Hearne Craton Research Articles

Circa 1.9 Ga Rae‐Hearne collision from Lu–Hf garnet chronology in eclogites

AbstractCanada's northern cratons record a complex poly‐metamorphic evolution in response to multiple orogenic events. The Snowbird Tectonic Zone (STZ), the principal suture between the Archean Rae and Hearne cratons, is a highly debated structure; it is argued to have accommodated modern‐style accretion in either the Neoarchean (2.6–2.5 Ga) or Palaeoproterozoic (ca. 1.9 Ga). To resolve this controversy, we use Lu–Hf garnet geochronology to date an eclogite collected from the East Athabasca mylonite triangle of the STZ. Garnet shows prograde trace‐element zoning, and we obtained an age of 1914.5 ± 3.4 Ma. The age is interpreted to date the timing of collision related to deep burial of crust along the STZ. This strongly supports Palaeoproterozoic terminal collision between the Rae and Hearne cratons, and that to constrain high‐pressure metamorphism during supercontinent assembly, dating eclogite itself is key.

Constraints on the evolution of the eastern margin of the Hearne craton: New data from the southwestern Rottenstone Domain, Trans-Hudson Orogen, Saskatchewan

The Rottenstone Domain (RD) is a complex lithotectonic supracrustal belt of the Paleoproterozoic Trans-Hudson Orogen in Saskatchewan, Canada, located between the Archean Hearne craton and the juvenile Paleoproterozoic Reindeer Zone (RZ). Its evolution and crustal affinity have been under considerable debate in the past few decades, with most interpretations considering it to be part of RZ, but some hinting that certain elements might have affinities to the Archean hinterland. On the basis of bedrock mapping, U-Pb, Sm-Nd and lithogeochemical data, the rocks of the southwestern RD were subdivided into several lithodemic entities, which were used to reconstruct a plate tectonic framework for the western part of the Trans-Hudson Orogen during Orosirian time.A U-Pb zircon crystallization age of 2518 ± 6 Ma and depleted mantle model age (TDM) of 2.8 Ga for an augen granite confirmed that the western part of RD is part of the Hearne craton. A newly discovered structural inlier at the eastern boundary of RD consists of a central unit of 2045 ± 9 Ma augen granite, with TDM ages between 2.8 and 2.6 Ga, that was intruded into gneissic granite with a TDM of 3.3 Ga. The augen granite was interpreted as representing a rift-related granite, emplaced into Archean rocks of the Hearne craton. Two lithotectonic assemblages, one dominated by feldspathic quartzite and psammite the other by migmatitic psammite-psammopelite, are in structural contact with the granitic inliers and have model ages of 2.6–2.4 Ga. One of the feldspathic quartzite samples contains detrital zircon age modes at 1910 and 1860 Ma, and lesser Archean modes. Both assemblages were likely deposited in a forearc basin to the 1.865–1.850 Ga Wathaman continental arc.Recognition of Archean components and metasedimentary assemblages with older model ages than those in the RZ to the east, suggest that this southern part of RD is more closely affiliated with the Hearne craton. The eastern edge of RD coincides with a mylonite zone (Howard Lake shear zone), which likely represents a fundamental, although multiply reactivated, structural break separating Archean granitoids and structurally overlying metasedimentary rocks from rocks of the more juvenile Reindeer Zone.

Regional geology and tectonic framework of the Southern Indian domain, Trans-Hudson orogen, Manitoba

The collaborative federal–provincial Southern Indian Lake project in north-central Manitoba covered an area of more than 3500 km2 of the Trans-Hudson orogen. Regional-scale geological mapping, sampling, and lithogeochemical, isotopic, and geochronological studies resulted in the identification of distinct assemblages of supracrustal rocks and varied episodes of plutonism. A granodiorite gneiss dated at ca. 2520 Ma is interpreted to represent the basement of the Southern Indian domain and is considered a separate crustal domain, named the Partridge Breast block. The Churchill River assemblage is composed of juvenile pillow basalt with intervening clastic sedimentary rocks, possibly a reflection of plume magmatism related to initial rifting of the Hearne craton margin. The Pukatawakan Bay assemblage consists mainly of massive to pillowed, juvenile metabasaltic rocks and associated basinal metasedimentary rocks. The Partridge Breast Lake assemblage is dominated by continental-arc volcanic and volcaniclastic rocks associated with basinal metasedimentary rocks. The Strawberry Island assemblage, consisting of arenite and polymictic conglomerate, is interpreted to have been deposited in a foreland-basin basin or intra-orogen pull-apart basin environment. The Whyme Bay assemblage is characterized by fluvial-alluvial orogenic sediments and is temporally linked to the Sickle Group rocks in the Lynn Lake greenstone belt. Granitoid rocks, dominantly monzogranite and granodiorite, range in age from ca. 1890 to 1830 Ma and occur throughout the Southern Indian domain, and intermediate and mafic intrusions of similar ages are also present. In this paper, we integrate these new data into a tectonic framework for the Southern Indian domain of the Trans-Hudson orogen in Manitoba.

A Palaeoproterozoic diamond-bearing lithospheric mantle root beneath the Archean Sask Craton, Canada

The recently recognised Sask Craton, a small terrane with Archean (3.3–2.5 Ga) crustal ages, is enclosed in the Paleoproterozoic (1.9–1.8 Ga) Trans Hudson Orogen (THO). Only limited research has been conducted on this craton, yet it hosts major diamond deposits within the Cretaceous (~106 to ~95 Ma) Fort à la Corne (FALC) Kimberlite Field. This study describes major, trace and platinum group element data, as well as osmium isotopic data from peridotitic mantle xenoliths (n = 26) from the Star and Orion South kimberlites.The garnet-bearing lithospheric mantle is dominated by moderately depleted lherzolite. Equilibration pressures and temperatures (2.7 to 5.5 GPa and 840 to 1250 °C) for these garnet peridotites define a cool geotherm indicative of a 210 km thick lithosphere, similar to other cratons worldwide. Many of the peridotite xenoliths show the major and trace element signatures of carbonatitic and kimberlitic melt metasomatism. The Re–Os isotopic data yield TRD (time of Re-depletion) model ages, which provide minimum estimates for the timing of melt depletion, ranging from 2.4 to 0.3 Ga, with a main mode spanning from 2.4 to 1.7 Ga. No Archean ages were recorded. This finding and the complex nature of events affecting this terrane from the Archean through the Palaeoproterozoic provide evidence that the majority of the lithospheric mantle was depleted and stabilised in the Palaeoproterozoic, significantly later than the Archean crust. The timing of the dominant lithosphere formation is linked to rifting (~2.2 Ga - 2.0 Ga), and subsequent collision (1.9–1.8 Ga) of the Superior and Hearne craton during the Wilson cycle of the Trans Hudson Orogen.

New insights on the Orosirian carbon cycle, early Cyanobacteria, and the assembly of Laurentia from the Paleoproterozoic Belcher Group

The Orosirian Period (2050–1800 Ma) of the Paleoproterozoic Era represents an interval between the Great Oxidation Event and Lomagundi-Jatuli Excursion, and the apparent environmental stability of the late Paleoproterozoic to mid-Mesoproterozoic (1800–1300 Ma). Here, we present stratigraphic data, carbon isotope chemostratigraphy, element abundances, and U–Pb zircon depositional ages from the Belcher Group in subarctic Canada, providing an unparallelled global reference section for the Orosirian Period. U–Pb dates of 2018.5 ± 1.0 Ma and 2015.4 ± 1.8 Ma for two tuffs in the Kasegalik Formation (lowest Belcher Group) provide the first robust age constraints for Earth's earliest unambiguous cyanobacterial fossil, Eoentophysalis belcherensis. A tuffaceous shale ∼4 km stratigraphically higher, at the contact of the Flaherty and Omarolluk formations (1854.2 ± 1.6 Ma), provides an age for foreland basin development associated with the collision of the Superior and Hearne cratons. Carbonate carbon isotope data show smooth fluctuations ranging from −2.3 to +3.6‰, reaching a maximum near ca. 1.88 Ga. This trend toward heavier carbon isotope values is interpreted as the result of global orogenesis during Nuna assembly that elevated nutrient supply and sedimentation rates, leading to an increase in the proportion of organic carbon burial relative to carbonate carbon burial.

Extending the realm of Archean crust in the Great Falls tectonic zone: Evidence from the Little Rocky Mountains, Montana

Two prominent features separate the Archean Wyoming and Hearne cratons: the Paleoproterozoic Great Falls tectonic zone (GFTZ) and the Medicine Hat block (MHB), neither of which is well defined spatially because of Phanerozoic sedimentary cover. Based on limited data, the MHB is thought to be a structurally complex mix of Archean (2.6–3.1 Ga) and Proterozoic (1.75 Ga) crust, but is recognized primarily by its geophysical signature, and its influence on the geochemistry of younger igneous rocks. Similarly, the GFTZ was recognized on the basis of broad differences in geophysical patterns, isopachs of Paleozoic sedimentary sections, and lineaments; however, juvenile arc rocks in the Little Belt Mountains (LBM) and strongly overprinted Archean rocks in southwestern Montana show it to be a dominantly Paleoproterozoic feature. The Little Rocky Mountains (LRM) of Montana provide access to exposures of the northeastern-most Precambrian crust in the MHB-GFTZ region. U/Pb ages of zircons from Precambrian rocks of the LRM range from 2.4 to 3.3 Ga, with most ages between 2.6 and 2.8 Ga. Whole-rock analyses yield Sm-Nd TDM from 3.1 to 4.0 Ga and initial εNd(T) values calculated at U-Pb zircon crystallization ages range from −0.9 to −10.5, indicating significant contributions from older Archean crust. The high proportion of 2.6–2.8 Ga U/Pb ages differentiates LRM crust from arc-related Paleoproterozoic magmatic rocks exposed in the LBM to the southwest. The age and isotopic composition of the LRM gneisses are similar to crust in the northern Wyoming Province (2.8–2.9 Ga), but Paleoproterozoic K-Ar cooling ages suggest crust in the LRM experienced the Paleoproterozoic metamorphism and deformation that characterizes the GFTZ. Consequently, its history differs markedly from the adjacent Beartooth-Bighorn magmatic zone of the northern Wyoming Province, which does not record Paleoproterozoic tectonism, but has a strong correlation with the Montana metasedimentary terrane that was strongly overprinted during the Paleoproterozoic Great Falls orogeny that defines the GFTZ. The LRM, therefore, likely provides a unique, and perhaps the only, opportunity to characterize Archean crust of the MHB.

Timing of sediment-hosted Cu-Ag mineralization in the Trans-Hudson orogen at Janice Lake, Wollaston Domain, Saskatchewan, Canada

The Janice Lake Cu-Ag mineralization in the Wollaston Domain of northern Saskatchewan is hosted by a metasedimentary sequence in the upper part of the Wollaston Supergroup of the Trans-Hudson orogen. The Wollaston Supergroup was deposited between ~ 2070 and 1865 Ma in a foreland basin setting constructed over Archean basement of the Hearne craton. The Trans-Hudson orogen underwent final collision and peak metamorphism at ~ 1810 Ma, during consolidation of Laurentia and its amalgamation with the Columbia supercontinent. Titanite is a common constituent of the post-peak metamorphic assemblages of Trans-Hudson lithotectonic units and accompanied disseminated sediment-hosted Cu sulfide mineralization at Janice Lake. Titanite crystals, intergrown with chalcocite over a strike-length of ~ 2 km of Cu-bearing stratigraphy, were dated by the ID-TIMS and LA-ICP-MS U-Pb methods, returning an age range from 1780 to 1760 Ma and a weighted average age of 1775 ± 10 Ma. The titanite ages effectively date the associated chalcocite-dominated sediment-hosted Cu-Ag mineralization and its formation during initial post-orogenic uplift and cooling, ~ 30 myr after peak metamorphism. The age-range and tectonic setting of the Janice Lake mineralization confirms that sediment-hosted Cu mineralization was an integral part of the metallogenic endowment of Columbia and that its emplacement coincided with the continental-scale Trans-Hudson orogeny rather than with diagenesis and extensional basin development ~ 100 myr earlier.

Characterising the southern part of the Hearne Province: A forgotten part of Canada’s shield revisited

The Hearne craton comprises the southeastern part of the western Churchill craton. The central part of the craton has been well studied in Nunavut and northeastern Saskatchewan in comparison with the southern part of the craton, which is mainly exposed south of the Athabasca Basin in Saskatchewan. Mapping and analytical results indicate that the southern Hearne craton is dominated by a felsic orthogneiss complex derived mainly from tonalites with crystallisation ages in the range 2.81–2.78 Ga and ca. 2.73 Ga. Sm-Nd isotopic analysis suggests that these evolved plutons (mean ƐNd = −3.3) were derived from a Mesoarchean to Paleoarchean source. Other components of the orthogneisses include leucotonalite intrusions dated at ca. 2.664 Ga and regionally extensive K-feldspar phyric granites emplaced between 2.63 and 2.60 Ga. Both of these suites have Sm-Nd isotopic signatures similar to the older tonalites. A suite of granodiorite and quartz diorite, one of which yielded a crystallisation age of 2.654 Ga, has unique Sm-Nd isotopic results with ƐNd values near CHUR. The felsic orthogneiss complex is unconformably overlain by the Black Birch Lake assemblage, which contains dominantly orthquartzites, mafic volcanic rocks and pelites. Detrital zircon signatures in the orthoquartzites are dominated by Archean sources and indicate that the supracrustal rocks were deposited after 2.60 Ga.The central Hearne Province extends into northeastern corner of Saskatchewan. It contains oceanic mafic volcanic rocks of the Ennadai Group that were intruded by juvenile tonalites at 2.71–2.70 Ga. The age and isotopic character of these tonalites contrasts markedly with those in the felsic orthogneiss complex of the southern Hearne Province. A comparison of plutonic suites and supracrustal assemblages of the central and southern Hearne Provinces suggests that there is heterogeneity between the two entities. Analysis of geophysical data, coupled with existing geological maps, suggest that an important tectonic boundary separates them. The current age relationships in the central and southern parts of the Hearne province imply that the two were juxtaposed after 2.60 Ga; however, the time of suturing remains equivocal.

Evidence for Neoarchean Ni-Cu-bearing mafic intrusions along a major lithospheric structure: A case study from the south Rae craton (Canada)

Deformed and metamorphosed mafic to ultramafic intrusive rocks are common along and adjacent to the Snowbird Tectonic Zone, a major Paleoproterozoic structure separating the Rae and Hearne cratons. On the Rae side of this major lithospheric structure, we investigated a series of mafic bodies that intrude high-grade paragneiss and are associated with Ni–Cu ± PGE mineralization. By linking field observations, geochronology and metamorphic petrology, our results indicate Archean (∼2.63 Ga) crystallization ages for these mafic intrusions over a minimum along-strike extent of ∼500 km and a later high-temperature (∼820 °C and ∼7.5 kbar), Paleoproterozoic metamorphic overprint at t < 1.94 Ga. The youngest reproducible ages of detrital zircon from the host paragneiss indicate a maximum depositional age of ∼2.73 Ga for the paragneiss. These results indicate that the sedimentary rocks hosting the mineralized mafic intrusions are older than the mixed siliciclastic and carbonate sedimentary units previously described along the Snowbird Tectonic Zone (<2.07 Ga). Furthermore, in two localities (Thye and Yotin-Shagory lakes, in Northwest Territories and northern Saskatchewan), zircon from both the mafic and pelitic samples exhibit an unusual within-run fluctuation in the lead isotope signal during SHRIMP analysis, suggesting inhomogeneous redistribution of radiogenic Pb within the crystal lattice. Redistribution is interpreted to have occurred during the ∼1.94–1.90 Ga tectonometamorphic event. This behaviour, documented only in ∼10 (Ultra)High-Temperature localities worldwide, has not been observed in the other two studied localities (Currie and Axis lakes in northern Saskatchewan), suggesting either i) a different metamorphic history or ii) different re-crystallization processes occurring during the Paleoproterozoic high-temperature evolution (or a combination of the two). Our data indicate that (U)HT metamorphism and the absence of a fluid/melt phase are not a requirement for lead redistribution in zircon.

Geology, U–Pb zircon geochronology, and geochemistry of PGE-bearing Neoarchean and Paleoproterozoic gabbroic rocks of the Peter Lake domain, southern Hearne craton, Canada

The Peter Lake domain, a component of the Hearne craton in Saskatchewan, includes several intrusive complexes ranging in age from 2.70 to 1.83 Ga. Two gabbroic complexes (Swan River and Porter Bay) together represent one of the largest accumulations of Precambrian mafic intrusion, both metamorphosed at amphibolite facies. Sulfide-poor PGE occurrences are found in both complexes and share many textural and lithological characteristics with magmatic contact-type PGE–(Ni–Cu) deposits such as the chaotic Lac des Iles pluton and with layered stratiform deposits of large layered intrusions such as Bushveld or Stillwater. Lithologically, both complexes are dominated by leucocratic gabbronorite and gabbro, locally characterized by cumulate layering, cross-bedding, brecciated, and pegmatitic textures. U–Pb zircon crystallization ages between 2562 and 2560 Ma were obtained for the Swan River complex. sulfide formation is interpreted to be of magmatic origin rather than remobilized, as had been speculated by exploration geologists, and is therefore of the same age as the host gabbro. Geochemical data support the interpretation of a mantle plume origin in a subduction environment for the Swan River complex. The Porter Bay complex is much more restricted in areal extent and a leucocratic gabbronorite yielded a U–Pb zircon age of 1913 ± 1 Ma, which is interpreted as the crystallization age and the time of sulfide formation. Major and trace element geochemical data of Porter Bay complex rocks show a considerably more evolved character than the Swan River complex, and is interpreted as indicating emplacement in a continental arc environment.

Finite-frequency P-wave tomography of the Western Canada Sedimentary Basin: Implications for the lithospheric evolution in Western Laurentia

The lithosphere beneath the Western Canada Sedimentary Basin has potentially undergone Precambrian subduction and collisional orogenesis, resulting in a complex network of crustal domains. To improve the understanding of its evolutionary history, we combine data from the USArray and three regional networks to invert for P-wave velocities of the upper mantle using finite-frequency tomography. Our model reveals distinct, vertically continuous high (>1%) velocity perturbations at depths above 200km beneath the Precambrian Buffalo Head Terrane, Hearne craton and Medicine Hat Block, which sharply contrasts with those beneath the Canadian Rockies (<−1%) at comparable depths. The P velocity increases from −0.5% above 70km depth to 1.5% at 330km depth beneath southern Alberta, which provides compelling evidence for a deep, structurally complex Hearne craton. In comparison, the lithosphere is substantially thinner beneath the adjacent Buffalo Head Terrane (160km) and Medicine Hat Block (200km). These findings are consistent with earlier theories of tectonic assembly in this region, which featured distinct Archean and Proterozoic plate convergences between the Hearne craton and its neighboring domains. The highly variable, bimodally distributed craton thicknesses may also reflect different lithospheric destruction processes beneath the western margin of Laurentia.

Age and geological setting of the Rankin Inlet greenstone belt and its relationship to the gold endowment of the Meliadine gold district, Nunavut, Canada

Archean greenstone belts host a significant proportion of the world's gold, typically in deposits that formed late during greenstone belt formation and cratonization. However, this is not always the case and, in the multiply reworked western Churchill Province (wCP), orogenic lode gold deposition post-dates greenstone belt formation by nearly one billion years. The spatial link between Proterozoic gold and Archean greenstone belts in the wCP is thus particularly striking although its significance is still not fully understood. The Meliadine gold district (2.8Moz contained Au in reserves, plus an indicated and inferred resources of 5.8Moz Au) represents an important example of this deposit style and is hosted within the Rankin Inlet greenstone belt (RIGB), which occupies a critical, but controversial position along the largely inferred boundary between the Hearne craton and the Chesterfield block.RIGB felsic volcanic rocks (ca. 2.66Ga) are structurally intercalated, and broadly coeval, with mafic volcanic and volcaniclastic rocks (2.66–2.64Ga), turbidite (≤2.66–2.64Ga), argillite, auriferous banded iron formation successions and syn-volcanic granodioritic to tonalitic intrusions (2.67–2.64Ga). Neoarchean basaltic to andesitic volcanic rocks possess calc-alkaline to primitive arc-like tholeiitic magmatic affinities along with lesser MORB-like basaltic compositions. Geochemically evolved lavas yield depleted 144Nd/143Nd ratios (ɛNd2.66Ga=−1.1 to +1.6) that reflect variable interaction with an evolved and hitherto undocumented Meso- to Neoarchean basement underlying the RIGB, whereas transitional, arc-like primitive tholeiitic and MORB-like basaltic samples overlap with the Nd isotopic composition of depleted mantle at ca. 2.66Ga (ɛNd2.66Ga=+1.6 to +2.7). These Neoarchean volcano-sedimentary panels represent the main auriferous rock package within the Meliadine gold district and are intercalated with deformed Paleoproterozoic conglomerate (≤2.50 and ≤2.155Ga). The latter are, in turn, unconformably overlain by a geochemically distinct pillowed-basalt sequence and a unique carbonate-siliciclastic package that presumably represent the remnants of Paleoproterozoic basins and are not known to host gold. The geological setting of gold deposits thus likely reflects this favourable Neoarchean lithostratigraphy in addition to metamorphism and fluid focusing along the reactivated faults during the collision of the Hearne and combined Chesterfield block-Rae craton at 1.90–1.85Ga.

The effect of a fatty acid-based diet on NK cells of sub-fertile women

The Paleoproterozoic (ca. 1900 Ma) Lynn Lake greenstone belt of northern Manitoba, Canada, has been previously characterized as comprising a series of tectonically juxtaposed intra-oceanic-derived metavolcanic rocks. The results of more recent local and regional studies, however, support a significant contribution of continental crust during formation of the metasedimentary, metavolcanic, and intrusive igneous rocks that comprise the majority of the Lynn Lake greenstone belt. The tectonic model previously proposed for the Lynn Lake greenstone belt, however, did not consider the geodynamics of the Lynn Lake greenstone belt in the context of all available data. In this study, we report the results of outcrop mapping and petrographic analysis, as well as major, minor, and trace element geochemical analyses for 54 samples from the Northern terrane, and integrate and compare the results with data from previously published studies. These data are used to recharacterize the metavolcanic rocks and to develop a new geodynamic model for the formation of the Lynn Lake greenstone belt.Ultramafic to intermediate rocks in the vicinity of the MacLellan Au–Ag deposit are characterized primarily by E-MORB-like trace element characteristics and Th–Nb–La systematics, which are interpreted to be the result of a primary, plume-derived melt interacting with continental lithosphere at a thinned (i.e., rifted) continental margin. Similarly, the majority of the mafic to intermediate rocks that comprise the Lynn Lake greenstone belt are characterized by flat to E-MORB-like trace element patterns and Th–Nb–La systematics, which are consistent with mantle plume-derived, contaminated, oceanic continental rift or rifted margin setting rocks. This study suggests that the metavolcanic rocks of the Lynn Lake greenstone belt were derived via rifting between the Superior and Hearne Cratons, which resulted in the formation and growth of the Manikewan Ocean. Alternatively, the metavolcanic rocks could have been derived from an upwelling plume generated from a back-arc-style subduction zone that formed between the Rae and Hearne Cratons during the closure of the Snowbird Ocean, which resulted in rifting of the passive Hearne margin.

Geochemistry of the metavolcanic rocks in the vicinity of the MacLellan Au–Ag deposit and an evaluation of the tectonic setting of the Lynn Lake greenstone belt, Canada: Evidence for a Paleoproterozoic-aged rifted continental margin

The Paleoproterozoic (ca. 1900Ma) Lynn Lake greenstone belt of northern Manitoba, Canada, has been previously characterized as comprising a series of tectonically juxtaposed intra-oceanic-derived metavolcanic rocks. The results of more recent local and regional studies, however, support a significant contribution of continental crust during formation of the metasedimentary, metavolcanic, and intrusive igneous rocks that comprise the majority of the Lynn Lake greenstone belt. The tectonic model previously proposed for the Lynn Lake greenstone belt, however, did not consider the geodynamics of the Lynn Lake greenstone belt in the context of all available data. In this study, we report the results of outcrop mapping and petrographic analysis, as well as major, minor, and trace element geochemical analyses for 54 samples from the Northern terrane, and integrate and compare the results with data from previously published studies. These data are used to recharacterize the metavolcanic rocks and to develop a new geodynamic model for the formation of the Lynn Lake greenstone belt.Ultramafic to intermediate rocks in the vicinity of the MacLellan Au–Ag deposit are characterized primarily by E-MORB-like trace element characteristics and Th–Nb–La systematics, which are interpreted to be the result of a primary, plume-derived melt interacting with continental lithosphere at a thinned (i.e., rifted) continental margin. Similarly, the majority of the mafic to intermediate rocks that comprise the Lynn Lake greenstone belt are characterized by flat to E-MORB-like trace element patterns and Th–Nb–La systematics, which are consistent with mantle plume-derived, contaminated, oceanic continental rift or rifted margin setting rocks. This study suggests that the metavolcanic rocks of the Lynn Lake greenstone belt were derived via rifting between the Superior and Hearne Cratons, which resulted in the formation and growth of the Manikewan Ocean. Alternatively, the metavolcanic rocks could have been derived from an upwelling plume generated from a back-arc-style subduction zone that formed between the Rae and Hearne Cratons during the closure of the Snowbird Ocean, which resulted in rifting of the passive Hearne margin.

Two Neoarchean supercontinents revisited: The case for a Rae family of cratons

A variety of early Paleoproterozoic supercontinents or supercratons have been proposed, including Kenorland, Superia and Vaalbara. Improved geologic knowledge of the Churchill Province, Canadian shield, suggests that the classical Kenorland configuration, with linked Rae and Hearne cratons, is invalid and represents a younger ca. 1.9Ga Nuna configuration. We review the early Paleoproterozoic tectonic history of the Rae craton and propose that the ca. 2.55–2.50Ga MacQuoid and 2.50–2.28Ga Arrowsmith orogens represent critical piercing points for supercontinent reconstruction. Utilizing the StratDB and DateView databases, we have identified nearly two dozen regions worldwide that show effects of these orogenic events, including the Rae, Gawler–Mawson, peninsular India, West Africa, Amazonia, North China, and Sask crustal blocks. We propose that these cratonic blocks form a distinct Rae family lineage. Utilizing new time-space plots for the Rae family and components of the Superia and Vaalbara supercratons, we demonstrate that the Rae family lacks the distinctive early Paleoproterozoic depositional and glaciogenic record characteristic of Superia and Vaalbara, but has an important contemporaneous record of juvenile crust formation and contractional orogenesis. We propose the collective history of the Rae family is best reconciled if it represents the now dispersed blocks of a third earliest Paleoproterozoic supercraton that we term Nunavutia. In this model the magmatic and tectonometamorphic record of these cratons includes an early 2.55–2.45Ga period of interior collisional orogenesis in which the supercraton amalgamated, and a later phase of long-lived peripheral orogenesis (2.45–2.28Ga) prior to its partial break-up at ca. 2.05Ga. Forming within an otherwise globally quiescent period, the history of the Rae family is important for assessment of constrasting models of early Paleoproterozoic geodynamic activity.

Structural, petrological and U–Pb SHRIMP geochronological study of the western Beaverlodge domain: Implications for crustal architecture, multi-stage orogenesis and the extent of the Taltson orogen in the SW Rae craton, Canadian Shield

The Beaverlodge domain is a polydeformed Precambrian terrane in the southwestern Rae craton whose rocks variably record the effects of Archean tectonism and four major Paleoproterozoic tectonic events: the 2.5–2.3Ga Arrowsmith orogeny, the 1.99–1.93Ga Taltson orogeny, the ca. 1.90Ga Snowbird orogeny and the ca. 1.87–1.80Ga Hudsonian orogeny. Structural–metamorphic analysis and in situ U–Pb SHRIMP zircon and monazite geochronology reported herein demonstrate that this domain consists of two main structural levels variably exposed due to complex interference folding: a lower level comprising mainly high-grade Archean (ca. 3.0 and 2.7–2.6Ga) metaplutonic and metasedimentary gneisses and a higher level comprising mainly ca. 2.33 to <2.17Ga supracrustal rocks of the Murmac Bay Group at more variable metamorphic grade. Whereas the older package underwent thermal and/or metamorphic events in the Archean (ca. 2.57Ga) and early Paleoproterozoic (ca. 2.34Ga; Arrowsmith orogeny), rocks at higher structural levels bear no record of these events, recording only younger (1.94–1.90Ga) metamorphism interpreted to reflect two further stages of orogenesis. The first stage, correlated with Taltson orogeny, involved tight to isoclinal folding (D1/2) that led to development of the composite ESE-trending (S0/S1/S2) transposition foliation followed by more open folding (D3). The absence of Taltson-age plutonic rocks, except for low-volume crustal melts, suggests that this cycle was driven by crustal thickening, consistent with a clockwise P–T–t path derived for one sample. The high degree of strain along the contact between Archean rocks and the Murmac Bay Group, along with local structural discordance, indicates some degree of translation of cover rocks over basement during this stage. Pelitic rocks at higher structural levels (low- to medium-grade zone) underwent prograde metamorphism to middle amphibolite facies whereas rocks at deeper structural levels (high-grade zone) attained upper amphibolite facies conditions. Monazite data bracket prograde metamorphism (and related garnet growth) between 1939±6Ma and 1934±5Ma. The second stage, dated most precisely at 1906±7Ma, involved refolding of D1 to D3 structures about NE-trending axes (F4) and coeval dextral shearing and is correlated with Snowbird orogenesis. Rocks of the low- to medium-grade zone were held at roughly the same crustal level whereas rocks in the high-grade zone were uplifted via dextral–oblique displacement along domain-bounding shear zones, instigating decompression (garnet to cordierite) reactions. This study demonstrates that after <2.17Ga deposition of the upper Murmac Bay Group, rocks along the entire west-southwestern Rae margin, from the Northwest Territories/northern Alberta to the Snowbird tectonic zone, were affected by Taltson (1940–1930Ma) orogenesis. The Taltson tectono-metamorphic cycle then appears to have transitioned, as rocks underwent further shortening and uplift in a dextral transpressive regime, into Snowbird tectonism at ca. 1910–1900Ma. Snowbird orogenesis is therefore best viewed as part of a semi-continuous 1940–1900Ma cycle of tectonic burial and exhumation, with its inception at ca. 1910Ma triggered by a change in boundary conditions, as yet not fully understood, but consistent with arrival of the Hearne craton proposed by others. While the Beaverlodge domain has many features in common with the Tantato domain, future work needs to explore why rocks in the Tantato domain lack evidence for Arrowsmith orogeny and failed to record the cycle of 1.94–1.93Ga tectonic burial.

The tectonometamorphic evolution of Southampton Island, Nunavut: Insight from petrologic modeling and in situ SHRIMP geochronology of multiple episodes of monazite growth

Petrologic modeling and in situ SHRIMP monazite geochronology reveal a complex, polycyclic tectonometamorphic history for Southampton Island, Nunavut, that provides further insights into the assembly of the Nuna supercontinent. Although early, cryptic metamorphic events at ca. 2.6–2.5 and 2.3Ga are indicated by monazite ages in an Archean metasedimentary sample, Paleoproterozoic tectonometamorphism is widespread and most profound. Monazite inclusions in garnet from two samples define a lower amphibolite- to upper amphibolite-facies M1–D1 event between 1883±9 and 1879±7Ma. These sampled rocks, as well as pre-1900Ma magmatic rocks, occur on the west side of the island's exposed Precambrian bedrock, outboard of the Hearne craton but on strike with its geophysically defined eastern edge beneath Hudson Bay. This timing and geographic distribution, as well as the orientation of S1 foliations, are consistent with the model of a ca. 1900Ma Rae–Hearne collision, with M1–D1 on Southampton Island reflecting reworking of the Rae upper plate following northeast-directed subduction.Textural variations among monazite dated in two samples tightly bracket the initiation of M2–D2 between 1868±5Ma and 1861±4Ma. M2–D2 monazite crystallized at 1851±5Ma in one sample with a more calcic composition interpreted to have shifted the allanite to monazite reaction to higher temperatures relative to less calcic rocks. Clockwise P–T paths in these recumbently folded high-grade gneisses support a collisional setting, with the timing of M2–D2 consistent with age constraints on Baffin Island for the accretion of Meta Incognita microcontinent to the southeast flank of the Rae craton. The Rae crust of Southampton Island is interpreted as the upper plate during this collision on the basis of the ∼10Myr earlier deformation on Southampton Island compared to the interpreted lower plate (Meta Incognita microcontinent), as well as textural observations in some samples that M2 metamorphism predated D2 deformation. Advective heat transfer would account for the relatively short time inferred between ca. 1870Ma collision and ca. 1868–1861Ma monazite growth during prograde metamorphism.Equant matrix monazite that overgrew the S2 foliation in one sample documents post-D2 metamorphism in the northern part of exposed basement at 1841±4Ma. Four samples from the south record post-D2 monazite growth between 1826±9 and 1815±7Ma. These ages highlight a thermal culmination associated with extensive crustal melting and ca. 1830–1820Ma plutonism attributed to a convergent setting culminating with ca. 1820Ma arrival of the Superior craton. Exhumation and cooling is dated by ca. 1790–1780Ma monazite associated with garnet breakdown.

The Paleoproterozoic Kaminak dykes, Hearne craton, western Churchill Province, Nunavut, Canada: Preliminary constraints on their age and petrogenesis

The NNW- to NNE-trending Kaminak diabase dykes, exposed in the Hearne craton of the western Churchill Province, Canada, are components of a Neoarchean–Paleoproterozoic LIP that forms an important pin in plate tectonic reconstructions impacting models for the assembly of the Nuna supercontinent. This study provides the first detailed information on the age, geochemistry and isotopic composition of these dykes.A precise U–Pb baddeleyite date of 2497.6±1.1Ma, obtained for one dyke sample, provides the best estimate for the timing of Kaminak mafic magmatism. The dykes are tholeiitic basalts to basaltic andesites with consistent compositions over an areal extent of ∼10,000km2. Major and compatible trace element variations indicate that the dykes were not in equilibrium with anhydrous peridotitic mantle and evolved through fractionation of Pl+Cpx±Ol from parental melts in the deep crust. All dykes record remarkably similar, incompatible element enriched, closely parallel multi-element profiles with variable HFSE troughs, comparable to Phanerozoic low-TiO2 continental tholeiites. Time-corrected Nd isotopic compositions overlap, or are slightly below bulk Earth (εNd2500 Ma=+0.4 to −0.8), and yield TDM values (2.75–2.87Ga) comparable to those of igneous crust in the Hearne craton. Implied short residence times, along with the geological, geochemical and isotopic data, are interpreted to imply that parental magmas of the dykes were low-TiO2 tholeiites generated via contamination of primary, depleted N-MORB-like magmas by low-degree, calc-alkaline lamprophyre melts in the continental lithospheric mantle. The melts homogenized in chambers near the Moho and underwent extensive fractional crystallization (Pl+Cpx±Ol), and possibly minor assimilation of lower crust, prior to injection into the upper crust. Paucity of evidence for contemporaneous lithospheric rifting, homogeneous lithogeochemical and Nd isotopic compositions, and petrochemical variations distinct from the Matachewan and Mistassini swarms of the Superior Province suggest the Kaminak dykes are the magmatic products of a Late Archean–Early Paleoproterozoic mantle thermal anomaly that extended, but did not break apart the Hearne craton. The results of this study do not support direct correlation of the Kaminak and Matachewan or Mistassini dyke swarms. Further evaluation of the proposed linkage of the Hearne and Superior cratons prior to Nuna supercontinent assembly requires additional geochronological, lithogeochemical and radiogenic isotopic constraints.

Seismic Architecture of the Archean North American Mantle and Its Relationship to Diamondiferous Kimberlite Fields

The seismic architecture of the average Archean-only mantle beneath exposed and subsurface Archean crust of the United States and Canada is presented here in three dimensions for the first time, using a high lateral resolution Rayleigh wave phase velocity model of the upper mantle (30- to 250-km depth). The morphology of the cratonic coherent mantle is compared with other regional and local geophysical models, geologic interpretations, and published xenolith barometric studies. In particular, the kimberlite magma source regions at the Lithosphere-Asthenosphere Boundary (LAB) inferred from xenolith data are consistent with the bottom topography of the Archean seismic mantle signature. The characteristic fast seismic response found beneath much of the exposed Archean crust is also found in Canada beneath some covered terranes, sedimentary basins, and Proterozoic mobile belts. The northeastern and northwestern parts of the Superior craton host, with the central Hearne craton, the deepest mantle roots of North America (225- to 240-km depth). However, the southern portion of the Superior craton is characterized by an east-west channel that is 30 percent slower in seismic velocity than its northern counterpart. This contrasting seismic signature correlates with the location of the southernmost Neoarchean greenstone belts and to their plume-driven subduction zones. The scar in the mantle produced by this early tectonothermal event has been reused by widespread and sporadic carbonatite and kimberlite magmatic events spanning from the Early Proterozoic to the Cretaceous, and as a consequence, the diamond stability field has been partially to totally overprinted. Almost all diamondiferous kimberlites in Canada are located vertically over an interval of 160- to 200-km depth in areas of steep slopes surrounding deep (180–240 km), relatively small, and flat-bottomed Paleo-Mesoarchean cratonic keels.

Timing of Paleoproterozoic granitoid magmatism along the northwestern Superior Province margin: implications for the tectonic evolution of the Thompson Nickel BeltThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

The U–Pb geochronology of three granitoid plutons and three granitic pegmatite dykes, largely from the Thompson Nickel Belt located along the northwestern Superior craton margin, was investigated to place constraints on the timing of felsic magmatism associated with closure of the Manikewan Ocean and final continent–continent collision to form the Trans-Hudson Orogen. These data indicate that 1840–1820 Ma granite magmatism along the Superior margin was more active than previously thought and that some magmatism extended beyond the Thompson Nickel Belt sensu stricto, including the 1836 ± 3 Ma Mystery Lake granodiorite, 1822 ± 5 Ma Wintering Lake granodiorite, and the 1825 ± 8 Ma Fox Lake granite located in the Split Lake Block. Granitic pegmatites within the Thompson Nickel Belt were emplaced late in the collisional history in the period 1.79–1.75 Ga and include a 1770 ± 2 Ma dyke exposed at the Thompson pit, a 1767 ± 6 Ma dyke at the Pipe Pit, and a 1786 ± 2 Ma dyke located at Paint Lake. The final stage of crustal amalgamation in the eastern Trans-Hudson Orogen involved Superior Province crustal thickening and partial melting forming 1.84–1.82 Ga granite magmas and then final collision at ∼1.8 Ga between the Superior Province and a continental block to the west consisting of the previously amalgamated Sask and Hearne cratons. Heating of the Superior craton margin and granitic magmatism continued past peak metamorphism (1790–1750 Ma); this thermal event is represented by the emplacement of numerous late pegmatite dykes and evidenced by cooling dates recorded by metamorphic minerals (e.g., titanite) in reworked Archean gneisses and Proterozoic intrusions.