Chronostratigraphic constraints on the genesis of Archean greenstone belts, northwestern Superior Province, Ontario, Canada

Precambrian Research 92 (1998) 277–295

Chronostratigraphic constraints on the genesis of Archean greenstone belts, northwestern Superior Province, Ontario, Canada Fernando Corfu a,*, Donald W. Davis a, Denver Stone b, Michelle L. Moore a,c a Jack Satterly Laboratory, Royal Ontario Museum, 100 Queen’s Park, Toronto, ON, Canada M5S 2C6 b Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, ON, Canada P3E 6B5 c Geoscience Laboratories, Ministry of Northern Development and Mines, 933 Ramsey Lake Road, Sudbury, ON, Canada P3E 6B5 Received 29 January 1998; received in revised form 9 June 1998; accepted 9 June 1998

Abstract The paper reports new U–Pb zircon ages for supracrustal assemblages of the Red Lake, McInnes Lake, Hornby Lake, North Spirit Lake and Favourable Lake greenstone belts of northwestern Superior Province and evaluates their significance within the regional stratigraphic and tectonic context. The new data confirm and refine the existing picture of a multistage evolution lasting ca 300 m.y., initiated at ca 3000 Ma by periods of coeval calc-alkalic, tholeiitic and komatiitic magmatic activity, followed by the eventual accretion of a microcontinental nucleus by ca 2900–2800 Ma, and concluded by an intense period of extensive tholeiitic and calc-alkalic volcanism, and by widespread plutonism and tectonism between 2750 and 2680 Ma. Volcanic sequences in the McInnes Lake and Hornby Lake greenstone belts of the Berens River Subprovince yield ages of 2974±2, 2969±3, 2928±2 and 2901±2 Ma, showing a correlation with the early stages of greenstone belt development in the region. Similarly, a sedimentary unit in the northern Red Lake greenstone belt was probably deposited at ca 2900 Ma, as indicated by a range of detrital zircon ages between 2989 and 2916 Ma, and the absence of younger components. By contrast, a volcanic unit in the center of the Red Lake greenstone belt, previously thought to be 2830 Ma, yields an age of 2745+7/−4 Ma, which correlates with some of the latest volcanic episodes in the Uchi Subprovince. A detrital zircon population in a sedimentary unit of the western Favourable Lake greenstone belt of the Sachigo Subprovince displays single grain ages ranging from 2843 to 2727 Ma, the youngest age corresponding to that of an unconformably underlying tonalite gneiss. The geological and geochronological relationships suggest that the North Spirit Lake and Favourable Lake greenstone belts represent back-arc basins that evolved between ca 2750 and 2730 Ma in a transtensional setting within the older North Caribou terrane; sedimentation was initiated during opening and deepening of the basins and accompanied most of the concluding compression and imbrication stages. The period between 2750 and 2680 Ma was also characterized by widespread granitoid plutonism that enveloped the McInnes Lake and Hornby Lake greenstone belts and older tonalites and formed the bulk of the Berens River Subprovince. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Absolute age; Archean; Chronostratigraphy; Kenoran orogeny; Superior Province; U–Pb

* Corresponding author. Tel.: +1 416 586 5811; Fax: +1 416 586 5814; e-mail: [email protected] 0301-9268/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S0 3 0 1 -9 2 6 8 ( 9 8 ) 0 0 07 8 - 3

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1. Introduction Greenstone belts of the northwestern Superior Province contain a record of over 300 m.y. of crustal evolution characterized by recurrent volcanic and plutonic episodes starting ca 3000 Ma ago and culminating with the Kenoran orogeny at ca 2700 Ma. A general framework for these events has been delineated by several decades of geological mapping and geochronology, enabling a progressive development and discussion of hypotheses concerning the evolution of the region. The dominant thought has evolved from sagduction-type evolutionary models towards the recently more favoured mobilistic concepts (e.g. Ayres and Thurston, 1985; Card, 1990; Thurston and Chivers, 1990; Stott, 1997), but the multifaceted and locally conflicting geological record remains the source of much uncertainty. In this study we explore some aspects of the stratigraphy of several greenstone belts in the Uchi, Berens River and Sachigo subprovinces of the northwestern Superior Province. Main objectives of the study concern the stratigraphic identity and tectonic significance of particular metasedimentary and volcanic assemblages in the Red Lake and Favourable Lake greenstone belts, and in the intervening McInnes and Horny Lake greenstone belts. The results, examined within the context of the existing geological and geochronological framework, permit us to revise the chronostratigraphic picture and to constrain more firmly the relationships between coeval but genetically different supracrustal packages across the region.

2. Geological setting The northwestern Superior Province has been subdivided into distinct subprovinces (Card and Ciesielski, 1986), including the volcano-plutonic Uchi and Sachigo subprovinces, the intervening, mainly plutonic Berens River Subprovince and the metasedimentary English River Subprovince [Fig. 1(a)]. These subprovinces include segments of pre-Kenoran (i.e. 2750–3100 Ma) crust, the oldest of which have been redefined as the North Caribou Terrane [Fig. 1(a); Thurston et al. (1991)]

an inferred pre-Kenoran cratonic nucleus affected by massive magmatic and tectonic accretion during the Kenoran orogeny (ca 2750–2670 Ma). These subprovinces also share a common history with parts of the northeastern Superior Province (Percival et al., 1994), together comprising the Uchi–Sachigo–Goudalie superterrane (Stott, 1997). The Uchi Subprovince is marked by an easttrending succession of greenstone belts engulfed in granitoid rocks [Fig. 1(a)]. The supracrustal rocks in the greenstone belts and some of the plutonic suites formed during intermittent episodes between ca 3000 and 2700 Ma. There is evidence for early periods of tectonism that deformed and consolidated segments of the crust prior to the concluding and dominant Uchian phase of the Kenoran orogeny (Stott and Corfu, 1991). Metasedimentary rocks are widespread at the southern edge of the subprovince and can be linked to the extensive turbiditic successions of the English River Subprovince (Corfu and Stott, 1993b; Corfu et al., 1995). Geological and geochronological evidence indicates a general southward younging trend of volcanism and plutonism within the subprovince, supporting the concept of an outward-growing Andean-type crustal evolution (Stott and Corfu, 1991). The Berens River Suprovince is a batholithic domain forming the hinterland of the Uchi Subprovince. It includes a few remnants of preKenoran greenstone belts and tonalitic gneisses that were invaded by tonalitic to granitic batholiths between 2750 and 2690 Ma [Fig. 2; Corfu and Stone (1998a)]. The Berens River Subprovince is partially in fault contact with the Sachigo Subprovince, which comprises a number of greenstone belts with a longevity and history broadly comparable to those of the Uchi Subprovince. 2.1. Red Lake greenstone belt The overall structure of the Red Lake greenstone belt is complex and still poorly understood. The belt can be subdivided into ca 3000–2900 Ma supracrustal assemblages along a central, west– southwest trending axis, and younger (2760– 2730 Ma) outward-facing assemblages on the

F. Corfu et al. / Precambrian Research 92 (1998) 277–295

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Fig. 1. (a) Geological map of a portion of the northwestern Superior Province showing main tectonic subdivisions and distribution of greenstone belts. Outline of North Caribou Terrane is from Thurston et al. (1991). (b)–(d ) Geological maps of the Favourable Lake and North Spirit Lake, Hornby Lake and McInnes Lake and Red Lake greenstone belts showing the main distribution of supracrustal assemblages and sample locations. Abbreviations refer to assemblage designations given in Fig. 2.

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Fig. 2. Schematic chronostratigraphic sections for the main greenstone belts and batholiths from the northwestern Superior Province in Ontario. The vertical bar on the zircon ages indicates the analytical uncertainty. See Fig. 1 for locations. Ages are from this paper and from Nunes and Thurston (1980), Corfu and Ayres (1984), Corfu et al. (1985), Corfu and Wallace (1986), Corfu and Wood (1986), Corfu and Andrews (1987), Scha¨rer (1989), Corfu and Ayres (1991), Corfu and Stott (1993a,b), Stevenson (1995) and Corfu and Stone (1998a).

northern and southern flanks, suggesting a general anticlinal structure [Fig. 1(d ); Pirie (1981)]. The margins of the surrounding batholiths, however, dip inwards toward the greenstone belt defining a synformal keel (Andrews et al., 1986). There are also several stratigraphic discontinuities that indicate internal tectonic juxtapositions of rock packages (Stott and Corfu, 1991; Stott, 1997). The dominant planar fabrics are generally subvertical. The belt is transected by northeast and east–

southeast trending high-strain zones which developed late in the tectonic evolution of the belt (Andrews et al., 1986). These high-strain zones are generally associated with highly altered rocks and are of particular economic significance because they represent the locus of several major gold deposits. Based on lithological and geochronological characteristics, the supracrustal rocks have been subdivided into several distinct assemblages. The

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Balmer assemblage [Fig. 1(d )Fig. 2] mostly comprises tholeiitic and komatiitic lava flows ( Tomlinson et al., 1998) with interbedded iron formation: local felsic pyroclastic and flow units yield U–Pb zircon ages of 2990–2960 Ma (Corfu and Wallace, 1986; Corfu and Andrews, 1987). The whole-rock Pb–Pb compositions of mafic and felsic volcanic rocks of the Balmer assemblage define isochron ages of 3070 and 2980 Ma, respectively (Gulson et al., 1993). The komatiites yield negative e values of ca −2 suggesting a proveNd nance from enriched mantle sources ( Tomlinson et al., 1998). The Ball assemblage consists of mafic volcanic flows, felsic pyroclastic rocks, and local chert and dolomitic marble beds with stromatolites (Hofmann et al., 1985), one of which is bracketed by volcanic units with U–Pb ages of 2940 and 2925 Ma. The Bruce Channel assemblage is dominated by basalts with minor felsic tuffs, clastic sedimentary rocks and iron formation; two samples have provided ages of 2894 Ma. This assemblage underlies the older Balmer assemblage implying that the two were tectonically juxtaposed prior to folding (Stott and Corfu, 1991). The Confederation assemblage consists dominantly of calc-alkalic pyroclastic units with subordinate tholeiitic components in the southern part, and yields a range of ages between ca 2760 and 2730 Ma. A small package of felsic volcanic rocks in the center of the belt [sample C11, Fig. 1(d )] had provided an age of ca 2830 Ma (Corfu and Wallace, 1986) suggesting an affinity with the Woman assemblage, which is widespread farther east in the central Uchi Subprovince (Stott and Corfu, 1991). The sample giving this age represented a heterolithic pyroclastic unit and yielded complex U–Pb results that could have meant either the presence of a multigenerational zircon population or complex Pb loss. To clarify the age and stratigraphic position of this unit, in this study we have revisited the same sample. The new grain by grain analyses presented below show that the 2830 Ma age is invalid, being the result of analyses of mixed populations with older inherited and younger magmatic zircon, and indicate that this unit is correlative with the Confederation assemblage. Another problem addressed in this study con-

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cerns the stratigraphic position and age of a sedimentary assemblage (herein termed Slate Bay assemblage) which occurs between the >2960 Ma Balmer mafic–ultramafic volcanic assemblage and the 2730 Ma felsic pyroclastic volcanic rocks of the Confederation assemblage in the northwest [Fig. 1(d)]. The Slate Bay assemblage consists of wacke-mudstones with intercalated polymictic conglomerates containing clasts of mixed volcanic origin, grading into more mature arkosic sandstones and conglomerate beds with abundant clasts of plutonic rocks (Pirie and Sawitzky, 1977; Wallace et al., 1986). Aside from local complications due to folding, the succession of Balmer, Slate Bay and Confederation assemblages youngs consistently to the northwest and displays apparently conformable stratigraphic relations suggesting that sedimentation had occurred sometime in the period intervening between 2960 and 2730 Ma ( Wallace et al., 1986; Corfu and Wallace, 1986). The alternative hypothesis was that the Slate Bay assemblage may be a Kenoran syn-orogenic deposit genetically related to the sedimentary successions found along the southern boundary of the Uchi Subprovince and in the English River Subprovince, but tectonically imbricated with older volcanic assemblage in the fashion observed, for example, in the Favourable Lake greenstone belt (Ayres and Corfu, 1991; Corfu and Ayres, 1991) or elsewhere in the Superior Province (Poulsen et al., 1980; Davis et al., 1989). This hypothesis was also supported by the unusual position of the overlying Confederation assemblage volcanic rocks to the north of the belt, that was suggestive of an allochthonous emplacement (Stott and Corfu, 1991). This second interpretation would imply deposition of the sandstones after ca 2720–2710 Ma, and, based on our experience elsewhere, we would then expect to find abundant detritus from syn-orogenic magmatic rocks. Such an hypothesis would necessarily also imply that the apparent conformity of the assemblages masks thrust contacts. The results presented below do not support the latter hypothesis, suggesting instead that the Slate Bay sedimentary package is related to the older Ball and/or Bruce Channel assemblages.

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2.2. McInnes Lake greenstone belt The narrow McInnes Lake greenstone belt belongs to a succession of generally small, northtrending greenstone slivers that can be traced through the Berens Subprovince from the northern edge of the Red Lake greenstone belt [Fig. 1(a) and (c)]. The supracrustal rocks have been metamorphosed at amphibolite facies conditions. They have been subdivided into three distinct stratigraphic assemblages (Cortis et al., 1988; Thurston et al., 1991), which are now consolidated into two based on the U–Pb results presented below (Davis and Moore, 1991; Stone, 1998). The Western McInnes assemblage comprises a lower unit of mafic flows and tuffs overlain by felsic to intermediate pyroclastic volcanic rocks and by epiclastic sedimentary units including turbiditic sandstones and mudstones with local quartz-rich sandstones and conglomerates. The assemblage displays a general northward fining trend suggesting the presence of the ancient volcanic center in the south, but internal variations in facing directions also point to structural complications. The overlying Power assemblage youngs to the east and consists of a basal unit of chemical sediments with iron formation and pelites, followed upwards by pillowed and massive basaltic flows and local komatiites, and by felsic pyroclastic volcanic rocks. The contact with the Western McInnes assemblage is conformable on the local scale but unconformable on the map scale. Swarms of mafic dykes and peridotitic and gabbroic sills locally present in the Western McInnes assemblage have been interpreted to be correlative with the mafic units of the overlying Power assemblage (Cortis et al., 1988). Three felsic volcanic units were sampled for geochronology. They represent the two sections of the Western McInnes assemblage [cycles 1 and 3 of Cortis et al. (1988)] and the Power assemblage [cycle 2 of Cortis et al. (1988)]. 2.3. Hornby Lake greenstone belt This greenstone belt comprises two assemblages [Fig. 1(c); Cortis et al. (1988)]. The Western Hornby assemblage comprises mafic and komatiitic rocks, mainly pillowed and massive flows, and

subordinate sills, with minor wacke, mudstones and iron formation. The Findlay assemblage, in the eastern part of the belt, comprises a turbiditic sequence of wackes, chert bands, mudstones and quartz arenites overlain by dacitic pyroclastic rocks. Facing directions at Findlay Lake are to the southeast but elsewhere tend to be inconsistent. One sample was collected from a thin felsic volcanic unit at the interface between Western Hornby and Findlay assemblages in the northern part of the belt. Although the unit is interpreted to represent the base of the Findlay assemblage its exact stratigraphic affinity remains uncertain. 2.4. Favourable Lake–North Spirit Lake greenstone belts The two greenstone belts appear to be dismembered parts of an originally continuous supracrustal belt [Fig. 1(b)]. They both exhibit wide, fanshaped greenstone segments broadening to the southeast and long, highly attenuated limbs to the northwest. The present geometry appears to follow to some degree the course of the dextral Bear Head Fault, which in this region consists of zshaped segments trending northwest and swinging off to the east and west. One such segment separates the two greenstone belts (Stone, 1998). The North Spirit Lake greenstone belt comprises a variety of ultramafic to felsic volcanic rocks and abundant metasedimentary rocks ( Wood, 1977, 1980a,b), that have been assigned to a number of assemblages by Thurston et al. (1991). The North Spirit assemblage in the northeastern part of the belt includes mafic flows, iron formation and felsic pyroclastic volcanic rocks yielding an age of 3023 Ma (Nunes and Wood, 1980; Corfu and Wood, 1986). The compositionally similar Disrupted assemblage is younger, with complex U–Pb zircon data suggesting deposition sometime in the interval 2950–2800 Ma (Corfu and Wood, 1986). The Nemakwis assemblage consists of quartz arenites, iron formation and komatiitic flows and displays some of the classical features of a platform sequence ( Thurston and Chivers, 1990). This assemblage is exposed in the eastern part of the belt and also appears in a window in the center of the belt below the unconformably overlying Makataiamik and Hewitt assemblages.

F. Corfu et al. / Precambrian Research 92 (1998) 277–295

The Makataiamik assemblage is characterized by basal polymict conglomerates overlain in the north by cross-bedded sandstone, mudstone and marble representing alluvial, fluvial and lacustrine deposits, whereas in the south the basal conglomerate is overlain by conglomerates, sandstones, mudstones and iron formation with the characteristics of channeled submarine fan deposits ( Wood, 1980a). The Hewitt assemblage comprises basaltic and dacitic flows and pyroclastic rocks overlain by arkoses, conglomerates, iron formation and marble and formed around 2740–2730 Ma. The youngest Bijou Point complex consists of fluviatile sedimentary rocks and volcanic/subvolcanic units with some indications of a shoshonitic affinity, one of them dated at 2731 Ma. The eastern part of the Favourable Lake greenstone belt has been subdivided into discrete stratigraphic sequences by Ayres (1977), with further refinements by Ayres (1988), Ayres and Corfu (1991) and Corfu and Ayres (1991). The oldest Setting Net assemblage ( Thurston et al., 1991) includes a basal sequence of subaqueous komatiites and basalts overlain by siltstone, sandstone, marble and ferruginous chert and by 2925 Ma intermediate pyroclastic rocks, lava flows, subvolcanic intrusions and minor sediments representing a calderafilling sequence. The upper part of the assemblage comprises basaltic and komatiitic flows and pyroclastic rocks. The South Trout and Eastern Trout assemblages consist of mixtures of basaltic flows, felsic pyroclastic volcanic rocks and sedimentary rocks and provided ages of ca 2870 and 2858 Ma, respectively. The Northwind assemblage comprises intermediate to felsic lava flows and pyroclastic rocks overlain by felsic domes, mafic flows and pyroclastic rocks yielding an age of ≤2734 Ma. The youngest North Trout assemblage has a basal sequence of pillowed basaltic flows followed upwards by alluvial fluvial conglomerate and by a turbidite fan with minor intercalated felsic tuff dated at 2725 Ma. The stratigraphy has been disrupted by tectonic imbrication that juxtaposed a thrust panel with the 2925 Ma Setting Net assemblage and the 2725 Ma North Trout assemblage on top of the ≥2734 Ma Northwind assemblage, whereas the 2870–2858 Ma South Trout and Eastern Trout assemblages form the smaller uppermost thrust sheets (Ayres and Corfu, 1991).

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Following thrusting, the greenstone belt was folded and metamorphosed at hornfels and amphibolite facies near the margins of the batholiths and greenschist facies at the center (Ayres, 1978). Stone (1998) extended some of the above assemblages and delineated new ones in the western limb of the Favourable Lake greenstone belt. The Gorman Creek assemblage is composed mainly of intermediate fragmental volcanic rocks with local tuffs and bedded sandstone and argillite and may correlate either with the Setting Net or with the Northwind assemblage. The Gorman assemblage consists of bedded sandstone and rare tuffs with local occurrences of marble intercalated with quartzite, quartz arenite and quartz-pebble conglomerate. This association suggests that the Gorman assemblage may represent a >2800 Ma platform-type deposit similar to the Nemakwis assemblage of the North Spirit Lake greenstone belt (Stone, 1998). The Azure assemblage is characterized by conglomerates transitional into sandstones with local felsic to intermediate tuffs, flows and breccias and mafic flows. On the north side and along most of the length of the western Favourable Lake greenstone belt there is a prominent conglomeratic unit containing clasts of most other supracrustal and plutonic rocks in the region suggesting that this is a late deposit, formed in a pull-apart basin related to faulting along the Bear Head Fault. The Azure assemblage overlies 2727±2 Ma tonalitic gneiss (Corfu and Stone, 1998a) and the transition is marked by a few meters of coarse granular and strongly foliated grit followed by conglomeratic units suggesting that this is an unconformity or a highly strained tectonic boundary. In the initial stages of the study it was unclear whether the conglomeratic unit was part of the Azure or, alternatively, an allochthonous part of the platformal Gorman assemblage. One sample of sandstone was analyzed to resolve this question and the data given below reveal the presence of Kenoran-age zircons consistent with unconformable deposition on the tonalite.

3. Analytical methods U–Pb zircon analyses were carried out by isotope dilution following the basic method of Krogh

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(1973) with such modifications as the use of smaller dissolution vessels and ion exchange columns, the use of a mixed 205Pb/235U spike, and the measurement of combined Pb and U loads. Analyses 33 and 34 were carried out without chemical separation. Garnet was processed using an HBr–HNO technique. Details are referenced 3 in other publications from this laboratory (e.g. Corfu and Stott, 1993a; Davis et al., 1989). Most blanks were ≤2 pg Pb and 0.1 pg U. Discordia lines have been calculated using the procedure of Davis (1982). Errors on the isotopic ratios and ages are given at the 2s level.

4. Results 4.1. Heterolithic lapilli tuff, McKenzie Island, Red Lake belt [sample C11, Fig. 1(d), Table 1] The original study (Corfu and Wallace, 1986) showed that this sample contains a morphologically complex population and the results yielded a considerable range of 207Pb/206Pb ages (Fig. 3). Because a set of analyses done on separate fractions of subrounded and euhedral grains all plotted in the same general age range it was concluded that the scatter was probably due to a combination of complex Pb loss superimposed on an originally somewhat heterogeneous population, and the range of the uppermost analyses was taken as the best indication of the age of the rock at 2830±15 Ma. Nevertheless, the data pattern remained perplexing. The (unpublished) detailed description of the fractions used in the original study indicated that the analyses with the youngest 207Pb/206Pb ages contained many crystals with prominently developed {211} and {110} crystal faces [ low A and T indices of Pupin (1980)]. Euhedral tips from such crystals were hand-picked, abraded and analyzed individually and they provide four concordant to 2.2% discordant analyses. These data define a discordia line with an upper intercept age of 2745+7/−4 Ma and a lower intercept age of 1500 Ma (Fig. 3; Table 2) showing that the unit is correlative with the Confederation rather than the Woman assemblage. The originally inferred age of 2830±15 Ma resulted from analyz-

ing mixtures of newly grown and xenocrystic material. 4.2. Quartz-rich sandstone, Slate Bay assemblage, Red Lake belt [sample C38, Fig. 1(d), Table 1] The zircon population exhibits a variety of morphologic characteristics. The grains used for the analyses were selected to reflect the range of variations and represent the most common types present. The data define three main groups with 207Pb/206Pb ages of 2989–2984, 2957 and 2926–2916 Ma (Fig. 3). All three age groups correspond to those of rocks present in the Balmer and Ball assemblages of the Red Lake belt (Fig. 2) indicating a local provenance of the detritus. The data indicate deposition after ca 2916 Ma. There is no strict lower constraint for the age of sedimentation, but the absence of younger components derived from the Woman, Confederation, or St. Joseph assemblages strongly suggests that the sediment predates these volcanic episodes. In fact all the Kenoran, syn-orogenic sedimentary rocks analyzed by us elsewhere in the northwestern Superior Province (e.g. Davis et al., 1990; Davis, 1996) contain abundant detrital zircons with ages close to that of deposition, presumably reflecting the fact that the youngest volcanic strata are the most easily affected by erosional processes during synorogenic compression. These arguments suggest that the Slate Bay sedimentary package is broadly correlative with the Ball or Bruce Channel assemblages ( Fig. 2). 4.3. Felsic volcanic rocks, Western McInnes assemblage, McInnes belt [samples OGS60, OGS80, Fig. 2(c), Table 1] During mapping of the belt, the Western McInnes assemblage had been subdivided into two separate cycles (Cortis et al., 1988). The separation was suggested by compositional differences such as the lower abundance of mafic units, lack of epiclastic sedimentary rocks and coarser grain and fragment size in the south with respect to the north and was supported by apparently contrasting though sparse stratigraphic facing directions. One

Mineral propertiesa

Th/Uc

Pbc (pg)d

207Pb/204Pbe

206Pb/238Uf

±2s

17.679 17.674 17.678 16.742

17.879 17.926 17.822 17.296 16.758 16.728 16.673 16.678

OGS60 (=OGS90-80) rhyolite, West McInnes assemblage (South=cycle 3), McInnes Lake greenstone belt 19 z [f ] 30 152 0.62 7.3 5163 0.5850 0.0036 20 z br [f ] 30 55 0.60 1.6 8318 0.5851 0.0045 21 z pbr [8] 10 79 0.66 3.3 1985 0.5850 0.0045 22 z br NA [f ] 30 103 0.65 2.7 8930 0.5589 0.0032

0.0029 0.0029 0.0028 0.0028 0.0028 0.0027 0.0026 0.0029

17.643 17.566 17.636 17.519 15.489 13.923

0.5862 0.5878 0.5863 0.5786 0.5715 0.5708 0.5696 0.5724 Lake greenstone belt 0.5857 0.0048 0.5831 0.0037 0.5852 0.0036 0.5834 0.0037 0.5317 0.0030 0.5399 0.0244

greenstone belt 2.3 536 0.7 2495 0.9 5863 0.6 17 775 0.6 1280 1.4 6158 3.2 14 237 27 133

OGS60 (=OGS90-60) rhyolite, West McInnes assemblage (North=cycle 1), McInnes 13 z [10] 10 40 0.42 51 79.44 14 z pbr eu eq [f ] 30 126 0.52 3.3 9287 15 z [20] 20 80 0.50 12 1079 16 z [30] 30 78 0.52 22 891 17 z NA [f ] 10 145 0.42 2.2 4779 18 garnet [f ] 358 0 — 12 21.795

C38 (=C-93-38) sandstone, Slate Bay assemblage, Red Lake 5 z eu sp [1] 8 18 0.36 6 z an eq [1] 10 20 0.49 7 z an eq [1] 34 20 0.54 8 z eu sp [1] 12 118 0.66 9 z eu sp [1] 2 50 0.35 10 z eu sp [1] 18 64 0.46 11 z an eq br [1] 10 599 0.76 12 z eu sp [1] 4 104 0.40

0.107 0.133 0.134 0.092

0.234 0.107 0.108 0.107 0.085 1.115

0.099 0.100 0.095 0.093 0.092 0.088 0.078 0.103

0.073 0.081 0.090 0.070

207Pb/235Uf ±2s

tuff, McKenzie Island, Confederation assemblage, Red Lake greenstone belt 4 163 0.51 1.3 3146 0.5284 0.0025 13.839 1 84 0.31 1.1 499 0.5278 0.0028 13.799 1 776 0.51 1.7 2924 0.5241 0.0033 13.655 1 674 0.49 10 402 0.5119 0.0023 13.184

Weight U (ppm)b (mg)b

C11 (=C-8-11) heterolithic lapilli 1 z eu tip LAT pk-br [1] 2 z eu tip LAT [1] 3 z eu tip LAT br [1] 4 z eu tip LAT br trl [1]

No.

Table 1 U–Pb data

2974.5 2973.8 2974.5 2960.6

2970 2969.7 2970.3 2964.3 2915.5 2716

2989.4 2989.3 2984.2 2957.0 2926.0 2925.0 2923.1 2915.7

2741.9 2738.9 2733.1 2714.3

(Ma)

207Pb/206Uf

0.3 0.3 0.7 2.2

Discordance (%)g

2.6 3.5 2.9 3.5

0.2 0.2 0.2 4.1

14 −0.1 3.5 0.4 2.7 0.0 3.6 0.1 3.5 7.0 112 −3.0

2.7 0.6 2.3 0.4 2.2 0.4 2.3 0.6 2.5 0.5 2.3 0.6 1.3 0.7 4.3 −0.1

2.5 3.0 3.2 2.6

±2s

F. Corfu et al. / Precambrian Research 92 (1998) 277–295 285

206Pb/238Uf

±2s

15.383 13.900 13.789 13.882 13.892 13.844 13.656

16.147 16.443 16.432

16.844 16.869 16.129

207Pb/235Uf

0.087 0.083 0.085 0.073 0.093 0.176 0.162

0.214 0.124 0.101

0.102 0.102 0.077

±2s

2843.0 2753.6 2752.0 2749.0 2738.5 2736.4 2727

2894 2901.5 2900.1

2927.8 2927.8 2912.0

(Ma)

207Pb/206Uf

2.8 2.3 1.3 2.4 3.3 6.2 13

14 3.5 3.0

2.9 2.9 2.3

±2s

0.4 1.1 1.7 0.8 −0.4 −0.3 0.1

0.9 −0.2 −0.2

0.2 0.0 2.8

Discordance (%)g

az, zircon, all minerals abraded prior to analysis ( Krogh, 1982) unless otherwise stated (NA, not abraded); eu, euhedral; sb, subhedral; an, anhedral; el, elongate; sp, shortprismatic; eq, equant; br, brown; pbr, pale brown; pk, pink; trl, translucent; LAT, HAT, low, high A and T indices (cf Pupin, 1980), [n], number of grains; f, fraction of >30 grains. bWeights known to better than 10% when >10 mg, and ca 50% when <2 mg; accuracy of U-concentration is roughly proportional to uncertainty of sample weight. cModel Th/U ratio estimated from 208Pb/206Pb ratio and age of sample. dTotal common Pb in sample, assuming all has blank isotopic composition. eCorrected for spike and fractionation. fCorrected for spike, fractionation, blank and initial common Pb, which is estimated using model of Stacey and Kramers (1975). gPercent discordance for the given 207Pb/206Pb age.

belt 0.5616 0.5693 0.5694

Varveclay Lake, Azure assemblage, Favourable Lake greenstone belt 2 147 0.45 1.7 1192 0.5521 0.0028 6 97 0.46 1.2 2967 0.5269 0.0029 4 979 0.09 4.5 5434 0.5232 0.0031 3 138 0.60 0.7 3524 0.5277 0.0025 0.6 46 0.56 0.5 397 0.5315 0.0033 0.8 13 1.82 0.4 188 0.5303 0.0063 1 120 0.77 15 64.62 0.5261 0.0041

207Pb/204Pbe

C54 (=C-93-54) sandstone, 29 z eu sp HAT [1] 30 z sb-an [1] 31 z sb-an br [1] 32 z sb-an [1] 33 z eu sp HAT [1] 34 z sb-an [1] 35 z eu sp HAT [1]

Pbc (pg)d

0.0046 0.0044 0.0035

Th/Uc

rhyolite, Findlay assemblage, Hornby Lake greenstone 5 69 0.55 45.1 73.42 15 39 0.61 2.5 1857 15 37 0.65 2.2 1929

U (ppm)b

OGS108B (=OGS90-108B) 26 z pbr [4] 27 z pbr el [1] 28 z pbr el [1]

Weight (mg)b

0.0035 0.0035 0.0026

Mineral propertiesa

OGS52 (=OGS90-52) rhyolite, Power assemblage (cycle 2), McInnes Lake greenstone belt 23 z [ f ] 31 249 0.55 3.1 19 387 0.5738 24 z eu eq br [14] 15 243 0.50 2.3 12 294 0.5747 25 z NA [f ] 58 292 0.52 35 3626 0.5548

No.

Table 1 (continued ) U–Pb data

286 F. Corfu et al. / Precambrian Research 92 (1998) 277–295

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Fig. 3. Concordia diagrams with zircon U–Pb data from Table 1. Ellipses indicate the 2s uncertainty.

Table 2 Summary of U–Pb ages Sample

Description

Upper intercept age (Ma)

Lower intercept age (Ma)

Prob. of fit (%)

No. of points

C-80-11 OGS90-60 OGS90-80

Red. L., Confederation McInnes L., Western McInnes (North) McInnes L., Western McInnes (South)

OGS90-52

McInnes L., Power

OGS90-108B

Hornby L., Findlay

2744.5+6.6/−4.1 2969.4+3.6/−3.2 2975.0+2.8/−2.2 2974.4±1.5 2928.3+4.4/−3.1 2927.7±1.9 2900.7±2.1

1503+210/−160 1166±93 726±200 0a 1011±220 0a 0a

74 47 96 94 76 91 54

4 5 4 3 3 2 2

aLower intercept constrained to pass through 0 Ma. Average 207Pb/206Pb age of concordant data.

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volcanic unit from each cycle was sampled for geochronology. Sample OGS60 from the northern part of the Western McInnes assemblage contains abundant, generally fine grained, euhedral short prismatic, highly fractured zircon. Five zircon fractions are collinear defining intercept ages of 2969±3 and 1166±93 Ma ( Fig. 3 and Table 2). One analysis of metamorphic garnet reveals very low U and radiogenic Pb contents seriously limiting the precision of the age determination. The garnet 207Pb/206Pb age of 2716±112 Ma ( Table 1) is nevertheless consistent with the timing of major plutonism that formed the surrounding Berens River batholiths (Corfu and Stone, 1998a) and metamorphosed this small greenstone belt (Stone, 1998; Ayres, 1978). Sample OGS80 represents the southern part of the assemblage. Zircon is abundant, of coarse grain size, colorless, euhedral and short prismatic. Four zircon analyses yield a very coherent discordia line with an upper intercept age of 2975+3/−2 Ma and a lower intercept age of 726±200 ( Fig. 3Table 2). Three of the analyses overlap on Concordia yielding a mean 207Pb/206Pb age of 2974±2 Ma, which is considered the best estimate for extrusion of the rhyolite. The age is just marginally older than that of the unit in the northern sequence indicating formation during essentially the same volcanic period. The geological and age relationships suggest that the southern sequence represents a proximal facies grading into a more distal facies in the northern parts of the assemblage (Stone, 1998). 4.4. Rhyolite, Power assemblage, McInnes belt [sample OGS52, Fig. 1(c), Table 1] The presence in the Western McInnes assemblage of mafic dykes and sills resembling basaltic flows of the Power assemblage, and the intervening basal unconformity inferred from the map pattern, suggest that the latter was built on the former during a younger volcanic episode. This interpretation is also supported by the zircon isotopic data for one felsic volcanic unit of the Power assemblage. The population comprises abundant coarse, euhedral, short prismatic zircons. Two concordant

and one discordant analyses are collinear defining intercepts of 2928+4/−3 and 1011±220 Ma. Because the precision of the upper intercept age is affected by the limited spread of the data along the line, the mean 207Pb/206Pb age of 2928±2 Ma of the two perfectly concordant analyses is taken as the preferred age of crystallization of the unit. 4.5. Felsic volcanic rock, Findlay assemblage, Hornby Lake belt [sample OGS108B, Fig. 1(c), Table 1] The sample contains a sparse population of rounded, short prismatic zircon grains varying from colorless, unfractured to brown and altered. Two analyses of single grains yield concordant data points with a mean 207Pb/206Pb age of 2901±2 Ma, which is overlapped by the age of a third, less precise and slightly discordant multigrain analysis. 4.6. Sandstone, Azure assemblage, western Favourable Lake belt [sample C54, Fig. 1(b), Table 1] The primary question regarding the age of this sample was whether it post-dated the underlying 2727 Ma tonalite or whether it represented an old sedimentary succession, similar to the Slate Bay assemblage and possibly correlative with the marble and quartz arenite bearing Gorman assemblage. For the isotopic analyses two groups of zircons were separated. One group consisted of euhedral to subhedral crystal showing relatively strongly developed {100} and {101} crystallographic faces [high A and T indices of Pupin (1980)], comparable to those dominating the population of the underlying tonalite. The second group consisted of an assortment of subhedral to anhedral grains representing main types present in the population. After abrasion three grains were measured from the first group yielding ages of 2843, 2739 and 2727 Ma. The latter analysis is relatively imprecise but corresponds within error to the age of the tonalite, whereas the other two crystals were evidently derived from older sources. Three of the four grains from the second group are 0.8–1.7% discordant and show similar

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207Pb/206Pb ages of ca 2750 Ma, while the fourth data point is concordant at ca 2736 Ma. The dominance of Kenoran-age zircons clearly proves that the sediment does not belong to one of the earlier platform-type successions. Most grains can be attributed to volcanic or plutonic sources found in the region (Fig. 2). Although imprecise, the youngest analysis overlaps the age of the underlying tonalite, supporting the previously discussed geological evidence in favour of an unconformity. The underlying tonalite contains titanite with an age of 2713 Ma which is a minimum age for the gneissification and metamorphism, and indirectly also for the preceding exhumation and erosion of the tonalite (Corfu and Stone, 1998a,b). This suggests that the Azure sedimentary assemblage is probable equivalent to the North Trout assemblage in the eastern Favourable Lake greenstone belt, which contains a volcanic unit dated at 2725 Ma (Fig. 2).

5. Discussion 5.1. Early pre-Kenoran stratigraphy The ages of 2974–2901 Ma obtained in the McInnes and Hornby Lake greenstone belts show that these sequences are correlative with the oldest assemblages present in the larger greenstone belts to the north and south ( Figs. 1 and 2). The 2974–2969 Ma Western McInnes assemblage can be linked to the Balmer assemblage in the Red Lake greenstone belt. The 2928 Ma Power assemblage correlates with the Ball assemblage at Red Lake, the Setting Net assemblage at Favourable Lake and is also correlative with a tonalite pluton on the northern shore of North Spirit Lake (Stevenson, 1995). The 2901 Ma Findlay assemblage of the Hornby Lake greenstone belt is close in age to volcanic units of the Bruce Channel assemblage in Red Lake. The metasedimentary unit in the Slate Bay assemblage was deposited after 2916 Ma and also received detritus from sources ca 2925, 2960 and 2990 Ma in age, which are all present in the Red Lake greenstone belt. The abundance of these early greenstone belt components and absence of

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younger detritus strongly suggests that sedimentation occurred relatively early in the development of the belt, probably in connection with development of the Bruce Channel assemblage. It is interesting to note that the ≤2916 Ma Slate Bay assemblage post-dates a very similar clastic succession present at the top of the Western McInnes assemblage, which must have formed prior to 2928 Ma, the age of the unconformably overlying Power assemblage. Both clastic successions include turbidites derived largely from the erosion of volcanic rocks, as well as quartz-rich sandstones that appear to have formed by local reworking of the host sediments and are atypical of platform-type successions (Cortis et al., 1988). Similar sedimentary units have also been described from the 2925 Ma Setting Net assemblage in the Favourable Lake greenstone belt (Ayres, 1977; Ayres and Corfu, 1991). It is more difficult to establish a direct comparison between these successions and the platformal quartz arenites of the Nemakwis assemblage at Favourable Lake. The Nemakwis quartz arenite zircons were not analyzed grain by grain but rather in multigrain fractions, yet the good collinearity and lack of dispersion between different analyses suggested the occurrence of a very restricted age spectrum in the population. Moreover, the 2986 Ma age of this population is similar to that of individual tonalite clasts found in conglomerate at the base of the Makataiamik assemblage [Figs. 1 and 2; Corfu and Wood (1986)]. This suggests that the quartz arenite was deposited at a time close to that of the detrital zircon age and derived from a relatively uniform nearby source. The area surrounding the quartz arenite is geologically complex and the relationship of this unit with the apparently much younger (2950–2800 Ma) Disrupted assemblage is still poorly understood; a proper resolution will require more extensive geological and geochronological work. The narrow detrital zircon age spread in these quartz arenites is consistent with the observations from grain-by-grain dating in other samples of the northwestern Superior Province that indicate very tight age groupings for detrital zircons from individual quartz arenites. These new data consolidate the picture emerging for the early period of greenstone belt magmatism

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between ca 3000 and 2870 Ma as a succession of discrete volcano-plutonic events including submarine extrusion of basaltic and komatiitic lavas, build-up of locally emergent arc volcanoes, and clastic deposition in flanking turbidite basins, with shallow water reworking to form local quartz-rich sedimentary strata and co-deposition of stromatolitic carbonate sequences. Some of the tonalite intrusions exposed at Favourable Lake (2950 Ma) and at North Spirit Lake (2925 Ma), and indirectly indicated by 3000–2950 Ma clasts in conglomerates of both greenstone belts, appear to represent exhumed deeper levels of the arc complexes. This early period of intermittent magmatic activity was probably concluded by a compressional orogenic event well established in the North Caribou Lake greenstone belt further to the northeast [Fig. 1(a)], where mixed volcanic-sedimentary sequences comparable in age and composition to those evaluated in the present study were metamorphosed, deformed and intruded at 2870 Ma by the posttectonic North Caribou pluton (Davis and Moore, 1991; Stott, 1997). One of the additional characteristics of the early greenstone sequences in the northwestern Superior Province is the widespread occurrence of komatiites, in contrast to the apparent absence of the latter from the Kenoran assemblages of the region. The available age constraints, however, do not limit the genesis of these komatiites to a single short-lived event; they occur within assemblages spanning a period of at least 100 m.y. Moreover, their association with typical arc complexes requires a mechanism that generated the komatiites in an arc setting. The occurrence of mafic dykes and mafic–ultramafic intrusions in the Western McInnes assemblage and their inferred role as feeders to the basaltic and komatiitic volcanic rocks of the overlying Power assemblage supports an arc setting for the ultramafic units. This is broadly comparable to the relationships farther south in the Superior Province, except that those komatiites and arc sequences intruded much later during the Kenoran events and spanned considerably shorter periods of time (Corfu, 1993; Jackson et al., 1994; Ayer and Davis, 1997; Corfu and Stott, 1998). Tomlinson et al. (1998) discuss various models for the genesis of komatiites of the

Balmer assemblage. One of the interpretations involves ridge subduction, which could explain the common association of komatiites and arc sequences; their preferred model, however, relates the genesis of the komatiites to plume activity. Just as komatiites are not all the same age, the early clastic sedimentary successions cannot be assigned to a single time marker, but appear to represent a number of discrete erosional periods related to volcanic activity. Examples include the <2916 Ma Slate Bay assemblage, the >2928 Ma Western McInnes assemblage, and a number of other sedimentary packages associated with various assemblages of the region, although their exact ages are generally poorly constrained. These observations suggest that sedimentation occurred in individual basins associated with active volcanic complexes, but there is good evidence for extensive emergence, resultant reworking of the sediments into quartz-rich sandstones and deposition of stromatolitic carbonate horizons. 5.2. Activity in the period 2870–2800 Ma This period appears to have been characterized by qualitatively similar but less intense magmatism than the preceding one, at least in the present area of study (Fig. 2). The volcanic units of the Nemakwis assemblage at North Spirit Lake could have formed in this interval. Two ages of 2870 Ma were obtained for baddeleyite and zircon in a mafic unit intruding the Balmer assemblage at Red Lake and for diorite in the South Trout assemblage at Favourable Lake, whereas a volcanic unit in the East Trout assemblage is slightly younger at 2858 Ma. A similar age was also obtained for tonalite south of the McInnes greenstone belt. Volcanic rocks of the Woman assemblage and tonalites with ages in the range 2840–2800 Ma are widespread in the central Uchi Subprovince and in the Trout Lake batholith (Noble, 1989; Scha¨rer, 1989; Corfu and Stott, 1993b). The present revision of the age of the pyroclastic volcanic unit on McKenzie Island, however, has eliminated the only presumed member of the Woman assemblage in the Red Lake greenstone belt. Gulson et al. (1993) reported a Pb–Pb age of ca 2865 for Cu–Ni sulfides from the Bridget Lake prospect located in the Ball

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assemblage of western Red Lake, and, based on a comparison with other data in the belt, they suggest that this event may have been significant for the genesis of some of the gold in the belt. Stromatolitic marbles are present on top of the Woman assemblage in the Birch–Uchi greenstone belt indicating the existence of shallow-water conditions (Hofmann et al., 1985). The region lacked significant magmatic activity between ca 2800 and 2750 Ma. In the Bamaji greenstone belt of the central Uchi Subprovince, however, local extension unroofed the 2805 Ma Bamaji pluton, shedding detritus into overlying conglomeratic sequences whose younger age limit is constrained by a 2781 Ma felsic pyroclastic unit (Scha¨rer, 1989). A similar extensional phase is also known in the northeastern Superior Province (Percival et al., 1994; Skulski and Percival, 1996). 5.3. Kenoran activity The best developed unconformity to disconformity is present at the base of the Makataiamik sequence in the North Spirit Lake greenstone belt [Fig. 1(b)Fig. 2; Wood (1980a,b)]. Although the presently available ages do not provide tight constraints for this period of erosion, the geological relationships between the Makataiamik and the metavolcanic Hewitt assemblage indicate that major sedimentation occurred mainly after ca 2750 Ma and was broadly correlative with extensive arc magmatism of the Confederation assemblage in the Uchi Subprovince and in the Favourable Lake greenstone belt. No volcanic activity occurred at this time in the Hornby Lake and McInnes Lake greenstone belts, although correlative plutons are found throughout the Berens River area. Volcanic units of the younger St. Joseph assemblage (Fig. 2) are widespread along the southern border of the Uchi Subprovince (Stott and Corfu, 1991) although they do not seem to be present in the Red Lake greenstone belt where this period was characterized mainly by the intrusion of highlevel plutons. The youngest known volcanic unit (2725 Ma) of the Favourable Lake greenstone belt occurs within the sedimentary North Trout assemblage, which represents a progression from shallow

291

water alluvial fluvial deposits to deep water turbidites overlain by basalts, and may be a lateral facies equivalent of the ≤2734 Ma Northwind assemblage (Ayres and Corfu, 1991). The 2727 Ma age of the tonalite unroofed prior to deposition of the Azure assemblage (Stone, 1998) and the presence of widespread Kenoran detritus in the latter suggest that the Azure and North Trout assemblages are lateral equivalents. The identity of the Bijou assemblage, the uppermost assemblage at North Spirit Lake, remains uncertain. Smith and Longstaffe (1974) suggested that volcanic–subvolcanic units associated with clastic sedimentary sequences of this assemblage are of shoshonitic affinity, inviting a comparison with the ‘Timiskaming-type’ association of the Abitibi and Shebandowan greenstone belts ( Thurston et al., 1991). Wood (1977), however, noted that the high degree of alteration may have biased the major element chemistry of these rocks, and Carter (1991), based on a field examination, also argued against an alkalic origin of the rock. Corfu and Stone (1998a) point out that the age of 2731 Ma for an intrusive member of the Bijou complex (Corfu and Wood, 1986) is anomalous with respect to the much younger ages of ca 2700 Ma for sanukitoid plutons in the region. The present understanding would seem to indicate that the Bijou assemblage is a lateral equivalent of the North Trout and Azure assemblages. Stone (1998) lists the polymict character, large variety of clasts, and continuity of conglomeratic units in the Azure assemblage and the latter’s position along the transcurrent Bear Head Fault, as arguments in favour of an origin in a late-tectonic pull-apart basin. The volcanic and sedimentary activity in the period starting ca 2750 Ma was accompanied by extensive plutonism that emplaced plutonic bodies inside the greenstone belts and generated the bulk of the batholithic material of the Berens River Subprovince [Fig. 2; Corfu and Stone (1998a)]. These plutons show a general progression from predominantly hornblende tonalite emplaced during the principal stages of volcanism, to increasing proportions of hornblende granodiorite and biotite granite intruded during the major period of compression. Sanukitoid plutons and local per-

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aluminous granites are among the youngest intrusions in the region at ca 2700–2690 Ma. Although most of the Kenoran magmatic and tectonic activity can be placed in a convergent plate setting, there are many features that point to significantly different tectonic settings controlling the evolution of separate subareas of the region. In general, the Uchi Subprovince appears to have undergone accretion by arc magmatism in marginal basins on and outboard of the leading edge of the North Caribou protocontinent (Stott and Corfu, 1991; Corfu and Stott, 1996; Stone, 1998). By contrast, the North Spirit Lake and Favourable Lake greenstone belts exhibit many features indicative of an extensional or transtensional setting. The Makataiamik assemblage is indicative of shallow to deep water sedimentation associated with local basaltic accumulations and followed by the mixed basaltic and intermediate to felsic volcanism interspersed with periods of sedimentation of the Hewitt assemblage. These relationships appear to reflect the opening of a basin. A similar trend is shown by the North Trout assemblage, which progresses from alluvial–fluvial to deeper water turbiditic sedimentation followed by basaltic magmatism (Ayres and Corfu, 1991). Felsic volcanic units from the Hewitt assemblage and the Northwind assemblage in the North Spirit Lake and Favourable Lake greenstone belts, respectively, contain abundant xenocrystic zircon (Corfu and Wood, 1986; Corfu and Ayres, 1991). Such occurrences are much less common in the Uchi Subprovince (Corfu and Stott, 1993b), the McKenzie Island tuff re-analyzed in this study (C11) being one the most significant exceptions. Hence, the North Spirit Lake and Favourable Lake greenstone belts are interpreted as having formed in back-arc settings (Stone, 1998) behind the main arc developing in the region of the Uchi Subprovince. It is uncertain how much of the present geometry of these greenstone belts still reflects the early stages of Kenoran development, because subsequent deformation and extensive plutonism probably affected the original relationships. The general shape of the North Spirit Lake and Favourable Lake greenstone belts resembles mega-tension gashes that may have opened during dextral

motion along a pre-existing zone of weakness, the ancestral Bear Head Fault ( Thurston et al., 1991). Continuing transpressional motion could explain the almost coeval occurrence of complex tectonic imbrication in the Favourable Lake greenstone belt (Ayres and Corfu, 1991) and the widespread deposition of clastic sedimentary rocks. Sedimentation appears to have spanned the whole period from inception of the basin (Makataiamik assemblage) to its final closure (Azure assemblage). In this context it is also useful to note that several of the tonalitic bodies emplaced ca 2740–2730 Ma in the northern Berens block, most prominently those in the vicinity of the Cherrington greenstone belt [Fig. 1(a)], display elongate northwest trending shapes subparallel to the Bear Head Fault (Stone, 1998). This suggests tectonically controlled emplacement in a transpressive regime as has also been suggested for intrusion of en-echelon plutons of the Sierra Nevada batholith ( Tikoff and de Saint Blanquat, 1997). These structures become less pronounced in the southern part of the Berens block, perhaps largely because of overprinting by the unoriented younger (2720–2700 Ma) granitic bodies, but presumably also due to a rotation of the local stress field. In the eastern Red Lake greenstone belt complex Au-hosting deformation zones formed under a regime of northeast directed shortening (Zhang et al., 1997). A similar northeast directed compression could also explain the southeast trending anti- and synforms of the Favourable Lake and North Spirit Lake greenstone belts. This local direction is perpendicular to the dominant, northwest compression of the Kenoran orogeny throughout the Superior Province (Stott, 1997). Thus, there was a progression from strike-slip tectonics along northwest directions, opening volcano-sedimentary basins and controlling the shape of plutonic bodies within the North Caribou terrane between 2750 and 2720 Ma, and a re-orientation of the Berens River area stress field to a northeast direction during the compressional orogeny between ca 2720 and 2710 Ma. A detailed assessment of the complex interaction between regional and local factors controlling the stress fields, and of their implications on the structural, magmatic, sedimentary and mineralizing processes, represents one of the main

F. Corfu et al. / Precambrian Research 92 (1998) 277–295

challenges for future geological and geochronological studies in the region.

6. Conclusions The new isotopic data and their integration into the regional framework consolidate our understanding of the regional greenstone belt stratigraphy in the northwestern Superior Province, confirming the existence of evolutionary periods with distinct magmatic and tectonic connotations. Ages of 2974±2 Ma and 2969±3 Ma for the Western McInnes assemblage, 2928±2 Ma for the Power assemblage, and 2901±2 Ma for the Findlay assemblage of the McInnes Lake and Hornby Lake greenstone belts in the Berens River batholithic block show that they are correlative with the older components of the Red Lake, North Spirit Lake and Favourable Lake greenstone belts, and an integral part of the larger pre-Kenoran North Caribou crustal block. The metasedimentary Slate Bay assemblage in the northern part of the Red Lake greenstone belt yields a range of detrital zircon ages between 2989 and 2916 Ma suggesting that it was deposited during or closely following volcanic activity of the Ball and Bruce Channel assemblages in the period 2920–2890 Ma. Similar sedimentary deposits, locally containing reworked shallow water, quartz rich sandstones and stromatolitic carbonates are widespread in the region, and appear to have evolved at different times as integral components of the volcanic complexes between ca ≥3020 and 2870 Ma. This intermittent arc volcanic activity was also accompanied by tholeiitic and komatiitic magmatism. The detrital zircon age spectrum of 2843–2727 Ma indicates syn-Kenoran deposition of the Azure assemblage in the western Favourable Lake greenstone belt. The abundance of Kenoran clastic sedimentary assemblages, their common shallow to deep water sedimentation trends, the association with strongly crustally contaminated calc-alkalic volcanic sequences, and the fact that their deposition accompanied the whole period of basin opening to closure with concomitant imbrication and deformation, suggest that the Kenoran sections of the North Spirit Lake and Favourable

293

Lake greenstone belts formed in transtensional to transpressional basins, presumably controlled by the ancestral Bear Head Fault. Development of these back-arc basins was oblique to the main arc accretion and compressional axis at the southern edge of the North Caribou terrane. A package of pyroclastic volcanic units in the center of the Red Lake greenstone belt, previously reported to be 2830±15 Ma in age (Corfu and Wallace, 1986) has been re-examined revealing the presence of a subpopulation of zircons with an age of 2745+7/−4 Ma, which indicates a correlation with Confederation assemblage arc magmatism in the region.

Acknowledgment This study was carried out at the Jack Satterly Laboratory of the Royal Ontario Museum under the auspices of geochronological programs supported by the Ontario Geological Survey. We thank A.L. Cortis, P.C. Thurston, G.M. Stott and B. Atkinson for help in sample collection and comments, and journal reviewers J.N. Lewry and N. Machado. R. Tahiste assisted with sampling and mineral separation. The paper is published with permission of the Ontario Geological Survey.

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