Geological Survey of Denmark and Greenland Bulletin 11, 33-52 33 Pre-Nagssugtoqidian crustal evolution in West Greenland: geology, geochemistry and deformation of supracrustal and granitic rocks north-east of Kangaatsiaq Jean-François Moyen and Gordon R. Watt The area north-east of Kangaatsiaq features polyphase grey orthogneisses, supracrustal rocks and Kan- gaatsiaq granite exposed within a WSW–ENE-trending synform. The supracrustal rocks are com- prised of garnet-bearing metapelites, layered amphibolites and layered, likewise grey biotite para- gneisses. Their association and geochemical compositions are consistent with a metamorphosed volca- no-sedimentary basin (containing both tholeiitic and calc-alkali lavas) and is similar to other Archae- an greenstone belts. The Kangaatsiaq granite forms a 15 × 3 km flat, subconcordant body of de- formed, pink, porphyritic granite occupying the core of the supracrustal synform, and is demonstra- bly intrusive into the amphibolites. The granite displays a pronounced linear fabric (L or L > S). The post-granite deformation developed under lower amphibolite facies conditions (400 ± 50°C), and is characterised by a regular, NE–SW-trending subhorizontal lineation and an associated irregular foli- ation, whose poles define a great circle; together they are indicative of highly constrictional strain. The existence of a pre-granite event is attested by early isoclinal folds and a foliation within the amphibo- lites that is not present in the granite, and by the fact that the granite cuts earlier structures in the supracrustal rocks. This early event, preserved only in quartz-free lithologies, resulted in high-temper- ature fabrics being developed under upper amphibolite to granulite facies conditions. Keywords: Archaean, deformation, supracrustal rocks, granite, Nagssugtoqidian _______________________________________________________________________________________________________ J.-F.M., Department of Geolog y, University of Stellenbosch, 7602 Matieland, South Africa. E-mail: jfmoyen@wanadoo.fr G.R.W., Marchmyres Cottage, Breda, Alford AB33 8NQ, Aberdeenshire, U.K. Introduction and regional geology The northern part of the Nagssugtoqidian orogen (NNO) is a domain of predominantly Archaean rocks that have been deformed and metamorphosed during Nagssugto- qidian orogenic activity at c. 1.8 Ga (Hollis et al. 2006, this volume; Mazur et al. 2006, this volume; Thrane & Connelly 2006, this volume; van Gool & Piazolo 2006, this volume). Palaeoproterozoic rocks are sparse, and ap- parently confined to some supracrustal belts, the most prominent one being the Naternaq supracrustal belt (Øster- gaard et al. 2002). A few small granitic-pegmatitic plugs and dykes are also related to the Palaeoproterozoic evolu- tion. Therefore, while the structures probably reflect Nags- sugtoqidian deformation, the rocks themselves (and their protoliths) largely reflect Archaean formations and evolu- tion. Among the Archaean units, the ubiquitous orthogneis- sic basement has previously been studied (Moyen et al. 2003a; Steenfelt et al. 2005); it is mostly made up of clas- sical tonalite-trondhjemite-granodiorite (TTG) gneisses, with minor components either related to TTG partial melting, or to the participation of peridotitic mantle in their petrogenesis. All these components are well known in the Archaean, and are probably related to the subduc- © GEUS, 2006. Geological Survey of Denmark and Greenland Bulletin 11, 33–52. Available at: www.geus.dk/publications/bull 34 tion of hot oceanic lithosphere in an arc setting (Martin 1986, 1994; Moyen et al. 2003b; Steenfelt et al. 2005). Several components of the gneissic basement have been dated (Kalsbeek & Nutman 1996; Connelly & Mengel 2000; Thrane & Connelly 2002, 2006, this volume), yield- ing ages in the range 2.9–2.6 Ga. Supracrustal assemblages are common, and have been mapped in many places in the Kangaatsiaq, Aasiaat and Kangersuneq map sheet areas (Marker et al. 1995; Men- gel et al. 1998; Østergaard et al. 2002; van Gool et al. 2002a; Garde 2004; Hollis et al. 2006, this volume). They are of two main types, either amphibolites or metasedi- mentary rocks, that may be either aluminous, biotite ± muscovite ± sillimanite ± garnet-bearing metapelites, or quartz-rich, psammitic rocks. The age of the supracrustal rocks is, however, poorly constrained. Whilst some of them are of demonstrated Palaeoproterozoic age (c. 1.95 Ga, the Naternaq supracrustal belt, Østergaard et al. 2002; Thrane & Connelly 2002, 2006, this volume), others are likely to be of Archaean age, for instance anatectic metapelites in Saqqarput fjord in the southern part of the Kangaatsiaq map sheet area (Thrane & Connelly 2006, this volume). Lastly, small intrusions, plugs and sheets of granite and pegmatite cut across the lithologies described above. Some of them have been dated (Kalsbeek & Nutman 1996; Thrane & Connelly 2002, 2006, this volume) and yield- ed late Archaean ages (2.7–2.6 Ga); it is commonly agreed that most magmatic activity in this region was related to late Archaean events, Palaeoproterozoic P–T conditions being such that anatexis was hardly achieved in the NNO (Mazur 2002; Piazolo 2002). The very homogeneous and porphyritic Kangaatsiaq granite north-east of Kangaat- siaq, 15 by 3 km in outcrop size, is among the largest granitic bodies of presumed late Archaean age in the south- ern Disko Bugt region. Altogether, the three components outlined above are representative of the usual trilogy of Archaean terranes (Windley 1995): grey TTG gneisses; volcanic and volcano-sedimentary deposits (greenstones); and late, K-rich granites. The area east and north-east of the town of Kangaatsiaq (Fig. 1) is dominated by a syn- form of supracrustal rocks (mafic and felsic volcanic rocks associated with sediments), into which the Kangaatsiaq granite was emplaced. It is, therefore, a good place to study the Archaean components and local history in the NNO. Greenland 68°19' 53°24' Kangaatsiaq granite Amphibolite Layered biotite gneiss of supracrustal origin and aluminous metapelite Orthogneiss Geological boundaries: Established Inferred Fault C D B A A 81 82 89 92 2 km Kangaatsiaq 61 73 52 85 64 58 57 80 75 Fig. 1. Geological map of the Kangaatsiaq granite and surrounding synform, with sample localities from Table 1. Geology mostly from 2002 field work; some parts are drawn from 2001 data (J.A.M. van Gool, G.I. Alsop, S. Piazolo and S. Mazur). A–A, approximate position of section on Fig. 2; B, loc. 89, see Fig. 3; C, locs 81–82, see Fig. 4; D, loc. 80, see Fig. 5. 35 Previous work Previous studies in the Kangaatsiaq area included recon- naissance mapping by Noe-Nygaard & Ramberg (1961), 1:250 000 scale mapping by Henderson (1969), and vis- its to key localities during the Danish Lithosphere Centre project (Marker et al. 1995; Mengel et al. 1998), as a result of which most published ages were obtained (Kalsbeek & Nutman 1996; Connelly & Mengel 2000). Mapping of the area was predominantly based on coastal exposures, while map information for large parts of the inland areas was based only on photogeological interpretation. There- fore, the Kangaatsiaq granite, which happens to crop out mostly inland and occupies the high grounds at the core of a synform, was at that time simply considered to be part of the polyphase gneissic basement. The Geological Survey of Denmark and Greenland (GEUS) and its partners undertook more detailed map- ping of the Kangaatsiaq map sheet in the summer of 2001. This included limited inland work, and the Kangaatsiaq granite was recognised for the first time as belonging to the group of the late Archaean intrusives. Its overall shape was mapped, as well as the supracrustal rocks into which it intrudes. Metasedimentary rocks in the area were also sampled, allowing for metamorphic studies (Mazur 2002; Piazolo 2002). Finally, re-evaluation of the area in the summer of 2002 by the present authors led to the refinement of geological boundaries and the production of the map of Fig. 1. Sam- pling of the whole supracrustal series was also undertak- en. Thin sections were cut at Université Claude-Bernard (Lyon, France), and samples were analysed at GEUS us- ing XRF as well as ICP-MS (Table 1). In addition, other supracrustal rocks from the same area (obtained from A.A. Garde, personal communication 2003) have been used for the interpretation presented here, as they show similar geochemical features. Map pattern As mentioned in the introduction, the studied area (Fig. 1) is mainly made up of three main components: base- ment orthogneisses discussed by Moyen et al. (2003a) and Steenfelt et al. (2005), a succession of supracrustal rocks which comprise a sequence of amphibolite and metasedi- mentary rocks described below (Figs 1, 2), and the Kan- gaatsiaq granite, an intrusion of pink, coarse-grained, strongly lineated (L or L > S fabric) granite with K-feldspar phenocrysts. The foliated basement gneisses and the supra- crustal rocks, together with early folds and structures, are refolded into a complex synform which is locally over- turned, in particular on its north-western rim (see below). The granite occupies the core of the synform; it is intru- sive within the top amphibolitic layer of the supracrustal sequence (Fig. 3) and is also folded together with the supra- crustal rocks. The geometry of the granite suggests that it constitutes a single sheet within the supracrustal unit, and that the original intrusion had an overall flat, laccolith- like shape (Fig. 2). We consider that the mapped contact always corresponds to the bottom of the laccolith, and that the top surface has been removed by erosion (Fig. 2). NW SE Kangaatsiaq granite Amphibolite 1 km Layered biotite gneiss of supracrustal origin with ultramafic layer (schematic) with gabbroic lens (schematic) Aluminous metapelites Basement orthogneiss Amphibolite layers and enclaves in other lithologies (schematic) Fig. 2. Schematic NNW–SSE cross- section across the Kangaatsiaq granite and the surrounding synform. The laccolith shape (dashed line) is inferred, see text for details. 36 A B C D photo JFM-2002-5-22 photo JFM-2002-5-22 photo JFM-2002-5-22 Photos JFM-2002-5-19–21Photos JFM-2002-5-19–21Photos JFM-2002-5-19–21 Fig. 3. Contact of the Kangaatsiaq granite and the south-western limb of the synform, loc. 89. The granite clearly intrudes the supracrustal pile, and at the same time occupies the core of the (here, slightly overturned) synform with apparently conformable relationships. A: Photomosaic of cliff face, facing east. B: Structural interpretation (stippled: pegmatites; rectangle: location of enlargement D). C: Lithological interpretation. crosses: granite; dark grey: amphibolite; light grey: layered gneiss. D: Detail of a small granitic apophysis which clearly cuts across the foliation of the amphibolite. 37 The early structures are associated with syntectonic aplites and pegmatites that cut across the amphibolite but occa- sionally occupy shear zones or fold hinges. The supracrustal series Stratigraphy The supracrustal rocks that define the synform occur as largely discontinuous layers (Figs 2–4), that could either correspond to an original, discontinuous geometry (there- fore suggesting lava flows), or simply be a result of tecton- ic stretching during the multiphase deformation witnessed by the area. Indeed, some of the contacts between the litho- logical units appear to be tectonic (Figs 4, 5), suggesting that the present-day ‘stratigraphy’ might not be original. Never- theless, our mapping suggests that three main units can be recognised, allowing the following tentative stratigraphic sequence (Figs 2, 4). 1. The lowermost, c. 100 m thick part consists of an asso- ciation of amphibolite interlayered with garnet-silliman- ite metapelites, sometimes with augen textures. Some of the amphibolites are garnet-bearing, while others contain centimetre-sized lenses of diopside-bearing gab- bro and small ultrabasic layers (pyroxenite or serpenti- nite, observed in the south-western part of the synform). The pelitic rocks seem to be more abundant in the north- ern limb and north-eastern extremity of the synform, while the ultramafic rocks and gabbros were found only in its south-western part. 2. The middle part is a sequence about 100 m thick of layered biotite gneiss, i.e. quartzo-feldspathic gneiss with no discriminant minerals and a compositional layering at a scale of c. 10 cm. The layered biotite gneiss is com- monly interstratified with layers and lenses of amphi- bolite 10–100 cm thick. The contact with the lower amphibolite is gradational. As will be discussed below, the layered biotite gneiss likely represents meta-rhyo- lite. The middle unit of layered biotite gneiss probably does not have a constant thickness; furthermore, in poor, inland outcrops, it is readily confused with basement orthogneisses. A detailed log of the lower and middle parts of the sequence as described in the foregoing was made in the overturned, north-eastern part of the syn- form, displaying its complex and composite nature (Fig. 3A, locs 81–82). 3. A horizon 50–100 m thick of fine grained, dark, lay- ered amphibolite forms the highest observed level. The Southern limb (locs 86–92) Northern limb (locs 81–83) Layered amphibolite Intrusive pink granite Possible peripheric intrusion? Tectonic contact ? Possible leucocratic intrusion? Layered biotite gneiss (metarhyolite ?) Occasional amphibolite layers Layered amphiboliteLayered amphibolite (garnet-bearing in places)(garnet-bearing in places) Layered amphibolite (garnet-bearing in places) Orthogneiss basement Garnet-sillimanite metapelite interlayered with amphibolite. Augen texture locally Unconformity or tectonic contact? Unconformity or tectonic contact? Unconformity or tectonic contact? 10 cm lenses of diopside- bearing gabbro Ultramafic layer Amphibolite and layered grey gneisses interstratified B. Generalised stratigraphic columns in the Kangaatsiaq syncline A. Coastal section, locs 81–82 SW NE loc. 82 loc. 81 538 537 536 535 533 532 530 100 m Fig. 4. Stratigraphic succession of the Kangaatsiaq synform. A: Detailed section of the overturned northern limb of the synform in its eastern extremity (locs 81–82), with sample numbers (all with prefix ‘485’). B: Inferred generalised logs in the north-eastern and south-western parts of the synform. Legend: see Fig. 2. 38 A B C Photo JFM-2002-4-14 Photo JFM-2002-4-13 Photo JFM-2002-4-12 Photos JFM-2002-4-06–08 Photos JFM-2002-4-06–08 Photos JFM-2002-4-06–08 Kangaatsiaq granite Amphibolite Layered biotite gneiss of supracrustal origin Basement orthogneiss Fig. 5. Photomosaic (A) and structural interpretation (B) of the cliff face at loc. 80 (photo facing east). Stippled: pegmatites; grey: high-strain zones. Evidence for pre- to syn-granite, apparently extensional deformation is preserved in the amphibolite bodies intruded by the granite. Details of the cliff face display the apparently extensive deformation in the amphibolite. Cross-cutting pegmatites (see photo 4-12) are occasion- ally affected by this deformation, suggesting that it is synchronous or nearly synchronous with granite emplacement. C: Schematic relationships between the granite, the early extensional deformation, and the supracrustal pile, inspired from loc. 80. 39 A1 A2 B1 B2 C1 C3 C2 C4 C5 Fig. 6. Field and thin sections photographs of lithologies of the supracrustal series (XPL: crossed polarised light; PPL: plane polarised light). Microphotographs are c. 5 mm across. A1: Outcrop of sillimanite- bearing metapelite, loc. 64 (sample 485525). Hammer is 80 cm long. A2: Thin section (XPL) of the same. B1: Outcrop of layered biotite gneiss interstratified with amphibolite at loc. 81 (sample 485537). Pen is 15 cm long. B2: Thin section (XPL) of same. C1: Outcrop of the top amphibo- lite at loc. 58 (sample 485523). Pocket knife is 10 cm long. C2: Thin section (PPL) of same. C3: Outcrop of gabbroic inclusions in the basal amphibolite layer at loc. 92 (sample 485541). Compass 5 cm wide. C4: Thin section (XPL) of clinopy- roxene cluster in amphibolite. C5: Thin section (XPL) of sample 485540 (ultrama- fic layer, same locality). 40 upper boundary of this unit is not observed, since it is everywhere intruded by the granite. This ‘top amphi- bolite’ is continuous and can be traced all around the exposed granite contact; it is also rather homogeneous, much more so than any of the other components of the supracrustal sequence. In loc. 80 (Fig. 5), it appears to be in tectonic contact with the underlying layered biotite gneiss. Field description and petrology As mentioned above, three main components are observed in the supracrustal succession: aluminous metapelite, lay- ered biotite gneiss and amphibolite. Field aspects togeth- er with photographs of thin sections are presented in Fig. 6. The aluminous metapelites occur as slaty, fine-grained (0.5–1 mm), grey to yellowish paragneisses (Fig. 6A1). Garnet or sillimanite is commonly seen in outcrop. In thin section, they display biotite, plagioclase and quartz with either sillimanite or poikiloblastic garnet (Fig. 6A2) cutting across an earlier weak foliation marked by pre- ferred orientation of biotite flakes and elongation of pla- gioclase crystals. The layered biotite gneisses appear as grey, relatively massive, fine grained (0.5–1 mm), finely layered rocks. They are interstratified at all scales with amphibolite (Figs 3C, 6B1) and generally form discontinuous bodies on a 100 m scale. They consist of quartz, plagioclase, K-feldspar and biotite; the foliation is defined by the preferred orien- tation of biotite and elongation of quartz grains (Fig. 6B2). The amphibolites are dark, massive rocks that also show a strong compositional banding (Fig. 6C1–C2). Regard- less of their mode of outcrop either as a thick continuous layer, as in the ‘top amphibolite’, or as discontinuous lay- ers interstratified with other lithologies, they are very sim- ilar in visual aspect and mineralogy. They mostly consist of a fine-grained (0.5–1 mm) hornblende-plagioclase as- semblage, with preferred orientation of minerals defining the foliation. Commonly, small clusters of clinopyroxene surrounded by felsic (mostly plagioclase) rims are observed (Fig. 6C4). At one locality, gabbroic lenses on a scale of 5–10 cm have been observed within the amphibolite (loc. 92, Fig. 6C3). They are medium grained (2–5 mm) and greenish in aspect, and composed of a clinopyroxene-plagioclase association with diffuse contacts with the neighbouring amphibolite (Fig. 6C4). At the same locality, an ultrama- SiOSiO2 2 A lk al in e Subalkaline/Tholeiitic Subalkaline/Tholeiitic Subalkaline/TholeiiticSubalkaline/TholeiiticSubalkaline/Tholeiitic 40 50 60 70 80 0 5 10 15 ● ● Paragneisses Paragneisses Paragneiss Na2O + K2O Basement Supracrustal sequence Kangaatsiaq granite Layered biotite gneiss Layered amphibolite Ultramafic rocks ●‘normal’ orthogneiss High-K orthogneiss Amphibolite enclaves Basaltic andesite Dacite Rhyolite Basalt Basalt Basalt Andesite SiOSiO2 2 SiO2 Fig. 7. Total alkali vs. silica (TAS) diagram (Le Maître et al. 1989) for the magmatic components of the supracrustal rocks and the surrounding orthogneisses. 41 fic layer c. 0.5 m thick has been observed. It is slightly coarser grained (2–5 mm) than the amphibolite, and solely consists of amphibole grains (Fig. 6C5), which are opti- cally similar to the hornblende in the surrounding am- phibolite. Geochemistry and origin Figures 7–8 and Table 1 summarise the major and trace element (especially REE) characteristics and relationships of the three main supracrustal components: amphibolites, metapelites and layered biotite gneisses. There is little, if any doubt of the fact that the amphibolites correspond to metamorphosed and deformed mafic igneous rocks. Else- where, similar field characteristics in amphibolites as those observed here have been interpreted as corresponding to transposition of former pillow lavas in high strain domains (e.g. Myers 2001). The metapelites obviously have a sedi- mentary origin and probably represent terrigeneous sedi- ments. The origin of the layered biotite gneisses, however, is less obvious. They could represent either sedimentary or felsic volcanic rocks. Therefore, they are plotted on ge- ochemical diagrams for both magmatic and sedimentary rocks (see below), allowing comparisons. Origin of the amphibolites The supracrustal amphibolites and their counterparts, enclaves in the basement orthogneisses, appear to be very similar in composition. They plot mostly as basalts in a TAS diagram (Fig. 7; Le Maître et al. 1989), and an AFM diagram (Fig. 9; Irvine & Baragar 1971) reveals that they belong to a tholeiitic series. This, together with their spectacularly flat REE pattern at about 10 times chon- dritic values (Fig. 8), is consistent with the amphibolites corresponding to former MORB basalts, possibly formed as part of an oceanic crust. Many discriminant diagrams for basaltic rocks have been proposed on geochemical grounds (e.g. Pearce 1982; Shervais 1982; Mullen 1983). However, some caution should be exercised when using such diagrams for the Archaean, since the existence of modern-style tectonic settings in the Archaean is not cer- tain, and the palaeogeodynamical contexts might not be similar to those of modern settings (Hamilton 1998; McCall 2003; van Kranendonk 2003). Nevertheless, in 100 10 1 0.1 Basement orthogneiss La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Supracrustal amphibolite Amphibolite as enclaves in orthogneisses Amphibolite100 10 1 0.1 La Ce Pr Sa m pl e / R EE c ho nd ri te Layered biotite gneiss100 10 1 0.1 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sa m pl e / R EE c ho nd ri te Kangaatsiaq granite La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 1000 100 10 1 0.1 Aluminous metapelites100 10 1 0.1 Sa m pl e / R EE c ho nd ri te Sa m pl e / R EE c ho nd ri te Sa m pl e / R EE c ho nd ri te La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Fig. 8. REE patterns (chondrite normalised, Boynton 1984) for the lithologies in and around the Kangaatsiaq synform. 42 such diagrams, the amphibolites plot either as MORB or as rocks originated in oceanic arcs (arc tholeiites), leaving some ambiguity about their original setting. Origin of the aluminous metapelites The geochemistry of metasedimentary rocks is common- ly used to discuss their source, in terms of (1) the nature of the original sediments, (2) the nature of the weathered/ eroded source material, and (3) the degree of weathering of the source (see e.g. Taylor & McLennan 1985; Herron 1988; Roser & Korsch 1988; Nesbitt & Young 1989; Bohlar et al. 2005). While several authors also use the geochemistry of sediments to discuss their geodynamical setting (Bhatia 1983; Bhatia & Crook 1986; Roser & Korsch 1988), some caution should be exercised when dealing with Archaean environments, as mentioned above. In terms of classification, the metasedimentary rocks from the Kangaatsiaq area plot mostly as shales or greywackes, using either of the two schemes proposed by Herron (1988). One of these is shown on Fig. 10A; the ambiguity and possible (chemical) confusion between the two groups, shales and greywackes, which are poorly separated by this diagram, has been outlined by these authors. Neverthe- less, the conclusion points to relatively immature sediments which have undergone limited transport from their source. The nature of the source itself can be discussed using major or trace elements. Roser & Korsch (1988) proposed a scheme for source determination of clastic sediments on the basis of major elements. In this instance, the studied samples straddle the P2–P3 boundary (Fig. 10B), suggest- ing a felsic to intermediate source. Also trace elements can be used to refine this conclusion. As pointed out by Taylor & McLennan (1985), some elements (high field strength elements, rare earth elements (REE), Y, Sc, Th) only undergo limited fractionation during sedimentary processes; thus, their ratios reflect the signature of their source. Plotting these elements against each other shows that the Kangaatsiaq metasedimentary rocks (Fig. 10E– H) have element ratios that are generally consistent with derivation from an orthogneissic source (amphibolites generally have too low trace element contents and incor- rect ratios to be a possible source). The only exception is for heavy REE (Figs 8, 10G). Indeed, the relatively high Yb contents of the metasedimentary rocks precludes their derivation solely from a low-Yb gneissic basement, and implies that they must, at least in part, have been derived from higher-Yb rocks such as the amphibolites; this is hardly a surprise, since amphibolite occurs as enclaves in- tercalated within the orthogneisses. Modelling the REE contents of such a mixture shows that mixing of ortho- A M F ● ● Granite Orthogneiss Tholeiite Series Calc-alkaline Series Supracrustal amphibolite Amphibolite enclave Ultramafic rocks Fig. 9. AFM diagram (Irvine & Baragar 1971) showing the tholeiitic affinity of both the supracrustal amphibolites and the enclaves in the gneisses. A, Na 2 O + K 2 O; F, FeO total ; M, MgO. The fields of the base- ment orthogneisses and the Kangaatsiaq granite are also shown for comparison. Facing page: Fig. 10. Major and trace element geochemistry (A–D and E–H) of the metasedimentary rocks (paragneisses, and layered biotite gneiss- es). Dotted fields show the compositions of the major regional lithol- ogies (orthogneiss and high-K orthogneiss, Moyen et al. 2003a; Steen- felt et al. 2005; amphibolite; Kangaatsiaq granite). A: Log(SiO 2 /Al 2 O 3 ) vs. log(Fe 2 O 3 /K 2 O), from Herron (1988). B: Discriminant diagrams for the metapelites, from Roser and Korsch (1988). The sources for each group are P1, mafic to intermediate volcanic rocks; P2, interme- diate (andesitic, dacitic, occasionally rhyolitic) volcanic rocks; P3, fel- sic volcanic rocks; P4, evolved sediments, sandstones, etc. The discri- minant functions are: F1 = –1.773 TiO 2 + 0.607 Al 2 O 3 + 0.760 Fe 2 O 3 – 1.500 MgO + 0.616 CaO + 0.509 Na 2 O – 1.224 K 2 O – 9.090; F2 = 0.445 TiO 2 + 0.070 Al 2 O 3 – 0.250 Fe 2 O 3 –1.142 MgO + 0.438 CaO + 1.475 Na 2 O + 1.426 K 2 O – 6.861. C, D: Triangular diagrams (from Nesbitt & Young 1989). Stars: theoretical mineral composi- tions; il, illite; ms, muscovite; pg, plagioclase; ksp, K-feldspar; cpx, clinopyroxene; hbl, hornblende; chl, chlorite; bt, biotite; sm, smec- tite. Dashed arrows: trends for (1) weathering and (2) K-metasoma- tism, after Nesbitt & Young (1989) and Bohlar et al. (2005). E, F: U vs. Th and Ti vs. Zr (log scale) diagrams, showing that the metasedi- mentary rocks have trace elements ratios comparable to the gneisses, but mostly different from the amphibolites. G, H: La/Yb vs. Yb and Ti/Zr vs. Ni (log scale) diagrams displaying the same relationships as E–F, also showing the mixing between an amphibolite-like and an orthogneiss-like source (ticks at 10% increments). 43 Amphibolite Yb La/Yb Ti/Zr U Ti Ni ZrTh 0 5 10 0.0 0.5 1.0 1.5 Th/U = 1 Th/U = 5 Th/U = 10 Orthogneiss Amphibolite Amphibolite AmphiboliteAmphiboliteAmphibolite Granite 10 100 200 100 200 500 10 20 50 1000 2000 5000 10000 0 20 40 60 80 100 120 140 20 50 100 200 Ti/Z r = 10 Ti/Z r = 50 Ti/Z r = 10 0 Ti/Z r = 20 0 Ti/Z r = 20 OrthogneissOrthogneissOrthogneissOrthogneissOrthogneiss Amphibolite Granite 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Orthogneisses Amphibolite Granite 0.1 0.5 1 5 10 50 100 500 Orthogneiss Amphibolite Granite E F F F G H Amphibolite Aluminous paragneiss Layered biotite gneiss Standard mineral compo- sitions for reference Compositional fields of regional lithologies (see figure text) Granite chl ksp, pg sm il bt cpx hbl ms pg ksp il, ms hbl cpx Al2O3 K2O Al2O3 CaO + Na2O + K2OCaO + Na2O FeOt + MgO 1 2 1 2 1 OrthogneissOrthogneiss C D GraniteGraniteGranite 0.0 0.5 1.0 1.5 2.0 -0.5 0.0 0.6 1.0 1.5 Fe -sh ale Sh ale Fe -sa nd Su bli th ar en ite Qu ar tz - ar en ite Su ba rk os e A rk os e W ac ke Lit ha re nit e lo g (F e 2 O 3/ K 2O ) log(SiO2/Al2O3)log(SiOlog(SiO2/Al/Al2O3)log(SiO2/Al2O3) AmphiboliteAmphiboliteAmphibolite Orthogneiss Granite A P4 P3 P2 P1 8 4 0 -4 -8 8 4 0 -4 -8 F1 F2 AmphiboliteAmphibolite OrthogneissOrthogneiss Amphibolite Orthogneiss Granite B 44 Si O 2 T iO 2 A l 2 O 3 Fe 2O 3* M nO M gO C aO N a 2 O K 2O P 2 O 5 H 2O K /N a M g# A /C N K C .I. A . V C r N i C o C u Z n R b Sr B a Y Z r N b Ta H f Sc G a C s Pb T h U T h/ U T i/Z r La C e Pr N d Sm Eu G d T b D y H o Er T m Y b Lu La /Y b Eu /E u* 69 .0 0 74 .0 8 72 .9 2 72 .8 8 70 .2 1 66 .0 4 0. 37 0. 18 0. 27 0. 21 0. 43 0. 62 15 .1 6 13 .2 2 13 .4 8 13 .8 7 15 .1 4 16 .0 1 2. 46 1. 33 1. 48 1. 61 2. 50 3. 94 0. 02 0. 01 0. 01 0. 02 0. 04 0. 01 0. 79 0. 31 0. 44 0. 46 0. 89 1. 27 2. 07 1. 01 1. 17 1. 29 2. 17 5. 07 4. 18 3. 63 3. 64 4. 06 3. 93 3. 31 4. 23 4. 56 4. 74 4. 28 4. 29 1. 07 0. 16 0. 05 0. 07 0. 08 0. 17 0. 39 0. 22 0. 16 0. 17 0. 26 0. 80 0. 67 0. 83 0. 86 0. 69 0. 72 0. 21 39 32 37 36 41 39 1. 00 1. 04 1. 02 1. 02 1. 01 1. 01 28 11 10 11 33 36 7 3 3 4 14 11 8. 1 3. 5 4. 4 5. 5 bd l 22 .7 37 .0 44 .6 79 .4 68 .8 50 .7 3. 3 2. 6 2. 6 1. 7 13 .0 32 9. 1 36 .2 21 .5 20 .6 27 .9 25 .0 31 .9 11 4. 7 11 5. 5 11 8. 7 12 5. 8 56 .0 96 .0 73 5 25 2 34 2 34 0 71 2 19 13 14 76 46 3 67 2 59 3 13 84 20 95 8. 5 2. 7 5. 7 5. 5 bd l 7. 2 17 8 10 4 13 2 14 0 21 9 20 3 8. 4 3. 8 6. 0 4. 8 bd l 7. 0 4. 2 3. 1 3. 5 3. 7 4. 6 5. 4 2. 2 10 .5 2. 6 4. 7 2. 4 0. 9 1. 5 1. 5 3. 9 19 .1 17 .6 17 .8 18 .2 21 .3 1. 0 0. 4 0. 6 1. 9 6. 2 15 .6 16 .5 12 .8 17 .3 6. 9 10 .8 6. 8 9. 8 9. 2 14 .0 0. 9 1. 4 1. 0 0. 9 2. 6 12 .1 5. 0 10 .0 9. 8 5. 4 12 .6 10 .4 12 .3 9. 0 11 .8 18 .2 35 .9 19 .8 35 .2 39 .1 29 .0 15 1. 7 81 .2 39 .7 68 .9 75 .1 10 5. 0 31 3. 7 9. 7 4. 2 7. 5 8. 2 37 .7 33 .0 12 .8 23 .3 25 .4 12 6. 4 5. 3 1. 6 3. 2 3. 1 13 .4 1. 5 0. 6 0. 7 0. 7 2. 4 5. 1 1. 9 3. 0 3. 2 12 .5 0. 44 0. 15 0. 27 0. 27 0. 83 1. 96 0. 62 1. 14 1. 10 2. 81 0. 28 0. 09 0. 17 0. 17 0. 25 0. 76 0. 27 0. 50 0. 49 0. 98 0. 10 0. 03 0. 07 0. 06 0. 07 0. 57 0. 24 0. 45 0. 43 0. 47 0. 09 0. 04 0. 06 0. 06 0. 06 63 .0 83 .3 79 .0 89 .8 32 0 0. 87 0. 96 0. 70 0. 71 0. 56 62 .2 9 70 .9 7 63 .1 0 66 .0 3 0. 68 0. 26 0. 61 0. 62 15 .6 9 14 .8 8 16 .9 7 18 .9 8 5. 68 1. 85 5. 92 4. 27 0. 09 0. 02 0. 10 0. 03 3. 13 0. 48 2. 17 1. 45 4. 71 2. 13 4. 60 2. 19 3. 31 4. 23 2. 94 3. 20 2. 03 3. 38 1. 41 1. 70 0. 22 0. 08 0. 07 0. 10 0. 98 0. 18 0. 69 0. 59 0. 40 0. 53 0. 32 0. 35 52 34 42 40 0. 97 1. 03 1. 15 1. 71 60 .9 5 60 .4 4 65 .4 7 72 .8 0 1 00 12 13 3 88 68 2 13 6 61 50 .1 5. 0 79 .1 25 .1 47 .8 86 .3 10 0. 8 10 3. 0 14 .2 3. 7 10 1. 4 14 .9 67 .0 34 .7 78 .3 57 .3 46 .9 83 .2 44 .7 79 .8 8 03 82 2 23 1 21 3 8 19 14 30 31 4 26 2 16 .2 3. 8 12 .5 9. 2 1 65 14 7 98 93 8. 3 3. 5 4. 5 5. 1 3. 9 3. 8 2. 7 2. 4 3. 6 3. 5 3. 3 2. 3 14 .2 2. 3 23 .4 13 .0 18 .6 19 .6 19 .7 24 .1 4. 7 1. 2 2. 2 1. 5 8. 6 15 .4 7. 1 7. 8 4. 7 4. 8 2. 7 3. 2 0. 8 0. 6 0. 8 0. 7 5. 6 8. 2 3. 3 4. 7 24 .8 10 .5 37 .0 39 .7 35 .3 27 .3 12 .9 21 .3 74 .4 61 .0 27 .2 46 .0 9. 3 6. 9 3. 4 5. 8 33 .4 22 .2 13 .0 21 .5 4. 9 2. 8 2. 5 3. 4 1. 3 0. 9 0. 7 0. 9 5. 0 2. 8 2. 6 3. 1 0. 57 0. 22 0. 37 0. 36 2. 99 0. 86 2. 13 1. 86 0. 53 0. 13 0. 43 0. 33 1. 51 0. 35 1. 20 0. 93 0. 22 0. 05 0. 18 0. 14 1. 41 0. 30 1. 23 0. 95 0. 22 0. 04 0. 18 0. 14 25 .0 91 .8 10 .5 22 .3 0. 77 0. 93 0. 87 0. 82 70 .8 1 70 .0 3 47 .5 2 0. 29 0. 31 0. 74 15 .0 8 15 .1 5 14 .9 4 2. 47 2. 06 12 .4 6 0. 02 0. 02 0. 19 0. 90 0. 64 8. 32 2. 88 2. 39 11 .2 2 4. 42 4. 61 2. 48 2. 02 3. 13 0. 83 0. 06 0. 15 0. 04 0. 18 0. 26 0. 54 0. 30 0. 45 0. 22 42 38 57 1. 03 0. 99 0. 59 25 20 25 4 22 3 33 2 12 .9 7. 6 14 7. 8 71 .4 46 .4 72 .3 5. 4 3. 2 40 .3 43 .6 42 .7 86 .2 74 .4 92 .0 7. 7 50 4 83 9 93 55 8 10 49 39 3. 0 4. 5 17 .6 86 2 11 7 33 4 5. 4 5. 6 2. 1 2. 5 3. 2 1. 0 11 .4 7. 9 3. 1 5. 0 3. 7 43 .8 20 .8 22 .4 15 .1 1. 0 1. 3 0. 0 9. 6 15 .6 2. 6 1. 9 5. 1 0. 2 0. 5 1. 7 0. 3 3. 8 3. 0 0. 7 20 .4 15 .9 13 4 10 .2 38 .4 2. 6 21 .4 82 .7 6. 3 2. 3 8. 9 1. 0 7. 8 30 .1 4. 9 1. 3 3. 8 1. 6 0. 6 1. 0 0. 6 1. 4 3. 9 2. 3 0. 13 0. 30 0. 43 0. 69 1. 35 2. 74 0. 11 0. 16 0. 63 0. 32 0. 52 1. 68 0. 04 0. 06 0. 28 0. 27 0. 38 1. 82 0. 03 0. 05 0. 26 37 .8 10 2 1. 44 1. 34 0. 78 0. 94 47 .0 5 47 .1 4 48 .7 7 48 .7 4 48 .0 8 45 .8 7 0. 95 0. 81 0. 87 0. 77 0. 72 0. 51 14 .6 8 14 .7 2 14 .6 7 15 .8 2 14 .6 9 11 .5 4 12 .9 9 10 .9 0 12 .6 0 12 .1 4 10 .7 7 11 .7 4 0. 22 0. 26 0. 24 0. 21 0. 20 0. 21 6. 80 4. 74 6. 70 7. 18 7. 90 14 .1 9 13 .3 0 17 .8 9 11 .5 1 12 .1 8 13 .1 4 11 .7 8 2. 02 1. 40 2. 67 1. 64 2. 55 1. 39 0. 53 0. 08 0. 76 0. 31 0. 52 0. 58 0. 06 0. 06 0. 06 0. 05 0. 03 0. 04 0. 36 1. 19 0. 51 0. 45 0. 30 0. 79 0. 17 0. 04 0. 19 0. 12 0. 13 0. 27 51 46 51 54 59 71 0. 52 0. 42 0. 56 0. 63 0. 51 0. 47 27 1 23 7 27 1 24 4 22 5 17 7 20 7 30 8 28 4 24 7 42 5 17 34 15 3. 1 16 7. 6 12 1. 7 17 5. 7 19 7. 5 72 8. 1 72 .1 59 .9 57 .1 57 .5 61 .0 81 .6 78 .7 73 .7 30 .3 12 .6 24 .1 2. 2 90 .1 76 .7 10 8. 2 77 .4 74 .8 10 3. 3 9. 1 1. 7 9. 4 32 .4 6. 5 14 .8 93 11 7 11 7 92 94 78 64 22 11 8 46 37 82 21 .1 18 .8 20 .6 18 .5 17 .1 11 .6 19 16 20 32 16 24 2. 8 2. 2 2. 4 4. 1 2. 7 1. 8 0. 9 0. 6 0. 9 1. 1 0. 7 0. 8 1. 1 0. 8 0. 8 0. 7 1. 3 0. 6 41 .6 38 .2 40 .9 41 .2 38 .3 31 .3 16 .4 14 .5 16 .1 16 .5 14 .8 11 .9 0. 1 0. 1 0. 0 0. 3 0. 0 0. 3 2. 9 2. 8 3. 5 1. 7 3. 2 1. 3 0. 4 0. 2 0. 3 0. 2 0. 2 0. 2 0. 2 0. 1 0. 1 0. 3 0. 1 0. 3 1. 8 3. 5 3. 2 0. 9 1. 4 0. 5 29 6 30 3 26 6 14 2 27 0 12 5 3. 0 2. 2 3. 0 2. 2 2. 9 2. 6 7. 7 6. 2 7. 2 6. 1 7. 9 5. 2 1. 2 1. 0 1. 2 1. 0 1. 3 0. 8 6. 3 5. 4 6. 1 4. 9 6. 1 3. 7 2. 1 1. 8 2. 0 1. 8 1. 8 1. 2 0. 7 0. 6 0. 7 0. 7 0. 6 0. 4 2. 5 2. 1 2. 5 2. 3 2. 1 1. 4 0. 49 0. 43 0. 48 0. 45 0. 41 0. 28 3. 12 2. 85 3. 10 2. 89 2. 60 1. 72 0. 72 0. 64 0. 73 0. 64 0. 60 0. 40 1. 96 1. 77 1. 95 1. 72 1. 62 1. 13 0. 31 0. 28 0. 33 0. 27 0. 26 0. 18 2. 12 1. 85 2. 08 1. 82 1. 70 1. 20 0. 31 0. 29 0. 31 0. 26 0. 26 0. 17 1. 42 1. 22 1. 43 1. 20 1. 68 2. 19 0. 96 1. 01 0. 90 1. 00 0. 87 0. 99 La ye re d bi o ti te g ne is s Pa le le uc o - cr at ic g ne is s B io ti te -b ea ri ng g ne is s A m ph ib o lit e K an ga at si aq g ra ni te A lu m in o us m et as ed . 48 55 31 48 55 35 48 55 25 Q z- B t- Si ll- G t sc hi st Q z- B t- G t gn ei ss 81 81 81 81 52 64 48 55 37 48 55 38 48 55 23 48 55 33 48 55 36 48 55 28 48 55 40 81 58 73 92 61 73 75 85 gi a 20 01 -2 17 81 A m ph ib o lit e in lo w er la ye r as so ci at ed w it h fe ls ic r o ck s Pe ri ph er ic gr an it ic d yk e M as si ve a m ph i- bo lit e, t o p U lt ra m af ic la ye r in a m ph ib o lit e 48 55 29 48 55 39 48 55 24 48 55 27 47 05 29 48 55 34 81 81 57 48 55 22 48 55 30 48 55 32 B as em en t M ai n gr an it ic m as s O rt ho gn ei ss A m ph ib o lit e en cl av e Sa m pl e N o . Lo ca lit y Ta bl e 1. C he m ic al a na ly se s o f ro ck s in a nd a ro un d th e K an ga at si aq s yn cl in e Sa m pl e nu m be rs r ef er t o G EU S da ta ba se s; lo ca lit ie s ar e sh o w n o n Fi gs 1 a nd 4 , e xc ep t gi a 20 01 -2 17 o ut si de t he m ap a re a (U T M 3 96 54 0, 7 57 59 84 ). M aj o r el em en ts in w t% ; t ra ce e le m en ts in p pm . A na ly ti ca l d at a o bt ai ne d at G EU S by X R F (m aj o r el em en ts a nd a pp ro xi m at e tr ac e el em en ts in 4 70 52 9) a nd IC P- M S (a ll o th er t ra ce e le m en ts ). K /N a: M o le cu la r K /N a ra ti o. M g# : M o le cu la r 10 0M g/ (M g + F e) . A /C N K : M o le cu la r A l/( C a + N a + K ). C .I. A .: C o nt in en ta l I nd ex o f A lt er at io n (N es bi tt & Y o un g 19 89 ); m et as ed .: m et as ed im en ta ry r o ck ; b dl : b el o w d et ec ti o n lim it . 45 gneisses with amphibolite (Fig. 10G, H) can explain the Yb contents of the sediments; since the amphibolites are, collectively, less enriched in trace elements than the gneiss- es, their involvement would only have little effect on the other incompatible elements in the metasedimentary rocks. In contrast, the latter display higher Ni and Cr contents than the orthogneisses, also consistent with a contribu- tion from amphibolite or its precursor rocks in their for- mation (Fig. 10H). Finally, the degree of alteration of the source can be discussed. The metasedimentary rocks dis- play C.I.A. values (Chemical Index of Alteration, Nesbitt & Young 1989) of c. 60–70 (Table 1), slightly lower than for shales or similar rocks (70–75, Taylor & McLennan 1985). In the triangular diagrams proposed by Nesbitt & Young (1989; Fig. 10C, D), they also depart only moder- ately from their protoliths, suggesting a relatively unweath- ered source. Very little or no evidence for secondary K- enrichment is observed. Taking the above-mentioned limitations into account, the geoynamic setting inferred from the geochemistry gives consistent results regardless of the classification scheme used. Both the major elements classifications of Bhatia (1983) and Roser & Korsch (1988) and the trace element systems of Bhatia & Crook (1986) suggest an oceanic or continental island-arc setting. However, this only reflects the characteristics outlined above: relatively immature sed- iment derived from poorly weathered felsic to intermedi- ate magmatic rocks, with a possible mafic component. Origin of the layered biotite gneiss (felsic volcanic rocks?) The two samples analysed of the layered biotite gneisses give ambiguous geochemical signatures and can be inter- preted either as sedimentary or igneous (Figs 7–10). In general, they seem to share more similarities with the gran- ite or the orthogneisses than with any other member of the supracrustal group. In particular, Fig. 10 (C, D) shows that if these rocks are of sedimentary origin, they are in- deed very similar to their source and were derived from a largely unweathered protolith. This implies that the lay- ered biotite gneiss can be interpreted in two ways. It may represent very immature sediment derived from a mostly unweathered protolith with a very similar bulk composi- tion, such as a conglomerate made of pebbles of unweath- ered orthogneiss, in which case the banding could be a trace of the transposed pebbles. Alternatively the layered biotite gneiss represents calc-alkali or TTG-type felsic la- vas, whose composition would of course be very similar to that of their plutonic counterparts. Origin of the supracrustal sequence as a whole Based on the foregoing discussion two interpretations can be proposed for the supracrustal sequence. 1. The succession could represent a dismembered ophio- lite sequence intermingled with clastic sediments erod- ed from a nearby continent. The combined sequence could then be interpreted as an accretionary prism. The likely tectonic nature of the contact between members of the sequence (see above and Figs 4, 5) supports this hypothesis. 2. The whole supracrustal pile consists of a bimodal, calc- alkaline, probably subduction-related volcanic suite associated with immature terrigeneous sediments di- rectly derived from their weathering. This is consistent with an arc situation, in which a back- or fore-arc ba- sin is being filled with both volcanic products and de- trital sediments largely derived from the weathering of these lavas. At the same time, tonalitic plutons are emplaced at depth from the same magmas. The plu- tonic rocks are quickly uplifted and eroded, and, be- sides intruding into the supracrustal pile, may in some cases also represent the basement for subsequent volca- no-detritic basin fill. In both cases, the rocks were formed in a convergent set- ting, probably above or close to an active subductions margin. In general, arc- or subduction-related origins for Archaean volcanic suites are preferred by most workers (e.g. Card 1990; Lowe 1994; Windley 1995; Chadwick et al. 1996), although the issue remains controversial (Ham- ilton 1998; McCall 2003; van Kranendonk 2003). Nature and origin of the Kangaatsiaq granite The Kangaatsiaq granite is pink, porphyritic, and displays a distinct rodding (Fig. 11A) showing that it has been intensely deformed (see below). While YZ sections (per- pendicular to the main stretching direction) display a pre- served magmatic texture, sections parallel to X clearly show the gneissic texture of the rock. The mineralogical para- genesis is K-feldspar + quartz + sodic plagioclase + biotite, with accessory zircon, titanite, apatite and oxides. The granite has moderate K/Na ratios (0.67–0.86), is slightly metaluminous with A/CNK ratios of 1.00–1.04, and has low Mg# of 32– 41 (Table 1). Ni and Cr contents are also low, while Rb, Sr and Ba contents are moderate; 46 this composition corresponds to the biotite-bearing gran- ites of Moyen et al. (2003b), which are interpreted to have been derived from partial melting of TTG gneisses. This conclusion is consistent with the highly migmatitic na- ture of the surrounding gneissic basement (van Gool et al. 2002a). Structure and deformation history As mentioned above, the granite displays a strong rod- ding and L > S fabrics (Fig. 11A). The strain pattern in the granite (Fig. 11B) is consistent with highly constric- tional deformation, with foliation poles plotting on a great circle, and lineations clustered near the pole of this great circle. This corresponds to subhorizontal, ENE–WSW stretching, consistent with the general orientation of the structures in Kangaatsiaq area (Fig. 1), and more general- ly with the structural grain of the region (van Gool et al. 2002a; Piazolo et al. 2004; Mazur et al. 2006, this vol- ume). The surrounding gneissic basement and supracrus- tal rocks show the same strain pattern when plotted (Fig. 11C), although in the field, the rocks commonly have a LS or S > L fabric. This suggests that pre-existing folia- tions have been reoriented during the latest constrictional deformation event, leading to their present distribution. The fact that intense constriction (rather than shorten- ing) can produce folded structures has previously been demonstrated by e.g. Leloup et al. (1995) in the Red Riv- er shear zone in Yunnan, China, where the ductile defor- mation in gneisses resulted in the development of elongate synclines and anticlines with axes parallel to the shear zone and the X-axis of deformation. The study of deformation-related textures allows the conditions of deformation to be roughly constrained. In the granite and felsic components of the supracrustal se- A Total data: 70 Equal area, lower hemisphere B. Within the granite Poles to foliation Lineation Poles to axial planes Fold axis C. Outside the granite Total data: 42 Fig. 11. A: Macroscopic view of the Kangaatsiaq granite at loc. 75 (correspond- ing to sample 485529), showing strong rodding. Hammer shaft about 4 cm wide. B, C: Stereograms of poles to foliation (circles) and lineations (squares) within and outside the granitic intrusion. The strain patterns are similar in both units and define a highly constrictional, NE–SW-trending and subhorizontal deformation. 47 A B C D E Fig. 12. Deformation textures either related to the latest, constrictional deformation (A–C) or not compatible with low-T deformation (D, E). See comments in the main text. A: Quartz ribbons in the Kangaatsiaq granite (sample 485527). B: Quartz subgrains in felsic supracrustal gneiss (sample 485531). C: Poikiloblastic garnet in metapelite cutting across an earlier foliation (sample 485535). D, E: High-temperature recrystalli- sation with 120° triple junctions in amphibolite (sample 485540) and felsic rocks (sample 485530). In E, the quartz also shows low-temperature deformational features such as undulating extinction and quartz subgrains, indicating that this rock witnessed two successive deformation events. 48 ries (Fig. 12A, B), the deformation led to the develop- ment of quartz subgrains and recrystallised quartz rib- bons. This corresponds to deformation under lower am- phibolite facies conditions (400 ± 50°C; Bouchez & Pécher 1976; Gapais & Barbarin 1986; Gapais 1989; Hirth & Tullis 1992; Vernon 2000). Under these conditions, only the quartz is ductile, such that all deformation is accom- modated by quartz recrystallisation or deformation. In the Al-rich lithologies, deformation-related textures are mostly seen in the formation of poikiloblastic, syn- to post-tec- tonic garnets (Fig. 12C). Piazolo (2002) estimated that the chemistry of garnet in similar pelites nearby is com- patible with a long duration of temperature conditions at around 500°C, which is in broad agreement with the above estimate. Willigers et al. (2002) described the cooling his- tory of the NNO close to our study area from Ar-Ar dat- ing of various minerals, and likewise concluded that the cooling history of the NNO was slow, from 400°C (mus- covite closure) at 1.7 Ga to 200°C (K-feldspar closure) at 1.5 Ga. Therefore, it can be considered that a constric- tional deformation event post-dating the granite emplace- ment occurred during cooling to lower amphibolite facies conditions. Since this event is apparently responsible for the regional-scale structures (Mazur 2002; van Gool et al. 2002b; Piazolo et al. 2004), and is of lower Proterozoic age (Willigers et al. 2002), we propose that it corresponds essentially to the Nagssugtoqidian deformation proper. However, some textures are not compatible with the above conditions. In amphibolites, high-temperature fab- rics with polygonal textures and 120° triple junctions are preserved (Fig. 12D). In some of the felsic supracrustal rocks or basement gneisses (but never in the granite), ev- idence is preserved for a similar high-temperature fabric, overprinted by later quartz recrystallisation (Fig. 12E). According to Kretz (1969), Gower & Simpson (1992), Kretz (1994) and Martelat et al. (1999), such fabrics are Poikiloblastic garnet cutting the D1 fabric Quartz subgrains, etc. Quartz subgrains, etc. High-temperature recrystallisation Country rocks Granite Pr e- N ag ss ug to qi di an D 1/ D 1b (g ra nu lit ic ) N ag ss ug to qi di an D 2 (a m ph ib o lit ic ) (not formed) No quartz - D2 deformation not expressed Quartzo-feldspathic amphibolitic Granite emplacement Fig. 13. Summary of the deformation history of the Kangaatsiaq synform and Kangaatsiaq granite. See comments in the main text. Photos from Fig. 12. 49 likely to develop under granulite facies conditions (600– 800°C). This points to the existence of one or more older (D1?) deformation event(s). Since no evidence for this deformation is found in the granite, we suggest that it was pre-granite, and therefore likely corresponds to late Archaean deformation. P–T estimates for metapelites and metabasites in the Kangaatsiaq area by Piazolo (2002) al- so indicated the existence of an early metamorphic phase with P–T conditions between 650°C, 3–5 kbar and 780°C, P unknown. This estimate is in good agreement with the textural evidence for D1 deformation under lower granu- lite facies conditions. The pre-granite deformation is also evidenced by the early isoclinal folds, the existence of a foliation within the supracrustal rocks that does not exist in the granite, and the fact that the granite apparently cuts earlier structures (Fig. 5). At loc. 80, the granite is clearly observed cutting across the foliation and shear bands in the amphibolite; these shear bands are injected by peg- matites that might also be cut by the granite. This sug- gests that there were actually two pre-granite events, the first of which corresponds to the granulite facies forma- tion of the foliation and isoclinal folds, and the second one to the pegmatite-injected shear bands. However, the floor of the granitic intrusion is also apparently offset by the shear bands (Fig. 5B). Furthermore, the geometry of the shear bands and the foliation suggests extensional defor- mation; since the cliff face studied here almost corresponds to a YZ section relative to the regional constrictional de- formation, this geometry is likely to correspond to the original, preserved pre-constriction geometry. Finally, the fact that the granite both cuts across, and is offset by the shear bands, suggests that the granite emplacement may actually have been syn-extension as sketched in Fig. 5C. Altogether, the simplest possible deformation history (with the smallest number of episodes) can be summa- rised as follows (Fig. 13). 1. A first deformation event (D1) under lower granulite facies conditions (c. 5 kb, 600–800°C), resulted in the development of granulitic (polygonal) textures in all the existing lithologies, the formation of a main folia- tion, and isoclinal folding. It probably corresponds to compression of the original, likely accretionaly wedge or arc sequence. 2. This may have been followed by a second event (D1b) of probably extensional deformation, maybe associat- ed with (or shortly followed by) the emplacement of the granite sheet. This event, only witnessed by the shear zones cutting the D1 foliation, e.g. at locality 80, is poorly recorded and probably just represents the final stage of D1 deformation. Assuming the granite has a late Archaean age, which is very likely in the regional context, this deformation could correspond to the lat- er stages of the evolution of an arc or active continental margin, with strain relaxation and syn-extension gran- ite emplacement. 3. A final event of constrictional deformation under low- er amphibolite conditions (D2). Due to the relatively low-temperature conditions, only the quartz-bearing lithologies were affected. Therefore, the granite shows strong recrystallisation, the felsic supracrustal rocks dis- play overprinting of the D1/D1b fabric by this event, and the quartz-free amphibolites were essentially unaf- fected by this event. The D2 event corresponds to the purely constrictional, regional structures which have been interpreted by Piazolo et al. (2004) and Mazur et al. (2006, this volume) as resulting from the indenta- tion of the NNO by a solid, north-moving block im- mediately north of the Arfersiorfik shear zone (for the latter, see e.g. Sørensen et al. 2006, this volume). This Palaeoproterozoic deformation gave the studied area its present synformal structure. Conclusions While the present-day synclinal structure of the Kangaat- siaq area essentially results from N60 constriction related to the Palaeoproterozoic Nagssuqtoqidian deformation, the lithologies together with early preserved structures give insight into the late Archaean crustal evolution. The base- ment gneisses genetically belong to the TTG suite (Moy- en et al. 2003a; Steenfelt et al. 2005), which is generally interpreted as generated by partial melting of a subduct- ing slab (e.g. Martin 1994). Some components of the base- ment display implications of mantle wedge involvement in their genesis (Steenfelt et al. 2005), which is unusual in the Archaean but nevertheless consistent with an active margin setting. The supracrustal succession is composed of discontinuous layers of mafic MORB-like or arc tho- leiite lavas, and together with immature, terrigeneous shales or greywackes derived from erosion of the basement TTG gneisses or volcanic counterparts to them, with a likely small contribution from tholeiitic lavas. Part of the succession could also have been felsic rocks derived from erosion of the basement TTG gneisses or volcanic coun- terparts to the latter, with a likely small contribution from tholeiitic lavas. The whole series is capped by a layer c. 100 m thick of mafic volcanic rocks likewise of tholeiitic affinity. All these lithological components are in good 50 agreement with either an arc-related setting, with a plu- tonic arc developing simultaneously with the filling of volcano-detritic basins with lavas of similar affinities and immature sediments; or with an accretionary wedge envi- ronment involving ocean floor juxtaposed together with similar sediments. In both cases, they correspond to an active subduction margin. Intense migmatisation of the basement is probably associated with the emplacement of the anatectic, likely synkinematic Kangaatsiaq granite. This was apparently synchronous with an early, lower granu- lite facies (D1/D1b) deformation event that may have ended with strain relaxation and exhumation of the rocks from the active margin at the end of the Archaean cycle. The supracrustal association and the sequence of events in the Kangaatsiaq area are comparable to the evolution of many Archaean greenstone belts (e.g. Card 1990; Chad- wick et al. 1996; Hunter et al. 1998). On the other hand, classical Archaean components such as orthochemical sed- iments and plume-related komatiites (Arndt 1994) or or- thochemical components (Lowe 1994) are completely missing from the Kangaatsiaq area. However, this appar- ently rather uncommon absence is known from other mid- to late Archaean greenstones, also in West Greenland (e.g. Garde 1997). The setting is sometimes interpreted as be- ing arc-related (Card 1990; Lowe 1994). In contrast, wide- spread melting and granite emplacement at the end of the Archaean is a very common situation, which has been described in many studies (among others, e.g. Gorman et al. 1978; Card 1990; Sylvester 1994; Windley 1995; Chadwick et al. 1996; Moyen et al. 2003b). Acknowledgements J.A.M. van Gool, G.I. Alsop, S. Piazolo and S. Mazur visited the area in 2001, and their work was used as a basis for the subsequent mapping. They also provided useful comments on the geology and metamorphic history of the region. A.A. 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