Nain Province Research Articles

Palaeoproterozoic, ∼ 1.88–2.0 Ga, organic matter from the Mugford/Kaumajet Mountain Group, northern Labrador

The Palaeoproterozoic, ∼ 1.88–2.0 Ga, Mugford Group of northern Labrador is a relatively undeformed cover sequence developed on Archaean crust of the Nain craton. The Cod Island Formation occurs at the base of the group and consists of mixed shale, chert, dolostone and siltstone. Thin layers of highly matured organic material similar to anthracite have been found within a shale and chert sequence. Based on the nonmigrated and sedimentological context of the layers, the material may represent some of the earliest intact, in situ, organic deposits known. δ 13C isotope ratios of the organic matter in this deposit range from −31.79 to −32.22‰ and the H/C atomic ratios from 0.05 to 0.08. The material represents the first truly biological remains known from the Nain Province of Labrador and the earliest organic material in northeastern North America.

Morphological features of multigeneration basic dykes near Nain, Labrador: clues to original emplacement mechanisms and subsequent deformation

The Archean rocks of the Nain Province near Nain, Labrador have been intruded by a multi-generation assemblage of basic dykes. The regional setting constrains their intrusion to between ∼2.5 Ga and ∼1.3 Ga; they may be, in fact, products of episodic basic magmatism over this whole 1200 million year span. They are unique among other mafic dykes in the Nain Province by their morphological diversity, mineral assemblages, and degree of recrystallization. They vary from straight-walled to cuspate-bordered intrusions, and they exhibit chilled to intimately mingled contacts against enveloping rocks; their emplacement is interpreted to have occurred under both brittle and ductile crustal conditions. The cuspate borders on the dykes are an enigma; they could be either primary features produced at time of emplacement or be deformation-induced by much later events (or both). Texturally, the dykes range from subophitic to granoblastic to strongly foliated. Foliated dykes commonly display sigmoidal fabrics indicative of both dextral and sinistral shear. The dykes have mineral assemblages implying variable recrystallization under amphibolite- and granulite-facies conditions, but no simple relationship to regional or contact metamorphic assemblages in enclosing gneisses is evident. It seems highly probable that some of the granular orthopyroxene-bearing dykes are primary gabbronoritic rocks. The multiplicity of intrusions evident within the basic dyke assemblage and the variable degree of deformation and recrystallization cannot presently be confidently correlated with other events in the surrounding region, but several options are considered. Among the probable regional scenarios that could account for the diversity and the multigeneration aspect of the assemblage are several discrete episodes of dyke intrusion: (i) Archean dykes related to a basic magmatic event yet unrecognized elsewhere in Labrador; (ii) Paleoproterozoic dykes equivalent to swarm snorth and south of Nain; (iii) Paleo- or Meso-proterozoic dykes related to newly recognized anorogenic plutons of these ages; and (iv) Mesoproterozoic dykes as leaks from a subscrustal magma reservoir that culminated in the formation of the mafic plutons within the anorogenic Nain Plutonic Suite.

U–Pb geochronology of a Paleoproterozoic continental magmatic arc on the western margin of the Archean Nain craton, northern Labrador, Canada

The Paleoproterozoic Torngat Orogen, in northernmost Labrador and northeastern Quebec, records the collision between the western margin of the Nain Province and the eastern margin of the Rae Province. Six samples from a suite of calc-alkaline plutonic rocks that intrude the Nain craton have been dated using the U–Pb method at 1895 ± 2, 1888 ± 2, [Formula: see text], 1885 ± 2 (two samples), and >1859 Ma, and support the interpretation that east-dipping subduction occurred below the Nain craton during this interval. A granitic dyke that truncates the sinistral shear fabric in the Abloviak shear zone is 1824 ± 2 Ma, placing a younger limit on the timing of this deformation in the area. A second granitic dyke, emplaced synchronously with dextral deformation in the bend in the Abloviak shear zone, is 1798 ± 3 Ma. A model is proposed in which subduction occurred beneath the Nain craton along its entire exposed length from 1.91 to 1.86 Ga, and collision occurred between 1.86 and 1.84 Ga, followed by oblique uplift of the southern part of the Rae craton from 1.80 to 1.71 Ga.

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.

U [sbnd]Pb geochronology of the northern Torngat Orogen, Labrador, Canada: a record of Palaeoproterozoic magmatism and deformation

The Palaeoproterozoic Torngat Orogen, located in northeastern Quebec and northern Labrador, Canada, records the sinistral transpressional collision of the western margin of the Archaean Nain Province and the eastern margin of the Rae Province. U Pb age determinations are presented for twelve samples from the northern part of the orogen, and used to evaluate models for its tectonic evolution. Voluminous granitoid plutonic rocks which in part intrude gneisses interpreted as the northern extension of the Nain Province and are possibly related to subduction below it, have zircon ages of 1910 ± 2 Ma, 1895 ± 3 Ma, 1891 ± 2 Ma, 1890 ± 2 Ma, 1869 +3 −2 Ma, and 1864 ± 2 Ma, suggesting at least 45 m.y. of convergence between the Rae and Nain Provinces. Granulite-facies conditions may have existed in the northern part of the area at ∼ 1.84 Ga. These plutonic rocks, as well as tonalitic gneisses of the Nain Province, were reworked at amphibolite-facies conditions within a north-south corridor referred to as the Komaktorvik zone. Zircon age determinations of two granitic veins which were intruded synchronously with ongoing deformation in the Komaktorvik zone are 1789 ± 2 Ma and 1719 ± 2 Ma, súggesting that deformation within the zone may have lasted for up to 70 m.y. Metamorphic titanite from samples within the Komaktorvik zone have ages of 1776 ± 4 Ma, 1774 ± 4 Ma, 1767 ± 4 Ma, 1710 ± 4 Ma and 1632 ± 4 Ma, and support the interpretation of long-lived deformation in the Komaktorvik zone. Palaeoproterozoic subduction beneath the Nain Province has not been widely recognized in southern parts of the orogen, suggesting that a distinct tectonic regime existed in the study area.

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.

Geophysical interpretation of the Torngat orogen along the North River – Nutak transect, Labrador

A 19.3 mGal (1 Gal = 1 cm/s2) positive Bouguer gravity anomaly and a broad aeromagnetic low coincide with the Tasiuyak domain of the early paleo-Proterozoic Torngat orogen. The domain evolved during the collision of the western margin of the Archean Nain Province with an inferred, eastward-facing, platform-to-rise, sedimentary wedge sited on a proto-continent of Churchill (Rae) Province.A density contrast of +0.065 g∙cm−3 was measured between rocks of the Tasiuyak domain and those of the flanking Lac Lomier complex of the orogen and Nain Province. Using this contrast, a two and one-half dimensional model shows that rocks of Tasiuyak domain constitute a triangular prismatic body with maximum thickness of 13 km adjacent to Nain Province, which thins westward to a feather edge. The model is compatible, qualitatively, with the aeromagnetic anomaly and consonant with geological interpretation of an eastward-facing and thickening continental slope deposit. Deep exhumation of the Torngat orogen may account for the absence of paired gravity anomalies characteristic of many sutures elsewhere in the Canadian Shield.

Petrological and whole rock isotopic characteristics of tectonically juxtaposed Archaean gneisses in the Okak area of the Nain Province, Labrador: relevance for terrane models

In a recent model (Schiøtte et al., 1990), the Archaean Nain Province of northern Labrador is described as a tectonic collage of terranes with different origins and metamorphic histories that was assembled in the late Archaean. 3200–3260 Ma granulite facies gneisses that are intercalated with the regional > 3500 Ma Uivak gneisses in the northern (Saglek) block of the Nain Province are interpreted as tectonic slivers of a terrane of mid-Archaean rocks represented by the Maggo gneisses of the Hopedale block 200 km farther south. This terrane is thought to correlate with the Akia terrane in southern West Greenland. In the Nain Province as opposed to West Greenland, the tectonic juxtaposition is thought to have taken place along subvertical planes. There is overlap in the range of chemical compositions exhibited by the early and mid-Archaean orthogneisses in the Okak area of the Saglek block, but on average the mid-Archaean gneisses are more mafic and less silica-rich. Their major element chemistry exhibits much simpler evolutionary trends. The REE chemistry is not related to the major element chemistry in any straightforward way in either early or mid-Archaean gneisses. REE patterns show some correlation with the local concentration of accessory REE-bearing minerals and are subject to metamorphic redistribution as late as 2560 Ma. In spite of this redistribution, late Archaean ϵNd-values can be used as a rough guide to the duration and complexity of the pre-juxtaposition history. ϵNd (2560 Ma) of individual samples ranges from −4.4 to −7.0 in the mid-Archaean gneisses as opposed to values < −7.7 in the early Archaean gneisses. The difference in metamorphic history also shows up in the whole rock Pb isotopic signature: the mid-Archaean granulite facies gneisses were juvenile intrusions, the source of which can be estimated from model μ1-values of ∼7.8–8.0. They crystallized in granulite facies sometimes between their emplacement at ∼3235 Ma and an event of granitoid veining at ∼2990 Ma. They do not exhibit the complexities or distinctly non-juvenile 207Pb204Pb ratios recorded from the early Archaean gneisses. The major volume of exposed early Archaean rocks in the Saglek block recrystallized under granulite facies metamorphism between 2780 and 2740 Ma. A suite of early Archaean granitic-granodioritic gneisses in granulite facies from the Okak area differs from previously investigated Uivak gneisses by showing evidence for larger pre-metamorphic variation in UPb (207Pb204Pb ratios that range from 14.02 to 16.30) and lower ThU. Secondary U loss during late Archaean metamorphism gave rise to a PbPb “palaeoisochron”. A late Archaean SmNd isochron with a strongly negative initial ϵNd has been obtained on this gneiss suite. The suite also exhibits other chemical peculiarities such as high Cr contents. A complex petrogenesis within a strongly differentiated early Archaean crust is suggested. There is no straightforward correlation between possible subterranes within terranes consisting of early Archaean rocks in the Nain Province and West Greenland.

Paleoproterozoic tectonic assembly of Northeast Laurentia through multiple indentations

Recent detailed geological mapping and UPb geochronological studies in the Torngat, Ungava and Baffin orogens of Northeast Laurentia have provided new information regarding the tectonic assembly of this region between ∼1.92 and 1.74 Ga. In contrast with an earlier hypothesis whereby the geometry of Archean continental fragments was thought to have resulted entirely from northerly indentation of the Superior Province and consequent extrusion tectonics, interpretation of recently acquired data shows that Superior Province indentation was the last of multiple collisional events during the formation of Northeast Laurentia. In the present model, we propose that Southeast Rae and North Rae provinces were separate prior to ∼1.92 Ga, when regional tectonism changed from extension to contraction. Westerly subduction and westward translation of the Archean Disko terrane and Nain Province between the North Rae and Southeast Rae provinces resulted in the development of a continental magmatic arc in the Cumberland batholithic complex and in the Torngat orogen, and deposition of the upper sequences of the Penrhyn-Piling-Karrat sequences in the Foxe and Rinkian belts of the Baffin orogen (1880–1895 Ma). Continental collision between the Disko terrane and North Rae Province produced shortening across the Rinkian and Foxe belts at, or before, 1876 Ma. Meanwhile, early arc magmatism in the Ungava orogen (1861–1898 Ma) accompanied foredeep or transtensional basin formation in the New Quebec orogen (∼1870–1883 Ma) and the onset of deformation in the southeastern Nagssugtoqidian orogen (∼1883 Ma). Nain-Southeast Rae provinces collision at ∼1860 Ma across the Torngat orogen caused renewed granitic magmatism and metamorphism in the Cumberland batholithic complex between 1857 and 1853 Ma. Initiation of felsic volcanism in the Makkovik-Ketilidian orogen at this time indicates a shift in the locus of plate subduction from the Southeast Rae Province to the southern margin of the Nain Province. In the Ungava orogen, northerly subduction changed to southerly subduction resulting in the generation of a second arc suite at between 1844 and 1826 Ma. Formation of the de Pas batholith in the hinterland of the New Quebec orogen at ∼1840 Ma accompanied arc-continent and Superior-Southeast Rae collision from 1845 to 1829 Ma. At this time, accretion of arc material in the Makkovik-Ketilidian orogen was contemporaneous with high-grade sinistral shearing along the reactivated Torngat orogen and the northwestern Nagssugtoqidian orogen. Southeast-directed thrusting along the southeastern margin of the Cumberland batholithic complex, and reactivation of the Rinkian belt is predicted to have also occurred at this time. At ⩽1826 Ma, the onset of arc-continent collision across the Ungava orogen initiated closure of the North Rae-Southeast Rae provinces juncture and generation of the Ford Lake plutonic suite. Subsequent Superior-Southeast Rae continental collision (∼1810 Ma) resulted in dextral “escape tectonics” in the Foxe and Rinkian belts, and thick-skinned folding of basement and cover sequences in the Ungava orogen. High-grade metamorphism and granitoid magmatism in the Makkovik-Ketilidian orogen also occurred. Reactivation of the Nain-Southeast Rae (east-directed thrusting) and Superior-Southeast Rae (west-directed thrusting) margins at ∼1790 Ma resulted in “pop-up” of the Southeast Rae Province between its more rigid neighbors. Final cooling and the emplacement of post-tectonic granitoid rocks took place at ∼ 1740–1790 Ma.

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.

U–Pb ages of single zircons within "Upernavik" metasedimentary rocks and regional implications for the tectonic evolution of the Archaean Nain Province, Labrador

Detrital zircons and their postdepositional overgrowths from three units of the "Upernavik" supracrustal association in the northern (Saglek) block of the Archaean Nain Province have been dated with the ion microprobe SHRIMP. In one unit, from the granulite-facies area in inner Saglek Fiord, the zircon population is dominated by early Archaean grains thought to be derived from the Uivak gneisses. Recrystallization and growth of new zircon within this metasediment took place during granulite-facies metamorphism at 2761 ± 12 Ma (2σ), which is also a younger limit on the age of deposition.In a second unit, from the amphibolite-facies area in outer Saglek Fiord, detrital zircons have predominantly mid- and late Archaean ages. The mid-Archaean zircons are comparable in age to the 3235 Ma Lister gneisses. The ages of the late Archaean detrital zircons (2800–2960 Ma) do not correspond to any known rock complex in the Saglek block, but plutonic rocks associated or correlative with the ca. 2840 Ma Kanairiktok Plutonic Suite of the southern (Hopedale) block are a possible source for many of the grains. Overgrowths were dated at 2690–2730 Ma in this sample.A third metasedimentary unit from the Okak Bay area, 100 km south of Saglek Fiord, also contains detrital zircons with ages comparable to that of the Lister gneisses (3235 Ma). The age of recrystallization and zircon overgrowths was dated at ca. 2560 Ma in this sample. A single grain dated at ca. 2780 Ma is considered most likely to be detrital, which would imply an age of deposition between ca. 2780 and 2560 Ma for this unit.The results show that although late Archaean depositional ages are possible for all three units, the "Upernavik" supracrustal association is composite and sediments in different units have widely different sources and metamorphic histories. These conclusions support a new model for the Nain Province according to which separate terranes were tectonically juxtaposed in the late Archaean. In this model, the age of plutonic and supracrustal rocks and their metamorphic histories prior to juxtaposition differ from one terrane to another.

Metallogenic and tectonic implications of Pb isotope data for galena separates from the Labrador Central mineral belt

Galena separates were collected from three Proterozoic supracrustal sequences comprising the Labrador Central mineral belt and also from minor occurrences in the Archean granitoid basement. Galena occurs in two environments: epigenetic quartz veins, and veins associated with uraninite occurrences. Fourteen samples from seven vein occurrences along the unconformity between shales of the ca. 2000-Ma Moran Lake Group and Archean basement and other spatially related rocks define a secondary isochron with a t 2 of 1163 Ma and a t 1 of 2473 Ma. These model ages suggest that Pb was mobilized from the Archean basement and deposited during the Grenvillian orogeny. Scatter in 208 Pb/ 206 Pb ratios reflects differing mixtures of source components. Separates from the Big Bight and Ford9s Bight showings in coastal exposures of the 1860-Ma Upper Aillik Group have Pb isotope ratios which reflect a source region with higher U/Pb and Th/Pb values than source regions for the Moran Lake Group galenas. In uraninite-associated Pb occurrences linked to post-tectonic granites elsewhere in the Upper Aillik Group, Pb isotope ratios reflect mixtures of country-rock Pb from low U/Pb sources (similar to that of the Moran Lake Group) and uranogenic Pb. Mobilization of the Pb from uraninite may have been the product of Grenvillian tectonism. The Moran Lake Group and uraninite-associated leads were derived from Archcan crust in the Nain province and this crust isotopically resembles Nuk gneiss of southern Greenland. The Big Bight-Ford9s Bight Pb isotope ratios indicate a more radiogenic Pb source to the southeast of the Central mineral belt and therefore the influence of another cratonic block outboard from the Nain province.

Major-, trace-, and rare-earth-element geochemistry of the Archaean Maggo gneisses, southern Nain Province, Labrador

The early middle Archaean Maggo gneisses of the southern Nain Province, Labrador, form the southwest portion of the once contiguous North Atlantic (Nutak) Craton (NAC). The gneisses and their late middle Archaean metamorphosed and migmatized equivalents are typical of grey gneiss terranes exposed worldwide. Geochemically the gneisses exhibit a continuous range of composition from 53.7 to 78.4 wt.% SiO2 and straddle the boundary between low- and high-Al trondhjemites. Major-element distributions are comparable to those of other Archaean-aged NAC gneisses (Amîtsoq, Uivak, and Nûk gneisses), however, the Na2O and K2O contents are scattered. The gneisses are depleted in K, Rb, and Ba, are enriched in Sr, and have high-field-strength-element distributions similar to those for NAC gneisses.Rare-earth-element (REE) patterns of Maggo gneisses can be subdivided, on the basis of the nature of the Eu anomaly, into two groups of samples: (i) with negative Eu anomalies and higher Σ REE contents and (ii) with positive to normal Eu anomalies and lower Σ REE contents. The subdivision reflects differentiation processes in the parent magma of the Maggo gneisses. REE patterns are similar to those reported for NAC grey gneiss complexes. On the basis of the (La/Yb)N and (Yb)N values, the Maggo gneisses parent magma is interpreted as being derived by partial-melting processes from preexisting, high-grade (granulite to amphibolite facies) sialic continental crust equivalent to the early Archaean lithologies preserved elsewhere in the NAC.

Rb–Sr geochronology of the Archaean Maggo gneiss from the Hopedale block, Nain Province, Labrador

The Maggo gneiss, a typical grey gneiss from the Hopedale block of coastal Labrador (southern Nain Province), preserves structures of two major Archaean tectonothermal events, the Hopedalian and the younger Fiordian. Rb–Sr whole-rock analyses of four suites representing the Hopedalian event (3140 ± 95 Ma (1σ), 3125 ± 43 Ma, 3028 ± 199 Ma, and 3025 ± 163 Ma) fall within the early middle Archaean and represent a period of cratonization of the Hopedale block.Two suites with a younger Fiordian structural fabric (2899 ± 116 Ma and 2804 ± 100 Ma) fall within the late middle Archaean and are interpreted as dating a period of reworking that locally reset the Rb–Sr isotopic systematics of the Maggo gneiss. Fiordian ages were also determined from four suites (2927 ± 50 Ma, 2884 ± 86 Ma, 2854 ± 83 Ma, and 2764 ± 89 Ma) that do not display the Fiordian structural fabric, indicating that chemical and mechanical components of the Fiordian event did not occur simultaneously. Age determinations of the Fiordian do not date specific geologic events but seemingly reflect varying degrees of protracted isotopic homogenization during chemical and mineralogical reequilibration.One suite of analyses yields an age of 3305 ± 75 Ma and is interpreted as dating a pre-Hopedalian event. This older age supports field evidence as to the existence of old crust in the Hopedale block, typical also of rocks of the North Atlantic Craton.

U–Th–Pb ages of single zircons in Archaean supracrustals from Nain Province, Labrador, Canada

Ion-probe U–Pb dating of igneous zircons from an acid metavolcanite belonging to the Nulliak supracrustal association of the Saglek area, northern Labrador, suggests an original age of 3776 ± 8 Ma (2σ), interpreted as the deposition age of the volcanite. In accordance with the field model, this is slightly but significantly older than the previously obtained ion-probe U–Pb age of igneous zircons from the main phase of the Uivak gneisses, which carries inclusions of the supracrustal association. A secondary event of zircon growth recorded in the metavolcanite has a maximum age of 3065 ± 8 Ma (2σ).A metapelite belonging to the Weekes amphibolite supracrustal association of the Hopedale area south of Nain contains rounded, low-U zircons of 3258 ± 24 Ma (2σ) age. Occasional textural evidence suggests that some of these zircons are most likely detrital grains of igneous origin, in which case 3258 ± 24 Ma is a minimum age of the source from which the sediment was derived and a maximum age for the deposition of the sediment. However, the possibility that the age represents recrystallization of the sediment under high-grade conditions cannot be ruled out. A 2841 ± 5 Ma (2σ) age obtained on a younger generation of euhedrally zoned high-U zircons from the metapelite is within error of the published bulk zircon U–Pb age of the tonalitic component of the Kanairiktok plutonic suite that intrudes the older orthogneiss and supracrustal units in the area. These younger zircons are most likely to have grown during migmatization of the metasediment at the time of emplacement of the suite.The zircon data give no positive evidence to support correlation between the Nulliak supracrustals and the Weekes amphibolite association.

Isotopic evidence from the eastern Canadian shield for geochemical discontinuity in the Proterozoic mantle

Most workers agree that Proterozoic anorthosite massifs represent the crystallization products of mantle-derived magmas1,2, although the composition of the parental melts is a major unsolved petrological problem3. As mantle-derived rocks, the massifs can be used as geochemical probes of their late Precambrian upper mantle sources. We report here Nd and Sr isotopic compositions of anorthosites and related rocks from the Grenville and Nain Provinces of the eastern Canadian shield. Here 75% of the Earth's known anorthosite is found in a 1,600-km belt from the Adirondack Mountains of northern New York State to the eastern coast of Labrador4 (Fig. 1). The results indicate that the massifs were derived from at least two distinct mantle source regions which were established before 1,650 Myr ago, and were episodically involved in magmatism over ∼500 Myr. One reservoir, below the Grenville Province, and probably below much of the eastern Superior Province, was isotopically similar to the depleted, modern-day mid-ocean ridge basalt (MORB) source. The other reservoir was chondritic to moderately enriched, and is most easily identified in the Nain Province, but may have occurred scattered throughout the Superior Province.

Significance of Folds and Mylonites at the Grenville Front in Ontario

The Grenville front is defined as the northwestern limit of the Grenville structural province of the Canadian shield (Stockwell, 1964; Wynne-Edwards, 1972). It strikes northeast-southwest and is approximately 1,600 km long, extending from Lake Huron to the Labrador Sea. The front separates the Grenville structural province from either the Nain, Churchill, Superior, or Southern structural province and is commonly associated with mylonitic fabrics (see Higgins, 1971 for terms usage) which strike approximately parallel to the front and dip steeply to the southeast.

Gravity anomalies, block-faulting and Andean-type tectonism in the Eastern Churchill Province

Evaluation of the tectonic evolution of Precambrian terrains may be greatly enhanced by selecting appropriate analogues from Phanerozoic examples where the geological history is better understood and where a knowledge of current geodynamic processes is sometimes available. The Churchill Structural Province in the eastern Canadian Shield apparently has a close analogue in the central Andes of Peru where the Nazca plate continues to thrust below the South American plate. We propose that this provides the basis for a tectonic framework that explains several geological features both within the Churchill Province and at the adjacent boundaries with the Superior and Nain provinces.

540-Myr-old anorthosite complex in the Grenville Province of Quebec, Canada

MASSIF-TYPE anorthosites and related rocks are an important component of the Grenville and Nain structural provinces of the Canadian Shield (Fig. 1), as well as in high grade metamorphic terrains in other parts of the world. There are many outstanding problems in the study of these rocks, not least of which is their age. Most geochronological data have been obtained from the associated granitic rocks of this suite as anorthosite itself contains very little rubidium and zirconium. Because of post-crystallisation deformation, however, the relationship between the anorthosite and the associated rocks is often tenuous and frequently disputed. Within the Grenville Province metamorphosed rocks believed to be genetically associated with anorthosites have yielded ages ranging from 1,020 to 1,130 Myr (refs 1–4). The high-grade of metamorphism surrounding the Morin and Lac St Jean anorthosites is believed by some researchers to be due to anorthosite emplacement. These ‘aureoles’ yield Rb–Sr ages5,6 of about 1,500 Myr. Anorthosite complexes in the Nain Province are undeformed, and the adamellite of the suite has yielded a U–Pb zircon age7 of about 1,300 Myr. Barton8 obtained a Rb–Sr isochron age of 1,418 Myr from a digested inclusion in the Nain anorthosite. Here we present geochronological data from the undeformed Sept Iles anorthosite, and show it to be Cambrian in age.

Contrasting types of bladed olivine in ultramafic rocks from the Archean of Labrador

Exceptionally coarse grained, interlocking olivine crystals up to 1 m long occur in small lenticular ultramafic bodies within the Nain Province of Labrador. The ultramafic rocks occur in association with Archean gneisses that vary in metamorphic grade from amphibolile fades to granulite facies. Some of the interlocking textures (i.e. near Hopedale) are interpreted as relict cumulate or harrisitic growths, mimicked by olivine regrowlh during metamorphism and preserved in structurally isotropic enclaves in their host rocks. Others (e.g. near Saglek) are regarded as solely of metamorphic origin. None of the olivine growths resemble spinifex textures reported in Archean ultramafic rocks elsewhere.