Thomas Skulski

Minto block, Superior province: Missing link in deciphering assembly of the craton at 2.7 Ga

Plate-tectonic models of the Superior province are rooted in the granite-greenstone and metasedimentary belts of the southern part of the craton. North-striking domains of the Minto block in the northeastern part of the province evolved at similar times, in 3.1-2.8, 2.725, and 2.69 Ga events, requiring expansion of models of late Archean assembly to accommodate Minto geology. Western and eastern protocratons (∼3.1-2.8 Ga) rifted at ∼2.79 Ga to produce an ocean basin that was mostly consumed by subduction at 2.725 Ga. The Leaf River plutonic suite of cale-alkalic hornblende + biotite ± orthopyroxene ±clinopyroxene granodiorite represents magmatic arcs built on the protocratons, whereas the intervening Goudalie domain—containing fault-bounded fragments of rifted continental crust, rift volcanics, primitive oceanic crust, 2724 Ma island-arc rocks, and a <2718 Ma back-arc assemblage—marks the suture. Terminal collision at ∼2.7 Ga led to thickening and crustally derived granitoid magmatism. The southern Superior province also experienced vigorous activity between 2.725 and 2.69 Ga as island arcs, oceanic plateaus, continental fragments, and accretionary prisms were amalgamated progressively from north to south in a regime of dextral transpression then stitched by granites. A northern proto-Superior craton had continental magmatic arcs built on its eastern and southern flanks in response to west-northwest-directed subduction; orthogonal convergence in the east produced wide plutonic arcs, in contrast to terrane-accretion tectonics in the more oblique regime along the southern margin.

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

Abstract 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.

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 La > Nb, and a range of ϵNd 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 ϵNd 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 ϵNd values of +2.6, whereas the andesite is LREE-enriched and has ϵNd 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.

STRUCTURAL CONSTRAINTS ON THE TECTONIC EVOLUTION OF A LATE ARCHEAN GREENSTONE BELT IN THE NORTHEASTERN SUPERIOR PROVINCE, NORTHERN QUEBEC (CANADA)

Abstract A detailed structural analysis shows that the Vizien greenstone belt of northeastern Superior Province records five generations (G1 to G5) of ductile deformation, as well as brittle faulting. G1 is indicated by the local presence of a pre-G2 foliation and is interpreted to be related to thrusting. G2 is responsible for the main penetrative foliation (S2), axial-planar to tight to isoclinal F2 folds and subparallel to major boundaries between lithotectonic panels. It is concentrated in a thrust with sense of shear opposite to that interpreted for G1. F3 and F4 folds are open to tight, with NNW-SSE- and E-W-trending axial surfaces, respectively. They warp the S2 foliation and major panel boundaries and dominate the map pattern. G5 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-G3 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 melange and deposition in a possible foreland basin. The G1 and G2 deformations are interpreted to be related to synthetic thrusting and back thrusting, respectively, during the collision.

Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis

Although the geological record indicates that eukaryotes evolved by 1.9–1.4 Ga, their early evolution is poorly resolved taxonomically and chronologically. The fossil red alga Bangiomorpha pubescens is the only recognized crown-group eukaryote older than ca. 0.8 Ga and marks the earliest known expression of extant forms of multicellularity and eukaryotic photosynthesis. Because it postdates the divergence between the red and green algae and the prior endosymbiotic event that gave rise to the chloroplast, B. pubescens is uniquely important for calibrating eukaryotic evolution. However, molecular clock estimates for the divergence between the red and green algae are highly variable, and some analyses estimate this split to be younger than the widely inferred but poorly constrained first appearance age of 1.2 Ga for B. pubescens. As a result, many molecular clock studies reject this fossil ex post facto. Here we present new Re-Os isotopic ages from sedimentary rocks that stratigraphically bracket the occurrence of B. pubescens in the Bylot Supergroup of Baffin Island and revise its first appearance to 1.047 +0.013/–0.017 Ga. This date is 150 m.y. younger than commonly held interpretations and permits more precise estimates of early eukaryotic evolution. Using cross-calibrated molecular clock analyses with the new fossil age, we calculate that photosynthesis within the Eukarya emerged ca. 1.25 Ga. This date for primary plastid endosymbiosis serves as a benchmark for interpreting the fossil record of early eukaryotes and evaluating their role in the Proterozoic biosphere.

Magnetotelluric investigations of the lithosphere beneath the central Rae craton, mainland Nunavut, Canada

New magnetotelluric soundings at 64 locations throughout the central Rae craton on mainland Nunavut constrain 2-D resistivity models of the crust and lithospheric mantle beneath three regional transects. Responses determined from colocated broadband and long-period magnetotelluric recording instruments enabled resistivity imaging to depths of > 300 km. Strike analysis and distortion decomposition on all data reveal a regional trend of 45–53°, but locally the geoelectric strike angle varies laterally and with depth. The 2-D models reveal a resistive upper crust to depths of 15–35 km that is underlain by a conductive layer that appears to be discontinuous at or near major mapped geological boundaries. Surface projections of the conductive layer coincide with areas of high grade, Archean metasedimentary rocks. Tectonic burial of these rocks and thickening of the crust occurred during the Paleoproterozoic Arrowsmith (2.3 Ga) and Trans-Hudson orogenies (1.85 Ga). Overall, the uppermost mantle of the Rae craton shows resistivity values that range from ~3000 Ω m in the northeast (beneath Baffin Island and the Melville Peninsula) to ~10,000 Ω m beneath the central Rae craton, to >50,000 Ω m in the south near the Hearne Domain. Near-vertical zones of reduced resistivity are identified within the uppermost mantle lithosphere that may be related to areas affected by mantle melt or metasomatism associated with emplacement of Hudsonian granites. A regional decrease in resistivities to values of ~500 Ω m at depths of 180–220 km, increasing to 300 km near the southern margin of the Rae craton, is interpreted as the lithosphere-asthenosphere boundary.