Nain Province Research Articles

Physical Controls Associated with the Distribution of Sulfidesin the Voisey's Bay Ni-Cu-Co Deposit, Labrador

The Voisey’s Bay Ni-Cu-Co sulfide deposits occur within troctolites and olivine gabbros of the 1.34 Ga Voisey’s Bay intrusion, formerly known as the Reid Brook Intrusive Complex. The Voisey’s Bay intrusion is the oldest known member of the predominantly anorthositic Nain Plutonic Suite, which straddles the ca. 1.85 Ga suture between Archean gneisses of the Nain province to the east and Paleoproterozoic rocks of the Churchill province to the west. The Reid Brook, Discovery Hill, Mini-Ovoid, and Ovoid zones are mineralized domains within a subvertical conduit dike system that appears to span between two large troctolite intrusions, the Eastern Deeps chamber and the lower Western Deeps chamber. The conduit trends east-west and extends from near the top of the Western Deeps chamber, at a depth of 700 m, toward the north margin of the Eastern Deeps chamber close to surface. All the mineralized zones in the Voisey’s Bay deposits occur as magmatic-textured sulfides associated with fragment-bearing troctolites and olivine gabbros related to the conduit dikes, rather than simply as basal accumulations at the floor of the chambers. Sulfides within the system are preferentially concentrated in traps where physical irregularities and changes in conduit morphology favor the precipitation, capture, and preservation of sulfides as a result of changes in the velocity and viscosity of a magma. There is a consistent relationship between different sulfide textures and the physical environment existing in the conduits where each of the mineralized zones occurs. This suggests that sulfides were deposited in their present sites by consistent magmatic processes within the conduits but were locally influenced by the physical parameters existing at these sites. Later remobilization or mass movement of sulfides would not generate the consistent textural relationships observed. Therefore, sulfide deposition is ultimately related to a complex interplay of dike geometry (i.e., changes in dike orientation and thickness) and the fluid dynamics in the fragment-bearing magma within a conduit system, i.e., the Ovoid mineralization appears to fill a bulge in the conduit, and the Reid Brook zone is located close to the axis of inflection along its strike. The Eastern Deeps zone, although it does occur at the base of the Eastern Deeps chamber, is more specifically associated with the line of entry for the feeder conduit close to a structural low at the base of the Eastern Deeps chamber. The Voisey’s Bay deposit does not conform to the traditional model for magmatic sulfide generation such as Sudbury, in which sulfides accumulate largely by gravitational settling within a magmatic chamber, following attainment of sulfur saturation as a result of local assimilation. In fact current evidence suggests, in the Eastern Deeps mineralized zone, that little gravitational settling took place because a large proportion of sulfides occur as high Ni tenor minerals trapped interstitially in sulfides in coarse phases of the variable-textured troctolite. Furthermore, sulfide saturation and generation of magmas with varying sulfide contents clearly appear to have occurred in chambers at a depth of several kilometers, rather than at the levels currently exposed. In contrast to gravitational settling, sulfide distribution within the Voisey’s Bay deposit is controlled by magma emplacement through multiple braids of a dynamic, channel-like conduit dike system.

The Nain-Churchill Boundary and the Nain Plutonic Suite: A Regional Perspective on the Geologic Setting of the Voisey’s Bay Ni-Cu-Co Deposit

Northern Labrador is the site of two major lithotectonic elements of eastern Canadian Shield geology. First, it encompasses a Paleoproterozoic continental suture zone: Archean rocks of the Nain province tectonically juxtaposed against Paleoproterozoic and reworked Archean rocks of the Churchill province. The Nain-Churchill suture evolved between 1860 and 1740 Ma, impressing upon the Archean and Paleoproterozoic rocks a regional metamorphic and tectonic signature referred to as the Torngat orogen. An important component of the Torngat orogen is the Tasiuyak gneiss, a Paleoproterozoic metasedimentary unit that occupies most of the eastern margin of the Churchill province. Second, northern Labrador is the site of a world-renowned terrane of Mesoproterozoic anorogenic igneous rocks, the Nain Plutonic Suite, that sits astride the suture and acts as a stitching batholith between the Nain and Churchill provinces. The Nain Plutonic Suite, comprising amalgamated granitic, anorthositic, dioritic, and troctolitic intrusions, largely obscures the suture for nearly half of its total length. The Suite evolved between 1350 and 1290 Ma and is interpreted as a product of magmatism associated with limited intracontinental rifting of the conjoined Nain-Churchill crust above a mantle plume. The Voisey’s Bay Ni-Cu-Co deposit, serendipitously discovered in 1993, resides within one of the oldest troctolitic intrusions of the Nain Plutonic Suite, a 1334 Ma intrusion that transgresses a contact between Archean and Paleoproterozoic gneisses. Genetic controls on the setting of mineralization have been built upon the Nain-Churchill boundary as a zone of weakness that was exploited by mantle magmas and have implicated the Tasiuyak gneiss as a crustal sulfur source. Although the Archean-Paleoproterozoic junction at Voisey’s Bay is presently interpreted as representing the Nain-Churchill boundary, in reality the provincial affinity of the Archean gneisses is equivocal. This uncertainty dictates caution in assigning the Nain-Churchill boundary as the convenient transcrustal conduit that allowed the Voisey’s Bay mafic intrusions rapid transit from their mantle staging area. Modeling the sulfide deposit as being solely a result of sulfur saturation of mafic magma by proximal Paleoproterozoic Tasiuyak paragneisses is also questionable because other genetic processes may have been responsible; for example, desilication of gneiss is strongly indicated by aluminous refractory minerals in vestiges of (fused?) Tasiuyak gneiss. The spatial distribution of the mineralized rock at Voisey’s Bay, apparently within the subchamber plumbing system feeding a larger intrusion and within remnants of the larger intrusion itself, may serve as a paradigm for similar deposits in the Nain area, but the preerosional comparative morphology of that larger intrusion relative to the better preserved ones may never be known. A thorough understanding of the setting and genesis of the Voisey’s Bay deposit will better facilitate establishing exploration targets in other parts of the Nain region, in Labrador as a whole, and in other geologically similar regions of the world.

Petrology of the Abloviak Aillikite dykes, New Québec: evidence for a Cambrian diamondiferous alkaline province in northeastern North America

A dozen ultramafic lamprophyre dykes have been identified in the eastern part of Ungava Bay, Québec. The dark grey dykes are composed of macrocrysts of olivine and phlogopite featuring tetraferriphlogopite rims. These minerals are included in a matrix consisting of fine-grained phlogopite, olivine, spinel, and interstitial carbonate. The geochemical composition, mineral assemblage, chemical analyses of the xenocrysts and phenocrysts and Sm-Nd isotopic signature indicate that these rocks are carbonated ultramafic lamprophyre dykes. The chemical zonation of the micas and the presence of andradite suggest a complex sequence of crystallization. An Ar isotope correlation analysis indicates an age of approximately 550 Ma. The dykes have intruded the Tasiuyak gneiss, a suite of Paleoproterozoic metasedimentary rocks. This unit occupies the Torngat Orogen collision zone between the Rae Province and the Nain Province. The dykes are considered to have been emplaced within tension gashes during reactivation of major Paleoproterozoic structures and fractures associated with the opening of the Iapetus Ocean. They are possibly related to a mantle plume that simultaneously caused both the rupture of the Laurentia and the formation of the ultramafic magma. The Abloviak lamprophyres have quite similar analogues in southwest Greenland. The age, geochemical characteristics, isotopic signature common to all these dykes, the geological environment of the southwest Greenland are all comparable with the data presented here for the Abloviak lamprophyres. Furthermore their pre-drift geographical proximity, prior to the opening of the Labrador Sea, suggests that they are related to a common magmatic event which constitutes a diamondiferous alkaline province located in northeastern North America.

Low mantle heat flow at the edge of the North American Continent, Voisey Bay, Labrador

Heat flow measurements in 4 deep drillholes near Voisey Bay, Labrador, have yielded the lowest value ever reported in the Canadian Shield, 22 mW m−2. This very reliable estimate is also one of the lowest continental heat flow values world wide. It requires the crust to be very poor in radioelements in this part of the Archean Nain Province. It also strongly supports the view that mantle heat flow is low (<15 mW m−2) throughout the Canadian Shield, with no trend of increasing mantle heat flow near the edges of the continent. It also raises questions about the controlling mechanism for rifting and the opening of the Labrador Sea at 100 Ma.

Emplacement of the Nain anorthosite: diapiric versus conduit ascent

Estimation of settling velocities of large orthopyroxene megacrysts, found within anorthosite intrusions, are calculated and compared with ascent rates achieved by diapirism and conduit propagation. Calculations suggest that diapirism is far too slow to be an appropriate ascent mechanism for anorthositic crystal mush and favour conduit emplacement. The intrusions of the Nain Plutonic Suite (NPS) are located along the Abloviak shear zone, which marks the boundary between the Nain and Churchill provinces, and within the zone of juxtaposition of the Saglek and Hopedale blocks of the Nain Province. These crustal weaknesses have probably controlled the emplacement and distribution of the intrusions. Contact relations between intrusions of anorthosite and their gneissic host rock provide evidence for two emplacement styles within the NPS, the first typified by strongly deformed and recrystallized rocks, and the second by an outer border zone of mafic rocks. It is proposed that these differences in intrusive style are due to differences in ductility contrast between the magma and its surrounding host rocks, such that those intrusions emplaced into the thermally softened shear zone have deformed margins, whereas those intruded into the cooler Archaean crust have undeformed margins.

Kanairiktok shear zone: the boundary between the Paleoproterozoic Makkovik Province and the Archean Nain Province, Labrador, Canada.

Kanairiktok shear zone: the boundary between the Paleoproterozoic Makkovik Province and the Archean Nain Province, Labrador, Canada.

Geochemical and radiogenic isotope (Sr-Nd) characteristics of Paleoproterozoic anorthositic and granitoid rocks in the Umiakoviarusek Lake region, Labrador, Canada.

Recent work in the north-central Labrador has identified Paleoproterozoic anorthositic and granitoid rocks that are spatially associated with, yet temporally distinct from, younger Mesoproterozoic intrusions of the Nain Plutonic Suite. The Umiakoviarusek Lake (UL) region of Labrador contains several of these Paleoproterozoic intrusions and provides an opportunity to study their geochemical and radiogenic isotope (Sr-Nd) characteristics. Geochemically, the anorthositic and granitoid rocks have features consistent with contemporary anorthositic and granitoid rocks from other anorthosite-mangerite-charnockite-granite complexes. The anorthositic rocks contain elevated contents of Al2O3, CaO, Sr, and Eu with low Ba, Rb, K, Zr, total rare earth elements (REE), and light REE. The granitoid rocks, on the other hand, contain lower concentrations of these elements along with elevated SiO2 and K2O. Isotopic data at 2050 Ma for the anorthositic rocks (ISr = 0.7048-0.7082; εNd = -4.1 to -15.9) and granitoid rocks (ISr = 0.7036-0.7094, εNd = -5.1 to -9.7) are consistent with variable crustal and mantle contributions to their genesis. The relatively unradiogenic Sr and slightly evolved Nd isotopic data from the UL granitoid rocks is consistent with a significant juvenile mantle component, possibly derived from an underplating mantle plume; this component may also be present in the anorthositic rocks. The Nd and Sr isotopic data are also consistent with crustal contamination from Archean source materials; however, based on the existing isotopic database for the Nain Province gneisses, it is not possible to delineate a specific gneiss component. Furthermore, it is also quite possible that an Archean source, unlike any described at present, was a crustal source component in the UL intrusive rocks.

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.

Age and tectonic implications of Paleoproterozoic granitoid intrusions within the Nain Province near Nain, Labrador

U-Pb (zircon) age determinations from three granitoid intrusions in the Nain region, formerly assigned to the well-known 1.35-1.29 Ga (Mesoproterozoic) Nain Plutonic Suite (NPS), indicate that they were emplaced, instead, during the Paleoproterozoic. Crystallization ages of 2109 ± 3 Ma for Sheet Hill granite, 2052 ± 4 Ma for Loon Island granite, and 2025 ± 7 Ma for Satok Island monzonite demonstrate a decreasing time of emplacement from north to south. Leucocratic basic rocks, superficially similar to those of the NPS, occur as inclusions within the Satok Island monzonite and are intruded by an 1873 ± 4 Ma granitic aplite dyke north of Webb's Bay. The ages clearly demonstrate granitic magmatism in the Nain area for at least 750 Ma, and anorthositic magmatism for at least 675 Ma, before the development of NPS. The earliest Paleoproterozoic magmatism (2109-2025 Ma) may have coincided with breakup of the North Atlantic craton, whereas the local magmatism represented by the 1873 Ma aplite dyke may be related to the embryonic stages of the continental collision that produced the 1860-1740 Ma Torngat Orogen. Leucocratic basic rocks associated with the Paleoproterozoic granitic rocks identified here could be Archean in age, but are more likely coeval with the granites. Other granitic and basic rocks in the Nain area may likewise be products of this bimodal plutonism. Identification of this possible geological duality suggests repetitive magmatism of "anorogenic type" over a significant time span in this part of Labrador. If the mantle-plume-related genesis accorded the Mesoproterozoic NPS is applicable to the Paleoproterozoic rocks, it implies multiple periods of similar lithosphere-asthenosphere interaction in this area, beginning at least in the Paleoproterozoic. The Paleoproterozoic magmas may have been emplaced into Nain Province crust along a linear intracontinental extension zone inboard from a rifted margin.

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.

Age and tectonic implications of Paleoproterozoic granitoid intrusions within the Nain Province near Nain, Labrador

U-Pb (zircon) age determinations from three granitoid intrusions in the Nain region, formerly assigned to the well-known 1.35-1.29 Ga (Mesoproterozoic) Nain Plutonic Suite (NPS), indicate that they were emplaced, instead, during the Paleoproterozoic. Crystallization ages of 2109 ± 3 Ma for Sheet Hill granite, 2052 ± 4 Ma for Loon Island granite, and 2025 ± 7 Ma for Satok Island monzonite demonstrate a decreasing time of emplacement from north to south. Leucocratic basic rocks, superficially similar to those of the NPS, occur as inclusions within the Satok Island monzonite and are intruded by an 1873 ± 4 Ma granitic aplite dyke north of Webb's Bay. The ages clearly demonstrate granitic magmatism in the Nain area for at least 750 Ma, and anorthositic magmatism for at least 675 Ma, before the development of NPS. The earliest Paleoproterozoic magmatism (2109-2025 Ma) may have coincided with breakup of the North Atlantic craton, whereas the local magmatism represented by the 1873 Ma aplite dyke may be related to the embryonic stages of the continental collision that produced the 1860-1740 Ma Torngat Orogen. Leucocratic basic rocks associated with the Paleoproterozoic granitic rocks identified here could be Archean in age, but are more likely coeval with the granites. Other granitic and basic rocks in the Nain area may likewise be products of this bimodal plutonism. Identification of this possible geological duality suggests repetitive magmatism of "anorogenic type" over a significant time span in this part of Labrador. If the mantle-plume-related genesis accorded the Mesoproterozoic NPS is applicable to the Paleoproterozoic rocks, it implies multiple periods of similar lithosphere-asthenosphere interaction in this area, beginning at least in the Paleoproterozoic. The Paleoproterozoic magmas may have been emplaced into Nain Province crust along a linear intracontinental extension zone inboard from a rifted margin.

Wide-angle seismic imaging of pristine Archean crust in the Nain Province, Labrador

A detailed refraction - wide-angle reflection seismic experiment was carried out in northern Labrador to determine the velocity structure of relatively unaltered Archean crust in the Nain Province. Six 3-component land seismometers were used to record an airgun source along a profile parallel to the coast. Forward modeling of traveltimes and amplitudes yields a P- and S-wave velocity model that shows two crustal blocks separated by a fault. Magnetic data suggest, but do not prove, that the fault is the offshore continuation of the Handy fault. A southwards thickening of the lower crust across the fault indicates that a transcurrent component might have been associated with the faulting. The total crustal thickness is 33 km to the north and 38 km to the south of the fault. The presence of PmS reflections imply a sharp transition at the Moho. Upper crustal velocities of 5.8-6.3 km/s and Poisson's ratios of 0.20 and 0.24, north and south of the fault respectively, are consistent with a gneissic composition, but suggest a higher quartz content in the northern block. Velocities in the middle crust increase to 6.5 km/s, where a discontinuity at a depth between 16 and 18 km marks the transition to the lower crust with velocities between 6.6 and 6.9 km/s. Poisson's ratios of 0.24 and 0.26 indicate, respectively, a felsic middle crust and an intermediate composition for the lower crust. The absence of a high-velocity basal layer is in accordance with other examples of Archean crust.

Crustal structures of Grenville, Makkovik, and southern Nain provinces along the Lithoprobe ECSOOT Transect: regional seismic refraction and gravity models and their tectonic implications

Crustal structures of the eastern Grenville, Makkovik, and southern Nain provinces are determined using seismic reflection-refraction and gravity data along the Lithoprobe Eastern Canadian Shield Onshore-Offshore Transect (ECSOOT). Within the Grenville Province, the velocity model contains a 5 km thick upper crust and a variable-thickness middle to lower crust. The total crustal thickness varies from 25 to 43 km, with the thickest crust in the south and thinnest crust in the north. A high-velocity, lower crustal wedge is coincident with a strong band of northward-dipping reflectors. The two-dimensional velocity structure is compatible with modelling of a 60 mGal gravity high over the Hawke River terrane. In the Makkovik Province, the thickness of upper crustal velocities increases to 17 km. The velocity decrease in the upper to middle crust from the Grenville Province to the Makkovik Province is similar to that of refraction models across the Grenville Front in Ontario and Quebec. It is possibly related to a decrease in metamorphic grade from south to north and (or) a larger volume of unmetamorphosed plutons in the Makkovik Province. A lower crustal layer is coincident with a region of increased reflectivity in the lower crust. There are no major crustal discontinuities associated with terrane boundaries within the Makkovik Province. The base of the crust is consistent with a change from north- to south-dipping reflectors beneath the Cape Harrison domain. Alternatively, it may consist of a thick zone of complex velocity variations, consistent with a zone of diffusive reflectivity observed to the north of the Allik domain.

Wide-angle seismic imaging of pristine Archean crust in the Nain Province, Labrador

Wide-angle seismic imaging of pristine Archean crust in the Nain Province, Labrador

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.

Crustal-scale flexural slip folding during late tectonic amplification of an orogenic boundary perturbation in the Paleoproterozoic Torngat Orogen, northeastern Canada

Large variations in metamorphic grade over short distances, disparate orientations and diverse kinematics of contemporaneous structures, and a previously unexplained, 90° counterclockwise bend in the orogenic boundary of the amphibolite- to granulite-facies northern segment of the Paleoproterozoic Torngat Orogen are shown to be the result of multiple tectonic events acting upon an orogenic boundary perturbation. The perturbation was initiated when a promontory on the Nain Province margin, composed of a 1910–1885 Ma continental magmatic arc (Burwell domain), indented the Rae Province hinterland during the onset of collisional orogeny at ca. 1870 Ma (Dn+1). Sinistral transpression at ca. 1845–1822 Ma (Dn+2) caused formation of the orogen-parallel Abloviak shear zone and oblique burial of the Nain Province margin beneath a tilted section of the hot, buoyant magmatic arc. Reactivation of the orogen at ca. 1798–1770 Ma (Dn+3) involved crustal-scale flexural slip folding of the perturbation and simultaneous exhumation of the Burwell domain and the previously buried Nain crust across the Komaktorvik shear zone, which represents a sheared, tightened fold train localized along the western limit of thinned Nain crust affected by preorogenic rifting, but which does not represent a fundamental plate boundary. The along-strike heterogeneities in the Torngat Orogen document the influence of geometrical and competency heterogeneities in the colliding margins on subsequent deformation and the fact that heterogeneities in the deep crust persist through high-grade metamorphism.

Igneous Charnockites and Related Rocks from the Bjerkreim-Sokndal Layered Intrusion (Southwest Norway): a Jotunite (Hypersthene Monzodiorite)-Derived A-type Granitoid Suite

The upper part of the Bjerkreim–Sokndal layered intrusion (BKSK) INTRODUCTION comprises a suite of charnockitic granitoids (mangerite, quartz Charnockitic granitoids are a major component of mangerite and charnockite) with some jotunites. The petrology and anorthosite–mangerite–charnockite–(rapakivi) granite geochemistry (major and trace elements) of eight main rock types (AMCG) complexes and therefore play a crucial role in (88 samples) are considered. The rocks form a suite of K2O-rich the formation and evolution of the Proterozoic crust. alkali–calcic granitoids of A-type, deriving from jotunitic magma. Their origin has been long debated. Though intimately Two trends of inferred liquids are distinguished: the main liquid associated with massif anorthosites, they are generally line of descent which passes from jotunites through two-pyroxene considered to be derived from distinct parental magmas, quartz mangerites to amphibole charnockites, the second one which generated in different sources (Emslie, 1978, 1991; Morse, encompasses olivine-bearing quartz mangerites and charnockites and 1982; Duchesne, 1984; Wiebe, 1992; Ashwal, 1993). It is rooted into mangeritic liquids. Comparison with the Tellnes rock has, however, been recognized that some charnockitic series reveals that contamination by leucogranitic material has taken rocks could also be derived from an intermediate monplace simultaneously with fractional crystallization in both BKSK zodioritic to ferrodioritic ( jotunitic) magma (Duchesne trends. Zr, Rb, Cs and Th are particularly sensitive to assimilation et al., 1989; Wilmart et al., 1989; Owens et al., 1993) and the higher water content in BKSK. Zircon and apatite have whose origin is also controversial. The jotunitic magma saturated at the liquidus of quartz mangerite around 900°C, close could either be derived from a crustal source by the to the fayalite–magnetite–quartz (FMQ) buffer. The two-pyroxene partial melting of basic rocks (Duchesne et al., 1989; quartz mangerites cannot have been equilibrated with the mangerite Duchesne, 1990), or be residual magmas after the forcumulates on which they rest. A major influx of magma took place mation of massif anorthosites which have been conwhen mangeritic cumulates were being deposited. It mingled and partly mixed with the resident contaminated magma. taminated to various degrees by crustal material (Emslie et al., 1994). The latter hypothesis seems to explain a large number of AMCG complexes in the Grenville and Nain provinces and elsewhere. Accepting that some charnockites are derived from jotunites, there is a logical

SmNd isotopic and geochemical characterisation of the Paleoproterozoic Torngat orogen, Labrador, Canada

The Torngat orogen is a Paleoproterozoic, granulite-facies belt that welds the Archean components of the Nain and Rae provinces in eastern Labrador. SmNd isotopic and supporting bulk rock major, trace and rare earth element data help elucidate the sources of the main Paleoproterozoic lithologies that comprise the orogen. In the foreland, Paleoproterozoic Ramah Group metasediments intercalated with reworked Nain Province gneiss show variable trace element and ∈ Nd(1.9 Ga) from −2.9 to −12.0, ranging between Nain Province crust and Paleoproterozoic Mugford Group basalts compositions. The Ramah Group data suggest derivation of Ramah detritus from Archean Nain Province gneisses and from the Mugford Group. In the Lac Lomier complex (Rae Province hinterland), metasediments with variable trace element distribution patterns akin to Rae Province crust, and ∈ Nd(1.9 Ga) from −2.9 to −10.5 suggest derivation from Rae Province crust and from an unspecified juvenile component. In the core of the orogen, pelitic and psammitic protoliths of the Tasiuyak gneiss (∈ Nd(1.9 Ga) from −1.9 to −2.6) were derived from crustal sources unknown in both Nain and Rae provinces, and may have been deposited in a trench environment starved of adjacent cratonic debris. Dioritic to granitic eastern and western metaplutonic suites intruding the Tasiuyak gneiss and the Lac Lomier complex display contrasting geochemical and SmNd isotopic compositions. The eastern metaplutonic suite has a tendency toward lower SiO 2, total alkalis, less fractionated REE and higher MgO, CaO and TiO 2 relative to the western metaplutonic suite. The eastern suite yields ∈ Nd(1.9 Ga) from +3.8 to −6.7 and negative f Sm Nd suggesting igneous precursors from Rae crust and a a LREE-enriched juvenile component. The western suite yields ∈ Nd(1.9 Ga) from −1.5 to −8.5, suggesting an important contribution from Rae crustal sources in the petrogenesis of the igneous protoliths. The presence of Rae Province Nd isotopic compositions, and the absence of Nain Province signatures in these intrusions favors the eastern Rae Province as the active margin during Nain-Rae convergence.

Late Archean evolution of the Nain Province, Nain, Labrador: imprint of a collision

Archean gneisses of the Nain Province in the Nain area, Labrador, comprise narrow septa between intrusions of the Mesoproterozoic Nain Plutonic Suite. This region occupies a critical position between the northern (Saglek) and southern (Hopedale) blocks of the Nain Province, which experienced distinct thermotectonic histories until the late Archean. Field and geochronological evidence are indicative of a strong late Archean thermotectonic overprint that is absent from most of the remainder of the Nain Province. Archean gneisses were intruded by granites and mafic dykes at 2578 ± 3 and [Formula: see text], respectively, and subsequently subjected to amphibolite- to granulite-facies regional metamorphism and ductile shearing at ca. 2550 Ma; granite veins and dykes related to the Nain Plutonic Suite were emplaced at ca. 1310 Ma. The Archean events are interpreted to represent the juxtapositioning and final docking of Saglek and Hopedale blocks to form a single, stable cratonic mass during the late Archean. Corroborative evidence indicates that the collisional boundary zone could extend at least 200 km to the north and 150 km to the south of Nain. This late Archean junction may have subsequently been exploited by several Paleoproterozoic granitic intrusions and some members of the Mesoproterozoic Nain Plutonic Suite.

Crustal structure across the Nain – Makkovik boundary on the continental shelf off Labrador from seismic refraction data

A detailed seismic refraction profile was shot along the continental shelf off Labrador, across the boundary between the Archean Nain Province to the north and the Proterozoic Makkovik orogenic zone to the south. A large air-gun source was used, with five ocean-bottom seismometers as receivers. The data were analysed by forward modelling of traveltimes and amplitudes and provided a well-determined seismic velocity structure of the crust along the profile. Within the Nain province, thin postrift sediments are underlain by crust with a P-wave velocity of 6.1 km/s, which increases with depth and reaches 6.6 km/s at about 8 km. Moho is at around 28 km, and there is no evidence for a high-velocity (>7 km/s) lower crust. The P- and S-wave velocity structure is consistent with a gneissic composition for the Archean upper crust, and with granulites becoming gradually more mafic with depth for the intermediate and lower crust. In the Makkovik zone, the sediments are thicker, and a basement layer of P-wave velocity 5.5–5.7 km/s is present, probably due to reworking of the crust and the presence of Early Proterozoic volcanics and metasediments. Upper crustal velocities are lower than in the Nain Province. The crustal thickness, at 23 km, is less, possibly due in part to greater crustal stretching during the Mesozoic rifting of the Labrador Sea. The crustal structure across the Nain–Makkovik boundary is similar to that across the corresponding Archean–Ketilidian boundary off southwest Greenland.