December 2010 LIP of the Month

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The Dubawnt Supergroup, Canada: a LIP with a LISP

Tony Peterson, Sally Pehrsson, Charlie Jefferson, Jeff Scott1, Robert Rainbird
Geological Survey of Canada (tpeterso@nrcan.gc.ca)
1Department of Earth Sciences, Carleton University, Ottawa

Introduction

Between about 1.85 and 1.6 Ga, a number of large intracontinental granite-minette igneous provinces developed within Archean cratons, in the hinterlands of major Paleo-Mesoproterozoic orogens. One of the best documented, and perhaps the largest, lies in the Western Churchill Province (WCP) of northern Canada, west of Hudson Bay (Figure 1, Figure 2). At 1830+/-20 Ma, approximately 5,000 km3 of potassic to ultrapotassic subaerial lamprophyric lavas (phlogopite-clinopyroxene minettes) and a feeder dyke swarm erupted over an area of 200,000 km2. At the same time, granodioritic laccoliths (the Hudson Suite) were emplaced at mid-crustal levels in a 850+ km-wide, 2000+ km-long swath. The lavas, intrusions and coeval sedimentary basins straddle the Snowbird Tectonic Zone, the Trans Hudson age suture between the Rae and Hearne subprovinces of the WCP, and occur to west and northwest into the Rae Subprovince hinterland. A bimodal mafic-felsic event (comprising the 1750 Ma rapakivi Nueltin Suite granites and Pitz Formation rhyolites/basalts, plus basaltic dykes) came 80 Ma later, within a portion of this same region.


Figure 1: Simplified Archean/Proterozoic tectonic elements of North America. WCP=Western Churchill Province. Other symbols: BF=Bathurst Fault, De=Dessert Lake Basin, MF=Macdonald Fault, Ho=Hornby Bay Basin, El=Elu Basin, Th=Thelon Basin, At=Athabasca Basin, STZ = Snowbird Tectonic Zone, THO=Trans-Hudson orogen.

Within the Western Churchill Province, the intrusive and extrusive suites are associated with an even larger intracontinental sedimentary province (hence, a LISP) that includes minor volcaniclastic rocks but is dominated by post-1750 Ma individual intracratonic basins of pebbly redbed quartz arenite to arkose (Thelon, Athabasca, Amundsen, and Elu basins: Gall and Donaldson, 2006). A minor post-Thelon (ca. 1.57 Ga) event of potassic, mafic volcanism (Kuungmi Formation: Chamberlain et al., 2010) strongly resembles the earlier Dubawnt minettes, further emphasizing the close integration of the sedimentary and igneous rocks. Together the entire package, spanning about 300 million years, comprises the Dubawnt Supergroup (DSG): a LIP with a LISP.


Figure 2: Simplified geological map (adapted from Paul et al., 2002) showing the Dubawnt Supergroup (DSG) within a portion of the Western Churchill Province. AK=Akluilak minette dyke (bearing subduction microdiamonds); AZ=Amer Mylonite Zone; B=Baker Lake and Baker Lake Basin; C=Chesterfield Inlet; K=Kiggavik uranium deposits; KG=Kaminak greenstone belt; KR=Kramanituar Complex; STZ=Snowbird Tectonic Zone; W=Wager Bay.

The magmatic component of the Dubawnt LISP is global in scale, with related minette-granodiorite and smaller rapakivi granite provinces extending nearly continuously across North America and Greenland into Baltica (e.g., Rämö and Haapala, 1990; Skerjnnaa, 1992; Kalsbeek and Nutman, 1996). We consider these rocks, and contemporaneous igneous/sedimentary packages of identical style in other Proterozoic hinterlands, to record events of crust/mantle interaction and intracontinental deformation that demarcate a change in Earth dynamics. The continental collisions that first produced these associations resulted in the formation of Nuna, which was probably the Earth’s first true supercontinent (Reddy, 2009).

In this contribution, we summarize the characteristics of this composite LIP/LISP and the known constraints on its origin, and present a tectonic model for its formation. The geographic extent and time span of the DSG and correlated sandstones are somewhat greater than that of most continental LIPs, and we will venture immediately that a plume, or a similar regional scale mantle disturbance, is not responsible for any part of it; basaltic rocks play a very subordinate role. The key to deciphering these rocks lies in recognizing the global – or rather, supercontinental - nature of the events that formed them.

Geological Summary

A simplified map (Figure 2) and stratigraphic section (Figure 3) summarize the wide extent in time and space of the DSG. The DSG comprises three volcano-sedimentary groups separated by erosional unconformities (Donaldson 1968; Rainbird et al. 2003; Aspler et al. 2004). Oldest is the Baker Lake Group (ca. 1840-1790 Ma; Rainbird et al. 2006), which contains the extrusive minettes (Christopher Island Formation, CIF) and a wide variety of immature continental redbeds (Hadlari et al. 2006). The Wharton Group (ca. 1760-1750 Ma; Rainbird and Davis; 2007) contains rhyolite and minor basalt of the Pitz Formation plus aeolian sandstone. The uppermost Barrensland Group contains quartzose conglomerate, alluvial and aeolian sandstone and mudstone of the Thelon Formation, capped by minor marine dolostone and 1.57 Ga Kuungmi mafic, potassic lavas and volcaniclastic rocks. Uranium-bearing fluorapatite cement in the Thelon Formation has been dated as a single basin-wide hydothermal-diagenetic event at 1670 +/- 5 Ma, synchronous with brittle faulting of lower Thelon and soft-sediment faulting of upper Thelon Formation strata (Davis et al., in press). Paragenetically similar fluorapatite in Athabasca Basin has a weighted mean age of 1609 +/- 30 Ma (ibid.). The cementation event is very close to the formation of world class unconformity associated uranium deposits in the Athabasca basin (Jefferson et al., 2007), which may originate from detrital monazite or basement granitic rocks.

For simplicity, hereafter we refer to the potassic igneous rocks (which include feeder dykes and minor plutons) as the Dubawnt minettes, and the Nueltin/Pitz granite-rhyolite event as the ‘rapakivi suite’.


Figure 3: Stratigraphic section for the DSG, depicting an idealized half-basin of the Baker Lake Group. Thicknesses are highly variable; maximum vertical thickness in such a basin would be approximately 2 km but apparent thicknesses due to stacking on active faults are up to 10 km. Baker Lake Group (ca. 1.83 Ga): LFM=lower felsic minette, MM=mafic minette, UFM=upper felsic minette, Ka=Kazan Formation arkose, Kf=Kunwak Formation (alluvial/fluvial redbeds), Hg=Hudson suite granodiorite, Ms=Martell syenite (mingled granite/minette). Wharton Group (ca. 1.75 Ga): Ng=Nueltin suite granite, P=Pitz Formation rhyolite. Barrensland Group (ca. 1.7 Ga): T=Thelon Formation (conglomerate, sandstone, mudstone). Not shown are minor dolostones/volcanics in the uppermost Barrensland Group, arenite/conglomerate which locally underlies the minette volcanic rocks, and the Amarook aeolian sandstone in the Wharton Group. The vertical extent of the Nueltin granite stocks, and the nature of the connection to their source region, are unknown.

As is usually the case for intracontinental minettes (which are sometimes confused with so-called ‘calc-alkaline’ lamprophyres of Andean-type volcanic belts, see e.g. Rock, 1991) the Dubawnt minettes are closely associated in time and space with granodioritic plutons; in the Dubawnt LIP, the granodiorites are termed the Hudson suite. Their contemporaneity is well established through geochronology (van Breemen et al., 2005; Rainbird et al., 2006). But because the minettes were mainly deposited in basins with active fault margins, and the granitoids are restricted to mid-crustal levels with no known extrusive equivalents, they are rarely observed in contact. However, mingled plutons (Martell Syenite) do occur, mostly along the northern edge of the minette field, near a line extending through and west of Chesterfield Inlet.

As was noted by Hoffman (1988) the orientations of the minette volcanic basins, and of the associated feeder dykes, reflect ongoing transpression and local stress regimes controlled by far-field compression related to the Superior-Churchill and Slave-Churchill collisional zones. The largest basin (Baker Lake basin) developed by E to NE dextral strike-slip and northerly extension, with lavas and volcaniclastic rocks stacked, in conveyor-belt fashion, against the southerly master fault (Hadlari and Rainbird, 2001). Lesser basins to the southwest are partly bounded by the Snowbird tectonic zone, an enigmatic structural/domain boundary in part separating the Rae and Hearne subprovinces, which may connect to the Baker Lake master fault. Dyke orientations either parallel these regional structural features, or have bimodal distributions parallel with the major NW and SW-trending Bathurst and Macdonald faults that developed from eastward indentation of the rigid Slave Province into the Churchill Province at this time. The overall effect was of tectonic escape to the northeast, with isolated blocks being either uplifted in response to shortening, or downdropped to accommodate local extension. Similarly, many plutons of the Hudson intrusive suite are associated with NE-trending shear zones and are typically syndeformational on their margins, with undeformed interiors. The majority of Hudson intrusions have a thin, sill-like aspect, reflecting a balance between small positive buoyancy, small magma volumes, and low ‘thermal inertia’, being emplaced close to their solidus temperatures and a small distance above their source regions (Peterson et al., 2003).

There is a clear erosional unconformity (typically, though not everywhere tilted) and a ca. 80 Ma hiatus in igneous activity, between the Dubawnt minettes and the Nueltin rapakivi event. There are two rapakivi domains. The larger southern one (the Nueltin domain) has undergone post-igneous tilting toward the north. Rhyolite flows and tuffs in contact with hypabyssal facies of granitic plutons outcrop at the northern edge of the domain and are closely associated with unconformity associated uranium deposits of the Thelon Basin (K in Figure 2; Scott et al., 2010). In contrast only deeper-seated, very coarse plutons are found at the southern end, near the type area of Nueltin Lake. The southern limit of the Nueltin domain has not been defined but it may extend beneath Phanerozoic cover in Manitoba and Saskatchewan. The northern boundary of the Nueltin domain is abrupt, and corresponds to a line extending through and west of Chesterfield Inlet. The northern rapakivi domain is represented mainly by a large pluton straddling the Amer Mylonite Zone; it includes both minette dykes and Hudson suite plutons but is not well characterized at this time. Although the areas intruded by rapakivi granite are relatively restricted, Rb-Sr isochron and K-Ar ages of 1.75 Ga are much more widespread throughout the WCP (e.g., Loveridge et al., 1988)

Prior to dating and characterization of the Kuungmi Formation, the sedimentary Thelon Formation had already been linked to the earlier igneous events by close association in time and space (Donaldson, 1968). The unconformity beneath the Thelon Formation displays intense chemical weathering and, although cobbles and grit derived from the Pitz Formation and Archean rocks are locally present above it, the formation is greatly dominated by well-rounded vein quartz pebbles and feldspathic quartz sand recycled in part from the Baker Lake and Wharton Group basins. The Thelon/Athabasca sandstone sequences, and those in other outlier basins (Amundsen, Elu: Figure 1) are mainly flat-lying and display a range of sedimentary structures consistent with deposition in continent scale river systems with local aeolian reworking (Rainbird et al., 2003). Comprehensive analyses of sequences, paleocurrents and fault systems have documented that the early sequences of these basins represent temporally close but independent lakes of sand constrained by basin controlling fault systems (Ramaekers et al., 2007; Yeo et al., 2007; Davis et al., in press). The uppermost sequences were part of a continent-wide transgressive depositional system which evidently continued to subside through the early Phanerozoic, e.g., the Thelon Basin preserves isolated outliers of Ordovician carbonate rocks 300 km west of their nearest correlatives in Hudson Bay.

Petrology of the Igneous Rocks

Dubawnt Minettes

Detailed discussions of the petrology of the potassic rocks exist (Peterson et al. 1994, 2003), and average elemental analyses are given in Table 1. Selected outcrop photographs and photomicrographs appear here in Figure 4. The volcanic rocks erupted in a felsic-mafic-felsic sequence. The first felsic eruptives bear obvious signs of crustal contamination, such as elevated SiO2 and partially melted crustal xenoliths; the younger felsic sequence is extremely enriched in incompatible elements. Some felsite dykes (bostonites) correlated with this unit contain a carbonatite phase greatly enriched in U, Th, and REE. The younger felsic volcanic rocks have compositional and mineralogical similarities to lamproites, and likely represent magmas fractionated at low pressure.

The intervening mafic minettes, by far the most voluminous in both lavas and feeder dykes, are phlogopite-clinopyroxene (+/-olivine) lamprophyres with abundant apatite and titanite in a groundmass dominated by potassium feldspar and magnetite. Olivine megacrysts occur in some dykes; leucite phenocrysts are rare. A mantle origin of the mafic minettes is beyond dispute: mantle xenocrysts, including lherzolitic spinels and even microdiamonds, have been observed in breccia intrusions and dykes (Peterson 2006, MacRae 1995). Ovoid xenoliths consisting of phlogopite megacrysts, battered into concentric ‘eggs’ by impacts against dyke walls during ascent, contain magnesiochromite, chromian diopside, and primary carbonate, and are interpreted as specimens of mantle source material from mica-rich veins (Peterson and LeCheminant, 1993). Although the mafic minettes have enriched trace element compositions, certain major element features, such as low CaO (average <6%) and high Mg/(Mg+Fetotal) (>0.8) are consistent with a depleted mantle signature. This feature has also been noted in lamproites (Mitchell and Bergman, 1991).

 

LFM

MM

UFM

Calvert H

SiO2

58.67

53.05

54.83

49.95

TiO2

0.65

0.82

1.24

2.62

Al2O3

12.98

11.39

13.53

14.25

Fe2O3

3.76

4.90

5.53

11.85

FeO

2.15

2.56

3.89

2.12

MnO

0.09

0.10

0.12

0.08

MgO

4.62

9.56

5.64

2.49

CaO

4.76

5.82

3.83

2.97

Na2O

2.65

1.74

2.43

0.56

K2O

5.95

6.52

5.76

8.53

P2O5

0.63

0.92

0.88

1.20

Rb

294

295

222

180

Sr

877

1309

1007

85

Ba

2512

4200

3369

1454

Y

18

26

46

58

Zr

269

394

533

398

Nb

18

20

36

36

La

68

113

124

81

Ce

135

233

237

175

Table 1. Average analyses of lower felsic, mafic, and upper felsic minettes of the DSG, and mafic rocks of the Calvert H association from the lower McArthur basin (n=163) after Scott et al. (2010).

  
  
  
Figure 4: Photographs of Dubawnt minettes. (a) Vesicular clinopyroxene-phlogopite lava; vesicles are filled with carbonate. (b) Subaerial olivine minette lapilli tuff interbedded with partially welded agglomerate; up is to the right. (c) Subaqueous flow breccia; base of chilled flow at the top. (d) Photomicrograph of mafic minette with altered olivine (black), leucite (red) replaced by potassium feldspar, and clinopyroxene. (e) Photomicrograph of typical phlogopite-clinopyroxene minette. (f) Photomicrograph of upper felsic minette with tetraferriphlogopite phenocrysts and leucite microphenocrysts.

Within the minette province, contemporaneous basaltic rocks are rare or absent. However, minor basaltic events have been noted near or outside its margins. Plutons containing spessartitic (i.e., alkali basaltic) melt mingled with Hudson granitoids are present along the north edge, south of Chesterfield Inlet (Sandeman et al., 2000). Hudson granitoid is mixed with diorite at the Amer Mylonite Zone (Scott and Peterson, in prep.). A minor basaltic swarm (Sparrow Dykes, 1827+/-4 Ma: Bostock and van Breemen, 1992) is located northwest of the Athabasca basin, near an outlying basin correlated with the Baker Lake Group (Martin Group: Mazimhaka and Hendry, 1984). However, the large relative volume of minette magma, and the absence of contemporaneous basaltic activity in the interior of the igneous province, indicates that voluminous melting of asthenosphere was not involved in its genesis.

The outstanding characteristic of these rocks, which presents both interpretive difficulties and strong constraints on tectonic models, is their Sm-Nd isotopic composition. In a large database of Nd model ages of these rocks and the surrounding crust (Peterson et al., 2010), it is seen that the Dubawnt minettes have an isotopic composition that is virtually indistinguishable from average Western Churchill lithosphere (Figure 5) - their modal depleted mantle model age is near 2.7 Ga. Their high LREE content precludes an explanation of this by crustal contamination. However, their potassic character, together with certain geochemical characteristics (such as high Ba/La, low Nb and overall depletion in HFSE) is consistent with an origin in subduction-enriched lithospheric mantle. This suggests two scenarios: (1) sublithospheric mantle was enriched by subducted juvenile components derived from accreting oceanic and continental fragments in the Archean, mostly at ca. 2.7 Ga (Cousens et al., 2001); or (2) sub-Churchill mantle was enriched by material eroded from exposed Churchill crust, and subducted during Proterozoic orogenesis immediately prior to 1.83 Ga (Peterson et al., 1994). Both hypotheses imply that REE in the source region were sourced almost entirely from subducted crustal material.


Figure 5: Histograms of Sm-Nd depleted mantle model ages (DePaolo, 1981) (after Peterson et al., 2010). ‘Proterozoic Granites’ includes Hudson suite and Nueltin suite/Pitz Formation. ‘All rocks’ includes the entire database of Archean through Proterozoic igneous rocks and orthogneisses for the Western Churchill Province. Note the strong similarity between the Dubawnt minettes and ‘All rocks’, including, within statistical uncertainty, the minor peaks at 2.9 Ga (most easily seen in the granite data set), 3.1 Ga, and 3.3 Ga.

The Archean subduction model is now in disfavour because it is inconsistent with current models of the amalgamation history of the Western Churchill Province (Berman et al., 2007). The minettes are found in two Archean domains (Rae Subprovince to the north, Hearne Subprovince to the southeast) which are now considered to have been juxtaposed at ca. 1.9 Ga. Strong evidence, in the form of correlative mafic dykes, indicates the Hearne Subprovince was rifted away from the (now) southern margin of the Superior Province at ca. 2.1 Ga (Bleeker and Ernst, 2010). It strains credulity that these separate fragments experienced the same type of metasomatic enrichment at a time long preceding their docking, and there are no comparable potassic rocks in the Superior Province to support that model.

The Proterozoic enrichment model, while more consistent with presently favored cladograms, also requires special pleading. As potassic intrusive rocks are absent immediately north of the Trans-Hudson orogen, north-directed subduction related to Churchill/Superior docking cannot have contributed to the enrichment. The only subduction/collision events available to provide enrichment are therefore those along the southeastern margin of the Rae Subprovince, where it meets the Hearne Subprovince (Berman et al., 2007). However, there is no preserved igneous root of a ca. 1.9 Ga Andean margin along the presumed suture south of Baker Lake, and Berman et al. (2007) concluded that the Rae and Hearne subprovinces were juxtaposed primarily by transpressive faulting in that region. The unusual geometry of the minette field, which is elongated mostly east-west, is at a high angle to the suture zone (Snowbird tectonic zone) - inconsistent with subduction enrichment along the length of the suture.

Finally, local variations in Nd model age and geochemistry are difficult to reconcile with a protracted process of erosion, transport and deposition, subduction, dissolution, and metasomatism, which should have created a relatively homogeneous isotopic composition. For example, in the Angikuni Lake area, exposed Neoarchean supracrustal rocks have relatively old (ca. 2.9 Ga) model ages (Peterson et al., 2010), as do several minettes of the area. Minettes on the northern edge of the Kaminak greenstone belt (central Hearne Province), characterized by a large volume of ca. 2.7 Ga oceanic basalts, have MgO contents well above the norm (Beaudoin, 1998). Such occurrences seem to require a degree of local crustal control on minette composition.

Below, we propose a more dramatic, yet conceptually simpler model that better reconciles these constraints: that the source of the minettes, which looks like mixed crust and mantle, was in imbricated lower and middle lithosphere, deformed and thickened during formation of early Nuna. Metasomatic subduction enrichment did not play a role in their genesis. Regardless of which enrichment model is preferred, the heat source to melt the metasomatized lithosphere is difficult to identify. The liquidus temperatures of mafic minettes are comparable to those of basalts (Esperanca and Holloway, 1987), which indicates that convective heat transfer was involved. Given that the Western Churchill Province was near or at the core of both Laurentia and Nuna, we speculate that subducted slab breakoff and sinking generated upward return flow in asthenospheric mantle, which transferred heat to the lower lithosphere. Minor basaltic magmatism on the periphery of the minette province may have been generated by this upward mantle flow.

Hudson Granitoid Suite

Selected outcrop photographs and photomicrographs of the Hudson suite are in Figure 6. The Hudson granitoids are relatively easy to interpret, because they clearly reflect low degrees of partial melting of local crust and commonly appear to have barely reached magmatic status. They are rich in inherited zircons and must be dated by ion microprobe analysis of thin magmatic overgrowths on inherited Archean cores (van Breemen et al., 2005). The margins of the plutons commonly bear an inherited foliation, mostly defined by biotite, and contacts with wall rocks are gradational in places. Compositions cluster around that appropriate for a minimum melt of average crustal composition, at a depth of about 20 km (Peterson and van Breemen, 1999) and are broadly calc-alkaline; they do not define any clear trends away from this composition that might have resulted from crystal fractionation. Many Hudson intrusions are located at flexures along syn-intrusive shear zones (e.g., MacLachlan et al., 2005) where their mobilization and emplacement were aided and localized by crustal deformation processes.

  
  
Figure 6: Photographs of Hudson suite granitoids. (a) Typical outcrop, showing horizontal cooling fractures parallel to sill contacts. (b) Martell Syenite – an admixture of granodiorite and Dubawnt minette – with typical red/purple weathering color. (c) Polished surface of typical granodiorite from the interior of a pluton, with medium to fine grained, equigranular texture. (d) Polished surface of typical granodiorite from the margin of a pluton, showing relict fabric defined by biotite, inherited from wall rocks.

The Hudson suite Nd isotopic compositions are essentially identical to those of their host Archean rocks, i.e., Nd model ages average near 2.7 Ga (Figure 5). However, some plutons along Chesterfield inlet and west of it have younger model ages (up to ca. 2.4 Ga) and tend to be relatively enriched in K, LREE, and other incompatible elements (Peterson et al., 2002).

The spatial extent of the Hudson suite throughout the Western Churchill Province is astonishing, being present virtually everywhere from the Trans-Hudson internides, and northward across the entire extent of the Rae subprovince. Two large batholiths within the Trans-Hudson reworked hinterland, the Wathaman (west of Hudson Bay) and the Cumberland (Baffin Island) are of the same age, as is a batholithic plutonic complex in the Wager Bay area (LeCheminant et al., 1987a). The only place where these plutons are absent is within the central Hearne, where a keel of greenschist facies volcanic/sedimentary rocks (Kaminak greenstone belt) apparently resisted melting.

Although dioritic rocks are locally associated with Hudson plutons, there is no evidence that there was a regional mafic trigger to convectively heat the crust to form granitic magmas. We are therefore forced to conclude that conductive heating, and/or radiogenic heat production, as well as local shear zone activity, were responsible for their generation. A non-convective trigger is also more consistent with the very limited degree of partial melting observed, and the absence of any known volcanic equivalents to the intrusions; they appear to be a phenomenon purely of the middle to lower crust. Temporally and petrologically they appear to be late syncollisional granitoids, in a setting similar to Himalayan or Cordilleran interior batholiths (Whalen et al., 2010).

Nueltin Suite and Pitz Formation

Selected outcrop photographs and photomicrographs of the Nueltin suite and Pitz Formation are in Figure 7. The intrusive Nueltin rapakivi granites, and the extrusive Pitz Formation rhyolites, are subaluminous rocks that record widespread local mingling with mafic magmas. Examples of the mafic rocks with little or no intermixed granitic melt have only recently been identified, and preliminary petrographic studies indicate they represent high-Ti alkali basalts (Scott and Peterson, in prep.). Although most rhyolites are porphyritic with phenocrysts mainly of quartz and sanidine but also anorthoclase and plagioclase, some banded rhyolite lava domes are crystal-free and represent superheated magma, erupted above its liquidus. Rapakivi texture is not universal, but widespread. The average Nueltin/Pitz isotopic composition is not distinguishable from that of the Hudson suite (Figure 5).

  
  
Figure 7: Photographs of Nueltin suite granites and Pitz Formation. (a) Porphyritic (sanidine) granite, Nueltin Lake. (b) Rapakivi-textured granite, Amer Mylonite Zone. (c) Bimodal basalt-rhyolite lithic/crystal tuff, Pitz Formation. (d) Porphyritic Pitz Formation lava; white crystals are dominantly sanidine.

In contrast to the Hudson Suite, Nueltin granites typically occur in stocks, have subvolcanic facies that grade into extrusive breccias, and display sharp, well-chilled contacts with wall rocks (LeCheminant et al., 1987b). In most respects, the Nueltin granites are typical anorogenic rapakivi granites, and the petrography and mineralogy of the Pitz Formation closely resembles the volcanic rapakivi suite on the Island of Suursaari, coeval with the rapakivi Wiborg batholith, in Finland (Rämö et al., 2008). The field relations of the WCP rapakivi suite are consistent with the most-cited genetic model for such rocks (e.g., Rämö and Haapala, 2005), which invokes crustal melting by injection of basaltic melt into the lower crust during an episode of extensional, mostly normal faulting.

The northern two thirds of the southern Nueltin domain are within the area encompassed by the Dubawnt minette swarm. Within this overlap area, few plutons of the mid-crustal Hudson suite are exposed, which probably reflects downdropping of crust within the Nueltin domain. As noted previously, the southern end of the Nueltin domain exposes coarse grained deep-seated plutons, and no basins are preserved there.

Tectonic Model

In this section, we present the hypothesis that the DSG is an example of a LIP-LISP tectonic environment that is as integral and predictable as, for example, that of back-arc basins. In this view, each of the lower, middle, and upper DSG sequences, though separated by periods of erosion and igneous lacuna lasting tens of millions of years, represents a distinct stage in a 300 Ma period of prolonged crust-mantle interaction directly resulting from supercontinent amalgamation.

Compressional tectonics and vertical crust movements from ca. 1.9 Ga - 1.83 Ga

Combined geochronological and metamorphic studies indicate that a large central region within the Western Churchill Province underwent high-pressure metamorphism at about 1.9 Ga (Berman et al., 2007). In Figure 8, the area of exposed crust showing this metamorphism is compared to the area encompassed by the DSG intrusions and basins, where it can be seen that the high-pressure domain runs through the center of the minette province and north of its boundary near Chesterfield Inlet. The high-pressure domain was uplifted along steeply dipping transpressional faults, and must persist at deeper levels to an unknown extent beyond those faults, particularly northward into the Rae Subprovince (Berman, pers. comm.).


Figure 8: Exposed areas subjected to high-pressure metamorphism at about 1.9 Ga (after Berman et al., 2007) compared to the distribution of the DSG. Rose diagrams depict orientations of minette dykes in various areas. B=Baker Lake, C=Chesterfield Inlet, K=Kramanituar granulite complex, STZ=Snowbird tectonic zone.

The high pressure metamorphism is attributed to crustal thickening as a result of collision between the Rae and Hearne subprovinces at ca. 1.9 Ga, along the trace of the Snowbird Tectonic Zone. A dramatic uplift event at this time north of Chesterfield Inlet involving rapid uplift and exposure of lower crustal granulites (Kramanituar complex: Sanborn-Barrie, 1994), is a clear example of regional scale vertical movements of crust. South of Chesterfield Inlet, subduction-type microdiamonds brought to the surface by a Dubawnt minette dyke (MacRae, 1995) indicate that crust was forced down to upper mantle depths immediately prior to 1.83 Ga. Data indicate that subduction-type diamonds can be closely associated with potassic metasomatism and ultrapotassic melt fluids derived from continental crust (e.g., Hwang et al., 2005; but see also Dobrzhinetskaya and Wirth, 2008).

These observations imply significant underthrusting within, and thickening of, continental lithosphere in the Western Churchill Province at about 1.9 Ga. The presence of subduction-type microdiamonds indicates that in at least one area, the magnitude of the underthrusting was sufficient to push crustal rocks into the diamond stability field. Less dramatic underthrusting would be required to place lower to middle crustal rocks beneath subcontinental upper mantle. In such an environment, with crustal rocks juxtaposed against hotter mantle, K and LILE-enriched fluids and melts could migrate into mafic upper mantle, producing mica-rich metasomes. The incompatible-element signature (and Nd isotopic composition) of melts derived from such metasomes should be similar to that of the crust, but compatible major element compositions would more closely resemble the mantle component. If this scenario is correct for the Dubawnt minettes, their composition is consistent with sub-Churchill lithospheric mantle being depleted in Ca and Fe, presumably by prior extraction of basaltic melts, prior to metasomatic enrichment.

Strong analogies have been described between the tectonic context, physiography, and sedimentology of the Western Churchill Province, and the Tibetan Plateau with its flanking basins (e.g., Dewey and Burke, 1973; Rainbird et al., 2003; Peterson, 1992). Computer simulations of lithospheric deformation in the Tibetan Plateau consistently indicate that crustal flow and thickening is concentrated in a weak lower crustal layer (Bendick and Flesch, 2007). Clark and Royden (2000) noted that basins flanking the northern edge of the plateau (Tarim and Sizchuan) are underlain by strong lithosphere, which resisted crustal flow that rose around the basins. In these models, although brittle faults bound the basins near the surface, uplift is driven by plastic flow in the lower crust.

In a study of teleseismic P waves on a cross-strike array, Nowack et al. (2010) indentified a central zone beneath the Tibetan Plateau, about 230 km in length, where the Moho is ‘disrupted’ (i.e., intercalated with lower crust) and the crust-mantle interface is distributed over depths of between 40 and 80 kilometres. A single broadly-spaced teleseismic and magnetotelluric profile within the Churchill Province (see Figure 1) (Jones et al., 2002) likely has insufficient resolution to reveal a discontinuous, micaceous conductive layer at the appropriate depth to be the source of the Dubawnt minettes. However, it does reveal lithosphere-scale (i.e., 100 km+) overlap of the leading edges of the Rae and Hearne subprovinces (Hearne is overthrust onto Rae), and juxtaposition of mantle blocks with strongly contrasting physical properties.

Post-1.83 Ga magma genesis and basin formation

Basins of the middle DSG (Wharton Group), containing the rhyolites and sandstones correlated with the rapakivi Nueltin granite suite, are neither as deep nor as well defined as those of the underlying Baker Lake Group. Middle DSG basins mostly developed by reactivation and tilting of the earlier basins, and were infilled by rhyolites and aeolian and alluvial redbeds (Rainbird et al., 2003). As outlined above, there is strong evidence for basaltic underplating and crustal melting to generate the silicic magmas at 1.75 Ga. A mechanism commonly invoked to promote asthenospheric melting and basalt injection in the formation of anorogenic granites is lithospheric mantle delamination (e.g., Wu, 2002; Whalen, 1996; Windley, 1991). This process is consistent with minor extension, block faulting, and tilting of the overlying crust. The primary rival hypothesis for basalt generation would be a mantle plume. Although we cannot disprove this hypothesis, critical plume-related features are lacking, such as a circular aspect to the rapakivi province, evidence for broad uplift prior to magmatism, and a centrosymmetric mafic dyke swarm. Basaltic dykes (McRae Lake Dyke and subsidiary dykes) are present, but they comprise an array sub-parallel to the dominant NE-trending tectonic fabric of the region.

Detailed geophysical studies are lacking to support lithospheric mantle delamination beneath the Wharton Group and the more extensive rapakivi province. However, a recent compilation of S-wave velocities beneath North America (Bedle and van der Lee, 2009) greatly clarifies the  nature of the mantle regionally beneath the Western Churchill Province. Figure 9, adapted from their depth-velocity maps, shows the current distribution of relatively fast velocity material at 90, 120, and 160 km depth. At 90 km, a region of fast mantle ends abruptly near an E-W line extending through and west of Chesterfield Inlet. Below 90 km, much of the fast mantle is absent beneath the Nueltin corridor and the Hearne Province.


Figure 9: Locations of the extent of fast S-wave mantle beneath the Western Churchill Province at 90, 120, and 160 km, adapted from Bedle and van der Lee (2009). Contours show the edge of mantle with velocities of 200+ m/s - faster than an average Earth model. Other symbols are as per Figure 1. In this area, the horizontal spatial resolution is rather low and can only resolve features on the order of 500 km across.

The distribution of fast mantle is too imprecise to make many conclusions regarding the architecture of sublithospheric mantle in this region; however, it is clear that significant boundaries in the mantle are at a high angle to the dominant NE-trending crustal fabric. The northern edge of fast mantle at 90 km corresponds approximately with a number of enigmatic features in the igneous rocks of the region, some of whose orientations bear no apparent relation to the high-level crustal fabric. Such features include: the abrupt northern termination of the southern (Nueltin) domain of the rapakivi suite; the northern margin of the Dubawnt minette dyke swarm; and widespread mafic mingling in Hudson granitoid intrusions, together with relatively young TDM and enriched K and LREE in those plutons. To this we can add the presence of rapidly exhumed lower crustal granitoid gneiss (Kramanituar) on the north side of Chesterfield Inlet, and the location of entrained subduction microdiamonds.

These observations imply that the crust and mantle were decoupled in this region, prior to ca. 1.83 Ga. The presence of a large terrane of uplifted crust involved in 1.9 Ga high-pressure metamorphism (see Figure 8) north of Chesterfield Inlet suggests comparisons with the observations of Clark and Royden (2000) and others for the Tibetan Plateau. Mechanically resistant mantle on the south side of Chesterfield Inlet may have impeded lower crustal flow, forced by indentation on the western margin of the Churchill Province, resulting in upward ‘oozing’ (the term of Clark and Royden) of lithosphere north of Chesterfield Inlet. In this model the Baker Lake Basin, if analogous to the Tarim basin on the north edge of the Tibetan Plateau, developed because it was underlain by more resistant lithosphere.

Deposition of the broad and mostly flat-lying Thelon and Athabasca basins was not accompanied by any significant igneous activity, although pre-, syn-, and post-depositional faulting was involved (Ramaekers et al., 2007; Yeo et al., 2007; Davis et al., in press). While far-field forces continued to reactivate local transextensional structures beneath these basins, regional subsidence would have been caused by the gradual sagging of lithosphere as it cooled after more than 100 million years of extensive volcanism, and sank into convecting mantle. The independent early evolutions of these basins may reflect local variations in lithosphere properties and history, replaced by long-wavelength subsidence to generate a continuous continent-scale sequence of quartz arenite – carbonaceous mudstone – dolostone near the end of the Dubawnt Large Igneous Province.

The Dubawnt LIP and LISP: a global phenomenon?

Previous authors (e.g., Kyser et al., 2000) have noted that broad, shallow paleo- to Mesoproterozoic basins similar to the upper DSG are present on other cratons, and also blanket world class unconformity associated uranium deposits. However, it is less well appreciated that these basins typically are associated with equivalents of the lower and middle DSG, which are present on six of the major cratons amalgamated during formation of Nuna. As individual provinces will have unique characteristics as a result of different Archean histories, and different sequences and geometries of Paleoproterozoic collisions, we would not expect precise duplicates of the tripartite DSG to be widely distributed or preserved. Nevertheless, very similar provinces exist elsewhere in the world. Figure 10 shows the locations of volcanic/sedimentary provinces that are similar to the DSG, based on a reconstruction of Nuna at ca. 1.7 Ga. One or more elements of late syntectonic Paleo- to Mesoproterozoic basins, ultrapotassic magmatism (extrusive and/or intrusive), and basalt-rapakivi provinces are present in the North China craton (Yu et al., 1994; stratigraphy summarized by Lamb et al., 2009), Siberia (Khudoley, 2004), Greenland (e.g., Whitehouse et al. 1998, Collinson 2008), and the North Australian and Sao Franciso cratons (below). In each case these provinces developed following accretion of cratons and microcontinents during the 2.1-1.8 Ga amalgamation of Nuna.


Figure 10: Paleoreconstruction of Laurentia, with elements of the supercontinent Nuna. Prominent occurrences of DSG-like igneous and sedimentary provinces are indicated (see text).

The Australian and Brazilian examples have pronounced similarity to the DSG. In the Sao Francisco craton, ultrapotassic plutons mingled with calc-alkaline mid-crustal granitoid intrusions were emplaced at about 2.0 Ga in reworked Archean rocks (Rosa et al., 1999). These are exposed west of the intracontinental Espinhaco Basin. The lower Espinhaco Group consists of fluvial-alluvial beds and bimodal mafic/felsic volcanic rocks in block-faulted basins, deposited at about 1.75 Ga (Uhlein et al., 1998). These rocks are overlain by fluvial sandstone, capped by shallow marine deposits, in a basin attributed to broad thermal subsidence (Martins-Neto, 2000). These igneous and depositional events took place during and after the Trans-Amazonian orogeny, at ca. 2.1-1.8 Ga (Akim and Marshak, 1998). Notwithstanding that the time gap between ultrapotassic and rhyolite volcanism was longer than in the DSG, the analogy between these volcano-sedimentary packages is striking.

The McArthur Basin in northern Australia is longer-lived than the DSG, and preserves a greater intensity of volcanic activity. Of interest are the earliest, three so-called ‘superbasin’ phases: the Leichardt (ca. 1830-1750 Ma), the Calvert (1740-1690 Ma), and the Isa (1690-1575 Ma). The volcanic stratigraphy and geochemistry is summarized by Scott et al. (2010), who attribute basin formation to continental collisions at the boundary of the North Australian Craton, and describe the formation of wrench basins analogous to the Tibetan Plateau. They ascribe igneous petrogenesis to mantle disturbances related to distal subduction activity. Volcanic activity in the Leichardt and Calvert intervals is overwhelmingly bimodal mafic-rhyolite, with rhyolite lavas consistently showing isotopic and inherited zircon indicators of Archean inheritance. The three earliest mafic associations – Leichhardt E, Calvert G, and Calvert H – have compositions very similar to the Dubawnt minettes (see Table 1), i.e., approximately 50% SiO2 with CaO<3%, K2O of 6-8.5% and Na2O<1%, with high Ba and LREE (average of 196 analyses, their table 5). Puzzlingly, they refer to these mafic rocks as ‘basalts’ and attempt to interpret them as such (though they conclude they have a lithospheric mantle origin) but in our view, they are typical transitional minette-lamproite. The sedimentary sequences are interpreted as initially rift-related, followed by a thermal sag basin (Jackson et al., 2000) but with approximately three minor marine incursions that deposited shallow water carbonate and mudstone sequences. A full characterization of the igneous suites in the McArthur basin now seems lacking and it is unclear if a rapakivi-type basalt-rhyolite sequence is present. Nevertheless, the broad similarities are apparent. Recent models have placed the North Australia craton adjacent to western North America at this time (e.g., Evans, 2009).

Although we are unaware of any example which rivals the DSG in extent and degree of preservation, a picture emerges of the occurrence of DSG-type LIP/LISP provinces across much of Nuna, on cratons now dispersed by supercontinent break-up. Although ultrapotassic rocks as old as Neoarchean are known (e.g., LaFleche et al., 1991) they are rare, seemingly limited to orogenic belts, and only locally developed. We speculate that the widespread development of Early Proterozoic hinterland minette/granodiorite-basalt/rapakivi granite provinces, in the context of large, long-lived intracratonic basins, represents a temporal evolution in the style of interaction between crust and mantle within cratons. In our model, the signature of this interaction is large-scale imbrication of lower crust and upper mantle within the core of Archean domains bounded by active orogens. The global Dubawnt-type LIP event resulted in widespread extraction of incompatible elements from the lower and middle crust to upper crustal levels, where they became available for mobilization by hydrothermal activity and weathering. The presence of world-class uranium deposits in Early to Mesoproterozoic sandstone basins may be the end result of this transference. A previous transference of U to the upper crust by K-rich magmas occurred in the period 3.1-2.2 Ga (Cuney, 2010) which provided unoxidized detrital minerals found in economic deposits in older basins, such as the Witwatersrand.

Numerous Phanerozoic examples of minette (transitional to lamproite)-granite associations are known and are petrologically similar to that of the Baker Lake Group; a large number of them are circum-Mediterranean (summarized in Peterson et al., 2002). The origin of circum-Mediterranean ultrapotassic rocks has for many years been a matter of conjecture, with camps divided between subduction-related enrichment and continental contamination. In view of a parallel history of conflicting ideas regarding the origin of the Dubawnt minettes, we find this unsurprising.

We have already referred to analogies drawn between the early Proterozoic Western Churchill Province and the Tibetan Plateau, and note that ultrapotassic rocks are currently erupting in the western Tarim Basin on the northern margin of the plateau (Pognante, 1990). In a detailed isotopic and mineralogical study of young (ca. 17-25 Ma) mafic ultrapotassic and silicic potassic volcanic rocks in southern Tibet, Miller et al. (1999) made several interpretations that parallel our own model. They concluded that ultrapotassic magmas resembling lamproites and bearing mantle xenocrysts, originated in metasomatized lithospheric mantle with a recycled crustal component, although they sought this in subducted sediment. Coeval silicic calc-alkaline (dacitic), potassic lavas, with much older inherited zircons, were generated by crustal anatexis, with no geological or petrographic evidence for contributions by basaltic melts. They speculated that melting of metasomatized lithosphere was a consequence of slab breakoff, or convective removal of the lower lithosphere (which we note, may not be mutually exclusive). As in our model for the Dubawnt LIP, this implies simultaneous melting at two levels of the lithosphere, by two different mechanisms.

Summary

The Dubawnt Supergroup (DSG) of the Western Churchill Province (WCP), deposited on dominantly Archean crust near the core of accreting Nuna, is a tripartite, large igneous/large intracontinental sedimentary province (LIP / LISP). The initial phase (Baker Lake Group) featured eruption of strongly potassic, mafic lavas (minettes) in active transtensional basins, at about 1.83 Ga. Concurrently, a bloom of granodioritic plutons (Hudson suite) was emplaced in the middle crust. At 1.75 Ga, after a protracted period of igneous inactivity and erosion, bimodal basalt-rhyolite volcanism (Wharton Group), and rapakivi granite plutonism, took place within a more restricted area that largely overlapped the older minette province. Further erosion, and intense chemical weathering, preceded deposition of conglomeratic arenites cemented by uraniferous fluorapatite at about 1.67 Ga (Barrensland Group). Preserved remnants of basins filled with similar-aged arenites record siliciclastic deposition in a continent-scale array of basins that were linked by big rivers and coalesced in their uppermost sequence.

Initial basin development is ascribed to uplift and strike-slip faulting generated by orogenic events on the margins of the growing supercontinent. The Hudson suite is ascribed to limited partial melting subsequent to crustal thickening, but the minettes must have originated in mantle previously metasomatized by crustal components, with an uncertain heat trigger. Subduction metasomatism is inconsistent with the geometry of the minette province and with current accretionary models for the WCP, and we suggest direct imbrication of lower crust and upper mantle to generate the source region of the minettes. Imbrication of this type has been seismically imaged within the Tibetan Plateau, where Miocene to Recent igneous suites identical to the lower DSG are prominent.

Middle DSG bimodal basalt-rhyolite activity was produced by injection of basalt into the lower crust, which may have been induced by mantle delamination. Although normal faulting was active during deposition of the upper DSG siliciclastic sequences, the continent-scale distribution of these basins, the regional scale of each basin, and the dominance of fluvial and aeolian facies are consistent with continent-scale thermal subsidence during ongoing transpression-transextension driven by far-field events on the margin of the supercontinent.

Numerous igneous/sedimentary provinces of similar style and age developed in Nuna, with examples in Brazil (Espinhaco Basin) and northern Australia (McArthur Basin) being most similar to the DSG. The repeated occurrence of these DSG-like LIP/LISPs indicates that a consistent sequence of events occurred in specific locations (i.e. Archean hinterlands to orogenic belts), such that they can be recognized as examples of crustal evolution within a well-defined tectonic environment. Strong analogies with the Tibetan Plateau indicate that this environment can develop on Earth today; however, it was apparently absent prior to about 2.0 Ga. The generation and upward migration of incompatible-element enriched ultrapotassic/potassic alkaline and granitic magmas represents a significant mobilization of incompatible elements to the upper crust in the Paleoproterozoic. Large uranium deposits beneath Paleo and Mesoproterozoic siliciclastic sequences in the DSG and beneath similar sequences elsewhere in North America and other continents, may have been made possible by this upward geochemical mobilization in an intracratonic setting.

Acknowledgements

Much of the material in this review was presented at a special session of the GEM Energy (Uranium) session at the 2010 GeoCanada annual conference (Peterson and Pehrsson, 2010 and related presentations). The authors wish to acknowledge the many discussions with fellow geologists which have helped form these ideas; of course, they do not necessarily agree with the conclusions given here and we are responsible for all errors in fact or argument. The first author wishes to especially acknowledge the contributions to studies of DSG magmatism by A. LeCheminant and A. Miller. Much of the isotopic data for the DSG was generated by Brian Cousens (Carleton University, Ottawa).

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