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

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Precambrian Research 242 (2014) 22– 38

Contents lists available at ScienceDirect

Precambrian Research

journa l h om epa ge : www.elsev ier .com/ locate /precamres

etamorphic record of accretionary processes during theeoarchaean: The Nuuk region, southern West Greenland

. Dziggela,∗, J.F.A. Dienerb, J. Kolbc, T.F. Kokfelt c

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

r t i c l e i n f o

rticle history:eceived 23 August 2013eceived in revised form2 December 2013ccepted 16 December 2013vailable online 27 December 2013

eywords:rchaean high-P metamorphismuality of thermal regimeslate tectonicsseudosection modelling

a b s t r a c t

The Nuuk region of southern West Greenland consists of several distinct terranes, including, from NWto SE, the Færingehavn, Tre Brødre, and Tasiusarsuaq terranes. Extensive high-pressure metamorphismand a clockwise P–T evolution of the Færingehavn terrane at ca. 2720–2710 Ma has been interpreted tobe a result of crustal thickening and thrusting of the Tasiusarsuaq terrane on top of the Tre Brødre andFæringehavn terranes. Pseudosection modelling constrains the P–T path for the Færingehavn terrane to becharacterised by initial burial, followed by heating at depth to peak conditions of ∼700 ◦C and 10 kbar andsubsequent isothermal decompression to conditions of 700 ◦C and 6 kbar. These data are consistent withthe results of previous studies, pointing to a relatively cool apparent geothermal gradient of <20 ◦C/kmduring prograde metamorphism. The tectonically overlying Tasiusarsuaq and Tre Brødre terranes recorda contrasting metamorphic history. Prior to final collision the Tasiusarsuaq terrane experienced granulitefacies metamorphism along a distinctly hotter apparent geothermal gradient of ∼35 ◦C/km, followed byprolonged isobaric cooling during NW-vergent thrusting to conditions of ∼700 ◦C and 6.5–7 kbar. Theseretrograde conditions are similar to the peak conditions of 620–660 ◦C and 6 kbar in the Tre Brødreterrane, which have been dated at 2751 ± 4 Ma. The contemporaneous existence of different thermalregimes and contrasting P–T paths, coupled to the strong structural evidence for regional-scale tectonicthickening 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 Brødreterrane 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 convergencerecorded in the Nuuk region does not seem to have resulted in deep subduction of crustal rocks, perhapsreflecting that Neoarchaean convergence rates were much slower than today or that subduction wasintermittent and inefficient.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

The processes that shaped the early continental crust aretill somewhat enigmatic and controversial (e.g. De Wit, 1998;amilton, 1998, 2011; Moyen et al., 2006; Bédard, 2006; Bédardt al., 2013). However, there appears to be an emerging consen-us that lateral, accretionary plate tectonic processes began inhe Meso- or Neoarchaean (Friend et al., 1988; Nutman et al.,

989; Brown, 2006, 2007, 2010; Moyen et al., 2006; Dziggel et al.,006; Nutman and Friend, 2007; Kisters et al., 2010; Næraa et al.,012), though some workers argue that the observed bulk crustal

∗ Corresponding author. Tel.: +49 2418095773.E-mail address: [email protected] (A. Dziggel).

301-9268/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.precamres.2013.12.010

shortening in many Archaean cratons can be equally explainedby mantle convection currents in the absence of lateral plate tec-tonics (Bédard et al., 2013). In most Archaean cratons, much ofthe details of these first tectonic events are cryptic and poorlyconstrained, partly due to the dearth of appropriate rocks in thegeological record, and partly because of the scarcity of detailedmetamorphic studies. For the younger Proterozoic and Phanerozoicparts of the rock record the metamorphic signature of accre-tionary tectonics is recognised as a thermal duality, where rockswith contrasting metamorphic histories are juxtaposed against oneanother (Miyashiro, 1961; Oxburgh and Turcotte, 1971; Brown,

2006, 2010). The juxtaposition involves a relatively high-pressure,low-temperature terrane that represents the converging and sub-ducting plate, and a relatively low-pressure, high-temperatureterrane that represents the overriding plate and volcanic arc.

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A. Dziggel et al. / Precambrian Research 242 (2014) 22– 38 23

uk reM

tcstNtb1aTNrriaspmietdadeFoa

2

ATt(t

Fig. 1. Geological map of the Nuodified from Escher and Pulvertaft (1995).

The Nuuk region of southern West Greenland exposes an excep-ionally well-preserved section through Archaean mid- to lowerontinental crust. The region has been recognised to consist ofeveral distinct and accreted terranes, making it the ideal loca-ion to examine early plate tectonic processes (Friend et al., 1988;utman and Friend, 2007; Næraa et al., 2012). The area includes

he lowermost Færingehavn terrane that is tectonically overlainy the Tre Brødre and Tasiusarsuaq terranes (Fig. 1; Friend et al.,988, 1996; Nutman et al., 1989; McGregor et al., 1991; Friendnd Nutman, 2005; Nutman and Friend, 2007; Kolb et al., 2012).he terranes are in fault contact with the Akia terrane in theW and the Kapisilik terrane in the E. Recent investigations have

ecognised that the metamorphic history of the Færingehavn ter-ane is characterised by relict high-pressure assemblages andsothermal decompression paths associated with terrane amalgam-tion at ca. 2715 Ma (Nutman and Friend, 2007). By contrast, thetructurally overlying Tasiusarsuaq terrane experienced medium-ressure granulite facies conditions, and the peak of regionaletamorphism was followed by a prolonged period of near-

sobaric cooling that eventually culminated in this amalgamationvent (Dziggel et al., 2012; Kolb et al., 2012). The recognition ofhese contrasting thermal regimes provides another line of evi-ence that terrane amalgamation occurred by horizontal tectonics,nd provides the opportunity to further elucidate and constrainetails of the processes involved. Here we use pseudosection mod-lling to determine new P–T estimates and P–T–t paths for theæringehavn and Tre Brødre terranes and examine the implicationsf these paths for the geodynamics of one of the oldest recognisedccretionary tectonic events on Earth.

. Geological setting

The Nuuk region of southern West Greenland is part of therchaean North Atlantic craton (Nutman and Friend, 2007; Fig. 1).

he area has a long tradition of Archaean geology research sincehe first comprehensive regional geological mapping by McGregor1973). The Nuuk region has a protracted Neoarchaean tectono-hermal history marked by polyphase amphibolite to granulite

gion, southern West Greenland.

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 conditionsof 7.5 kbar and 850 ◦C for the Tasiusarsuaq terrane, and foundthat peak metamorphism, dated by Crowley (2002) and Kolb et al.(2012) at ca. 2825–2800 Ma, was followed by an extended period ofnear-isobaric cooling to ∼700 ◦C and 6.5–7 kbar until final collisionat ca. 2720–2700 Ma. This is significantly different from the condi-tions and timing of high-pressure metamorphism in the Færinge-havn terrane (8–12 kbar and 700–750 ◦C at 2720–2700 Ma) deter-mined by Nutman et al. (1989). In addition, rocks of the Færinge-havn terrane evolved along a clockwise P–T path and experiencedisothermal decompression to conditions of ∼5 kbar and 700 ◦C dur-ing the later stages of accretion. Metamorphic conditions for the TreBrødre terrane that separates the Færingehavn and Tasiusarsuaqterranes have not been quantified, but U–Pb zircon and monaziteages suggest that the terrane experienced upper amphibolite faciesmetamorphism at 2720–2700 Ma (Crowley, 2002).

2.1. Færingehavn terrane and Simiutat supracrustal sequence

High-pressure rocks have been studied and sampled onthe islands of Qilanngarssuit and Simiutat (Figs. 1 and 2).The Færingehavn terrane is dominated by ca. 3850–3600 Matonalite–trondhjemite–granodiorite (TTG) gneisses of the ItsaqGneiss Complex (Nutman et al., 1996, 2004). The TTG gneisses areoverlain by, and in tectonic contact with, a sequence of supracrustalrocks that was previously referred to as the Malene supracrustalsequence (Chadwick and Nutman, 1979; Nutman and Friend,2007). Historically, however, the Malene supracrustal sequencealso included similar supracrustal units from other terranes suchas the Tre Brødre terrane, and is therefore no longer in use. We pro-pose here a new term, the Simiutat supracrustal sequence, to namesupracrustal rocks that structurally overly the Færingehavn ter-rane. The Simiutat supracrustal sequence includes meta-ultramafic

rocks, amphibolites derived from basaltic protoliths, and aluminousgneisses that originated from ca. 2840 Ma old volcanic protoliths(Friend et al., 1996). Bulk rock geochemical data indicate thatthe aluminous gneisses mainly consist of SiO2, Fe2O3(tot), Al2O3,

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

ilannM

a(ttfThArft

b

Fig. 2. Geological map of the island Qodified from Chadwick and Coe (1983b).

nd MgO, and have low concentrations of CaO, Na2O, and K2ODymek and Smith, 1990). This unusual bulk composition is similaro so-called cordierite-orthoamphibole gneiss, a rock type charac-erised by low-variance mineral assemblage considerably differentrom those typically found in metapelites (Diener et al., 2008).he contact between the supracrustal rocks and the Færinge-avn terrane is always tectonic and typically marked by foldedrchaean mylonites. The TTG gneisses and overlying supracrustalocks were metamorphosed to amphibolite- and, locally, granulite

acies conditions and record a polyphase tectono-metamorphic his-ory (Nutman and Friend, 2007).

Early Archaean medium-pressure granulite facies (M1) assem-lages and D1 fabrics are locally preserved in the gneisses of the

gaarssuit, showing sample localities.

Færingehavn terrane, with metamorphic and igneous zircon froma granulite-facies ferrodiorite recording ages between ca. 3400 and3650 Ma. This range suggests that the crystallisation of this rockwas followed by a complex Palaeo- to Eoarchaean metamorphichistory (Nutman and Friend, 2007).

M1 high-temperature metamorphism was succeeded by aca. 2.7 Ga high-pressure metamorphic event (M2), that hasbeen described at localities on southern Qilanngaarsuit andinner Ameralik (Fig. 1; Nutman and Friend, 2007). The high-

pressure assemblages are recorded in both the Færingehavnterrane and the overlying Simiutat supracrustal sequence. Typ-ical mineral assemblages in the Simiutat supracrustal sequenceinclude garnet + clinopyroxene + plagioclase + quartz ± hornblende

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A. Dziggel et al. / Precambrian Research 242 (2014) 22– 38 25

Fig. 3. Field occurrence of mafic supracrustal rocks and cordierite-orthoamphibole gneisses in the southern part of the island Qilanngarssuit. (a) Typical field occurrence ofmesosomes with irregularly shaped melt patches. View on foliation plane. (b) Melt escape structure in mafic supracrustal rock, view on foliation plane. The leucosome isa d plagc (d) Sit le.)

ia(gosbsbmvvefsciaDdSt

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of a pervasive S2 foliation at lower-pressure amphibolite faciesconditions of 5–7 kbar and 700–750 ◦C (Nutman and Friend,2007). Decompression is indicated by hornblende-plagioclasesymplectites rimming garnet in the high-P mafic rocks, and the

ssociated with coarse-grained garnet and diopside, and mainly consists of quartz anm large garnet porphyroblasts. Please note the presence of abundant leucosomes.o colour in this figure legend, the reader is referred to the web version of this artic

n mafic supracrustal rocks, and relict garnet + kyanite + rutilessemblages in the strongly retrogressed aluminous gneissesNutman and Friend, 2007). During field work, relict diopside-arnet assemblages in mafic supracrustal rocks have locally beenbserved as lenses and boudins in highly retrogressed and stronglychistose to mylonitic amphibolites on southern Qilanngarssuit,ut these high-P domains in mafic supracrustal rocks are best pre-erved at locality G03/38 of Nutman and Friend (2007; Fig. 3a and). The rocks contain abundant leucosomes with spectacular partialelting and melt segregation textures. The leucosomes occur in a

ariety of textural settings such as melt patches, veinlets and largereins, giving the rock a migmatitic appearance. Relict high-P min-ral assemblages dominated by garnet and kyanite have also beenound in strongly schistose to mylonitic aluminous gneisses in theouthern part of the island (Fig. 3c and d). The aluminous gneissesommonly contain thin, quartzo-feldspathic stringers, which arenterpreted to reflect former leucosomes. High-P metamorphismnd associated crustal thickening during M2 correlate with regional2 deformation (Nutman and Friend, 2007). Thrust imbricationuring D2 is marked by Itsaq gneisses structurally overlying theimiutat supracrustal sequence, which, in turn, structurally overly

he Itsaq gneisses (Fig. 4).

Exhumation of the Færingehavn terrane and Simiutatupracrustal sequence occurred immediately after peak meta-orphism, as zircon associated with the high- and low-pressure

ioclase. (c) Close-up of a cordierite-orthoamphibole gneiss containing up to severalllimanite associated with light-green kyanite. (For interpretation of the references

assemblages yield indistinguishable ages of ca. 2715 Ma (Nutmanand Friend, 2007). Exhumation is associated with the development

Fig. 4. Block diagram illustrating the structural geology of the island Qilan-ngaarssuit.

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evelopment of cordierite and sillimanite replacing garnet and/oryanite in the aluminous gneisses. The S2 foliation is axial planaro isoclinal F2a folds (Kolb et al., 2013), and the associated mineraltretching lineation plunges at shallow to moderate angles to the

and SW. The S2 foliation has been folded into upright, open toight F2b folds with wavelength of 0.5–3 km and E–W trendingxial traces (Fig. 4).

Subsequent deformation (D3) is marked by the developmentf km-scale synform–antiform pairs of upright, open to tightolds with north-trending axial traces, locally forming sheath foldsChadwick and Nutman, 1979; Fig. 4). The F3 fold axes mainlylunge at shallow to moderate angles to the S and SW, parallelo the L2 mineral stretching lineation (Fig. 4). Ductile shearinguring F3 flexural slip folding was associated with the formationf gold-bearing quartz veins at lower amphibolite facies condi-ions (540–620 ◦C and 4.5 ± 1 kbar; Koppelberg et al., 2013). Theiming of the D3 deformation is not well constrained; however,egmatites considered to have formed at ca. 2560 Ma (Chadwicknd Nutman, 1979; Nutman et al., 1989) crosscut the mineralisa-ion, bracketing the D3 event to between 2715 and 2560 Ma (Kolbt al., 2013).

.2. Tre Brødre terrane

The Tre Brødre terrane forms a ∼5 km wide thrust sheet thats situated between the Færingehavn and Tasiusarsuaq terranesFig. 1; Nutman and Friend, 2007). It mainly consists of felsicupracrustal rocks, TTG gneisses (the 2829 ± 11 to 2817 ± 9 Makkattoq gneiss; Friend et al., 2009; Nutman and Friend, 2007), and

dismembered, sheet-like gabbro-anorthosite complex (Chadwicknd Coe, 1983a). Mafic supracrustal rocks locally occur as thin lay-rs and boudins of amphibolite, with a maximum thickness of 10 m.he sequence has been intruded by pegmatite dyke swarms thatrosscut all lithological units. In contrast to the Færingehavn andasiusarsuaq terranes, the metamorphic grade of the Tre Brødreerrane never exceeded amphibolite facies conditions (Nutmant al., 1989). Typical mineral assemblages in the felsic supracrustalocks are quartz + garnet + biotite + orthoamphibole + cordierite ±illimanite ± plagioclase ± staurolite, whereas rocks with a maficulk composition contain hornblende + plagioclase + quartz ±arnet ± orthoamphibole (Kolb et al., 2009).

The general map pattern of the Tre Brødre terrane is markedy two different structural domains. The southern part of the ter-ane is exposed as a north-trending high-strain belt known as theæringehavn straight belt (Chadwick and Coe, 1983a). In the north,he supracrustal rocks and anorthosites define an open to tightpright fold pattern with southerly trending axial traces. This foldattern represents the regional D3 deformation. As in the Færinge-avn and Tasiusarsuaq terranes, three deformation events can beistinguished. The earliest fabric preserved is a locally developed S1oliation that has been preserved in the fold hinges of rare, rootlesssoclinal intrafolial folds in the Ikkattoq gneiss (Kolb et al., 2009).he main fabric in the Tre Brødre terrane is a pervasive, S to SEipping, S2 foliation that has been interpreted as a result of ter-ane amalgamation following the intrusion of the Ikkattoq gneissn the Neoarchaean (Nutman et al., 1989). The S2 foliation is definedy peak metamorphic minerals such as sillimanite and biotite, andhe associated mineral stretching lineation plunges at shallow to

oderate angles to the S and SE. Shear sense indicators such as–C fabrics point to a reverse sense of movement broadly to the N

nd NW. The formation of north-trending F3 folds during D3 and theubsequent intrusion of NE-trending pegmatite dyke swarms markhe end of tectonic and igneous activity in the Tre Brødre terraneKolb et al., 2009).

esearch 242 (2014) 22– 38

2.3. Tasiusarsuaq terrane

The Tasiusarsuaq terrane is the largest terrane in the Nuukregion. The terrane is dominated by TTG gneisses with numer-ous enclaves of mafic and ultramafic supracrustal rocks (Fig. 1;Chadwick and Coe, 1983a,b; Friend et al., 1996). The oldest igneousactivity in the Tasiusarsuaq terrane is recorded in the FiskenæssetComplex, a layered and highly dismembered anorthosite complexthat intruded into mafic supracrustal rocks in the southern partof the terrane (Escher and Myers, 1975; Polat et al., 2009). TheFiskenæsset Complex has a Sm–Nd errorchron age of 2973 ± 28 Ma,and has been interpreted to have formed in a supra-subductionzone geodynamic setting (Polat et al., 2009, 2010). The intru-sion of the complex was followed by the emplacement of TTGgranitoid rocks between 2920 and 2820 Ma, with a maximumbetween 2860 and 2840 Ma (Compton, 1978; Crowley, 2002; Friendand Nutman, 2001; McGregor et al., 1991; Næraa and Scherstén,2008; Nutman and Friend, 2007; Schjøtte et al., 1989; Kokfeltet al., 2011). The last igneous event in the Tasiusarsuaq ter-rane is marked by the emplacement of the ca. 2800 Ma Ilivertalikgranite/charnockite at granulite facies conditions (Pidgeon andKalsbeek, 1978).

Details of the metamorphic history and the structural evolu-tion of the Tasiusarsuaq terrane are given by Dziggel et al. (2012)and Kolb et al. (2012). In general, the structural evolution of theTasiusarsuaq terrane from the granulite facies peak to amphibo-lite facies cooling was associated with regional-scale thrusting. D1fabrics are only preserved in the fold hinges of later isoclinal F2folds, and are defined by granulite facies mineral assemblages. Thedominant thrust-related D2 fabric is a SE-dipping foliation formedunder amphibolite to granulite facies conditions. Shear sense indi-cators point to a reverse sense of movement broadly to the NW. Thesubsequent D3 deformation is marked by the formation of amphi-bolite facies N-trending strike-slip shear zones and upright F3 foldsduring E–W shortening (Kolb et al., 2012).

3. Analytical techniques

Five samples, including mafic supracrustal rocks, TTG gneissand aluminous gneiss from the Færingehavn and Tre Brødreterranes were chosen for a detailed petrological and mineral equi-libria study. The sample localities are shown in Figs. 1 and 2,and the mineral assemblages are summarised in Table 1. Wholerock major element data were obtained by X-Ray fluorescencespectroscopy using a Phillips PW 1400 energy-dispersive spec-trometer at the Institute of Mineralogy and Economic Geology atRWTH Aachen University. Electron microprobe analysis was car-ried out using a JEOL JXA-8900R electron microprobe, which isequipped with 5 wavelength dispersive spectrometers. The accel-eration voltage was 15 kV and the beam current 20 nA, a beamsize of 1–2 �m for ferromagnesian minerals and 10 �m for pla-gioclase was used. Only natural mineral standards were usedfor calibration. ZAF corrections were applied to the data, andrepresentative analyses are given in Tables 2–5. Ferric iron in ferro-magnesian minerals was estimated following the scheme of Droop(1987).

In order to constrain the age of volcaniclastic activity andregional metamorphism in the Tre Brødre terrane, laser ablationsector-field-inductively coupled plasma mass spectrometry (LA-SF-ICP-MS) U-Pb zircon dating was carried out on a sample ofaluminous gneiss (sample 515128, Fig. 1), following the meth-

ods outlined by Frei and Gerdes (2009) and Kolb et al. (2012).The laser was operated at a repetition rate of 10 Hz and nom-inal energy output of 45%, corresponding to a laser fluency of3.5 J cm−2 (see Table A.1 for detailed running conditions). All data

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

Sample Peak metamorphic phases Retrograde phases Remarks

515221 g + di + hb + q + pl + ilm + sph hb + pl hb–pl symplectites around g; leucosome-bearing515219 g + ky + q + bi + ged + ilm cd + sill + bi + mt; chl + pl + st + q + mu + mt Two different retrograde assemblages preserved

wcat2

i2

mssfitiePo2

i(tmaAi

TR

508445 g + hb + pl + bi + q + ilm None

515220 relict di and g hb + pl + q

515113 g + hb + anth + pl + q + ilm None

ere acquired with a single spot analysis on each individual zir-on grain with a beam diameter of 30 �m and a crater depth ofpproximately 15–20 �m. For the spot diameter of 30 �m and abla-ion times of 30 s the amount of ablated material approximates00–300 ng.

Supplementary material related to this article can be found,n the online version, at http://dx.doi.org/10.1016/j.precamres.013.12.010.

The ablated material was analysed on an Element2 (Ther-oFinnigan, Bremen) single-collector, double focussing, magnetic

ector ICPMS with a fast field regulator for increased scanningpeed. The total acquisition time for each analysis was 60 s, with therst 30 s used to measure the gas blank. Full analytical details andhe results for all quality control materials analysed are reportedn Table A.1 in the electronic supplementary material. Long termxternal reproducibility was monitored by repeated analyses of thelesovice zircon standard (Sláma et al., 2008), yielding an averagef 339.3 ± 0.7 Ma based on 238U/206Pb and 343.1 ± 1.8 Ma based on07Pb/206Pb ratios (N = 543 zircons, 2 standard deviations), which isn perfect agreement with reported value by ID-TIMS of 338 ± 1 MaAftalion et al., 1989). Plotting of concordia diagrams and calcula-ion of ages and their associated uncertainties from either weighted

eans or from unmixing of multiple age components were done inn off-line Excel sheet using Isoplot/Ex version 3.22 (Ludwig, 2003).ll uncertainties are reported at the 2� level or 95% confidence

nterval.

able 2epresentative electron microprobe data of garnet.

Sample 515221 515221 515221 515219 515219 515219Analysis 55 62 58 1 3 16

Location Rim Core Core Rim Core Core

Mineral g g g g g g

SiO2 37.94 38.003 38.17 37.44 37.435 37.414

TiO2 0.04 0.081 0.059 0 0 0

Al2O3 20.476 20.28 20.168 21.175 21.204 20.781

Cr2O3 0.042 0.066 0.069 0 0 0.006

FeO 28.345 26.462 27.232 35.008 33.351 33.967

MnO 1.465 1.047 1.207 0.993 0.584 0.698

MgO 2.328 3.296 3.228 4.247 6.156 5.098

CaO 9.748 10.621 10.008 1.083 0.84 1.533

Na2O 0 0.009 0 0.007 0.009 0.014

K2O 0 0 0.013 0.074 0.059 0

Total 100.384 99.865 100.154 100.027 99.646 99.513

TSi 6.012 6.001 6.026 5.99 5.936 5.976

TAl 0 0 0 0.01 0.064 0.024

AlVI 3.821 3.771 3.75 3.98 3.898 3.886

Fe3+ 0.13 0.191 0.165 0.02 0.156 0.128

Ti 0.005 0.01 0.007 0 0 0

Cr 0.005 0.008 0.009 0 0 0

Fe2+ 3.626 3.304 3.43 4.664 4.266 4.408

Mg 0.55 0.776 0.76 1.014 1.456 1.214

Mn 0.197 0.14 0.161 0.134 0.078 0.094

Ca 1.655 1.797 1.693 0.186 0.142 0.262

Na 0 0.003 0 0.002 0.002 0.004

Cations 16 16 16 16 16 16

No. Ox. 24 24 24 24 24 24

XAlm 0.60 0.55 0.57 0.78 0.72 0.74

XGrs 0.27 0.30 0.28 0.03 0.02 0.04

XPy 0.09 0.13 0.13 0.17 0.25 0.20

XSps 0.03 0.02 0.03 0.02 0.01 0.02

Leucosome-bearingLeucosome-bearingLeucosome-bearing

4. Petrology

4.1. Færingehavn terrane and Simiutat supracrustal sequence

4.1.1. High pressure domains in mafic supracrustal rocks (sample515221)

Sample 515221 was collected from a high-P boudin hostedby sheared and retrogressed amphibolites at locality G03/38 ofNutman 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 equilibratedwith the mineral phases of the leucosomes. The leucosomes aremade up of plagioclase, quartz, and minor biotite, pointing to abroadly trondhjemitic composition. Hornblende locally replacesdiopside, or occurs as hornblende-plagioclase symplectitic over-growths on garnet (Fig. 5b). This suggests that at least someof it formed during decompression via a reaction such as gar-net + plagioclase 1 + H2O = plagioclase 2 + hornblende.

Garnet is unzoned or slightly zoned, and locally contains inclu-sions of quartz, plagioclase, calcite, and, locally, chlorite. The com-position of larger grains is Alm55–58Grs27–29Pyr11–13Sps03; smallergrains have a composition of Alm57–60Grs23–25Pyr09–17Sps02–05. In

the smaller grains, pyrope decreases towards the rims, whereasgrossular, spessartine, and XFe increase (Table 2). The XFe inhornblende varies between 0.47 and 0.52 (Table 3). Plagioclaseoccurs in a number of textural settings and exhibits strong

508445 508445 515220 515220 515113 5151131 6 3 20 51 52Rim Core Core Rim Core Rimg g g g g g

37.416 37.622 38.196 38.165 37.726 37.5370 0 0.1 0.025 0 0

21.082 21.377 20.401 20.596 21.218 21.1460 0 0 0 0 0.002

28.856 27.909 27.128 28.589 31.142 32.2942.732 2.405 0.945 1.166 1.413 1.7575.561 5.957 4.378 3.372 5.4 4.3963.868 4.594 8.956 8.226 3.561 2.9740.008 0 0.006 0 0.004 00 0.004 0 0.009 0 0

99.550 99.868 100.11 100.148 100.464 100.106

5.919 5.917 5.996 6.035 5.928 5.9660.081 0.083 0.004 0 0.072 0.0343.847 3.839 3.768 3.836 3.854 3.9230.225 0.232 0.202 0.076 0.208 0.0990 0 0.012 0.003 0 00 0 0 0 0 03.593 3.251 3.359 3.705 3.885 4.1931.311 1.497 1.025 0.795 1.265 1.0410.366 0.283 0.126 0.156 0.188 0.2370.656 0.898 1.506 1.394 0.599 0.5060.002 0 0.002 0 0.001 0

16 16 16 16 16 1624 24 24 24 24 24

0.61 0.55 0.56 0.61 0.65 0.700.11 0.15 0.25 0.23 0.10 0.080.22 0.25 0.17 0.13 0.21 0.170.06 0.05 0.02 0.03 0.03 0.04

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

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

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A. Dziggel et al. / Precambrian Research 242 (2014) 22– 38 29

Table 3Representative electron microprobe data of amphibole.

Sample 515221 515221 515219 515219 515219 508445 508445 515220 515220 515113 515113 515113 515113Analysis 49 107 100 87 91 16 17 10 2 46 54 47 52Location Matrix Matrix Matrix Matrix Incl. Matrix Incl. Matrix Matrix Matrix Matrix Matrix MatrixMineral hb hb ged ged ged hb hb hb hb hb hb anth anth

SiO2 45.698 45.034 40.518 41.628 41.25 43.783 45.067 44.458 42.044 44.775 41.69 54.071 47.525TiO2 0.448 0.39 0.155 0.2 0.15 0.52 0.376 1.057 1.412 0.477 0.404 0.014 0.211Al2O3 10.101 11.377 18.913 18.384 18.54 14.926 13.099 11.785 13.395 13.472 16.986 1.516 11.005FeO 19.092 17.578 22.618 23.423 22.52 14.769 15.81 17.284 17.959 17.815 17.315 22.33 22.835Cr2O3 0.07 0.051 0 0 0 0.068 0.049 0.007 0 0.02 0 0 0MnO 0.081 0.086 0.186 0.181 0.17 0.278 0.29 0.082 0.151 0.199 0.113 0.344 0.385MgO 9.279 9.813 12.782 12.257 12.67 10.843 12.933 9.897 8.883 11.408 8.626 18.715 14.985CaO 11.97 11.823 0.243 0.374 0.32 10.977 9.154 11.518 11.532 8.632 10.918 0.297 0.538Na2O 1.014 1.264 1.812 1.839 1.94 1.365 1.09 1.371 1.622 1.046 1.429 0.061 0.978K2O 0.321 0.245 0 0.014 0.00 0.241 0.198 0.439 0.55 0.205 0.205 0.006 0.004Total 98.082 97.699 97.26 98.378 97.61 97.77 98.066 97.898 97.548 98.049 97.686 97.354 98.466

Si 6.808 6.699 6 6.131 6.10 6.419 6.54 6.619 6.33 6.563 6.184 7.871 6.922TAl 1.192 1.301 2 1.869 1.90 1.581 1.46 1.381 1.67 1.437 1.816 0.129 1.078CAl 0.581 0.692 1.298 1.32 1.33 0.996 0.778 0.685 0.705 0.888 1.151 0.13 0.81CCr 0.008 0.006 0 0 0.00 0.008 0.006 0.001 0 0.002 0 0 0CFe3+ 0.148 0.104 0.147 0 0.00 0.03 0.251 0 0.065 0.106 0.126 0 0CTi 0.05 0.044 0.017 0.022 0.02 0.057 0.041 0.118 0.16 0.053 0.045 0.002 0.023CMg 2.061 2.176 2.822 2.691 2.79 2.37 2.798 2.197 1.994 2.493 1.907 4.061 3.254CFe2+ 2.152 1.978 0.716 0.967 0.86 1.539 1.126 1.999 2.076 1.458 1.771 0.807 0.913BFe2+ 0.079 0.105 1.938 1.918 1.93 0.241 0.541 0.152 0.12 0.62 0.251 1.911 1.869BMn 0.01 0.011 0.023 0.023 0.02 0.035 0.036 0.01 0.019 0.025 0.014 0.042 0.047BCa 1.911 1.884 0.039 0.059 0.05 1.724 1.423 1.837 1.86 1.356 1.735 0.046 0.084ANa 0.293 0.365 0.52 0.525 0.56 0.388 0.307 0.396 0.474 0.297 0.411 0.017 0.276AK 0.061 0.046 0 0.003 0.00 0.045 0.037 0.083 0.106 0.038 0.039 0.001 0.001Cations 15.354 15.411 15.52 15.528 15.56 15.433 15.343 15.479 15.579 15.336 15.45 15.018 15.277

ccamitt(p

TR

No. Ox. 23 23 23 23 23 23

XFe 0.52 0.49 0.48 0.52 0.50 0.43

ompositional variation, ranging between An47 and An85. Plagio-lase inclusions in garnet are either oligoclase (XAn = 0.25–0.30), orndesine–labradorite (XAn = 0.48–0.60; Table 4). Plagioclase rim-ing garnet as coronas or as hornblende-plagioclase symplectites

s mainly bytownite (XAn = 0.65–0.85). Matrix plagioclase records

wo distinctly different compositions: the most abundant genera-ion is andesine (XAn = 0.46–0.50), whereas subordinate labradoriteXAn = 0.60–0.64) is also found. Andesine is interpreted to be theeak metamorphic generation as it is locally rimmed by labradorite.

able 4epresentative electron microprobe data of plagioclase.

Sample 515221 515221 515221 515221 515221 51522Analysis 41 77 80 97 86 102

Location Incl. Incl. Corona Corona Leucos. MatrixMineral pl pl pl pl pl pl

SiO2 56.391 61.853 51.428 47.427 52.525 56.34TiO2 0 0.03 0 0 0.035 0

Al2O3 28.11 24.187 31.617 34.039 30.767 28.47FeO 0.344 0.109 0.462 0.266 0.294 0.01MnO 0.025 0 0.001 0.036 0 0

BaO 0.033 0 0 0.026 0.016 0.04CaO 9.654 5.11 13.432 16.718 12.505 9.85Na2O 5.687 8.19 3.728 1.906 4.386 5.68K2O 0.08 0.024 0.018 0.023 0.002 0.04Total 100.324 99.503 100.686 100.441 100.53 100.46

Si 10.097 10.994 9.289 8.668 9.474 10.06Al 5.927 5.063 6.725 7.327 6.535 5.99Ti 0 0.004 0 0 0.005 0

Fe2+ 0.052 0.016 0.07 0.041 0.044 0.00Mn 0.004 0 0 0.006 0 0

Ba 0.002 0 0 0.002 0.001 0.00Ca 1.852 0.973 2.599 3.274 2.417 1.88Na 1.974 2.823 1.306 0.675 1.534 1.96K 0.018 0.005 0.004 0.005 0 0.01Cations 19.928 19.878 19.993 20 20.011 19.92No. Ox. 32 32 32 32 32 32

XAn 0.48 0.26 0.67 0.83 0.61 0.49

23 23 23 23 23 23 23

0.37 0.49 0.52 0.45 0.51 0.40 0.46

Labradorite is also the dominant plagioclase in the leucosomes, sug-gesting that it crystallised in conjunction with the melt. Diopsidehas an XFe of 0.34–0.40 and an Al content of up to 0.17 a.p.f.u. anda 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 assemblagesare best-preserved in sample 515219. The rock consists of garnet,

1 515219 508445 515220 515220 515113 51511330 6 26 32 74 77

Matrix Matrix Corona Leucos. Core Rimpl pl pl pl pl pl

1 60.051 55.062 58.148 56.9 55.073 50.0630 0 0.022 0 0 0

9 25.418 28.963 27.371 27.822 29.396 31.6422 0.125 0.153 0.116 0.101 0 0.047

0 0.024 0 0 0 07 0.019 0.037 0 0.077 0 0.0571 5.574 10.466 8.234 9.119 11.057 14.3385 7.995 5.436 6.466 6.037 5.159 3.2868 0.023 0.028 0.11 0.073 0.035 0.0233 99.205 100.169 100.467 100.129 100.72 99.456

3 10.74 9.899 10.338 10.185 9.846 9.17 5.354 6.132 5.731 5.865 6.189 6.826

0 0.003 0 0 02 0.019 0.023 0.017 0.015 0 0.007

0 0.004 0 0 0 03 0.005 0.003 0 0.005 0 0.0045 1.068 2.016 1.568 1.749 2.118 2.8149 2.773 1.895 2.229 2.095 1.788 1.1671 0.005 0.006 0.025 0.017 0.008 0.0056 19.964 19.978 19.911 19.936 19.949 19.993

32 32 32 32 32 32

0.41 0.52 0.41 0.45 0.54 0.71

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

Table 5Representative electron microprobe data of diopside, biotite, chlorite, staurolite, and cordierite.

Sample 515221 515221 515220 515219 515219 515219 515219 515219 515219 515219 515219Analysis 40 41 16 2 33 6 1 44 46 12 103Location Matrix Matrix Matrix Incl. Matrix Matrix Incl. Incl. Incl. Matrix MatrixMineral di di di bi bi chl chl chl chl st crd

SiO2 50.931 52.105 51.842 36.135 37.133 24.272 25.305 26.319 23.878 27.406 48.543TiO2 0.31 0.078 0.133 1.162 1.209 0.059 0.074 0.035 0 0.186 0Al2O3 2.593 0.846 1.084 18.559 18.737 25.208 24.471 24.719 26.594 57.008 34.904FeO 12.748 12.739 11.576 15.029 13.635 15.631 16.819 13.371 32.083 12.686 5.079Cr2O3 0.054 0.022 0.004 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.MnO 0.15 0.157 0.139 0.01 0 0.007 0.026 0.009 0.038 0.04 0.022NiO 0.044 0 0 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.MgO 10.018 10.888 11.652 15.104 16.288 21.456 21.121 23.276 4.446 2.396 9.781CaO 23.347 23.38 23.654 0.022 0 0.015 0 0.028 0.097 0 0.032Na2O 0.325 0.172 0.278 0.566 0.583 0.009 0.01 0.001 0.011 0.035 0.315K2O 0.001 0.019 0 8.159 8.197 0.022 0 0.009 0 0 0ZnO n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.903 n.a.Total 100.52 100.40 100.36 94.756 95.782 86.679 87.826 87.767 87.147 100.66 98.676

TSi 1.928 1.974 1.951 5.379 5.42 4.912 5.076 5.177 5.207 3.691 4.921TAl 0.072 0.026 0.048 2.621 2.58 3.088 2.924 2.823 2.793AlVI 0.044 0.011 0 0.632 0.641 2.92 2.856 2.903 4.037Al 0.116 0.037 0.048 3.253 3.221 6.008 5.78 5.726 6.83 9.048 4.167Ti 0.009 0.002 0.004 0.13 0.133 0.009 0.011 0.005 0 0.019 0Fe3 0.033 0.023 0.062 0 0 0 0 0 0 0 0Cr 0.002 0.001 0 0 0 0 0 0 0 0 0Mg 0.565 0.615 0.654 3.352 3.544 6.473 6.316 6.825 1.445 0.481 1.478Ni 0.001 0 0 0 0 0 0 0 0 0 0Fe2 0.371 0.381 0.302 1.871 1.664 2.645 2.821 2.199 5.851 1.429 0.431Mn 0.005 0.005 0.004 0.001 0 0.001 0.004 0.001 0.007 0.005 0.002Ca 0.947 0.949 0.954 0.004 0 0.003 0 0.006 0.023 0.000 0.003Na 0.024 0.013 0.02 0.163 0.165 0.004 0.004 0 0.005 0.009 0.062K 0 0.001 0 1.549 1.526 0.006 0 0.002 0 0.000 0Zn 0 0 0 0 0 0 0 0 0 0.090 0Cations 4 3.999 4 15.70 15.67 28.061 28.012 27.941 27.368 14.771 11.064

kcnasiaggcmcpcaftbawpcd

ArtpTbar

No. Ox. 6 6 6 22 22

XFe 0.40 0.38 0.32 0.36 0.32

yanite, cordierite, quartz, biotite, gedrite, sillimanite, ilmenite,hlorite, staurolite, plagioclase, muscovite, and magnetite. Gar-et forms up to 1 cm large lens-shaped porphyroblasts that areligned along a well-developed S2 foliation, and contains inclu-ions of chlorite, quartz, gedrite, kyanite, and biotite. The matrixs dominated by quartz, gedrite, cordierite, ilmenite, magnetite,nd minor amounts of biotite and sillimanite. Cordierite replacesarnet, kyanite, and gedrite along the rims (Fig. 5d), or forms fine-rained aggregates that are aligned along the foliation. Locally,hlorite, plagioclase, staurolite and magnetite replace both the peaketamorphic minerals such as garnet and kyanite and retrograde

ordierite (Fig. 5c, d), suggesting a second retrograde metamor-hic event following cordierite crystallisation. An assemblage ofhlorite, quartz, staurolite, and, locally, muscovite, also occurs as

fine-grained rosette-like intergrowth which replace all earlier-ormed minerals along rims and fractures (Fig. 5c, d). Based onhese textures, we infer that the peak metamorphic mineral assem-lage is garnet + kyanite + quartz + biotite + gedrite + ilmenite. Thessemblage is interpreted to have formed in equilibriumith melt (Fig. 3c). Cordierite and sillimanite are inter-reted to form an early retrograde assemblage, with thehlorite + staurolite + plagioclase ± muscovite assemblage forminguring later retrogression.

Garnet in sample 51219 has a composition oflm71–78Grs02–05Pyr19–25Sps01–02. The cores of the grains areelatively unzoned (Alm71–74Grs02–03Pyr21–25Sps01–02), whereashe outermost rims are enriched in almandine and depleted inyrope component (Alm74–78Grs03–05Pyr19–22Sps01–02; Table 2).

he composition of gedrite is relatively constant; the XFe variesetween 0.48 and 0.52 (Table 3). Gedrite inclusions in garnet have

Na content of 0.56–0.57 a.p.f.u., whereas gedrite in contact withetrograde chlorite and staurolite is less sodic (0.52–0.53 a.p.f.u.).

36 36 36 36 23 18

0.29 0.31 0.24 0.80 0.71 0.23

Plagioclase is oligoclase with XAn of ∼0.4 (Table 4). Biotite inclu-sions in garnet are relatively Fe-rich (XFe ∼ 0.36–0.37), whereasmatrix biotite has an XFe between 0.29 and 0.34. Chlorite recordsa wide range in composition. Most matrix grains have an XFebetween 0.27 and 0.29. Chlorite inclusions in kyanite and chloritereplacing biotite inclusions in garnet have an XFe between 0.31 and0.32 (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 to0.74 and Zn content of 0.08–0.11 a.p.f.u. Cordierite is Mg-rich withXFe 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 Simiu-

tat Island south of Qilanngarssuit (Fig. 1). The rock is medium-to coarse-grained, and consists of garnet, hornblende, plagioclase,biotite, quartz and ilmenite (Fig. 5e). It contains abundant leuco-somes that are oriented parallel to a well-developed S2 foliation.Garnet forms up to 2 cm large, poikiloblastic crystals that containnumerous inclusions of quartz, hornblende, plagioclase, biotite, andilmenite. The matrix is dominated by plagioclase and hornblende.Garnet is slightly zoned, with the cores having a compositionof Alm55–59Grs11–16Pyr23–26Sps04–06. Almandine and spessartinecontents increase towards the rims, whereas grossular and pyropedecrease (Alm61–64Grs9–12Pyr19–22Sps06–08; Table 2). Matrix horn-blende has an XFe of 0.41–0.45 (Table 3). Hornblende inclusions ingarnet and some of the rims of matrix hornblende record slightlylower values (XFe = 0.37–0.38). Plagioclase is unzoned, and its com-position varies between An49 and An53 (Table 4).

4.1.4. Retrograde amphibolites (sample 515220)Sample 515220 is from the schistose to mylonitic amphi-

bolite surrounding the high-pressure lenses on southern

Page 10

rian R

Qssa(lt(gtamd(atgschA(

Fca

A. Dziggel et al. / Precamb

uilanngaarssuit (Fig. 2), and is therefore interpreted to be atrongly retrogressed equivalent of sample 515221. The rock con-ists of garnet, hornblende, quartz, ilmenite, plagioclase, diopside,nd titanite and contains abundant, coarse-grained leucosomesFig. 5f). Garnet commonly contains inclusions of quartz, andocally, titanite, and ilmenite, and is slightly zoned. The cores ofhe grains have compositions of Alm52–60Grs25–31Pyr11–18Sps02–03Table 2). Although the absolute values vary between individualrains, the grossular and pyrope contents generally decreaseowards the rims, whereas almandine increases. The rims have

composition of Alm57–62Grs23–25Pyr11–16Sps02–03. Hornblendeainly occurs as large grains surrounding garnet, and locally

isplays symplectitic intergrowths with plagioclase at garnet rimsFig. 5f). Hornblende often shows exsolution lamellae of ilmenite,nd has an XFe of 0.48–0.53 (Table 3). Plagioclase either occurs inhe leucosomes or as thin (a few micrometre thick) rims aroundarnet (Fig. 5f), but is otherwise absent from the matrix of theample. Plagioclase in the garnet coronas and symplectites has a

omposition of An41 to An52, whereas plagioclase in the leucosomeas XAn of 45–47 (Table 4). Diopside has an XFe of 0.28–0.34, anl content of up to 0.155 a.p.f.u., and only contains traces of Na

Table 5).

ig. 6. Calculated pseudosections for rocks from the Færingehavn terrane and Simiutat suordierite–orthoamphibole gneiss sample 515219; (c) garnet-bearing TTG gneiss sample

nd garnet abundance. Assemblages referred to in the text are highlighted in bold.

esearch 242 (2014) 22– 38 31

4.2. Mafic supracrustal rocks from the Tre Brødre terrane (sample515113)

The sample investigated was taken from a ∼1 m thick boudinhosted by aluminous felsic schists in the northern part of theTre 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). Garnetforms up to ∼1 cm large porphyroblasts that contains inclusionsof quartz, ilmenite, hornblende, anthophyllite, plagioclase, and,locally, chlorite and biotite. The matrix is dominated by plagioclaseand amphiboles, which define a weakly-developed foliation.

Garnet in sample 515113 is slightly zoned (Table 2).The cores of the grains have a composition ofAlm0.65–0.68Grs0.09–0.12Pyr0.19–0.22Sps0.03–0.04. Almandine increasestowards the rims, whereas grossular and pyrope decrease. The rimhas a composition of Alm0.66–0.70Grs0.07–0.10Pyr0.16–0.19Sps0.03–0.04.The XFe in hornblende is between 0.42 and 0.51, whereas antho-

phyllite records XFe values between 0.40 and 0.47 (Table 3). Plagio-clase is complexly zoned and its composition varies between An51and An83. Most grains are relatively Ca-poor in their core (An50–60)and XAn increases towards the rims (Table 4). The composition of

pracrustal sequence. (a) Mafic granulite sample 515221, with isopleths for XAn. (b)508445; (d) retrograde amphibolite sample 515220, with isopleths for hornblende

Page 11

32 A. Dziggel et al. / Precambrian R

Ft

mttIittr

5

NMi(T(2saheeasP2(R(

coii2Tcamoattwsa

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

ost of these rims is between An65 and An75, although values of upo An82 have been analysed. Other grains display an inverse zona-ion, with a Ca-rich core (An75–82) and Ca-poorer rims (An60–70).t is not immediately clear what the cause for this complex zonings, but both types of zoning occur in contact with, or close to,he leucosomes, suggesting plagioclase grew in equilibrium withhe melt. The complexly zoned grains may therefore reflect ae-equilibration of plagioclase due to changes in melt composition.

. Phase diagram modelling

Modelling calculations were performed in thea2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCF-ASHTO) chemical system for mafic supracrustal rocks and

n the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3NCKFMASHTO) system for the cordierite-orthoamphibole- andTG gneisses. Calculations used THERMOCALC version 3.33Powell and Holland, 1988, updated June 2009) and the November003 updated version of the Holland and Powell (1998) dataet (file tc-ds55.txt). The phases considered in the calculationsnd references to the activity-composition models used are:ornblende, cummingtonite, gedrite and anthophyllite (Dienert al., 2007, updated by Diener and Powell, 2012), diopside (Greent al., 2007, updated by Diener and Powell, 2012), garnet, biotitend silicate melt (White et al., 2007), cordierite, epidote andtaurolite (Holland and Powell, 1998), plagioclase (Holland andowell, 2003), orthopyroxene and spinel-magnetite (White et al.,002), muscovite-paragonite (Coggon and Holland, 2002), chloriteHolland et al., 1998) and ilmenite-hematite (White et al., 2000).utile, titanite, the aluminosilicates, quartz and aqueous fluidH2O) are pure end-member phases.

Bulk rock compositions determined by X-ray fluorescence wereonverted to the model systems by disregarding the minor amountsf MnO, K2O and Cr2O3 in the mafic bulk compositions, and convert-ng ∼20% of total Fe to Fe3+, consistent with the observations thatlmenite is the main oxide phase in these rocks (Diener and Powell,010). For the modelling of the cordierite-orthoamphibole- andTG gneiss, Mn and Cr were disregarded and 10% of the total Fe wasonverted to Fe3+ in samples in which magnetite was either rare orbsent. Even though these rocks exhibit evidence of partial melting,elt was only explicitly included in calculations for the cordierite-

rthoamphibole- and TTG gneisses as the current melt model is notppropriate for melting of mafic rocks. All samples were assumedo be fluid-saturated during prograde-to-peak metamorphism and

he fluid content for suprasolidus parts was constrained from theet solidus. The bulk compositions used to construct the pseudo-

ections are presented in Table 6, and the calculated pseudosectionsre presented in Figs. 6 and 7.

esearch 242 (2014) 22– 38

5.1. Færingehavn terrane and Simiutat supracrustal sequence

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 quartzoccurs 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 Pand lower T, whereas titanite is lost to higher T and garnet to lowerP. Plagioclase is calculated to have a composition of An50–An56 atthese conditions, broadly consistent with the analysed composition(An46–An50) of matrix plagioclase in this sample.

Calculated isopleths for plagioclase composition show a largevariability in anorthite content within the lower-temperatureepidote-bearing assemblages (stippled lines, Fig. 6a). It is likelythat the more albitic (An25–An30) plagioclase inclusions foundin garnet are a reflection of these conditions. The intersectionof the garnet-in phase boundary and albitic plagioclase contoursoccurs at 10–11 kbar at approximately 640 ◦C and likely reflect theconditions under which garnet was introduced during progrademetamorphism (Fig. 6a). The more anorthite-rich (∼An50) plagio-clase inclusions that also occur in garnet could have been trappedat similar pressure while the rock was heated to peak conditions(Fig. 6a). Labradorite (An60–An64) occurs in the leucosome and asplagioclase associated with garnet breakdown. The pseudosectionshows that garnet breakdown occurs through decompression at9 kbar. Plagioclase at these conditions is calculated to be the mostanorthite-rich for the P–T range considered (An56; Fig. 6a), but tohave lower anorthite content than that observed for the garnetbreakdown textures (An65–An85). The mismatch between the cal-culated and observed compositions could either be a reflection ofcompositional domains that formed and achieved only local chem-ical equilibrium during retrogression, or can be explained by thisplagioclase generation having formed through a melting reactionthat cannot be quantitatively modelled. Both of these options arelikely, as plagioclase that is rimming garnet shows a large com-positional variability, whereas plagioclase in the leucosome has acomposition 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 largevariety of mineral assemblages and a number of low-variance fieldsat low pressure (Fig. 6b). The solidus occurs at 660–680 ◦C at highpressure, but shifts to higher temperatures up to 750 ◦C at lowerpressure. The inferred peak assemblage of garnet, gedrite, kya-nite, biotite, quartz, ilmenite and silicate melt occurs between 8and 11.5 kbar and 670–750 ◦C (Fig. 6b). Gedrite is lost to higherT, whereas sillimanite, magnetite and cordierite are introduced tolower P. The compositional isopleths in the stability field of the peakassemblage (not shown for clarity) are consistent with the mineralcompositions analysed; however, the slopes of most isopleths aregenerally steep and are therefore not useful for a tighter pressureconstraint. Nevertheless, the section shows that the replacement ofpeak metamorphic garnet and kyanite by cordierite and sillimaniterequires decompression to conditions at or below 6 kbar (Fig. 6b).The second retrograde assemblage of staurolite, plagioclase, mag-netite, chlorite, and, locally, muscovite suggests subsequent coolingto ∼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 atpressure above 7–8 kbar. The inferred peak assemblage of garnet,hornblende, biotite, plagioclase, quartz, ilmenite and silicate melt

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A. Dziggel et al. / Precambrian Research 242 (2014) 22– 38 33

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

SiO2 TiO2 Al2O3 FeO MgO CaO Na2O K2O O H2O

Fig. 7a (515221) 54.90 0.705 7.735 9.135 8.291 15.728 2.611 – 0.894 ExcessFig. 7b (515219) 79.10 0.319 6.106 10.060 4.263 0.349 0.239 0.070 0.498 1a

Fig. 7c (508445) 50.95 1.020 10.701 11.083 7.976 10.444 2.830 0.133 0.762 4.1a

Fig. 7d (515220) 49.19 1.680 7.671 17.926 9.184 11.855 0.727 – 1.768 Excess1

o9tfow2bsviHtt

5

hdPsspptgrfaoitp

5

haq6atbtl(r

6

a4e

Fig. 8 (515113) 53.55 2.532 8.586 14.472

a H2O was taken as in excess for the subsolidus part of the diagram.

ccurs over a large stability range, extending from the solidus at kbar and 700 ◦C to higher and lower pressure with increasingemperature (Fig. 6c). Minor diopside (<0.5 vol.%) is calculated toorm part of the peak assemblage at these conditions, but is notbserved in the rock. This discrepancy is likely due to uncertaintyithin the activity-composition relations (e.g. Diener and Powell,

012). Titanite bounds the assemblage to higher pressure, whereasiotite and garnet are lost to higher temperature and lower pres-ure, respectively. Calculated plagioclase compositions (not shown)ary from An52 at 8 kbar to An43 at 11 kbar for the temperatures ofnterest, consistent with the analysed compositions in this sample.owever, isopleths for grossular content in garnet are sensitive to

emperature, and are calculated to have higher values (Grs0.3–0.35)han those analysed (Grs0.1–0.15).

.1.4. Retrograde amphibolites (sample 515220)The pseudosection for this sample is dominated by large,

igh-variance fields, and the inferred peak assemblage of garnet,iopside, hornblende, quartz and ilmenite is stable over a large–T range above 7.5 kbar and 660 ◦C (Fig. 6d). This range is con-istent with the better-constrained estimates obtained from otheramples (see above). However, this sample can be expected torovide information on retrograde conditions and the retrogradeath. Retrogression in this sample is characterised by the consump-ion of garnet, the introduction of plagioclase as coronas aroundarnet, and an increase in the abundance of hornblende, whicheplaces peak metamorphic diopside (Fig. 5f). Calculated isoplethsor the abundance of garnet and hornblende have shallow slopesnd are dominantly dependent on pressure (Fig. 6d), such that thebserved textures are developed by decompression. Plagioclase isntroduced at 7–7.5 kbar for near-peak temperatures, suggestinghat retrogression of this sample records near-isothermal decom-ression to conditions below at least 7 kbar.

.2. Mafic rocks from the Tre Brødre Terrane

The pseudosection for sample 515113 is characterised by large,igh-variance hornblende-bearing fields, but the inferred peakssemblage of garnet, hornblende, orthoamphibole, plagioclase,uartz and ilmenite occurs over a small range, between 620 and60 ◦C at 6 kbar (Fig. 7). This field is bound by orthoamphibole-bsent assemblages to higher pressure, the introduction of chloriteo lower T, and garnet-absent, cummingtonite-bearing assem-lages to lower P (Fig. 7). Calculated mineral compositions matchhose analysed in this sample, with garnet composition calcu-ated as Alm0.74Grs0.11Pyr0.15. Plagioclase is calculated to be Ca-richAn82), similar to the analysed composition of some plagioclaseims.

. Geochronology

One sample of aluminous gneiss (sample 515128) was sep-rated for zircons and analysed by LA-SF-ICP-MS. The up to00 �m × 100 �m large zircons are generally an- to subhedral andlongate, with high aspect ratios of 1:3–1:4 (Fig. 8). The grains

0.525 7.934 1.079 – 1.385 Excess

usually constitute a partially resorbed BSE-dark/U-poor core over-grown or replaced by a variably broad rim zone that is relativelyBSE-bright/U-rich. The core domains are often intensely crackedcompared to the rims and contain few inclusions of a Fe–Mg–Al–Simineral (presumably amphibole). The rim domains show relativelyabundant inclusions of albite, quartz and monazite; xenotime isseen as up to 60 �m large grains intergrown with rim zircon (Fig. 8).The texture of the rim is generally nebulitic, heterogeneous (withrespect to BSE-brightness), suggesting an uneven distribution ofU, Th and Hf throughout the rim zones. In contrast to the rims,the cores are more homogeneous in BSE-colour. Shown examplesdemonstrate (from left to right) increasing degrees of resorbtion ofcores (Fig. 8).

In the Terra–Wasserburg diagram (Fig. 9a) the respective rimand core domains correspond to two distinct age populations withdifferent Th/U ratios and U concentrations (Fig. 9a and b). The 19core domain analyses constitute an approximate age of ca. 2.80 Ga;however based on the U ppm vs. Pb–Pb age diagram (Fig. 9b), thereis evidence for mixing of age domains in some of the analyses, sug-gesting 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 Færingehavnterrane and Simiutat supracrustal sequence overlap to constrainpeak metamorphic conditions for this terrane at ∼10 kbar and700 ◦C (Fig. 10). The shape of the prograde path can be constrainedfrom the plagioclase inclusions in garnet from sample 515221,which indicate that prograde metamorphism likely intersectedconditions of 10.5 kbar at 640 ◦C (an apparent geothermal gradi-ent of 17.5 ◦C/km) and maintained this pressure during heating topeak metamorphic conditions (Fig. 10).

Retrograde conditions for the Færingehavn terrane and Simi-utat supracrustal sequence are estimated at 6 kbar and ∼700 ◦C,indicating that initial retrogression was dominated by decom-pression of more than 4 kbar (Fig. 10). The later parts of theretrograde path are not as well-constrained, but appear to havebeen dominated by cooling with only minor attendant decom-pression. The P–T–t path for the Færingehavn terrane determinedhere is similar to that proposed by Nutman and Friend (2007),and is characterised by a high-pressure, clockwise trajectory withsignificant decompression subsequent to peak metamorphism.The peak and retrograde assemblages have indistinguishable agesof ca. 2715 ± 5 Ma (Nutman and Friend, 2007), indicating thatdecompression was rapid and followed immediately after peakmetamorphism.

By contrast, P–T conditions estimated for the Tre Brødre ter-rane are at much lower pressure but similar temperatures thanthe 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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34 A. Dziggel et al. / Precambrian Research 242 (2014) 22– 38

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) 207Pb/206Pb age versus U-concentration (ppm) of zircons in sample 515128, illustrating the presence oftwo different age domains.

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A. Dziggel et al. / Precambrian Research 242 (2014) 22– 38 35

F ic cona Dzig(

fptBp

rcgaTpct

7

tbatePrtcrNdiGtirafi

ig. 10. Contrasting P–T–t paths for various terranes of the Nuuk region. Metamorphre from this study, whereas those for the Tasiusarsuaq terrane (black stars) are from2012).

or this terrane are currently unknown. The 2751 ± 4 Ma zircon ageresented above indicate a slightly older age of metamorphism forhe Tre Brødre terrane; however, the peak conditions from the Trerødre terrane are comparable to those experienced during post-eak decompression in the Færingehavn terrane (Fig. 10).

Furthermore, the P–T path for the overlying Tasiusarsuaq ter-ane was dominated by slow, near-isobaric cooling from peakonditions of 7.5 kbar and 850 ◦C, reflecting an elevated apparenteotherm of ∼35 ◦C/km, to 6.5–7 kbar at 700 ◦C during reworkingt 2770–2720 Ma (Fig. 10; Kolb et al., 2009; Dziggel et al., 2012).herefore, the Færingehavn, Tasiusarsuaq and Tre Brødre terranesreserve distinctly contrasting P–T histories and P–T paths that allonverge at the same mid-crustal, upper amphibolite facies condi-ions of ∼6–7 kbar and 650–700 ◦C at ca. 2715 Ma (Fig. 10).

.2. Implications for Archaean tectonics

One of the hallmarks of one-sided subduction is a duality ofhermal environments, i.e. the presence of “penecontemporaneouselts of contrasting type of metamorphism that record differentpparent thermal gradient, one warmer and the other colder, jux-aposed by plate-tectonic processes” (Brown, 2006, 2009). Thexistence of distinctly different thermal regimes and contrasting–T paths in the Færingehavn, Tre Brødre and Tasiusarsuaq ter-anes, the strong structural evidence for regional-scale tectonichickening, as well as the good correlation between the timing ofollision, high-P metamorphism and exhumation make the Nuukegion a very convincing candidate for being the oldest preservedeoarchaean paired metamorphic belt (Fig. 10, and below). Theata presented above clearly show how the various terranes orig-

nated from very different geodynamic settings during the 2.8–2.7a terrane accretion event to be juxtaposed in the mid-crust during

he latter stages of it (Fig. 10). Given the convergence of P–T paths,

t appears likely that the conditions of juxtaposition can be taken toeflect a relatively stable thermal regime for the continental crustt this time. These conditions yield a geotherm of ∼30 ◦C/km, con-rming that burial of the Færingehavn terrane occurred along a

ditions for the Færingehavn (grey shading) and Tre Brødre terranes (black shading)gel et al. (2012). Ages are from this study, Nutman and Friend (2007) and Kolb et al.

significantly depressed apparent geotherm (∼17.5–20 ◦C/km) andmetamorphism of the Tasiusarsuaq terrane involved a slightly ele-vated geotherm (35 ◦C/km).

Further evidence for Neoarchaean subduction in the Nuukregion has recently emerged from the preservation of 2.7 ± 0.3 Gaeclogitic xenoliths within kimberlites present the Tasiusarsuaq ter-rane on Nunataq-1390 (Fig. 1; Tappe et al., 2011). The eclogiteshave a highly refractory major and trace element geochemistry,suggesting that they represent residues of TTG melts extracted froma basaltic precursor. In addition, elevated garnet �18O values andnegative Eu-anomalies point to seafloor-altered oceanic crust asthe most likely protolith (Tappe et al., 2011).

Despite the recognition of a paired metamorphic belt in theNuuk region, Neoarchaean crustal convergence apparently didnot result in the extreme thickening observed in many youngeraccretionary orogenic belts, and the high-P rocks preserved in theFæringehavn terrane and Simiutat supracrustal sequence at beststraddle the eclogite or high-P amphibolite stability fields (Fig. 10).The rocks record an apparent geothermal gradient that is signif-icantly warmer than that of modern subduction environmentswhich may be due to the generally higher mantle temperaturesin the Archaean. The apparent geotherm is, however, depressed by10–13 ◦C/km relative to the background, which is comparable tothat 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 ofNutman and Friend (2007), and involves the southwards sub-duction of the Faeringehavn terrane and Simiutat supracrustalsequence (lower plate) underneath the Tre Brødre and Tasuisar-suaq terrranes (upper plate) (Fig. 11a and b). The earliest record of

subduction-related igneous activity in the Tasiusarsuaq terrane isreflected by the intrusion the ca. 2973 ± 28 Ma Fiskenæsset Com-plex (Escher and Myers, 1975; Polat et al., 2009, 2010). The intrusionof the complex was followed by voluminous TTG magmatism

Page 15

36 A. Dziggel et al. / Precambrian R

Fr

b2a1dot(ittsb

BtiWiTm2isrvtgmue

Archaean oceanic crust was rheologically weak and unstable (van

ig. 11. (a and b) Sketches illustrating the proposed tectonic evolution of the Nuukegion. See text for discussion.

etween 2920 and 2820 Ma (Fig. 11a; Compton, 1978; Crowley,002; Friend and Nutman, 2001; McGregor et al., 1991; Næraand Scherstén, 2008; Nutman and Friend, 2007; Schjøtte et al.,989; Kokfelt et al., 2011). The younger TTG ages overlap with theeposition of the ∼2840 Ma felsic volcaniclastic rocks structurallyverlying the Faeringehavn terrane (Friend et al., 1996), as well ashe intrusion of the ∼2825 Ma Ikkattoq gneiss in Tre Brødre terraneFig. 11a; Crowley, 2002; Nutman and Friend, 2007). Thus, the datandicate a younging of igneous activity from south (Tasiusarsuaqerrane) to north (Tre Brødre terrane; Fig. 11a), and we interprethis northwards shift of the centre of igneous activity to be due tolab rollback, slab breakoff or backward movement of subductionefore final accretion.

In a plate-tectonic scenario, one can therefore think of the Trerødre terrane as reflecting the leading edge of the Tasiusarsuaqerrane (then the centre of igneous activity) or as a newly formedsland arc between the Tasiusarsuaq and Faeringehavn terranes.

e prefer the former because at the time of and following thentrusion of the Ikkattoq gneiss (between 2825 and 2800 Ma), theasiusarsuaq terrane underwent medium-pressure granulite faciesetamorphism (Crowley, 2002; Kolb et al., 2012; Dziggel et al.,

012). The results presented here further indicate ongoing ign-mbrite formation in the Tre Brødre terrane at ca. 2820 Ma (Fig. 9b),uggesting that the felsic volcaniclastic rocks in the Tre Brødre ter-ane reflect the volcanic equivalents of the Ikattoq gneiss. Afterolcanic activity ceased, the Tasiusarsuaq terrane was exposedo a prolonged period of near-isobaric cooling to a more stableeotherm (Dziggel et al., 2012). The amphibolite facies reworking

ay have begun as early as 2770 Ma, and lasted for at least 50 Ma

ntil collision with the Faeringehavn terrane (Fig. 12a and b; Kolbt al., 2012). The isobaric cooling and lack of exhumation of the

esearch 242 (2014) 22– 38

Tasiusarsuaq terrane mean that the granulite facies metamorphismlikely occurred in a hinterland setting where the rocks could coolafter TTG plutonism had ceased. The conditions and timing of thepeak of amphibolite facies metamorphism in the Tre Brødre terraneare similar to the conditions of amphibolite facies reworking in theTasiusarsuaq terrane, consistent with our interpretation that theTre Brødre terrane represents the leading edge of the Tasiusarsuaqterrane, prior to, and during final collision.

The earliest record of Neoarchaean metamorphism in rocksof the lower plate is given by rare metamorphic zircon ages of2740–2760 Ma in aluminous gneisses of the newly named Simi-utat supracrustal sequence (Nutman and Friend, 2007). This age isremarkably similar to the age of metamorphism in the Tre Brødreterrane, and suggests that it marks some point on the burial pathof lower plate rocks reflected by the Faeringehavn terrane andSimiutat supracrustal sequence (Fig. 11a and b). The subsequentpeak of high-P metamorphism and in situ partial melting corre-lates with terrane amalgamation, as indicated by the age of granitesheets intrusive into all three terranes (Friend et al., 1996; Nutmanand Friend, 2007). After this, the Færingehavn terrane was rapidlyexhumed, with 12–15 km of uplift occurring within error of theoverlapping U–Pb zircon ages. This corresponds to a conservativeaverage uplift rate on the order of 1–10 mm/a, similar to thosedetermined for exhumation of the high-grade parts of the Mesoar-chaean Barberton greenstone belt (Diener et al., 2005), and com-parable to uplift rates from younger orogenic terranes (e.g. Abbottand Silver, 1997). Willett (2010) estimated that erosion-controlledexhumation 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 estimatedvia 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 asubduction channel is more rapid, approximately 10–30 mm/a (e.g.Rubatto and Hermann, 2001; Rubatto et al., 2011). The uplift ratesestimated for the Faeringehavn terrane and Simiutat supracrustalsequence 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 amalgam-ation in the Nuuk region during the Neoarchaean had muchin common with modern accretionary processes. Specifically,accretion involved the juxtaposition of terranes with distinctlydepressed and elevated apparent geotherms and contrasting meta-morphic histories, with the estimated rates of crustal recovery alsobeing comparable to modern-day examples. However, despite theTTG record in the Tasiusarsuaq and Tre Brødre terranes indicatingthat convergence could have been as long-lived as 250 Ma (Fig. 11),the lack of the extreme P–T conditions typical of modern accre-tionary environments suggests that not all aspects of Neoarchaeantectonics can be directly compared to modern examples. At facevalue, the observation that long-lived convergence did not result insignificant burial can be taken to mean that Archaean convergencerates were much slower than today, perhaps due to a much weakerslab pull. However, this does not fit with a hotter and more vigor-ously convecting Archaean mantle. Instead, we speculate that thestyle of convergence in the Nuuk region indicates that subductionwas inherently inefficient and characterised by weak slab pull, cou-pled to frequent slab break-off and rollback (Fig. 11; Kisters et al.,2012). This is consistent with numerical models showing that the

Hunen and van den Berg, 2008; Sizova et al., 2010), which is likelythe key difference between Neoarchaean and modern accretionaryenvironments.

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cknowledgements

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

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