Metamorphic record of accretionary processes during the Neoarchaean: The Nuuk region, southern West Greenland

Metamorphic record of accretionary processes during the Neoarchaean: The Nuuk region, southern West Greenland

Precambrian Research 242 (2014) 22–38 Contents lists available at ScienceDirect Precambrian Research journal homepage: www.elsevier.com/locate/preca...

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Precambrian Research 242 (2014) 22–38

Contents lists available at ScienceDirect

Precambrian Research journal homepage: www.elsevier.com/locate/precamres

Metamorphic record of accretionary processes during the Neoarchaean: The Nuuk region, southern West Greenland A. Dziggel a,∗ , J.F.A. Diener b , J. Kolb c , T.F. Kokfelt c a b c

Institute of Mineralogy and Economic Geology, RWTH Aachen University, Wüllnerstrasse 2, 52062 Aachen, Germany Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa Department of Petrology and Economic Geology, Geological Survey of Denmark and Greenland, DK-1350 Copenhagen K, Denmark

a r t i c l e

i n f o

Article history: Received 23 August 2013 Received in revised form 12 December 2013 Accepted 16 December 2013 Available online 27 December 2013 Keywords: Archaean high-P metamorphism Duality of thermal regimes Plate tectonics Pseudosection modelling

a b s t r a c t The Nuuk region of southern West Greenland consists of several distinct terranes, including, from NW to SE, the Færingehavn, Tre Brødre, and Tasiusarsuaq terranes. Extensive high-pressure metamorphism and a clockwise P–T evolution of the Færingehavn terrane at ca. 2720–2710 Ma has been interpreted to be a result of crustal thickening and thrusting of the Tasiusarsuaq terrane on top of the Tre Brødre and Færingehavn terranes. Pseudosection modelling constrains the P–T path for the Færingehavn terrane to be characterised by initial burial, followed by heating at depth to peak conditions of ∼700 ◦ C and 10 kbar and subsequent isothermal decompression to conditions of 700 ◦ C and 6 kbar. These data are consistent with the results of previous studies, pointing to a relatively cool apparent geothermal gradient of <20 ◦ C/km during prograde metamorphism. The tectonically overlying Tasiusarsuaq and Tre Brødre terranes record a contrasting metamorphic history. Prior to final collision the Tasiusarsuaq terrane experienced granulite facies metamorphism along a distinctly hotter apparent geothermal gradient of ∼35 ◦ C/km, followed by prolonged isobaric cooling during NW-vergent thrusting to conditions of ∼700 ◦ C and 6.5–7 kbar. These retrograde conditions are similar to the peak conditions of 620–660 ◦ C and 6 kbar in the Tre Brødre terrane, which have been dated at 2751 ± 4 Ma. The contemporaneous existence of different thermal regimes and contrasting P–T paths, coupled to the strong structural evidence for regional-scale tectonic thickening indicate that these terranes of the Nuuk region are a Neoarchaean paired metamorphic belt. Here we propose a new tectonic model for the Nuuk region that involves the southwards subduction of the Færingehavn terrane underneath the Tre Brødre and Tasiusarsuaq terranes. In our model, the Tre Brødre terrane is not regarded as a separate tectonic entity, but rather as the leading edge of the upper plate, prior to, and during terrane amalgamation in the Neoarchaean. The prolonged period of convergence recorded in the Nuuk region does not seem to have resulted in deep subduction of crustal rocks, perhaps reflecting that Neoarchaean convergence rates were much slower than today or that subduction was intermittent and inefficient. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The processes that shaped the early continental crust are still somewhat enigmatic and controversial (e.g. De Wit, 1998; Hamilton, 1998, 2011; Moyen et al., 2006; Bédard, 2006; Bédard et al., 2013). However, there appears to be an emerging consensus that lateral, accretionary plate tectonic processes began in the Meso- or Neoarchaean (Friend et al., 1988; Nutman et al., 1989; Brown, 2006, 2007, 2010; Moyen et al., 2006; Dziggel et al., 2006; Nutman and Friend, 2007; Kisters et al., 2010; Næraa et al., 2012), though some workers argue that the observed bulk crustal

∗ Corresponding author. Tel.: +49 2418095773. E-mail address: [email protected] (A. Dziggel). 0301-9268/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.precamres.2013.12.010

shortening in many Archaean cratons can be equally explained by mantle convection currents in the absence of lateral plate tectonics (Bédard et al., 2013). In most Archaean cratons, much of the details of these first tectonic events are cryptic and poorly constrained, partly due to the dearth of appropriate rocks in the geological record, and partly because of the scarcity of detailed metamorphic studies. For the younger Proterozoic and Phanerozoic parts of the rock record the metamorphic signature of accretionary tectonics is recognised as a thermal duality, where rocks with contrasting metamorphic histories are juxtaposed against one another (Miyashiro, 1961; Oxburgh and Turcotte, 1971; Brown, 2006, 2010). The juxtaposition involves a relatively high-pressure, low-temperature terrane that represents the converging and subducting plate, and a relatively low-pressure, high-temperature terrane that represents the overriding plate and volcanic arc.

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Fig. 1. Geological map of the Nuuk region, southern West Greenland. Modified from Escher and Pulvertaft (1995).

The Nuuk region of southern West Greenland exposes an exceptionally well-preserved section through Archaean mid- to lower continental crust. The region has been recognised to consist of several distinct and accreted terranes, making it the ideal location to examine early plate tectonic processes (Friend et al., 1988; Nutman and Friend, 2007; Næraa et al., 2012). The area includes the lowermost Færingehavn terrane that is tectonically overlain by the Tre Brødre and Tasiusarsuaq terranes (Fig. 1; Friend et al., 1988, 1996; Nutman et al., 1989; McGregor et al., 1991; Friend and Nutman, 2005; Nutman and Friend, 2007; Kolb et al., 2012). The terranes are in fault contact with the Akia terrane in the NW and the Kapisilik terrane in the E. Recent investigations have recognised that the metamorphic history of the Færingehavn terrane is characterised by relict high-pressure assemblages and isothermal decompression paths associated with terrane amalgamation at ca. 2715 Ma (Nutman and Friend, 2007). By contrast, the structurally overlying Tasiusarsuaq terrane experienced mediumpressure granulite facies conditions, and the peak of regional metamorphism was followed by a prolonged period of nearisobaric cooling that eventually culminated in this amalgamation event (Dziggel et al., 2012; Kolb et al., 2012). The recognition of these contrasting thermal regimes provides another line of evidence that terrane amalgamation occurred by horizontal tectonics, and provides the opportunity to further elucidate and constrain details of the processes involved. Here we use pseudosection modelling to determine new P–T estimates and P–T–t paths for the Færingehavn and Tre Brødre terranes and examine the implications of these paths for the geodynamics of one of the oldest recognised accretionary tectonic events on Earth.

2. Geological setting The Nuuk region of southern West Greenland is part of the Archaean North Atlantic craton (Nutman and Friend, 2007; Fig. 1). The area has a long tradition of Archaean geology research since the first comprehensive regional geological mapping by McGregor (1973). The Nuuk region has a protracted Neoarchaean tectonothermal history marked by polyphase amphibolite to granulite

facies metamorphism during at least three phases of deformation (Nutman and Friend, 2007; Kolb et al., 2012). Dziggel et al. (2012) recently constrained peak metamorphic granulite facies conditions of 7.5 kbar and 850 ◦ C for the Tasiusarsuaq terrane, and found that peak metamorphism, dated by Crowley (2002) and Kolb et al. (2012) at ca. 2825–2800 Ma, was followed by an extended period of near-isobaric cooling to ∼700 ◦ C and 6.5–7 kbar until final collision at ca. 2720–2700 Ma. This is significantly different from the conditions and timing of high-pressure metamorphism in the Færingehavn terrane (8–12 kbar and 700–750 ◦ C at 2720–2700 Ma) determined by Nutman et al. (1989). In addition, rocks of the Færingehavn terrane evolved along a clockwise P–T path and experienced isothermal decompression to conditions of ∼5 kbar and 700 ◦ C during the later stages of accretion. Metamorphic conditions for the Tre Brødre terrane that separates the Færingehavn and Tasiusarsuaq terranes have not been quantified, but U–Pb zircon and monazite ages suggest that the terrane experienced upper amphibolite facies metamorphism at 2720–2700 Ma (Crowley, 2002). 2.1. Færingehavn terrane and Simiutat supracrustal sequence High-pressure rocks have been studied and sampled on the islands of Qilanngarssuit and Simiutat (Figs. 1 and 2). The Færingehavn terrane is dominated by ca. 3850–3600 Ma tonalite–trondhjemite–granodiorite (TTG) gneisses of the Itsaq Gneiss Complex (Nutman et al., 1996, 2004). The TTG gneisses are overlain by, and in tectonic contact with, a sequence of supracrustal rocks that was previously referred to as the Malene supracrustal sequence (Chadwick and Nutman, 1979; Nutman and Friend, 2007). Historically, however, the Malene supracrustal sequence also included similar supracrustal units from other terranes such as the Tre Brødre terrane, and is therefore no longer in use. We propose here a new term, the Simiutat supracrustal sequence, to name supracrustal rocks that structurally overly the Færingehavn terrane. The Simiutat supracrustal sequence includes meta-ultramafic rocks, amphibolites derived from basaltic protoliths, and aluminous gneisses that originated from ca. 2840 Ma old volcanic protoliths (Friend et al., 1996). Bulk rock geochemical data indicate that the aluminous gneisses mainly consist of SiO2 , Fe2 O3(tot) , Al2 O3 ,

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Fig. 2. Geological map of the island Qilanngaarssuit, showing sample localities. Modified from Chadwick and Coe (1983b).

and MgO, and have low concentrations of CaO, Na2 O, and K2 O (Dymek and Smith, 1990). This unusual bulk composition is similar to so-called cordierite-orthoamphibole gneiss, a rock type characterised by low-variance mineral assemblage considerably different from those typically found in metapelites (Diener et al., 2008). The contact between the supracrustal rocks and the Færingehavn terrane is always tectonic and typically marked by folded Archaean mylonites. The TTG gneisses and overlying supracrustal rocks were metamorphosed to amphibolite- and, locally, granulite facies conditions and record a polyphase tectono-metamorphic history (Nutman and Friend, 2007). Early Archaean medium-pressure granulite facies (M1 ) assemblages and D1 fabrics are locally preserved in the gneisses of the

Færingehavn terrane, with metamorphic and igneous zircon from a granulite-facies ferrodiorite recording ages between ca. 3400 and 3650 Ma. This range suggests that the crystallisation of this rock was followed by a complex Palaeo- to Eoarchaean metamorphic history (Nutman and Friend, 2007). M1 high-temperature metamorphism was succeeded by a ca. 2.7 Ga high-pressure metamorphic event (M2 ), that has been described at localities on southern Qilanngaarsuit and inner Ameralik (Fig. 1; Nutman and Friend, 2007). The highpressure assemblages are recorded in both the Færingehavn terrane and the overlying Simiutat supracrustal sequence. Typical mineral assemblages in the Simiutat supracrustal sequence include garnet + clinopyroxene + plagioclase + quartz ± hornblende

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Fig. 3. Field occurrence of mafic supracrustal rocks and cordierite-orthoamphibole gneisses in the southern part of the island Qilanngarssuit. (a) Typical field occurrence of mesosomes with irregularly shaped melt patches. View on foliation plane. (b) Melt escape structure in mafic supracrustal rock, view on foliation plane. The leucosome is associated with coarse-grained garnet and diopside, and mainly consists of quartz and plagioclase. (c) Close-up of a cordierite-orthoamphibole gneiss containing up to several cm large garnet porphyroblasts. Please note the presence of abundant leucosomes. (d) Sillimanite associated with light-green kyanite. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

in mafic supracrustal rocks, and relict garnet + kyanite + rutile assemblages in the strongly retrogressed aluminous gneisses (Nutman and Friend, 2007). During field work, relict diopsidegarnet assemblages in mafic supracrustal rocks have locally been observed as lenses and boudins in highly retrogressed and strongly schistose to mylonitic amphibolites on southern Qilanngarssuit, but these high-P domains in mafic supracrustal rocks are best preserved at locality G03/38 of Nutman and Friend (2007; Fig. 3a and b). The rocks contain abundant leucosomes with spectacular partial melting and melt segregation textures. The leucosomes occur in a variety of textural settings such as melt patches, veinlets and larger veins, giving the rock a migmatitic appearance. Relict high-P mineral assemblages dominated by garnet and kyanite have also been found in strongly schistose to mylonitic aluminous gneisses in the southern part of the island (Fig. 3c and d). The aluminous gneisses commonly contain thin, quartzo-feldspathic stringers, which are interpreted to reflect former leucosomes. High-P metamorphism and associated crustal thickening during M2 correlate with regional D2 deformation (Nutman and Friend, 2007). Thrust imbrication during D2 is marked by Itsaq gneisses structurally overlying the Simiutat supracrustal sequence, which, in turn, structurally overly the Itsaq gneisses (Fig. 4). Exhumation of the Færingehavn terrane and Simiutat supracrustal sequence occurred immediately after peak metamorphism, as zircon associated with the high- and low-pressure

assemblages yield indistinguishable ages of ca. 2715 Ma (Nutman and Friend, 2007). Exhumation is associated with the development of a pervasive S2 foliation at lower-pressure amphibolite facies conditions of 5–7 kbar and 700–750 ◦ C (Nutman and Friend, 2007). Decompression is indicated by hornblende-plagioclase symplectites rimming garnet in the high-P mafic rocks, and the

Fig. 4. Block diagram illustrating the structural geology of the island Qilanngaarssuit.

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development of cordierite and sillimanite replacing garnet and/or kyanite in the aluminous gneisses. The S2 foliation is axial planar to isoclinal F2a folds (Kolb et al., 2013), and the associated mineral stretching lineation plunges at shallow to moderate angles to the S and SW. The S2 foliation has been folded into upright, open to tight F2b folds with wavelength of 0.5–3 km and E–W trending axial traces (Fig. 4). Subsequent deformation (D3 ) is marked by the development of km-scale synform–antiform pairs of upright, open to tight folds with north-trending axial traces, locally forming sheath folds (Chadwick and Nutman, 1979; Fig. 4). The F3 fold axes mainly plunge at shallow to moderate angles to the S and SW, parallel to the L2 mineral stretching lineation (Fig. 4). Ductile shearing during F3 flexural slip folding was associated with the formation of gold-bearing quartz veins at lower amphibolite facies conditions (540–620 ◦ C and 4.5 ± 1 kbar; Koppelberg et al., 2013). The timing of the D3 deformation is not well constrained; however, pegmatites considered to have formed at ca. 2560 Ma (Chadwick and Nutman, 1979; Nutman et al., 1989) crosscut the mineralisation, bracketing the D3 event to between 2715 and 2560 Ma (Kolb et al., 2013).

2.2. Tre Brødre terrane The Tre Brødre terrane forms a ∼5 km wide thrust sheet that is situated between the Færingehavn and Tasiusarsuaq terranes (Fig. 1; Nutman and Friend, 2007). It mainly consists of felsic supracrustal rocks, TTG gneisses (the 2829 ± 11 to 2817 ± 9 Ma Ikkattoq gneiss; Friend et al., 2009; Nutman and Friend, 2007), and a dismembered, sheet-like gabbro-anorthosite complex (Chadwick and Coe, 1983a). Mafic supracrustal rocks locally occur as thin layers and boudins of amphibolite, with a maximum thickness of 10 m. The sequence has been intruded by pegmatite dyke swarms that crosscut all lithological units. In contrast to the Færingehavn and Tasiusarsuaq terranes, the metamorphic grade of the Tre Brødre terrane never exceeded amphibolite facies conditions (Nutman et al., 1989). Typical mineral assemblages in the felsic supracrustal rocks are quartz + garnet + biotite + orthoamphibole + cordierite ± sillimanite ± plagioclase ± staurolite, whereas rocks with a mafic bulk composition contain hornblende + plagioclase + quartz ± garnet ± orthoamphibole (Kolb et al., 2009). The general map pattern of the Tre Brødre terrane is marked by two different structural domains. The southern part of the terrane is exposed as a north-trending high-strain belt known as the Færingehavn straight belt (Chadwick and Coe, 1983a). In the north, the supracrustal rocks and anorthosites define an open to tight upright fold pattern with southerly trending axial traces. This fold pattern represents the regional D3 deformation. As in the Færingehavn and Tasiusarsuaq terranes, three deformation events can be distinguished. The earliest fabric preserved is a locally developed S1 foliation that has been preserved in the fold hinges of rare, rootless isoclinal intrafolial folds in the Ikkattoq gneiss (Kolb et al., 2009). The main fabric in the Tre Brødre terrane is a pervasive, S to SE dipping, S2 foliation that has been interpreted as a result of terrane amalgamation following the intrusion of the Ikkattoq gneiss in the Neoarchaean (Nutman et al., 1989). The S2 foliation is defined by peak metamorphic minerals such as sillimanite and biotite, and the associated mineral stretching lineation plunges at shallow to moderate angles to the S and SE. Shear sense indicators such as S–C fabrics point to a reverse sense of movement broadly to the N and NW. The formation of north-trending F3 folds during D3 and the subsequent intrusion of NE-trending pegmatite dyke swarms mark the end of tectonic and igneous activity in the Tre Brødre terrane (Kolb et al., 2009).

2.3. Tasiusarsuaq terrane The Tasiusarsuaq terrane is the largest terrane in the Nuuk region. The terrane is dominated by TTG gneisses with numerous enclaves of mafic and ultramafic supracrustal rocks (Fig. 1; Chadwick and Coe, 1983a,b; Friend et al., 1996). The oldest igneous activity in the Tasiusarsuaq terrane is recorded in the Fiskenæsset Complex, a layered and highly dismembered anorthosite complex that intruded into mafic supracrustal rocks in the southern part of the terrane (Escher and Myers, 1975; Polat et al., 2009). The Fiskenæsset Complex has a Sm–Nd errorchron age of 2973 ± 28 Ma, and has been interpreted to have formed in a supra-subduction zone geodynamic setting (Polat et al., 2009, 2010). The intrusion of the complex was followed by the emplacement of TTG granitoid rocks between 2920 and 2820 Ma, with a maximum between 2860 and 2840 Ma (Compton, 1978; Crowley, 2002; Friend and Nutman, 2001; McGregor et al., 1991; Næraa and Scherstén, 2008; Nutman and Friend, 2007; Schjøtte et al., 1989; Kokfelt et al., 2011). The last igneous event in the Tasiusarsuaq terrane is marked by the emplacement of the ca. 2800 Ma Ilivertalik granite/charnockite at granulite facies conditions (Pidgeon and Kalsbeek, 1978). Details of the metamorphic history and the structural evolution of the Tasiusarsuaq terrane are given by Dziggel et al. (2012) and Kolb et al. (2012). In general, the structural evolution of the Tasiusarsuaq terrane from the granulite facies peak to amphibolite facies cooling was associated with regional-scale thrusting. D1 fabrics are only preserved in the fold hinges of later isoclinal F2 folds, and are defined by granulite facies mineral assemblages. The dominant thrust-related D2 fabric is a SE-dipping foliation formed under amphibolite to granulite facies conditions. Shear sense indicators point to a reverse sense of movement broadly to the NW. The subsequent D3 deformation is marked by the formation of amphibolite facies N-trending strike-slip shear zones and upright F3 folds during E–W shortening (Kolb et al., 2012).

3. Analytical techniques Five samples, including mafic supracrustal rocks, TTG gneiss and aluminous gneiss from the Færingehavn and Tre Brødre terranes were chosen for a detailed petrological and mineral equilibria study. The sample localities are shown in Figs. 1 and 2, and the mineral assemblages are summarised in Table 1. Whole rock major element data were obtained by X-Ray fluorescence spectroscopy using a Phillips PW 1400 energy-dispersive spectrometer at the Institute of Mineralogy and Economic Geology at RWTH Aachen University. Electron microprobe analysis was carried out using a JEOL JXA-8900R electron microprobe, which is equipped with 5 wavelength dispersive spectrometers. The acceleration voltage was 15 kV and the beam current 20 nA, a beam size of 1–2 ␮m for ferromagnesian minerals and 10 ␮m for plagioclase was used. Only natural mineral standards were used for calibration. ZAF corrections were applied to the data, and representative analyses are given in Tables 2–5. Ferric iron in ferromagnesian minerals was estimated following the scheme of Droop (1987). In order to constrain the age of volcaniclastic activity and regional metamorphism in the Tre Brødre terrane, laser ablation sector-field-inductively coupled plasma mass spectrometry (LASF-ICP-MS) U-Pb zircon dating was carried out on a sample of aluminous gneiss (sample 515128, Fig. 1), following the methods outlined by Frei and Gerdes (2009) and Kolb et al. (2012). The laser was operated at a repetition rate of 10 Hz and nominal energy output of 45%, corresponding to a laser fluency of 3.5 J cm−2 (see Table A.1 for detailed running conditions). All data

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Table 1 Mineral assemblages and replacement textures in the samples investigated. Sample

Peak metamorphic phases

Retrograde phases

Remarks

515221 515219 508445 515220 515113

g + di + hb + q + pl + ilm + sph g + ky + q + bi + ged + ilm g + hb + pl + bi + q + ilm relict di and g g + hb + anth + pl + q + ilm

hb + pl cd + sill + bi + mt; chl + pl + st + q + mu + mt None hb + pl + q None

hb–pl symplectites around g; leucosome-bearing Two different retrograde assemblages preserved Leucosome-bearing Leucosome-bearing Leucosome-bearing

were acquired with a single spot analysis on each individual zircon grain with a beam diameter of 30 ␮m and a crater depth of approximately 15–20 ␮m. For the spot diameter of 30 ␮m and ablation times of 30 s the amount of ablated material approximates 200–300 ng. Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.precamres. 2013.12.010. The ablated material was analysed on an Element2 (ThermoFinnigan, Bremen) single-collector, double focussing, magnetic sector ICPMS with a fast field regulator for increased scanning speed. The total acquisition time for each analysis was 60 s, with the first 30 s used to measure the gas blank. Full analytical details and the results for all quality control materials analysed are reported in Table A.1 in the electronic supplementary material. Long term external reproducibility was monitored by repeated analyses of the Pleˇsovice zircon standard (Sláma et al., 2008), yielding an average of 339.3 ± 0.7 Ma based on 238 U/206 Pb and 343.1 ± 1.8 Ma based on 207 Pb/206 Pb ratios (N = 543 zircons, 2 standard deviations), which is in perfect agreement with reported value by ID-TIMS of 338 ± 1 Ma (Aftalion et al., 1989). Plotting of concordia diagrams and calculation of ages and their associated uncertainties from either weighted means or from unmixing of multiple age components were done in an off-line Excel sheet using Isoplot/Ex version 3.22 (Ludwig, 2003). All uncertainties are reported at the 2 level or 95% confidence interval.

4. Petrology 4.1. Færingehavn terrane and Simiutat supracrustal sequence 4.1.1. High pressure domains in mafic supracrustal rocks (sample 515221) Sample 515221 was collected from a high-P boudin hosted by sheared and retrogressed amphibolites at locality G03/38 of Nutman and Friend (2007). The melanosomes consist of garnet, diopside, hornblende, quartz, plagioclase, ilmenite, and titanite (Fig. 5a and b). Garnet and diopside are texturally well equilibrated with the mineral phases of the leucosomes. The leucosomes are made up of plagioclase, quartz, and minor biotite, pointing to a broadly trondhjemitic composition. Hornblende locally replaces diopside, or occurs as hornblende-plagioclase symplectitic overgrowths on garnet (Fig. 5b). This suggests that at least some of it formed during decompression via a reaction such as garnet + plagioclase 1 + H2 O = plagioclase 2 + hornblende. Garnet is unzoned or slightly zoned, and locally contains inclusions of quartz, plagioclase, calcite, and, locally, chlorite. The composition of larger grains is Alm55–58 Grs27–29 Pyr11–13 Sps03 ; smaller grains have a composition of Alm57–60 Grs23–25 Pyr09–17 Sps02–05 . In the smaller grains, pyrope decreases towards the rims, whereas grossular, spessartine, and XFe increase (Table 2). The XFe in hornblende varies between 0.47 and 0.52 (Table 3). Plagioclase occurs in a number of textural settings and exhibits strong

Table 2 Representative electron microprobe data of garnet. Sample Analysis Location Mineral

515221 55 Rim g

515221 62 Core g

515221 58 Core g

515219 1 Rim g

515219 3 Core g

515219 16 Core g

508445 1 Rim g

508445 6 Core g

515220 3 Core g

515220 20 Rim g

515113 51 Core g

515113 52 Rim g

SiO2 TiO2 Al2 O3 Cr2 O3 FeO MnO MgO CaO Na2 O K2 O Total

37.94 0.04 20.476 0.042 28.345 1.465 2.328 9.748 0 0 100.384

38.003 0.081 20.28 0.066 26.462 1.047 3.296 10.621 0.009 0 99.865

38.17 0.059 20.168 0.069 27.232 1.207 3.228 10.008 0 0.013 100.154

37.44 0 21.175 0 35.008 0.993 4.247 1.083 0.007 0.074 100.027

37.435 0 21.204 0 33.351 0.584 6.156 0.84 0.009 0.059 99.646

37.414 0 20.781 0.006 33.967 0.698 5.098 1.533 0.014 0 99.513

37.416 0 21.082 0 28.856 2.732 5.561 3.868 0.008 0 99.550

37.622 0 21.377 0 27.909 2.405 5.957 4.594 0 0.004 99.868

38.196 0.1 20.401 0 27.128 0.945 4.378 8.956 0.006 0 100.11

38.165 0.025 20.596 0 28.589 1.166 3.372 8.226 0 0.009 100.148

37.726 0 21.218 0 31.142 1.413 5.4 3.561 0.004 0 100.464

37.537 0 21.146 0.002 32.294 1.757 4.396 2.974 0 0 100.106

6.012 0 3.821 0.13 0.005 0.005 3.626 0.55 0.197 1.655 0 16 24

6.001 0 3.771 0.191 0.01 0.008 3.304 0.776 0.14 1.797 0.003 16 24

6.026 0 3.75 0.165 0.007 0.009 3.43 0.76 0.161 1.693 0 16 24

5.99 0.01 3.98 0.02 0 0 4.664 1.014 0.134 0.186 0.002 16 24

5.936 0.064 3.898 0.156 0 0 4.266 1.456 0.078 0.142 0.002 16 24

5.976 0.024 3.886 0.128 0 0 4.408 1.214 0.094 0.262 0.004 16 24

5.919 0.081 3.847 0.225 0 0 3.593 1.311 0.366 0.656 0.002 16 24

5.917 0.083 3.839 0.232 0 0 3.251 1.497 0.283 0.898 0 16 24

5.996 0.004 3.768 0.202 0.012 0 3.359 1.025 0.126 1.506 0.002 16 24

6.035 0 3.836 0.076 0.003 0 3.705 0.795 0.156 1.394 0 16 24

5.928 0.072 3.854 0.208 0 0 3.885 1.265 0.188 0.599 0.001 16 24

5.966 0.034 3.923 0.099 0 0 4.193 1.041 0.237 0.506 0 16 24

0.60 0.27 0.09 0.03

0.55 0.30 0.13 0.02

0.57 0.28 0.13 0.03

0.78 0.03 0.17 0.02

0.72 0.02 0.25 0.01

0.74 0.04 0.20 0.02

0.61 0.11 0.22 0.06

0.55 0.15 0.25 0.05

0.56 0.25 0.17 0.02

0.61 0.23 0.13 0.03

0.65 0.10 0.21 0.03

0.70 0.08 0.17 0.04

TSi TAl AlVI Fe3+ Ti Cr Fe2+ Mg Mn Ca Na Cations No. Ox. XAlm XGrs XPy XSps

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Fig. 5. (a and b) Photomicrographs of sample 515221. The peak assemblage (garnet + diopside + plagioclase + quartz + ilmenite + titanite ± hornblende) is variably replaced by hornblende along diopside rims and hornblende-plagioclase symplectites rimming garnet. (c) Peak metamorphic garnet in sample 515219 containing inclusions of gedrite and biotite. (d) Replacement of garnet and kyanite by a fine-grained assemblage of chlorite, plagioclase, cordierite, and staurolite (sample 515219). (e) Peak assemblage of garnet + hornblende + plagioclase + quartz + biotite + ilmenite in sample 508445. (f) Photomicrograph of sample 515220. Garnet is rimmed by plagioclase and hornblendeplagioclase symplectites, a texture typical of isothermal decompression. (g) Boudin of mafic supracrustal rock in the Tre Brødre terrane (sample 515113), containing abundant leucosomes. (h) Photomicrograph illustrating the peak assemblage of sample 515113.

A. Dziggel et al. / Precambrian Research 242 (2014) 22–38

29

Table 3 Representative electron microprobe data of amphibole. Sample Analysis Location Mineral

515221 49 Matrix hb

515221 107 Matrix hb

515219 100 Matrix ged

515219 87 Matrix ged

515219 91 Incl. ged

508445 16 Matrix hb

508445 17 Incl. hb

515220 10 Matrix hb

515220 2 Matrix hb

515113 46 Matrix hb

515113 54 Matrix hb

515113 47 Matrix anth

515113 52 Matrix anth

SiO2 TiO2 Al2 O3 FeO Cr2 O3 MnO MgO CaO Na2 O K2 O Total

45.698 0.448 10.101 19.092 0.07 0.081 9.279 11.97 1.014 0.321 98.082

45.034 0.39 11.377 17.578 0.051 0.086 9.813 11.823 1.264 0.245 97.699

40.518 0.155 18.913 22.618 0 0.186 12.782 0.243 1.812 0 97.26

41.628 0.2 18.384 23.423 0 0.181 12.257 0.374 1.839 0.014 98.378

41.25 0.15 18.54 22.52 0 0.17 12.67 0.32 1.94 0.00 97.61

43.783 0.52 14.926 14.769 0.068 0.278 10.843 10.977 1.365 0.241 97.77

45.067 0.376 13.099 15.81 0.049 0.29 12.933 9.154 1.09 0.198 98.066

44.458 1.057 11.785 17.284 0.007 0.082 9.897 11.518 1.371 0.439 97.898

42.044 1.412 13.395 17.959 0 0.151 8.883 11.532 1.622 0.55 97.548

44.775 0.477 13.472 17.815 0.02 0.199 11.408 8.632 1.046 0.205 98.049

41.69 0.404 16.986 17.315 0 0.113 8.626 10.918 1.429 0.205 97.686

54.071 0.014 1.516 22.33 0 0.344 18.715 0.297 0.061 0.006 97.354

47.525 0.211 11.005 22.835 0 0.385 14.985 0.538 0.978 0.004 98.466

Si TAl CAl CCr CFe3+ CTi CMg CFe2+ BFe2+ BMn BCa ANa AK Cations No. Ox.

6.808 1.192 0.581 0.008 0.148 0.05 2.061 2.152 0.079 0.01 1.911 0.293 0.061 15.354 23

6.699 1.301 0.692 0.006 0.104 0.044 2.176 1.978 0.105 0.011 1.884 0.365 0.046 15.411 23

6 2 1.298 0 0.147 0.017 2.822 0.716 1.938 0.023 0.039 0.52 0 15.52 23

6.131 1.869 1.32 0 0 0.022 2.691 0.967 1.918 0.023 0.059 0.525 0.003 15.528 23

6.10 1.90 1.33 0.00 0.00 0.02 2.79 0.86 1.93 0.02 0.05 0.56 0.00 15.56 23

6.419 1.581 0.996 0.008 0.03 0.057 2.37 1.539 0.241 0.035 1.724 0.388 0.045 15.433 23

6.54 1.46 0.778 0.006 0.251 0.041 2.798 1.126 0.541 0.036 1.423 0.307 0.037 15.343 23

6.619 1.381 0.685 0.001 0 0.118 2.197 1.999 0.152 0.01 1.837 0.396 0.083 15.479 23

6.33 1.67 0.705 0 0.065 0.16 1.994 2.076 0.12 0.019 1.86 0.474 0.106 15.579 23

6.563 1.437 0.888 0.002 0.106 0.053 2.493 1.458 0.62 0.025 1.356 0.297 0.038 15.336 23

6.184 1.816 1.151 0 0.126 0.045 1.907 1.771 0.251 0.014 1.735 0.411 0.039 15.45 23

7.871 0.129 0.13 0 0 0.002 4.061 0.807 1.911 0.042 0.046 0.017 0.001 15.018 23

6.922 1.078 0.81 0 0 0.023 3.254 0.913 1.869 0.047 0.084 0.276 0.001 15.277 23

0.52

0.49

0.48

0.52

0.43

0.37

0.49

0.52

0.45

0.51

0.40

0.46

XFe

0.50

compositional variation, ranging between An47 and An85 . Plagioclase inclusions in garnet are either oligoclase (XAn = 0.25–0.30), or andesine–labradorite (XAn = 0.48–0.60; Table 4). Plagioclase rimming garnet as coronas or as hornblende-plagioclase symplectites is mainly bytownite (XAn = 0.65–0.85). Matrix plagioclase records two distinctly different compositions: the most abundant generation is andesine (XAn = 0.46–0.50), whereas subordinate labradorite (XAn = 0.60–0.64) is also found. Andesine is interpreted to be the peak metamorphic generation as it is locally rimmed by labradorite.

Labradorite is also the dominant plagioclase in the leucosomes, suggesting that it crystallised in conjunction with the melt. Diopside has an XFe of 0.34–0.40 and an Al content of up to 0.17 a.p.f.u. and a Na content below 0.04 a.p.f.u. (Table 5). 4.1.2. Aluminous gneisses with relict high-pressure assemblages (sample 515219) In the samples investigated, relict high-pressure assemblages are best-preserved in sample 515219. The rock consists of garnet,

Table 4 Representative electron microprobe data of plagioclase. Sample Analysis Location Mineral

515221 41 Incl. pl

515221 77 Incl. pl

515221 80 Corona pl

515221 97 Corona pl

515221 86 Leucos. pl

515221 102 Matrix pl

515219 30 Matrix pl

508445 6 Matrix pl

515220 26 Corona pl

515220 32 Leucos. pl

515113 74 Core pl

515113 77 Rim pl

SiO2 TiO2 Al2 O3 FeO MnO BaO CaO Na2 O K2 O Total

56.391 0 28.11 0.344 0.025 0.033 9.654 5.687 0.08 100.324

61.853 0.03 24.187 0.109 0 0 5.11 8.19 0.024 99.503

51.428 0 31.617 0.462 0.001 0 13.432 3.728 0.018 100.686

47.427 0 34.039 0.266 0.036 0.026 16.718 1.906 0.023 100.441

52.525 0.035 30.767 0.294 0 0.016 12.505 4.386 0.002 100.53

56.341 0 28.479 0.012 0 0.047 9.851 5.685 0.048 100.463

60.051 0 25.418 0.125 0 0.019 5.574 7.995 0.023 99.205

55.062 0 28.963 0.153 0.024 0.037 10.466 5.436 0.028 100.169

58.148 0.022 27.371 0.116 0 0 8.234 6.466 0.11 100.467

56.9 0 27.822 0.101 0 0.077 9.119 6.037 0.073 100.129

55.073 0 29.396 0 0 0 11.057 5.159 0.035 100.72

50.063 0 31.642 0.047 0 0.057 14.338 3.286 0.023 99.456

10.097 5.927 0 0.052 0.004 0.002 1.852 1.974 0.018 19.928 32

10.994 5.063 0.004 0.016 0 0 0.973 2.823 0.005 19.878 32

9.289 6.725 0 0.07 0 0 2.599 1.306 0.004 19.993 32

8.668 7.327 0 0.041 0.006 0.002 3.274 0.675 0.005 20 32

9.474 6.535 0.005 0.044 0 0.001 2.417 1.534 0 20.011 32

10.063 5.99 0 0.002 0 0.003 1.885 1.969 0.011 19.926 32

10.74 5.354 0 0.019 0 0.005 1.068 2.773 0.005 19.964 32

9.899 6.132 0.023 0.004 0.003 2.016 1.895 0.006 19.978 32

10.338 5.731 0.003 0.017 0 0 1.568 2.229 0.025 19.911 32

10.185 5.865 0 0.015 0 0.005 1.749 2.095 0.017 19.936 32

9.846 6.189 0 0 0 0 2.118 1.788 0.008 19.949 32

9.17 6.826 0 0.007 0 0.004 2.814 1.167 0.005 19.993 32

0.48

0.26

0.67

0.83

0.61

0.49

0.41

0.52

0.41

0.45

0.54

0.71

Si Al Ti Fe2+ Mn Ba Ca Na K Cations No. Ox. XAn

30

A. Dziggel et al. / Precambrian Research 242 (2014) 22–38

Table 5 Representative electron microprobe data of diopside, biotite, chlorite, staurolite, and cordierite. Sample Analysis Location Mineral

515221 40 Matrix di

515221 41 Matrix di

515220 16 Matrix di

515219 2 Incl. bi

515219 33 Matrix bi

515219 6 Matrix chl

515219 1 Incl. chl

515219 44 Incl. chl

515219 46 Incl. chl

515219 12 Matrix st

515219 103 Matrix crd

SiO2 TiO2 Al2 O3 FeO Cr2 O3 MnO NiO MgO CaO Na2 O K2 O ZnO Total

50.931 0.31 2.593 12.748 0.054 0.15 0.044 10.018 23.347 0.325 0.001 n.a. 100.52

52.105 0.078 0.846 12.739 0.022 0.157 0 10.888 23.38 0.172 0.019 n.a. 100.40

51.842 0.133 1.084 11.576 0.004 0.139 0 11.652 23.654 0.278 0 n.a. 100.36

36.135 1.162 18.559 15.029 n.a. 0.01 n.a. 15.104 0.022 0.566 8.159 n.a. 94.756

37.133 1.209 18.737 13.635 n.a. 0 n.a. 16.288 0 0.583 8.197 n.a. 95.782

24.272 0.059 25.208 15.631 n.a. 0.007 n.a. 21.456 0.015 0.009 0.022 n.a. 86.679

25.305 0.074 24.471 16.819 n.a. 0.026 n.a. 21.121 0 0.01 0 n.a. 87.826

26.319 0.035 24.719 13.371 n.a. 0.009 n.a. 23.276 0.028 0.001 0.009 n.a. 87.767

23.878 0 26.594 32.083 n.a. 0.038 n.a. 4.446 0.097 0.011 0 n.a. 87.147

27.406 0.186 57.008 12.686 n.a. 0.04 n.a. 2.396 0 0.035 0 0.903 100.66

48.543 0 34.904 5.079 n.a. 0.022 n.a. 9.781 0.032 0.315 0 n.a. 98.676

TSi TAl AlVI Al Ti Fe3 Cr Mg Ni Fe2 Mn Ca Na K Zn Cations No. Ox.

1.928 0.072 0.044 0.116 0.009 0.033 0.002 0.565 0.001 0.371 0.005 0.947 0.024 0 0 4 6

1.974 0.026 0.011 0.037 0.002 0.023 0.001 0.615 0 0.381 0.005 0.949 0.013 0.001 0 3.999 6

1.951 0.048 0 0.048 0.004 0.062 0 0.654 0 0.302 0.004 0.954 0.02 0 0 4 6

5.379 2.621 0.632 3.253 0.13 0 0 3.352 0 1.871 0.001 0.004 0.163 1.549 0 15.70 22

5.42 2.58 0.641 3.221 0.133 0 0 3.544 0 1.664 0 0 0.165 1.526 0 15.67 22

4.912 3.088 2.92 6.008 0.009 0 0 6.473 0 2.645 0.001 0.003 0.004 0.006 0 28.061 36

5.076 2.924 2.856 5.78 0.011 0 0 6.316 0 2.821 0.004 0 0.004 0 0 28.012 36

5.177 2.823 2.903 5.726 0.005 0 0 6.825 0 2.199 0.001 0.006 0 0.002 0 27.941 36

5.207 2.793 4.037 6.83 0 0 0 1.445 0 5.851 0.007 0.023 0.005 0 0 27.368 36

3.691

4.921

9.048 0.019 0 0 0.481 0 1.429 0.005 0.000 0.009 0.000 0.090 14.771 23

4.167 0 0 0 1.478 0 0.431 0.002 0.003 0.062 0 0 11.064 18

XFe

0.40

0.38

0.32

0.36

0.32

0.29

0.31

0.24

0.80

0.71

0.23

kyanite, cordierite, quartz, biotite, gedrite, sillimanite, ilmenite, chlorite, staurolite, plagioclase, muscovite, and magnetite. Garnet forms up to 1 cm large lens-shaped porphyroblasts that are aligned along a well-developed S2 foliation, and contains inclusions of chlorite, quartz, gedrite, kyanite, and biotite. The matrix is dominated by quartz, gedrite, cordierite, ilmenite, magnetite, and minor amounts of biotite and sillimanite. Cordierite replaces garnet, kyanite, and gedrite along the rims (Fig. 5d), or forms finegrained aggregates that are aligned along the foliation. Locally, chlorite, plagioclase, staurolite and magnetite replace both the peak metamorphic minerals such as garnet and kyanite and retrograde cordierite (Fig. 5c, d), suggesting a second retrograde metamorphic event following cordierite crystallisation. An assemblage of chlorite, quartz, staurolite, and, locally, muscovite, also occurs as a fine-grained rosette-like intergrowth which replace all earlierformed minerals along rims and fractures (Fig. 5c, d). Based on these textures, we infer that the peak metamorphic mineral assemblage is garnet + kyanite + quartz + biotite + gedrite + ilmenite. The assemblage is interpreted to have formed in equilibrium with melt (Fig. 3c). Cordierite and sillimanite are interpreted to form an early retrograde assemblage, with the chlorite + staurolite + plagioclase ± muscovite assemblage forming during later retrogression. Garnet in sample 51219 has a composition of Alm71–78 Grs02–05 Pyr19–25 Sps01–02 . The cores of the grains are relatively unzoned (Alm71–74 Grs02–03 Pyr21–25 Sps01–02 ), whereas the outermost rims are enriched in almandine and depleted in pyrope component (Alm74–78 Grs03–05 Pyr19–22 Sps01–02 ; Table 2). The composition of gedrite is relatively constant; the XFe varies between 0.48 and 0.52 (Table 3). Gedrite inclusions in garnet have a Na content of 0.56–0.57 a.p.f.u., whereas gedrite in contact with retrograde chlorite and staurolite is less sodic (0.52–0.53 a.p.f.u.).

Plagioclase is oligoclase with XAn of ∼0.4 (Table 4). Biotite inclusions in garnet are relatively Fe-rich (XFe ∼ 0.36–0.37), whereas matrix biotite has an XFe between 0.29 and 0.34. Chlorite records a wide range in composition. Most matrix grains have an XFe between 0.27 and 0.29. Chlorite inclusions in kyanite and chlorite replacing biotite inclusions in garnet have an XFe between 0.31 and 0.32 (Table 5). Other chlorite inclusions in garnet are either Fe-rich (XFe = 0.80) or Fe-poor (XFe = 0.24). Staurolite has an XFe of 0.71 to 0.74 and Zn content of 0.08–0.11 a.p.f.u. Cordierite is Mg-rich with XFe that varies between 0.17 and 0.26 (Table 5). 4.1.3. Garnet-bearing TTG gneiss (sample 508445) The high-pressure TTG gneiss has been sampled from Simiutat Island south of Qilanngarssuit (Fig. 1). The rock is mediumto coarse-grained, and consists of garnet, hornblende, plagioclase, biotite, quartz and ilmenite (Fig. 5e). It contains abundant leucosomes that are oriented parallel to a well-developed S2 foliation. Garnet forms up to 2 cm large, poikiloblastic crystals that contain numerous inclusions of quartz, hornblende, plagioclase, biotite, and ilmenite. The matrix is dominated by plagioclase and hornblende. Garnet is slightly zoned, with the cores having a composition of Alm55–59 Grs11–16 Pyr23–26 Sps04–06 . Almandine and spessartine contents increase towards the rims, whereas grossular and pyrope decrease (Alm61–64 Grs9–12 Pyr19–22 Sps06–08 ; Table 2). Matrix hornblende has an XFe of 0.41–0.45 (Table 3). Hornblende inclusions in garnet and some of the rims of matrix hornblende record slightly lower values (XFe = 0.37–0.38). Plagioclase is unzoned, and its composition varies between An49 and An53 (Table 4). 4.1.4. Retrograde amphibolites (sample 515220) Sample 515220 is from the schistose to mylonitic amphibolite surrounding the high-pressure lenses on southern

A. Dziggel et al. / Precambrian Research 242 (2014) 22–38

Quilanngaarssuit (Fig. 2), and is therefore interpreted to be a strongly retrogressed equivalent of sample 515221. The rock consists of garnet, hornblende, quartz, ilmenite, plagioclase, diopside, and titanite and contains abundant, coarse-grained leucosomes (Fig. 5f). Garnet commonly contains inclusions of quartz, and locally, titanite, and ilmenite, and is slightly zoned. The cores of the grains have compositions of Alm52–60 Grs25–31 Pyr11–18 Sps02–03 (Table 2). Although the absolute values vary between individual grains, the grossular and pyrope contents generally decrease towards the rims, whereas almandine increases. The rims have a composition of Alm57–62 Grs23–25 Pyr11–16 Sps02–03 . Hornblende mainly occurs as large grains surrounding garnet, and locally displays symplectitic intergrowths with plagioclase at garnet rims (Fig. 5f). Hornblende often shows exsolution lamellae of ilmenite, and has an XFe of 0.48–0.53 (Table 3). Plagioclase either occurs in the leucosomes or as thin (a few micrometre thick) rims around garnet (Fig. 5f), but is otherwise absent from the matrix of the sample. Plagioclase in the garnet coronas and symplectites has a composition of An41 to An52 , whereas plagioclase in the leucosome has XAn of 45–47 (Table 4). Diopside has an XFe of 0.28–0.34, an Al content of up to 0.155 a.p.f.u., and only contains traces of Na (Table 5).

31

4.2. Mafic supracrustal rocks from the Tre Brødre terrane (sample 515113) The sample investigated was taken from a ∼1 m thick boudin hosted by aluminous felsic schists in the northern part of the Tre Brødre terrane (Fig. 5g and h). The rock (sample 515113) contains abundant leucosomes, and consists of garnet, hornblende, anthophyllite, plagioclase, quartz, and ilmenite (Fig. 5g, h). Garnet forms up to ∼1 cm large porphyroblasts that contains inclusions of quartz, ilmenite, hornblende, anthophyllite, plagioclase, and, locally, chlorite and biotite. The matrix is dominated by plagioclase and amphiboles, which define a weakly-developed foliation. Garnet in sample 515113 is slightly zoned (Table 2). The cores of the grains have a composition of Alm0.65–0.68 Grs0.09–0.12 Pyr0.19–0.22 Sps0.03–0.04 . Almandine increases towards the rims, whereas grossular and pyrope decrease. The rim has a composition of Alm0.66–0.70 Grs0.07–0.10 Pyr0.16–0.19 Sps0.03–0.04 . The XFe in hornblende is between 0.42 and 0.51, whereas anthophyllite records XFe values between 0.40 and 0.47 (Table 3). Plagioclase is complexly zoned and its composition varies between An51 and An83 . Most grains are relatively Ca-poor in their core (An50–60 ) and XAn increases towards the rims (Table 4). The composition of

Fig. 6. Calculated pseudosections for rocks from the Færingehavn terrane and Simiutat supracrustal sequence. (a) Mafic granulite sample 515221, with isopleths for XAn . (b) cordierite–orthoamphibole gneiss sample 515219; (c) garnet-bearing TTG gneiss sample 508445; (d) retrograde amphibolite sample 515220, with isopleths for hornblende and garnet abundance. Assemblages referred to in the text are highlighted in bold.

32

A. Dziggel et al. / Precambrian Research 242 (2014) 22–38

5.1. Færingehavn terrane and Simiutat supracrustal sequence

Fig. 7. Calculated pseudosection for a mafic supracrustal rock from the Tre Brødre terrane (sample 515113). The inferred peak assemblage is shown in bold.

most of these rims is between An65 and An75 , although values of up to An82 have been analysed. Other grains display an inverse zonation, with a Ca-rich core (An75–82 ) and Ca-poorer rims (An60–70 ). It is not immediately clear what the cause for this complex zoning is, but both types of zoning occur in contact with, or close to, the leucosomes, suggesting plagioclase grew in equilibrium with the melt. The complexly zoned grains may therefore reflect a re-equilibration of plagioclase due to changes in melt composition. 5. Phase diagram modelling Modelling calculations were performed in the (NCFNa2 O–CaO–FeO–MgO–Al2 O3 –SiO2 –H2 O–TiO2 –Fe2 O3 MASHTO) chemical system for mafic supracrustal rocks and in the Na2 O–CaO–K2 O–FeO–MgO–Al2 O3 –SiO2 –H2 O–TiO2 –Fe2 O3 (NCKFMASHTO) system for the cordierite-orthoamphibole- and TTG gneisses. Calculations used THERMOCALC version 3.33 (Powell and Holland, 1988, updated June 2009) and the November 2003 updated version of the Holland and Powell (1998) data set (file tc-ds55.txt). The phases considered in the calculations and references to the activity-composition models used are: hornblende, cummingtonite, gedrite and anthophyllite (Diener et al., 2007, updated by Diener and Powell, 2012), diopside (Green et al., 2007, updated by Diener and Powell, 2012), garnet, biotite and silicate melt (White et al., 2007), cordierite, epidote and staurolite (Holland and Powell, 1998), plagioclase (Holland and Powell, 2003), orthopyroxene and spinel-magnetite (White et al., 2002), muscovite-paragonite (Coggon and Holland, 2002), chlorite (Holland et al., 1998) and ilmenite-hematite (White et al., 2000). Rutile, titanite, the aluminosilicates, quartz and aqueous fluid (H2 O) are pure end-member phases. Bulk rock compositions determined by X-ray fluorescence were converted to the model systems by disregarding the minor amounts of MnO, K2 O and Cr2 O3 in the mafic bulk compositions, and converting ∼20% of total Fe to Fe3+ , consistent with the observations that ilmenite is the main oxide phase in these rocks (Diener and Powell, 2010). For the modelling of the cordierite-orthoamphibole- and TTG gneiss, Mn and Cr were disregarded and 10% of the total Fe was converted to Fe3+ in samples in which magnetite was either rare or absent. Even though these rocks exhibit evidence of partial melting, melt was only explicitly included in calculations for the cordieriteorthoamphibole- and TTG gneisses as the current melt model is not appropriate for melting of mafic rocks. All samples were assumed to be fluid-saturated during prograde-to-peak metamorphism and the fluid content for suprasolidus parts was constrained from the wet solidus. The bulk compositions used to construct the pseudosections are presented in Table 6, and the calculated pseudosections are presented in Figs. 6 and 7.

5.1.1. High-pressure mafic rocks (sample 515221) The pseudosection for sample 515221 is dominated by large, high-variance fields, but the inferred peak assemblage of garnet, diopside, hornblende, plagioclase, titanite, ilmenite and quartz occurs over a narrow range between 9 and 10.7 kbar at 690–740 ◦ C (Fig. 6a). Ilmenite is lost and epidote becomes stable to higher P and lower T, whereas titanite is lost to higher T and garnet to lower P. Plagioclase is calculated to have a composition of An50 –An56 at these conditions, broadly consistent with the analysed composition (An46 –An50 ) of matrix plagioclase in this sample. Calculated isopleths for plagioclase composition show a large variability in anorthite content within the lower-temperature epidote-bearing assemblages (stippled lines, Fig. 6a). It is likely that the more albitic (An25 –An30 ) plagioclase inclusions found in garnet are a reflection of these conditions. The intersection of the garnet-in phase boundary and albitic plagioclase contours occurs at 10–11 kbar at approximately 640 ◦ C and likely reflect the conditions under which garnet was introduced during prograde metamorphism (Fig. 6a). The more anorthite-rich (∼An50 ) plagioclase inclusions that also occur in garnet could have been trapped at similar pressure while the rock was heated to peak conditions (Fig. 6a). Labradorite (An60 –An64 ) occurs in the leucosome and as plagioclase associated with garnet breakdown. The pseudosection shows that garnet breakdown occurs through decompression at 9 kbar. Plagioclase at these conditions is calculated to be the most anorthite-rich for the P–T range considered (An56 ; Fig. 6a), but to have lower anorthite content than that observed for the garnet breakdown textures (An65 –An85 ). The mismatch between the calculated and observed compositions could either be a reflection of compositional domains that formed and achieved only local chemical equilibrium during retrogression, or can be explained by this plagioclase generation having formed through a melting reaction that cannot be quantitatively modelled. Both of these options are likely, as plagioclase that is rimming garnet shows a large compositional variability, whereas plagioclase in the leucosome has a composition that is distinct from the matrix. 5.1.2. Aluminous gneisses with relict high-pressure assemblages (sample 515219) The pseudosection for this sample is characterised by a large variety of mineral assemblages and a number of low-variance fields at low pressure (Fig. 6b). The solidus occurs at 660–680 ◦ C at high pressure, but shifts to higher temperatures up to 750 ◦ C at lower pressure. The inferred peak assemblage of garnet, gedrite, kyanite, biotite, quartz, ilmenite and silicate melt occurs between 8 and 11.5 kbar and 670–750 ◦ C (Fig. 6b). Gedrite is lost to higher T, whereas sillimanite, magnetite and cordierite are introduced to lower P. The compositional isopleths in the stability field of the peak assemblage (not shown for clarity) are consistent with the mineral compositions analysed; however, the slopes of most isopleths are generally steep and are therefore not useful for a tighter pressure constraint. Nevertheless, the section shows that the replacement of peak metamorphic garnet and kyanite by cordierite and sillimanite requires decompression to conditions at or below 6 kbar (Fig. 6b). The second retrograde assemblage of staurolite, plagioclase, magnetite, chlorite, and, locally, muscovite suggests subsequent cooling to ∼500–550 ◦ C (Fig. 6b). 5.1.3. Garnet-bearing TTG gneiss (sample 508445) The pseudosection calculated for the TTG gneiss is presented in Fig. 6c. The wet solidus in this sample occurs at 740 ◦ C at 4 kbar, and extends lower T at higher P, whereas garnet is present at pressure above 7–8 kbar. The inferred peak assemblage of garnet, hornblende, biotite, plagioclase, quartz, ilmenite and silicate melt

A. Dziggel et al. / Precambrian Research 242 (2014) 22–38

33

Table 6 Bulk compositions (in mol.%) used to construct the pseudosections.

Fig. 7a (515221) Fig. 7b (515219) Fig. 7c (508445) Fig. 7d (515220) Fig. 8 (515113) a

SiO2

TiO2

Al2 O3

FeO

MgO

CaO

Na2 O

K2 O

O

H2 O

54.90 79.10 50.95 49.19 53.55

0.705 0.319 1.020 1.680 2.532

7.735 6.106 10.701 7.671 8.586

9.135 10.060 11.083 17.926 14.472

8.291 4.263 7.976 9.184 10.525

15.728 0.349 10.444 11.855 7.934

2.611 0.239 2.830 0.727 1.079

– 0.070 0.133 – –

0.894 0.498 0.762 1.768 1.385

Excess 1a 4.1a Excess Excess

H2 O was taken as in excess for the subsolidus part of the diagram.

occurs over a large stability range, extending from the solidus at 9 kbar and 700 ◦ C to higher and lower pressure with increasing temperature (Fig. 6c). Minor diopside (<0.5 vol.%) is calculated to form part of the peak assemblage at these conditions, but is not observed in the rock. This discrepancy is likely due to uncertainty within the activity-composition relations (e.g. Diener and Powell, 2012). Titanite bounds the assemblage to higher pressure, whereas biotite and garnet are lost to higher temperature and lower pressure, respectively. Calculated plagioclase compositions (not shown) vary from An52 at 8 kbar to An43 at 11 kbar for the temperatures of interest, consistent with the analysed compositions in this sample. However, isopleths for grossular content in garnet are sensitive to temperature, and are calculated to have higher values (Grs0.3–0.35 ) than those analysed (Grs0.1–0.15 ). 5.1.4. Retrograde amphibolites (sample 515220) The pseudosection for this sample is dominated by large, high-variance fields, and the inferred peak assemblage of garnet, diopside, hornblende, quartz and ilmenite is stable over a large P–T range above 7.5 kbar and 660 ◦ C (Fig. 6d). This range is consistent with the better-constrained estimates obtained from other samples (see above). However, this sample can be expected to provide information on retrograde conditions and the retrograde path. Retrogression in this sample is characterised by the consumption of garnet, the introduction of plagioclase as coronas around garnet, and an increase in the abundance of hornblende, which replaces peak metamorphic diopside (Fig. 5f). Calculated isopleths for the abundance of garnet and hornblende have shallow slopes and are dominantly dependent on pressure (Fig. 6d), such that the observed textures are developed by decompression. Plagioclase is introduced at 7–7.5 kbar for near-peak temperatures, suggesting that retrogression of this sample records near-isothermal decompression to conditions below at least 7 kbar. 5.2. Mafic rocks from the Tre Brødre Terrane The pseudosection for sample 515113 is characterised by large, high-variance hornblende-bearing fields, but the inferred peak assemblage of garnet, hornblende, orthoamphibole, plagioclase, quartz and ilmenite occurs over a small range, between 620 and 660 ◦ C at 6 kbar (Fig. 7). This field is bound by orthoamphiboleabsent assemblages to higher pressure, the introduction of chlorite to lower T, and garnet-absent, cummingtonite-bearing assemblages to lower P (Fig. 7). Calculated mineral compositions match those analysed in this sample, with garnet composition calculated as Alm0.74 Grs0.11 Pyr0.15 . Plagioclase is calculated to be Ca-rich (An82 ), similar to the analysed composition of some plagioclase rims. 6. Geochronology One sample of aluminous gneiss (sample 515128) was separated for zircons and analysed by LA-SF-ICP-MS. The up to 400 ␮m × 100 ␮m large zircons are generally an- to subhedral and elongate, with high aspect ratios of 1:3–1:4 (Fig. 8). The grains

usually constitute a partially resorbed BSE-dark/U-poor core overgrown or replaced by a variably broad rim zone that is relatively BSE-bright/U-rich. The core domains are often intensely cracked compared to the rims and contain few inclusions of a Fe–Mg–Al–Si mineral (presumably amphibole). The rim domains show relatively abundant inclusions of albite, quartz and monazite; xenotime is seen as up to 60 ␮m large grains intergrown with rim zircon (Fig. 8). The texture of the rim is generally nebulitic, heterogeneous (with respect to BSE-brightness), suggesting an uneven distribution of U, Th and Hf throughout the rim zones. In contrast to the rims, the cores are more homogeneous in BSE-colour. Shown examples demonstrate (from left to right) increasing degrees of resorbtion of cores (Fig. 8). In the Terra–Wasserburg diagram (Fig. 9a) the respective rim and core domains correspond to two distinct age populations with different Th/U ratios and U concentrations (Fig. 9a and b). The 19 core domain analyses constitute an approximate age of ca. 2.80 Ga; however based on the U ppm vs. Pb–Pb age diagram (Fig. 9b), there is evidence for mixing of age domains in some of the analyses, suggesting that the real age of the relict cores is closer to 2.82 Ga. This age is interpreted as magmatic age of the gneiss protolith. The BSE-bright rim domains give a well-defined age of 2751 ± 4 Ma (MSWD = 1.04), interpreted as the timing of peak metamorphism.

7. Discussion 7.1. Contrasting P–T–t paths for Neoarchaean metamorphism The inferred peak assemblages in rocks from the Færingehavn terrane and Simiutat supracrustal sequence overlap to constrain peak metamorphic conditions for this terrane at ∼10 kbar and 700 ◦ C (Fig. 10). The shape of the prograde path can be constrained from the plagioclase inclusions in garnet from sample 515221, which indicate that prograde metamorphism likely intersected conditions of 10.5 kbar at 640 ◦ C (an apparent geothermal gradient of 17.5 ◦ C/km) and maintained this pressure during heating to peak metamorphic conditions (Fig. 10). Retrograde conditions for the Færingehavn terrane and Simiutat supracrustal sequence are estimated at 6 kbar and ∼700 ◦ C, indicating that initial retrogression was dominated by decompression of more than 4 kbar (Fig. 10). The later parts of the retrograde path are not as well-constrained, but appear to have been dominated by cooling with only minor attendant decompression. The P–T–t path for the Færingehavn terrane determined here is similar to that proposed by Nutman and Friend (2007), and is characterised by a high-pressure, clockwise trajectory with significant decompression subsequent to peak metamorphism. The peak and retrograde assemblages have indistinguishable ages of ca. 2715 ± 5 Ma (Nutman and Friend, 2007), indicating that decompression was rapid and followed immediately after peak metamorphism. By contrast, P–T conditions estimated for the Tre Brødre terrane are at much lower pressure but similar temperatures than the Færingehavn terrane. Peak metamorphism is constrained at ∼6 kbar and 620–660 ◦ C (Fig. 10), but further details of the P–T path

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Fig. 8. Backscattered electron (BSE) images of zircon in sample 515128. See text for explanation.

Fig. 9. (a) Terra–Wasserburg U–Pb diagram of sample 515128. (b) two different age domains.

207

Pb/206 Pb age versus U-concentration (ppm) of zircons in sample 515128, illustrating the presence of

A. Dziggel et al. / Precambrian Research 242 (2014) 22–38

35

Fig. 10. Contrasting P–T–t paths for various terranes of the Nuuk region. Metamorphic conditions for the Færingehavn (grey shading) and Tre Brødre terranes (black shading) are from this study, whereas those for the Tasiusarsuaq terrane (black stars) are from Dziggel et al. (2012). Ages are from this study, Nutman and Friend (2007) and Kolb et al. (2012).

for this terrane are currently unknown. The 2751 ± 4 Ma zircon age presented above indicate a slightly older age of metamorphism for the Tre Brødre terrane; however, the peak conditions from the Tre Brødre terrane are comparable to those experienced during postpeak decompression in the Færingehavn terrane (Fig. 10). Furthermore, the P–T path for the overlying Tasiusarsuaq terrane was dominated by slow, near-isobaric cooling from peak conditions of 7.5 kbar and 850 ◦ C, reflecting an elevated apparent geotherm of ∼35 ◦ C/km, to 6.5–7 kbar at 700 ◦ C during reworking at 2770–2720 Ma (Fig. 10; Kolb et al., 2009; Dziggel et al., 2012). Therefore, the Færingehavn, Tasiusarsuaq and Tre Brødre terranes preserve distinctly contrasting P–T histories and P–T paths that all converge at the same mid-crustal, upper amphibolite facies conditions of ∼6–7 kbar and 650–700 ◦ C at ca. 2715 Ma (Fig. 10). 7.2. Implications for Archaean tectonics One of the hallmarks of one-sided subduction is a duality of thermal environments, i.e. the presence of “penecontemporaneous belts of contrasting type of metamorphism that record different apparent thermal gradient, one warmer and the other colder, juxtaposed by plate-tectonic processes” (Brown, 2006, 2009). The existence of distinctly different thermal regimes and contrasting P–T paths in the Færingehavn, Tre Brødre and Tasiusarsuaq terranes, the strong structural evidence for regional-scale tectonic thickening, as well as the good correlation between the timing of collision, high-P metamorphism and exhumation make the Nuuk region a very convincing candidate for being the oldest preserved Neoarchaean paired metamorphic belt (Fig. 10, and below). The data presented above clearly show how the various terranes originated from very different geodynamic settings during the 2.8–2.7 Ga terrane accretion event to be juxtaposed in the mid-crust during the latter stages of it (Fig. 10). Given the convergence of P–T paths, it appears likely that the conditions of juxtaposition can be taken to reflect a relatively stable thermal regime for the continental crust at this time. These conditions yield a geotherm of ∼30 ◦ C/km, confirming that burial of the Færingehavn terrane occurred along a

significantly depressed apparent geotherm (∼17.5–20 ◦ C/km) and metamorphism of the Tasiusarsuaq terrane involved a slightly elevated geotherm (35 ◦ C/km). Further evidence for Neoarchaean subduction in the Nuuk region has recently emerged from the preservation of 2.7 ± 0.3 Ga eclogitic xenoliths within kimberlites present the Tasiusarsuaq terrane on Nunataq-1390 (Fig. 1; Tappe et al., 2011). The eclogites have a highly refractory major and trace element geochemistry, suggesting that they represent residues of TTG melts extracted from a basaltic precursor. In addition, elevated garnet ␦18 O values and negative Eu-anomalies point to seafloor-altered oceanic crust as the most likely protolith (Tappe et al., 2011). Despite the recognition of a paired metamorphic belt in the Nuuk region, Neoarchaean crustal convergence apparently did not result in the extreme thickening observed in many younger accretionary orogenic belts, and the high-P rocks preserved in the Færingehavn terrane and Simiutat supracrustal sequence at best straddle the eclogite or high-P amphibolite stability fields (Fig. 10). The rocks record an apparent geothermal gradient that is significantly warmer than that of modern subduction environments which may be due to the generally higher mantle temperatures in the Archaean. The apparent geotherm is, however, depressed by 10–13 ◦ C/km relative to the background, which is comparable to that of modern subduction environments (e.g. Molnar and England, 1990; Peacock, 1996). 7.3. A refined tectonic model Our proposed tectonic model is a refinement of the work of Nutman and Friend (2007), and involves the southwards subduction of the Faeringehavn terrane and Simiutat supracrustal sequence (lower plate) underneath the Tre Brødre and Tasuisarsuaq terrranes (upper plate) (Fig. 11a and b). The earliest record of subduction-related igneous activity in the Tasiusarsuaq terrane is reflected by the intrusion the ca. 2973 ± 28 Ma Fiskenæsset Complex (Escher and Myers, 1975; Polat et al., 2009, 2010). The intrusion of the complex was followed by voluminous TTG magmatism

36

A. Dziggel et al. / Precambrian Research 242 (2014) 22–38

Fig. 11. (a and b) Sketches illustrating the proposed tectonic evolution of the Nuuk region. See text for discussion.

between 2920 and 2820 Ma (Fig. 11a; Compton, 1978; Crowley, 2002; Friend and Nutman, 2001; McGregor et al., 1991; Næraa and Scherstén, 2008; Nutman and Friend, 2007; Schjøtte et al., 1989; Kokfelt et al., 2011). The younger TTG ages overlap with the deposition of the ∼2840 Ma felsic volcaniclastic rocks structurally overlying the Faeringehavn terrane (Friend et al., 1996), as well as the intrusion of the ∼2825 Ma Ikkattoq gneiss in Tre Brødre terrane (Fig. 11a; Crowley, 2002; Nutman and Friend, 2007). Thus, the data indicate a younging of igneous activity from south (Tasiusarsuaq terrane) to north (Tre Brødre terrane; Fig. 11a), and we interpret this northwards shift of the centre of igneous activity to be due to slab rollback, slab breakoff or backward movement of subduction before final accretion. In a plate-tectonic scenario, one can therefore think of the Tre Brødre terrane as reflecting the leading edge of the Tasiusarsuaq terrane (then the centre of igneous activity) or as a newly formed island arc between the Tasiusarsuaq and Faeringehavn terranes. We prefer the former because at the time of and following the intrusion of the Ikkattoq gneiss (between 2825 and 2800 Ma), the Tasiusarsuaq terrane underwent medium-pressure granulite facies metamorphism (Crowley, 2002; Kolb et al., 2012; Dziggel et al., 2012). The results presented here further indicate ongoing ignimbrite formation in the Tre Brødre terrane at ca. 2820 Ma (Fig. 9b), suggesting that the felsic volcaniclastic rocks in the Tre Brødre terrane reflect the volcanic equivalents of the Ikattoq gneiss. After volcanic activity ceased, the Tasiusarsuaq terrane was exposed to a prolonged period of near-isobaric cooling to a more stable geotherm (Dziggel et al., 2012). The amphibolite facies reworking may have begun as early as 2770 Ma, and lasted for at least 50 Ma until collision with the Faeringehavn terrane (Fig. 12a and b; Kolb et al., 2012). The isobaric cooling and lack of exhumation of the

Tasiusarsuaq terrane mean that the granulite facies metamorphism likely occurred in a hinterland setting where the rocks could cool after TTG plutonism had ceased. The conditions and timing of the peak of amphibolite facies metamorphism in the Tre Brødre terrane are similar to the conditions of amphibolite facies reworking in the Tasiusarsuaq terrane, consistent with our interpretation that the Tre Brødre terrane represents the leading edge of the Tasiusarsuaq terrane, prior to, and during final collision. The earliest record of Neoarchaean metamorphism in rocks of the lower plate is given by rare metamorphic zircon ages of 2740–2760 Ma in aluminous gneisses of the newly named Simiutat supracrustal sequence (Nutman and Friend, 2007). This age is remarkably similar to the age of metamorphism in the Tre Brødre terrane, and suggests that it marks some point on the burial path of lower plate rocks reflected by the Faeringehavn terrane and Simiutat supracrustal sequence (Fig. 11a and b). The subsequent peak of high-P metamorphism and in situ partial melting correlates with terrane amalgamation, as indicated by the age of granite sheets intrusive into all three terranes (Friend et al., 1996; Nutman and Friend, 2007). After this, the Færingehavn terrane was rapidly exhumed, with 12–15 km of uplift occurring within error of the overlapping U–Pb zircon ages. This corresponds to a conservative average uplift rate on the order of 1–10 mm/a, similar to those determined for exhumation of the high-grade parts of the Mesoarchaean Barberton greenstone belt (Diener et al., 2005), and comparable to uplift rates from younger orogenic terranes (e.g. Abbott and Silver, 1997). Willett (2010) estimated that erosion-controlled exhumation rates in orogenic belts are in the range of 0.5–2 mm/a. The response of the middle crust to such erosion rates is estimated via depth-time data from metamorphic rocks to have similar values (0.5–1 mm/a; Berger et al., 2011; Malusà et al., 2011). By contrast, tectonic exhumation of high-pressure rocks by reverse flow in a subduction channel is more rapid, approximately 10–30 mm/a (e.g. Rubatto and Hermann, 2001; Rubatto et al., 2011). The uplift rates estimated for the Faeringehavn terrane and Simiutat supracrustal sequence correspond to the lower end of this range, and are, thus, considerably higher than erosion-controlled exhumation.

8. Conclusions In conclusion, we have demonstrated that terrane amalgamation in the Nuuk region during the Neoarchaean had much in common with modern accretionary processes. Specifically, accretion involved the juxtaposition of terranes with distinctly depressed and elevated apparent geotherms and contrasting metamorphic histories, with the estimated rates of crustal recovery also being comparable to modern-day examples. However, despite the TTG record in the Tasiusarsuaq and Tre Brødre terranes indicating that convergence could have been as long-lived as 250 Ma (Fig. 11), the lack of the extreme P–T conditions typical of modern accretionary environments suggests that not all aspects of Neoarchaean tectonics can be directly compared to modern examples. At face value, the observation that long-lived convergence did not result in significant burial can be taken to mean that Archaean convergence rates were much slower than today, perhaps due to a much weaker slab pull. However, this does not fit with a hotter and more vigorously convecting Archaean mantle. Instead, we speculate that the style of convergence in the Nuuk region indicates that subduction was inherently inefficient and characterised by weak slab pull, coupled to frequent slab break-off and rollback (Fig. 11; Kisters et al., 2012). This is consistent with numerical models showing that the Archaean oceanic crust was rheologically weak and unstable (van Hunen and van den Berg, 2008; Sizova et al., 2010), which is likely the key difference between Neoarchaean and modern accretionary environments.

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Acknowledgements This study was supported by the Geological Survey of Denmark and Greenland (GEUS) and the Bureau of Minerals and Petroleum (BMP) in Nuuk. Comments by two anonymous reviewers clarified and improved the manuscript, and are gratefully acknowledged. This paper is published with permission from the Geological Survey of Denmark and Greenland.

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