Archean Cratons Research Articles

Impact origin of Archean cratons

Archean cratons consist of crustal granite-greenstone terrains (GGTs) coupled to roots of strong, buoyant cratonic lithospheric mantle (CLM). Although this association is unique to the Archean and formed from ca. 4.0 to 2.5 Ga, the origins of terrestrial cratons are debated. I propose that crustal plateaus, quasi-circular craton-like features (∼1400–2400 km diameter, 0.5–4 km high), on Earth’s sister planet Venus might serve as analogs for Archean cratons. Crustal plateaus, which are isostatically supported by a compositionally controlled low-density root, host a distinctive surface called ribbon-tessera terrain. Ribbon-tessera also occurs as arcuate-shaped inliers in the Venus lowlands, widely interpreted as remnants of rootless crustal plateaus. Within each crustal plateau, surface ribbon-tessera terrain comprises a vast igneous province analogous to terrestrial GGTs, and the plateau root is analogous to CLM. Crustal plateaus and ribbon-tessera terrain collectively represent Venus’ oldest preserved features and surfaces, and they formed during an ancient period of globally thin lithosphere. To explain the linked features of crustal plateaus, a bolide impact hypothesis has been proposed in which a large bolide pierces ancient thin lithosphere, leading to massive partial melting in the sublithospheric mantle. In this model, melt escapes to the surface, forming an enormous lava pond, which evolves to form ribbon-tessera terrain; mantle melt residue forms a strong, resilient buoyant root, leading to plateau support and long-term stability of an individual crustal plateau. Building on the similarity of GGT–CLM and Venus crustal plateaus, I propose an exogenic hypothesis for Archean craton formation in which a large bolide pierces thin Archean lithosphere, causing localized high-temperature, high-fraction partial melting in the sublithospheric mantle; melt rises, forming an igneous province that evolves to form a GGT, and melt residue develops a complementary CLM. By this mechanism, Archean cratons may have formed in a spatially and temporally punctuated fashion at a time when large bolides showered Archean Earth.

Lithospheric architecture beneath Hudson Bay

AbstractHudson Bay overlies some of the thickest Precambrian lithosphere on Earth, whose internal structures contain important clues to the earliest workings of plate formation. The terminal collision, the Trans‐Hudson Orogen, brought together the Western Churchill craton to the northwest and the Superior craton to the southeast. These two Archean cratons along with the Paleo‐Proterozoic Trans‐Hudson internides, form the core of the North American craton. We use S to P converted wave imaging and absolute shear velocity information from a joint inversion of P to S receiver functions, new ambient noise derived phase velocities, and teleseismic phase velocities to investigate this region and determine both the thickness of the lithosphere and the presence of internal discontinuities. The lithosphere under central Hudson Bay approaches ∼350 km thick but is thinner (∼200–250 km) around the periphery of the Bay. Furthermore, the amplitude of the LAB conversion from the S receiver functions is unusually large for a craton, suggesting a large thermal contrast across the LAB, which we interpret as direct evidence of the thermal insulation effect of continents on the asthenosphere. Within the lithosphere, midlithospheric discontinuities, significantly shallower than the base of the lithosphere, are often imaged, suggesting the mechanisms that form these layers are common. Lacking time‐history information, we infer that these discontinuities reflect reactivation of formation structures during deformation of the craton.

Differences of Structure in Mid‐Lower Crust between the Eastern and Western Blocks of the Sichuan basin

AbstractThe Sichuan basin is the main part of the Middle‐upper Yangtze craton, which has experienced a long tectonic evolution since Archean. It was regarded as a stable block until the collision with the Cathaysia block in late Neoproterozoic. As one of the largest and most complex Archean cratons in China, the Yangtze craton consists of Archean‐Paleoproterozoic crystalline basement (mostly covered) outcropped in the western and southwestern margins and the Three Gorge area surrounded by late Mesoproterozoic to early Neoproterozoic fold belts, which are locally unconformably overlain by middle Neoproterozoic weakly metamorphosed strata and late Neoproterozoic unmetamorphosed Sinian and Phanerozoic cover. These strata limit the understanding of the nature and distribution characteristics of the middle‐lower crust beneath the sedimentary rocks. Gravity anomalies can reveal the overall regional structure. In this paper we obtain the middle‐lower crustal gravity anomalies by removing the gravity anomalies induced by the sedimentary rocks and the mantle beneath the Moho, which shows that the mid‐lower crustal structure of the Sichuan Basin can be divided into eastern and western parts. Combined with the deep seismic reflection data and other geochemical and geophysical data, we propose that the boundary of mid‐lower crust between the eastern and western parts is along the Chongqing‐Huaying line, which may indicate that there are two continental nucleuses beneath the Yangtze block.

Significance of the whole rock Re–Os ages in cryptically and modally metasomatised cratonic peridotites: Constraints from HSE–Se–Te systematics

The Re–Os isotopic system is the geochronometer of choice to constrain the timing of lithospheric mantle root formation and reconstruct the evolution of Earth’s dynamics from the “mantle” perspective. In order to constrain the effects of metasomatic processes on the Re–Os isotopic system, eleven peridotites from the Letlhakane kimberlite pipe were investigated for whole rock major and trace elements, highly siderophile elements (HSE), Se, Te and 187Os/188Os signatures. These spinel peridotites (SP), garnet peridotites (GP), garnet-phlogopite peridotites (GPP) and phlogopite peridotites (PP) experienced cryptic metasomatism and the GP–GPP–PP additionally constitute a sequence of increasing modal metasomatism.The cryptically metasomatised SP appear devoid of base metal sulphides (BMS) and show suprachondritic Se/Te ratios (15–40) and extremely Pd- and Pt-depleted HSE patterns. These features are characteristic of high-degree partial melting residues. Their 187Os/188Os signatures are thus considered to be inherited from the partial melting event. This implies a Neoarchean (2.5–2.8Ga, TRD eruption) stabilisation of the Letlhakane mantle root and supports the Letlhakane mantle root being a westerly extension of the Zimbabwe cratonic root.The modally metasomatised peridotites contain BMS whose abundance significantly increases from the GPP to the GP and PP. The BMS-poor GPP are only slightly richer in Pt and Pd than the BMS-free SP but have similarly high Se/Te ratios. The BMS-rich GP and PP exhibit significant enrichments in Pt, Pd, Se, Te resulting in HSE–Se–Te signatures similar to that of the Primitive Upper Mantle (PUM). Addition of 0.001–0.05wt.% metasomatic BMS±PGM (platinum group minerals, i.e., Pt-tellurides) to highly refractory residues, such as the Letlhakane SP, reproduce well the HSE–Se–Te systematics observed in the BMS-poor and BMS-rich modally metasomatised peridotites. In the GPP, the negligible addition of metasomatic BMS±PGM did not disturb the 187Os/188Os signatures, indicating TRD eruption ages identical with those of the SP. By contrast, the PP and GP, which suffered significant BMS±PGM addition, have 187Os/188Os considerably shifted toward more radiogenic values. As a result, unrealistically young TRD eruption ages are obtained that cannot be used to constrain the time frame of the stabilisation of the cratonic roots and the mechanisms of craton formation.The Se–Te and incompatible HSE (i.e., Pt, Pd) are powerful geochemical tools to assess the robustness of the Re–Os isotopic system in mantle peridotites. Specifically, Se/Te and Pd/Ir ratios provide complementary insights into the complex metasomatic history of mantle peridotites. The Se/Te ratio is very sensitive to Os-free PGM metasomatism producing highly variable Se/Te >15 at low, relatively constant Pd/Ir< 0.4 but does not affect the 187Os/188Os systematics. This contrasts with BMS metasomatism, where Pd/Ir becomes more variable (>0.4), at constant but low Se/Te <10 and which is accompanied by modification of the Os isotopic composition of the host peridotite. As they are sensitive indicators of BMS±PGM metasomatism, HSE, Se and Te should be systematically considered when investigating the timing of stabilisation of lithospheric mantle and its petrogenetic history. This would allow a more robust assessment of the Re–Os ages obtained and provide firmer constraints on the evolution and formation of Archean cratons and early Earth dynamics.

Interpretations and implications of LA ICP-MS analysis of chert for the origin of geochemical signatures in banded iron formations (BIFs) from the Meadowbank gold deposit, Western Churchill Province, Nunavut

Among the many types of mineral deposits within Archean cratons, gold mineralization is an important economic commodity with over 20,000metric tons of gold produced from greenstone belts in 2001. Of the Archean–early Paleoproterozoic gold deposits, several different types of mineralization are known, which includes the important Algoma-type banded iron formation (BIF) where gold is locally associated with sulfide-facies zones within regionally extensive oxide-facies. It is commonly accepted that the shale-normalized chemical signature of REE+Y of chert bands in Algoma-type BIFs may reflect one of the three processes, each of which may be relevant to the nature and origin of the gold mineralization: (1) direct seawater precipitation; (2) involvement of and contribution from hydrothermal fluids; and (3) replacement of precursor volcanic units due to silicification. An essential question in regard to the mineralization is, therefore, whether the gold mineralizing fluids have a preference for one geochemical type of iron formation versus another. In order to assess the relevance of these competing models, we report herein the results of a LA ICP-MS study of chert samples within different Algoma-type BIFs from the Meadowbank deposit (24.5Mt proven/probable ore reserves grading 2.8g/t (2011)) hosted in the Neoarchean Woodburn Lake Group of the Rae Domain of the western Churchill Province, Canada. This study used 39 carefully selected and characterized (i.e., petrography and SEM-EDS imaging) chert samples from the main deposit, the Central BIF, and four additional BIFs, the Far West, West, East and Grizzly zones, with data collected using line traverses along the chert bands. The geochemical data indicate that an ambient seawater signature (i.e., enrichment in HREE relative to LREE associated with positive La and Y anomalies) dominates the samples with a lesser hydrothermal component (characterized by a positive Eu anomaly) and the influence of detrital contamination can also be detected. These initial results indicate that the methodology and protocol employed provides a reliable means to assess and interpret the chemical signature of BIFs hosting gold mineralization. In the present case, the results for the Meadowbank deposit suggest that chert from mineralized BIF units does not record a typical chemical signature that may be used as a vector for potential gold mineralization.

Transformation of an Archean craton margin during Proterozoic basin formation and magmatism: The Albany–Fraser Orogen, Western Australia

The Albany–Fraser Orogen is a well-preserved example of Proterozoic modification of an Archean craton margin. The formation of two successive basin systems accompanied by magmatism along the southern and southeastern Yilgarn Craton margin in Western Australia reflect distinct changes in tectonic regimes, resulting in significant transformations of the Archean craton crust. Provenance analysis from detrital zircons indicates that the first basin system – the c. 1815–1600Ma Barren Basin – was dominantly filled with Neoarchean detritus derived from the Yilgarn Craton, and Paleoproterozoic detritus derived from coeval voluminous, mostly felsic magmatism. The abundance of locally derived sediment deposited onto a reworked Archean Yilgarn Craton substrate, coupled with the progressively more juvenile isotopic signature of magmas, indicates a largely extensional tectonic setting, consistent with either a continental rift basin or a long-lived backarc basin system. This extensional regime led to the formation of a passive margin and oceanic basin along the craton edge – the c. 1600–1305Ma Arid Basin – and formation of an ocean-continent transition in the most heavily modified, outboard region of the craton margin (the eastern Nornalup Zone). A change in tectonic regime is denoted by the formation of an oceanic-arc – the c. 1410Ma Loongana Arc, preserved in the adjoining Madura Province. Oceanic subduction led to accretion of the Loongana Arc onto the craton edge, recorded by the presence of exotic detritus derived from this arc deposited into a foreland basin (uppermost Arid Basin). This accretion event triggered crustal thickening and the earliest phase of magmatism of Stage I of the Albany–Fraser Orogeny at c. 1330Ma. A change to west-dipping subduction beneath the accreted portion of the Loongana Arc placed the orogen into a backarc setting, into which the remainder of the Recherche Supersuite mafic and felsic magmas were intruded. Thus, the Albany–Fraser Orogen is regarded as an Archean craton margin that preserves a long history of Proterozoic transformation dominated by extensional processes that resulted in the formation of orogen-wide, basin systems, accompanied by magmatism. The recognition that the Albany–Fraser Orogen has always been part of the West Australia Craton, and the tectonic regimes that have affected it, provide important constraints on reconstructions of the Nuna and Rodinia supercontinents.

The affinity of Archean crust on the Yilgarn—Albany–Fraser Orogen boundary: Implications for gold mineralisation in the Tropicana Zone

Craton margins can be subject to a wide array of gold genesis and redistribution processes, although high-grade terrains on craton margins are frequently viewed as less prospective than lower-grade counterparts. In contrast to this, the high-grade Tropicana Zone, a newly defined Archean crustal component on the eastern margin of the Yilgarn Craton within the Albany–Fraser Orogen (AFO), contains a significant Proterozoic gold deposit. This deposit and zone comprise mid-amphibolite to granulite-facies gneissic rocks with evidence of partial melting and granite injection. The Tropicana Zone contains significant low-Si, LILE-enriched, granites classed as sanukitoids. Along with the distinctive compositions, the rarity of these rocks within any Archean craton suggests that the granitoid protoliths represent a single suite, emplaced during one event.Due to the intense granulite-facies overprinting of the Tropicana Zone rocks, determination of the magmatic protolith age for these sanukitoids is challenging. Nonetheless, the best age estimate for magmatism is 2692±16 Ma, based on the youngest zircons preserving textural evidence of growth within a viscous silicate melt. This age is older than compositionally similar magmatism found within the Yilgarn Craton, although a sanukitoid in the Northern Foreland of the AFO has a similar age. Furthermore, the granulite-facies metamorphic zircon growth in the Tropicana Zone at 2718–2554 Ma was prolonged compared to that in the Yilgarn Craton. Nonetheless, the Hf isotopic signature of the Tropicana Zone zircon shares strong similarity to that from the Eastern Goldfields Superterrane of the Yilgarn Craton. This implies that the Tropicana Zone reflects a deeper crustal level of the Yilgarn Craton, exhumed and thrust NW an unknown distance over the craton edge. In addition, we observe that the granulite-facies zircons have a less radiogenic Hf-isotope signature than the preserved pre-metamorphic zircon cores. Based on correlations with alpha dose, U and Th content and 176Hf/177Hf we suggest this reflects the preferential destruction and release of unradiogenic Hf from inherited zircon whereas the protolith sanukitoid zircon, with lower U and Th content, was more resistant to mobilisation during high-grade metamorphism. We note this situation may be a more general response of the Hf isotopic system, in which zircon grown in a more mafic melt is less likely to contribute to the metamorphic Hf reservoir than its felsic counterpart.Juvenile granitic veins dated at c. 1780 Ma intruded into the Tropicana Zone indicate that the Tropicana Zone was structurally emplaced at or before c. 1780 Ma, given similar Proterozoic magmatic events are well documented from (para)autochthonous adjacent units. Re–Os dating of pyrite coeval with one generation of gold in these rocks indicates model ages of c. 2100 Ma, supportive of a Palaeoproterozoic age of mineralisation. This mineralisation event is distinct from major Proterozoic tectonothermal events elsewhere in the AFO. Sanukitoid magmas are well-known for gold fertility and were likely the original source of gold in the Tropicana Zone, which was subsequently concentrated into brittle structures during several episodes. Gold mineralisation post-dated peak metamorphic conditions and is significantly younger than gold mineralisation within other parts of the adjoining Yilgarn Craton.

Crustal velocity structure of the Neoarchean convergence zone between the eastern and western blocks of Dharwar Craton, India from seismic wide-angle studies

The Dharwar Craton in the southern part of the Indian shield comprises two distinct blocks viz. the Meso-Neoarchean western and the predominantly Neoarchean eastern blocks, which are popularly referred to as Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC). The boundary between the WDC and EDC is marked by a prominent shear zone, the Chitradurga eastern margin shear zone (CSZ). Several studies suggested Neoarchean convergence between the EDC and WDC, although there is diversity of opinion on the tectonic process. We present here the results of a new seismic refraction/wide-angle reflection experiment along a 200km long profile across the WDC and EDC encompassing the CSZ to elucidate the crustal velocity structure of the convergence zone. Travel time inversion and amplitude modeling of the seismic data delineate a five-layered crustal velocity model for the Dharwar Craton with a 42km (minimum) thick crust in the WDC grading to a distinctly thinner crust up to 38km, in the EDC. The transect is also characterized by a high-velocity (7.0–7.1kms−1) mafic lower-crustal layer, but with distinctly variable thickness, 10km in WDC and 8km in EDC. The transition from thick to thinner crust coincides with the CSZ. The thick crust with a high-velocity basal layer is atypical of Archean cratons, but more akin to Proterozoic terrains. The crustal velocity structure across the EDC–WDC boundary zone is consistent with the Neoarchean plate convergence model that invoke tectonic scenarios, such as accretion of a hot orogen, the EDC and/or westward subduction of the latter beneath the WDC. Spatial variation in the high-velocity lower-crustal layer across the craton is ascribed to post-accretional mafic magmatism and underplating mainly during the Paleoproterozoic and Phanerozoic.

A review of structural patterns and melting processes in the Archean craton of West Greenland: Evidence for crustal growth at convergent plate margins as opposed to non-uniformitarian models

The Archean craton of West Greenland consists of many fault-bounded Eoarchean to Neoarchean tectonic terranes (crustal blocks). These tectonic terranes are composed mainly of tonalite–trondhjemite–granodiorite (TTG) gneisses, granitic gneisses, metavolcanic-dominated supracrustal belts, layered anorthositic complexes, and late- to post-tectonic granites. Rock assemblages and geochemical signatures in these terranes suggest that they represent fragments of dismembered oceanic island arcs, consisting mainly of TTG plutons, tholeiitic to calc-alkaline basalts, boninites, picrites, and cumulate layers of ultramafic rocks, gabbros, leucogabbros and anorthosites, with minor sedimentary rocks. The structural characteristics of the terrane boundaries are consistent with the assembly of these island arcs through modern style of horizontal tectonics, suggesting that the Archean craton of West Greenland grew at convergent plate margins. Several supracrustal belts that occur at or near the terrane boundaries are interpreted as relict accretionary prisms. The terranes display fold and thrust structures and contain numerous 10cm to 20m wide bifurcating, ductile shear zones that are characterized by a variety of structures including transposed and redistributed isoclinal folds. Geometrically these structures are similar to those occurring on regional scales, suggesting that the Archean craton of West Greenland can be interpreted as a continental scale accretionary complex, such as the Paleozoic Altaids. Melting of metavolcanic rocks during tectonic thickening in the arcs played an important role in the generation of TTGs. Non-uniformitarian models proposed for the origin of Archean terranes have no analogs in the geologic record and are inconsistent with structural, lithological, petrological and geochemical data collected from Archean terranes over the last four decades. The style of deformation and generation of felsic rocks on outcrop scales in the Archean craton of West Greenland and the Mesozoic Sulu orogenic belt of eastern China are similar, consistent with the formation of Archean continental crust by subduction zone processes.

An imbricate midcrustal suture zone: The Mojave-Yavapai Province boundary in Grand Canyon, Arizona

The Paleoproterozoic Mojave and Yavapai crustal provinces in southwestern Laurentia contain evolved and juvenile crust, respectively, but the nature of the province boundary remains uncertain. 1.78–1.35 Ga crystalline basement rocks of the Mojave Province preserve an evolved isotopic signature reflecting an Archean crustal component in several isotopic systems (Nd, Pb, Hf). However, no Archean rocks have been found, and hence the origin and tectonic significance of this Archean component are also unclear. This paper analyzes the U-Pb age and Hf isotopic composition of zircons from both the oldest granodiorite plutons (1.84–1.71 Ga) and the oldest metasedimentary rocks (1.75 Ga Vishnu Schist) across a 180-km-long cross-strike transect in Grand Canyon. This transect crosses the Crystal shear zone, which has been proposed as the location of a suture separating the provinces. Our results show that the characteristically bimodal population of detrital zircons in the Vishnu Schist (2.5 Ga and 1.8 Ga modes) yields mixed ɛHf(t) values, primarily between +5 to –5, that are uniform across the transect. Another new finding is that the 1.84 Ga Elves Chasm pluton, on which the Vishnu Schist was deposited, yields juvenile ɛHf(t) values of +5 to +12 and was not the dominant source for the ca. 1.85 Ga peak in the 1.75 Ga Vishnu Schist. Instead, the Vishnu Schist was derived from an Archean craton mixed with intermediate to evolved 1.85 Ga crust. Metasediments show no evidence in support of the proposed suture. Paradoxically, plutons east and west of the Crystal shear zone do support models for a crustal suture. Plutons east of the Crystal shear zone dated at 1.74–1.71 Ga yield juvenile ɛHf(t) values of +5 to +12 that are characteristic of the Yavapai Province. Plutons west of the Crystal shear zone show juvenile to evolved Paleoproterozoic grains (ɛHf(t) of –5 to +10) as well as xenocrystic Archean and 1.85 Ga grains (ɛHf(t) of –12 to +10). These data support the proposition that the Crystal shear zone marks a sharp boundary between the Mojave and Yavapai crustal provinces. However, the overlapping Vishnu Schist suggests a more complicated crustal architecture. The depositional setting of the Vishnu Schist remains unclear; however, we interpret the ultimate geometry of the transect to reflect an ∼200-km-wide middle-crustal duplex system in which the 1.75 Ga Vishnu Schist was deposited across both Mojave and Yavapai crust. This system was subsequently imbricated in an accretionary complex. The ultimate architecture is of a distributed boundary with slivers of plutons that carry the isotopic signature of their respective provinces imbricated within metasediments.

Episodic and multistaged gravitational instability of cratonic lithosphere and its implications for reactivation of the North China Craton

Archean cratons are the most stable tectonic units and their lithospheric mantle is chemically depleted and buoyant relative to the underlying mantle. The chemical depletion leads to high viscosity that maintains the long-term stability of cratons. However, the eastern part of the North China Craton (∼1200 km in horizontal length scale) had been extensively reactivated and modified over a time scale of ∼100 Myr in the Mesozoic and Cenozoic. While the causes for the weakening of the North China Craton, a necessary condition for its reactivation, are still in debate, we investigate gravitational instability of compositionally buoyant lithosphere, by computing 2-D thermochemical convection models with different buoyancy number, lithospheric viscosity, and rheology. We find that the gravitational instability of cratonic lithosphere can happen over a larger range of buoyancy numbers with non-Newtonian rheology, but lithospheric instability with Newtonian rheology only happens with relatively small buoyancy numbers. For cratonic lithosphere with non-Newtonian rheology and relatively weak temperature-dependent viscosity, the instability starts in the cold, shallow part of the lithosphere and has small horizontal length scale (<300 km), leading to efficient thermal and chemical mixing with the underlying mantle. For cratonic lithosphere such as the eastern North China Craton, the instability process is episodic and consists of multiple instability events that may last for ∼100 Myr. The instability process revealed from our study explains the observations of episodic magmatism/volcanism events, geochemical mixing, and time scales associated with the reactivation of the North China Craton.

Upper-mantle anisotropy beneath the south Indian Shield: Influenced by ancient and recent Earth processes

ABSTRACT We obtained shear-wave splitting parameters from core-refracted phases like SKS, SKKS, and PKS at 75 digital broadband seismic stations almost uniformly spread over the south Indian Shield representing varied geological terrains, including the western Dharwar craton, the eastern Dharwar craton, the Southern granulite terrain, and the continental margins along the west and east coasts. A majority of the sta-tions over the Dharwar craton show delay times of 1–1.6 s, indicative of a 150–200-km-thick anisotropic layer correlating with the lithospheric root, while segments like the Pan-African Southern granulite terrain have delay times of 0.5–0.7 s, suggesting an internally deformed and thin anisotropic layer, possibly due to recent plate-tectonic and geodynamic processes. The average direction of anisotropy is generally ~N30°E, correlating with the present-day plate motion, with local deviations where the direction of anisotropy correlates with the orienta-tion of the regional shear zones. Stations close to the continental margin show large time delays (up to 2 s) with the fast axis parallel to the rift axis. Further, we infer a layered anisotropic lithosphere in the south Indian Shield as revealed in 90° periodicity of the two anisotropy parameters (fast direction and delay time). The upper lithosphere represents the depleted Archean mantle, while the lower lithosphere could be transformed to more fertile mantle due to subsequent deformation. This study suggests that the observed anisotropy over the south Indian Shield is the result of complex interplay between the architecture of an Archean craton and its subsequent deformation in different geological domains due to deep Earth processes.

The building and stabilization of an Archean Craton in the Superior Province, Canada, from a heat flow perspective

AbstractHow continental lithosphere responds to tectonic stresses and mantle convective processes is determined in large part by its mechanical strength and temperature distribution, which depend on crustal heat production. In order to establish reliable crustal and thermal models for the Superior Craton, Canadian Shield, new measurements of heat flux and heat production in 28 deep boreholes at 16 sites are combined with a larger set of older data. The Superior Province was assembled by the docking of volcanic/plutonic and metasedimentary terranes and continental fragments to the southern margin of an older core around 2.7 Ga. The average heat flux is much lower in the craton core than in the accreted terranes, 31 versus 43 mW m−2. The major accreted volcanic/plutonic belts share the same heat production characteristics, testifying to the remarkable uniformity of crust‐building mechanisms. The marked difference between the crusts of the core and the accreted belts supports the operation of two different crust‐forming processes. The crust of the craton core has an enriched upper layer, in contrast to that of the younger belts which lack marked internal differentiation. At the end of amalgamation, the lithosphere of the craton core was colder and mechanically stronger than the lithosphere beneath newly accreted material. Surrounding the craton core with weaker belts may have ensured its stability against tectonic and mantle convection perturbations. This large strength contrast accounts for the lack of lithospheric imbrication at the edge of the craton core as well as for the different characteristics of seismic anisotropy in the lithospheres of the craton core and the younger terranes.

Radially anisotropic 3-D shear wave structure of the Australian lithosphere and asthenosphere from multi-mode surface waves

A new radially anisotropic shear wave speed model for the Australasian region is constructed from multi-mode phase dispersion of Love and Rayleigh waves. An automated waveform fitting technique based on a global optimization with the Neighbourhood Algorithm allows the exploitation of large numbers of three-component broad-band seismograms to extract path-specific dispersion curves covering the entire continent. A 3-D shear wave model is constructed including radial anisotropy from a set of multi-mode phase speed maps for both Love and Rayleigh waves. These maps are derived from an iterative inversion scheme incorporating the effects of ray-path bending due to lateral heterogeneity, as well as the finite frequency of the surface waves for each mode. The new S wave speed model exhibits major tectonic features of this region that are in good agreement with earlier shear wave models derived primarily from Rayleigh waves. The lateral variations of depth and thickness of the lithosphere–asthenosphere transition (LAT) are estimated from the isotropic (Voigt average) S wave speed model and its vertical gradient, which reveals correlations between the lateral variations of the LAT and radial anisotropy. The thickness of the LAT is very large beneath the Archean cratons in western Australia, whereas that in south Australia is thinner. The radial anisotropy model shows faster SH wave speed than SV beneath eastern Australia and the Coral Sea at the lithospheric depth. The faster SH anomaly in the lithosphere is also seen in the suture zone between the three cratonic blocks of Australia. One of the most conspicuous features of fast SH anisotropy is found in the asthenosphere beneath the central Australia, suggesting anisotropy induced by shear flow in the asthenosphere beneath the fast drifting Australian continent.

The diversity and evolution of late-Archean granitoids: Evidence for the onset of “modern-style” plate tectonics between 3.0 and 2.5Ga

The end of the Archean aeon (3.0–2.5 Ga) was a period of fundamental change in many aspects of the geological record. In Archean cratons, this timespan is marked by a considerable diversification in both the nature and petrogenesis of granitoid rocks. In this article, we review the nature, petrogenesis and global evolution of late-Archean granitoids and discuss their geodynamic significance. Late-Archean granitoids can be classified into four groups: (1) volumetrically-dominant and juvenile tonalites, trondhjemites and granodiorites (TTGs), whose geochemistry is consistent with an origin through partial melting of meta-igneous mafic rocks at various pressures; (2) Mg-, Fe- and K-rich, metaluminous (monzo)diorites and granodiorites, referred to as sanukitoids s.l., which derive primarily from hybridization between mantle peridotite and a component rich in incompatible elements; (3) peraluminous and K-rich biotite- and two-mica granites, formed through melting of older crustal lithologies (TTGs and meta-sediments, respectively); and (4) hybrid high-K granites with mixed characteristics from the first three groups. The chronology of granitoid emplacement in late-Archean times is different from one craton to another but, in general, follows a very specific two-stage sequence: (1) a long period (0.2–0.5 Ga) of TTG emplacement; (2) a shorter period (0.02–0.15 Ga) during which all other granitoid types were generated. We propose that this sequence represents the first global subduction–collision cycle in the Earth's history. Although possibly present in the geological record prior to 3.0 Ga, such mechanisms became progressively prevalent on a planetary scale only between 3.0 and 2.5 Ga, indicating that the late-Archean geodynamic changes resulted from the global initiation of “modern-style” plate tectonics. The Archean–Proterozoic transition thus represents a major change in the mechanisms of the Earth's heat loss: before 3.0–2.5 Ga, it took place by large-scale magmatic differentiation characterized by generation of proto-continents that underwent crustal maturation locally, but without obvious cyclic activity on a planetary scale. After this, heat loss became accommodated by plate tectonics and global Wilson subduction–collision cycles. These changes were the consequence of the Earth's cooling, which in turn controlled a number of different parameters locally (thickness, temperature, volume and rheology of the crust). This explains why the changes took place over a short timespan (~ 0.5 Ga) relative to the Earth's history, but at different times and with different characteristics from one craton to another.

Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons

The mechanisms of growth and destruction of continental lithosphere have been long debated. We define and test a unifying plate tectonic driving mechanism that explains the numerous petrological, geophysical, and geological features that characterize the destruction of cratonic lithospheric roots. Data from three Archean cratons demonstrate that loss of their roots is related to rollback of subducted flat slabs, some along the mantle transition zone, beneath the cratons. During flat slab subduction dehydration reactions add water to the overlying mantle wedge. As the subducting slabs roll back, they suck in mantle material to infill the void space created by the slab roll back, and this fertile mantle becomes hydrated. The roll-back causes concomitant lithospheric thinning of the overlying craton so the flux of newly hydrated mantle material inevitably rises causing adiabatic melting, generating new magmas that gradually destroy the roots of the overlying craton through melt–peridotite reactions. Calculated fluxes of new mantle material beneath cratons that have lost their roots range from 2.7 trillion to 70 million cubic kilometers, which is sufficient to generate enough melt to completely replace the affected parts of the destroyed cratons. Cratonic lithosphere may be destroyed in massive quantities through this mechanism, warranting a re-evaluation of continental growth rates with time.

The Samapleu mafic-ultramafic intrusion and its Ni-Cu-PGE mineralization: an Eburnean (2.09 Ga) feeder dyke to the Yacouba layered complex (Man Archean craton, western Ivory Coast)

AbstractThe Yacouba layered complex intrudes the Archean (3.5–2.7 Ga) Kenema-Man craton in the Samapleu-Yorodougou area, western Ivory Coast. In Samapleu area, the complex was recognized in drill holes at three locations: Samapleu Main (SM); Samapleu Extension 1 (E1) and Yorodougou (Yo). It comprises websterites, peridotites and gabbro-norites arranged symmetrically with mafic layers at the center and ultramafic layers at both margins. The complex is inclined at 70–80° to the SE. The thickness of individual layers varies from 2 to 60 m and the total thickness is 120 to 200 m. At the E1 site, the complex extends to depths &amp;gt; 500 m.Contacts with the country rock gneiss are characterized by a hybrid zone that is a few meters thick and composed of plagioclase-orthopyroxene bearing metabasites, and locally (E1 site) a metamorphic assemblage of sapphirine-cordierite-sillimanite-spinel ± rutile. This assemblage is attributed to contact metamorphism during intrusion of the complex in the lower crust at a depth of about 25 km. Zircons in country rock gneisses and granulites, as well as in the hybrid facies, yield Archean ages of ~ 2.78 Ga, similar to ages reported in the Man craton. Rutiles in the hybrid zone give a U-Pb age of 2.09 Ga, which is interpreted as the age of contact metamorphism and emplacement of the intrusion.The Samapleu Main and Samapleu Extension 1 sites contain Ni and Cu sulfide deposit with reserves estimated as more than 40 million tons grading 0.25% Ni and 0.22% Cu (Sama Nickel-CI, August 2013). The Ni-Cu mineralization is composed of pentlandite, chalcopyrite, pyrrhotite and rare pyrite, which is disseminated mainly in pyroxenite or occurs as subvertical and semi-massive to massive sulfide veins. The sulfide textures range from matrix ore, net-textured, droplets or breccia textures. Zones enriched in PGM, particularly Pd, are associated with the sulfides and several chromite bands are also present. These observations suggest that an immiscible sulfide liquid formed from a parental silicate liquid and percolated through the crystal pile.The parental melt composition, determined using the Chai and Naldrett [1992] method, has a SiO2-rich mafic composition with 53% SiO2 and 10% MgO. This result, the presence of the hybrid zone, and the trace-element signature determined using the Bedard [1994] method, suggest a mantle-derived basaltic parental magma that had assimilated abundant continental crust.These observations indicate that Samapleu intrusion corresponds to a magmatic conduit of the Yacouba complex as at Jinchuan (China), Voisey’s bay (Canada), Kabanga (Tanzania) or Nkomati (South Africa).

The geochemistry and petrogenesis of the Paleoproterozoic du Chef dyke swarm, Québec, Canada

The du Chef dyke swarm in southern Québec, Canada is composed of numerous northeast trending, greenschist-amphibolite facies, gabbronoritic dykes that crop out either side of the Grenville Front. The age of the du Chef swarm (2408±3Ga) has led previous authors to suggest a genetic link between the du Chef dykes and coeval swarms (including the Ringvassøy, Scourie, Widgemooltha and Sebanga) preserved on other Archean cratons. These now disparate dyke swarms are proposed to have formed in response to mantle plume-induced continental breakup during the early Proterozoic. This work represents the first geochemical study of the du Chef dykes and shows that the swarm evolved through fractional crystallisation of a tholeiitic parent magma that remained largely uncontaminated during its residence in, and ascent through, the crust. We also show that the primary magma for the du Chef swarm was derived through partial melting of an enriched region of the mantle, with a similar trace element composition to the modern-day HIMU reservoir and that the magma produced was significantly hotter than the ambient mantle at the time. We contend that the du Chef dykes are the product of early Proterozoic mantle plume magmatism and may help pinpoint an ancient hotspot centre that initiated continental break up along the margin of the Superior Craton at ∼2.4Ga. Other dyke swarms proposed to be genetically linked with the du Chef dykes record a distinctly different petrogenetic history to that of the du Chef dykes, as evidenced by their more volcanic arc-like geochemical signature. These contrasting geochemical signatures in supposedly cogenetic continental tholeiitic rocks may be evidence of early Proterozoic mantle heterogeneity sampled by the rising du Chef mantle plume.

Are Wilson Cycles preserved in Archean cratons? A comparison of the North China and Slave cratons

A review and comparison of the tectonic history of the North China and Slave cratons reveal that the two cratons have many similarities and some significant differences. The similarities rest in the conclusion that both cratons have a history of a Wilson Cycle, having experienced rifting of an old continent in the late Archean, development of a rift to passive margin sequence, collision of this passive margin with arcs within 100–200 Ma of the formation of the passive margin, reversal of subduction polarity, then eventual climactic collision with another arc terrane, microcontinental fragment, or continent. This cycle demonstrates the operation of Paleozoic-style plate tectonics in the late Archean. The main differences lie in the later tectonic evolution. The Slave’s post-cratonization history is dominated by subduction dipping away from the interior of the craton, and later incorporation into the interior of a larger continent, whereas the North China Craton has had a long history of subduction beneath the craton, including presently being located above the flat-lying Pacific slab resting in the mantle transition zone, placing it in a broad back-arc setting, with multiple mantle hydration events and collisions along its borders. The hydration enhances melting in the overlying mantle, and leads to melts migrating upwards to thermochemically erode the lithospheric root. This major difference may explain why the relatively small Slave craton preserves its thick Archean lithospheric root, whereas the eastern North China Craton has lost it.

Age and evolution of the lower crust beneath the western Churchill Province: U–Pb zircon geochronology of kimberlite-hosted granulite xenoliths, Nunavut, Canada

Zircon from granulite-facies xenoliths can preserve unique geodynamic records of deep and mostly inaccessible levels of the continental crust within Archean cratons. We present the first such records from beneath two Archean crustal blocks that were affected by multiple episodes of regional-scale Proterozoic metamorphism, and are located within the western Churchill Province, Nunavut, Canada. Zircon U–Pb geochronology and trace element analysis were carried out on samples that originated in the deep crust at the time of kimberlite eruption, as confirmed with Raman spectroscopy. Zircon was recovered from four metabasites (grt–cpx–plg) from the Rankin Inlet area of the Chesterfield block, and five metabasites (grt–cpx–plg±hbl±opx) and three metatonalite/anorthosites (plg–cpx–plg±hbl±opx) from the Repulse Bay area of the Rae craton. SEM-CL imaging reveals several types of growth zoning diagnostic of igneous and metamorphic zircon crystallization. The latter are related to local high-temperature recrystallization and micro-fracturing, and are spatially associated with zones of apparent Pb-loss and/or intracrystalline Pb-variability. Evidence for the formation of Paleoarchean/Mesoarchean to Neoarchean crust is preserved in oscillatory zoned, igneous cores having 207Pb/206Pb ages of ca. 3.5Ga and ca. 3.0–2.6Ga. Two periods of Paleoproterozoic metamorphism and zircon growth followed, with the oldest episode at ca. 2.0–1.9Ga in the Chesterfield block. Metamorphic zircon rims from all twelve xenoliths have an age range of ca. 1.75 to 1.70Ga. The trace element compositions of the metamorphic rims differ from those of older igneous and metamorphic cores, with the rims characterized by lower ΣHREE contents and low (Lu/Gd)N, suggesting that garnet crystallization occurred syn- to pre-rim growth at ca. 1.75Ga. The Archean and Paleoproterozoic ages correspond broadly to the upper crustal histories of the western Churchill Province; however, the prevalence of ca. 1.75–1.70Ga metamorphic rims indicates regional-scale metamorphic reworking of the lower crust across the region, during a period of intracratonic extension and associated magmatism. Regional magmatic underplating and/or remobilization of the lithosphere at ca. 1.75–1.70Ga, to an extent not widely appreciated, represents a terminal stage in western Churchill cratonization, including stabilization of reworked Archean crust relative to underlying diamondiferous mantle.