Rae Province Research Articles

The oldest part of the Rae craton identified in western Greenland

New U-Pb zircon data from a range of Archean rocks from the Paleoproterozoic Rinkian Belt in the North-West and central West Greenland provide new constraints on the crystallisation and formation of the Archean basement to this Paleoproterozoic orogen. The results show that the protoliths of the oldest orthogneisses were emplaced in the central part of the Rinkian Belt at c. 3150–3100 Ma. This was followed in the southern part of the belt by the crystallisation of igneous rocks at c. 3000–2900 Ma, including rhyolites that are well preserved. This event is not recorded in the northern part of the belt and may represent southward growth away from a cratonic core at this time. The orthogneisses across the entire belt were subsequently affected by a metamorphic event at c. 2730–2660 Ma, which included intrusions of granites and northward cratonic growth. A few samples also yield evidence of a younger Paleoproterozoic overprint associated with the formation of the 1.90–1.80 Ga Rinkian Belt. The Archean basement rocks of the Rinkian Belt have previously been correlated with similar rocks exposed in northeast Canada and, consequently, they have been referred to as the Greenland part of the Rae craton. The new data support this correlation in general but reveal that Greenland contains the oldest rocks of the Rae craton discovered so far. The new data also show that the Rinkian Belt has a significantly different Archean history than the basement underlying the Nagssugtoqidian orogen to the south. This supports former models that envision two separate Archean cratons colliding during the Paleoproterozoic and that a suture of this age is situated in the central Disko Bugt area.

Timing and provenance of Paleoproterozoic supracrustal rocks in the central Thelon tectonic zone, Canada: implications for the tectonic evolution of western Laurentia from ca. 2.1 to 1.9 Ga

Depositional ages and provenance of metasedimentary rocks provide constraints on the architecture of the interface between the Slave and Rae cratons and processes related to the Thelon Orogen. Clastic rocks analysed from the central Thelon tectonic zone are Paleoproterozoic in age and not remnants of the Archean Yellowknife Supergroup (Slave Province), as originally considered. Two assemblages are recognized. An older clastic assemblage deposited after 2.09 Ga contains detrital zircon age modes of 2.3 and 2.17 Ga, with subordinate Neoarchean and Paleoarchean detritus. Its deposition is interpreted to predate Thelon magmatic activity given that (1) it lacks ca. 2.01–1.97 Ga detritus of Thelon magmatic origin, and (2) correlative clastic rocks occur as inclusions in Thelon plutons and contain ca. 2.0 Ga metamorphic monazite. This assemblage is correlative with both the Mary Frances and Rutledge River groups, establishing a >800 km long basin at ca. 2.1 Ga that received detritus from the western Rae and (or) Buffalo Head terrane(s). Separation from the Slave craton at this time is consistent with the absence of any Slave-affinity detritus. A younger assemblage deposited after 1.95 Ga and prior to 1.91 Ga contains mainly 2.02–1.95 Ga detrital zircon, age modes comparable with adjacent Thelon convergent-margin plutonic rocks. The younger assemblage records deposition of the uplifted and eroded Thelon magmatic arc in an intermontane or foreland basin setting during the later stages of post-collisional convergence. These U–Pb zircon data support a tectonic model for western Laurentia that reconciles differences between the Thelon and Taltson magmatic zones involving ca. 2.1 Ga rifting, ca. 2.01–1.97 Ga convergence, followed by <1.95 Ga thrust-driven exhumation.

Oxidation state and metasomatism of the lithospheric mantle beneath the Rae Craton, Canada: strong gradients reflect craton formation and evolution

We present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield. In combination with major and trace element compositions for garnet and clinopyroxene, we assess the relationship between oxidation state and metasomatic overprinting. The sample suite comprises peridotite xenoliths from the central part (Pelly Bay) and the craton margin (Somerset Island) providing insights into lateral and vertical variations in lithospheric character. Our suite contains spinel, garnet-spinel and garnet peridotites, with most samples originating from 100 to 140 km depth. Within this narrow depth range we observe strong chemical gradients, including variations in oxygen fugacity (ƒO2) of over 4 log units. Both Pelly Bay and Somerset Island peridotites reveal a change in metasomatic type with depth. Observed geochemical systematics and textural evidence support the notion that Rae SCLM developed through amalgamation of different local domains, establishing chemical gradients from the start. These gradients were subsequently modified by migrating melts that drove further development of different types of metasomatic overprinting and variable oxidation at a range of length scales. This oxidation already apparent at ~ 100 km depth could have locally destabilised any pre-existing diamond or graphite.

Post-1.9 Ga evolution of the south Rae craton (Northwest Territories, Canada): A Paleoproterozoic orogenic collapse system

The Trans-Hudson Orogen (THO), formed from the convergence between the Superior craton and the composite Churchill Upper Plate (CUP), is one of the best-preserved examples of a collisional orogen in the Paleoproterozoic. Similar to modern collision systems such as the Himalayan orogen, it is characterized by a composite upper plate in which terrane accretion established a continental plateau that was tectonically and magmatically active for >100 myr. Our study presents new petrological and geochronological data for four samples collected in three lithotectonic domains of the south Rae craton (one of the CUP terranes). The results presented here allow us to re-define the previously proposed extent of THO reworking in the CUP and afford the opportunity to study and compare the evolution of various fragments that illustrate differing levels of a collapsed plateau in the CUP hinterland. The new data indicate that the south Rae craton locally preserves evidence for burial at 1.855–1.84 Ga with peak metamorphic conditions at approximately 790 °C and 9.5–12.5 kbar followed by rapid cooling and decompression melting (P < 6 kbar) at ca. 1.835–1.826 Ga.These results, which provide important and so far missing Pressure-Temperature-time (P-T-t) constraints on the evolution of the south Rae craton in the Northwest Territories at Trans-Hudson time, coupled with existing regional geochronological and geochemical data, are used to propose an updated model for the post-1.9 Ga THO collision and extensional collapse. Our results reveal that: i) initial thickening in the upper plate started at Snowbird time (ca. 1.94 Ga), then continued via Sask collision (with high-grade metamorphism recorded in the south Rae craton, ca. 1.85 Ga), and ended with Superior collision (ca. 1.83 Ga); ii) the extent of the THO structural and metamorphic overprint in the SW CUP is much broader across strike than previously recognized, and iii) T-t data in the south Rae are indicative of relatively fast cooling rates (8–25 °C/Ma) compared to other known Precambrian orogens. We suggest that the Paleoproterozoic THO represents the first record of a major ‘modern-style’ orogenic plateau collapse in Earth’s history.

The Patterson Lake corridor of Saskatchewan, Canada: defining crystalline rocks in a deep-seated structure that hosts a giant, high-grade Proterozoic unconformity uranium system

The Patterson Lake corridor in the Athabasca Basin region of Saskatchewan, Canada, hosts a large-scale uranium system with two major deposits already delineated. The corridor developed in crystalline rocks of the SW Rae Province, which host all of the known uranium endowment. Orthogneisses along with voluminous pegmatites are the hosts of the uranium mineralization. These rocks, however, underwent significant open-system metasomatic–hydrothermal modification. Principal amongst these alterations is early and pervasive quartz flooding that resulted in the development of widespread secondary quartzites and associated rock types. These secondary quartzites and their altered host rocks suffered ductile deformation, typically focused at silicification fronts. Late carbonatite dykes exploited the associated shear zones. Semi-brittle deformation zones nucleated near the previously developed ductile high-strain zones. Graphite and associated iron-sulfides precipitated in a semi-brittle structural regime. These graphitized zones provided the necessary structural architecture to focus the uranium system, which exploited the conduit hundreds of millions of years later at c. 1.425 Ga. Host rocks of the Patterson Lake corridor prove that metasedimentary rocks are not a requirement for the development of giant Proterozoic unconformity uranium deposits. Crustal-scale fault zones with access to the mantle (i.e. carbonatites) should be considered a key parameter in the exploration model for Proterozoic unconformity uranium deposits. Given the similarity of the mineral assemblages in the crystalline basement rocks of the main exploration corridor in the eastern Athabasca Basin region, it is likely that a similar, cryptic geological boundary focused the giant uranium system in that region. Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

Provenance, stratigraphic and precise depositional age constraints for an outlier of the 1.9 to 1.8 Ga Nonacho Group, Rae craton, Northwest Territories, Canada

The Nonacho Group comprises six formations of continental clastic rocks that were deposited between 1.91 and 1.83 Ga. The Nonacho Group is part of a broader assemblage of conglomerate and sandstone that was deposited atop the Rae craton in response to the amalgamation of Laurentia and supercontinent Nuna, but the details of its tectonic setting are contentious. This paper documents an outlier of Nonacho Group rocks ∼50 km east of the main Nonacho basin. Field observations and LA-ICPMS (laser ablation inductively coupled plasma mass spectrometry) U-Pb detrital zircon geochronology are integrated with previous studies of the main basin to better understand the group’s depositional history, provenance and tectonic setting. The lithology and detrital zircon age spectra of the outlier allow for its correlation to the upper two formations of the Nonacho Group. CA-ID-TIMS (chemical abrasion isotope dilution thermal ionization mass spectrometry) analyses of two fragments of the youngest detrital zircon provide a maximum depositional age of 1901.0 ± 0.9 Ma. A felsic volcanic cobble dated at ca. 2.38 Ga provides evidence of volcanism during the Arrowsmith orogeny. Detrital zircon dates recovered from the outlier (ca. 3.4–3.0, 2.7, 2.5–2.3 and 2.0–1.9 Ga) are consistent with derivation from topography of the Taltson and/or Thelon orogens on the western margin of the Rae craton. Taltson-Thelon (2.0 to 1.9 Ga) aged detritus is only abundant in the upper two formations of the Nonacho Group, marking a change in provenance from the lower formations. This change in provenance may have coincided with a period of renewed uplift and the unroofing of Taltson-Thelon plutons. The detrital zircon provenance and depositional age of the Nonacho Group is consistent with models that link its deposition to the Taltson and/or Thelon orogens. However, tectonism associated with the 1.9 to 1.8 Ga Snowbird and Trans-Hudson orogens to the east could also have affected basin formation or the change in provenance from the lower to upper Nonacho Group. This study highlights the importance of CA-ID-TIMS in establishing accurate and precise maximum depositional ages for sedimentary successions.

Shale and pyrite Re-Os ages from the Hornby Bay and Amundsen basins provide new chronological markers for Mesoproterozoic stratigraphic successions of northern Canada

Here we present Re-Os geochronological data from two carbonaceous shale units from the Hornby Bay and Amundsen basins that provide important chronological markers for Mesoproterozoic stratigraphic successions within northern Canada. Shale from the basal Escape Rapids Formation yields a robust depositional age of 1067.3 ± 13.5 Ma for the lowermost Shaler Supergroup of Amundsen basin with a remarkably unradiogenic initial 187Os/188Os value (0.15), implying that it derived from weathering of juvenile source material, such as basalt. Sedimentological features, indicative of large basin conditions, suggest deposition within an extensive epeiric sea that was intermittently mixed with waters from an exterior ocean. This age, and the 720 Ma age of the uppermost Shaler Supergroup, constrain deposition of the ∼4 km-thick succession to a period of approximately 350 myr, recording sedimentation through a complete supercontinent cycle - amalgamation through to break-up of Rodinia. Further, it indicates that the Amundsen basin formed at the same time as the Borden, and Fury and Hecla basins on northern Baffin Island, and the Thule basin in northwestern Greenland, and that these basins were perhaps connected as part of a much larger basin situated in the interior of Rodinia. Diagenetic pyrite from sandstone dykelets in carbonaceous shales of the Fort Confidence Formation yielded a Re-Os model age of 1438 ± 8 Ma, providing a minimum age for deposition of the lower Dismal Lakes Group of the Hornby Bay basin. This age strengthens proposed regional correlation of the Hornby Bay basin with the large intracontinental Athabasca and Thelon basins, located on the Rae craton to the southeast, and is evidence for a Laurentia-wide marine flooding episode ca. 1500 Ma. Both new ages place important constraints on newly discovered eukaryotic microfossils from the Shaler Supergroup and Dismal Lakes Group and calibration points for critical chemostratigraphic data.

>2.7 Ga metamorphic peridotites from southeast Greenland record the oxygen isotope composition of Archean seawater

The δ18O value of seawater is presently buffered by high-temperature hydrothermal alteration and low-temperature weathering of lithospheric rocks. It is much debated whether the δ18O of seawater has been buffered to this steady-state value throughout Earth's history or, alternatively, whether it gradually increased towards the present value since the Archean. A third possibility is that the δ18O of seawater was buffered at a higher value before continents emerged, and has been buffered at its current value (δ18O ∼ −1‰) since the emergence of continents. In this contribution, we reconstruct the δ18O of Archean seawater from triple oxygen isotope (δ17O, δ18O) variations in >2.7 Ga ultramafic rocks that reacted with fluids that, in turn, had been derived from seawater in the Archean. The samples that were studied are peridotites from the Ivnartivaq complex, an ultramafic lens in the Archean Kuummiut terrane (Rae craton, southeast Greenland) that have unusually low δ18O values for peridotites (in olivine, 1.7‰≤δ18O≤4.6‰, relative to VSMOW). Bulk rock trace element concentrations, mineral compositions and U-Pb dating of zircon grains in the peridotites indicate that these samples are metamorphic peridotites, that formed by the dehydration of serpentinites ∼2.7 Ga ago. The serpentinite protoliths, in turn, had formed by alteration of ultramafic cumulate rocks at high temperatures (250–450°C) by fluids that were derived from seawater. Triple oxygen isotope variations in olivine from the peridotites indicate that the fluids could not have been derived from seawater with a significantly higher or lower δ18O value than that of seawater in the Phanerozoic, but could have been derived from seawater with a value of δ18O ∼ −1‰, i.e., the modern (ice-free) seawater composition. We conclude that the δ18O of seawater had reached its current steady state value by 2.7 Ga ago.

Localisation of the brittle Bathurst fault on pre-existing fabrics: a case for structural inheritance in the northeastern Slave craton, western Nunavut, Canada

The north–northwest-striking Bathurst fault in the northeastern Slave craton displaced the 1.9 Ga Kilohigok basin and the ca. 2.02–1.96 Ga Thelon tectonic zone, and projects beneath the 1.7 Ga Thelon basin where unconformity-associated uranium deposits are spatially associated with basement faults. Here we investigate the deformation–temperature–time history of the Bathurst fault rocks using structural and microstructural observations paired with U–(Th–)Pb and 40Ar/39Ar geochronology. Highly strained hornblende-bearing granitoid rocks, the predominant rock type on the northeastern side of the Bathurst fault in the study area, show ambiguous sense of shear suggesting flattening by coaxial deformation. Quartz and feldspar microstructures suggest ductile deformation occurred at ≥500 °C. Along the main fault trace, brittle features and hydrothermal alteration overprint the pervasive ductile flattening fabric. In situ U–Th–Pb dating of synkinematic monazite suggests ductile fabric formation at ca. 1933 ± 4 Ma and ca. 1895 ± 11 Ma, and zircon from a cross-cutting dyke constrains the brittle deformation to ≤1839 ± 14 Ma. 40Ar/39Ar dating of fabric-defining minerals yield cooling ages of ca. 1920–1900 Ma and ca. 1900–1850 Ma for hornblende and muscovite, respectively, and a maximum cooling age of ca. 1840 Ma for biotite. We suggest the ca. 1933–1895 Ma ductile flattening fabric developed during orthogonal collision and indentation of the Slave craton into the Thelon tectonic zone and Rae craton. Brittle deformation on the Bathurst fault was localised parallel to the ductile flattening fabric after ca. 1840 Ma and preceded Thelon basin deposition. Brittle deformation features in Bathurst fault rocks preserve evidence for fluid–rock interaction and enhanced basement permeability, suggesting the fault is a possible conduit structure for mineralising fluids.

Interpretation of buried basement in the southwestern Athabasca Basin, Canada, from integrated geophysical and geological datasets

Recent discoveries of basement-hosted uranium deposits in the Patterson Lake corridor in the southwestern Athabasca Basin of Canada have brought vigorous exploration interest to the region. New lithostratigraphic constraints, geochronology and airborne geophysical surveys have dramatically improved the understanding of the host basement geology, warranting a re-examination of the remote predictive mapping and geophysical responses of the buried basement rocks. This study took a two-step approach to examine the regional basement geology and architecture. First, a mosaic of the long-wavelength response of potential field (gravity and magnetic) datasets was examined to divide the basement into regional domains based on bulk physical property variations. The interpretive geological model was then refined using textural and lineament analysis of new airborne gravity and magnetic datasets, geological drill hole logs and magnetic susceptibility measurements. The new basement map identifies and updates major features including a crustal-scale structure that separates the southern Tantato Domain from the newly defined eastern Taltson Domain. This structure may have played a role in localizing fluid flow in the Patterson Lake corridor, defining the spatial extents of structurally controlled buried felsic intrusions, and redefines the boundaries of the Taltson, Clearwater and Tantato Domains. In addition, the potential field enhancements delineated significant regional faults that controlled the geometry of Paleoproterozoic cover sequences and have implications for understanding the crustal architecture of the southern Rae Province. These new interpretations shed light on the tectonic history of the region to support on-going exploration activities and delineate regionally prospective areas in this understudied area of the Canadian Shield. Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

In situ U-Pb geochronology, Lu-Hf and Sm-Nd isotope systematics of the Hoidas Lake REE deposit, northern Saskatchewan, Canada

The diopside-allanite veins and apatite breccia veins of the Hoidas Lake LREE deposit cross-cut the deformed Archaean and Paleoproterozoic granitoid gneisses of the Rae Subprovince, in northern Saskatchewan, Canada. The timing and source of this unusual and complex REE deposit are not well-constrained. Here we report on the results of LA-ICP-MS U-Pb geochronology of zircon and titanite from diopside-allanite veins and green apatite and monazite from apatite breccia veins, Hf isotopic data for zircon, and Sm-Nd isotopic data for titanite, apatite and monazite, in an attempt to constrain the age and duration of crystallization of REE-bearing phases, and provide implications on the source of the mineralization. Geological relationships and in situ U-Pb data indicate that the REE veins formed after peak metamorphism at ca. 1.9 Ga. However, there are zircon crystals in the diopside-allanite veins with concordant U-Pb dates as old as ~2350 Ma, the oldest of which may have been inherited from Arrowsmith Orogen granitoids. Zircon rims show a concordant cluster of U-Pb dates around 1911 Ma, which are interpreted to represent new zircon growth, based on textural features and their distinct initial ɛHf values (26.9 and 34.3), compared to the inherited zircon cores, which range from −9.5 to −2.0. Younger U- Pb Concordia ages of titanite (ca. 1860 Ma and 1830 Ma) and monazite (ca. 1843 Ma), and discordant apatite 207Pb/206Pb dates (ca. 1840 Ma), are considered to be the best constraints on the initiation of REE mineralization. The initial ɛNd values of titanite, green apatite and monazite (−15.2 to −10.7) are comparable to, but lower than the ɛNd values for the Martin Group alkali basalts (Beaverlodge Domain) and the ultrapotassic rocks of the Christopher Island Formation (Baker Lake Basin), both of which also yielded similar U-Pb dates to those of the youngest Hoidas Lake REE-bearing minerals. These regionally occurring alkali units originated from a similar source, most probably an ancient enriched lithospheric mantle reservoir. The melts and fluids, from which the Hoidas Lake deposit formed, then interacted with Rae craton crust as they utilized structures while they were transported from the source region to the depositional site.

Conditions and timing of low-pressure – high-temperature metamorphism in the Montresor Belt, Rae Province, Nunavut

Pressure–temperature–time (P–T–t) estimates for the Montresor Belt, obtained using phase equilibria and geospeedometry modelling integrated with in situ U–Th–Pb monazite geochronology, shed new light on the tectonometamorphic effects of the Snowbird phase of the Trans-Hudson orogeny. Typical metapelitic assemblages of the lower Montresor group consist of white mica, biotite, plagioclase, quartz, and andalusite, which in some rocks is partly or completely pseudomorphed by white mica. The observed assemblages reflect peak P–T conditions centring at approximately 575 °C and 3 kbar. Rocks with high bulk Fe/Mg contents contain compositionally zoned garnet, permitting the addition of further constraints on the conditions of metamorphism in the Montresor Belt: Core compositions of earliest-grown garnets indicate initial garnet crystallization at approximately 535 °C and 2.3 kbar, suggesting a nearly isobaric P–T path of prograde metamorphism with a gradient of approximately 50 °C·kbar–1. Chemical age-dating of monazite inclusions in garnet yields ages of ca. 1870 ± 9 to 1837 ± 9 Ma. Retrograde, pseudomorphic andalusite replacement by white mica at approximately 540 °C is inferred to have been controlled by variations in bulk rock chemistry. Morphologically corroded and chemically heterogeneous monazite adjacent to white mica pseudomorphs suggests that andalusite replacement took place at ca. 1792 ± 10 Ma, possibly associated with extension and movement along the detachment fault separating the upper and lower Montresor groups. Simulations of diffusion across chlorite- and biotite-filled cracks in garnet assumed to be coeval with andalusite replacement suggest that the rocks have experienced the retrograde event for at least 20 My.

Multiple burial–exhumation episodes revealed by accessory phases in high-pressure granulite-facies rocks (Rae craton, Nunavut, Canada)

The Big Lake shear zone (BLsz) is a 60 km long deep crustal structure adjacent to the northern segment of the Snowbird Tectonic zone (STZ: Nunavut, Canada) in a region characterized by intensely sheared high-pressure granulite-facies rocks. The units exposed here provide an exceptional record of the lower crust that preserves crucial evidence bearing on interpretation of superimposed Neoarchean and Paleoproterozoic tectono-metamorphic events. Deformation along the BLsz postdates 2650 Ma, the age of a mylonitized monzogranite, and predates 2190 Ma, the age of cross-cutting MacQuoid mafic dykes. Metamorphic assemblages and P–T modeling suggest crustal thickening in the Neoarchean with peak conditions of 800 °C and 14–15 kbar at ca. 2530 Ma. Mylonitic fabrics developed at ca. 2505 Ma (T ~ 800 °C, P 10 °C/Ma) and partial exhumation post-ca. 2505 Ma followed by re-burial at 1900 Ma, rather than slower isobaric cooling at depth. These results indicate that the long-term residence models proposed by several authors for domains in the central STZ are not applicable to the BLsz region, and are more compatible with a ca. 1.9 Ga collisional setting.

High-grade metamorphism flying under the radar of accessory minerals

Uranium-lead (U-Pb) accessory mineral petrochronology has been increasingly used to constrain the timing of tectonometamorphic events. However, because mafic rocks commonly lack minerals with a high U/Pb ratio, they may be underrepresented in the chronologic record. This study on polymetamorphic mafic granulites from the Archean Rae craton (northern Canada) provides a striking example of a metamorphic cycle that has been entirely overlooked. We utilized Lu-Hf garnet geochronology and equilibrium phase diagram modeling to characterize two high-pressure granulite-facies mineral assemblages that affected the 2.6 Ga protolith. Zircon and garnet recrystallization occurred at 1.87 Ga within a gneissic foliation, while a coarse-grained garnet precursor nucleated 230 m.y. earlier during a stage of high heat flow within thickened lower crust, the latter of which is nearly absent in the zircon and monazite age record except for rare igneous occurrences. Combined garnet geochronology and petrological modeling reinforce a ca. 1.9 Ga age for high-grade overprinting in the southern Rae craton and clearly show within the same sample that U-Pb accessory minerals did not grow during a newly identified 2.11 Ga granulite-facies event.

Early Paleoproterozoic magmatism in the Yangtze Block: Evidence from zircon U-Pb ages, Sr-Nd-Hf isotopes and geochemistry of ca. 2.3 Ga and 2.1 Ga granitic rocks in the Phan Si Pan Complex, north Vietnam

Our understanding of the early evolution of the Yangtze Block is limited by the sparsely dispersed nature of pre-Neoproterozoic exposures. New, integrated petrographic, zircon U-Pb age and Hf-Nd isotope analyses, and whole-rock geochemical data for early Paleoproterozoic granites in the Phan Si Pan Complex provides new insights into the evolution of the Yangtze Block as well as its role in the Pre-Nuna supercontinent. LA-ICP-MS zircon U-Pb dating of magmatic zircons from quartz monzonite and gneissic granite yielded 207Pb/206Pb ages of 2306 ± 12 Ma and 2096 ± 15 Ma, respectively. Zircons from the quartz monzonite have εHf(t) values ranging from −4.1 to −2.1, corresponding to TDM2 model ages of 3002–2890 Ma, whereas zircons in the gneissic granite have εHf(t) values between −0.95 and +1.72 and corresponding TDM2 model ages of 2660–2516 Ma, which are consistent with their whole-rock Nd isotope values. Geochemically, the quartz monzonites are I-type granites. Combined with their relatively high Sr/Y ratios and low Y concentrations, as well as fractionated REE patterns with relatively high LREE but low HREE concentrations, they were probably generated by partial melting of the thickened middle-lower crust under elevated temperature. Geochemical and isotopic signatures suggest that the ca. 2.1 Ga gneissic granites are high-K calc-alkaline, ferroan A-type granites formed by partial melting of juvenile crustal source at high temperature and low pressure with little involvement of ancient crustal material. The Phan Si Pan complex has a distinct early Paleoproterozoic crustal evolution history compared with the other crustal provinces of the Yangtze Block, suggesting independent histories that were not unified until the late Paleoproterozoic during the assembly of Nuna. Moreover, the magmatism and tectonic evolution of the north Vietnam region is broadly similar to that of the Arrowsmith Orogen of the Rae craton in Laurentia suggesting a potential spatial linkage. The geologic record of the Yangtze Block does not support an early Paleoproterozoic shutdown of plate tectonics.

Deformation and extensional exhumation of 1.9 Ga high-pressure granulites along the Wholdaia Lake shear zone, south Rae craton, Northwest Territories, Canada

Deformation and extensional exhumation of 1.9 Ga high-pressure granulites along the Wholdaia Lake shear zone, south Rae craton, Northwest Territories, Canada

Neoarchean convergent margin Ni-Cu mineralization? Axis Lake and Nickel King Ni-Cu deposits in the south Rae craton of the Canadian Shield

The southern portion of the Rae craton in the Canadian Shield is host to multiple ca. 2.63 Ga Ni-Cu-rich metamorphosed mafic intrusions with noritic, noritic-anorthositic and pyroxenitic compositions, including those hosting the undeveloped Axis Lake and Nickel King deposits found in northern Saskatchewan and southeast Northwest Territories.In order to understand the setting of these intrusions, and their concomitant mineralization, we investigated their geochemical and isotopic characteristics. Mineralized intrusions at Axis Lake and Nickel King are metamorphosed up to granulite facies, with local partial melting. The intrusions preserve whole-rock major element geochemistry consistent with a sub-alkaline basaltic composition (e.g., SiO2 = 46–54 wt%; TiO2 < 0.5 wt%). In terms of their trace elements, these intrusions have: LREE to HREE enrichments (La/YbPM = 2–16), low HREE fractionation (Gd/YbPM = 1.0–2.6), well defined negative Nb (Nb/Nb∗ = 0.06–0.31) and Zr (Zr/Zr∗ = 0.24–0.41) and variable negative to positive Ti anomalies (Ti/Ti∗ = 0.25–1.38), and Th/Yb versus Nb/Yb distributions consistent with formation in an arc-like setting.The Nd isotope values of the Axis Lake intrusion (ɛNdt2630 = +0.3 to +2.1) are more juvenile and closer to the depleted mantle value, relative to those of the Nickel King deposit (ɛNdt2630 = −2.4 to −0.4). This agrees with their also distinctive Th/Yb and Nb/Yb distributions suggesting oceanic and continental arc-like settings, respectively. The Axis Lake and Nickel King Nd isotope compositions are indistinguishable from other arc-like 2.61 to 2.58 Ga mafic to felsic intrusions and volcanic rocks recognized in the Rae craton and the neighboring Chesterfield block. The age, spatial distribution and geochemistry of the studied intrusions coincide with the initial stages of a recently defined 2.63–2.60 Ga continental arc, the roots of which are found in the south Rae craton.Our data indicate that Neoarchean intrusions with an arc-like geochemistry may be a significant exploration target in the Rae craton. This broadens possible host suites beyond the established Paleoproterozoic rift-related rocks most commonly considered as the prime target for Ni-Cu-PGE mineralization.

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).

Correction to: U–Pb geochronology of Paleoproterozoic gneiss from southeastern Ellesmere Island: implications for displacement estimates on the Wegener fault

The Wegener fault is a sinistral transform in Nares Strait, which lies between Greenland and Ellesmere Island; it connects the opening of the Eurasian Basin with spreading in Baffin Bay after ≈ 56 Ma (magnetic anomaly 24) and before 36 Ma (magnetic anomaly 13). The amount of horizontal slip on the Wegener fault system can be estimated from the offset of the boundary between the Archean Rae craton to the south and Paleoproterozoic gneisses of the Thelon Tectonic Zone to the north. This boundary is best defined in Greenland due to recent U–Pb zircon geochronology, but is less well known on Ellesmere Island. We have dated zircon in five samples of Precambrian granitic to tonalitic gneiss from southeastern Ellesmere Island using secondary ion mass spectrometry. Cathodoluminescence images reveal complex zircons with oscillatory zoning that lack evidence of xenocrystic cores and are commonly overprinted by recrystallization and surrounded by homogeneous rims. Trace-element patterns show the range of disturbance in zircon. The intrusive age is Paleoproterozoic and ranges from 2007 ± 14 to 1958 ± 14 Ma. In one sample, a well-defined lower intercept age of 357 ± 16 Ma on late overgrowths corresponds to a disturbance during the Ellesmerian orogeny. The results enhance the sparse data set of U–Pb ages in southeast Ellesmere Island and corroborate the previous placement of the boundary with Archean gneiss to the south near Devon Island. The boundary is offset by a minimum of 215 km across Nares Strait, corresponding to the minimum offset on the Wegener fault.

Deep komatiite signature in cratonic mantle pyroxenite

AbstractWe present new and compiled whole‐rock modal mineral, major and trace element data from extremely melt depleted but pyroxenite and garnet(‐ite)‐bearing Palaeoarchean East Greenland cratonic mantle, exposed as three isolated, tectonically strained orogenic peridotite bodies (Ugelvik, Raudhaugene and Midsundvatnet) in western Norway. The studied lithologies comprise besides spinel‐ and/or garnet‐bearing peridotite (dunite, harzburgite, lherzolite) garnet‐clinopyroxenite and partially olivine‐bearing garnet‐orthopyroxenite and ‐websterite. Chemical and modal data and spatial relationships between different rock types suggest deformation to have triggered mechanical mixing of garnet‐free dunite with garnet‐bearing enclosures that formed garnet‐peridotite. Inclusions of olivine in porphyroclastic minerals of pyroxenite show a primary origin of olivine in olivine‐bearing variants. Major element oxide abundances and ratios of websterite differ to those in rocks expected to form by reaction of peridotite with basaltic melts or silica‐rich fluids, but resemble those of Archean Al‐enriched komatiite (AEK) flows from Barberton and Commondale greenstone belts, South Africa. Websterite GdN/YbN, 0.49–0.65 (olivine‐free) and 0.73–0.85 (olivine‐bearing), overlaps that of two subgroups of AEK, GdN/YbN 0.25–0.55 and 0.77–0.90, with each of them being nearly indistinguishable from one another in not only rare earth element fractionation but also concentration. Websterite MgO content is high, 22.7–29.0 wt%, and Zr/Y is very low, 0.1–1.0. The other, non‐websteritic pyroxenites overlap—when mechanically mixed together with garnetite—in chemistry with that of AEK. It follows an origin of websterite and likely all pyroxenite that involves melting of a garnet‐bearing depleted mantle source. Pyroxene exsolution lamellae in the inferred solidus garnet in all lithological varieties require the pyroxenites to have crystallized in the majorite garnet stability field, at 3–4 GPa (90–120 km depth) at minimum 1,600°C. Consequently, we interpret the websterites to represent the first recognized deep plutonic crystallization products that formed from komatiite melts. The other pyroxenitic rocks are likely fragments of such crystallization products. An implication is that a mantle plume environment contributed to the formation of (one of) the worldwide oldest lithospheric mantle underneath the eastern Rae craton.