Trans-Hudson Orogen Research Articles

On the Statherian–Calymmian palaeogeography of northwestern Laurentia

The c. 1.75 to 1.27 Ga Hornby Bay intracontinental basin, exposed in northwestern Canada, NE of Great Bear Lake, includes the Big Bear, Mountain Lake and Dismal Lakes groups. The Big Bear group comprises mainly immature clastic rocks deposited by high-energy rivers in restricted fault-bounded basins formed during the late-stage assembly of supercontinent Nuna. Detrital zircon analysis of the Big Bear group suggests provenance from a combination of local sources including recycling of sedimentary rocks, which were originally derived from sources further east, such as the Thelon orogen. The overlying Mountain Lake group was deposited by westerly flowing rivers over a much broader region, with less evidence for syndepositional faulting, suggesting thermal subsidence. It displays provenance via recycling of strata from the proximal Coronation Supergroup and the more distal and correlative Goulburn Supergroup of Kilohigok Basin. Following tectonic uplift and erosion, basal clastic rocks of the Dismal Lakes Group were deposited in fluvial and then shallow-marine to paralic environments. Detrital zircon from these strata displays prominent late Archean peaks demonstrating provenance from adjacent rocks of the Slave craton and recycling of the Mountain Lake group. Detrital zircon geochronology of sandstones from the Hornby Bay Basin supports stratigraphic correlation with the Wernecke Supergroup in the Wernecke Mountains and the Muskwa assemblage, located further southward along the ancient western margin of Laurentia. Our data, along with Nd and C isotopes from corresponding shale and carbonate units, provide robust evidence for a palaeogeographical model whereby these successions represent the terrestrial and marine components of a west-facing, passive-margin clastic wedge, considered the westward extension of an Amazon-scale, braided fluvial system that originated in the foreland of the Trans-Hudson orogen, to the east. Elements of this system have been identified from inliers in northeastern Australia, which was attached to northwestern Laurentia prior to the break-up of supercontinent Nuna. Supplementary material: U–Pb and Hf results, and Hf isotopic compositions of reference zircons are available at https://doi.org/10.6084/m9.figshare.c.5980940

Crustal structure and subsidence mechanisms of the Williston Basin: New constraints from receiver function imaging

Mechanisms responsible for the long-term subsidence of intracontinental basins such as the Williston Basin in North America remain enigmatic, partly due to the thick sedimentary layer commonly found in the basins that prevents reliably imaging the deep crustal and upper mantle structures using some of the most-commonly employed seismic methods such as receiver function analysis. In this study, we used receiver functions recorded by 274 USArray and other stations in the Williston Basin and adjacent areas to investigate the layered structure of the crust in the hydrocarbon-rich intracontinental basin. After the removal of strong reverberations on the receiver functions associated with a low-velocity sedimentary layer using a recently developed time-domain deconvolution approach, two positive arrivals representing downward increases of seismic velocities are imaged beneath the basin and the area to the west. The top interface has a depth of about 40 km at the depocenter of the basin, and gradually shallows eastward to about 30 km beneath the Superior Craton, and the deeper interface has a mean depth of about 65 km beneath the Williston Basin. The layer between the two interfaces may represent an eclogitized or garnet-rich lower crustal layer. The results are consistent with the hypothesis that continuous retrograde metamorphic reactions in the previously-thickened lower crust during the Paleoproterozoic Trans-Hudson Orogeny resulted in the subsidence of the intracontinental Williston Basin.

A lithospheric-scale Arrowsmith (2.4 Ga) detachment system with major Trans-Hudson (1.8 Ga) reactivation documented in the Howard Lake shear zone, Rae craton, Canada

This study characterizes the polyphase tectonometamorphic history of a major shear system in the Rae craton, the Howard Lake shear zone (HLsz), which exhumed high-grade rocks in the distant hinterland of the Paleoproterozoic Trans-Hudson Orogen. Prior work established a Mesoarchean mantle model-age discrepancy in basement rocks across the HLsz, which we interpret to have contributed a fundamental lithospheric rheology contrast that allowed for HLsz localization. Our work shows that the HLsz has a long-lived history starting in the early Paleoproterozoic as it abruptly separates 2.36 Ga granulite-facies metamorphic units to the east from 2.4 Ga greenschist-facies metamorphic basement rocks to the west. Subsequent reactivation during Trans-Hudson Orogen time between 1.86 and 1.82 Ga accommodated re-burial and re-exhumation of high-grade rocks in its footwall coeval with Sask craton collision. Strong penetrative overprinting deformation in metaigneous rocks and a newly discovered < 2.04 Ga schist preserve evidence for right-lateral transpression within the HLsz at ca. 1.82 Ga, which is coincident with the terminal collision of the Superior craton. Our work demonstrates that the HLsz is one of several major crustal-scale anisotropies in the south Rae craton and it is broadly analogous to major detachment fault systems documented in numerous Phanerozoic orogenic systems. Furthermore, the polycyclic deformation history of the HLsz, along which regional burial and exhumation was strongly localized, demonstrates the considerable influence crustal scale anisotropies can have on the evolution of craton architecture such as that presently found in the south Rae craton of the Canadian Shield.

Understanding the Paleoproterozoic Circum-Superior Large Igneous Province constrains the thermal properties of Earth’s mantle through time

Basalts and komatiites in the ∼1880 Ma Paleoproterozoic Circum-Superior Large Igneous Province are associated in space and time with the closure/subduction of the Manikewan ocean and formation of the Trans-Hudson Orogen during the construction of Laurentia and the supercontinent of Nuna/Columbia. In one model, the igneous rocks formed by the melting of ambient mantle during contemporaneous closure of the Manikewan ocean and rifting of the margins of the Superior craton. In another model melting took place in a mantle plume. Resolution to this problem can help in the understanding of the role of LIPS in supercontinent assembly or breakup over the last 2 billion years of Earth history. It can also constrain the thermal properties of ambient mantle and mantle plumes over the last 3.5 billion years. The contrasting hypotheses were tested by petrological modeling of a large database of basalts and komatiites from the Circum-Superior Large Igneous Province. Results reveal limited variability in mantle potential temperature, from 1540 to 1570 °C, over ∼12 to 15 million years in time and over 1500 km in space from the Cape Smith Belt in the north to the Winnipegosis Belt in the southwest. The essential problem with the mantle plume model for the Circum-Superior LIP is that it predicts thermal heterogeneity, in contrast with the evidence presented here; there is no Phanerozoic mantle plume analog. The limited long wavelength variation in TP is characteristic of ambient mantle magmatism, evidence supporting the plate tectonic model for the Circum-Superior LIP. Results are consistent with ambient mantle thermal models characterized by sluggish mantle convection in the past and with a present-day Urey ratio of about 0.38 (Korenaga, J. 2008. Urey ratio and the structure and evolution of Earth's mantle. Rev. Geophys. 46, RG2007). In contrast, most high MgO komatiites in Paleoproterozoic and Archean greenstone belts melted from sources that were ∼100–200 °C higher. This temperature excess is similar to 100–250 °C for most Phanerozoic mantle plumes, demonstrating approximate constancy of thermal properties of mantle plume structures through time.

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.

Crustal structure around the margins of the eastern Superior craton, Canada, from receiver function analysis

The Precambrian domains of eastern Canada preserve a remarkable record of continental crust formation and evolution spanning at least 3 billion years, and thus present an excellent opportunity to investigate the imprints of early tectonic processes on the present-day continental crust. Archean domains in the region include the eastern Superior craton, part of one of the largest cratons on Earth, the western portion of the North Atlantic craton, and a collage of continental fragments. The Paleoproterozoic Trans-Hudson Orogen and its eastern branches played a fundamental role in the assembly of Laurentia. In the southeast, a succession of orogenic events culminated in the Mesoproterozoic Grenville Orogen and assembly of supercontinent Rodinia. To better understand crustal formation and evolution in this complex region, we use teleseismic P-wave receiver functions recorded by a network of broadband seismographs distributed across Precambrian eastern Canada. At each station, back-azimuthal variations in receiver function waveforms indicate significant lateral crustal heterogeneity, leading us to model different representative directions separately. The stations situated on Archean terranes present a relatively simple crustal structure with a well-defined Moho at ∼33–46 km depth. In contrast, those on Proterozoic domains show a more complex structure, higher velocities in the lower crust and a deeper, more diffuse Moho at ∼46–55 km depth. Across the entire region, bulk crustal composition is largely felsic to intermediate (Vp/Vs ∼1.66–1.76), except for a station situated in an area dominated by anorthosite massifs whose composition is significantly more mafic (Vp/Vs ∼1.85). We combine our new models with those from previous natural- and active-source seismic studies of eastern Canada, and compare our results to those from other Precambrian regions. We suggest that the present-day crustal structure of Precambrian eastern Canada is more strongly influenced by the tectonic processes that assembled Laurentia than secular variations in initial crustal formation.

Deep crustal structure and Palaeoproterozoic evolution of the supercontinent Lauroscandia: 3D model of Trans-Hudson Orogen and new insight based on LITHOPROBE, FIRE and 1-EU reflection seismic and HuBLE tomography imaging

The Palaeoproterozoic Lauroscandia supercontinent received fundamental geological and geodynamic support based on the 3D characteristics of the deep structure of key tectonic units in the North American and East European megacratons. The deep structure of the western Reindeer zone of Trans-Hudson orogen (THO) most closely resembles the structure of sedimentary–volcanic belts along the eastern boundary of the Karelia craton formed during continental rifting. These belts were accompanied by vast depressions filled by the thick epicontinental volcano-sedimentary sequences, which were later affected together with the Archaean basement by high-temperature metamorphism. Some of the rifts could later be transformed into narrow short-lived oceans. The deep structure of the eastern Reindeer zone of THO is similar to Svecofennian accretionary orogen: structures of the crust and crust-mantle boundary point to a significant breakup of the continent, the origin of an ocean, and its later closure, involving a number of subduction zones, which operated almost simultaneously. The Sask craton should be viewed as a fragment of a Neoarchaean megacraton of North America. The main feature of the THO, Svecofennian Accretionary Orogen and Lauroscandia as a whole during Palaeoproterozoic evolution is the causal link with development of the Hudson Bay superplume that initialized a succession of huge events of plate-tectonic type. The head of the Hudson Bay superplume reached about 3000 km in the north-south direction and from 3000 to 4500 km in the east-west direction at different stages of evolution; 4500 km likely is a minimal estimate of the greatest dimension. The width of the Manikewan ocean reached more 1000 km, the width of the Svecofennian–Pre-Labradorian Ocean of ~2000 km that apparently also corresponds to a minimal value. The most significant temporal intervals in the Lauroscandia Palaeoproterozoic evolution (~2.5 Ga and 2.2–1.8 Ga ago) coincide with events of global rank.

When Wyoming Became Superior: Oblique Convergence Along the Southern Trans‐Hudson Orogen

AbstractThe Trans‐Hudson orogen (THO) is one of the best‐preserved Proterozoic orogens on Earth, largely unaffected by subsequent tectonism, yet its southern extent lies concealed beneath the North American Central Plains. A new 3D resistivity model over the southern orogen is developed and interpreted alongside borehole, potential field, and seismic reflection data. We present the first synoptic crustal view of the southern THO and a new tectonic model of the orogen. Our model reveals high‐conductivity belts marking paleo‐subduction zones while the orogen center consists of deeply exhumed relatively dense and mostly magnetic juvenile crust preserved between the deformed margins of the Wyoming and Superior cratons. Complex structure along the western margin suggests convergence began with oblique subduction and the northward transport of severed fragments of the Wyoming Province. High‐conductivity belts are in places offset from upper‐crustal geophysical boundaries, consistent with the thrusting of accreted arcs over the Archean margins during terminal closure.

Thermotectonic events recorded by U-Pb geochronology and Zr-in-rutile thermometry of Ti oxides in basement rocks along the P2 fault, eastern Athabasca Basin, Saskatchewan, Canada

Abstract Basement rocks below the Athabasca Basin, Saskatchewan, have been intensely altered through paleoweathering and multiple hydrothermal events, including the formation of world-class unconformity-type uranium deposits. Here, we demonstrate the utility of Ti-oxide thermochronology for identifying thermotectonic events in these altered rocks leading to uranium mineralization along basement structures. Rutile grains along the P2 fault, a major fault in the eastern Athabasca Basin, exhibit 207Pb/206Pb ages of ca. 1850–1700 Ma, with a weighted mean of 1757 ± 6 Ma (mean square of weighted deviation [MSWD] = 1.4, n = 116). The older ages (&amp;gt;1770 Ma) record regional metamorphism reaching a temperature of 875 °C during the Trans-Hudson orogeny. Pb diffusion modeling indicates that metamorphic rutile should exhibit cooling ages of 1760–1750 Ma. Rutile grains showing young ages, &amp;lt;1750 Ma, reflect isotopic resetting during regional asthenospheric upwelling between 1770 and 1730 Ma related to the emplacement of the Kivalliq igneous suite to the north. This thermotectonic event (temperature &amp;gt; 550 °C) promoted hydrothermal activity to produce silicified rocks, i.e., “quartzite,” along the P2 fault, which later focused mineralizing fluids for unconformity-type uranium deposits. The young rutile ages also indicate that the basement rocks remained hot until 1700 Ma, providing the maximum age for the deposition of the Athabasca sediments. Anatase yields a concordia age of 1569 ± 31 Ma (MSWD = 0.30, n = 5), which is within uncertainty of the oldest ages for uraninite of the McArthur River deposit. This age corresponds to the incursion of basinal fluids in the basement along the P2 fault during uranium mineralization.

Long-lived anatexis in the exhumed middle crust of the Torngat Orogen: Constraints from phase equilibria modeling and garnet, zircon, and monazite geochronology

Middle and lower orogenic crust exposed at the Earth's surface represent an exceptional opportunity to investigate accretionary and collisional processes in various tectonic settings. In particular, deciphering their metamorphic history in terms of evolving P-T-t-D paths enables an assessment of lower crustal behavior and the role of granulitic terranes throughout orogeny. In this paper, we use field work, average P-T thermobarometry, phase equilibria modeling, and zircon and monazite UPb dating coupled with detailed LuHf and SmNd garnet dating to investigate the style, the timing, the duration, the extent, and the significance of metamorphism in the lower plate of the Torngat Orogen, a transpressional orogen that is part of the larger-scale Paleoproterozoic Trans-Hudson Orogen in Canada. Supracrustal rocks in the region record clockwise P-T paths with peak conditions gradually increasing from 8.7 kbar at 815 °C to 11 kbar at 915 °C from west to east, and homogeneous near-solidus retrograde conditions at ~6–8 kbar and ~ 775–850 °C. LuHf garnet geochronology indicates that prograde metamorphism was ongoing at c. 1885 Ma, marking the onset of crustal thickening earlier than previously thought. Zircon and monazite UPb ages suggest protracted growth during melt crystallization along the retrograde metamorphic path between c. 1859 and 1811 Ma. The minimum duration of anatexis is estimated to be ~50 Myr. We show that the metamorphic effect of the Torngat orogeny is wider than previously thought, resulting in a large mid-pressure, high-temperature domain with widespread anatexis. Our dataset provides an example of the utility of multi-geochronometer dating for deciphering the metamorphic history of granulitic terranes.

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.

Author Correction: Record of modern-style plate tectonics in the Palaeoproterozoic Trans-Hudson orogen

Author Correction: Record of modern-style plate tectonics in the Palaeoproterozoic Trans-Hudson orogen

Geology and crystallization conditions of the Särkiniemiintrusion and related nickel-copper ore, central Finland – implications for depth of emplacement of 1.88 Ga nickel-bearing intrusions

Several Ni-Cu deposits occur within the Kotalahti area, central Finland, in proximity to an Archaean gneiss dome surrounded by a Palaeoproterozoic craton-margin supracrustal sequence comprising quartzites, limestones, calc-silicate rocks, black schists and banded diopside amphibolites. The geology of the area and age of the Ni-bearing intrusions (1.88 Ga) are similar to the Thompson Ni belt in the Canadian Trans-Hudson Orogen. The small mafic-ultramafic and Ni-Cu -bearing Särkiniemi intrusion, closely associated with the Archaean basement core of the Kotalahti Dome, is composed of a western peridotite and eastern gabbro body, both of which are mineralized. The eastern gabbro has a contact aureole several meters thick, consisting of orthopyroxene +/- cordierite bearing hornfels between the intrusion and the migmatites. Geochemically, the Särkiniemi intrusion shares many features in common with other Svecofennian mafic-ultramafic intrusions, including crustal contamination and nickel depletion. The related Ni-Cu deposit has a low Ni/Co value (15) and low nickel content in the sulphide fraction (2.8 wt.%), together with a low estimated magma/sulphide ratio of around 170. Svecofennian 1.88 Ga mafic-ultramafic intrusions occur in terrains of variable metamorphic grade (from low-amphibolite to granulite facies) and are likely to represent emplacement at different crustal depths. Multi-equilibrium thermobarometry indicates that the contact aureole at Särkiniemi reached equilibrium at pressures of 4.5–6 kbar (15–20 km depth) and temperatures of 600–670 °C. Combined with the results of earlier research on the Svecofennian intrusions, this study indicates that a depth of 15–20 km crustal level was favourable, along with other critical factors, for nickel sulfide deposition at 1.88 Ga.

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.

A reinterpretation of the Snow Lake gold camp, Trans-Hudson Orogen, Canada: the use of cleavages as markers to correlate structures across deformed terranes

The Snow Lake gold camp is located within amphibolite facies volcanic rocks of the ca. 1.88–1.87 Ga Flin Flon – Glennie Complex (FFGC) in the Trans-Hudson Orogen, Manitoba. During thrusting and collision with the Archean Sask craton, volcanic rocks were interleaved with turbidites of the ca. 1.855–1.84 Ga Burntwood Group and sandstone and conglomerate of the ca. 1.845–1.835 Ga Missi Group. The main cleavage in the turbidites was previously attributed to thrusting and used as a marker for correlating structures across the camp. A re-examination of this cleavage suggests that it overprints the thrust faults and formed during later collision between the FFGC and the Archean Superior craton. This has important implications as it further suggests that (1) previously unrecognized, early brittle thrust faults repeat volcanic stratigraphy and may have created the boundary conditions that enabled the formation of ductile thrust faults, fold nappes, and mega sheath folds; (2) shear sense indicators along ductile thrust faults formed during their reactivation as sinistral shear zones rather than during thrusting; and (3) peak metamorphic conditions were caused by thrusting and stacking during collision with the Sask craton but were attained later during collision with the Superior craton due to the time lag between orogenesis and the re-equilibration of regional isotherms. Results from this study may be applicable to other complexly deformed terranes where the dominant regional cleavage differs in expression in mixed volcanic and sedimentary successions and has been used as a marker for correlating structures.

Metamorphic and structural evolution of the Flin Flon – Athapapuskow Lake area, west-central Manitoba

The Flin Flon – Athapapuskow Lake area, situated in the Flin Flon Greenstone Belt, Manitoba, consists of ocean-floor and island-arc assemblages, deformed and metamorphosed during the Trans-Hudson Orogeny (∼1.86–1.69 Ga). A new map of metamorphic mineral assemblages and isograds has been compiled that reveals a largely coherent regional metamorphic sequence increasing in metamorphic grade from prehnite–pumpellyite to amphibolite facies. Regional metamorphism postdates most of the deformation within the area, with the exception of the reactivation of major block-bounding faults. The regional prograde sequence has been subdivided into 10 metamorphic zones, separated by 9 isograds, that describe the transition from prehnite–pumpellyite to greenschist to amphibolite facies. The formation of contact metamorphic aureoles, pre-dating regional metamorphism, record conditions up to amphibolite facies. Equilibrium phase diagrams for the island-arc (low-Mg) and ocean-floor (high-Mg) assemblages were calculated and allow for the evaluation of the modelling techniques and determination of pressure–temperature conditions. Discrepancies between the modelling predictions and natural observations occur due to (1) limitations in the thermodynamic models for some of the complex minerals (e.g., amphibole); and (2) metastable persistence of some minerals to higher grade due to sluggish reaction kinetics. Notwithstanding these discrepancies, the modelling suggests that metamorphosed mafic rocks in the Flin Flon – Athapapuskow Lake area reached about 430–480 °C and 3.0–4.5 kbar. Peak metamorphic conditions within contact aureoles that preceded regional metamorphism did not exceed 500 °C (at a pressure between 2.7 and 4.4 kbar). The metamorphic field gradient records a transition from 250–300 °C/1.5–2.3 kbar to 430–480 °C/3–4.5 kbar (100–150 °C/kbar), defining a geothermal gradient of approximately 25–31 °C/km.

Crustal structure of the metasedimentary Kisseynew domain and bounding volcanic–plutonic domains, Trans-Hudson orogen, Canada

The Kisseynew domain (KD) is the largest component of the Paleoproterozoic Trans-Hudson orogen (THO) in Saskatchewan and Manitoba. It is bounded to the north by the Lynn Lake – Leaf Rapids (LL-LR) domain and to the south by the Glennie – Flin Flon (G-FF) complex. The THO was the focus of one of the study areas of Lithoprobe, Canada’s national Earth science research project (1984–2005). To further investigate the crustal structure of the KD and its bounding domains, this study reprocesses reflection line S3a across its northern boundary, analyses four 2.5-D gravity profiles, carries out 3-D gravity inversions for two areas, and replicates results from reflection lines 7 and 10 across the southern boundary of the KD. The reprocessed seismic section enhances the continuity of reflections within the crust. The reflectivity is representative of the complex tectonic development of the boundary zone and clearly identifies a subsurface deformation zone consistent with the boundary. The reflection section also shows that a lower plate (at about 30 to 50 km depth), interpreted as remnant lower crust of the G-FF complex, extends 30 km further northward than in the original section. Lines 7 and 10 illustrate the complex nature of the transition from the KD to the G-FF complex. The gravity analyses show that the variability and complexities of the boundary region between the LL-LR domain and KD, and the G-FF complex and KD, as indicated by the geological and Bouguer gravity maps, extend at depth throughout the crust.

The Medicine Hat Block and the Early Paleoproterozoic Assembly of Western Laurentia

The accretion of the Wyoming, Hearne, and Superior Provinces to form the Archean core of western Laurentia occurred rapidly in the Paleoproterozoic. Missing from Hoffman’s (1988) original rapid aggregation model was the Medicine Hat block (MHB). The MHB is a structurally distinct, complex block of Precambrian crystalline crust located between the Archean Wyoming Craton and the Archean Hearne Province and overlain by an extensive Phanerozoic cover. It is distinguished on the basis of geophysical evidence and limited geochemical data from crustal xenoliths and drill core. New U-Pb ages and Lu-Hf data from zircons reveal protolith crystallization ages from 2.50 to 3.28 Ga, magmatism/metamorphism at 1.76 to 1.81 Ga, and εHfT values from −23.3 to 8.5 in the Archean and Proterozoic rocks of the MHB. These data suggest that the MHB played a pivotal role in the complex assembly of western Laurentia in the Paleoproterozoic as a conjugate or extension to the Montana Metasedimentary Terrane (MMT) of the northwestern Wyoming Province. This MMT–MHB connection likely existed in the Mesoarchean, but it was broken sometime during the earliest Paleoproterozoic with the formation and closure of a small ocean basin. Closure of the ocean led to formation of the Little Belt arc along the southern margin of the MHB beginning at approximately 1.9 Ga. The MHB and MMT re-joined at this time as they amalgamated into the supercontinent Laurentia during the Great Falls orogeny (1.7–1.9 Ga), which formed the Great Falls tectonic zone (GFTZ). The GFTZ developed in the same timeframe as the better-known Trans-Hudson orogen to the east that marks the merger of the Wyoming, Hearne, and Superior Provinces, which along with the MHB, formed the Archean core of western Laurentia.

Petrological and geochemical comparison of the Eisler and Laonil Lake Intrusive Complexes, Glennie Domain, northern Saskatchewan

The Seabee Gold Operation, located in northern Saskatchewan, ~125 km northeast of La Ronge, has been in continuous operation since 1991. The host rocks for the gold mineralization are the volcanic and plutonic rocks of the Pine Lake greenstone belt, which represent a volcanic island arc terrane that was accreted onto proto-North America and was metamorphosed during the Trans-Hudson orogen ca. 1.8 billion years ago. An older volcanic package (“A”) is separated from a younger volcanic package (“B”) by a regional unconformity. The Laonil Lake Intrusive Complex, which hosts the Seabee gold deposit, and the Eisler Intrusive Complex are thought to be part of Assemblage A, based on an imprecise age determination of the Laonil Lake Intrusive Complex and by initial comparative observation of the Laonil and Eisler Intrusive Complexes. This study aims to better understand the age and tectonic setting of the Laonil Lake Intrusive Complex and its relationship to the nearby Eisler Intrusive Complex—which might represent the same igneous event, and therefore be of interest as an exploration target. Methods to test their correlation include whole rock geochemistry, petrography, Sm-Nd isotopes, and U-Pb geochronology. Preliminary modal mineralogy of samples was used to derive rock names (using pre-metamorphic names); the Laonil Lake Intrusive Complex samples are quartz gabbro and tonalite, whereas the Eisler Intrusive Complex samples are tonalite, quartz gabbro, gabbro, and pyroxenite. The preliminary age determinations of the Laonil Lake and Eisler Intrusive Complexes are ca. 1877 Ma and 1884 Ma, respectively. The geochemical signatures of all samples suggest derivation in a volcanic arc setting. The samples exhibit a slight LREE enrichment and epsilon Nd values (+3.61 to +4.65) that suggest a depleted mantle source for the magmas. The ages of Laonil Lake and Eisler Intrusive Complexes suggest that both are part of Assemblage A. The results of this research will provide constraints on the correlation between the Pine Lake greenstone belt and other volcanic belts in the Trans-Hudson orogeny.

Contrasting P-T-t paths reveal a metamorphic discontinuity in the New Quebec Orogen: Insights into Paleoproterozoic orogenic processes

The large scale Paleoproterozoic Trans-Hudson Orogen exposed in Canada is recognized to record one of the first complete Wilson cycle. Although it is considered as a prototype of modern accretionary orogen, the strain pattern, thermal state, and architecture in its wedge are still poorly constrained. Part of it, the accretionary New Quebec Orogen (NQO, 1.82–1.77 Ga) resulted from oblique collision between the Superior Craton lower plate and a composite upper plate including the Kuujjuaq Domain (KD) and the Core Zone, and exposes supracrustal sequences of the Rachel-Laporte Zone (RLZ), a continental forearc basin that evolved in a collisional foreland basin upon collision. We performed an integrated analysis, using petrography, phase equilibria modelling and Lu-Hf garnet, and U-Pb zircon, monazite and rutile chronology on supracrustal rocks in the central segment of the New Quebec Orogen. On the one hand, the RLZ experienced a tight clockwise metamorphic evolution, with a peak at 650 °C/0.7 GPa. Prograde garnet and monazite growth is estimated at 1804 ± 8 Ma and 1796 ± 4 Ma respectively. On the other hand, the metamorphic peak for restitic granulites from the KD is estimated at 790 °C/0.72 GPa and follows isothermal decompression. Geochronology yields ages of 1836 ± 5 Ma for prograde garnet growth, and 1807 ± 4 Ma for crystallization of zircon from anatectic melt, which was followed by rapid cooling recorded by rutile at 1798 ± 16 Ma. We propose that the Lac Turcotte Fault that separates both domains has acted as a tectonometamorphic discontinuity that limits contrasting P-T-t paths. We suggest sequential burial of supracrustal sequences along an apparently continuous Barrovian sequence, with progressive recycling of inner zones in the foreland basin and subsequent involvement of the latter in the orogenic wedge.