Archaean Cratonization and Deformation in the Northern Superior Province, Canada: an Evaluation of Plate Tectonic Versus Vertical Tectonic Models Jean H

Precambrian Research 127 (2003) 61–87

Archaean cratonization and deformation in the northern Superior Province, Canada: an evaluation of plate tectonic versus vertical tectonic models Jean H. Bédard a,∗, Pierre Brouillette a, Louis Madore b, Alain Berclaz c a Geological Survey of Canada, Division Québec, 880, ch.Ste-Foy, Quebec City, Que., Canada G1S 2L2 b Géologie Québec, Ministère des ressources naturelles du Québec, 5700, 4e Avenue Ouest, Charlesbourg, Que., Canada G1H 6R1 c Géologie Québec, Ministère des ressources naturelles du Québec, 545 Crémazie Est, bureau 1110, Montreal, Que., Canada H2M 2V1 Accepted 10 April 2003

Abstract The Archaean Minto Block, northeastern Superior Province, is dominated by tonalite–trondhjemite, enderbite (pyroxene tonalite), granodiorite and granite, with subordinate mafic rocks and supracrustal belts. The plutons have been interpreted as the batholithic roots of Andean-type plate margins and intra-oceanic arcs. Existing horizontal-tectonic models propose that penetrative recrystallization and transposition of older fabrics during terrane assembly at ∼2.77 and ∼2.69 Ga produced a N-NW tectonic grain. In the Douglas Harbour domain (northeastern Minto Block), tonalite and trondhjemite dominate the Faribault–Thury complex (2.87–2.73 Ga), and enderbite constitutes 50–100 km-scale ovoid massifs (Troie and Qimussinguat complexes, 2.74–2.73 Ga). Magmatic muscovite and epidote in tonalite–trondhjemite have corroded edges against quartz + plagioclase, suggesting resorption during ascent of crystal-charged magma. Foliation maps and air photo interpretation show the common development of 2–10 km-scale ovoid structures throughout the Douglas Harbour domain. Outcrop and thin-section scale structures imply that many plutons experienced a phase of syn-magmatic deformation, typically followed by high temperature sub-magmatic overprints. Thermobarometric data for plutons indicate near-solidus recrystallization at 4–6 kbar pressures. The common preservation of syn-magmatic fabrics in plutons of different ages seems incompatible with the origin of these fabrics through superimposed regional orogenesis. The broad uniformity of intrusion ages and lithologies throughout the Minto Block, and the rarity of shallowly-dipping planar fabrics, also seem inconsistent with accretion of disparate older terranes, each of which should preserve distinct histories. A possible alternative explanation for these features is provided by vertical tectonic models, whereby buoyant felsic magmas ascended as crystal slurries, while dense supracrustal rocks (and solidified felsic intrusions emplaced into them) subsided as cold fingers (10–20 km-scale instabilities). Shear between upwelling and downwelling limbs would have concentrated in the weak intrusions, generating steeply-plunging syn-magmatic fabrics, and producing ductile overprints in solidified rocks. © 2003 Elsevier B.V. All rights reserved.

Keywords: Archaean; Orogeny; Vertical tectonics; Minto Block; Superior Province; Cratonization

∗ Corresponding author. Tel.: +1-418-654-2671; fax: +1-418-654-2615. E-mail addresses: [email protected] (J.H. Bedard),´ [email protected] (L. Madore), [email protected] (A. Berclaz).

1. Introduction and regional framework Skulski, 2000; Percival et al., 2001), which were interpreted to represent cratonic nuclei. Juvenile isotopic Most recent Archaean crustal growth models in- signatures of Qalluviartuuq belt lavas (2.84–2.83 Ga) voke a near-uniformitarian process of subduction, were considered evidence for an intra-oceanic arc arc maturation, and lateral accretion of oceanic arcs setting (Skulski et al., 1996). A 2.81 Ga shear zone and plateaux (e.g. Condie, 1986; Card, 1990; de Wit, in the Qalluviartuuq belt was interpreted as an 1998; Percival et al., 2001). However, recent work intra-oceanic accretionary thrust (D1: Percival and in a number of cratons has favored vertical tectonic Skulski, 2000). However, analogous D1 structures models (e.g. Chardon et al., 1996; Collins et al., are younger in the Vizien belt (<2.718 Ga), and so 1998), leading to a revival of this classic debate. The D1 structures cannot record a unique tectonic event Minto Block (Fig. 1) is the largest plutonic-dominated (Percival and Skulski, 2000). Percival et al. (2001) terrane of the Superior Craton, and is an ideal proposed that a composite cratonic basement was place to investigate how Archaean crust formed and formed when oceanic and continental terranes docked stabilized. at about 2.77 Ga (Fig. 2), with crustally contaminated The Minto Block was poorly known prior to pio- neering work by Percival et al. (1992, 1994, 2001), who subdivided it into lithotectonic domains (Fig. 1), and developed tectonic models. The oldest Minto Block rocks are maﬁc and ultramaﬁc lavas and felsic tuffs (3.825 Ga) from the Porpoise Cove greenstone belt, embedded in younger (2.75 Ga) tonalitic rocks of the Inukjuak domain (David et al., 2002). The next oldest Minto Block rocks are tonalite and trondhjemite with embedded supracrustal belts from the Goudalie and Douglas Harbour domains (3–2.8 Ga: Stern et al., 1994; Madore et al., 1999; Percival and

Fig. 1. Simplified map of Minto Block adapted from Percival Fig. 2. Cartoon illustrating existing plate-tectonic scenarios et al. (1997).PN= Pelican´ Nantais belt, Ko = Kogaluc belt, (Percival et al., 2001) for origin and assembly of Minto Block Vz = Vizien belt. domains. J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87 63 calc-alkaline volcanics in the Kogaluc belt, Lac Minto Minto Block (D2) is interpreted as a 2.69 Ga over- domain, representing development of a successor print by Percival and Skulski (2000), then how is arc at 2.77–2.76 Ga (Skulski et al., 1996). Younger the pre-Leaf River suite accretion event (∼2.77 Ga) (<2.748 Ga), unconformable greywacke and iron for- manifested in terms of fabric development and meta- mations that contain ancient detritus (Percival et al., morphism, and what was the impact of the 2.69 Ga 1995), and quartzites and ultramafic lavas in the Farib- event on the older fabrics? ault belt (Douglas Harbour domain), were interpreted To clarify these issues, we present field, structural, as a continental overlap sequence (Percival et al., and petrographic data from the poorly-known north- 1997), linked to intra-arc extension by Skulski et al. eastern part of the Minto Block, recently mapped on (1994). a 1:250,000 scale (Figs. 3 and 4) by the Ministère This older composite basement was intruded by des Ressources Naturelles du Québec. After com- voluminous granodiorite and granite (subordinate paring our data with observations from elsewhere in tonalite, enderbite, pyroxene tonalite and mafic intru- the Minto Block, we discuss the relative merits of sions) of the Leaf River suite (2.73–2.72 Ga: Percival different tectonic models to explain the characteristic et al., 1994; Stern et al., 1994), which constitutes fabrics and lithological assemblages associated with most of the Lac Minto and Utsalik domains. Relict Archaean cratonization in this area. zircon cores and Nd-isotopic signatures indicate recycling of older continental crust, and so the Leaf River suite was interpreted to be the plutonic root zones 2. Douglas Harbour domain of Andean-type continental arcs (Stern et al., 1994; Percival et al., 1994). The great areal extent of syn- We divide the Douglas Harbour domain into the chronous magmatism and heterogeneous Nd-isotopic Faribault–Thury, Troie and Qimussinguat plutonic signatures were explained in terms of two simultane- complexes (FTC, TC and QC) (Figs. 3–5), each of ously active subduction zones (Fig. 2; Percival et al., which contains supracrustal belts. In the west, the FTC 2001). is cut by granodiorite, granite, pyroxene tonalite and Lin et al. (1996) originally proposed that collision enderbite of the Lepelle complex (2.729–2.727 Ga), and amalgamation of the Lake Minto, Goudalie and which envelops screens and enclaves of rocks equiv- Utsalik arcs generated the dominant N-NW foliation of alent to the FTC (Fig. 3, Kapijuq and Bottequin a regionally distributed, tectono-metamorphic episode suites). To the northeast, Diana complex gneisses (D2). Subsequently, Percival and Skulski (2000) dated (2.78–2.76 Ga) were juxtaposed against the Douglas a D2 fabric at 2.693–2.675 Ga and re-interpreted Harbour rocks during Proterozoic dextral compres- D2 as an overprint related to overthrusting of the sion (Madore and Larbi, 2000), which overprints the 2.71–2.70 Ga Tikkerutuk continental arc onto the eastern FTC (Figs. 1, 3 and 5). To the south, Troie amalgamated Lac Minto + Goudalie + Utsalik + complex rocks are intruded by Utsalik domain granite Douglas Harbour proto-craton (Fig. 2, at 2.7 Ga). and granodiorite (Berclaz et al., 2001; Leclair et al., They further proposed that deformation and amphibo- 2001a). lite to granulite facies metamorphism in supracrustal The FTC is dominated by hornblende tonalite and belts of the west-central Minto Block resulted from biotite trondhjemite (Fig. 6a), with subordinate dior- this crustal thickening event, with transposition and ite, granodiorite and granite. Western FTC tonalites recrystallization of older fabrics into the dominant yield 2.88–2.86 Ga crystallization ages, while eastern N-NW grain, and production of crustally-derived plu- FTC tonalites yield younger ages between 2.81 and tons and diatexites (2.69 Ga). Subsequent activity is 2.77 (Madore et al., 1999; Madore and Larbi, 2000; manifested as a series of less penetrative deformation Percival et al., 2001). Inherited zircon cores yield ages events (D3–D5: Lin et al., 1996; Percival and Skulski, up to 3 Ga (Percival et al., 2001), indicating recy- 2000) and late- to post-tectonic granitoid and syen- cling of older sialic material. The age of FTC pluton- ite intrusions (2.696–2.645 Ga: Stern et al., 1994; ism overlaps with ages from embedded supracrustal Skulski et al., 1996; Percival and Skulski, 2000). belts (∼2.82–2.78 Ga: Madore et al., 1999; Madore Since the dominant tectono-metamorphic pulse of the and Larbi, 2000). 64 J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87

Fig. 3. Simplified geological map of the Douglas Harbour domain, adapted from Madore et al. (1999, 2001), Madore and Larbi (2000), and Cadieux et al. (2002).K= Kapijuq and Bottequin suite, strongly deformed tonalite and trondhjemite, equivalent to the Faribault–Thury complex. L = Lepelle complex, syn-kinematic sheet-like intrusions of granite, granodiorite and tonalite, with steep dips. M = McMahon suite, enderbite, diorite and gabbro-norite, which straddle the Lepelle/Utsalik boundary. Among the late granitic suites, we distinguish the monzonitic rocks from the Troie complex and a series of late granite and granodiorite from the extreme northeast of the Faribault–Thury complex (Madore and Larbi, 2000). Note that associated granites are included with the monzonitic suite rocks in this figure, unlike Fig. 5. All other late granitoid rocks have been grouped together since there has yet to be a systematic investigation of their chemistry and petrography. Cu = Curotte belt, Fa = Faribault belt, Ha = Hamelin belt, Ki = Kimber belt, Ta = Tasiaalujjuaq belt, Th = Thury belt. J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87 65

Fig. 4. Simpliﬁed map of structural subdivisions of the Douglas Harbour domain, with stereonets showing mineral lineations (dots) and contoured plots (Schmidt) of mineral foliation and layering. In the case of the Troie complex, ﬁlled dots represent the core of the massif, while open dots are from the periphery. Areas to the northeast have been overprinted by Proterozoic orogenesis.

The TC and QC are dominated by enderbite (py- 2.74–2.73 Ga were obtained from TC enderbites roxene tonalite and trondhjemite), with subordinate (inherited cores 2.83–2.8 Ga: Madore et al., 1999; pyroxene granodiorite, pyroxene granite, hornblende Percival et al., 2001), and a QC trondhjemite (2.73 Ga, tonalite and biotite trondhjemite (Fig. 6a). Ages of inherited cores 2.81–2.80 Ga: Madore et al., 1999). 66 J.H. B´edard et al. / Precambrian Research 127 (2003) 61–87

Fig. 5. Map of the Lac Peters mapsheet (SE Douglas Harbour), showing the trend of foliations defined from field observations and interpretation of air photographs. Plotted strike and dips are averaged values for each sub-area. Supracrustal belts also define a similar, swirly pattern, whereas late- to post-kinematic monzonitic intrusions strike N-S. Inset shows schematic cross-section. J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87 67

Fig. 6. Thematic maps of the Lac Peters area (same area as Fig. 5) showing the distribution of: (a) the nature of felsic plutonic rocks determined in thin section, using stained slabs, or where ﬁeld determinations were unambiguous. TTG = tonalite, trondhjemite, granodiorite. Granite and monzonite are not shown. (b) Metamorphic assemblages in metabasalts. (c) The distribution of eastern and western FTC tonalite and trondhjemite (TT) and diorite. Granodiorite and granite are not represented. W-FTC rocks have distinct geochemical signatures (higher U–Th-HREE) and it is from this domain that the oldest age dates originate. (d) Presence of igneous epidote and of (e) igneous(?) muscovite. 68 J.H. B´edard et al. / Precambrian Research 127 (2003) 61–87

Poorly exposed contacts between the enderbitic TC is the age of the eastern FTC rocks, we infer that or QC, and the tonalitic FTC, are marked by zones tonalite and trondhjemite within the TC and QC are of ductile deformation and strongly deformed gran- mostly septa of FTC rocks engulfed by enderbitic ite and granodiorite intrusions. The QC and TC magmas. are separated by a corridor of FTC-type rocks, the The TC and QC are invaded by gabbronorite, Thury Deformation Zone (TDZ, Figs. 3–5). Rocks pyroxene + mica diorite, granodiorite, monzonite, from the TDZ are younger than the rest of the FTC, quartz monzonite and granite intrusions (Figs. 3 and yielding a 2.734 Ga crystallization age, with inher- 5). Among the younger intrusions, a hornblende ited cores at 3.013–2.762 Ga (Percival et al., 2001). gabbronorite from the TC yields an average Pb–Pb Thus, to a ﬁrst approximation, emplacement of the age of 2.73 Ga (Madore et al., 1999), similar to that QC, TC and TDZ magmas (2.74–2.73 Ga) is coeval of TC enderbites. A quartz monzonite sheet from and nearly age-correlative with the Leaf River suite the TC yields a 2.697 Ga U/Pb zircon age (inher- (∼2.73–2.72 Ga: Percival et al., 1994; Stern et al., ited cores 2.73–2.72 Ga: Madore et al., 1999). A 1994). However, tonalite and trondhjemite bodies pyroxene + mica diorite dyke cuts this quartz mon- in the QC are signiﬁcantly older (2.77 Ga, inher- zonite, implying that these diorites are younger than ited cores 2.83 Ga: Madore et al., 1999). Since this the hornblende gabbronorites.

Fig. 7. (A) Layered trondhjemite to leuco-trondhjemite with segregations of leuco-trondhjemite (Thury Deformation Zone). A structural control on segregation is apparent. Lenscap 55 mm for scale. Black spots are lichen. (B) Folds in banded iron formation (quartz/magnetite) (E-FTC). Lenscap for scale. Irregular black and white spots are lichen. (C) Trondhjemite 98-3230A (QC/E-FTC). Note the euhedral antiperthitic plagioclase phenocryst, and the high-energy feldspar–feldspar grain boundaries. Crossed polarizers, 5 mm across. (D) Melatonalite 98-5064A (W-FTC). Note the serrated, high-energy feldspar–feldspar and feldspar–quartz grain boundaries, and presence of subgrains and neoblasts at grain contacts. Crossed polarizers, 5 mm across. J.H. B´edard et al. / Precambrian Research 127 (2003) 61–87 69

3. Field relationships and lithological assemblages granodiorite may contain quartz and alkali feldspar phenocrysts, while muscovite, epidote, allanite, ap- 3.1. The Faribault–Thury complex atite, titanite and rare garnet are concentrated in mafic schlieren with biotite and hornblende. Tonalite and trondhjemite in the FTC are commonly Mafic ‘dioritic’ schlieren are common, but larger interlayered on a centimeter- to meter-scale, and con- bodies (∼100–200 m) are rare, and are engulfed by tain deformed enclaves of amphibolite, paragneiss, tonalite–trondhjemite. Diorite is massive to weakly and pyroxenite. The absence of melanosome sheaths foliated, and heterogeneous in mode and grain size. separating tonalite from trondhjemite, or of preserved Feldspars are lath-shaped. Hornblende, biotite and metatexite domains, suggests that these are probably minor clinopyroxene relicts constitute 30–50%. Ac- not in-situ migmatites. Outcrop-scale structures indi- cessory apatite, titanite, magnetite and ilmenite are cate that magmatism and deformation overlapped in ubiquitous and can be abundant. Minor allanite and time: e.g. trondhjemite may occupy strain shadows quartz may be present. and fill shear bands, and trondhjemitic breccia contains abundant tonalitic to dioritic enclaves in various 3.2. The Troie and Qimussinguat complexes states of transition from angular blocks to elongate schlieren (Fig. 7A). Layered tonalite–trondhjemite do- Enderbite contains plagioclase + quartz+ ortho- mains alternate on a 1–5 km-scale with more homoge- pyroxene ± clinopyroxene + biotite + magnetite + neous trondhjemite or tonalite intrusions, which also apatite+zircon±minor interstitial hornblende. Pyrox- commonly display penetrative fabrics. ene granodiorite and granite are uncommon. Ender- Mineral foliation generally parallels layering and bite may form massive intrusions showing little modal is typically steeply dipping. Regionally, the foliation or textural variation over many kilometers, but more strikes N-NW (Fig. 4), but in detail the foliation and commonly, mela- and leuco-enderbite are interlay- layering define complexly swirling, 2–10 km-scale ered on every scale (like FTC tonalite–trondhjemite). ovoid patterns (Figs. 5 and 8). Lineations charac- Veins of unfoliated or weakly foliated leuco-enderbite teristically plunge steeply (Fig. 4), and L-tectonites may form reticulated networks that crosscut com- are locally prominent in the vicinity of supracrustal positional layering, occupy strain shadows around belts. In the TDZ (Figs. 3–5), N-NW-trending fabrics xenoliths, or are injected along shear zones (see are reoriented to an E-W grain with sub-vertical lin- Bédard, 2003). These features are interpreted to eations. Proterozoic orogenesis affected the eastern signify that deformation was syn-magmatic, with FTC (Fig. 4), with partial preservation of Archaean structural control on migration of evolved residual fabrics and textures between deformation corridors leuco-enderbite melt. Melanosome sheaths sepa- (Madore and Larbi, 2000). rating mela- and leuco-enderbite have never been Tonalite contains 10–30% hornblende, biotite, observed, suggesting that these are probably not in and Fe–Ti-oxides, with accessory epidote, allan- situ migmatites. Swarms of pyroxene diorite and ite, titanite and apatite (Fig. 6d). Feldspar-phyric mela-enderbite enclaves in leuco-enderbite matrices facies are rare. Trondhjemite has <10% ferromag- are common. Calc-silicate and partially-reacted am- nesian minerals, principally biotite, with subordi- phibolite enclaves with pyroxene-rich reaction rims nate muscovite, and the same accessory phases as also occur. Steeply-dipping foliations generally strike tonalite (Fig. 6e). Trondhjemite pegmatites asso- N-NW (Fig. 4), but in detail, they define swirling, ciated with meta-pelitic supracrustal rocks contain 2–10 km-scale ovoid patterns (Fig. 5). Lineations tourmaline + garnet + muscovite + biotite. Granite generally plunge steeply, but plunges are slightly and granodiorite (<15% of the FTC) occur as dis- shallower near contacts with the FTC (Fig. 4). crete bodies that are distinctly more massive and less In the east and north of the TC, extensive domains deformed than tonalite–trondhjemite hosts, as small of homogeneous, massive granite with blue quartz are bodies of augen gneiss associated with the edges of attributed to the monzonitic suite. Small, strongly de- the TC and QC, or as widely-distributed, essentially formed and heterogeneous bodies of granite, granodi- undeformed, granitic pegmatite dykes. Granite and orite, tonalite and trondhjemite are common near the 70 J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87

Fig. 8. Detail map of the Hamelin belt, located in the W-FTC. edges of the TC and QC (Fig. 6a). The tonalite and nocrysts (1–10 cm) of alkali feldspar, plagioclase and trondhjemite are geochemically and petrographically locally quartz. Most intrusions are undeformed, but similar to FTC equivalents, and may be cut by ender- in some bodies, phenocrysts have ovoid shapes and bite intrusions, which suggests that many are engulfed define a contact-parallel foliation. Hornblende, biotite slivers of the FTC. However, some tonalites and trond- and Fe–Ti-oxides are the dominant mafic minerals. hjemites may be intrusions from a younger phase of Minor apatite, titanite and zircon are common. There magmatism similar to that of the TDZ. are isolated igneous microgranular enclaves, and thin (∼1 m), micro-monzonite dykes. Clinopyroxene 3.3. Troie complex monzonites, quartz monzonites, monzodiorite and biotite granite have sharp intru- granites, gabbronorites and diorites sive contacts against monzonite or quartz monzonite. Granular textured, fine- to medium-grained biotite North-trending, steeply-dipping, intrusive sheets of granite forms extensive, homogeneous bodies. It has porphyritic monzonite to quartz monzonite (5–10 km few phenocrysts, and commonly contains blue quartz. wide, up to 40 km long) constitute ∼15% of the Troie Small intrusions of pyroxene + mica diorite and complex. Similar rocks occur in the Quimussinguat hornblende gabbronorite are dispersed throughout the complex, but have not been systematically examined, Troie complex and also occur in the Quimussinguat so that we will focus on the Troie complex as a type complex (Madore et al., 1999). Abundant (∼10–15%) example of the late intrusive rocks. The monzonites Fe–Ti-oxides are diagnostic. Hornblende gabbronorite and quartz monzonites are typically massive and is more strongly deformed than pyroxene+mica dior- homogeneous, and are crowded with euhedral phe- ite, with fabrics being similar to those in adjoining J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87 71 supracrustal and plutonic rocks. Pyroxene + mica 2001). Associated coarse to pegmatitic leuco- diorite contains plagioclase and biotite phenocrysts trondhjemite pods probably represent segregations of aligned parallel to chilled contacts, indicating mag- anatectic melts. In the TC and QC, metabasalt is trans- matic flow. Isolated gabbronorite to diorite bodies formed into mafic granulite with individual foliation- may be massive and homogeneous, with the grain parallel laminae having different feldspar/pyroxene size coarsening inwards. Agmatitic complexes are and hornblende/pyroxene ratios. Laminae are strongly common, with gradations from dykes with chilled transposed and may define steeply plunging isoclinal margins to swarms of cm- to m-scale ovoid or lobate fold closures. Hornblende is commonly concentrated bodies embedded in enderbite, tonalite or monzonite. into anastomozing amphibolitic gabbro laminae within The agmatites are interpreted to be disaggregated the hornblende-poor mafic granulite. Metabasalt in intra-plutonic dykes (Hibbard and Watters, 1985; the TDZ varies from amphibolite to granulite grade Pitcher, 1991). Rare potassic pyroxenite and harzbur- (Fig. 6b). gite form discrete lensoid bodies (sheared dykes or Komatiitic flows, subvolcanic sills and peridotitic, metalavas?) in felsic hosts, or are associated with pyroxenitic, and gabbroic cumulates are interlay- supracrustal belts (feeders or sills?). ered with metabasaltic lavas. Rhyodacitic to dacitic metatuffs are thin (<1 m) and laterally continu- 3.4. Supracrustal belts ous (>20 m). Coarser blocky tuffs and pyrite-rich gossans occur locally. Metapelite is the most com- All three Archaean complexes contain remnants of mon metasedimentary rock, forming 1–10 m thick supracrustal belts, generally <3 km wide and <15 km bands of migmatitic paragneiss dominated by quartz, long (Fig. 3). Decameter-scale fold closures are ap- garnet and biotite ± sillimanite. Well-preserved parent (Fig. 8), with ubiquitous sub-vertical fold axes meta-greywacke, meta-quartzite and pebbly sandstone and stretching lineations. Centimeter-scale folds are occur in the Hamelin belt (Fig. 8). Bands of iron visible in the laminated quartz–magnetite ironstone formation are typically 1–10 m thick, with up to 50 m (Fig. 7B). Fabric orientations in supracrustal and ad- of apparent thickness in fold closures. Rare marble joining plutonic rocks are similar. A coherent stratig- and calc-silicate rocks form recessive layers 1–10 m raphy is rarely preserved in supracrustal belts, and thick. there are marked along-strike contrasts in the domi- Considering the nature of the lithologies present, nant lithology. For example, in the Thury belt (eastern an intra-cratonic or peri-continental environment FTC: Fig. 3) a section of 100% paragneiss+ironstone seems likely for most of these supracrustal belts (cf. changes within <20 m into a section dominated Ayres and Thurston, 1985; Thurston and Chivers, by metabasalt. This variability implies significant 1990; Lowe, 1994; Percival et al., 1997, p. 211). internal deformation and probable tectonic sliver- This is consistent with the observation of mafic and ing. Detail mapping of the Hamelin belt (western ultramafic dykes cutting tonalites (Percival et al., FTC) reveals a complex fold interference pattern 1992) and of basal unconformities in other Minto (Fig. 8) and ubiquitous injection of syn-kinematic Block belts (Moorhead, 1989; Skulski and Percival, tonalite–trondhjemite sheets (cf. Pawley and Collins, 1996). 2002). Tholeiitic metabasalts form sequences hundreds of 3.5. Proterozoic rocks and overprints meters in thickness (Fig. 8), or are interbedded on a meter-scale with paragneiss and komatiitic metalava. Proterozoic dykes in the eastern FTC are affected Pillow lavas are preserved locally, but more typically, by amphibolite-grade regional metamorphism and FTC metabasalt is converted to massive or lami- deformation. Proterozoic deformation is concentrated nated amphibolite, commonly with a well-developed, into corridors hundreds of meters wide, characterized steeply-plunging, hornblende lineation. Common by dextral shear (Madore and Larbi, 2000). Brittle, leuco-trondhjemitic, foliation-parallel leucosomes west-verging reverse faults near the eastern TC/FTC (1 mm–1 cm) have millimeter-thick hornblendite contact may represent distal manifestations of the rinds, suggesting an anatectic origin (Kriegsman, Proterozoic deformation (Fig. 5; Madore and Larbi, 72 J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87

2000). Proterozoic supracrustal packages are klippe, Hornblende may also be concentrated into centimet- with kinematic indicators implying that they were ric lamellae or veins that alternate with granulitic thrust towards the south. assemblages. Retrogression to amphibolite grade caused by channelized penetration of water seems likely. 4. Petrographic and mineral-chemistry Ultramafic lavas contain porphyroclastic metamor- constraints phic(?) olivine (Fo77) and minor chromite, surrounded by clinochlore (Mg-chlorite) needles in a matrix of Many rocks show partial retrogression to the iddingsite, tremolitic hornblende, and epidote. As- greenschist grade of original igneous, or high-grade sociated ultramafic cumulates are harzburgite (sub- metamorphic assemblages. Hydrothermal greenschist ordinate wehrlite and lherzolite) containing olivine facies metamorphism is more intense in the vicinity (Fo88.5) and either green spinel or brown chromite, of major greenschist facies fractures. These fractures both surrounded by poikilitic pyroxene. Pyroxenite are spaced several kilometers apart, on average, and contains both ortho- and clino-pyroxenes, olivine, extend as much as 100 km, but are of unknown age Fe–Ti-oxides, amphibole, mica, plagioclase and al- and origin. kali feldspar. Pale brown, interstitial phlogopitic mica is ubiquitous, and may be abundant. With increas- 4.1. Supracrustal rocks ing proportions of poikiloblastic hornblende, rocks grade towards hornblendite and feldspathic horn- Metabasaltic rocks in the FTC are dominated blendite. by hornblende and plagioclase, with minor quartz, biotite, Fe–Ti-oxides, pyrite, titanite, epidote and 4.2. Gabbronorite and diorite of the Troie complex garnet. Relict clinopyroxene and olivine phenocryst pseudomorphs are uncommon. Textures range from Hornblende gabbronorite has a mosaic texture, and granoblastic polygonal, to nematoblastic, to por- is dominated by pyroxene and plagioclase (An26–56). phyroclastic. Generally, elongate hornblende grains It is distinguished from metabasaltic granulite by hav- in a thin section share the same pleochroism and ing abundant apatite (1–3%), magnetite and ilmenite extinction angle (see Fig. 3e in Bédard, 2003), im- (∼10–15%). Green hornblende (15–30%) replaces py- plying extensive syn-deformational recrystallization. roxene or forms oikocrysts. Pyroxene + mica dior- Plagioclase feldspar forms a polygonal mosaic with ite commonly preserves euhedral, zoned, antiperthitic, ◦ straight grain boundaries and 120 triple junctions. plagioclase phenocrysts (≤15%, An24–32; see Fig. 12 Plagioclase may also be concentrated with quartz in Bédard, 2003). Abundant red biotite (5–15%) oc- into foliation-parallel anatectic veinlets. Amphibolitic curs as phenocrysts or small flakes. Hornblende is metabasalt affected by lower-temperature deformation subordinate (<5%). The groundmass is a texturally has a porphyroclastic texture, with feldspar and horn- equilibrated, polygonal mosaic of two pyroxenes and blende augen, deformation twins, marginal granula- plagioclase, together with magnetite, ilmenite, minor tion of feldspar into a fine-grained quartzo–feldspathic sulphide, titanite and quartz. matrix, oblique shear bands, and concentrations of quartz + chlorite in pressure shadows and frac- 4.3. Monzonite, quartz monzonite, monzodiorite, tures. and granite of the Troie complex In granulitic metabasalt of the TC and QC, pyroxene and feldspar have straight or gently curved grain Monzonite and quartz monzonite have large boundaries and 120◦ triple junctions, indicating com- (1–5 cm), generally euhedral, perthitic alkali feldspar plete re-equilibration. Some rocks preserve evidence phenocrysts, commonly with inclusions of euhedral of prograde granulitization of amphibolite. Where plagioclase (An25) and/or quartz. Quartz phenocrysts present, hornblende typically makes up a polygo- may be embayed. Myrmekite is common. Monzodior- nal mosaic with pyroxenes and feldspar, with rare ite contains relics of clinopyroxene within hornblende, rims on pyroxene that suggest a replacement origin. and has little quartz or alkali feldspar. J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87 73

4.4. Felsic plutonic rocks in the Faribault–Thury internal growth zoning that is truncated by corrosion complex structures, features to which we attribute an igneous origin (Zen and Hammarstrom, 1984, 1986; Evans and Tonalite and trondhjemite are generally not por- Vance, 1987; Dawes and Evans, 1991). Epidote is eu- phyritic, though a few contain sub-euhedral, anhedral when completely embedded in biotite or mus- tiperthitic plagioclase phenocrysts (1–5 cm, 5–15%), covite, but shows embayments and wormy textures or micro-phenocrysts (Fig. 7C), and deformed phe- when exposed to the quartzo–feldspathic matrix (Fig. nocrysts (phenoclasts) are equally rare. Plagio- 9A; cf. Fig. 3a–d in Bédard, 2003), which we interpret clase composition ranges from An15 to An30, with as the result of magmatic corrosion or dissolution. Ig- the tonalite/trondhjemite transition at ca. An25. neous epidote in contact with chlorite is replaced by Plagioclase–plagioclase contacts are typically ser- deeper green (Fe-rich) metamorphic epidote rims. rated high-energy boundaries showing subgrain de- Allanite is generally metamict, but rare grains pre- velopment (Fig. 7C and D). Weakly-deformed rocks serve fine, concentric lamellar growth zoning. Other contain quartz with deformation bands and undula- allanite grains have complex internal zonations that tory extinction, yet which locally retains an inter- probably reflect multiple growth/dissolution events, stitial habit. More typically, quartz is recrystallized locally associated with the development of Th- and into neoblasts or annealed ribbons. Microcline is rare rare-earth minerals. Allanite also forms isolated (<5%), and occurs interstitially, as exsolutions, or as prismatic grains with scalloped edges suggesting small neoblastic subgrains. Micro-perthite to micro- magmatic resorption (see Bédard, 2003). With pro- cline exsolutions are generally undeformed (Fig. 7C), gressive solid-state deformation, the relatively rigid and myrmekitic plagioclase + quartz intergrowths are epidote, allanite and titanite grains tend to break up common. In rocks affected by submagmatic, solid- into domino structures (Kruse and Stünitz, 1999) state deformation, plagioclase forms twinned augen, and micro-boudins, and eventually evolve into slivers mortar textures develop, and the matrix is neoblastic. enclosed within biotite or muscovite fish. Tonalite contains subequal proportions of horn- Muscovite has the high FeO∗ (4.5–6 wt.%) and blende and biotite, typically concentrated into TiO2 (0.5–1.7 wt.%) contents typical of igneous schlieren with oxides, titanite, apatite, allanite and muscovite (Miller et al., 1981; Speer, 1984; Zen, epidote. Hornblende grains in a thin section do not 1988). Muscovite intergrown with biotite occurs as have common extinction angles and pleochroism (un- well-crystallized, faceted crystals. Muscovite that is like the amphibolites), suggesting substantial grain in direct contact with quartz or plagioclase, on the rotation or translation during deformation, or simply other hand, has irregular terminations, with wormy an absence of dynamic recrystallization. Some of textures (Fig. 9B) that Bédard (2003) attributed to the larger hornblende grains may be recrystallized magmatic resorption. phenocrysts. Trondhjemite has <10% ferromagne- Apart from a higher proportion of alkali feldspar, sian minerals, principally biotite, with subordinate granodiorite resembles tonalite in most respects (in- muscovite, and traces of the same accessory phases cluding the types of trace phases), while granite as tonalite. Apatite (∼1%) is commonly included in resembles trondhjemite. Granite and granodiorite magnetite and ilmenite (1–4%), which occur as inter- may contain minor Fe–Mn-garnet with biotite inclu- growths or as discrete grains. Titanite is ubiquitous (1– sions, and/or minor prismatic tourmaline. The garnet 5%), as rims on Fe–Ti-oxides and as anhedral grains falls in (or near) the igneous garnet field of Clarke with complex internal zoning. Euhedral to rounded (1981). Trondhjemitic pegmatite is associated with zircons contain inherited cores and multiple igneous metapelite-dominated supracrustal packages, and overgrowths separated by dissolution surfaces. contains almandine-rich garnet, muscovite, biotite Epidote is nearly ubiquitous in FTC tonalite and and tourmaline. Dichroic tourmaline has blue cores trondhjemite (0.5–5%, Fig. 9A). It is pale green in thin (schorl) and more dravitic green rims. The presence section, commonly contains a core of allanite (Figs. of tourmaline implies a metasedimentary component 3b–d and 5 in Bédard, 2003), has a narrow, typically (London, 1999), supporting local derivation from, or igneous, compositional range (Ps23–29), and may show contamination by, paragneiss. 74 J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87

Fig. 9. (A) Resorbed magmatic epidote from tonalite 98-3094A (E-FTC). Note the absence of secondary magnetite from the spongy domains, and the presence of U–Th-LREE enriched high-reﬂectibity veinlets, which suggest a late deuteric(?) ﬂuid-migration event (Bea, 1996). (B) Trondhjemite 98-2200A (E-FTC). B = biotite. Note that muscovite (M) is corroded when in contact with the quartzo–feldspathic matrix, but has euhedral faces when protected by biotite. Plane polarized light, 5 mm across. (C) Enderbite 98-3266 (TC). Euhedral antiperthitic plagioclase (P) with apatite (A), zircon (z) and magnetite (mt) inclusions. Interstitial quartz (Q). Crossed polarizers, 5 mm across. (D) Enderbite 98-1064D (TC). Euhedral orthopyroxene (O) and magnetite (black). Biotite (B) is abundant, as is interstitial hornblende (H). Quartz and feldspar are white. Plane polarized light, 5 mm across.

Rocks of the eastern FTC are typically more de- 4.5. Felsic plutonic rocks in the Troie and formed than those in the west as a result of Proterozoic Qimussinguat complexes orogenesis. Strong, solid-state deformation caused development of plagioclase and quartz neoblasts, re- Textures of feldspars and quartz in TC and QC en- crystallization of biotite, concentration of muscovite derbites do not differ significantly from those in the into phacoidal cleavages, and development of pris- FTC. Plagioclase (An27–34) generally has high-energy, matic epidote. There is also extensive retrogression high-temperature, serrated grain boundaries and is to the greenschist facies. As a result, igneous textures commonly antiperthitic. In relatively undeformed and minerals are less well preserved in the eastern rocks, plagioclase may be euhedral and quartz inter- FTC, though still recognizable in low-strain domains. stitial (Fig. 9C; cf. Fig. 6c in Bédard, 2003), unam- Some of the metamorphic epidote may represent biguously indicating an igneous origin. Phenoclasts recrystallized igneous phases. The occurrences of ig- are rare. With progressive deformation, quartz de- neous epidote shown in Fig. 6d represent cases where velops deformation bands, sub-grains, and finally is an igneous origin could be established based on tex- reduced to a neoblastic mosaic. tural considerations (growth zoning, igneous allanite Orthopyroxene generally forms stubby, euhe- cores, marginal resorption structures). dral, pleochroic prisms (Fig. 9D). Clinopyroxene is J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87 75 commonly euhedral and prismatic, but may also be ing, we discuss the implications of our observations interstitial to orthopyroxene. Fine exsolution lamellae with regard to the interplay of igneous, metamorphic occur in either pyroxene. Minor green hornblende and deformational processes active in the Douglas is interstitial (Fig. 9D) and rims clinopyroxene. Harbour domain and other parts of the Minto Block. Fe–Ti-oxide granules and TiO2-rich biotite (bright We then compare the geology of the Douglas Harbour red in thin section) are ubiquitous and abundant. Mi- domain and the rest of the Minto Block with An- nor apatite and zircon are common (Fig. 9C), but dean/Cordilleran accretionary orogens. The compari- accessory igneous titanite and epidote are absent. son highlights problems with application of actualistic Hornblende tonalite and biotite trondhjemite in the models to Archaean craton formation, leading us to TC and QC are identical to those found in the FTC. consider vertical tectonic models in the final section.

6.1. Metamorphic or plutonic origin of minerals and 5. Geothermobarometric constraints fabrics in plutons

Bédard (2003) presented geothermobarometric cal- Minto Block supracrustal rocks have tectono- culations on these rocks. The Al-in-hornblende geo- metamorphic fabrics and assemblages (Percival and barometer (Hammarstrom and Zen, 1986; Schmidt, Skulski, 2000; Bédard, 2003) and display leucosome– 1992) yielded 3–6.4 kbar pressures for tonalite of the melanosome pairs and diatexitic/metatexitic struc- FTC, QC and TC in agreement with other estimates tures that indicate anatexis. Existing genetic models from Minto Block plutons (3.5–5.6 kbar: Percival for the Minto Block attribute deformation and meta- et al., 1992; Percival and Mortensen, 2002), and morphism of supracrustal rocks to overprinting, supracrustal belts (generally 2–5 kbar: Percival and regional-scale orogenic/metamorphic events that Berman, 1996; Madore et al., 1999; Percival and completely transposed and recrystallized pre-existing Skulski, 2000). Granulite-grade supracrustal rocks fabrics into the dominant N-NW grain (Fig. 2). How- may show fragmentary preservation of a higher ever, during a regional-scale orogenic overprint, the pressure history (8–12 kbar: Percival and Skulski, plutonic rocks in which the deformed and metamor- 2000; Cadéron et al., 2003). The QUILF thermome- phosed supracrustal slivers were embedded must have ter (Andersen et al., 1993) gave 697–911 ◦C for experienced the same events, and so should exhibit hornblende gabbronorite, pyroxene + mica diorite, microstructures and mineral assemblages consistent and enderbite of the TC. The Blundy and Holland with transposition and prograde recrystallization at (1990) plagioclase–hornblende geothermometer temperatures and pressures similar to those of the yields near-solidus temperatures (711–745 ◦C) for supracrustal rocks. Do the plutonic rocks record this FTC felsic plutons. A hornblende-bearing TC en- overprinting tectono-metamorphic event? derbite records a higher temperature (790 ◦C), as Many Douglas Harbour (and other Minto Block) do TC gabbronorite and pyroxene + mica diorite plutonic rocks preserve relict magmatic textures and (760–826 ◦C). Reconstructed pre-exsolution compo- structures, such as: (1) variably deformed tonalitic sitions of antiperthite grains (Bédard, 2003) yield (or mela-enderbitic) enclaves in trondhjemitic (or minimum ternary plagioclase liquidus tempera- leuco-enderbitic) breccia complexes; (2) mingled tures of 550–870 ◦C in tonalite–trondhjemite, and melatonalite–trondhjemite intra-plutonic dykes (cf. 810–1045 ◦C in enderbite. Fig. 2b in Bédard, 2003) showing varying degrees of deformation, with fabric orientations similar to those observed in adjacent tonalitic hosts; (3) euhedral 6. Discussion twinned laths of plagioclase in diorite (cf. Vernon, 2000); (4) quartz with interstitial habits (Fig. 9C); (5) Existing plate tectonic models for the Minto Block enderbite with coarse, massive, homogeneous textures (Percival et al., 2001) have great predictive value, (Percival and Skulski, 2000), locally with aligned since the geology can be compared directly with phenocrysts indicating magmatic flow (Percival et al., well-studied Phanerozoic environments. In the follow- 1997); (6) epidote (Fig. 9A) and allanite with textures 76 J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87 and compositions (Bédard, 2003) identical to those at high temperatures (Tullis and Yund, 1985; Lafrance from undeformed and unmetamorphosed Phanerozoic et al., 1996; Vigneresse et al., 1996; Rosenberg, plutons (e.g. Zen and Hammarstrom, 1984, 1986; 2001). These high-temperature feldspar microstruc- Zen, 1985); (7) an absence of coronitic structures tures in the plutons contrast with the straight, well (compare with St-Onge and Ijewliw, 1996); (8) oc- equilibrated feldspar grains (120◦ triple junctions) in casional euhedral plagioclase phenocrysts, which are metabasalt, which indicate a very different, and more only rarely transformed into phenoclasts. clearly ‘metamorphic’, prograde recrystallization The presence of leucocratic segregations occupy- history (Kretz, 1966; Vernon, 1999). Since geother- ing strain shadows and shear zones within tonalite or mometry yields near-solidus temperatures for most mela-enderbite (Figs. 2, 6 and 7a in Bédard, 2003; cf. mineral phases in the plutonic rocks (Bédard, 2003), Sawyer, 2000; Pawley and Collins, 2002) implies that we infer that the plastic deformation textures gener- deformation was syn-magmatic. The fact that mafic ally represent a continuation of syn-magmatic defor- minerals generally do not share crystallographic axes, mation beyond the rheological locking point into the unlike the hornblende grains in metabasalt from ad- sub-subsolidus, sub-magmatic regime (e.g. Hibbard, joining supracrustal belts, is consistent with this in- 1987; Paterson et al., 1989; Vigneresse et al., 1996), ference, as is the homogeneity of the fabrics, which rather than an overprinting orogenic/metamorphic suggests a viscous (mushy) medium (Gapais, 1989). event. Later overprinting fabrics exist, but tend to be At the granulite to amphibolite grade condi- concentrated in relatively narrow corridors with steep tions preserved in supracrustal rocks (peaks at foliations and sub-horizontal lineations (Cadieux ∼800 ◦C from Percival and Skulski, 2000), biotite + et al., 2002), and are not obviously associated with muscovite-bearing granitoids should be migmatized the development of the penetrative N-NW fabrics (e.g. Thompson, 2001). However, felsic plutons of the characterizing the Douglas Harbour domain and most Douglas Harbour domain (and elsewhere in the Minto of the central-western Minto Block. Block: Bédard, 2003) show few of the hallmark traits of migmatites (e.g. Kriegsman, 2001; Brown, 2001), 6.2. Melt ascent and emplacement in the Douglas and commonly record temperatures higher than those Harbour domain of adjacent supracrustal belts (Bédard, 2003). The common preservation of outcrop- and Recognition of relict igneous textures and of the micro-scale structures indicating that many fabrics syn-kinematic nature of Douglas Harbour magmatism (mafic schlieren, lineation, foliation, preferred crystal allows partial reconstruction of the melt ascent and orientation) began to form at the magmatic or submag- emplacement history. Textures in igneous epidote and matic stage, the preservation of delicate exsolution muscovite from FTC rocks are important, because textures (Figs. 7C and 9C) recording near-liquidus the stability of these minerals is sensitive to pressure. temperatures; and the rarity of phenoclasts imply that Epidote commonly has embayed and spongy margins most plutons did not experience penetrative, prograde, when in contact with the quartzo–feldspathic matrix tectono-metamorphic recrystallization. Most of the (e.g. Fig. 9A), but faceted faces when armored by bi- textural and mineralogical characteristics of the Dou- otite or muscovite. Identical partial magmatic resorp- glas Harbour felsic plutons are more consistent with tion structures in epidote from Phanerozoic tonalites slow cooling of a syn-kinematic igneous system, with are attributed to syn-magmatic decompression (Zen gradual development of solid-state fabrics as plutons and Hammarstrom, 1984, 1986; Zen, 1985; Schmidt continued to deform in the sub-magmatic stage. The and Thompson, 1996). A 6 kbar threshold is com- fact that feldspar behaved as plastically as quartz in monly suggested (Zen and Hammarstrom, 1984, 1986; the felsic plutons suggests that this solid-state de- Zen, 1985), but Schmidt and Thompson (1996) con- formation occurred at high (>900 ◦C) temperatures cluded that epidote could remain stable to 3 kbar in (Gapais, 1989; Dell’Angelo and Tullis, 1996). The oxidized magmas. Thermobarometric calculations for ubiquitous serrated, high-energy, feldspar–feldspar Douglas Harbour rocks (Bédard, 2003) suggest that a grain boundaries and subgrains (Fig. 7C and D) sug- 6 kbar breakdown pressure may be applicable. Mus- gest recrystallization by fast grain boundary migration covite also exhibits evidence of magmatic resorption J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87 77

(Fig. 9B, also Bédard, 2003). Experimental data indi- required to generate the bulk of the Minto Block. cate a 2–4 kbar lower limit for the stability of igneous Percival et al. (1994) circumvented this problem by muscovite (Chatterjee and Johannes, 1974; Anderson proposing the existence of two synchronous subduc- and Rowley, 1981), though variations in oxygen and tion zones. Slab rollback may generate extensional volatile activity may shift this threshold (Miller et al., accretionary orogens characterized by wider plutonic 1981; Weidner and Martin, 1987; Zen, 1988). belts (Collins, 2002), and represent another possible Despite uncertainty about the absolute pressure analogue for the Minto Block plutonism. However, of phase destabilization, the common presence of the basalts associated with extensional accretionary corroded epidote and muscovite in Douglas Harbour orogens have prominent arc-related geochemical sig- tonalite–trondhjemite (Fig. 6d and e) indicates that natures, unlike typical Minto Block tholeiites (Madore these magmas ascended rapidly in the form of crys- et al., 1999), and their closure is characterized by tal slurries from a depth where these phases were thin-skinned thrusting and duplexing (see references stable on the liquidus (cf. Zen, 1985; Brandon et al., in Collins, 2002), which is not seen in the Minto 1996). Identical igneous epidote and muscovite oc- Block. cur in tonalite–trondhjemite from most Minto Block Secondly, the structural pattern in the Minto Block domains (Bédard, 2003), suggesting that these con- is very different from that of Phanerozoic accretionary clusions are generally applicable. Ascent of melt is orogens. Terrane accretion in the North and South an efficient heat transport mechanism, and Bédard American Cordilleras is characterized by the devel- (2003) proposed that the metamorphism observed opment of synchronous, shallowly-dipping thrusts in Minto Block supracrustal rocks is pluton-driven and steeply-dipping strike-slip faults (Chardon et al., (DeYoreo et al., 1989), which removes the need for 1999; Scheuber and Gonzalez, 1999; Brown et al., a regional, Barrovian metamorphic episode linked to 2000). This contrasts with the extreme rarity of orogenesis. shallowly-dipping planar fabrics in the Minto Block (e.g. Fig. 4). Domains characterized by syn-magmatic 6.3. Do voluminous tonalite–trondhjemite– fabrics with steep plunges exist in Phanerozoic accre- granodiorite–granite (TTG) plutons imply tionary orogens (McClelland et al., 2000), but form subduction under an Andean-type margin? narrow belts (10’s of km) embedded within dominant shallowly-dipping structures. While a pure flattening It has been proposed that the closest modern ana- fabric associated with terrane accretion or collision logues to Archaean TTG suites are Andean mar- could produce steeply-dipping and plunging L–S gin cordilleran-type batholiths constructed on a and S–L fabrics on a local scale (Choukroune et al., pre-existing sialic crust (Weaver and Tarney, 1981; 1995, 1997), it is difficult to imagine a collisional Condie, 1986; Stern et al., 1994; Martin, 1999). or accretionary process that repeatedly generated However, the differences between Minto Block TTG such fabrics without also generating coeval foreland and Andean margins leads us to question whether thrust-and-fold belts. this proposal is correct (cf. Maaløe, 1982; Hamilton, Another problem is the scattered distribution of 1998; Smithies, 2000). supracrustal belts (Figs. 3 and 5), a pattern which Firstly, the broad areal distribution of TTG in Ar- seems inconsistent with a marginal accretion or chaean cratons (∼500 km wide distribution of the ‘obduction’ zone, as proposed for the southern Supe- 2.73 Ga Leaf River suite in the Minto Block, Fig. 1) rior Province (Card, 1990; Kimura et al., 1993; Calvert and large volumes produced, are probably incon- and Ludden, 1999). The absence of thrust faults with sistent with a single, linear source of arc magma consistent vergence also seems inconsistent with such (Reymer and Schubert, 1984; Hamilton, 1998). Note models. Furthermore, many Minto Block supracrustal that the plutonic belts of the North and South Amer- rocks do not resemble oceanic crust, being more ican Cordilleras are only ∼150 km wide (Chardon similar to platform sequences (Thurston and Chivers, et al., 1999; Scheuber and Gonzalez, 1999; Brown 1990; Lowe, 1994; Percival et al., 1997). One possi- et al., 2000), even though subduction operated nearly ble alternative to obduction is subcretion, or lateral continuously for 200 Ma, a timescale similar to that accretion of soft, largely melted slabs to the margins 78 J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87 of the continent (Kusky and Polat, 1999). However, must have traveled in exactly the same direction as this type of lateral accretion should also produce the preceding one, with near-orthogonal convergence obliquely dipping fabrics with a consistent vergence in every case. While not impossible, this appears (Beaumont et al., 1996; Barr et al., 1999). too fortuitous. Finally, the common preservation of syn-magmatic fabrics in all Minto Block domains 6.4. Origin of the dominant North-Northwest- suggests that amalgamation must have occurred while trending fabric of the Minto Block each terrane was still magmatically active, a conclu- sion at odds with the proposed very young age of the In the terrane accretion model originally proposed D2 event. for the Minto Block (Percival et al., 1992; Stern In their revised model, Percival and Skulski (2000) et al., 1994; Lin et al., 1996), the amalgamation of proposed that metamorphic effects (seen mostly several distinct oceanic and continental arcs gener- in supracrustal rocks) decrease eastward from a ated N-NW-striking foliations and steeply dipping granulite-facies peak at 8 kbar and 850 ◦C, down to lineations at, or near, the metamorphic peak. In the the amphibolite facies in the Goudalie domain, with revised model (Percival and Skulski, 2000), a first only weak evidence of metamorphism in the Utsalik amalgamation event at ca. 2.77 Ga (Fig. 2) created domain (zircon overgrowths). Comparison with other a composite cratonic basement, while collision and examples of crustal thickening and exhumation (e.g. overthrusting of the Tikkerutuk arc onto the western Frisch et al., 2000; Streepey et al., 2000; Hynes, 2002) and central Minto (2.70–2.69 Ga) thickened the crust suggests that prominent exhumation-related struc- and produced the dominant N-NW fabric through tures should separate structural panels with distinct transposition and recrystallization synchronous with pressure–temperature histories. Furthermore, the east- metamorphism. Percival and Skulski (2000) also ern limit of the reactivated orogen should correspond suggested that subsequent exhumation and erosion to a marked contrast in metamorphic grade, structural removed most of this overthrust terrane, except for style, and depth of exhumation, and be characterized the topmost lavas in the Vizien belt. by prominent structures (thrust-and-fold belt and/or Terranes accreted to a margin or entrapped between major strike-slip faults). However, recent mapping in colliding blocks in Phanerozoic orogens typically the Minto Block has not discovered evidence of such retain distinctive lithological, deformational, and exhumation-related structures, or of an eastern struc- pressure–temperature histories, with terrane bound- tural boundary to the deformation and metamorphism aries being recognizable either as high-strain zones, caused by a Tikkerutuk accretion event. Finally, as or zones where oceanic crustal fragments and ac- noted previously, textures of plutons in the central cretionary prisms occur (e.g. Williams and Hatcher, Minto Block (Bédard, 2003) do not seem to record 1983; Coney, 1989; Bluck and Dempster, 1991; this orogenic event. Monger, 1993; Huang et al., 2000). Preservation of terrane boundaries in the Proterozoic Grenville oro- 6.5. Vertical tectonic models? gen (Rivers, 1997; Indares et al., 2000) suggests that these boundaries should still be recognizable, despite The preceding discussion underscores the problems deep burial and intense deformation. The limited involved in applying uniformitarian plate-tectonic lithological variations, and apparent lack of tectonic models to Archaean craton formation. Could vertical terrane boundaries in the Minto Block, suggests to us tectonic models provide an alternative framework in that this was not an accretionary orogen. which to interpret our observations? Greenstone belts In the context of a collisional or accretionary oro- in many Archaean cratons occur as anastomizing syn- genic scenario, homogeneous and orthogonal ENE- forms between large domiform TTG bodies, a pattern WSW compression of the Minto Block would be originally interpreted in terms of diapiric ascent of required to generate a uniform, N-NW-striking grain, granitoids into pre-existing or pene-contemporaneous with extensive crustal thickening to generate sub- supracrustal sequences (e.g. Macgregor, 1951; vertically-plunging lineations. To keep fabrics uni- Blackburn, 1981; Hickman, 1983; Ayres and Thurston, form during repeated terrane accretion, each terrane 1985). These vertical tectonic models fell into disfavor J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87 79 principally because repetition of supracrustal pack- at the base of the migrating supracrustal sequences ages, and the presence of nappe- and thrust-like (e.g. Chardon et al., 1996). Further increments of structures, were interpreted as being due to marginal deformation might be expected as material flowed accretion processes (Myers, 1976; de Wit, 1982; down into constrictions between impinging upwelling Williams, 1990; Kusky and Kidd, 1998; Polat and zones, and as sheath folds developed in the deeper, Kerrich, 1999; the ‘D1’ event of Percival and Skulski, ductile portion of the downwelling limbs. 2000); while the ‘dome-and-basin’ structures were We now discuss how vertical tectonic models reinterpreted as fold interference patterns (Snowden could be used to interpret the formation of fabrics and Bickle, 1976; Drury et al., 1984; Myers and in the Douglas Harbour rocks. The steep fabrics Watkins, 1985). and near-ubiquitous ∼2–10 km-scale ‘swirly’ foli- Vertical tectonic models have been revived because ation patterns found within the TC, QC, and FTC many studies have revealed a general absence of fab- (Figs. 4 and 5) can be interpreted either as: (1) an rics compatible with fold interference in dome cores interference pattern generated by near-orthogonal (Schwerdtner et al., 1979; Chardon et al., 1996, 1998; compressional deformation events (horizontal tec- Choukroune et al., 1995, 1997; Collins et al., 1998), tonics); or (2) as a section through steeply-plunging and showed that there are no structural repetitions L-tectonite or diapiric bodies (vertical tectonics). The in some of these so-called ‘accretionary’ sequences, intimate interpenetration and similar fabric orienta- many of which are now recognized as being essen- tions of felsic plutonic and supracrustal rocks implies tially autochthonous (Chardon et al., 1998; Bleeker that both were deformed together, with many of et al., 1999; Van Kranendonk, 2001; Thurston, 2002; the plutonic facies recording a supra- to sub-solidus Ayer et al., 2002; Van Kranendonk et al., 2002, continuum. The absence of orthogonal, overprinting 2003, this issue). Many objectors to vertical tectonic cleavages, and the preservation of syn-magmatic tex- models have invoked the absence of radial ‘diapiric’ tures in most plutons, are difficult to reconcile with structural patterns in the domal massifs. However, superimposed, mutually perpendicular, orogenic de- natural and experimental diapirs show complex in- formation events, and so we favor a vertical tectonics ternal structures, particularly where viscosity con- model. trasts between diapir and host are small (Jackson and In a vertical tectonic model, repeated deformation Talbot, 1989). In any case, petrologic studies imply of supracrustal rocks would occur as supracrustal that ascent of material within TTG ‘domes’ mostly in- packages migrated towards, and then into, the zone(s) volved movement of magma in large conduits (Ayres of downwelling (Fig. 10B and C). The steeply dipping et al., 1991; Collins et al., 1998; this paper), so that and plunging syn-magmatic fabrics in the plutonic their lack of resemblance to simple diapiric structures rocks, on the other hand, could originate in a variety should come as no surprise (see also Van Kranendonk of ways. Given the evidence for rapid melt ascent in et al., 2003, this issue; and Pawley et al., 2003, large conduits (resorbed epidote), and for common this issue). syn-magmatic deformation in Douglas Harbour intru- Proponents of vertical tectonic models argue sions, it seems probable that most of these fabrics were that structures in the synclinal troughs, including acquired deep within the crust. Newly emplaced intru- ‘accretionary’ or thrust faults, can also be explained sions would tend to concentrate strain (down-dip shear, as a consequence of the complex flow pattern ex- Fig. 10B, ‘a–b’) between downwelling and upwelling pected in a downwelling limb (e.g. Bouhallier et al., zones, since in rheological terms they would be the 1995; Chardon et al., 1996, 1998; Collins et al., weakest part of the crust (Pavlis, 1996; Vauchez et al., 1998; Van Kranendonk, 2001; Van Kranendonk et al., 1997; McCaffrey et al., 1999). Subsequent intrusions 2003, this issue). In this context, it seems proba- would create new weak zones, shifting the locus of ble that complex overprinting structures and intense maximum strain and preserving the fabrics devel- compression would develop as supracrustal packages oped in older intrusions (Schwerdtner et al., 1979). flowed into convergence zones above downwelling Alternatively, some of these fabrics may have been limbs (Fig. 10; cf. Dixon and Summers, 1983), and acquired due to magmatic overpressure from younger prominent decollement structures would be expected intrusions emplaced into partially consolidated older 80 J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87 J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87 81 plutons (cf. McNulty et al., 1996). A more radical hy- these two domical masses (cf. Bouhallier et al., 1995; pothesis involves slow convective overturn of the en- Chardon et al., 1996, 1998; Collins et al., 1998; Van tire lower crust (Fig. 10B and C), which is supported Kranendonk, 2001). by fluid dynamic models that imply bulk convection Given that fabrics developed in the Douglas Har- of the largely molten lower part of the crust during bour domain plutons (Figs. 4 and 5) were originally the peak magmatic flux (Ridley and Kramers, 1990; syn-magmatic, then they must reflect the stress field Ridley, 1992). However, convective overturn need not that existed at the time of intrusion. Since this N-NW be complete (Collins et al., 1998; Van Kranendonk grain is shared by the western FTC (2.85–2.87 Ga), the et al., 2003, this issue), and only modest amounts eastern FTC (2.8–2.77 Ga), the enderbites of the TC of shear might be sufficient to develop the observed and QC (2.73–2.74 Ga), and the TC monzonitic suite fabrics. Detailed mapping and structural studies are (2.69 Ga), it seems as though this stress field persisted needed to verify these suggestions and discriminate nearly unchanged in the Douglas Harbour domain for between mechanisms. almost 200 million years. What was the nature of this We suspect that the broad domical structures of the stress field, how was it produced, and how could it re- Troie and Quimussinguat complexes (Figs. 3 and 5) main so uniform, so long? As pointed out previously, formed in a late warping event, since the presence of Phanerozoic accretionary orogens commonly exhibit shear zones at the margins of the domes indicates that strike-slip faults and shallowly-dipping fabrics, fea- the crust was rather stiff and cold at this time. Concen- tures which appear to be inevitable consequences of trations of granite at sheared contacts to the TC and non-orthogonal subduction and terrane accretion. Con- QC domes suggest that the bounding structures also sequently, it is difficult to account for the long-lived guided the ascent of late K2O-rich melts and fluids uniformity of the Douglas Harbour stress field (and its (cf. Smit and van Reenen, 1997; Kramers et al., 2001). fabrics) in terms of multiple compressional orogenic The zone of E-W-trending Archaean fabrics that sepa- pulses associated with docking of a variety of terranes rates the TC and QC (Thury Deformation Zone) could against a proto-cratonic core. On the other hand, represent a crush zone related to upward movement of differential vertical movement (shear) in an overall

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Fig. 10. Vertical tectonics scenario for the Douglas Harbour domain, which may also be applicable to the central and western Minto Block. Dotted line is sea level. Note the progressive horizontal extension of the crust from (A) to (C). (A) Extensive mantle melting produces thick oceanic plateaux. Iceland or Ontong-Java would be good analogues (e.g. Maaløe, 1982; Kröner, 1991; Tejada et al., 2002). Collisions between plateaux may cause local thickening and compressional tectonics (D1 of Percival and Skulski, 2000). Different mantle domains yield lavas (tholeiites and komatiites) with distinct trace element and isotopic signatures, such as volcanic terranes 1 and 2 (VT1 and VT2). Small dark arrows indicate movement of mafic–utramafic melt out of the mantle residuum. Continued under- and intra-plating by basalt and komatiite (UKT) eventually causes the base of the lava plateau to melt, yielding a first generation (TTG1) of tonalite/trondhjemite magmatism. Inheritance from the lavas (VT1 vs. VT2) gives the resulting TTGs distinct isotopic and trace element signatures. (B) Underplated cumulates (UKTC) and restites delaminate (Jull and Kelemen, 2001; Zegers and van Keken, 2001), allowing fresh mantle to well up, decompress, and generate another major pulse of mafic–ultramafic magmatism, much of which underplates/intraplates the crust. As a result, TTG1 (and host rocks) melt to give a second generation of TTG magmas. When the proportion of TTG2 magma reaches a critical threshold, the overlying volcanics become unstable and density-driven convective instabilities develop. Compressional forces in the downgoing limbs cause folding and thrusting that may correspond to some of the D1 events seen in Minto Block supracrustal rocks. Ascent of TTG magma is dominantly as intrusions (‘a’ and ‘b’), which focus shear between the downwelling limb and the ascending TTG intrusions, producing the steep D2 fabrics. Diapiric ascent (‘d’), and thermally-driven convective instabilities (‘c’) may also occur, both of which would contribute to generation of steep, D2-like fabrics. (C) Zones where underplated cumulates and restites delaminate anew receive a fresh pulse of mafic–ultramafic magmatism, generating a third TTG event which may correspond to domains dominated by the Leaf River suite (2.73 Ga, Utsalik, Lac Minto). Instabilities such as those illustrated in part B (a–d) would be repeated, reinforcing the D2 structural overprint. Domains where the underplated material does not, or only partly, delaminates would only develop minor amounts of late magmas that could fill ‘tensional’ fractures or shear zones (e.g. Troie complex monzonitic suite), with lower magmatic fluxes generating only a broad ‘domal’ movement, rather than pervasive vertical instabilities. Note that the supracrustal relicts in TTG1 and TTG2 have been omitted for clarity. The central trough structure includes subsiding lavas and sediments, as well as intruded, or adjoining, older TTGs, and would represent the older, more tectonized domains (Goodalie, Faribault–Thury). 82 J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87 extensional stress field would explain the absence of 7. Conclusions shallowly-dipping thrusts and transpressive structures (cf. Barros et al., 2001) and simplifies the space prob- Archaean tonalite, trondhjemite, enderbite, gran- lem associated with generation and emplacement of odiorite and granite of the Minto Block, northeast- the huge volumes of magma involved. Although most ern Superior Province, have been interpreted as a Phanerozoic Cordilleran-type batholiths are unde- deeply-eroded collage of Andean and intra-oceanic formed (Zen, 1985), emplacement-related structures arcs, with the dominant N-NW-trending foliation are locally prominent, preserving sub-vertical fabrics resulting from orogenic transposition and metamor- (e.g. McNulty et al., 1996; Miller and Paterson, 2001), phism associated with terrane accretion. However, some of which have been related to emplacement outcrop scale structures and microstructures im- of successive sub-vertical sheets during a magmatic ply that deformation was initially syn-magmatic in extensional phase (Campbell-Stone et al., 2000). In most Minto Block plutons. Magmatic muscovite principle, the continental magmatic extension scenario and epidote grains have resorbed edges that sug- proposed here (Fig. 10) resembles seafloor-spreading, gest ascent of magma as crystal slurries from higher where the crust forms by repetitive intrusion per- pressures. Thermobarometry indicates final equi- pendicular to the principal extensional stress. We libration at ∼4−6 kbar during near-solidus defor- speculate that the driving force was extensional col- mation, with several intrusive phases preserving lapse of the weak continental crust (Rey et al., 2001; high-temperature, near-liquidus signatures. Preserva- Liu, 2001), a scenario that would be favored by on- tion of syn-magmatic fabrics and of fossil liquidus going mafic–ultramafic underplating and the partially temperatures in the plutons is probably not consis- molten nature of the lower crust (e.g. Sandiford, 1989; tent with development of the dominant, penetrative, Fountain, 1989; Laube and Springer, 1998). N-NW fabrics of the Minto Block during a late, Thus, the bulk of the evidence is compatible with orogenic, tectono-metamorphic overprint. Further- an origin of the dominant ‘D2’ fabric of the Douglas more, the broad uniformity of these high-temperature Harbour domain as a result of diachronous vertical fabrics in rocks of widely different ages, the over- tectonic processes associated with cratonization. The all lithological similarity of rocks from different broad domical structures (TC and QC) could repre- Minto Block domains, and apparent absence of ma- sent later, cooler manifestations of the same processes jor domain-limiting structures, also seem inconsistent acting on a more rigid crust (Fig. 10C, right side). A with terrane accretion models. Instead, we infer an vertical tectonic model can account for the regional overall extensional regime, with dense supracrustal extent and temporal persistence of the N-NW-trending assemblages and associated subsolidus intrusions sub- fabric, the common occurrence of steeply-plunging siding as cold fingers, while felsic magmas ascended lineations, the rarity of shallowly-dipping planar initially as syn-kinematic intrusions. Shear between fabrics, the syn-magmatic nature of the early de- ascending and descending limbs, or partial, possibly formation, the scattered distribution of supracrustal cyclical, convective overturn, would have generated belts and their pluton-driven metamorphism. We hy- the dominant steeply-dipping and plunging fabrics. pothesize that vertical tectonic models might explain Broader domical structures would have developed as many of the problematic features of the western and the crust stiffened, with bounding shear zones guiding central Minto Block also. In this context, changes ascent of late granitoids and fluids. in the locus of peak magmatic flux in the Minto Block between 2.9 and 2.7 Ga (Leclair et al., 2001b) would reflect shifts in the locus of convective insta- Acknowledgements bility, rather than the presence of subduction zones. This hypothesis is also consistent with the provin- This study was funded by the Geological Survey ciality of intrusion and inherited zircon ages (Leclair of Canada (contribution # 2002-145) and Ministère et al., 2001b), and explains why domain bound- des Ressources Naturelles du Québec (contribution # aries are principally intrusive contacts, rather than 2002-5130-04). Patrice Rey, Steve Sheppard, Alain tectonic ones. Leclair, Léopold Nadeau and John Percival provided J.H. Bédard et al. / Precambrian Research 127 (2003) 61–87 83 many valuable comments and kept us on the straight Blundy, J.D., Holland, T.J.B., 1990. Calcic amphibole equilibria and narrow path. Marc Choquette assisted with the and a new amphibole–plagioclase geothermometer. Contrib. Mineral. Petrol. 104, 208–224. microprobe analyses. We also wish to thank the field Bouhallier, H., Chardon, D., Choukroune, P., 1995. 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