A model of ocean-crust accretion for the Superior province, Canada

Lithos, 30 (1993) 337-355

337

Elsevier Science Publishers B.V., Amsterdam

A model of ocean-crust accretion for the Superior province, Canada G. Kimura a, J.N. Ludden b, J-P.

D e s r o c h e r s b a n d R. H o r i c

aDepartment of Earth Sciences, Kagawa University, Takamatsu, 760 Japan bDPpartement de GPologie, Universitk de MontrOal, 6128, Succursale ".4 ", Montrkal, Qukbec, H3C 3J7, Canada CDepartment of Earth Sciences, Ehirne UniversiO,, Matsuyama, 790 Japan (Received December 7, 1992; revised and accepted January 15, 1993)

LITHOS

A.ST.AC One of the keys to understanding the origin of Archaean greenstone belts lies in the geological relationships between mafic and ultramafic greenstones, felsic to intermediate volcanic rocks and terrigenous sediments. Traditional models for greenstone belt evolution have been based on in-situ stratigraphic relationships. Most of these models, for example an oceanic island-arc developed on oceanic basement, back-arc basins, and the recently popular plume model, predict concordant stratigraphic relationships among the various greenstone belt lithologies. However, rather than being depositional in nature, several authors have indicated that many of the relationships between the different lithologies in greenstone belts are in fact tectonic, suggesting an allochthonous origin for most greenstone sequences. All of these latter models make analogies to Phanerozoic tectonic processes involving accretion of oceanic materials with volcanism related to both plate subduction and rifting. In this paper, we have evaluated the geological relationships between volcanic rocks and sediments in three regions in the Superior province, where the accretion of oceanic material can be documented, and direct comparisons are made to geological processes in Phanerozoic accretionary complexes. In the Malartic area in the southeastern Abitibi Subprovince, 3 to 4 km thick slices of komatiite and tholeiite, with intercalated terrigenous sediment, are tectonically imbricated and are overlain by calc-alkaline volcanics which postdate tectonic stacking. In both the Larder Lake region of the southwestern Abitibi belt and in the Beardmore-Geraldton belt, at the south-eastern limit of the Wabigoon belt, slices of iron-rich tholeiite and chemical sediments of an oceanic origin are tectonically imbricated with terrigenous sediment. The Malartic-Val d'Or area is considered to be an example of accretion of an Archaean oceanic plateau, while the Larder Lake and the Beardmore-Geraldton regions are potentially typical of accretion of normal oceanic crust in an arc-environment. Phanerozoic accretion of oceanic crust is accompanied by a step-back in subduction, and in this paper we suggest that oceanic crust accretion may have been the principal mechanism by which the locus of subduction migrated towards the south of the Superior province. Asthenospheric upwelling associated with the isolated sinking plate may have been responsible for widespread late-magmatism. This scenario requires that magmas be erupted through previously accreted volcanic, plutonic and sedimentary material. Furthermore, later ridge subduction will result in transpressional tectonics and eruption of mafic sequences over mature and immature volcano-plutonic sequences. The combined result of the plate tectonic scenario envisaged would result in the well-described "cyclic stratigraphy" of many granite greenstone sequences.

Introduction O n e o f the keys to u n d e r s t a n d i n g the origin o f Archaean greenstone belts lies in the geological rela-

tionships between m af i c and u l t r a m a f i c greenstones, felsic to i n t e r m e d i a t e volcanic rocks and terrigenous sediments. T r a d i t i o n a l m o d e l s for greenstone belt evolution have been based on in-situ

0024-4937/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

338

G. KIMURA ET AL.

stratigraphic relationships. Most of these models, for example: the "sagduction" models (Goodwin, 1979; Jensen and Langford, 1985 ); oceanic island-arc developed on oceanic basement (Windley, 1984; Dimroth et al., 1982); back-arc basins (Burke et al., 1975; Tarney et al., 1976); the recently revised plume model (Campbell et al., 1989; Hill et al., 1991 ), predict conformable stratigraphic relationships among the various greenstone belt lithologies. Several recent papers indicate that, in many cases, rather than being depositional in nature many of the relationships between the different lithologies in greenstone belts are in fact tectonic, suggesting an

exotic origin for many greenstone sequences (Ludden et al., 1986; Davis et al., 1988; Corfu and Ayres, 1991; Jackson and Fyon, 1991; Thurston et al., 1991 and Desrochers et al., 1993, for the Superior province; lsozaki et al., 1992; Kimura et al., 1992a; Maruyama et al., 1992, for Pilbara Craton; de Wit et al., 1982, 1992 and Lowe et al., 1985, for the Barberton region). All of these authors draw analogies to Phanerozoic tectonic processes involving accretion of oceanic materials with volcanism related to both plate subduction and rifting. In this paper we evaluate the geological relationships between volcanic rocks and sediments in three

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Fig. 1. Map of the Superior province showing the various subprovinces, based on Card and Ciesielski ( 1986 ) and Thurston and Chivers (1990). The locations of three regions studied in this paper are shown. The sections used to construct the age progressions are given as vertical lines and the age data are projected onto these sections in Fig. 13.

339

MODEL OF OCEAN-CRUST ACCRETION

regions of the Superior province, where we can document the accretion of oceanic material, and make direct comparisons to geological processes in accretionary complexes in the Phanerozoic. In the Val d'Or Malartic area, in the southeastern Abitibi Greenstone belt (Fig. 1 ), 3-4 km thick slices ofkomatiite and tholeiite with intercalated terrigenous sediment are tectonically imbricated and are overlain by calc-alkaline volcanics which post date tectonic-stacking (Desrochers et al., 1993). In both the Larder Lake region of the southwestern Abitibi belt and in the Beardmore-Geraldton belt, at the southern limit of the Wabigoon belt (Fig. 1 ), slices of iron-rich tholeiite and chemical sediments are tectonically imbricated with terrigenous sediment. The Malartic area is considered to be an example of accretion of Archaean oceanic plateau material, while the Larder Lake region and the Beardmore-Geraldton region are potentially typical of accretion of normal oceanic crust in an Archaean arc environment. The role of accretion of oceanic crust and consequent step-back in subduction are evaluated in terms of the tectonic evolution of the Superior province.

Geological features of accreted oceanic fragments in the Abitibi belt

(i) the Malartic Composite Block (MCB) The MCB is located in the Southern Abitibi greenstone belt in the Matagami-Val d'Or region (Fig. 1 ). It is a 140 km by 20 km lozenge-shaped block composed of komatiitic, tholeiitic and calcalkaline flows with intercalated terrigenous sedimentary units metamorphosed to greenschist facies. Previous models for the MCB invoked a stratigraphically conformable sequence of a komatiitic and tholeiitic platform upon which was built a calcalkaline island-arc (Dimroth et al., 1982). A U - P b zircon age of 2705+ 1 Ma (Wong et al., 1991 ) was determined from a rhyolite of the calc-alkaline suite.

(a) Field observations for the MCB A recent structural analysis by Desrochers et al. (1993) indicates that the MCB is composed of five lithotectonic domains (Fig. 2). Four of these domains (Northern, Vassan, Central and Southern domains) comprise tholeiitic and komatiitic flows

and are overlain by the calc-alkaline units of the Val d'Or domain. Despite similarities, which may reflect formation in a similar source region, the rocks of the four mafic-ultramafic domains are stratigraphically unrelated (Desrochers et al., 1993). Deformation events recorded in the rocks of each domain are the key to understanding the relationships between the lithotectonic units. The four mafic-ultramafic domains have Dl tectonic fabrics which are specific to each of the four lithotectonic units. In the Northern, Vassan and Southern domains the Dl event tilted the strata into an E-W striking overturned position, whereas in the Central domain it developed NW-SE trending folds (Fig. 2). The importance of the D1 event is best illustrated in the Central domain where Dl fold traces are truncated by sheared and mylonitized zones which separate the domains. These relationships suggest that at least some of the four blocks were separate entities before their accretion. The respective D~ events took place either during the collision of each of the fragments on the growing Superior province margin, or by collision of an assemblage of fragments that was later accreted to the continental margin. In the southeastern part of the MCB, Desrochers et al. ( 1993 ) have shown that Bourlamaque pluton, which is comagmatic with the volcanic rocks of the Val d'Or domain, truncates primary bedding and DI tectonic fabrics of the Central and Southern domains (Fig. 2a and b). The rocks of the Val d'Or domain and the Bourlamaque pluton, must have been emplaced after the D~ event recorded in the other four domains. A D2 event is represented by an E-W foliation and is recorded in the rocks of the five domains representing a common transpressive tectonic stage.

(b) Tectonic interpretation of the MCB The lithologies and the tectonic relationships in the MCB are consistent with a model of oceanic plateau accretion. The komatiitic and tholeiitic suites of the MCB are comparable to those described in the oceanic plateau of the island of Gorgona (Storey et al., 1991, and section on geochemical constraints, this paper). The mafic-ultramafic fragments of the MCB represent accreted allochthonous material which developed different Dl tectonic fabrics as a consequence of the direction of

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Fig. 2. Malartic Blockgeologyand interpretive diagram showinggeologicrelations between different lithologies from Desrochers et al., 1993. Note the difference in the first-phase deformation of the mafic-ultramafic blocks and the discordant relationships between these blocks and the Val d'Or formation volcanics.

collision. After the accretion of the mafic-ultramafic fragments, the calc-alkaline volcanism erupted through the accreted material to form the Val d'Or domain units and the Bourlamaque pluton. This volcanic activity is interpreted to have taken place in local extensional basins, similar to those as described in the Eocene sequence of the western coast of North America by Babcock et al. ( 1992 ), which are related to ridge subduction.

(ii) the Larder Lake Group The Larder Lake Group ( L L G ) is located in the southwestern Abitibi belt south of Kirkland Lake (Fig. 1 ). The group comprises mafic volcanics, felsic pyroclastics and sedimentary rocks (Fig. 3). Previous models for the LLG imply a depositional relationship among the different lithologies (Jensen, 1978; Jensen and Landford, 1985). A U - P b zircon age of 2705 + 2 Ma (Corfu et al., 1989) from felsic tufts at the base of the LLG is considered to be the age of the LLG.

(a) Field observations for the LLG Our results indicate a tectonic, or unconformable relationship, between the basaltic rocks and terrigenous turbidites and an unconformity between the

basic rocks and felsic pyroclastics on the basis of the following field observations. (i) The greenstones, which are dominated by pillowed basalt, are deformed to the same extent as the sedimentary rocks and contacts between the two lithologies are tectonic. (ii) There is no evidence for interdigitation between terrigenous sediments and pillow basalts. The absence of inter-pillow deposition of terrigenous sediments indicates that the eruption site of the pillow lavas was remote from any site of terrigenous sediment deposition. Our results indicate that the tectonic stacking of the pillow laves and sediments by layer-parallel faults is the cause of the cyclic basaltic magmatism and repeated sedimentation. Similar sequential repetition by layer-parallel faults has been reported for greenstones in the Pilbara craton (Isozaki et al., 1992; Kimura et al., 1992a; Maruyama et al., 1992) and in modern accretionary complexes by Isozaki et al. (1990). (iii) Banded iron formations (BIF) in the Adams Mine area have been considered to be contemporaneous with volcanism (Goodwin, 1979; Blum and Crocker, 1992). Where observable, all of the contacts between the BIF and volcanics are tectonic, except for those between tuff and the basal part of BIF. For example in Fig. 4a, a BIF is strongly

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Fig. 3. Geologic map of the Larder Lake Group in the Larder Lake region, reinterpreted from Jensen and Langford, 1985 and our mapping. The location of the section in Fig. 6 is indicated. Note the thrust imbrication indicated for the Adams mine region and the oceanic sequence in Fig. 6.

folded and truncated by pillow lava. Foliations are developed in the pillow lavas and the boundary is a several centimeter thick mylonite. BIF's show a general composition change: the lower part of the BIF is dominantly composed of sulphide-rich deposits alternating with marie volcanogenic sediments or tuff; the middle section comprises alternation of iron oxide with chert; the upper section includes the distal parts of terrigenous turbidites (Fig. 5). The chert of the BIF exhibits gray to red color transitions and the thickness ranges from several millimeters up to 1 cm, including very fine ( < 1 ram) lamination. Gradual change

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Fig. 4. Photograph of the tectonic boundary between the BIF and overlying pillow basalt in the Adams mine region.

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Fig. 5. Interpreted oceanic plate stratigraphy observed in the Adams Mine. Similar sections from sulphide rich sediments through pelagic sediments and to terrigenous turbidite sequences are documented in the Japanese accretionary complexes (see text).

342

G. KIMURA ET AL.

from sulphide-rich BIF to chert-rich BIF is observed. This sequential change of BIF suggests a progressive change of depositional environment, with initial accumulation resulting from active hydrothermal circulation near a ridge-crust and evolution towards a quiescent open ocean environment involving pelagic chert deposition. Terrigenous sediments are deposited as distal turbidites as the oceanic plate approaches a continental or volcanic margin. The change in sedimentary environment is very similar to the well documented "oceanic plate stratigraphy" of pelagic to hemipelagic sediments within the accretionary complexes in Japan (Taira et al., 1989; Matsuda and Isozaki, 1991 ). (iv) Felsic sequences are sandwiched between mafic lavas and terrigenous sedimentary rocks of the LLG, and field relationships (Fig. 6) indicate that the felsic pyroclastics and reworked sediments unconformably overlie the mafic volcanics. The boundary between the pyroclastic rocks and the underlying komatiite is irregular and the spinifex structures are eroded along the unconformity with fragments of the komatiite being included in the pyroclastic rocks. This unconformable contact is re-

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peated twice by faulting and a basal conglomerate including mafic pebbles is observed in an adjacent outcrop. The pyroclastic rocks evolve gradually to reworked sediments which are composed of volcaniclastic materials. The sediments are undeformed and primary structures are well preserved. Well-developed cross bedding of coarse sandstone and nonturbidite sedimentary structure (rhythmic banded laminae) suggest a shallow deltaic, non turbulent, depositional environment.

(b) Tectonic interpretation of the LLG These field observations require revision of the current models for deposition of the LLG. Mafic rocks of this group can be divided into two parts: (1) the western LLG which includes the Adams mine region; (2) an eastern part characterised by alternating occurrences of pillowed basalts, felsic pyroclastics with reworked sediments and terrigenous turbidites. Based on the tectonic relationships between turbidites and pillowed volcanics and the transition from sulphide-rich to pelagic and finally terrigenous sediments in the BIF's, our interpretation of the western LLG is that it represents a sequence of pillowed basalt and overlying hydrothermal, pelagic and hemipelagic sediments which has been repeated by layer-parallel stacking following formation and transport in an oceanic basin. Fault-stacking and accretion at a subduction zone is an appropriate tectonic scenario. The eastern LLG indicates fault-related stacking of pillow-lava and terrigenous turbidite packages. Felsic pyroclastic sequences and related reworked sediment overlie these rocks. The lack of inter-pillow sediment implies the the pillow-lavas were erupted well outside the limits of turbidite deposition. We propose that the turbidites are trench-fill sediments that were tectonically interlayered with oceanic basement along layer-parallel faults during accretion in a fore-arc environment. Where the chemical sediments are less well developed, we infer a less active hydrothermal system, a shorter oceanic travel history, or tectonic removal of the upper volcanic-sedimentary sequence. The felsic pyroclastics that unconformably overlie the western LLG greenstone-turbidite package most probably represent distal fore-arc deposits. Similar fore-arc pyroclastic rocks overlie accretionary deposits in Japan, for example the Late Creta-

343

MODEL OF OCEAN-CRUST ACCRETION

ceous Shimanto accretionary complex (Isozaki et al., 1990).

(iii) The southern Wabigoon and BeardmoreGeraldton belts Recent studies by Devaney and Williams (1989), Williams ( 1989, 1990) and Williams et al. ( 1991 ) indicate that the southernmost part of the Wabigoon greenstone belt and the northern Quetico belt in the Beardmore-Geraldton region (The Beardmore-Geraldton Belt - BGB - Fig. 1 ) display features very similar to Phanerozoic accretionary complexes. Our observations in the region suggest an exotic origin for the greenstones and overlying chemical sediments and the tectonic interleaving of terrigenous turbidites and overlying arc-related felsic and intermediate units (Fig. 7 ). All of these units

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and the terrigenous sediments are intruded by tonalitic and granodioritic plutons and related porphyries.

(a) Field observations in the BeardmoreGeraldton region Four east-west trending greenstone packages (units 1 to 4) are recognised in the region (Fig. 7; Stott, 1984; Kresz and Zayachivisky, 1991 ) and deformed terrigenous sediments are distributed between each greenstone unit. These greenstone units were described as repeated stacks of oceanic basement by Williams ( 1990); the geological features of each unit, however, differ considerably. The northernmost unit (unit 1 ) is composed of strongly deformed greenstones of various kinds: pillowed basalts, basic volcanogenic sediments or tuff including iron-rich beds, gabbro and serpentinised peridotites. All of these components are metamorphosed, strongly deformed and are in contact with a deformed tonalite of the Wabigoon belt (Fig. 7). The second greenstone unit (Unit 2) is the largest package of greenstones in the BGB and can be subdivided into two sub-units: (i) strained pillow basalts and potentially comagmatic gabbros which dominate the unit; (ii) undeformed mafic pillowlava associated with clast-supported breccia with an origin as fault-scarp terrace deposits. The different deformation characteristics of the sub-units suggest that the lower greenstones were deformed, potentially in a tectonic event related to accretion, and the upper sub-unit was deposited at a later stage

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Fig. 7. Geologicalmap of the Geraldton area based on Williams et al. ( 1989), Stott ( 1984) and Kresz and Zayachrviski (1991).

Fig. 8. Photograph showingthe tectonic relationshipbetween greenstones and banded-iron formations in the Geraldton region.

344

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formed gabbro and mafic volcanogenic sediments is overlain by sediments with iron sulphides which give way to a terrigenous turbidite sequence, displaying a coarsening upward sequence with interleaved bands of iron oxide. The entire sequence is repeated by layer-parallel faults in which small scale duplex structures are observed (Fig. 9). Mappable scale distributions of thin greenstones and accompanying iron-rich sediments have been repeated by the same duplex formation that is observed on the outcrop scale.

(b) Tectonic interpretation of the BGB

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As interpreted by Williams (1990), and as described above, the geological features are typically those of accretionary complexes related to subduction of an oceanic plate. Oceanic crust, composed of pillowed basaltic sequences overlain by hydrothermal sediments and pelagic sediments were intruded by differentiated Fe-rich gabbroic complexes (Kresz and Zayachivisky, 1991 ). Terrigenous sediments, with coarsening upwards sequences, cover the oceanic basement. These rocks were stacked tectonically by layer-parallel faults both on a district scale and also on a layer by layer scale. Terrigenous sediments resemble modern trenchfill turbidites, with the transition upwards to coarser deposits duplicating modern ocean plate stratigraphy (Isozaki et al., 1990). Pelagic and hydrothermal sequences are not as well developed as those described above for the Adams mine. This may reflect a shorter travel history for the oceanic plate or tectonic peeling of the upper oceanic crust as is recorded in Phanerozoic accretionary complexes (Kimura and Mukai, 1990; Matsuda and Isozaki, 1991; Kimura et al., 1992b, c, d). Post-accretionary volcanism related to extension was responsible for the relatively undeformed sequences observed in the uppermost sub-unit of unit 2. Both volcanic units predate later felsic porphyry intrusion and tonalite-granodiorite pluton emplacement.

r$1~BasiCGabbro~SChist Fig. 9. Interpreted oceanic plate stratigraphy in the Geraldton area and field occurrence of layer-parallel shear making duplex repetition of stratigraphy (locality shown in Fig. 7).

above the deformed greenstones. The southern boundary between Unit 2 greenstones and terrigenous sediments is highly foliated and defined by a dextral reverse fault. The third and fourth units (units 3 and 4) are described from the area of the town of Geraldton. Between the two units several east-west trending gabbroic and tuffaceous greenstone packages are sandwiched between terrigenous sediments. The detailed geological observations are observed in a trenched outcrop west of Long Lake (Figs. 8 and 9). A metamorphosed sequence comprising de-

Geochemical constraints

Most of the volcaniclastic assemblages in the Superior province overlie mafic-ultramafic volcanic sequences. In many cases these relationships are considered to be conformable and this has led to the subdivision ofvolcanics into platformal (mafic-ul-

345

MODELOF OCEAN-CRUSTACCRETION

tramafic) and emergent volcanic settings (commonly calc-alkaline) by Thurston and Chivers, 1990. The geological observations that we have described above indicate that the platformal sequences are generally formed in an oceanic environment, which is isolated from sources of terrigenous sediment, and are later accreted in forearc accretionary complexes. The emergent sequences are superimposed on accreted oceanic assemblages as a result of back-stepping of the locus of subduction following accretion. Later transpression-related tectonics in pull-apart basins, potentially related to ridge subduction associated with oblique convergence, would result in post-early deformation "second or third cycle" mafic volcanism. Recognition of true oceanic-ridge or oceanic plateau volcanism remains problematic in the Archaean. Oceanic plateau assemblages in the modern oceans are comparable or higher in FeO than MORB tholeiites and commonly have similar or higher Zr/ La, Nb/La and Nb/Th relative to normal MORB. Their trace element characteristics contrast with those of arc-derived magmas for which high field strength element abundances (HFS--Nb, Zr, Ta) are generally low relative to the rare-earth elements (REE) and large-ion lithophile elements (Th, K, Rb) in arc-related magmas. This is due either to mineral-liquid partitioning characteristics of hydrous melts, enrichment of light-REE by a fluid phase or reaction with refractory mantle after melting (e.g. Keleman et al., 1992). Thus, if mantle compositions and melting regimes were generally similar to those characteristic of the Phanerozoic, ocean-ridge tholeiites would be typified by higher FeO contents at a given MgO than calc-alkaline island-arc suites and generally higher Zr/La, Nb/La values. Figure 10 summarises the FeO vs. MgO variations for the Archaean assemblages in the Superior province. The tholeiites and komatiites lie on a fractionation trend that mirrors that of MORB, but is displaced towards FeO contents more typical of oceanic islands. However, in contrast to modern oceanic islands, these volcanics are clearly depleted in incompatible trace elements relative to chondritic abundances (Fig. 11 ). Recent spark-source mass-spectrometric determinations for komatiites and tholeiites from the southern Abitibi (Jochum et al., 1991 ) do not show abnormal Nb/La abundances. Rhyodacitic se-

GENERALISED GEOCHEMICAL TRENDS BASED ON DATA COMPILATION FOR THE SUPERIOR PROVINCE

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quences intercalated with the Malartic komatiitetholeiite sequence which are demonstrably comagmatic (Parent, 1985; Desrochers et al., 1992) define chondritic Nb/La. Geochemical data from amphiboles which are tectonically interspersed with the Pontiac metagreywackes (Camire et al., 1993) do not show depressed HFS/REE abundances. All of these results indicate that these Archaean oceanic assemblages are generally richer in FeO than modern MORB, but have comparable HFS/REE relative to MORB and oceanic plateaus (ie. Storey et al., 1991 ). The high-Fe tholeiite, komatiite and komatiitic basalt are more readily formed in oceanic rift-zones associated with relatively deep mantle upwelling compared to modern MORB genesis. They may be explained by active upwelling for the Archaean in contrast to passive rifting for modern MORB (Sinton and Derrick, 1992). The following modern oceanic environments may be comparable to the Archaean oceanic environment: (i) thick oceanic crust formed along the ridge axes (e.g. the Koblenski ridge north of Iceland; Schilling, 1986); (ii) with oceanic crust generated in high heat flow

346

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parable to modern arc-related plutonic suites (i.e. Ludden et al., 1986; Bickle et al., 1983). We consider these plutons, and where preserved their volcanic carapaces, as representative of volcanism above Archaean subduction zones. What is critical to the unequivocal proof for allochtonous relationships and better comprehension of the tectonic evolution of Archaean greenstone belts is to establish the stratigraphic, geochronological and tectonic relationships of the accreted oceanic assemblages and subduction related volcanism. In all of the regions described, pyroclastic sequences which appear to be related to calc-alkaline plutons overlie previously accreted oceanic basement, pelagic sedimentary sequences and tectonically interleaved terrigenous sediments. Given active ridge-crest activity and smaller oceanic plate size, these relationships are probably the norm in the Archaean and are discussed in the following section with reference to the tectonic evolution of the Superior province.

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Fig. 11. Rare earth element variation for rocks from the Malartic Block (Parent, 1982 ); Northern Pontiac (Camir6 et al., 1992); Larder Lake region (Blum and Crocket, 1992). K komatiite; T - tholeiite; E - T - evolved tholeiite; R - rhyodacite; C-A -eale-alkaline units from the Val d'Or formation.

regimes at the onset of oceanic rifting (e.g. Eissen et al., 1988 for the Red Sea rift and Ludden and Dionne, 1992; Kean et al., 1990, for old ocean crust close to rifted continental margins); (iii) crust generated in association with a hot-spot and rifting (e.g. Cocos and Carnegie ridges, Galapagos, Storey et al., 1991). Despite the arguments above for relatively low HFS/REE as evidence for arc-related volcanism, there are clear examples of suites of high AlzO3, low FeO, calc-alkaline, plutons in the Superior province and many early- and late-Archaean greenstone belts elsewhere, that display depressed HFS/REE com-

A progression in ages of volcanic and plutonic rocks has been described for the Superior province (Thurston et al., 1991; Williams et al., 1991 ) in Fig. 12. The ages of volcanic rocks have been projected onto north-south sections across the different subprovinces of the Superior province. These sections are shown in Fig. 1 and the ages are compiled in the recent OGS "Geology of Ontario" and supplemented with data from the Quebec region. In the following section we consider the implications for age progression across the Superior province.

(i) Tectonic models (a) Arc-arc collision The most widely accepted model for Superior evolution is an arc-arc collision model involving paired arcs and associated sedimentary prisms (Fig. 12A, Hoffman, 1989; Percival and Williams, 1989; Williams, 1990; Thurston and Chivers, 1990). Progressive initiation of magmatism in evolving arcs would be fortuitous, and age progression should be shown by a succession of magmatic events. This is demonstrated in Fig. 12a for the situation of paired

347

MODEL OF OCEAN-CRUST ACCRETION

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o

L

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14~ no constraints for

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~ younging of volcanism cessation

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,~ arc-arc

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1 no constraintsfor 4~ volcanismlnitlation

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model

younging of volcanism Initiation younging of volcanism cessation

4 •

..

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..

.

rerrane

~n

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..

accretion

distance

model

2

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strike-slip duplex model

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Fig. 12. Cartoons illustrating the expected age progression pattern from various tectonic models involving plate accretion.

arcs with the same subduction polarity, and for those with the opposite subduction polarity. Detrital zircons in sediments in the accretionary prism cannot be younger than the youngest igneous rocks in the paired arc.

(b) Accretionao' model In contrast, the accretion of oceanic or continental fragments to construct greenstone belts, as described in this paper and by Hoffman ( 1991 ), requires that both the initiation and the cessation of arc magmatism show an oceanward migration (Fig. 12B). Zircons in accretionary sediments could represent a range from the youngest age of volcanic arc magmatism against which accretion is taking place to the oldest age of zircons in the accretionary package. Where volcaniclastic rocks are intercalated with mafic accreted sequences the U / P b zircon date of

the volcaniclastics will not provide the age of mafic volcanism, as is commonly implied in models invoking concordant growth of volcanic arcs.

(c) Strike-slip duplex model As displayed in Fig. 12c, oblique subduction will result in the stacking of arc-accretion complexes of the same age with strike-slip boundaries between greenstone belts which may later be reactivated. Oceanward migration of the locus of subduction will result in superposition of arc volcanics and accreted fragments on the accretionary complexes.

(d) Plume models Given the recent "experimental" results on thermal plumes (Hill et al., 1992), models for komatiite genesis have been invoked as involving deepseated mantle plumes. Komatiites occur through-

348 out the western Superior (Thurston and Chivers, 1990) and are particularly abundant in the southern Abitibi. The age progression in initiation of volcanism in the Superior would therefore have to be accidental, or involve a migrating plume head or migration of the initiation of volcanism associated with progressive deviation of the plume head around pre-existing continental fragments.

Age progression in the Superior province Age progression in the Superior province has been examined by a series of age-distance diagrams. The locations of the sections are given on Fig. 1 and the results presented in Fig. 13. These results are summarised below: (i) Uchi-Sachigo subprovince: The ages young northward, from 2992 to 2892 Ma, and then invert to the south (2892-2850 Ma). There is an intermission in volcanic activity from about 2850 Ma to 2750 Ma, following which resurgent volcanic activity continued to about 2710 Ma. The oldest age recorded is 3012 Ma. and is interpreted as a remnant of a volcanic sequence formed prior to 2990 Ma. (ii) Wabigoon subprovince: Volcanism started at about 2745 Ma and evolved southward ending at about 2710 Ma. Volcanism migrated to the south and started again at approximately 2730 Ma. Fragments of crust that are older than 2745 Ma occur in the Wabigoon i.e. the Steeprock region (Wilks and Nisbet, 1988), the Caribou-O'Sullivan lakes (Sutcliffe, 1986) and the kumby Lake (Davis and Jackson, 1988) and are considered to be exotic, as is the Winnipeg river subprovince which has ages ranging from 3070 2775 Ma. (iii) Wawa subprovince: Volcanism initiated at approximately 2745 Ma, which is almost equivalent to the oldest volcanism in the Wabigoon. The southern margin of the Wawa belt records volcanism at 2730 Ma. Synchronous activity is recorded in the northern Abitibi belt. Volcanism in both the Wawa and the Abitibi belts show southward younging, although isolated older volcanic complexes of upto 2745 Ma, of a poten-

G. KIMURA ET AL.

tially exotic character, are located in the southern Abitibi. (iv) English River, Quetico and Pontiac subprovinces: Sedimentary rocks of the Quetico and Pontiac subprovinces include zircons as young as 2690 Ma (Gariepy et al., 1984; Davis et al., 1989, 1990; Davis, 1992). These ages indicate deposition of these sediments after the major stages of volcanism. Plutonic rocks in the sedimentary subprovinces can be broadly divided into two stages: 2725-2710 Ma, in the Uchi-Sachigo subprovince, an age which is coincident with volcanism in the southern Wabigoon and Wawa Abitibi belt; and 2690 and 2660 Ma in the southern Superior, ages that are common to both the sedimentary subprovinces and the greenstone belts. (v) General conclusions for age progression in the Superior province: - I f exceptions which we interpret as exotic fragments are ignored, both initiation and cessation of volcanism show a younging trend. In the Uchi-Sachigo this trend is northward. -Subduction switched to the south in Sachigo province and initiation and cessation of volcanism shows a progression towards the south from Wabigoon subprovince to Abitibi. This zoned growth of greenstone belts is well developed for all subprovinces. -Exotic fragments are ubiquitous in the greenstone belts. The age of the fragments appears to young from north to south: approximately 3075 Ma in Uchi subprovince: 3000 Ma in Wabigoon subprovince; 2745 Ma in Abitibi subprovince. Although the oldest fragmemt in the Winnipeg river subprovince is 3180 Ma. -Few volcanic rocks are preserved between 2810 and 2750 Ma. The 2810 Ma age corresponds to cessation of volcanism in Uchi and the active margin following in the period 2810 may this period must have been removed tectonically. -Two of the major sedimentary sequences (Quetico and Pontiac) do not pair with the expected ages in the adjacent greenstone belts. With the exception of the last two points, the age progression in the volcanic belts, the presence of exotic fragments and the migration of the locus of subduction following accretion of exotic fragments are consistent with models of accretion in Phanerozoic island arcs.

349

M O D E L OF O C E A N - C R U S T A C C R E T I O N

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Tectonic evolution of the Superior province

Some of the consequences of the oceamc accretion model for the Superior province are summar-

ised in the c a r t o o n s in Fig. 14. In c o n t r a s t to the a r c - a r c collision model, these m o d e l s c o n s t r a i n the following features o f greenstone belt evolution.

350

G. KIMURA ET AL. 2992Ma

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shown to have been deformed with the pillow-lava sequences, should always yield ages older than the adjacent arc volcanics. Furthermore, ages of intercalated pyroclastic sequences would date the time of oceanic accretion rather than the age of the volcanic group (i.e. for the Larder Lake Group, Corfu et al., 1989). Intermediate to felsic volcanism in greenstone subprovinces is generally divisible into older units, representing arc volcanism initiation, and younger units which reflect migration of the arc oceanward. The time gap between the volcanic units should reflect accretion of oceanic materials.

(ii) Ridge subduction

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Fig. 14. A schematic plate-tectonic model of the Uchi-Sachigo Subprovinces incorporating the observed polarity of the age progression shown in Fig. 13. Note the role of accreted oceanic assemblages in construction of crust and backstepping of the locus of subduction.

(i) Oceanic-crust accretion Mafic greenstones can be subdivided into two general settings in greenstone belts: strongly deformed older greenstones overlain by less deformed maflc volcanics, and often, mafic-intermediate volcaniclastics (i.e. in the Wabigoon-Beardmore Geraldton, described in this paper, and the Malartic region described by Desrochers et al., 1993 ). The older sequences are interpreted as accreted fragments of oceanic plateau or oceanic basement. Intensely deformed turbidites occur together with the greenstones and are considered to be trench fill turbidites. A test of this model would require U/Pb dating of detrital zircons associated directly with terrigenpus sediments in the accreted material; ages should, in these cases, pair with volcanism in the arc. U/Pb age dates in layered gabbroic sills, which can be

The change from orthogonal to oblique subduction in many cases, reflects ridge subduction in the Phanerozoic (e.g. Engebretson et al., 1985). The consequences for Superior province evolution are summarised in Fig. 15: -Stepwise accretion of arc material or previously accreted fragments will be enhanced by strike-slip duplex formation. This may explain the similar volcanism initiation ages in the southern Wabigoon and Wawa subprovinces; -Accretion of exotic oceanic material is enhanced resulting in rapid migration of the locus of subduction; -Spreading may develop in extensional basins above an oblique subduction zone resulting in an Andaman Sea environment (e.g. Curray et al., 1982). Tholeiitic volcanism in these basins may evolve to calc-alkaline volcanism during arc evolution (i.e. the Matagami region and Chibougamau region of northern Abitibi ); -Ridge subduction would result in mafic-felsic (generally bi-modal) volcanism in pull-apart basins overlying previously accreted oceanic fragments and trench-fill sediments. The isolated volcanic centres in the Wabigoon belt (Blackburn et al., 1991 ), the undeformed mafic volcanics overlying the northernmost accreted unit in the BeardmoreGeraldton belt (this paper) and the Val d'Or domain volcanics which overlie the Malartic accretionary complex (Desrochers et al., 1993) are cited as examples; -Slab detachment, following back stepping of the subduction zone, and consequent asthenospheric upwelling in the "slab window" would result in plu-

MODEL OF OCEAN-CRUST ACCRETION

Stage1(2730Ma)

351

p~

(iii) Sedimentation

.....arcv ° l c a ~

Sediments associated with trench-fill deposits are intercalated with accreted oceanic fragments. We have described examples from the Malartic, Larder Lake and Beardmore-Geraldton regions. To date no U/Pb studies have been completed on detrital zircons from these sedimentary assemblages. The elongated sedimentary belts that traverse the southern Superior, the English River, Quetico and Pontiac, include a young detrital zircon population and

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tonism in the accreted hinterland. In the case of the Superior province this may explain the correlation between mafic volcanism and plutonism in the southern Wabigoon and Wawa-northern Abitibi and the Uchi respectively. Similarly, the plutonism in the southern Superior province may correspond to slab detachment and ridge subduction at, or before, 2710 Ma.

..

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Fig. 15. A plate-tectonic model of the Wabigoon-Wawa-Abit i b i Subprovinces. In which accretion of immature oceanic material and the development of younger arc complexes are demonstrated.

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~/'~ Delaminationandsinkingof oceanicslab Fig. 16. Final stage in the tectonic evolution of the Superior province based on the interpretation of the Pontiac as a late stage syn-orogenic turbidite fan associated with a prograding orogen. Based on observations for the western Superior and following suggestions by Hoffman (1989), the Minnesota landmass is shown as the potential colliding continent. The scenario must have been more complex as ages in the Grenville province south of the Pontiac indicate a collision with a landmass younger than in the Minnesota region. The Pontiac, Quetico and potentially the English River sediments are predicted as belonging to the same sedimentary system.

352

cannot be paired with the greenstone belts in an accretionary prism. LITHOPROBE seismic results suggest a consistent positioning of sedimentary material as imbricates under the greenstone belts (Ludden et al., this volume; Hubert et al., 1992), an interpretation which is supported by metamorphic constraints discussed in Feng and Kerrich (1991). The exposed sedimentary belts are therefore best represented as tectonic windows resulting from imbrication beneath the greenstone belts followed by north-south shortening of the greenstone-sedimentary package. In Fig. 16 we indicate that these sediments may have originated as a clastic fan deposited during collision and uplift in the south-western Superior province. We consider it most likely, in terms of tectonic and metamorphic constraints, that these sediments were imbricated underneath the greenstone belts prior to the north-south folding, or synchronous with an easterly prograding collision.

Conclusions The quantity of high-precision U/Pb age determinations in the Superior province have been the single most important contribution to the construction of tectonic models for this vast segment of dominantly juvenile crust that was stabilised between approximately 3.1 Ga and 2.65 Ga. Coupled with petrological data that indicate a geochemical affinity comparable to modern island-arc and continental arc sequences, the most accepted model for the Superior province evolution involves southward accretion of island arc sequences. Given an Archaean regime involving higher than present average heat-flow from the mantle, which would be reflected in greater numbers of spreading centres, smaller average plate size and thicker oceanic crust (Sleep, 1979; Sleep and Windley, 1982; Abbott and Hoffman, 1984; Hoffman and Ranalli, 1988 etc.), plate-tectonic models proposed for Superior province do not adequately account for the role of the oceanic plate in its evolution. Relative to modern oceanic crust which, on average, is 8-10 km thick and locally, in areas of oceanic plateau, may approach 20 km (e.g. Hussong et al., 1979), Archaean oceanic crust is estimated to average 20 km in thickness (Hoffman and Ranalli, 1988). Subduction-related delamination and accretion of oceanic

G. KIMURA ET AL.

crust in the Archaean would therefore be comparable to that for oceanic plateaus in the modern platetectonic cycle. Accretion of over-thickened oceanic crust may "choke" the subduction zones, resulting in wholesale accretion of oceanic material, backstepping of subduction and isolation of the previously subducted plate. Actively upwelling spreading centres may require a predominant role for ridge-push tectonics in the Archaean earth, resulting in enhanced accretion, increased chances of ridge subduction with the consequence of mafic and ultramafic magmas being erupted on top of recently accreted, oceanic material of island-arc or oceanic crust origin. Oceanic accretion may have been the principal mechanism by which the locus of subduction migrates towards the south of the Superior province. Asthenospheric upwelling associated with the isolated sinking plate may be responsible for widespread late magmatism. This model requires that magmas may be erupted through previously accreted volcanic, plutonic and sedimentary material. Furthermore, later ridge subduction will result in transpressional tectonics and eruption of mafic sequences over mature and immature volcano-plutonic sequences. The combined result of the active plate-tectonic scenario envisaged could explain the tectonic evolution and the stratigraphy of many granite greenstone sequences.

Acknowledgements G. Kimura acknowledges support from an NSERC Japan-Canada exchange program. G. Kimura and R. Hori also acknowledge field support from the "Evolving Earth Project". J. Ludden received support from NSERC-Canada and FCARQuebec and travel support from the "Evolving Earth Project", Japan. J.-P. Desrochers acknowledges a support from MER-Quebec. We appreciate constructive reviews from M. de Wit, D. Davis and P. Hoffman and discussions with C. Hubert on structural constraints in the Abitibi belt and D. Francis on magmatism on the early Earth.

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MODEL OF OCEAN-CRUST ACCRETION

of subducting oceanic lithosphere and their effects on the origin and evolution of continents. Tectonics, 3: 429-448. Babcock, R.S., Burmester, R.F., Engebretson, D.C., Warhock, A. and Clark, K.P., 1992. A rifted margin origin for the Crescent basalts and related rocks in the Northern Coast Range Volcanic Province, Washington and British Columbia. J. Geophys. Res., 97:6799-6821. Bickle, M.J., 1983. A 3500 Ma plutonic and volcanic calc alkaline province in the Archaean east Pilbara block. Contrib. Mineral. Petrol., 84: 25-35. Blackburn, C.E., Johns, G.W., Ayer, J. and Davis, D.W., 1991. Wabigoon Subprovince. In: Geology of Ontario. Ont. Geol. Surv. Spec. Vol., 4( 1 ): 303-381. Blum, N. and Crocket, J.H., 1992. Repetitive cyclical volcanism in the Late Archaean Larder Lake Group near Kirkland Lake, Ontario: implications of geochemistry of magma genesis. Precambrian Res., 54:173-194. Burke, K., Dewey, D.F. and Kidd, W.S.F., 1975. Dominance of horizontal movements, arc and microcontinental collisions during the later permobile regime. In: B.F. Windley (Editor), The Early History of the Earth. Wiley-lnterscience, Bristol, pp. 113-129. Card, K.D., 1990, A review of the Superior Province of the Canadian Shield, a product of Archaean accretion. Precambrian Res., 48: 99-156. Card, K.D. and Ciesielski, A., 1986. DNAG ~ 1. Subdivisions of the Superior province of the Canadian Shield. Geosci. Can., 13: 5-13. Camire G., Ludden, J.N. and Lafl6che, M., 1993. Mafic and ultramafic amphibolites from the northwestern Pontiac subprovince: chemical characterisation and implications on tectonic setting. Can J. Earth Sci., In press. Campbell, I.H., Griffiths, R.W. and Hill, R.I., 1989. Melting in an Archaean mantle plume: Heads it's basalts, tails it's komatiites. Nature, 339: 697-699. Condie, K.C., 1981. Archaean Greenstone Belts. Elsevier, Amsterdam, 434 pp. Corfu, F and Ayres, L.D., 1991. Stacking of disparate volcanic and sedimentary unitsby thrusting in the Archaean Favourable Lake greenstone belt, central Canada. Precambrian Res., 50: 165-186. Corfu, F., Krogh, T.E., Kwok, Y.Y. and Jensen, L.S., 1989, U-Pb zircon geochronology in the southwestern Abitibi greenstone belt, Superior Province. Can. J. Earth Sci., 26: 1747-1763. Curray, J.P., Emmel, F.J., Moore, D.G. and Raitt, R.W., 1982. Structure, Tectonics and Geological history of the Northeastern Indian ocean. In: A.M. Nairin and F.G. Stehli (Editors), The Ocean Basins and Margins, Vol. 6. Indian Ocean. Plenum Press, New York, pp. 399-450. Davis, D, 1992. Age constraints on deposition and provenance of Archaean sediments in the southern Abitibi and Pontiac subprovinces from U - P b analysis of detrital zircons. LITHOPROBE Rep. 25, Univ. British Columbia, pp. 147-150. Davis, D.W., Pessutto, F. and Ojakangas, R.J., 1990. The age and provenance of metasedimentary rocks in the Quetico Subprovince, Ontario, from single zircon anlyses: implications for Archaean sedimentation and tectonics in the Superior Province. Earth Planet. Sci. Lett., 99: 95-105.

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