 500 km long De Pas batholith. Orma domain appears to have escaped Paleoproterozoic metamorphism and deformation, suggesting that a "Hudsonian" tectono-thermal front separates it from Crossroads and McKenzie River domains.

Tracking the sources of icebergs with lead isotopes: The provenance of ice‐rafted debris in Heinrich layer 2

The provenance of ice‐rafted debris (IRD) deposited in the North Atlantic before, during, and after Heinrich event 2 has been determined through measuring the lead isotopic composition of single feldspar grains and multiple‐grain composites from the larger than 150‐µm size fraction, from cores from the eastern and western North Atlantic and from the Labrador Sea. Single‐grain analyses are used to identify the specific continental sources of the IRD, whereas composite samples are used to assess the relative IRD contributions from different sources. All single grains from Heinrich layer 2 (H 2) as well as H 2 composites plot along a correlation line on a 207Pb/204Pb versus 206Pb/204Pb diagram characteristic of the Churchill province of the Canadian shield. This is yet another strong piece of evidence that this Heinrich event was dominated by a massive iceberg discharge of the Laurentide ice sheet lobe located over Hudson Bay. In contrast, single grains from the ambient glacial sediment (above and below H 2) have multiple sources: many of them also lie along the correlation line with H 2 grains, but many others have Pb signatures consistent with derivation from the Grenville province and the Appalachian range in North America and possibly from Scandinavia and Greenland. Composites from the ambient sediment generally lie well to the right of the H 2 reference line in agreement with the results of the single‐grain analyses. The evidence provided by lead isotopes regarding the dominant role played by the Hudson Bay lobe of the Laurentide ice sheet in the development of the Heinrich events lends support to the binge/purge model advanced by MacAyeal [1993a, b] that invokes trapping of geothermal heat by the base of the icecap and subsequent basal melting as the mechanism that triggered the Heinrich events.

Evidence from 40Ar/39Ar Ages for a Churchill province source of ice-rafted amphiboles in Heinrich layer 2

Abstract40Ar/39Ar ages of most single ice-ratted amphiboles from Heinrich layer 2 (H2) from a core in the Labrador Sea, a core in the eastern North Atlantic and a core in the western North Atlantic range from 1600 to 2000 Ma. This range is identical to that for K/Ar ages from the Churchill province of the Canadian Shield that outcrops at Hudson Strait and forms the basement of the northern part of Hudson Bay. The ambient glacial sediment includes some younger and older grains derived from Paleozoic, Mesoproterozoic and Archean sources, but still the majority of the amphiboles have ages in the 1600–2000 Ma interval. The Ca/K ratios of these 1600–2000 Ma old amphiboles, however, have a bimodal distribution in contrast with the uniformity of the Ca/K ratios of H2 amphiboles. This indicates that 1600–2000 Ma old amphiboles of the ambient sediment were derived from an additional Early Proterozoic source besides Churchill province. In H2, Churchill-derived grains constitute 20–40% of the ice-rafted debris (IRD). The fraction in the ambient glacial sediment is 65–80%. Results presented here are consistent with the hypothesis that Heinrich events were produced by a sudden intensification of the iceberg discharge through Hudson Strait that mixed, in the North Atlantic, with icebergs that continued to calve from other ice sheets. The shift from mixed sources in the background sediment to a large dominance of Churchill province grains in H2 indicates that, even if calving of other ice sheets intensified during the Heinrich episode, the increase in the iceberg discharge via Hudson Strait from the Hudson Bay drainage basin of the Laurentide ice sheet was by far the largest.

Evidence from 40Ar/39Ar Ages for a Churchill province source of ice-rafted amphiboles in Heinrich layer 2

Abstract40Ar/39Ar ages of most single ice-ratted amphiboles from Heinrich layer 2 (H2) from a core in the Labrador Sea, a core in the eastern North Atlantic and a core in the western North Atlantic range from 1600 to 2000 Ma. This range is identical to that for K/Ar ages from the Churchill province of the Canadian Shield that outcrops at Hudson Strait and forms the basement of the northern part of Hudson Bay. The ambient glacial sediment includes some younger and older grains derived from Paleozoic, Mesoproterozoic and Archean sources, but still the majority of the amphiboles have ages in the 1600–2000 Ma interval. The Ca/K ratios of these 1600–2000 Ma old amphiboles, however, have a bimodal distribution in contrast with the uniformity of the Ca/K ratios of H2 amphiboles. This indicates that 1600–2000 Ma old amphiboles of the ambient sediment were derived from an additional Early Proterozoic source besides Churchill province. In H2, Churchill-derived grains constitute 20–40% of the ice-rafted debris (IRD). The fraction in the ambient glacial sediment is 65–80%. Results presented here are consistent with the hypothesis that Heinrich events were produced by a sudden intensification of the iceberg discharge through Hudson Strait that mixed, in the North Atlantic, with icebergs that continued to calve from other ice sheets. The shift from mixed sources in the background sediment to a large dominance of Churchill province grains in H2 indicates that, even if calving of other ice sheets intensified during the Heinrich episode, the increase in the iceberg discharge via Hudson Strait from the Hudson Bay drainage basin of the Laurentide ice sheet was by far the largest.

The evolution of the Grenville Province in eastern Labrador, Canada

Abstract Rifting at c. 1.71 Ga separated the Mealy Mountains terrane from pre-Labradorian Laurentia and resulted in a short-lived (backarc?) basin, followed by southward subduction giving rise to 1.68-1.66 Ga Labradorian calc-alkaline arcs, partly built on older crust in the Mealy Mountains terrane. The arcs and subduction were also short lived, terminating when accretion against the Makkovikian and Eastern Churchill provinces occurred at 1.65 Ga. The Trans-Labrador batholith developed as a result of crustal thickening during suturing of the 1.68-1.66 Ga arc-related rocks to pre-Labradorian Laurentia. Further bimodal magmatism, in response to crustal thickening, continued until c. 1.62 Ga, followed by waning granitic magmatism until 1.60 Ga. The Labrador Orogen and older rocks in the Mealy Mountains terrane are flanked to the south by Pinwarian (1.51-1.45 Ga) granitoid rocks, possibly the inboard manifestation of a northward-subducting continental-margin arc at the southern margin of Laurentia. Extension, mostly coincident with the 1.71 Ga zone of weakness, started at c. 1.45 Ga. Although rifting was successful further west, it is only represented by basaltic magmatism (Michael and Shabogamo gabbros) along a linear zone in Labrador. Similar tectonism characterized the same zone periodically for at least the next 150 million years during which time the 1.27 Ga Harp-Nutak dykes were emplaced and slightly younger rocks of the Seal Lake Group formed north of the Grenville Province. After c. 1.23 Ga tectonic conditions changed, partly in response to Elzevirian accretion of arc terranes preserved in the southwest Grenville Province, that are speculated to have once also existed south of the exposed eastern Grenville Province. Alternating compressional and extensional conditions terminated at c. 1.0 Ga during collisional orogenesis associated with the final stages of development of the Grenville Orogen. Grenvillian deformation in Labrador (1.08-0.97 Ga) resulted in northwesterly thrusting, the effects of which are focused in the Exterior Thrust Belt, and widespread plutonism in the Interior Magmatic Belt. Thrusting in the Exterior Thrust Belt is sited along the zone of 1.71 and 1.43 Ga rifting. Uplift and cooling continued until 0.90 Ga as crustal stability was gradually achieved.

The Palaeoproterozoic Southeastern Churchill Province of Labrador-Quebec, Canada: orogenic development as a consequence of oblique collision and indentation

Abstract Palaeoproterozoic orogenic development in the northeastern Canadian Shield was controlled by the successive, oblique collision of the Archaean Nain (North Atlantic) and Superior cratons with a southwards projecting promontory of the Archaean Rae Province (part of the northern Churchill Province hinterland). By this process the Rae Province became sutured to the Nain craton by the Torngat Orogen and to the Superior craton by the New Quebec Orogen, resulting in the formation of a 400 km wide orogenic belt known as the Southeastern Churchill Province (SECP). Initial rifting at 2.45–2.1 Ga, and early (arc?) magmatism and deformation at 2.3–1.9 Ga, were restricted to the Rae Province. They were followed by arc magmatism in the Torngat Orogen at c. 1.91–1.88 Ga and Rae/Nain collision 1.87–1.86 Ga, which resulted in the formation of an orogen with east- and west-verging structures. Arc magmatism in the New Quebec Orogen commenced at c. 1.845 Ga and was succeeded by Rae/Superior collision and widespread deformation across the SECP at c. 1.83 Ga. Deformation at this time was dominated by west-vergent thrusting in the New Quebec Orogen and Rae Province, and by renewed east-vergent thrusting in Torngat Orogen. Deformation was then progressively transferred to major sinistral (1.845–1.82 Ga in the eastern SECP) and dextral (1.83–1.80 Ga? in the western SECP) shear systems that accommodated continued northwards motion of the Nain and Superior cratons relative to the Rae Province. Juvenile crust expelled by thrusting effectively doubled crustal thickness in parts of the Rae Province and the northwestern edge of the Nain craton. Late stage development (1.8–1.71 Ga), was restricted to the margins of the SECP, where deformation was associated with renewed outward-directed overthrusting and transcurrent shear in conjunction with uplift of the orogenic core.

Proterozoic orogens of the northeastern Canadian Shield: new information from the Lithoprobe ECSOOT crustal reflection seismic survey

As part of the Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT), Lithoprobe acquired 1250 km of deep seismic reflection data along the coast of Labrador and across Ungava Bay, to image evidence of Proterozoic crustal accretion to the Archean nuclei of the Nain and Superior provinces of the Canadian Shield. The relatively pristine Archean crust of the Nain Province has low reflectivity and generally lacks systematic reflector orientations. Reworking of Archean crust on the margins of the Makkovik Province has little effect on this weak signature. In contrast, the Archean crust in the Eastern Churchill (Rae) Province appears to have been overprinted by a strongly developed, whole-crustal, easterly dipping reflection fabric, interpreted to result from Proterozoic collision of the Nain and Superior provinces in the paired New Quebec and Torngat orogens. Juvenile Proterozoic crust in the Makkovik and Grenville provinces also shows strong whole-crustal dipping reflection fabrics, interpretable as outwardly verging structures associated with collisional mobile belts. Crustal thickness varies from 35 to 45 km in Proterozoic provinces, except where thinner in areas probably affected by Mesozoic extension associated with rifting of the Labrador Sea. Dipping reflectors in the mantle are commonly associated with strong lower-crustal dipping reflections in a manner similar to that observed in some modern orogens. The ECSOOT data show that Proterozoic crust in this area has structural forms comparable with those of modern orogens and, inferentially, its tectonic development was controlled by very similar collisional processes.

A comparison of the receiver structure beneath stations of the Canadian National Seismograph Network

Three-component broadband data from the recently deployed Canadian National Seismograph Network provide a new opportunity to examine the structure of the crust and upper mantle beneath the Canadian landmass. Receiver function analysis is an ideal method to use with this data set, as it can provide constraints on the S-velocity structure beneath each station of this seismograph network. This analysis method is particularly useful in that it provides site-specific information (i.e., within 5–15 km of the station), low-velocity layers can be identified, and it is possible to examine structure to upper mantle depths. In this study, receiver functions were computed for each of the 19 stations that made up the seismograph network in June 1994. Five stations, sampling a variety of tectonic environments, including the Appalachian Orogen, the Canadian Shield, the Western Canadian Sedimentary Basin, and the Cascadia subduction zone, were chosen for detailed modelling. The results presented here are the first estimates of the S-velocity structure beneath these five stations. For those stations where comparisons can be made with seismic reflection and refraction results, there is excellent agreement. In eastern Canada, simple receiver functions and clear Moho Ps conversions at most stations indicate a relatively transparent crust and a Moho depth of 40–45 km. In northwestern Canada, Moho Ps phases indicate a crustal thickness of 33–38 km. Beneath Inuvik, Northwest Territories, the Moho is interpreted as two velocity steps separated in depth by 5 km, and an upper mantle low-velocity zone is near 47 km depth. In western Canada, the data indicate a mid-crustal low-velocity zone beneath Edmonton. The Moho of the subducting Juan de Fuca plate is interpreted at 52 km depth beneath southern Vancouver Island. Several stations exhibiting complex receiver functions warrant further study. They include stations at Schefferville, Quebec, in the Canadian Shield; Deer Lake, Newfoundland, on the boundary of the Grenville Province and the Appalachian Orogen; and Yellowknife, Northwest Territories, at the intersection of the Churchill and Slave provinces and the Western Plains.

A Diamondiferous Lamprophyre Dike, Gibson Lake Area, Northwest Territories

The Akluilak dike is a narrow lamprophyre intrusion of ultrapotassic composition that has yielded a large content of diamonds; it is the first multidiamond occurrence noted from the central Churchill Province. Xenocrysts of apatite containing monazite have been dated at 1832 ± 28 Ma, thus confirming the correlation with Christopher Island Formation volcanic and intrusive rocks. Only 1 or 2 diamonds per 1000 are greater than 0.5 mm, and the majority are less than 0.075 mm. They occur primarily as octahedra and tetrahexahedroids, but cubes, macles, aggregates, and fragments also are present. Their occurrence points to the presence of a “mantleroot-friendly” lithosphere, and confirms the “highly diamond prospective” nature of the area suggested by previous studies.

Petrogenesis of a Mid-Proterozoic Anorthosite-Mangerite-Charnockite-Granite (AMCG) Complex: Isotopic and Chemical Evidence from the Nain Plutonic Suite

The Nain Plutonic Suite (NPS), about 1.30 Ga, underlies some 19,000 km², intruding the boundary between Middle to Late Archean rocks in the east (Nain Province) and Late Archean and Early Proterozoic rocks in the west (Churchill Province). Nd isotopic compositions in most igneous units of the NPS reflect their geographic position relative to the inferred boundary between the Nain and Churchill Provinces. Rare exceptions are local intrusions of olivine-bearing basic rocks with highest $$\epsilon_{Nd}$$ @ 1.3 Ga (near -3.0), present in both eastern and western sectors of the NPS. Anorthositic rocks have variable and significant contributions of crustal Nd but notably lower $$I_{Sr}$$ than granitoid rocks. Ferrodiorites are similar isotopically to the anorthositic group and have chemical and mineral compositions more akin to the anorthositic group than the granitoids. Aluminous orthopyroxene megacrysts in NPS anorthositic rocks indicate crystallization pressures of 6.2 to 11.0 kbar, consistent with lower crustal to upper mantle depths. The granitoid rocks crystallized from magmas derived in large part by crustal partial melting. These rocks show the influence of sources similar to the basement gneisses, but were augmented by materials with much shorter crustal residence time, offering support for contemporaneous basaltic underplating of the lower crust. Nd and Sr isotope data, trace element geochemistry, and mineral compositions in major rock units of the NPS, together with knowledge of the timing of intrusive events, permits assessment of the essential features of petrogenesis and development of a paradigm that may have wider application. Separation of early granitoid melts left depleted, hot, plagioclase-pyroxene granulite crustal residues assimilated by mantle-derived basaltic magmas. That process developed large volumes of anorthositic magma in upper levels of deep crustal to uppermost mantle magma chambers by plagioclase flotation. Concurrently, fractionation moved residual liquids toward ferrodiorite compositions. Buoyant upward movement of anorthositic magmas followed crustal paths preheated by earlier passage of granitoid magmas. Only small volumes of high density, Fe-rich, ferrodiorite melts were expelled to higher crustal levels.

Geochemistry and origin of the Proterozoic ultrapotassic rocks of the Churchill Province, Canada

Early Proterozoic ultrapotassic dikes, lava flows, and pyroclastic rocks of the Christopher Island Formation (CIF) erupted throughout an area 600 × 300 km within the Churchill Province of the Canadian Shield at 1.84 Ga. The rocks range from mafic lamprophyres (mg # ⩾ 60; SiO2 47–54%, mean K2O/Na2O > 4) with phenocrysts of phlogopite + diopside + apatite ± olivine ± magnetite, to phenocryst-poor felsic rocks and sanidine porphyries (SiO255–69%). All samples have high incompatible element contents and display large depletions of high field strength elements relative to K, Rb, Sr, Ba, and Th. The CIF has geochemical and petrographic characteristics of both minettes and lamproites, but overall most closely resembles young Mediterranean lamproites. Felsic rocks of the CIF were produced by crystal fractionation and crustal contamination of mafic ultrapotassic magma, and include both high-silica lamproites strongly enriched in Zr, U, and Th, and weakly potassic to sodic rocks of trachytic composition. Flows and feeder dikes have relatively homogeneous ɛNd, 1840 Ma (−6 to −11) but highly variable ES., 1840 Ma (−40 to + 100); samples classified as lamproites have higher average ɛSr. Dike samples have highly variable present-day Pb isotope compositions, ranging from moderately to strongly nonradiogenic. Geochemical and isotopic data are consistent with contributions from depleted Archean lithospheric mantle, and OIB-type convecting mantle, both metasomatized by subduction-related processes during the Early Proterozoic. The lithospheric mantle probably contained Archean enriched domains as well. Proterozoic enrichment may have accompanied shallow underplating of subducted oceanic lithosphere beneath the Churchill Province during amalgamation of the Laurentian supercontinent. There are strong analogies in isotopic composition, and interpreted source region history, between the CIF and lamproites and minettes of the Wyoming Province and western Greenland, which suggest the existence of a Laurentian ultrapotassic “superprovince”.

Fluorite-bearing early and middle proterozoic granites, Okak Bay area, labrador: Geochronology, geochemistry and petrogenesis

Three discrete age groups of relatively fractionated, fluorite-bearing, post-tectonic Proterozoic granitoid rocks have been identified in the Okak Bay area of Labrador on the basis of U-Pb zircon geochronology. Two of the groups occur entirely within the Archean Nain Province; the third occurs largely within the Early Proterozoic Churchill Province (Torngat Orogen). None of the three episodes of granitoid magmatism can be directly linked to orogenic activity in its immediate surroundings. Although all three granite groups are broadly similar, petrologically and chemically, to the felsic rocks of the anorogenic Elsonian (≈1450−1290 Ma) Nain Plutonic Suite (NPS), only one of the dated units can be correlated confidently with Elsonian magmatism. The other two groups represent significantly older anorogenic or postorogenic magmatism. The oldest unit, the Wheeler Mountain granites of the Nain Province has yielded a U-Pb age of about 2135 (2134± 3 1, 2137±2) Ma. A major tectonothermal event of this age has not been previously identified in northern Labrador, and the origin of the granites is uncertain. Three small intrusions on offshore islands (White Bear, Saddle, Opingiviksuak) in the Nain Province were intruded about 1775 (1774± 2 1,1776± 4 2, 1774± 2 1) Ma. These granites may have been intruded post-kinematically into a stable Archean foreland representing a delayed effect of a preceding collisional event (Torngat Orogeny) between Nain and Churchill Province to the west. The youngest unit dated, at about 1318 (1319±2, 1316± 2 3) Ma, is Umiakovik Lake batholith which crops out largely within the eastern Churchill Province, and is one of the largest granitoid intrusions of the NPS. The age confirms that it formed an integral part of the Elsonian magmatic event, but both Umiakovik Lake batholith and nearby 1320 Ma Makhavinekh Lake pluton are significantly older than some basic members of the NPS in the coastal area to the southeast. Nd and Sr isotopic data demonsrate that Archean protoliths formed significant components of the source materials for all three of these Proterozoic granitoid magmas. This source was supplemented by one or more Proterozoic episodes of lower crustal intrusion and underplating by juvenile mantle-derived magmas that occurred in the region.

Nd Isotopic Evolution of the Taltson Magmatic Zone, Northwest Territories, Canada: Insights into Early Proterozoic Accretion along the Western Margin of the Churchill Province

The Taltson magmatic zone is a north-trending belt of Early Proterozoic granitoids exposed over an area exceeding in the Northwest Territories of Canada. Three petrologically and geochronologically discrete intrusive suites comprise the Taltson zone. These are the 1986 Ma Deskenatlata suite, the 1955 to 1925 Ma Slave suite, and the 1935 Ma Konth suite. The Deskenatlata suite is composed of biotite-hornblende diorite to granite and displays a more mafic geochemical character relative to the other two intrusive suites. Initial values for the Deskenatlata suite range from -2.7 to -3.6. The Slave and Konth suites are compositionally heterogeneous to homogeneous leucocratic granitoids that contain numerous xenoliths of dominantly pelitic high-grade paragneiss and display a peraluminous granite mineralogy and geochemistry. Initial values for the Konth suite cluster within -4.7 and -5.5 and are completely overlapped by the -4.1 to -6.0 range of the pelitic xenoliths. The Slave suite has initial values ranging from -4.4 to -8.6, overlapping with part of the Nd isotopic evolution path of gneisses of the 2.0-2.3 Ga Buffalo Head terrane and western Archean Rae Province. Similar to its northern extension (the Thelon orogen) the Taltson zone is considered to be a composite pre-collisional continental arc (Deskenatlata suite) and a continent-continent collision orogen (Konth and Slave suites). Collision between the Buffalo Head terrane to the west and the Rae Province to the east would have caused tectonic thickening and melting of pelitic metasediments, Rae Province and possibly Buffalo Head terrane crust manifested as the Konth and Slave suites.