Structure of the Palaeoproterozoic Nagssugtoqidian Orogen, South-East Greenland: Model for the tectonic evolution

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Precambrian Research

jou rn al h om epa ge: www.elsev ier .com/ locate /precamres

tructure of the Palaeoproterozoic Nagssugtoqidian Orogen,outh-East Greenland: Model for the tectonic evolution

ochen Kolb ∗

epartment of Petrology and Economic Geology, Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen K, Denmark

r t i c l e i n f o

rticle history:eceived 22 July 2013eceived in revised form6 December 2013ccepted 26 December 2013vailable online xxx

eywords:alaeoproterozoicagssugtoqidianreenlandrogenyigh-pressure metamorphismaired metamorphic belt

a b s t r a c t

The Palaeoproterozoic Nagssugtoqidian Orogen extends over 250 km along the east coast of Greenlandaround the settlement of Tasiilaq. The orogen includes Archaean rocks from the adjoining Rae Cratonto the north and the North Atlantic Craton to the south, and Palaeoproterozoic rocks. The Rae Cratonconsists of orthogneiss and amphibolite included in the Schweizerland and Kuummiut Terranes, and istectonically overlain in the Kuummiut Terrane by ca. 2100–2200 Ma units that include marble, meta-pelite and -psammite and amphibolite assigned to the Helheim and Kuummiut units. The KuummiutTerrane was probably subducted underneath the SE-trending ca. 1885 Ma Ammassalik Intrusive Complex,which has a high-temperature metamorphic halo and is characterised by a change from sinistral faultingto pure shear deformation. The southern Isortoq Terrane consists of medium-pressure amphibolite faciesbimodal meta-volcanic and <1910 Ma meta-sedimentary rocks assigned to the Kap Tycho Brahe unit,which is in tectonic contact with orthogneiss and amphibolite of the North Atlantic Craton.

The rocks of the Kuummiut Terrane were tectonically imbricated in ENE-verging structures during ca.1870 Ma high-pressure metamorphism. This was followed by NE–SW convergence and close to ortho-gonal extrusion in the weakened crust, which is characterised by partial melting during decompression.The rocks of the Isortoq Terrane were imbricated in a SE-vergent thrust and ramp system either duringoblique subduction of the Kuummiut Terrane or an earlier tectonic stage elsewhere. NE–SW compression,as in the northern terranes, formed SW-vergent thrust systems and folds. This was most likely causedby a change in the regional stress field during collision between ca. 1870 and 1820 Ma. In the north,the Schweizerland Terrane was juxtaposed to the Kuummiut Terrane in southeasterly direction, causing
refolding of earlier structures in the lower amphibolite facies. This hinteland-type of deformation waspossibly related to tectonism in western Greenland. The latest recognised deformation event was dur-ing ca. 1740–1680 Ma associated with NE–SW extension, which is interpreted as orogenic collapse. Thecomplex structural evolution of the orogen was caused by oblique convergence during WSW-directedsubduction, the convergence of irregularly shaped cratons and the change of the regional stress field from
ring p
ENE–WSW to NW–SE du
. Introduction

There is a long tradition for geological research in South-Eastreenland and attempts to correlate the Archaean and Palaeopro-

erozoic history with that in the Lewisian Complex of Scotland andhe Trans-Hudson Orogen in Canada (Andrews et al., 1973; Eschert al., 1976; Myers, 1987; Bridgwater et al., 1990; Kalsbeek et al.,993; Friend and Kinny, 2001; van Gool et al., 2002; Nutman et al.,008; St-Onge et al., 2009). The early attempts were based on: (1)

Please cite this article in press as: Kolb, J., Structure of the Palaeoproterotectonic evolution. Precambrian Res. (2014), http://dx.doi.org/10.1016/j.pr

he general geology of the region; (2) the geometry and timing ofrecambrian mafic dykes; and (3) the hypothesis that there waseformation extending several hundred million years forming a

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rogressive collisional tectonics between the two Archaean cratons.© 2014 Elsevier B.V. All rights reserved.

mobile belt. Recent correlations are based on geochronological dataand general tectonic trends.

The Nagssugtoqidian Orogen forms a ∼250 km wide belt at thenorthern edge of the North Atlantic Craton in South-East Greenland(Fig. 1). Earlier work in the region is summarised in Kalsbeek (1989)and Kalsbeek et al. (1993). The general structural fabric in theorogen is parallel to intrusions assigned to the Ammassalik Intru-sive Complex (AIC), which trends to the southeast (Wright et al.,1973; Myers, 1984; Chadwick et al., 1989). This and south-vergentstructures led to the interpretation that the orogen formed dur-ing northward subduction (Kalsbeek et al., 1993; Nutman et al.,2008). In addition, scattered eclogite facies metamorphic assem-

zoic Nagssugtoqidian Orogen, South-East Greenland: Model for theecamres.2013.12.015

blages reported from mafic rocks to the north of the AIC wereinterpreted as indicative of collisional orogeny in the Palaeopro-terozoic (Nutman et al., 2008). However, northward subductionunderneath the AIC and the presence of high-pressure rocks to the

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orth of the AIC are at odds with the general understanding of theeometry of paired metamorphic belts in orogens.

A clear understanding of the geometry of the Nagssugtoqid-an Orogen and its plate tectonic evolution are essential for betterstablishing larger regional correlations and the interpretation ofarge-scale orogenies. In this paper, structural data are presentedndicating previously unrecognised ENE-vergent structures relatedo high-pressure metamorphism. The literature is reviewed andhe Palaeoproterozoic strata and the metamorphic evolution of theegion are re-interpreted based on own field data collected in 2010.his work has allowed the recognition of a paired metamorphicelt with high-pressure metamorphism to the north of the AIC,nd high-temperature metamorphism in intrusion halos assignedo the AIC. These data are used to suggest a WSW-directed obliqueubduction underneath the complex. Detailed structural field datare used to reveal a complex collision history with rotation of theegional stress field from WSW-ENE to NW–SE during orogeny,hich can be correlated with the evolution of the Nagssugtoqidianrogen and Rinkian Fold Belt of western Greenland.

. Regional structural geology

The Nagssugtoqidian Orogen in South-East Greenland was for-erly known as the Nagssugtoqidian, Ammassalik or Angmagssalikobile Belt, because the link between the Palaeoproterozoic oro-


ens of west and east Greenland, which are separated by Inlandce, has been a matter of discussion for decades (Wright et al., 1973;

yers, 1984, 1987; Chadwick et al., 1989; Kalsbeek et al., 1993; vanool et al., 2002; Nutman et al., 2008; St-Onge et al., 2009).

the Archaean and Proterozoic terranes (modified after, St-Onge et al., 2009).

The boundaries of the Nagssugtoqidian Orogen to the north andsouth are defined by near-vertical, dextral shear zones assigned tothe “Nag.1” deformation, and are located at Umivik and Kap JapetusSteenstrup (Fig. 1; Myers, 1984, 1987). An eastward trending set oflocally ophitic, ∼10–50 m wide dolerite dykes were interpreted asbeing Palaeoproterozoic, and their deformed and metamorphosednature was used to define the extent of the orogen (Bridgwater andMyers, 1979; Myers, 1984, 1987; Escher et al., 1989). The north-ern boundary was redefined as an approximately 50 km wide zonein the same area where Myers (1984) originally defined it (Fig. 1;Dawes et al., 1989b). The “Nag.2” deformation is characterised byregional-scale folds and north-dipping shear zones (Myers, 1984,1987).

The Nagssugtoqidian Orogen was originally interpreted as anintracratonic mobile belt formed over a period of ∼700 millionyears (Myers, 1984, 1987). Modern geochronological techniques,better insight into orogenic processes and detailed field workin the orogen, however, are indicative of a collisional orogenicsetting where juvenile Palaeoproterozoic igneous rocks intrudedArchaean units and where Palaeoproterozoic sedimentary and vol-canic rocks were deposited (Chadwick et al., 1989; Kalsbeek et al.,1993; Nutman et al., 2008). The orogen is now seen as a ∼250 kmwide, southeastward trending belt that includes diverse Archaeanand Palaeoproterozoic rocks affected by polyphase deformationand high-grade metamorphism (Figs. 1 and 2). The north-dippingstructures, probably correlating with Nag.2 structures of Myers


(1984, 1987), have been interpreted to be formed during south-directed tectonic transport and northward subduction beneaththe AIC that has a continental-arc signature (Nutman et al.,2008).




J. Kolb / Precambrian Research xxx (2014) xxx– xxx 3

Fig. 2. Schematic geological map of the Nagssugtoqidian Orogen in the Tasiilaq area of South-East Greenland showing major structures present (modified after, Escher, 1990)a he KuS

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nd geological cross-section and division into different terranes. The structures in tchweizerland Terrane are D3.

The structural evolution of the western segment of the Nagssug-oqidian Orogen in Greenland is characterised by WNW–ESEompression during ca. 1860–1840 Ma associated with an obliqueollision and high-grade metamorphism, which resulted in WNW-ergent thrusts in the central orogen and ESE-vergent structures ints southern hinterland (van Gool et al., 2002). At ca. 1825 Ma, therientation of the regional stress-field changed to N–S, resulting inarge-scale eastward trending folds formed at peak metamorphiconditions (van Gool et al., 2002). The subsequent evolution is char-cterised by ca. 1775 Ma sinistral strike-slip shear zones (van Goolt al., 2002).

It has been suggested that the Nagssugtoqidian Orogenxtended to the west to the Churchill Domain and the Transudson Orogen in Canada, which formed by Palaeoproterozoicccretion of several terranes and final collision of the Rae Cratonnd Hearne Province in the north with the Superior Craton in the


outh (Fig. 1; St-Onge et al., 2009; Garde and Hollis, 2010). Cor-elation with the Palaeoproterozoic orogens of Scandinavia is nottraight forward and requires further investigation (St-Onge et al.,009).

ummiut and Isertoq terranes are D2 shear zones. The shear zones at the base of the

The Nagssugtoqidian Orogen in western Greenland was devel-oped at the southern margin of the Rae Craton by northwardaccretion of microcontinents including the Aasiaat Domain. Theorogen formed between ca. 1880 and 1865 Ma followed by colli-sion of the Rae and North Atlantic cratons between ca. 1860 and1840 Ma (Fig. 1; St-Onge et al., 2009; Garde and Hollis, 2010). Byextrapolation, this implies that the Archaean rocks to the north ofthe AIC are part of the Rae Craton (Fig. 1).

3. Regional geology of the Nagssugtoqidian Orogen ofSouth-East Greenland

The Nagssugtoqidian Orogen of South-East Greenland consistsof Meso- to Neoarchaean orthogneiss, amphibolite and ultramaficrocks of the North Atlantic and Rae cratons, which form the hin-terland and foreland of the orogen, respectively (Fig. 2; Kalsbeek


et al., 1993). The arc-like, southeast-trending, autochtonous AICis located around Tasiilaq and consists of noritic to dioritic intru-sions and a contact metamorphic halo (Fig. 2; Andersen et al., 1989;Friend and Nutman, 1989). Palaeoproterozoic mafic dykes and


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ayered mafic-ultramafic intrusions intruded the Archaean rocks,hich are tectonically overlain by Palaeoproterozoic sedimen-

ary and volcanic rocks of the Síportôq Supracrustal AssociationHall et al., 1989a, 1989b). The thickness of the association variesetween 100 m and over 3 km (Wright et al., 1973). Post-tectonicranite, diorite, pegmatite and gabbro have a sharp contact withhe surrounding gneiss and a narrow (metre-scale) hornfels aure-le (Wright et al., 1973; Friend and Nutman, 1989; Nutman et al.,008). Three major intrusion centres have been recognised, withwo north of AIC and one to the south at Isertoq (Fig. 2). U–Pb zirconata from the three largest granite intrusions at Ikasartivaq, Johanetersen Fjord and Isortoq (Fig. 2) yield a TIMS upper intercept agef 1680 +10/−8 Ma interpreted as the intrusion age (Kalsbeek et al.,993). The Rb–Sr and Pb isotopic data indicate a crustal source ofhe melts in the genesis of the granite (Kalsbeek et al., 1993).

Based on different lithology and tectonometamorphic history,our terranes are newly defined in the Nagssugtoqidian Orogen ofouth-East Greenland (Fig. 2), namely from south to north: (1) thesertoq Terrane; (2) the Ammassalik Intrusive Complex (AIC); (3)he Kuummiut Terrane; and (4) the Schweizerland Terrane.

.1. The southern hinterland and the Isertoq Terrane

The Thrym Complex of the North Atlantic Craton forms the sub-trate to the south of the Nagssugtoqidian Orogen (Fig. 1; Bagast al., 2013; Kolb et al., 2013). The polyphase orthogneiss of thekjoldungen area records precursor intrusion ages between ca.850 and 2750 Ma (Kolb et al., 2013). Earlier intrusions are meta-orphosed at granulite facies between ca. 2790 and 2750 Ma, and

he younger ones intruded at granulite facies levels (charnock-te; Kolb et al., 2013). Associated meta-sedimentary rocks wereeposited after ca. 2850 Ma and subsequently metamorphosed atranulite facies grades at ca. 2750 Ma (Berger et al., submitted forublication). Alkaline intrusions of the Skjoldungen area intrudedetween 2720 and 2700 Ma into upper crustal levels at <1.5 kbarNutman and Rosing, 1994; Blichert-Toft et al., 1995; Berger et al.,ubmitted for publication). The latest Archaean magmatic event isarked by the ca. 2665 Ma Singertât Complex (Blichert-Toft et al.,

995; Kolb et al., 2013).Rocks from the area immediately south of the AIC are sparsely

tudied and consist of polyphase mainly granodioritic orthogneissnd narrow bands of mafic and ultramafic rocks and local parag-eiss (Bridgwater et al., 1978; Pedersen and Bridgwater, 1979;alsbeek and Taylor, 1989; Bagas et al., 2013; Kolb et al., 2013).hey form the Isertoq Terrane, together with the Palaeoprotero-oic rocks (Fig. 2). One sample of orthogneiss has a Sm–Nd wholeock model age of 3050 Ma (Kalsbeek et al., 1993), suggesting thathe source of the orthogneiss’ protolith is ca. 3050 Ma. Migmatiticanding and orthopyroxene–hornblende–feldpar–quartz assem-lages indicate granulite facies grade that is, locally, retrogressed inhe amphibolite facies. Metre-scale xenoliths consist of metamor-hosed gabbro, leucogabbro, pyroxenite and anorthosite (Wrightt al., 1973; Myers, 1984).

The mafic dykes in the Isertoq Terrane are not dated but, basedn crosscutting relationships, are interpreted as Palaeoproterozoicy Bridgwater et al. (1990), who have recognised early Mg-richnd younger Fe-tholeiitic swarms. Mafic dykes at the northerndge of the North Atlantic Craton in western Greenland, namelyangâmiut Dykes, have an age of 2050–2030 Ma (Nutman et al.,999), supporting a possible Palaeoproterozoic age in a similaretting in eastern Greenland. The mafic dykes in the Isertoq Ter-ane are weakly deformed and metamorphosed, eastward trending


olerite dykes with ophitic fabric and preserved magmatic mineralssemblages (Escher et al., 1989). Mafic dykes to the east of Isertoqre least deformed and contain a metamorphic assemblage of pla-ioclase, garnet, clinopyroxene and quartz at 7.0–8.5 kbar, which

PRESSh xxx (2014) xxx– xxx

is overprinted by a retrograde assemblage of hornblende, plagio-clase, quartz and minor garnet at ∼650–660 ◦C (Wright et al., 1973;Nutman and Friend, 1989; Nutman et al., 2008).

A succession of meta-sedimentary rocks and amphibolite formsdoubly folded belts in the Isertoq Terrane (Figs. 2, 3a and 4). Thisis the type locality for the Síportôq Supracrustal Association. Thelithostratigraphic sequence, here termed the Kap Tycho Brahe unit,consists of three lower amphibolite units that are separated byultramafic and calc-silicate rocks including possible marble (Fig. 4;Wright et al., 1973). A polymictic breccia contains 0.3–1 m widemeta-sedimentary, meta-gabbroic and amphibolitic fragments ina plagioclase–hornblende matrix (Hall et al., 1989a). The brecciais interpreted as a pyroclastic breccia based on its compositionand new field data. Ultramafic lenses consist of olivine, tremolite,amphibole, anthophyllite, chlorite and talc, and the amphiboliteand meta-gabbro consist of hornblende, plagioclase, garnet and,locally, corundum, kyanite and tremolite (Hall et al., 1989a). Theserocks are overlain by quartzite and meta-pelitic and -semipeliticrocks (Fig. 4; Wright et al., 1973). The meta-pelitic rocks arequartz–mica gneiss with small feldspar augen and locally preservedcross-bedding, containing garnet, kyanite, sillimanite and graphitein places (Hall et al., 1989a). Locally, thin <0.5 m thick amphibolitedykes crosscut the meta-sedimentary rocks (Wright et al., 1973).The metamorphic mineral assemblage of the meta-pelites is indica-tive of amphibolite facies conditions in the temperature range of500–700 ◦C and pressures of 4–8 kbar. The absence of staurolite insuch an assemblage suggests stabilisation at the higher tempera-ture end of the range, but no detailed data are available.

3.2. The Ammassalik Intrusive Complex (AIC)

Three 20 km long and 10–15 km wide mafic to intermediateintrusions aligned in a SE-trend constitute the AIC in the Tasiilaq-Kulusuk-outer Sermilik area (Fig. 2; Wright et al., 1973; Friend andNutman, 1989). Similar rocks form a conjugate set of N- and SE-trending dykes between the intrusions (Friend and Nutman, 1989).The rocks are coarse-grained, hypersthene-bearing with local felsiclayers containing quartz and K-feldspar on Kulusuk Island (Fig. 2;Wright et al., 1973). The intrusions are complex containing layeredmelagabbro followed by a homogeneous anorthositic rock con-taining lenses of ultramafic and mafic rocks (Wright et al., 1973;Andersen et al., 1989). They are crosscut by three generations ofpegmatitic anorthosite dykes (Wright et al., 1973).

A diorite from the quarry at Tasiilaq was dated at 1886 ± 2 Ma bybulk zircon analysis (Hansen and Kalsbeek, 1989) and has a sensi-tive high-resolution iron microprobe (SHRIMP) Pb–Pb zircon dateof 1881 ± 10 Ma (Nutman et al., 2008). U–Pb, Sm–Nd and Rb–Srisotopic data indicate a contribution from older material, suggest-ing the mixing of juvenile magma with Archaean crustal materialduring the genesis of the AIC (Kalsbeek et al., 1993). The Tasiilaqintrusion is magmatically layered with local felsic quartz–feldspar-dominated rocks and <20 m wide layers containing disseminatedsulphides (such as pyrrhotite and chalcopyrite). The layering is par-allel to a near-vertical, SE-trending magmatic foliation defined bymafic minerals and the alignment of xenoliths.

The temperature of the AIC magma is estimated at ∼1100 ◦Cusing the average composition and clinopyroxene–orthopyroxenethermometry (Andersen et al., 1989). PT data on a diorite suggesttemperatures of 830–850 ◦C and pressures of ∼7.5 kbar (Nutmanand Friend, 1989). Fluid inclusion microthermometry in igneousand contact aureole samples was used to estimate a pressure of6–8 kbar for the intrusion and a retrograde overprint at 550 ◦C at a


pressure of 2–3 kbar (Andersen et al., 1989).Wall rocks in the AIC are contact metamorphosed forming a

several km wide aureole containing migmatite where schlierenand nebulitic structures merge into one another (diatexite) (Fig. 2;


Please cite this article in press as: Kolb, J., Structure of the Palaeoproterozoic Nagssugtoqidian Orogen, South-East Greenland: Model for thetectonic evolution. Precambrian Res. (2014), http://dx.doi.org/10.1016/j.precamres.2013.12.015




Fig. 3. (a) Cross-section through a profile showing the tectonic contact between Archaean orthogneiss and amphibolite and the Palaeoproterozoic Síportôq SupracrustalAssociation (SSA), Isertoq Terrane, which generally show brownish weathering colours due to abundant paragneiss; (b) Migmatitic orthogneiss in the bottom of SermilikFjord showing mafic enclaves in polyphase felsic gneiss. The sharp boundary between different units indicates melt-present deformation (Kuummiut Terrane). (c) Maficdykes in Archaean orthogneiss of the Kuummiut Terrane that are crosscut by a set of en echelon pegmatites ∼5 km south of Tiniteqilaaq. These dykes preserved thehigh-pressure metamorphic mineral assemblage in the centre. (d) Cross-section close to Kuummiut showing the sheared contact between Archaean orthogneiss and theparagneiss–amphibolite succession of the SSA. (e) Characteristic appearance of the grey, well-foliated paragneiss of the Kuummiut unit. Foliation and lineation are definedby sillimanite. Note the pencil fabric in this particular outcrop, which is interpreted as representing an extensional shear zone. (f) Tectonostratigraphy of the syncline southof Helheim Fjord with the garnet–biotite gneiss in the footwall overlain by marble and amphibolite (Helheim unit). (g) Typical crumbly weathering of the garnet–biotitegneiss when it is graphite-rich and has abundant sulphides (Helheim unit). (h) Marble and calc-silicate rocks with the characteristic boudinage of more competent dolomiticlayers (Helheim unit).





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riend and Nutman, 1989). The aureole is characterised by bandedarnet gneiss with local pods of amphibolite, calc-silicate andltramafic rocks (Wright et al., 1973). The AIC’s contact with sur-ounding rocks is either sharp and tectonic or characterised byingling and veining in a several hundred metre wide zone (Wright

t al., 1973). Geothermobarometry in meta-pelitic rocks fromhe contact aureole yields 720–840 ◦C and 7.5 kbar for cores and10–570 ◦C for rim analyses, which is supported by garnet–biotiteata (Andersen et al., 1989; Nutman and Friend, 1989). Zirconges in the contact aureole overlap with the intrusion age of ca.886 ± 2 Ma (Kalsbeek et al., 1993).

Detrital zircons from meta-sedimentary rocks in the contactureole yield mostly ages of ca. 1950–1910 Ma with one Archaeanrain and a few dating ca. 2050 Ma (Fig. 4; Nutman et al., 2008).he Sm–Nd, Rb–Sr and Pb–Pb data for the whole rocks yieldedrrochrons (arrays with high MSWD) with slopes giving apparentalaeoproterozoic ages (Kalsbeek et al., 1993).

.3. The high-pressure Kuummiut Terrane

The orthogneiss in the Kuummiut Terrane is migmatitic witholyphase leucosomes and pegmatitic bands, and consists ofuartz, plagioclase, K-feldspar, biotite and hornblende (Fig. 3b;awes, 1989). One sample from the northern Sermilik area has


n upper intercept TIMS U–Pb age of 2636 +22/−18 Ma (Fig. 2;alsbeek et al., 1993). The Blokken Gneiss, at Blokken (Fig. 2), pre-iously regarded as a younger intrusion by Myers (1984, 1987) isrchaean with a Sm–Nd model age of ca. 3010 Ma, upper intercept

he areas (not to scale). The profiles are interpretations from the literature and own

TIMS U–Pb ages of ca. 2920 and 2960 Ma (Kalsbeek et al., 1993), anda 207Pb/206Pb weighted mean age of 3035 ± 14 Ma for concordantdata interpreted as a magmatic protolith age, plus 2723 ±49 Marims grown during Neoarchaean metamorphism (Nutman et al.,2008). In the latter study, Palaeoproterozoic zircon growth wasnot detected. The gneiss consists of a suite of less deformed mafictonalitic, granodioritic and dioritic rocks composed of quartz, pla-gioclase, K-feldspar, biotite and hornblende (Dawes et al., 1989a).Amphibolites form 10 to 100 m wide lenses and layers withpinch-and-swell structures parallel to the foliation in the Kuum-miut Terrane (Dawes, 1989). The amphibolite is variable in modalabundance of plagioclase, hornblende and quartz, and locally hasgreenish, clinopyroxene-rich centres in the bands and lenses.

Swarms of discordant mafic dykes cut the Archaean rocksand have a relict clinopyroxene–hornblende–garnet assemblagein their cores, which is retrogressed to hornblende and pla-gioclase (Fig. 3c; Nutman and Friend, 1989). The dykes arestrongly deformed and represented by lenses and boudins inorthogneiss (Wright et al., 1973). Reaction textures, such asclinopyroxene–albite and plagioclase–orthopyroxene symplectitessurrounding garnet, indicate high-pressure metamorphism andsubsequent decompression (Nutman and Friend, 1989). CalculatedPT conditions are 660–760 ◦C and 8–11 kbar (Nutman and Friend,1989; Nutman et al., 2008). Zircons from a high-pressure amphi-


bolite facies dyke yield an imprecise Pb–Pb age of ca. 1867 ± 28 Ma,which is the date of the high-pressure event, whereas Rb–Sr andSm–Nd data indicate metamorphism at ca. 1820 Ma and probablyare related to decompression in the amphibolite facies (Kalsbeek


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t al., 1993; Nutman et al., 2008). Sm–Nd age data suggest that theykes were emplaced after ca. 2400–2200 Ma (Fig. 4, Bridgwatert al., 1990; Kalsbeek et al., 1993). One dyke has a 2015 ± 15 Mab–Pb zircon age (Nutman et al., 2008), which suggests that theykes were emplaced at ca. 2015 ± 15 Ma, but also may compriseifferent generations of dykes.

The Ivnartivaq Complex is an approximately 300 by 800 m lay-red intrusion located east of Sermilik and 50 km north of Tasiilaqn the Kuummiut Terrane (Fig. 2; Brooks and Stenstrop, 1989). Theomplex has a dunitic centre rimmed by serpentinite. The duniteocally contains layers and lenses of amphibolite, which are asso-iated with a ∼230 m2 magnetite-rich lens (Brooks and Stenstrop,989). The hornblende has an Ar-Ar age of 1955 ± 28 Ma, which is

nterpreted as the age of emplacement (Fig. 4, Brooks and Stenstrop,989).

Two major intrusions of diorite and tonalite are present to theest and east of Sermilik and a smaller one to the south of the

6 September Gletscher (glacier; Fig. 2). The tonalite to the westf Sermilik has a Pb–Pb zircon age of 1901 ± 9 Ma (Fig. 4, Nutmant al., 2008), which is consistent with a whole rock errorchronMSWD = 3.25) date of 1906 ± 75 Ma (Kalsbeek et al., 1993).

Paragneiss and schist are the dominant Palaeoproterozoic rocksorth of Tasiilaq and east of Sermilik (Kuummiut unit), whichonsist of quartz, biotite, muscovite, feldspar, and minor garnet,yanite, sillimanite and graphite (Figs. 2 and 3d, e; Hall et al., 1989a).he metamorphic mineral assemblage is characteristic of amphi-olite facies conditions in the temperature range of 500–700 ◦Cnd pressures of 4–8 kbar. Sillimanite pseudomorphing kyanite isommon and is consistent with decompression recorded in theafic dykes. Zircons in these rocks have complex rims, suggesting,

ogether with Rb–Sr, Sm–Nd and Pb–Pb isotope data, a meta-orphic overprint between 1870 and 1740 Ma (Kalsbeek et al.,

993; Nutman et al., 2008). Narrow layers of grey graphitic marblere locally present (Fig. 4; Hall et al., 1989a), which are com-only boudinaged and hosted by muscovite–biotite–calcite schist.mphibolite, containing plagioclase, hornblende, biotite, garnetnd minor quartz, forms 0.3–100 m thick sheets parallel with theithological layering in the meta-sedimentary rocks (Fig. 3d; Hallt al., 1989a). Local felsic pods of quartz, clinopyroxene and plagio-lase are interpreted as in situ partial-melt pockets and indicateetamorphic conditions of the higher amphibolite or granulite

acies. Ultramafic rocks and meta-gabbro form lenses similar tohose in the Kap Tycho Brahe unit (Fig. 4).

Rb–Sr whole rock data do not give conclusive ages, but are inhe broad range of the Palaeoproterozoic and indicate sedimen-ation after ca. 2200–2100 Ma (Fig. 4, Kalsbeek et al., 1993). Them–Nd data yields Archaean model ages between ca. 2880 and750 Ma indicative of an Archaean source, which is supported byeoarchaean (2800–2600 Ma) U–Pb detrital zircon ages (Kalsbeekt al., 1993; Nutman et al., 2008). In contrast to meta-sedimentaryocks from the AIC contact aureole, these rocks completely lackalaeoproterozoic detrital zircons.

The Síportôq Supracrustal Association to the north and west ofermilik (Helheim unit) is characterised by meta-diorite, amphi-olite, marble, quartzite and garnet–biotite gneiss in severaloubly folded belts (Figs. 2, 3f–h and 4; Hall et al., 1989a). Theeta-sedimentary rocks consist of quartz, biotite, garnet, kyanite,

illimanite and locally graphite, and the meta-diorite and amphibo-ite contain hornblende, plagioclase, quartz and garnet (Hall et al.,989a). The metamorphic conditions were estimated at ∼640 ◦Cnd ∼5.3 kbar (Baden, 2013).

The ca. 1680 Ma granite and diorite form a set of verti-


al, 10–30 m wide, eastward trending dykes. Both, diorite dykesmplaced in granite and mingling textures between dioritic andranitic melts are observed, indicating that the diorite is syn- toate-granite intrusion. The contact between granite and gabbro

PRESSh xxx (2014) xxx– xxx 7

has a cauliflower structure with granitic xenoliths in the gabbroand gabbro veins and gabbroic xenoliths in the granite, suggestingmagma mingling. A sample of meta-pelite from a contact aureole ofa ca. 1680 Ma granite yields temperature estimates of 580–650 ◦Cand pressure estimates of ∼2.5 kbar during the time of contactmetamorphism, which indicates that the granite was emplacedat a depth of ∼7–8 km (Nutman and Friend, 1989). SE-trendingaplite dykes crosscut all other intrusions indicating that they arethe youngest intrusive rocks recognised.

3.4. The northern foreland and the Schweizerland Terrane

Archaean orthogneiss and narrow bands of mafic rocks ofthe Rae Craton form the Schweizerland Terrane to the northof the AIC in the hanging wall of the Niflheim Thrust (Fig. 2;Bridgwater and Myers, 1979; Myers, 1984, 1987; Dawes et al.,1989b). The orthogneiss is brown and consists of quartz, plagio-clase, K-feldspar, and clino- and orthopyroxene, which indicatesgranulite facies metamorphism (Dawes, 1989; Escher and Hall,1989). The orthogneiss was dated at 2835 +6/−8 Ma (U–Pb TIMSupper intercept; Kalsbeek et al., 1993). Other samples have SHRIMPzircon U–Pb dates of 2863 ± 11 Ma, 2756 ± 6 Ma and 2734 ± 34 Ma,confirming the polyphase nature of the orthogneiss, and nosigns of Palaeoproterozoic overprint on the zircons is recognised(Kalsbeek et al., 1993; Nutman et al., 2008). Pegmatites with Rb–Srwhole rock dates of 2630 ± 65 Ma intruded the orthogneiss dur-ing retrograde amphibolite facies metamorphism (Pedersen andBridgwater, 1979).

4. Structural geology

Archaean deformation fabrics are preserved in the orthogneissand the amphibolite units that were subsequently refolded andsheared during the Palaeoproterozoic (Wright et al., 1973). The AICwas not involved in the different folding episodes present in thesurrounding terranes. The complex has a near-vertical contact withthe wall rocks in the north and a NE-dipping contact in the south(Friend and Nutman, 1989). The contact between the Archaeanrocks and the Síportôq Supracrustal Association is always tectonicand suggests large-scale imbrication (Wright et al., 1973).

Detailed structural investigations forming part of this studymake it necessary to re-interpret the Palaeoproterozoic structuralevolution of the eastern Nagssugtoqidian Orogen, which furtheridentifies deformation stages that are not easily correlated withthe two main phases of folding (NF1/2, Nag. 1/2, F1/2; abbreviationsused by the different authors) proposed by earlier investigators(Wright et al., 1973; Myers, 1984, 1987; Chadwick and Vasudev,1989; Friend and Nutman, 1989).

4.1. Structures of the Isertoq Terrane

Archaean orthogneiss and amphibolite of the North AtlanticCraton in the Isertoq Terrane have an early SA1 foliation that isfolded into isoclinal, recumbent folds with north-plunging fold axes(Fig. 5a). This type of isoclinal fold is not found in the Palaeopro-terozoic rocks and so the early SA1 foliation is interpreted to predatethe Síportôq Supracrustal Association.

4.1.1. Small-scale structuresAxial planar S1 foliation associated with isoclinal folds is pen-

etrative in all rock units and is refolded into upright, W-vergent,


shallow, open, north-plunging F1 folds at various scales, whichplunge moderately to the NW (Figs. 5a and 6a). Locally, 10–50 mwide shear zones have developed characterised by a closely spaced,NW-dipping S1 foliation with a down-dip lineation (Fig. 5b). Shear





Fig. 5. (a) Early foliation and fold structures in Archaean orthogneiss east of Isortoq. (b) Reverse shear zone in Palaeoproterozoic paragneiss north of Isortoq. S1 is thep from nN m tha

s(deDTeaSsa

4

AvDttpzam(

enetrative foliation. (c) Archaean orthogneiss in a D1 dextral strike-slip shear zoneote the book-shelf structures in feldspar. (e) Panorama view of the D1 thrust syste

ense indicators suggest a SE sense of reverse movement during D1Fig. 6b). Closely spaced, near-vertical foliation has developed iniscrete dextral strike-slip shear zones with near-horizontal min-ral stretching lineation (Figs. 5c and 6b). Moderately NE-dipping2 shear zones are developed locally along the limbs of F2 folds.hese D2 shear zones are characterised by NE- to N-plunging min-ral stretching lineation and a reverse sense of shear (Fig. 6b). Inddition, orthogneiss forms protomylonites and mylonites with-C fabrics (Fig. 5d), and feldspar forms porphyroclasts and book-helf structures, indicating brittle–ductile deformation of feldspart lower amphibolite facies conditions (Fig. 5d).

.1.2. Large-scale structuresLarge-scale thrusts juxtaposed the Kap Tycho Brahe unit and

rchaean orthogneiss–amphibolite rocks against each other in a SE-ergent imbricated thrust system south of the AIC (Figs. 2 and 5e).extral strike-slip shear zones located south of Isortoq connect

he reverse shear zones, forming lateral and frontal ramp geome-ries typical of thrust systems (Fig. 2). Large-scale fold interferenceatterns are obvious by the map pattern of the Palaeoprotero-


oic rocks in the Kap Tycho Brahe area (Fig. 2). These F2 foldsre developed between SW-vergent D2 thrust zones, becomeore open to the south and are not observed south of Ikertivaq

Fig. 2).

orth of Ikeq. (d) D2 shear zone and S2 foliation from the thrust northeast of Isortoq.t imbricated Archaean and Palaeoproterozoic strata in the Kap Tycho Brahe area.

4.2. Structures of the Ammassalik Intrusive Complex

The intrusive rocks of the AIC were initially interpreted to bepost-tectonic or as being emplaced late in the structural evolution,because no deformation fabrics were observed in the unit (Wrightet al., 1973; Chadwick et al., 1989; Friend and Nutman, 1989).Nutman et al. (2008) reinterpreted the AIC as an allochthonouscomplex bound to the north and south by shear zones. Detailedobservations indicate an alignment of pyroxene and xenoliths par-allel to a moderately to steeply dipping foliation (Fig. 6c). Thefoliation appears to undulate according to the ellipsoidal shapeof the intrusions suggesting sinistral rotation (Fig. 2). The intru-sion centres are commonly non-foliated. Foliation towards the rimsbecomes pronounced, and a closely-spaced foliation has devel-oped at the contact with the host rocks. The structural fabrics indeformed rocks are symmetric, consistent with pure shear defor-mation. In addition, m-scale shear zones are locally developed(Fig. 7), and S-C and S-C′ fabrics and quartz veins indicate solid-statedeformation and suggest a normal sense of deformation.


4.3. Structures of the Kuummiut Terrane

Orthogneiss and local mafic granulite–amphibolite lenses inthe Kuummiut Terrane have an early foliation defined by the





Fig. 6. Stereoplots of representative structural readings (lower hemisphere, equal area projections). (a) Poles to S1 and S2, and related fold axis from the Kap Tycho Brahe area(Fig. 2). (b) Poles to S1 and S2 with lines indicating mineral stretching lineation and arrows indicating sense of movement (Hoeppener diagram; Isertoq Terrane). (c) Poles tothe foliation in discrete sinistral shear zones and the Ammassalik Igneous Complex (AIC). (d) Poles to S1 foliation and mineral stretching lineation with sense of shear for theKuummiut Terrane (Hoeppner diagram). (e) Poles to the S1 foliation suggesting fold interference geometry resulting from the interaction of F1 and F2 folds. (f) Poles to the S2

foliation forming the conjugate set of extensional shear zones in orthogneiss of the Kuummiut Terrane (Fig.8b). (g) Hoeppener diagram for D2 structures of the KuummiutTerrane showing sense of normal and sinistral displacements. (h and i) Poles to S2 foliatde

FC


iagram of D3 structures in the Niflheim Thrust and the structural footwall, showing revextension.

ig. 7. Approximately 2 m wide shear zone in norite of the Ammassalik Igneousomplex west of Tasiilaq.

ion and b2 fold axes from Palaeoproterozoic belts west of Sermilik. (j) Hoeppenerrse movement to the SE. (k) Poles to S4 foliation and pegmatites, indicating NE–SW

alignment of mafic minerals and compositional layering. The ageof the foliation is not certain, but as Palaeoproterozoic metamor-phic conditions reached very high grades in the terrane, the firstfoliation in the rocks of the Síportôq Supracrustal Associationis parallel to the orthogneiss foliation and the mafic dykes aretransposed into this foliation locally, the early foliation is probablyPalaeoproterozoic in age.

4.3.1. Small-scale structuresThe early (S1) foliation dips moderately to shallowly to the

SW with a near-down-dip lineation and shear sense indicators ofreverse shearing to the NE-ENE (Fig. 6d). Isoclinal intrafolial folds(F1) are observed with fold axes plunging moderately W in places(Fig. 6e). The second set of open to close (F2) folds with near-horizontal, eastward trending fold axes refolds earlier structures(Figs. 6e and 8a). The second (S2) foliation is variably developed


in different D2 deformation zones (Fig. 6d–i). A widely spacedconjugate set of S2 foliations is locally formed in the Archaeanorthogneiss, which marks NE–SW extension (Figs. 6f and 8b).Discrete shear zones containing S2 fabrics are characterised by:





F lanars prote

(rIata(tSsi

4

sP(pd

Fts

ig. 8. (a) Folded banded orthogneiss with leucosomes developed along the axial phear zones in orthogneiss. Note the leucosomes parallel to S2. (c) F3 folds in Palaeo

1) a steeply S-SW-dipping S2 with a down-dip lineation andeverse shear sense indicators south of Tiniteqilaaq and west ofvnartivaq (Fig. 2); (2) eastward-trending, near vertical S2 with

near-horizontal mineral stretching lineation and oblique sinis-ral shear sense indicators south of Kuummiut (Figs. 2 and 6g);nd (3) shallow W-plunging L > S fabrics in the Kuummiut areaFigs. 3e and 6g). The S2 foliation is F3 folded (Fig. 6h–i), andhe S2 foliation is cut by a shallow to moderately NW-dipping3 foliation with down-dip mineral stretching lineation in theurroundings of the Niflheim Thrust (Figs. 2 and 8c,d). Shear sensendicators point to reverse sense of movement to the SE (Fig. 6j).

.3.2. Large-scale structuresDeformation associated with D1 resulted in the formation of

hear zones along the contact between Archaean orthogneiss andalaeoproterozoic rocks and large-scale imbrication of the two


Figs. 2 and 9). The fabrics in the mylonites suggest nappe trans-ortation to the NE-ENE (Fig. 6d). The nappes are progressivelyeformed by subsequent deformation stages (Fig. 9).

ig. 9. Sketch of ENE–NE vergent D1 thrust juxtaposing orthogneiss and Palaeopro-erozoic rocks. Refolding of the thrust system formed near isoclinal upright synclinaltructures during D2 and D3.

S2 foliation, indicating partial melting during D2. (b) Conjugate set of extensionalrozoic paragneiss south of Helheim. (d) S3 foliation in the Niflheim Thrust.

The D2 deformation formed different structures: (1) reverse, NE-vergent shear zones; (2) near-vertical, sinistral oblique-slip shearzones; and (3) normal W-vergent shear zones. The reverse shearzones are located 10–15 km NE of the AIC south of Tiniteqilaaqand west of Ivnartivaq (Fig. 2). They are several 10 s of metreswide and are characterised by mylonitic orthogneiss. To the northof the ca. 1680 Ma granite complex, several 10 s of metres wideshear zones have developed in orthogneiss and rocks of the Kuum-miut unit (Fig. 2). The shears are subvertical, eastward trendingmylonites with moderately WSW-plunging mineral stretching lin-eation and a sinistral sense of movement (Fig. 6g). North of theseshear zones in the Kuummiut area, L-type tectonites with lineationsplunging moderately W indicate normal sense of shear to the W(Fig. 6g).

Refolding of the S2 fabrics is best seen by the outline of the largerSíportôq Supracrustal Association belts (Figs. 2, 8c and 9). The F3folds are regionally open upright structures with fold axes plungingshallow to the SE. Folds closer to the Niflheim Thrust are tighter andassociated with a new S3 foliation (Figs. 6h–j).

A conjugate set of cm-scale shear zones is locally syn-kinematically intruded by pegmatites associated with the ca.1680 Ma granite complex and is spatially associated with the gran-ite (Fig. 3b). There geometry indicates NE–SW extension in avertical stress field (Fig. 6k).

4.4. Structures of the Schweizerland Terrane

Archaean structures are present in the hanging wall of thePalaeoproterozoic Niflheim Thrust. These are a foliation in theorthogneiss and retrograde amphibolite facies shear zones, which


contain syn-tectonic pegmatites dated at ca. 2630 Ma (Pedersenand Bridgwater, 1979; Myers, 1984, 1987). The Niflheim Thrust andthe related major thrust system (Fig. 2) have a shallow to moder-ately NW-dipping S3 foliation with down-dip mineral stretching


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ineation (Figs. 2 and 8c, d). Shear sense indicators point to reverseense of movement to the SE (Fig. 6j).

. Discussion

During the 1980s, Archaean and Palaeoproterozoic rocks ofhe Tasiilaq area were interpreted as forming in a long-lived∼700 million year long) mobile belt (Bridgwater and Myers, 1979;

yers, 1984, 1987; Chadwick et al., 1989). Modern geochrono-ogical techniques have distinguished between Archaean andalaeoproterozoic evolutions assigning deformation, magmatismnd metamorphism to Palaeoproterozoic orogeny (Kalsbeek et al.,993; Nutman et al., 2008). This advance in the understanding ofhe geological evolution of the region has made it possible to cor-elate the Nagssugtoqidian Orogen in South-East Greenland withrogens in central West and North-West Greenland, Canada andcotland (Kalsbeek et al., 1993; Friend and Kinny, 2001; Nutmant al., 2008; St-Onge et al., 2009).

The tectonic reconstruction of the Tasiilaq area by Nutmant al. (2008) suggests northward transport and subduction of theorth Atlantic Craton, the development of the AIC as Palaeo-roterozoic arc between the North Atlantic and Rae cratons andubsequent Palaeoproterozoic metamorphism and deformationucceeding collision. In the following, structural evidence andegional data is presented that are at odds with such a simple modelor the Nagssugtoqidian Orogen of South-East Greenland and givevidence for WSW-directed subduction and subsequent N–S accre-ion, similar to that in western Greenland (van Gool et al., 2002;arde and Hollis, 2010).

.1. Stratigraphic and magmatic evolution

All so-called Palaeoproterozoic supracrustal rocks have beenummarised under the Síportôq Supracrustal Association (Hallt al., 1989a). Detailed observations and age data, however, indi-ate at least three different stratigraphic units (Fig. 4). Psammiticnd semipelitic rocks, marl, carbonates and amphibolite thattructurally overlay Archaean orthogneiss, amphibolite and maficranulite characterise the Helheim unit in the north. An ultra-afic layered intrusion in the Ivnartivaq Complex was emplaced

t ca. 1955 Ma and a tonalite intruded at ca. 1900 Ma (Brooksnd Stenstrop, 1989; Nutman et al., 2008). The eastern Kuummiutnit is composed of psammitic and pelitic rocks, local carbo-ates, amphibolite sheets of probable basaltic composition andltramafic lenses. The siliciclastic rocks have an Archaean proven-nce and were deposited after ca. 2200–2100 Ma (Kalsbeek et al.,993; Nutman et al., 2008). They also structurally overlay Archaeanocks, which are cut by mafic dykes that were emplaced sometimeetween ca. 2400 and 2050 Ma (Kalsbeek et al., 1993; Nutman et al.,008). The rocks of the Helheim and Kuummiut units are similarnd could represent a lateral facies variation. The sedimentationredates the AIC and tonalite emplacement (ca. 1900–1885 Ma),ut directly postdates the intrusion of the mafic dykes and thevnartivaq Complex into the Archaean substrate. The siliciclasticnd carbonate rocks of the Helheim and Kuummiut units are, there-ore, interpreted as passive continental margin sediments with anrchaean provenance. The mafic dykes and meta-volcanic rocksay indicate Palaeoproterozoic spreading and basin formation intohich the sediments were deposited.

Similar Palaeproterozoic strata in western Greenland is rep-esented by the Maligiaq (2850–2100 Ma detrital zircons) and


kertooq (2400–1900 Ma detrital zircons) suites to the south of theasiaat Domain, which are characterised by siliciclastic and nar-ow marble units (Fig. 1, Marker et al., 1999). They were probablyeposited on Archaean units assigned to the North Atlantic Craton


during Palaeoproterozoic spreading and basin formation (Markeret al., 1999; van Gool et al., 2002). The Kuummiut and Helheim unitsare best correlated with the Mârmorilik and Qeqertarssuaq forma-tions of the Karrat Group to the north of the Aasiaat Domain (Fig. 1)in North-West Greenland. The Mârmorilik and Qeqertarssuaq for-mations are meta-siliciclastic rocks and marbles deposited in a riftand platform setting (Henderson and Pulvertaft, 1987; Garde andHollis, 2010). The Qeqertarssuaq Formation is overlain by a meta-volcanic unit similar to that at Helheim (Kalsbeek et al., 1998).

The Kap Tycho Brahe unit located to the south of the AIC tecton-ically overlays Archaean rocks (Figs. 2 and 4). The unit consists ofmetamorphosed felsic (dacitic) and mafic (basaltic) volcanic rockswith lenses of ultramafic rocks. These are covered by pelitic andpsammitic siliciclastic rocks with a ca. 1950–1910 Ma provenance(Kalsbeek et al., 1993; Nutman et al., 2008), which are cut byamphibolite dykes (Fig. 4). The ca. 1950–1910 Ma provenance isonly represented by the ca. 1900 Ma tonalite intrusion in the Hel-heim unit and is otherwise unknown from the area so far. Basedon the siliciclastic nature and the young provenance, the meta-sedimentary rocks of the Kap Tycho Brahe unit are interpreted asflysch deposits sourced from the erosion of a developing volcanic-arc, and the bimodal nature of the meta-volcanic rocks would becomparable with such a volcanic-arc setting. These rocks may havetheir western correlation in the <1920 Ma Nordre Strømfjord suite(2200–1950 Ma detrital zircons), which was deposited in the Aasi-aat Domain as detritus from an exotic arc (Fig. 1; Marker et al., 1999;Nutman et al., 1999; van Gool et al., 2002). The rocks of the SíportôqSupracrustal Association, thus, represent heterogeneous strata ofvarious ages that probably formed in various tectonic settings of theevolving Nagssugtoqidian Orogen. The Helheim and Kuumiut unitswere probably deposited on Archaean substrate of the Rae Cratonin a Palaeoproterozoic passive margin setting. The Kap Tycho Braheunit represents flysch deposits of the Palaeoproterozoic continentalarc that was developed on Archaean substrate of the North AtlanticCraton.

The AIC intruded at ca. 1885 Ma into Archaean and Palaeo-proterozoic rocks pre-dating regional metamorphism by some 15million years (Fig. 2; Hansen and Kalsbeek, 1989; Nutman et al.,2008). The complex was interpreted as a Palaeoproterozoic arcbased on major element characteristics and similar rocks in WestGreenland (Nutman et al., 2008). The ca. 1680 Ma granitic rocksformed late in the Palaeoproterozoic evolution of the orogen ascrustal melts during D4 NE–SW extension.

5.2. Tectonometamorphic evolution

The metamorphism and pressure–temperature–time (PTt) evo-lution has not been adequately investigated, but granulite andamphibolite facies metamorphism has been described from thestudy area (Wright et al., 1973). The Archaean rocks to the northof the AIC, in particular those in the Schweizerland Terrane, areat granulite facies (Wright et al., 1973; Dawes et al., 1989b; Escherand Hall, 1989). Isotopic data indicate an Archaean age for the gran-ulite facies metamorphism older than ca. 2635 Ma and probably ca.2720 Ma (Kalsbeek et al., 1993; Nutman et al., 2008). These ages aresimilar to what has been observed in the Thrym Complex (Bagaset al., 2013; Kolb et al., 2013), the Lewisian Complex in Scotland(Love et al., 2004) and in the Rae Craton in Canada (St-Onge et al.,2009).

Palaeoproterozoic structures and metamorphic conditions varyconsiderably between the terranes north and south of the AIC.Structures of at least four deformation stages and symplectites


indicating high-pressure metamorphism are restricted to theKuummiut Terrane (Nutman et al., 2008). The AIC and the areato the south are structurally relatively simple and record maxi-mum pressures of approximately 8 kbar (Nutman et al., 2008). Two


ARTICLE ING Model


12 J. Kolb / Precambrian Researc

Fig. 10. Schematic block diagram, illustrating the complex geometry of structures inthe Kuummiut Terrane. D1 structures are represented by ENE-vergent shear zonesas

mw

tpuANapcScntEhaa

rfmpiwmf

direction beneath the SE-trending AIC (Fig. 11a). This formed the

FttG

nd folds, D2 structures are the sinistral strike-slip shear zones and folds, and D3

hear zones are SW-vergent.

ajor deformation stages are distinguished in the Isertoq Terrane,hereas only one is preserved in the structural fabric in the AIC.

The earliest Palaeoproterozoic metamorphic stage is related tohe emplacement of the AIC at ca. 1885 Ma, resulting in medium-ressure amphibolite facies metamorphism (600 ◦C, 7 kbar) andp to granulite facies in the vicinity of the intrusions of bothrchaean and Palaeoproterozoic strata (Andersen et al., 1989;utman and Friend, 1989). The intrusions are interpreted to beutochthonous (Fig. 2), whereas it is unclear whether the Palaeo-roterozoic rocks were deposited over Archaean rocks or, as is thease in the northern and southern terranes, have a tectonic contact.imilar medium-pressure amphibolite facies peak metamorphiconditions are contemporaneous with NW–SE compression andappe transport to the southeast in the Isertoq Terrane (Fig. 5e). Inhe Kuummiut Terrane, thrust imbrication to the east-northeast inNE–WSW compression is most probably related to the ca. 1870 Maigh-pressure metamorphic stage, because younger structures arelready retrograde (Figs. 9 and 10). The near vertical AIC may havected as a link between the two terranes as a sinistral shear zone.

The D2 structures formed during NE–SW compression in all ter-anes, except the northernmost Schweizerland Terrane. They areolds, reverse-oblique-slip and normal shear zones in the Kuum-

iut Terrane (Fig. 10). The oblique-slip shear zones formed inrogressively steepened fold limbs. The normal shear zones are

nterpreted as near-orogen-parallel E–W extrusion fabrics in the


eakened crust due to high-grade metamorphism and partialelting of felsic lithologies (Figs. 3b and 8a). The leucosomes in

elsic rocks and garnets rimmed by plagioclase in mafic rocks of

Rae Craton

North AtlanticCraton

AasiaatDomain

Met

aIn

cogn

ita

AmmassalikIntrusiveComplex

Rinkianfold belt

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ig. 11. Schematic sketch of the larger tectonic situation for the Nagssugtuqidian Orogenook place in eastern Greenland and subduction to the ESE in the west underneath the

he change of the regional stress field to more N–S directions in eastern and western Greereenland. The D3 stage is shown with the Niflheim Thrust and is possibly related to defo


the Kuummiut Terrane probably indicate near isothermal decom-pression and associated decompression melting during terraneexhumation until ca. 1820 Ma, which is consistent with PT esti-mations by Nutman et al. (2008). Amphibolite facies pure-shearfabrics in the AIC, and D2 folds and S-SW-vergent shear zones inthe Isertoq Terrane formed at the same time.

The Niflheim Thrust at the northern margin of the KuummiutTerrane juxtaposed the Schweizerland Terrane to the KuummiutTerrane during NW–SE compression at amphibolite facies condi-tions, forming a set of shear zones in the hanging wall of the thrustand fold structures in the Kuummiut Terrane (Figs. 2 and 10). Thiscaused further retrogression of the rocks in the Kuummiut Terranelate in the metamorphic evolution, either at ca. 1820 or ca. 1740 Ma.No D3 structures have been observed south of the Kuummiut Ter-rane.

The D4 orogen-normal extension and pegmatite intrusionsin the Kuummiut Terrane formed during NE–SW extension latein the deformation history of the orogen and are crosscut bythe ca. 1680 Ma granites, representing the last Palaeoproterozoictectonometamorphic stage observed in the Tasiilaq area.

5.3. Plate tectonic and orogenic interpretation

The general E- to SE-trend of the Nagssugtoqidian Orogen andcorrelation with Palaeoproterozoic orogens in Canada led to theinterpretation that the Nagssugtoqidian Orogen formed by north-ward accretion of terranes onto the Rae Craton and northwardsubduction beneath the AIC in South-East Greenland (Nutman et al.,2008). The relative timing of deformation and metamorphism sug-gests, however, a different scenario.

The Kuummiut Terrane represents a ca. 1870 Ma high-pressuremetamorphic terrane, whereas the ca. 1885 Ma AIC is characterisedby high-temperature metamorphism and the Isertoq Terrane is amedium-pressure amphibolite facies terrane, where only Archaeanrocks are at granulite facies (Fig. 2). The D1 ENE-vergent deforma-tion stage in the Kuummiut Terrane is related to the high-pressuremetamorphism and subsequent deformation caused decompres-sion and exhumation. The Kuummiut Terrane, thus, represents thelower plate during the orogeny, whereas the AIC and the Iser-toq Terrane are the upper plate. The Nagssugtoqidian Orogen inSouth-East Greenland formed in an oblique subduction setting,where the northern Kuummiut Terrane was subducted in a WSW


ENE-vergent D1 thrusts in the Kuummiut Terrane and steep struc-tures in the AIC, which had a sinistral component caused by theoblique convergence and could explain the rotational asymmetry

Rae Craton

North AtlanticCraton

Met

aIn

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ita

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Rinkianfold belt

D2

D2

D2

D3/NiflheimThrust

b

: (a) Tectonic situation of the subduction stage (D1), where subduction to the WSWNorth Atlantic Craton, forming fold and thrust belts. (b) Collision stage (D2) withnland, causing the characteristic orogenic double-vergent geometry in South-Eastrmation of similar sense in western Greenland.


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A

dAfmh



f the elliptic intrusions (Fig. 2). The SE-vergent D1 thrust sys-em in the Isertoq Terrane is orthogonal to the strike of the AICFig. 11a). It could be related to: (1) the oblique convergence andinistral deformation along a SE-trend causing local NW–SE com-ression to the south; or (2) to earlier deformation elsewhere inhe southern terranes. The ca. 1860–1840 Ma WNW–ESE conver-ence and WNW-vergent deformation in western Greenland cane correlated with the deformation in the Isertoq Terrane, wherehe latter represents the hinterland to the Nagssugtoqidian Oro-en in the west (Fig. 11a). Alternatively, D1 in the Isertoq Terrane iselated to orogenic processes further southeast, now in Scandinaviand Scotland.

The orientation of the stress field in the eastern Nagssugtoqidianrogen during collision changed to a NE–SW direction during D2,

esulting in the deformation of all of the terranes and pure shearabrics in the AIC (Fig. 11b). The terranes underwent regional lowerressure metamorphism during ca. 1870–1820 Ma (Nutman et al.,008). The orogen received its doubly vergent geometry with NE-ergent thrusts in the Kuummiut Terrane and SW-vergent thrustsn the Isertoq Terrane (Fig. 2). Such a change is also observed in

estern Greenland at ca. 1825 Ma, where the regional stress fieldotated to and was orientated N–S (Fig. 11b; van Gool et al., 2002).

hether the subsequent ca. 1775 Ma sinistral strike-slip deforma-ion (van Gool et al., 2002) can be correlated with the sinistraltrike-slip shear zones of the Kuummiut Terrane, is presently unre-olved.

The NE–SW collision is followed by the D3 southeast-ward jux-aposition of the Schweizerland Terrane, which required furtherotation of the regional stress field to a NW–SE orientation eithert 1820 or 1740 Ma (Figs. 2 and 11b). Since the D3 overprint is notbserved in the southwestern Kuummiut Terrane and further tohe southwest, it is possibly related to an orogeny further westn the northern Aasiaat Domain and the Rinkian fold belt (Gardend Hollis, 2010). This hinterland deformation in the Schweizer-and Terrane is only followed by NE–SW extension, which can bexplained by orogenic collapse following convergence and collisionf terranes.

. Conclusions

The Nagssugtoqidian Orogen in South-West Greenland has aomplex structural history associated with superimposed defor-ation due to changes in the regional stress field. The SE-trending

elt of intrusions forming the AIC represents a continental-arcituated on Archaean rocks in the Isertoq Terrane as part of theorth Atlantic Craton. The high-pressure Kuummiut Terrane of theae Craton to the northeast represents the lower plate during aa. 1885–1870 Ma oblique WSW-directed subduction. The AIC andhe Kuummiut Terrane form the paired metamorphic belt of therogen. Collision between the two Archaean cratons resulted intress-field rotation and NE–SW compression orthogonal to theormer arc. The northern Schweizerland Terrane was juxtaposedo the southeast in a foreland setting of an orogeny in westernreenland. Orogenic collapse and intrusion of post-orogenic gran-

tes between ca. 1740 and 1680 Ma represent the final stages of theagssugtoqidian orogeny in South-East Greenland.

cknowledgements

I thank the members of the 2010 and other GEUS expeditions foriscussions in the field and the Precambrian geology of Greenland.


. Garde, B.M. Stensgaard and in particular L. Bagas are thankedor comments on earlier versions, which helped improving this

anuscript. Comments by reviewers C.R.L. Friend and A. Nutmanelped improving the manuscript significantly. The Geological


Survey of Denmark and Greenland and the Bureau of Minerals andPetroleum financially supported this study.

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Documents

Structure of the Palaeoproterozoic Nagssugtoqidian Orogen, South-East Greenland: Model for the tectonic evolution