Hottah Terrane Research Articles

A Paleoproterozoic Andean-type iron oxide copper-gold environment, the Great Bear magmatic zone, Northwest Canada

Iron oxide copper-gold (IOCG) and associated iron-oxide apatite (IOA) styles of metallic mineralization are recognized throughout the Paleoproterozoic Great Bear magmatic zone of the northwest Canadian Shield. The Great Bear magmatic zone was constructed between ca. 1876 and 1855Ma on top of the older Hottah terrane, which preserves continental arc magmatism that began around ca. 2.0 to 1.97Ga and continued between ca. 1.93 and 1.89Ga. The Great Bear represents the final stages of ca. 150 million years of intermittent and pulsed magmatism related to an evolving continental orogenic belt. The preserved geology supports a dramatic geodynamic change in the subduction zone process at ca. 1875Ma, a key driving mechanism for magma and metal mobilization, and was rapidly followed by a large-scale introduction of felsic-intermediate plutons. The overall tectonic setting is partially constrained from new and previously published geochemical data that show that the volcanic and plutonic rocks are high-K calc-alkaline to shoshonitic in nature (e.g., high K2O, Th/Yb, and Ce/P205). They also have suprasubduction-zone geochemical signatures, including primitive mantle normalized positive Th and negative Nb, P, and Ti anomalies. The data support the primary melts were derived from a GLOSS-modified mantle wedge. Three-dimensional rendering of geophysical datasets suggest that two (of four) preserved surfaces within the upper mantle lithosphere, at 70 to 120km depths, represent frozen, subducted oceanic slabs, and likely were the drivers for the bulk of Hottah and Great Bear arc magmatism. The older slab is northwest-striking and dips 12° to 15° northeast, whereas the younger is deeper and north-striking, dipping 13° east. The geometry of the surfaces are comparable with 4D modeling, where a subduction zone is temporarily shut down due to plateau collision, and then steps oceanward and re-initiates; there is no need for polarity reversal of the subduction system. This new geometry and the related inferences about process should be the focus of future research in the region, but for the time-being it can be stated that these subduction and collisional processes were the first order control on lithospheric evolution, and therefore metallic mineralization. Overall, the Great Bear magmatic zone IOCG and related mineralization is not comparable to other Proterozoic IOCG belts, such as those in Australia. However, the complexity of mineralization styles, the spatial-temporal relationship between IOA and IOCG mineralization, the suprasubduction zone environment, and a major change in tectonic regime are features similar to Andean-type IOCG mineralization, as well as Cordilleran alkali porphyry Cu-Au deposits. This further establishes the linkages between subduction zone processes and IOCG formation, as well as relationships in the IOCG-porphyry deposit continuum model.

Chronostratigraphy of the Hottah terrane and Great Bear magmatic zone of Wopmay Orogen, Canada, and exploration of a terrane translation model

The Paleoproterozoic Hottah terrane is the westernmost exposed bedrock of the Canadian Shield and a critical component for understanding the evolution of the Wopmay Orogen. Thirteen new high-precision U–Pb zircon crystallization ages are presented and support field observations of a volcano-plutonic continuum from Hottah terrane through to the end of the Great Bear magmatism, from >1950 to 1850 Ma. The new crystallization ages, new geochemical data, and newly published detrital zircon U–Pb data are used to challenge hitherto accepted models for the evolution of the Hottah terrane as an exotic arc and microcontinent that arrived over a west-dipping subduction zone and collided with the Slave craton at ca. 1.88 Ga. Although the Hottah terrane does have a tectonic history that is distinct from that of the neighbouring Slave craton, it shares a temporal history with a number of domains to the south and east — domains that were tied to the Slave craton by ca. 1.97 Ga. It is interpreted herein that Hottah terrane began to the south of its current position and evolved in an active margin over an always east-dipping subduction system that began prior to ca. 2.0 Ga and continued to ca. 1.85 Ga, and underwent tectonic switching and migration. The stratigraphy of the ca. 1913–1900 Ma Hottah plutonic complex and Bell Island Bay Group includes a subaerial rifting arc sequence, followed by basinal opening represented by marginal marine quartz arenite and overlying ca. 1893 Ma pillowed basalt flows and lesser rhyodacites. We interpret this stratigraphy to record Hottah terrane rifting off its parental arc crust — in essence the birth of the new Hottah terrane. This model is similar to rapidly rifting arcs in active margins — for example, modern Baja California. These rifts generally occur at the transition between subduction zones (e.g., Cocos–Rivera plates) and transtensional shear zones (e.g., San Andreas fault), and we suggest that extension-driven transtensional shearing, or, more simply, terrane translation, was responsible for the evolution of Bell Island Bay Group stratigraphy and that it transported this newly born Hottah terrane laterally (northward in modern coordinates), arriving adjacent to the Slave craton at ca. 1.88 Ga. Renewed east-dipping subduction led to the Great Bear arc flare-up at ca. 1876 Ma, continuing to ca. 1869 Ma. This was followed by voluminous Great Bear plutonism until ca. 1855 Ma. The model implies that it was the westerly Nahanni terrane and its subducting oceanic crust that collided with this active margin, shutting down the >120 million year old, east-dipping subduction system.

Characterization of the Paleoproterozoic Hottah terrane, Wopmay Orogen using multi-isotopic (U-Pb, Hf and O) detrital zircon analyses: An evaluation of linkages to northwest Laurentian Paleoproterozoic domains

The Paleoproterozoic Wopmay orogen is modeled as a simple arc-continent collision involving the Archean Slave craton and an enigmatic continental arc terrane termed the Hottah terrane. Hottah is a composite terrane that includes a juvenile 2.6-2.1 Ga basement and a younger 1.93-1.90 Ga continental arc. The extent and crustal age structure of the terrane is poorly known which limits interpretations as to its origin, and eventual role in the development of the orogen. Multi-isotopic (U-Pb, Hf, O) analyses of detrital zircon from rocks of the Hottah terrane and its cover sequence are used to indirectly evaluate its age and crustal characteristics. The data confirm that ca 2.6-2.1 Ga crust is an important component of the terrane but Hf isotopic data indicate mostly non-juvenile compositions consistent with recycled Neoarchean crust sources. Ages of detrital zircon are distinct from those expected of the Slave craton but are similar to adjacent 2.0-1.90 Ga Paleoproterozoic terranes of northwestern Laurentia. The similarity to terranes such as the Ksituan/Buffalo Head and Taltson-Thelon magmatic zones, south and east of the Wopmay Orogen indicate that Hottah could represent a terrane that initially evolved adjacent to the earliest assembled parts of the Nuna continent between 2.0 and 1.9 Ga, and was rifted prior to the collision with the Slave margin that produced the Calderian orogeny.

U–Pb zircon ages and field relationships of Archean basement and Proterozoic intrusions, south-central Wopmay Orogen, NWT: implications for tectonic assignments

New field and U–Pb zircon data from the south-central Wopmay Orogen (south of 65°N) establish crystallization ages of basement and plutonic phases and bring to the forefront questions on the architecture of the orogen. The complex and extensive >3300–2575 Ma basement domain was derived from the adjacent Slave craton. Paleoproterozoic supracrustal rocks, dominated by an extensive pelitic–psammitic sequence with basal arenite, conglomerate, and carbonate, unconformably overlie this basement. Pre- to post-kinematic Paleoproterozoic plutons intrude both basement and overlying strata and crystallized at 1877, 1867–1862, and 1858–1850 Ma. The first pulse of plutonism constrains an early generation of metamorphism and deformation to younger than 1877 Ma and is corroborated by metamorphic zircon in Archean basement. The interval from 1867 to 1862 Ma brackets ductile deformation in granite and granodiorite intrusions; although corresponding structure in the metasedimentary strata is equivocal. Post-kinematic ca. 1858–1850 Ma plutons were coeval with localized high-grade metamorphism and concomitant recrystallization of Archean basement. The absence of older (>1880 Ma) phases of the Hepburn intrusive suite in south-central Wopmay Orogen demonstrates a previously unrecognized north–south plutonic dichotomy and that the historical assignment of Calderian metamorphism to ca. 1885 Ma may be too old. The designation of part of the area to a klippe of Hottah terrane is not compatible with field and zircon age data which shows that basement and the overlying strata can be tied directly to the Slave craton. The study raises questions regarding the tectonic relationship between the Hottah terrane and Slave craton.

Two detrital zircon signatures for the Cambrian passive margin of northern Laurentia highlighted by new U-Pb results from northern Canada

In time, Laurentia was bounded on all sides by passive margins, providing an opportunity to establish a Cambrian of detrital zircon provenance for the craton during a relatively quiescent tectonic stage. U-Pb detrital zircon ages from laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) and sensitive high-resolution ion microprobe (SHRIMP) analysis of seven sandstone samples from northern Northwest Territories are consistent with generally local provenance. Samples from Mackenzie Mountains were most likely derived from the Proterozoic Mackenzie Mountains Supergroup and are dominated by detrital zircon ages of ca. 1020–1516 Ma, but also contain populations of ca. 1615–1692 Ma and ca. 2699–2752 Ma. sandstones from south of Great Bear Lake have a complementary set of detrital zircon age populations dominated by ca. 1810–2010 Ma, with smaller relative probability peaks at 2100 Ma, 2300 Ma, and 2600 Ma, which are consistent with derivation from the Hottah terrane, Great Bear magmatic zone, and Slave craton. Comparison of the signatures from Mackenzie Mountains and Great Bear Lake south suggests that there were two distinct sources of detrital zircon available to the northern Northwest Territories during the Cambrian. Detrital zircon ages from strata of Victoria Island appear to be a mixture of Shaler Supergroup, Slave craton–like, Taltson-Thelon orogen, and Wopmay orogen sources. Following the dichotomy of detrital zircon age spectra presented by samples from Mackenzie Mountains and Great Bear Lake south allows subdivision of other published detrital zircon ages from across northern Laurentia into two groups: Laurentia type 1 suite reflects the Archean cratons and Paleoproterozoic orogens of the Precambrian Shield. Laurentia type 2 suite is dominated by Grenvillian and older ages (1000–1500 Ma) and includes relative probability peaks at 1600–1700 Ma and 2700–2750 Ma. We find, therefore, that this baseline mirrors the Precambrian tectonic fabric of Laurentia from Archean cratonization and Paleoproterozoic through Mesoproterozoic orogenesis, culminating in assembly of the supercontinent Rodinia. Furthermore, the Neoproterozoic breakup of Rodinia and establishment of passive margins surrounding Laurentia provided little new zircon available to the sedimentary system. The detrital zircon spectra for the of northern Laurentia are intended to provide a reference to test the affinity of suspect terranes and potentially exotic detrital zircon within Paleozoic foreland basins.

Birthdate for the Coronation paleocean: age of initial rifting in Wopmay orogen, CanadaThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

The 1.9 Ga Coronation “geosyncline” to the west of Slave craton was among the first Precambrian continental margins to be identified, but its duration as a passive margin has long been uncertain. We report a new U–Pb (isotope dilution – thermal ionization mass spectrometry (ID–TIMS)) 207Pb/206Pb date of 2014.32 ± 0.89 Ma for zircons from a felsic pyroclastic rock at the top of the Vaillant basalt, which underlies the passive margin sequence (Epworth Group) at the allochthonous continental slope. A sandstone tongue within the basalt yields Paleoproterozoic (mostly synvolcanic) and Mesoarchean detrital zircon dates, of which the latter are compatible with derivation from the Slave craton. In contrast, detrital zircon grains from the Zephyr arkose in the accreted Hottah terrane have Paleoproterozoic and Neoarchean dates. The latter cluster tightly at 2576 Ma, indistinguishable from igneous zircon dates reported here from the Badlands granite, which is faulted against the Vaillant basalt and underlying Drill arkose. We interpret these data to indicate that Badlands granite belongs to the hanging wall of the collisional geosuture between Hottah terrane and the Slave margin, represented by the Drill–Vaillant rift assemblage. If 2014.32 ± 0.89 Ma dates the rift-to-drift transition and 1882.50 ± 0.95 Ma (revised from 1882 ± 4 Ma) the arrival of the passive margin at the trench bordering the Hottah terrane, the duration of the Coronation passive margin was ∼132 million years, close to the mean age of extinct Phanerozoic passive margins of ∼134 million years (see Bradley 2008).

Paleoproterozoic subduction in northwestern Canada from near-vertical and wide-angle seismic reflection data

Near-vertical incidence and refraction – wide-angle reflection seismic data, recorded as part of Lithoprobe studies in the Paleoproterozoic–Archean domains of Canada’s Northwest Territories, show remarkable reflections from within the upper mantle. A parallel pair of reflectors imaged by the near-vertical data can be traced from Moho levels (∼33 km) down to ∼70 km depth. In a previous study, the reflectors were interpreted as the top and bottom of an ∼1.8 Ga subducted oceanic crust beneath the Hottah terrane. Further inboard, where the seismic line changes its direction from east–west to nearly north–south, another pair of reflectors extends subhorizontally for about 100 km at ∼70 km depth before dipping downward. The subhorizontal reflectors were not correlated with the dipping slab; instead they were interpreted as a separate feature. However, they roughly coincide with a horizontal interface modeled from wide-angle data by an earlier study. Considering the crooked line acquisition geometry, we re-examined both near-vertical incidence and wide-angle reflection data using 2-dimensional (2-D) and 3-D forward and inverse modeling algorithms. Our results demonstrate that the subhorizontal reflectors are the continuation of the relict subducted slab, which now extends laterally for 300 km. Its base is the source of the wide-angle data. The apparent flattening for the near-vertical data is most likely an artifact of projecting a 3-D geometry onto a 2-D cross section. The shallowly subducted slab probably contributed to the thickening and stabilization of the subcrustal lithosphere below the Wopmay orogen.

The Calderian orogeny in Wopmay orogen (1.9 Ga), northwestern Canadian Shield

The Wopmay orogen is a Paleoproterozoic orogenic belt formed in part by the accretion of Hottah terrane, an east-facing continental magmatic arc, to the western margin of the Archean Slave craton at ca. 1.88 Ga. The arc-continent collision was responsible for the Calderian orogeny. Just prior to the collision, arc volcanism of the Hottah terrane had migrated trenchward and changed in composition from an aphyric calc-alkaline to a bimodal tholeiitic suite. The change in magmatism, along with subsidence and consequent high-temperature–low-pressure metamorphism of rocks on the upper plate, is attributed to extension and asthenospheric upwelling during rollback of the lower plate. The upwelling led to regional heating of the crust, melting, and generation of the metaluminous to peraluminous Hepburn intrusive suite within the arc just prior to accretion of Hottah terrane onto the Slave craton. During the collision, a 200-km-long, orogen-parallel swarm of mafi c sills was intruded into the lower plate and trench-fill sediments at the passive-margin shelf-slope break. The mafi c magmatism may have been induced by slab breakoff. The Calderian orogeny provides an excellent example of the magmatic history of an arc-continent collision. The Calderian orogeny was short-lived. Within a few million years, an east-dipping subduction zone formed outboard of the accreted terrane, creating volcanic rocks that were erupted atop the eroded HottahSlave collision zone. The short duration of the Calderian orogeny seems typical of arccontinent collisions, perhaps because the attenuated arc crust, coupled with the extended crust of the lower plate, never generates severely overthickened crust. Rise of asthenospheric mantle due to slab breakoff generates rapid uplift, erosion, and collapse of the mountains built during collision.

Lithospheric geometry of the Wopmay orogen from a Slave craton to Bear Province magnetotelluric transect

Two‐dimensional inversions of lithospheric‐probing magnetotelluric (MT) data at a total of 20 sites acquired along an approximately east–west 300‐km‐long profile across the Wopmay orogen in the Northwest Territories, Canada, provide electrical resistivity models of the boundary between the Archean Slave craton and the adjacent Proterozoic Bear Province. An analysis of distortion effects and structural dimensionality indicates that the MT responses are primarily one‐dimensional or only weakly two‐dimensional with a depth‐independent geoelectric strike angle of N32°E, consistent with regional structural geology. The regional‐scale model, generated from the longer period responses from all of the sites along the profile, reveals significant lateral variations in the lithospheric mantle. Resistive cratonic roots are imaged to depths of ∼200 km beneath both the Slave craton and the Hottah terrane of the Bear Province. These are separated by a less resistive region beneath the Great Bear magmatic zone, which is speculatively interpreted as a consequence of a decrease in the grain size of olivine in the Wopmay mantle, caused by localized shearing, compared to its neighboring cratonic roots. Focused two‐dimensional models, from higher frequency responses at sites on specific sections of the profile, reveal the resistivity structure at crustal depths beneath the region. These suggest that the root of the Slave craton crosses beneath the Wopmay orogen, and that the Wopmay fault zone does not penetrate into the lower crust. A comparison of these results with those obtained during the Lithoprobe project farther south shows striking along strike variations in the conductivity structure associated with the Wopmay orogen.

Ages of zircons from Jurassic sediments of Bluefish River slope, NWT Canada, and the possible age of kimberlite activity on the Lena West property

This work is based on a study of 30 zircons extracted from the heavy mineral fraction of a sedimentary rock sample with anomalously high contents of well-preserved kimberlite indicator minerals (KIM). The sample was taken from the basal horizon of Jurassic‐Cretaceous sandstones on the slope of Bluefish River valley. The Bluefish River, located in the Northwest Territories (Canada) north of Great Bear Lake, is the right tributary of the Mackenzie River. The Bluefish River basin is a part of the large Lena West property that occupies an area of about 40 000 km 2 . Based on available tectonic data [1, 2], the area is situated within the Paleoproterozoic Hottah terrane and is separated from the Slave Craton to the east by Wopmay orogen. The Lena West property represents the sedimentary basin of a stable platform, which is filled with a thick sequence of mainly continental Cambrian‐Cretaceous sediments. The Bluefish area is dominated by undivided Jurassic‐Cretaceous rocks (clay shales, siltstones, and sandstones), which make up the Parsons Series and represent terrigenous sediments of the continental margin. Undivided Cambrian‐Devonian rocks are exposed only at the lower reaches and in the immediate vicinity of the river valley. Glacial deposits are subordinate. During prospecting of this area for diamonds, anomalously high contents of KIMs (primarily, Cr-pyrope and picroilmenite) were found in the alluvial samples of the middle reaches of the Bluefish River. Sedimentary rocks were sampled on the slope in detail (Fig. 1). The highest KIM contents (hundreds of grains) were found in samples from the basal horizon of Jurassic‐ Cretaceous sandstones. The Jurassic basal horizon rests on the limonitized gray siltstones of the Devonian Hare Indian Formation. The basal horizon (about 30 cm thick) includes three lithologically diverse interbeds: quartz gravelstones (upper interbed), limonitized coarse-to medium-grained sandstones (middle interbed), and yellow-green clays with a small admixture of pebbly material (lower interbed).

Geoelectric structure of the Proterozoic Wopmay Orogen and adjacent terranes, Northwest Territories, Canada

Magnetotelluric (MT) soundings were made along a transect in northern Canada crossing the Proterozoic Wopmay Orogen, Fort Simpson basin, and adjacent parts of the Slave craton and the Nahanni terrane. The results are used to define the geoelectric structure and constrain the crustal and lithospheric structure and evolution. Across the Wopmay Orogen, geoelectric strikes at crustal depths average N34°E and are interpreted to be related to transcurrent faulting that occurred during late distal collisions at the western margin of the orogen. Weak two-dimensionality in the Fort Simpson basin is interpreted to be due to the sedimentary rocks in the basin. At longer periods, geoelectric strikes across the Wopmay Orogen rotate from ∼N43°E at uppermost mantle penetration to ∼N62°E at a depth of 100 km. The uppermost mantle strikes are interpreted to be due to ductile shearing linked to the transcurrent faulting in the overlying crust. The deeper strikes may be caused by shearing at the base of the present-day lithosphere. Within the Wopmay Orogen, the MT results image a conductor at the margin of the Fort Simpson and Hottah terranes interpreted to be related to the collision of these terranes. Conductive crust beneath the western margin of the Great Bear magmatic arc suggests correlative rocks of the Coronation margin extend south of the Slave craton. Lastly, decreased resistivity in the Hottah terrane at mantle depths is interpreted to be caused by the introduction of graphitic or sulphidic rocks during subduction prior to the Hottah–Slave and Fort Simpson – Hottah collisions.

SHRIMP U–Pb geochronology of detrital zircons from the Treasure Lake Group — new evidence for Paleoproterozoic collisional tectonics in the southern Hottah terrane, northwestern Canadian Shield

The recently defined Treasure Lake Group is a platformal sedimentary sequence more than 3 km thick that was deformed and metamorphosed prior to formation of the Great Bear continental arc during 1875–1845 Ma. The group is exposed as remnants in the southern part of the arc. Its basement is not exposed, but is interpreted to comprise the Paleoproterozoic Hottah terrane, which is exposed at Hottah Lake in the northern part of the arc. Studies in that region show that the Hottah terrane collided with the Archean Slave craton to the east ca. 1883 Ma following reversal of subduction polarity from west to east. Continued eastward subduction under the collision zone formed the Great Bear arc. The present study was undertaken to determine the age and provenance of the Treasure Lake Group. Thirty-nine detrital zircon grains from a sample of a quartz arenite unit in the middle part of the Treasure Lake Group were analyzed by SHRIMP (sensitive high-resolution ion microprobe). They show an age range from ca. 2729 to ca. 1880 Ma, with main modes at ca. 1895, 1985, 2090, and 2330 Ma. These data confirm a mainly Paleoproterozoic provenance for the group. The youngest group of five zircon analyses places the maximum depositional age at 1886 ± 8 Ma. The Treasure Lake Group is close to the north-trending Wopmay medial zone formed during collision of the Hottah terrane and the Slave craton. Deformation and metamorphism of the group can now be attributed to the collisional event. The group may in part be coeval with the Bell Island Bay and Akaitcho groups of the northern Hottah terrane, the lower parts of which were deposited ca. 1900 Ma. The wide range of detrital zircons ages found in this study is interpreted to reflect a complex Paleoproterozoic evolution of the Hottah terrane.

Magnetotelluric response and geoelectric structure of the Great Slave Lake shear zone

The Great Slave Lake shear zone (GSLsz) is a northeast-trending 25-km-wide dextral continental transform fault from the foothills of the Rocky Mountains in northeast British Columbia to the southeast side of Great Slave Lake. Based on its magnetic expression the GSLsz can be correlated for at least 1300 km, mostly in the subsurface. Magnetotelluric (MT) soundings were made at 60 sites in the southwest part of the Northwest Territories, Canada, along the LITHOPROBE Slave–Northern Cordillera Lithospheric Evolution (SNORCLE) Transect Corridors 1 and 1A, in the Summer and Autumn of 1996. Of these, 15 sites were along Corridor 1A which crossed the GSLsz, the Great Bear magmatic arc and Hay River terrane to the northwest of the shear zone, and the Buffalo Head terrane to the southeast. The primary objective of the Corridor 1A deployments was to image the structure of the GSLsz. Analysis of MT data indicates that along the Corridor 1A the resistivity structure is approximately 1D at shallow depth (<1 km) corresponding to Phanerozoic sedimentary rocks, 2D in the upper to mid crust with a strike ∼N30°E, and approximately 2D in the lower crust to lithospheric mantle with a strike of ∼N60°E. The direction in the upper crust is interpreted to represent the local-scale (<50 km) horizontal strike of the GSLsz whereas the direction in the mantle is parallel to the larger-scale strike of the GSLsz. 2D inversions of the MT data reveal that the GSLsz forms a crustal-scale resistive zone (>5000 Ωm) that is spatially correlated with a magnetic low. The GSLsz comprises greenschist to granulite facies mylonites. Its high resistivity is interpreted to be due to the resistive nature of the granitic protolith of the mylonites and that mylonites are dominated by rocks deformed in the ductile regime. To the northwest of GSLsz the MT profile reveals crustal conductors beneath the Great Bear magmatic arc and Hay River terrane. The enhanced conductivity occurring beneath the Great Bear magmatic arc is interpreted to be caused by electronic conduction within deformed and metamorphosed rocks of the Hottah terrane or the Coronation Supergroup. The MT results also reveal a mantle conductor beneath the margin of the Hottah terrane and Great Bear magmatic arc that is interpreted to be associated with the subduction of oceanic lithosphere. A second mantle conductor to the southeast is truncated at the GSLsz suggesting an older source for the enhanced conductivity and that the GSLsz includes significant strike–slip motion of sub-crustal lithosphere.

Magmatic evolution of the southern Great Bear continental arc, northwestern Canadian Shield: geochronological constraints

The calc-alkaline Great Bear continental arc in the Wopmay Orogen developed after a collision ca. 1890 Ma of the Archean Slave craton with the Paleoproterozoic Hottah terrane to the west. U–Pb zircon dating of three volcanic and six intrusive rocks from the southern part of the arc shows four stages of development: (i) intrusion of a few small sodic leucogranite plutons at 1873 ± 2 Ma into a previously folded metasedimentary sequence; (ii) abundant calc-alkaline felsic volcanism and subvolcanic intrusions during the period 1870–1866 Ma, bracketed by five ages; (iii) intrusion of large calc-alkaline granitic plutons, including the Marian River batholith, dated by zircon and titanite from two samples at 1866 +2–3 Ma; and (iv) emplacement of the potassic Faber Lake rapakivi granite at 1856 +2–3 Ma. The arc was developed on the Hottah terrane due to easterly subduction of an oceanic plate under the amalgamated Slave craton – Hottah terrane. The oldest exposed rocks in the southern part of the arc are remnants of a Paleoproterozoic platformal sequence. They were regarded previously as equivalents of the initial passive margin sequence on the Slave craton, but are interpreted here as part of the Hottah terrane. The ages reported here are comparable with earlier data from the northern part of the arc, which show an age range from 1875 to 1840 Ma and also identify two suites of compositionally and temporally distinct granites. The age constraints show that the Great Bear arc evolved rapidly in time from sodic through calc-alkaline, and then, with a pause, to potassic composition.

Magmatic evolution of the southern Great Bear continental arc, northwestern Canadian Shield: geochronological constraints

The calc-alkaline Great Bear continental arc in the Wopmay Orogen developed after a collision ca. 1890 Ma of the Archean Slave craton with the Paleoproterozoic Hottah terrane to the west. U–Pb zircon dating of three volcanic and six intrusive rocks from the southern part of the arc shows four stages of development: (i) intrusion of a few small sodic leucogranite plutons at 1873 ± 2 Ma into a previously folded metasedimentary sequence; (ii) abundant calc-alkaline felsic volcanism and subvolcanic intrusions during the period 1870–1866 Ma, bracketed by five ages; (iii) intrusion of large calc-alkaline granitic plutons, including the Marian River batholith, dated by zircon and titanite from two samples at 1866 +2–3 Ma; and (iv) emplacement of the potassic Faber Lake rapakivi granite at 1856 +2–3 Ma. The arc was developed on the Hottah terrane due to easterly subduction of an oceanic plate under the amalgamated Slave craton – Hottah terrane. The oldest exposed rocks in the southern part of the arc are remnants of a Paleoproterozoic platformal sequence. They were regarded previously as equivalents of the initial passive margin sequence on the Slave craton, but are interpreted here as part of the Hottah terrane. The ages reported here are comparable with earlier data from the northern part of the arc, which show an age range from 1875 to 1840 Ma and also identify two suites of compositionally and temporally distinct granites. The age constraints show that the Great Bear arc evolved rapidly in time from sodic through calc-alkaline, and then, with a pause, to potassic composition.

Nd isotopic evidence from Wopmay Orogen for 2.0–2.4 Ga crust in western North America

Nd isotopic data from the 1.9 Ga Wopmay Orogen, N.W.T., Canada, strongly suggest the involvement of crust in the range of 2.0–2.4 Ga. Remnants of three compositionally diverse magmatic arcs (1940–1902 Ma, 1890–1880 Ma, and 1875–1840 Ma) are preserved in Wopmay Orogen. Each of the three arcs originated west of the limit of exposed Archean crust. Nineteen samples of plutonic and volcanic rocks from the three arcs yield a narrow range of initial ε Nd from −1.8 to +0.3. There is no apparent correlation between ε Nd and geographic distribution, nor with whole-rock chemistry ( SiO 2 = 54–75% ). Two basalt samples from west of the Archean have initial ε Nd's between +2 and +3. Two 1.87 Ga granites which intrude through Archean crust have much lower initial ε Nd's of −5.9 and −8.6. The data from the three magmatic arcs are best interpreted in terms of a source region which had a very limited range of ε Nd at 1.9 Ga which we interpret as 2.0–2.4 Ga crust, presumably Hottah Terrane basement; Wopmay Fault Zone is therefore the surface manifestation of a suture between 2.0–2.4 Ga Hottah Terrane and the Archean Slave Craton. There is a growing body of data which suggests that much of western North America may be underlain by 2.0–2.4 Ga crust. Geochronologic and isotopic data from other Precambrian cratons also suggest that 2.0–2.4 Ga crust may be much more widespread than previously thought. Models of crustal growth which interpret 1.9–1.7 Ga crust as mixtures of Archean crust and Proterozoic depleted mantle may be overestimating the amount of juvenile material added to the crust in this time period.