Northeastern Superior Province Research Articles

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Reply to the comments on “Late Archean sedimentary basins in the northeastern Superior Province, Canada: Plume-generated crustal tears or syn-convergent accretionary belts?” of

Exploring the link between spatiotemporal patterns of plutonism and geodynamic regimes at the end of Archean: an example from the northeastern Superior Province, Canada

This paper presents new U–Pb zircon ages obtained using laser ablation ICP-MS from various plutonic units that intruded along the southern margin of the Bienville domain, a presumed Neoarchean magmatic arc in the northeastern Superior Province, Québec, Canada. The U–Pb ages indicate that the arc plutons were emplaced episodically between ca. 2745 Ma and ca. 2697 Ma, synchronous with regional transpressional deformation. Furthermore, the observed intra-pluton age variations show that the largest examined intrusion, the Radisson pluton, grew incrementally over a period of at least 15 My, perhaps in a series of elongated, sheet-like magma batches straddling the presumed arc margin and emplaced successively, younging inwards. This emplacement style is comparable (though not unique) to modern arc plutons emplaced along or within regional transpressional shear zones. The new ages are then complemented with a statistical analysis of previously published U–Pb pluton zircon ages from the adjacent units (La Grande, Opinaca, Opatica), which revealed a subtle southward-younging trend (i.e., towards the presumed trench) in otherwise broadly coeval and areally extensive plutonism characterized by diverse compositions. The observed spatiotemporal pattern of plutonism is transitional between unfocused, plume-related magmatic centers and focused, unidirectionally migrating magmatism of modern arcs, perhaps as a result of hotter environment and complex interplay between the waning plume-related regime and increasingly dominating modern-style plate tectonics at the end of Archean.

Distributed crustal shortening followed by transpressional shearing in the Superior Province, northeastern Canada: A Late Archean analogy to modern accretionary plate margins?

The Canadian Superior Province has become one of the key test pieces to discuss tectonic processes and mechanisms of crustal growth in the Late Archean. The Province consists of a >2.8 Ga proto-cratonic core intruded by voluminous arc-like plutons and surrounded by a series of narrow, elongate ca. 2.8–2.7 Ga juvenile belts, also referred to as terranes or domains. The terranes seem to wrap around the proto-cratonic core and generally young outward, but the kinematics and geodynamic causes of their assembly remain debated. In this paper, we examine the Radisson pluton in northeastern Québec, which intruded the southern, outer edge of the presumed magmatic arc (Bienville domain) along its ~WNW–ESE-trending tectonic boundary with the proto-cratonic crust (La Grande domain). The pluton, dominated by porphyritic monzogranite to quartz monzonite, was emplaced at around 2712 Ma and exhibits complex internal structure resulting from superposed magmatic to solid-state deformations. An early margin-parallel ~WNW–ESE magmatic foliation containing a steep lineation, recognized by the anisotropy of magnetic susceptibility (AMS), is interpreted as recording vertical stretching and horizontal flattening of highly crystallized magma, either due to emplacement and/or pure shear dominated transpression. More widespread, however, is a horizontal lineation within the same foliation that is interpreted as recording post-emplacement, but still syn-magmatic, tectonic strain (~NNE–SSW shortening and boundary-parallel stretching). Upon cooling, localized dextral S–C mylonite zones accommodated further shortening within the pluton whereas undeformed late-stage felsic dikes cross-cut the solid-state fabric at an angle to the pluton margins. We suggest that this structural succession, also reproduced by numerical fabric modeling, is a local-scale signal of a two-stage assembly of the northeastern Superior Province: the frontal, NNE-directed terrane convergence and attachment to the cratonic nucleus, operating in a ‘hot’ regime with voluminous arc-like plutonism, was followed by more localized dextral shearing parallel to terrane boundaries. The latter phase is recorded at the proto-craton margin but also in the outboard Abitibi greenstone belt virtually at the same time (ca. 2700–2690 Ma). In combination, the two-stage evolution and similar deformation distributed over a broad region resemble modern large hot orogens formed in a plate-tectonic regime.

DEEP CRUSTAL STRUCTURE AND MODEL OF NEORCHAEAN EVOLUTION OF THE NORTH AMERICAN CRATON

The structure of the Archaean crust of the North America has been studied based on the synthesis of geolo‐ gical and geophysical data, including seismic sections along LITHOPROBE Geotransects, magnetic and gravity anomaly maps, and seismic tomography data. The authors rely on the experience gained in the Russian Program of the deep geological and geophysical studies of the East European Craton. The juvenile Neoarchaean crust, containing the frag‐ ments of reworked Meso‐ and Paleoarchaean rocks, forms an asymmetric round‐oval‐shaped domain, wherein the geophysical, structural, and metamorphic parameters display a concentric zoning pattern. The Central zone occupies the Hudson Bay basin. The Internal zone (the northeastern and northern Superior Province) is mainly composed of the granulite facies of metaplutonic, metavolcanic and metasedimentary rocks. The External zone encompasses the southern Superior Province together with Hearne and Rae Provinces. This paper presents 3D crustal models of sou‐ thern Superior Province. The crust development resulted from rifting and a partial disruption of the continental crust, short‐term opening of the linear oceans, successive northward subduction and accretion of the ancient continental and juvenile Neoarchaean oceanic and island‐arc terranes between ~2.78 and ~2.70 Ga. Subsequent events in the epicontinental environment, including formation of the metasedimentary belts, granulite facies metamorphism and intense ore formation processes, took place within the range from ~2.71 to ~2.63 Ga. The SCLM morphology within the limits of the Archaean North American Craton can be represented as a flattened overturned cone with a vertical axis (down to a depth of ~350 km). The Hudson Bay basin is located right above the lithospheric keel. A number of the main features of the structure and evolution of the Archaean crust of the North American Craton, primarily the oval‐ concentric zoning, the important role of high‐temperature magmatic and metamorphic processes and mainly in‐ tracontinental magmatism and sedimentation, indicates the leading role of the mantle‐plume type processes. The Neoarchaean evolution of the North American craton represents the plate‐tectonic processes initiated by a super‐ plume. The Neoarchaean North American Craton is one of a series of similar phenomena that occurred ~2.75 Ga ago in a number of continental regions. The most important features, repeated to a certain degree in tectonic units of this type, are: (1) synchronous formation between 2.79 and 2.58 Ga; (2) mainly intracontinental development; (3) the prevalence of oval‐shaped synformal tectonic structures of different ranks with some form of concentric zoning; (4) high‐temperature magmatism (usually with the participation of enderbite‐charnockites and gabbro‐anorthosites) and metamorphism of the granulite facies; (5) a frequently repeated combination of high‐grade (granulite and high‐ temperature amphibolites facies) and low‐ or moderate‐grade (greenschist and epidote‐amphibolite facies) meta‐ morphic rocks; (6) the lower‐crust granulite‐basaltic layer that had formed and was deformed at the final stage of endogenic activity; (7) a thick lithosphere (the lithospheric keel reaches a depth of 250–350 km).

The Paleoproterozoic Otish Gabbro suite and coeval dyke swarms of the Superior Province: Probing the ca. 2.17 Ga mantle

The uraniferous, Proterozoic Otish Basin of the southeastern Superior Province hosts shallow-dipping sills and northeast to northwest-trending dykes of the Otish Gabbro suite and the north-trending Matoush dyke. Otish Gabbro sills comprise two distinct geochemical groups that reflect different degrees of crystal accumulation in a common olivine-tholeiitic parental magma. Group 1 Otish gabbros approach liquid compositions, whereas Group 2 gabbros are crystal cumulates of olivine+plagioclase+clinopyroxene. The high Nb/Yb, La/Yb, Th/Yb and low Zr/Nb ratios of the Otish Gabbro sills reflect trace element-enriched, mantle-derived parental liquids, modified by the addition of a crustal component. A north-trending Matoush dyke records a second magmatic event in the Otish Basin, genetically unrelated to the emplacement of the Otish Gabbro sills. In contrast to the gabbros, the strongly altered Matoush dyke has more fractionated trace element patterns and strong enrichments in light rare earth elements (LREE) that are indicative of a lamprophyric affinity.The Otish Gabbro sills are part of a large ca. 2.17Ga magmatic event that also generated the extensive Biscotasing and Payne River dyke swarms in the south-central and northeastern Superior Province, respectively. The contemporaneous mafic dyke swarms experienced relatively minor crustal contamination and their high Nb/Zr and Nb/Yb ratios suggest derivation from fertile mantle sources. Furthermore, the low Ti/Yb ratios of the ca. 2.17Ga magmas are consistent with melting at relatively shallow depths (<100km), implying removal of the depleted Archean lithosphere from a large portion of the Superior Province and replacement by a more fertile Proterozoic mantle. The variable geochemical composition of the ca. 2.17Ga mafic magmas indicates melting of a compositionally heterogeneous mantle and appears inconsistent with a common distal mantle plume source. A more suitable petrogenetic model for the ca. 2.17Ga dyke swarms of the Superior Province is vertical rise of mantle-derived magmas in a rifting environment.

Neoarchean (c. 2·7 Ga) Plutons of the Ungava Craton, Québec, Canada: Parental Magma Compositions and Implications for Fe-rich Mantle Source Regions

Mafic to ultramafic intrusions of the Qullinaaraaluk suite (Q-suite) were emplaced into the Ungava craton of the Northeastern Superior Province during an episode of intense igneous activity and crustal reworking from c. 2 74 to 2 70 Ga. Orthopyroxene-rich Q-suite intrusions from the Hudson Bay Terrane and southwestern Rivie' re ArnaudTerrane, and orthopyroxene-poor Q-suite intrusions from the north^central Rivie' re Arnaud Terrane indicate the existence of at least two Q-suite magma types: a subalkaline magma parental to the orthopyroxene-rich intrusions and a transitional magma parental to the orthopyroxene-poor intrusions. Both types of intrusions are characterized by light rare earth element (LREE)-enriched, high field strength element (HFSE)depleted trace element profiles that reflect, in large part, contamination by the tonalite^trondhjemite^granodiorite-dominated crust. Near-chondritic to strongly sub-chondritic initial eNd(2 72 Ga) values (þ2 to ^10) of the Q-suite intrusions reflect the combined effects of both the amount of crustal contamination and the agedependent isotopic composition of the contaminant. The inferred trace element profiles of the uncontaminated Q-suite magmas were probably flat to LREE-depleted. The transitional magmas that produced the least evolved dunitic cumulates of the Q-suite were ferropicrites (MgO 14 wt %, FeOTOT 17 wt %). In contrast, the magmas parental to the primitive Q-suite harzburgites were Fe-rich, high-Mg basalts (MgO 11wt %; FeO 14 wt %). The high Fe contents of the Q-suite magmas are incompatible with derivation from a pyrolitic mantle [Mg-number 0 90, Mg/ (MgþFeTOT)] and require sources significantly enriched in iron (Mg-number 0 79). Both magma types are also characterized by relatively low Ni contents suggesting derivation from source regions depleted in Ni relative to pyrolitic mantle peridotite. Differences in the major element compositions of the subalkaline and transitional parental magmas may reflect compositional diversity among the Fe-rich mantle sources. Comparisons with melting experiments on compositions analogous to the Martian mantle suggest that the Q-suite magmas may rather be generated by different degrees of melting of a common source with an Fe content slightly lower than that of the Homestead L5 ordinary chondrite (Mg-number1⁄4 0 77). The Fe-rich picritic to high-Mg basaltic magmas last equilibrated with garnet-free harzburgitic to lherzolitic residues at upper mantle pressures ( 5 GPa). The craton-wide occurrence of c. 2 72^2 70 Ga Q-suite mafic to ultramafic plutons suggests that underplating by Fe-rich mantle melts may have had a key role in the c. 2 74^2 70 Ga cratonization of the Northeastern Superior Province.

Paleo- to Mesoarchean basement recycling and terrane definition in the Northeastern Superior Province, Québec, Canada

The Northeastern Superior Province (NESP) is a 500 km × 700 km segment of plutonic Archean crust generated by 1.2 Ga of tectonomagmatic activity. The NESP exhibits the classical plutonic series of Archean terranes, low-K tonalite–trondhjemite–granodiorite (TTG) and high-K granodiorite–granite–monzogranite (GGM) with unusually large volumes of Px-TTGs (mostly enderbites and subordinate charnockites). Volcanic rocks constitute <10% of the NESP and are dominated by magnesian basalt-komatiite assemblages and bimodal mafic tholeiitic and calc-alkaline felsic tuffs. Mapping, U/Pb geochronological, and Nd isotopic data suggest the presence of two terranes with distinct magmatic histories: the Hudson Bay to the SW, and the Rivière Arnaud to the NE. These terranes are separated by a metasedimentary basin, which may be a paleosuture. The Hudson Bay terrane is a Paleo- to Mesoarchean TTG-greenstone craton (3.8–2.9 Ga), intruded by 2.76–2.74 Ga low-K TTGs and younger 2.72–2.69 Ga high-K GGM and Px-TTGs that have low Nd isotopic values (ɛNd T = +0.45 to −5.03). Mafic to felsic volcanic rocks and mafic to ultramafic plutonic rocks <2.76 Ga also have enriched ɛNd T = +0.40 to −6.90. These radiogenic signatures (as well as inherited zircon cores) record recycling of Mesoarchean (3.8–2.9 Ga) crust, either through direct melting, or by coupled assimilation-fractional crystallization processes. In comparison, the Rivière Arnaud terrane is underlain by younger (<3.0 Ga), more juvenile TTG crust, that is intruded by plutons of low-K TTGs (2.79–2.76 Ga), high-K GGM (2.73–2.72 Ga), and Px-TTGs (2.74–2.72 Ga). Most of these rocks have positive to moderately negative ɛNd T values (+2.34 to −0.67), indicating either a juvenile character, or recycling of fairly juvenile Late Mesoarchean-Early Neoarchean crust. Overall, geochemical and isotopic data of the Late Neoarchean (<2.76 Ga) low-K TTG and Px-TTG series of the NE Superior Province are not consistent with partial melting solely of juvenile basaltic crust in an oceanic setting. The data are most consistent with remelting of a composite crust constituted of older, isotopically enriched mafic–ultramafic rocks and more juvenile metabasaltic rocks. The basaltic component may have been introduced either by crustal underplating, intra-plating or foundering of supracrustal belts. The mafic melts injected into the maturing cratonic nuclei assimilated the older rocks by an AFC-type process and provided heat. The isotopic and geochemical data imply that the high-K GGM series melts were produced by melting at shallower depth of an older composite TTG crust, with less involvement of the metabasaltic component.

Evidence for a widespread mafic cover sequence and its implications for continental growth in the Northeastern Superior Province

Archean greenstone belts of the Northeastern Superior Province (NESP) were emplaced over a 160 Ma period (2.88–2.72 Ga), spanning a major episode of crustal reworking in which early tonalite–trondhjemite plutonism evolved to dominant granite–granodiorite and pyroxene-bearing felsic plutonism. The numerous greenstone belts that crop up across the craton contain lavas belonging to three mafic volcanic suites: (1) Mg-tholeiites that have chemical compositions typical of many Archean basalts, with 4–10 wt.% MgO, 9–15 wt.% Fe 2O 3t, 0.4–1.2 wt.% TiO 2 and La/Sm ratios between 1 and 3. (2) Fe-tholeiites that have similar MgO contents and La/Sm ratios, but markedly higher Fe 2O 3t (11–20 wt.%), TiO 2 (1.0–2.6 wt.%), and Gd/Yb ratios that may reflect derivation from a distinct garnet-bearing mantle source. (3) Light rare earth element (LREE)-enriched rocks that have the chemical characteristics typical of ‘calc-alkaline’ mafic magmas, with higher SiO 2, Al 2O 3 and La/Sm ratios, but lower Fe 2O 3t and TiO 2 than the tholeiitic suites at a given MgO content. The presence of unconformities within single greenstone belts, numerous inheritance ages in volcanic rocks replicating those of older volcanic–plutonic assemblages, and the recognition of widespread, geographically separate, yet coeval (2.78 Ga) extrusive assemblages containing the distinctive Fe-tholeiites, do not support a plate tectonic model for a large portion of the NESP. The correlations in composition and ages of greenstone belts are more consistent with a model in which they represent the remnants of an extensive autochthonous mafic cover sequence. Nd isotopic evidence for the presence of evolved crust in some syn-volcanic felsic plutons and volcaniclastic rocks imply that this mafic cover sequence was erupted onto an older granitoid crust. The amalgamation of two isotopically distinct terranes at ca. 2.76–2.74 Ga created a proto-cratonic mass that consisted of a composite tonalite–trondhjemite–greenstone crust. Underplating by mafic mantle melts and widespread insulation after 2.75 Ga increased temperatures sufficiently to produce extensive melting of this crust to generate pyroxene-bearing felsic rocks and granodiorite to granite partial melts. The isotopically enriched character of mantle-derived magmas after 2.75 Ga may reflect more extensive contamination by a felsic crust affected by widespread partial melting. Furthermore, a decrease of Nb/Th ratios in the LREE-enriched mafic rocks younger than 2.75 Ga reflects a change in the nature of the crustal contaminant, from tonalite–trondhjemite prior to 2.75 Ga, to granite–granodiorite afterwards.

Metamorphic Gradient: A Regional-Scale Area Selection Criterion for Gold in the Northeastern Superior Province, Eastern Canadian Shield

ABSTRACT One hundred years after the first gold discoveries in the Abitibi subprovince, the Archean James Bay region to the north is experiencing a major exploration boom. Poor geologic coverage in this part of the northeastern Superior province has hindered the application of traditional Abitibi exploration criteria such as crustal-scale faults and “Timiskaming-type” sedimentary rocks. New area selection criteria are needed for successful greenfield exploration in this frontier region, and the use of steep metamorphic gradients is presented as a possible alternative. The statistical robustness of the metamorphic gradient area selection criterion was confirmed by using the curve of the receiver operating characteristic (ROC) to estimate the correlation between metamorphic fronts and the distribution of known Abitibi orogenic gold producers. The criterion was then applied to the James Bay region during a first-pass craton-scale exploration program. This was part of the strategy that led to the discovery of the Eleonore multimillion-ounce gold deposit in 2004.

A catalytic delamination-driven model for coupled genesis of Archaean crust and sub-continental lithospheric mantle

There is no consensus on the processes responsible for near-coeval formation of Archaean continental crust (dominantly tonalite–trondhjemite–granodiorite: TTG), greenstone belts dominated by komatiitic to tholeiitic lavas (KT), and sub-continental lithospheric mantle (SCLM). The Douglas Harbour domain (2.7–2.9 Ga) of the Minto Block, northeastern Superior Province, has two TTG suites, the western and eastern Faribault–Thury (WFT and EFT), with embedded KT greenstones. Tonalites of both suites have high light/heavy rare-earth element ratios (L/HREE), high large ion lithophile element (U–Th–Rb–Cs–La: LILE) contents, positive Sr–Pb anomalies, and negative Nb–Ta–Ti anomalies. Such typical Archaean TTG signatures are commonly explained by melting of subducted oceanic crust, but could also originate by melting the base of thick basaltic plateaux formed above mantle upwellings (plumes), leaving behind restites containing pyroxene, garnet, and rutile. Field relationships (in situ segregation veins), phase equilibria (hornblende stabilized at lower crustal pressure), petrography (corroded epidote and muscovite phenocrysts, rare plagioclase phenocrysts), and trace element models, all imply that FT tonalite to trondhjemite evolution reflects hornblende-dominated fractional crystallization, not partial melting of subducted crust. The geochemistry of parental FT tonalites can be modeled by 15–30% melting of FT tholeiitic metabasalts, with residues of eclogite, garnet-websterite, or hornblende-garnet websterite. A minor residual Ti-phase such as rutile is also needed to generate negative Ti–Nb–Ta troughs in the TTGs. However, large volumes of eclogitic restites complementary to TTG are not observed either at the base of Archaean crustal sections, or in the SCLM. Additional problems with slab-melting models include: (a) the rarity of lithologies and associations characteristic of active margins (ophiolites, andesites, blueschists, accretionary mélanges, molasse, flysch, high-pressure belts, and thrust-and-fold belts); (b) the need to deliver plume-derived KT melt through the slab; and (c) extracting enough TTG melt from a subducting slab in the time available (200–300 my). In the plateau-melting model, heat for crustal anatexis is supplied by ongoing KT magma derived from mantle upwellings. However, SCLM rocks differ from predicted 1-stage mantle melting residua; and the voluminous residual eclogites complementary to TTG generation somehow need to be removed. These two problems might solve one another if the dense crustal restites disaggregated and mixed into the underlying depleted mantle. Mantle melting slows upon exhaustion of Ca–Al-rich phases, with large temperature increases needed to extract more melt from harzburgite residua. Physical addition of delaminated crustal restites would refertilize the refractory mantle, allowing extraction of additional melt increments, and might explain the ultra-depleted and orthopyroxene-rich nature of the SCLM. A hybrid source composed of 10% eclogitic restite of EFT tonalite generation, mixed with harzburgitic residues from 25% melting of primitive mantle, yields model melts with trace element signatures resembling typical Munro komatiites. Variations in the mineralogy and geochemistry of the delaminated component might account for the diversity of komatiite types. Degassing of hornblende-rich delaminated restites would transfer LILE to surrounding depleted mantle and could generate boninites. Fusion of undepleted metabasalt sandwiched among denser restites could generate sanukitoids. Mantle melt pulses generated by catastrophic delamination events would underplate nascent TTG crust and trigger renewed crustal melting, followed by delamination of newly formed eclogitic restites, triggering additional mantle melting, and so on. I posit that delamination of crustal restites catalyzed multi-stage melting of the SCLM and maturation of the Archaean continental crust. Thus, Archaean crust and SCLM are genetically inter-linked, and both form above major mantle upwellings.

Sm-Nd, Lu-Hf and Pb-Pb signatures of gneisses and granitoids from the La Grande belt: extent of late Archean crustal recycling in the northeastern Superior Province, Canada

Sm-Nd, Lu-Hf and Pb-Pb isotopic signatures were determined for gneisses and felsic plutons of the La Grande granite-greenstone and the Nemiscau metasedimentary belts of Canada. The northern part of the La Grande belt exposes gneisses of the Langelier complex formed between ca. 2.8 and 2.9 Ga. The gneisses yielded εNd (T) values between −2 and +1, εHf (T) between −1 and +3, initial 207Pb/ 204Pb ratios of ∼14.9, higher than the value of 14.6 derived from the Stacey-Kramers growth curve, and crustal extraction ages ≥3.0 Ga. Modeling shows that the syn- to late-tectonic intrusions of granitoid rocks record at least 20%, and up to 40%, recycling of this gneissic basement. The southern part of the La Grande belt, where no basement gneisses are exposed, records lower proportions (5–20%) of this crustal end-member. In both parts of the La Grande belt, the post-tectonic plutons record the largest proportion of recycled crust, likely related to southeastward thrusting events in the Superior craton, crustal thickening and concomitant partial melting. The La Grande plutons have isotopic signatures more radiogenic than those of similar intrusions that formed concurrently, to the south, in the Abitibi greenstone belt. This reflects an ancient crust environment for the former vs. an oceanic setting for the latter. Mantle reservoirs with a protracted history of incompatible element depletion were present beneath the whole eastern Superior Province, in the late Archean.

Archaean cratonization and deformation in the northern Superior Province, Canada: an evaluation of plate tectonic versus vertical tectonic models

The Archaean Minto Block, northeastern Superior Province, is dominated by tonalite–trondhjemite, enderbite (pyroxene tonalite), granodiorite and granite, with subordinate mafic rocks and supracrustal belts. The plutons have been interpreted as the batholithic roots of Andean-type plate margins and intra-oceanic arcs. Existing horizontal-tectonic models propose that penetrative recrystallization and transposition of older fabrics during terrane assembly at ∼2.77 and ∼2.69 Ga produced a N-NW tectonic grain. In the Douglas Harbour domain (northeastern Minto Block), tonalite and trondhjemite dominate the Faribault–Thury complex (2.87–2.73 Ga), and enderbite constitutes 50–100 km-scale ovoid massifs (Troie and Qimussinguat complexes, 2.74–2.73 Ga). Magmatic muscovite and epidote in tonalite–trondhjemite have corroded edges against quartz+plagioclase, suggesting resorption during ascent of crystal-charged magma. Foliation maps and air photo interpretation show the common development of 2–10 km-scale ovoid structures throughout the Douglas Harbour domain. Outcrop and thin-section scale structures imply that many plutons experienced a phase of syn-magmatic deformation, typically followed by high temperature sub-magmatic overprints. Thermobarometric data for plutons indicate near-solidus recrystallization at 4–6 kbar pressures. The common preservation of syn-magmatic fabrics in plutons of different ages seems incompatible with the origin of these fabrics through superimposed regional orogenesis. The broad uniformity of intrusion ages and lithologies throughout the Minto Block, and the rarity of shallowly-dipping planar fabrics, also seem inconsistent with accretion of disparate older terranes, each of which should preserve distinct histories. A possible alternative explanation for these features is provided by vertical tectonic models, whereby buoyant felsic magmas ascended as crystal slurries, while dense supracrustal rocks (and solidified felsic intrusions emplaced into them) subsided as cold fingers (10–20 km-scale instabilities). Shear between upwelling and downwelling limbs would have concentrated in the weak intrusions, generating steeply-plunging syn-magmatic fabrics, and producing ductile overprints in solidified rocks.

Water-deficient Calc-alkaline Plutonic Rocks of Northeastern Superior Province, Canada: Significance of Charnockitic Magmatism

Calc-alkaline batholiths of the Archaean Minto block, northeastern INTRODUCTION Superior Province, Canada, have pyroxeneand hornblende-bearing Since Holland’s (1900) initial description of south Indian mineral assemblages inferred to have crystallized from hot, watercharnockites as orthopyroxene-bearing rocks of granitic undersaturated magmas at 2·729–2·724 Ga. A regional amcomposition, the term charnockite has been applied to phiboliteto granulite-facies tectonothermal event at 2·70 Ga rocks of widely divergent origin: granitic rocks metaresulted in mild to negligible metamorphic effects on the dominantly morphosed to the granulite facies (metamorphic chargranodioritic units. Geochemical, textural and thermobarometric nockites); and rocks whose pyroxene crystallized studies define the crystallization history in compositions ranging directly from a magma (igneous charnockites). Unmetafrom cumulate pyroxenite through quartz diorite, granodiorite, granite, morphosed, discrete plutons such as the Barrington Tops and syn-magmatic gabbroic dykes. Early magmatic assemblages batholith (Eggins & Hensen, 1987), Kleivan granite (Peinclude orthopyroxene, clinopyroxene, plagioclase, biotite, Fe–Ti tersen, 1980), Mawson charnockite (Young & Ellis, 1991; oxides and ternary feldspar, indicating crystallization from magmas Young et al., 1997; Zhao et al., 1997) and Ballachulish containing 100°C than at present, producing large volumes of distinguish igneous and metamorphic origins of pyroxenewater-deficient magma. bearing granitoids. Field observations such as dykes of

Crustal growth through successive arc magmatism: reconnaissance U–Pb SHRIMP data from the northeastern Superior Province, Canada

North of the La Grande subprovince in northern Quebec, the Superior Province is dominated by plutonic rocks of the Bienville subprovince and Minto block, which have been further subdivided into domains on the basis of reconnaissance geological mapping and geochronology, and projected along strike using aeromagnetic anomalies. To test the validity of the extrapolations, U–Pb ages were obtained through SHRIMP analyses of zircon from 18 small archival samples selected from a suite of ∼2000 specimens collected during a helicopter reconnaissance survey conducted in the 1960s. The results reveal a history of successive arc magmatism between 3.07 and 2.70 Ga, followed by metamorphism, emplacement of crustally-derived granites (2.702–2.685 Ga) and late hydrothermal events (ca. 2.65 Ga). In the northeastern Minto block, the Douglas Harbour domain, previously extrapolated from dated localities to the northeast, is substantiated through an age of 2.87 Ga. The older tonalitic crust is punctured by 2.734–2.725 Ga granodioritic plutons of the Leaf River suite that locally carry 3.01 Ga inherited zircon. The northern Utsalik domain is made up of pyroxene and hornblende-bearing Leaf River plutons with consistent 2.725–2.723 Ga ages and local inheritance of 2.82 and 2.77 Ga. However, the southern extension of Utsalik domain contains pyroxene-bearing granodiorites both older (2.762 Ga) and younger (2.692 Ga) than the Leaf River suite. A tonalitic rock from the southern Goudalie domain with an age of 2.833 Ga supports a linkage to 2.84–2.83 Ga units of the Qalluviartuuq domain along strike to the north. Tikkerutuk domain represents a north-trending, 50-km wide calc-alkaline magmatic arc with ages between 2.71 and 2.70 Ga, that extends southward into the Bienville subprovince on the basis of aeromagnetic anomalies and geochronology. Zircon inheritance ages indicate that the Tikkerutuk arc was built upon antecedent arcs of 2.84, 2.77 and 2.725 Ga. The Inukjuak domain in the west consists mainly of granite, dated in one locality at 2.72 Ga.

Structural constraints on the tectonic evolution of a late Archean greenstone belt in the northeastern Superior Province, northern Quebec (Canada)

A detailed structural analysis shows that the Vizien greenstone belt of northeastern Superior Province records five generations (G 1 to G 5) of ductile deformation, as well as brittle faulting. G 1 is indicated by the local presence of a pre-G 2 foliation and is interpreted to be related to thrusting. G 2 is responsible for the main penetrative foliation (S 2), axial-planar to tight to isoclinal F 2 folds and subparallel to major boundaries between lithotectonic panels. It is concentrated in a thrust with sense of shear opposite to that interpreted for G 1. F 3 and F 4 folds are open to tight, with NNW-SSE- and E-W-trending axial surfaces, respectively. They warp the S 2 foliation and major panel boundaries and dominate the map pattern. G 5 is associated with dextral transcurrent movement along a NNW-SSE-trending shear zone. Through understanding of its geometry and deformation history, the Vizien belt has been unfolded into its much simpler pre-G 3 geometry. This greatly helps in the investigation of relationships between the different panels (proposed terranes), their tectonic settings and the kinematics of panel amalgamation. The structural study, in concert with geological and geochemical work, leads to a tectonic model for the evolution of the greenstone belt, which involves subduction of oceanic lithosphere and continental arc volcanism at ∼2724 Ma, intra-arc rifting at ∼2722 Ma, and collision between ∼2718 and ∼2693 Ma with associated obduction of oceanic plateau crust, formation of mélange and deposition in a possible foreland basin. The G 1 and G 2 deformations are interpreted to be related to synthetic thrusting and back thrusting, respectively, during the collision.

Allochthonous 2.78 Ga oceanic plateau slivers in a 2.72 Ga continental arc sequence: Vizien greenstone belt, northeastern Superior Province, Canada

Embedded within the vast granitoid terrane of the Minto block of northeastern Superior Province are Late Archean greenstone belts of the Goudalie domain that preserve a long-lived record of continent-ocean interaction. The Vizien greenstone belt is one such belt and it contains four fault-bounded structural panels. The 2786 Ma mafic-ultramafic sequence is an allochthonous package of pillowed basaltic andesite, komatiite and volcaniclastic rocks cut by peridotite and gabbro sills. The mafic rocks are LREE-depleted tholeiites which have primitive mantle (PRIM)-normalized abundances of Th < Nb < La, and ϵ Nd values of +1.5 to + 3.2 reflecting extraction from a depleted mantle source. The 2724 Ma lac Lintelle continental calc-alkaline volcanic sequence consists of massive basalt, plagioclase-porphyritic andesite, dacite, rhyolite, capped by quartz-rich sandstones/conglomerates with 2.97 Ga Nd model ages. Lac Lintelle volcanic rocks are LREE enriched, with low TiO 2 (< 1%) and Zr (< 200 ppm), PRIM-normalized enrichment in Th > La > Nb, and a range of ϵ Nd values from −0.1 to +1.7. The ~ 2722 Ma lac Serindac bimodal, subaerial tholeiitic volcanic sequence contains andesite (locally with tonalite xenoliths), basalt, gabbro sills, lenses of quartz-rich sedimentary rocks and a thick, upper rhyolite sequence. The lac Serindac tholeiites are LREE-enriched, have PRIM-normalized Th > La > Nb, high Zr (to 300 ppm) and Ti contents, and low ϵ Nd values from +0.8 in basalt to −1.4 in rhyolite. The < 2718 Ma basement-cover sequence comprises 2.94 Ga tonalitic gneiss unconformably overlain by clastic sediments and a thin upper sequence of 2700 Ma gabbro, siliceous high-Mg basalt (SHMB) and andesite. The SHMB are characterised by LREE depletion and ϵ Nd values of +2.6, whereas the andesite is LREE-enriched and has ϵ Nd values of −0.3. The 2786 Ma mafic-ultramafic sequence is interpreted as a sliver of plume-related oceanic plateau crust. The 2724 lac Lintelle sequence represents a continental arc formed on the eastern protocraton. The ~ 2722 Ma lac Serindac volcanic sequence represents late continental rift deposits. The various 2.8-2.7 Ga supracrustal sequences were accreted, deformed and metamorphosed to mid-amphibolite facies during late-stage assembly of the Minto block between 2.718 and 2.693 Ga.

Geochemical evolution of the Minto block: a 2.7 Ga continental magmatic arc built on the Superior proto-craton

Abstract A 400 km-long by 100 km-wide geological transect across the Minto block of northeastern Superior Province reveals five plutonic suites and minor amphibolite- to granulite-grade supracrustal remnants. The oldest regionally significant plutonic rocks include 3.0±0.1 Ga foliated to gneissic tonalite characterized by high Na K ratios, high Al contents, light REE enrichment, and initial geNd= +1.8. The Leaf River plutonic suite is the most widespread, with crystallization ages of about 2725 ± 5 Ma across the transect. The suite comprises massive to foliated pyroxene- and hornblende-bearing granodiorite, tonalite, granite, diorite, and minor gabbro-pyroxenite and syn-plutonic mafic dykes. The rocks are I-type, calc-alkaline, have variable Na K ratios, moderate to high Ba and Sr contents, and REE patterns with variable light REE enrichment depletion of heavy REEs with increasing silica, and neutral, negative, or positive Eu anomalies. Initial ϵNd values for the suite range from −0.5 to +1.3 (mean = +0.4), and initial 87 Sr 86 Sr ratios (ISr) are ∼0.7020. The diatexite plutonic suite (2713 Ma) occurs within a 150 km-wide zone, and consists of orthopyroxene±garnet granodiorite containing 25–50% inclusions of granulite-grade mafic, supracrustal, and tonalitic inclusions. The variable major- and trace-element chemistry of the suite, with generally high Ni and Cr contents, reflects heterogeneous source materials and the presence of non-liquid components. Initial ϵNd values are from +0.1 to −1.7. Monzogranite and orthopyroxene granite dykes, plugs and plutonic masses, both ∼2690 Ma, comprise the last important plutonic suites. Both biotite monzogranite and orthopyroxene granite are light REE-enriched, but the former have negative Eu anomalies, and the latter positive. Initial ϵNd values range from −0.7 to −3.2 for monzogranite (ISr=0.7016 to 0.7067), and +0.7 to −0.2 for orthopyroxene granite (ISr=0.701 to 0.702). The initial ϵNd and ISr values of the ∼ 2.7 Ga plutonic suites are lower and higher, respectively, than estimatesf of the contemporaneous depleted mantle, suggesting significant and wide-scale reworking of older, tight REE-enriched, high Rb SR lithosphere. Early tonalite orthogneiss from Goudalie domain may be derived by partial melting of mafic crust at high pressures. The lithological and geochemical diversity of the 2725 Ma Leaf River plutonic suite indicate a range of parental magma compositions and petrogenetic processes, but fractional crystallization of mantle-derived basaltic melts which had assimilated small amounts of significantly older lower crust may have been the dominant mechanism. Crustal-inclusion-charged granodioritic diatexite (2713 Ma) was probably derived by partial melting of heterogeneous lower crustal materials under relatively dry conditions. Late biotite monzogranite was generated by partial melting of older, sialic crust, and orthopyroxene granite by deeper melting or fractional crystallization of more juvenile source materials with garnet as a residual mineral. We infer from the presence of coeval mafic dykes in all plutonic suites that heat for melting was ultimately derived from intrusion of basaltic magmas at depth. The ∼ 2.7 Ga magmatism in the Minto block occurred during a period of active production and accretion of juvenile oceanic terranes in the southern Superior Province. The northwest-directed subduction regime inferred from the southern Superior Province arc terranes may also have been responsible for the magmatism that acted on 3 Ga lithosphere in the northern Superior Province. Melting within the mantle wedge beneath the proto-cratonic lithosphere and in the lower crust occurred within a tectonic setting similar to that of modem continental magmatic arcs.

Giant granulite terranes of northeastern Superior Province: the Ashuanipi complex and Minto block

The Ashuanipi complex and Minto block of the Superior Province are large regions that have been classified as "high-grade gneiss" terranes on the basis of the presence of orthopyroxene-bearing units. Like the granite–greenstone and metasedimentary subprovinces of the southern Superior Province, the two terranes consist predominantly of intrusive rocks, but are distinguished by their primary magmatic orthopyroxene. Both "high-grade" and "gneiss" are misnomers because granulite-facies gneisses are only sparingly present and the regions consist dominantly of massive, unmetamorphosed plutonic rock.The Ashuanipi complex consists of a deformed, metamorphosed package of metasedimentary rocks and primitive, early tonalite cut by widespread orthopyroxene ± garnet granodiorite (diatexite), as well as plutons of tonalite, granite, and syenite. Based on its lithological and chronological similarity and on-strike position, the complex appears to be the continuation of metasedimentary subprovinces such as the Quetico. Its evolution involved deposition of immature greywacke in an accretionary prism, early arc (tonalitic) magmatism and deformation, followed by widespread intracrustal magmatism in the period 2700–2670 Ma. Both metamorphic and igneous rocks record equilibration under granulite-facies conditions (700–835 °C; 0.35–0.65 GPa; [Formula: see text] ~0.3) and indicate exposure levels of ~20 km.The Minto block at the latitude of Leaf River consists of several north-northwest-trending domains of similar scale and diversity to the east-trending subprovinces of the southern Superior Province. The central Goudalie domain is dominantly amphibolite-facies tonalitic rocks including some with ages &gt;3 Ga, with small belts of volcanic and sedimentary origin. Lake Minto domain contains poorly preserved supracrustal remnants in a plutonic complex comprising hornblende granodiorite, clinopyroxene ± orthopyroxene granodiorite, orthopyroxene–biotite diatexite, and granite. The hornblende granodiorite suite constitutes most of the Utsalik and Tikkerutuk domains and is thought to represent continental arc magmatism. On the basis of their distinct aeromagnetic and lithological character, two additional domains are evident north of the Leaf River area, the Inukjuak domain in the west and the Douglas Harbour domain in the east.The northerly grain of the Minto block appears to have been established in situ with respect to the easterly belts of the southern Superior Province (i.e., no large-scale block rotation) during the same interval of time (3.0–2.7 Ga). Modification of the tectonic framework for the Superior Province is required to explain Minto arc magmatism. In the interval ~2730–2690 Ma ago, a continental magmatic arc built the Berens River and Bienville subprovinces and Minto block on the southern and eastern edges, respectively, of a northern protocratonic foundation. In the same period, primitive volcanic arcs and accretionary prisms developed outboard on oceanic crust and were accreted to form a southern tectonic regime.