A new fragment of the early earth crust: the Aasivik ...directory.umm.ac.id/Data Elmu/jurnal/P/Precambrian... · Precambrian Research 105 (2001) 115–128 A new fragment of the early

Precambrian Research 105 (2001) 115–128

A new fragment of the early earth crust: the Aasivik terraneof West Greenland

Minik T. Rosing a,b,*, Allen P. Nutman c, Linea Løfqvist a,b

a Kobenha6ns Uni6ersitet, Geologisk Museum, Øster Voldgade 5-7, DK-1350 Kobenha6n K, Denmarkb Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Kobenha6n K, Denmark

c RSES, The Australian National Uni6ersity, Canberra ACT 0200, Australia

Received 12 May 1999; accepted 27 August 1999

Abstract

The Aasivik terrane is a �1500 km2 complex of gneisses dominated by �3600 Ma components, which has beendiscovered in the Archaean craton of West Greenland, �20–50 km south of the Paleoproterozoic Nagssugtoqidianorogen. The Aasivik terrain comprises granulite facies tonalitic to granitic gneisses with bands of mafic granulite,which include disrupted mafic dykes. Four gneiss samples of the Aasivik terrain have given imprecise SHRIMP U–Pbzircon ages of 3550–3780 Ma with strong loss of radiogenic lead and new growth of zircon probably associated witha granulite facies metamorphic event(s) at �2800–2700 Ma. To the Southeast, the Aasivik terrane is in tectoniccontact with a late Archaean complex of granitic and metapelitic gneisses with apparently randomly distributed maficand ultramafic units, here named the Ukaleq gneiss complex. Two granitic samples from the Ukaleq gneiss complexhave U–Pb zircon ages of 2817 9 10 and 2820 9 12 Ma and tzircon oNd values of 2.3–5.4. Given their compositionand positive oNd values, they probably represent melts of only slightly older juvenile crust. A reconnaissance SHRIMPU–Pb study of a sample of metasedimentary rock from the Ukaleq gneiss complex found �2750–2900 Ma zirconsof probable detrital origin and that two or more generations of 2700–2500 Ma metamorphic zircons are present. Thisgneiss complex is provisionally interpreted as a late Archaean accretionary wedge. A sample of banded granulitefacies gneiss from a complex of banded gneisses south of the Aasivik terrain here named the Tasersiaq gneiss complexhas yielded two zircon populations of 3212 9 11 and 3127 9 12 Ma. Contacts between the three gneiss complexesare mylonites which are locally cut by late-post-kinematic granite veins with SHRIMP U–Pb zircon ages of �2700Ma. The isotopic character and the relationships between the lithologies from the different gneiss complexes suggestthe assembly of unrelated rocks along shear zones between 2800 and 2700 Ma. The collage of Archaean gneisscomplexes were intruded by A-type granites, here named the Umiatsiaasat granites, at �2700 Ma, later than thetectonic intercalation of the gneiss complexes. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Aasivik terrane; SHRIMP U–Pb; Ukaleq gneiss complex; Umiatsiaasat granites

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* Corresponding author. Tel.: +45 3532 2345; fax: +45 3532 2325.E-mail address: [email protected] (M.T. Rosing).

0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

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M.T. Rosing et al. / Precambrian Research 105 (2001) 115–128116

1. Introduction

Rock complexes with ages in excess of 3600 Mahave been identified in all the continents (Black etal., 1971, 1986; McGregor, 1973; Moorbath et al.,1972, 1973; Baadsgaard, 1973; Bowring et al.,1989; Bridgwater and Schiøtte, 1991; Myers, 1988;Nutman et al., 1991; Kinny and Nutman, 1996;Krylov et al., 1989; Song et al., 1996; Compstonand Kroner, 1988). However, the ancient compo-nents of the continents are in general found assmall enclaves, which are commonly migmatisedby younger felsic components. The largest contin-uous \3600 Ma complex is the West GreenlandItsaq Gneiss Complex, which underlies �3000km2. Friend et al. (1988) and Nutman et al. (1996)showed that the \3500 Ma rocks were confinedto the so-called Akulleq terrane, which is boundedto the Northwest by the Akia terrain and to theSoutheast by the Tasiusarssuaq terrain (Friend etal., 1988; Nutman et al., 1996). The Akulleqterrain is dominated by tonalite–trondhjemite–granodiorite (TTG) gneisses metamorphosed atamphibolite facies during the early Archaean. TheEarly Archaean rocks of the Godthabsfjord areathus represent allochtonous mid-crustal segmentsfrom an early Archaean continent. Further insightinto the thickness structure and composition ofthis early Archaean continental mass can begained by finding further segments of \3600 Marocks within the West Greenland Archaean block.

The three terrains of the Godthab region areseparated by Archaean mylonites and have sharedthe same geologic history since �2700 Ma, whichcan be regarded as the age of final accretion ofthat cratonic block. Currently, the northern limitof the Akia terrain is uncertain. Further north-wards, Archaean rocks are penetratively deformedin the southern part of the PaleoproterozoicNagssugtoqidian orogen (Hanmer et al., 1997 andreferences therein) (Fig. 1).

The region bordering the inland ice �50 kmsouth of the Nagssugtoqidian front and �100km north of the northern most exposure of theAkulleq terrain can be divided into a number ofdomains, based on Landsat image texture, streamsediment geochemistry (Steenfelt, 1994), occur-rence and deformation state of Paleoproterozoic

Fig. 1. Geologic map of central West Greenland showing thelocations of early Archaean terranes in the Akulleq belt andthe inferred extend of the Aasivik terrane shown in dark grey.

mafic dykes (Escher et al., 1970) and occurrenceof Mesozoic kimberlite and carbonatite intrusions(Larsen et al., 1983). A large domain shown as theAasivik terrane in Fig. 2, shows contrasts in all

Fig. 2. Map showing the distribution of topographic linea-ments, kimberlite intrusions and the inferred outline of theAasivik terrane.

M.T. Rosing et al. / Precambrian Research 105 (2001) 115–128 117

Fig. 3. Preliminary geologic map of the early Archaean Aasivik terrane and adjoining gneiss complexes.

these characters relative to the surrounding do-mains, and it is separated from bordering do-mains by pronounced topographic lineaments. Wesuggest that this domain is a tectonic terranedominated by early Archaean rocks.

Through most of this millennium the area hasbeen favoured by Eskimo caribou hunters, andtraces of their summer camps are frequently en-countered. We have therefore chosen the nameAasivik (= summer camp) for the terrane.

2. Geology

We have identified five crustal components inthe studied area (Figs. 2 and 3) (1) The Aasivikterrane, (2) the Ukaleq gneiss complex, (3) TheTasersiaq gneiss complex, (4) The Umiatsiaasat

granite plutons and (5) Mafic dykes tentativelycorrelated with the Kangamiut dykes in the ad-joining areas, but here referred to as ‘‘metadoler-ite dykes’’.

2.1. Aasi6ik terrain

The Aasivik terrain is dominated by garnetbearing two pyroxene granulite facies or-thogneisses. The gneisses have a layered appear-ance defined by 1–50 m thick, planer felsic macrounits with slightly varying colour index, interca-lated with boudinaged mafic units ranging from afew centimetres to 50 m thick. At one locality, thegneisses are crosscut by fractures each rimmed byup to 2 m in wide haloes of amphibolite faciesretrogression. In these zones, hornblende and bi-otite outline a mm–cm scale banding, which re-


veal very complicated fold patterns internally inthe macro units. The retrogression was not ac-companied by any deformation other than frac-turing. We therefore assume that the large gneissunits were generally strongly deformed relative totheir protolith, but that the granulite facies meta-morphic recrystallisation obscured complex struc-tures formed before or during metamorphism.This has implications for geochemical sampling,since even apparently homogeneous granulite fa-cies gneiss samples may represent a mixture ofseveral chemical and chronostratigraphic endmembers.

The structures outlined by the alternation ofmafic and felsic macro units is much simpler, anddefine large open folds with km-scale amplitude.This may suggest that the mafic bodies representdykes intruded into an already strongly deformedgneiss complex.

2.2. Ukaleq gneiss complex

The Ukaleq gneiss complex forms a package ofgranulite facies rocks dominated by garnet–bi-otite–feldspar–quartz 9 sillimanite rocks span-ning greywacke–pelite composition, interruptedby 0.5–1 m thick layers of garnet bearing leuco-granite, and rare thin diorite sheets. The felsicrocks contain abundant pods of hornblende–augite–hypersthene–plagioclase–quartz metaba-sites and olivine–tremolite–diopside–spinelultramafic rocks. Due to deformation and exten-sive anatexis of the felsic rocks, the complex hasthe appearance of a melange, and no sedimentarystructures have been observed. Based on its com-position, we interpret this gneiss complex asmainly supracrustal in origin, and dominated bymetasediments.

2.3. Tasersiaq gneiss complex

The Tasersiaq gneisses are dominated bybanded granulite facies orthogneisses and are sep-arated from the Aasivik gneiss complex and theUkaleq gneiss complex by E–W and NW–SEtrending topographic lineaments. The gneisseshave been retrogressed along the E–W lineamentsthat separate them from the Aasivik gneiss com-

plex. Tasersiaq gneisses are found within 100 mfrom a Umiatsiaasat granite dome, but it has notbeen demonstrated that the granite intruded theTasersiaq gneisses. The Tasersiaq gneisses are in-truded by the metadolerite dykes.

2.4. Umiatsiaasat granites

The supracrustal rocks of the Ukaleq gneisscomplex were intruded by granite plutons exposedin large topographic domes typically 3–5 km longand 1–2 km across. The axises of the domes trend140°, which is also the strike of foliations andlithologic layering in banded basement gneissesfound in the cusps between domes. In most cases,the granites are hosted by gneisses which can beidentified as belonging to the Ukaleq gneiss com-plex, but in a number of outcrops the bandedgneiss host cannot be assigned to any particulardefined unit. One granite dome extends across thelineament that separates the Aasivik and Ukaleqgneisses, which suggests that the granites wereemplaced later than the assembly of the gneisscomplexes.

2.5. Lineaments and Archaean mylonites

Marked topographic lineaments dissect the en-tire region in a nearly orthogonal pattern (Fig. 2).These lineaments coincide with the boundariesbetween the Aasivik gneiss complex, the Ukaleqgneiss complex and the Tasersiaq gneiss complex.Where examined, these lineaments also coincidewith zones of high strain, including mylonites.The mylonites have synkinematic growth of gar-net suggesting that they were formed at granulitefacies. The mylonites are generally annealed, andquartz and feldspar grains show no strain, butrecrystallisation at lower metamorphic grade isuncommon in the studied examples. In a numberof cases, the mylonites have experienced partialmelting at various stages in the deformation pro-cess. One prominent set of mylonites with folia-tions of 140°/30°NE and a sinistral sense ofdisplacement, and a less prominent set with folia-tion of 80°/70°SE and steeply plunging lineationhave been observed. However, systematic struc-tural studies of the high strain zones have not yet


been carried out. The high strain zones are hostsof a diffuse network of red granitic veins. In someplaces, these veins consist of mylonite or finegrained annealed mylonite. In other places, theveins are distinctly magmatic and consist ofisotropic garnet bearing myrmekitic granite.Gneisses in the vicinity of the veins are affected bymetasomatic infiltration, seen as a red colourationof the gneisses, which only partially overprint thecompositional banding. We interpret the redgranitic rocks as syn- to post-kinematic relative tothe high strain zones. They could have beenformed by flux melting within these zones as aresponse to fluid introduction during deforma-tion, or be preferentially intruded into these zonesfrom an external source.

2.6. Metadolerite dykes

The gneiss complexes were intruded by rareE–W trending metadolerite dykes. Such dykeshave not been observed in the post-tectonic gran-ites. However, the dykes show no signs of dis-placement along lineaments within the Aasivikgneiss complex. We interpret the dykes as em-placed later than the juxtaposition of the twocomplexes, but preferentially intruded into thegneissic roks for structural and rheological rea-

sons. The dolerite dykes show no signs of penetra-tive deformation, and the igneous mineralassemblage is widely preserved, although amphi-bolite facies metadoleritic domains are common.In places, the metamorphic hornblende–plagio-clase assemblage is replaced by garnet andclinopyroxene along fractures. We interpret thetwo metamorphic assemblages as formed duringone event of static metamorphism in which am-phibolite facies metamorphism was followed bydehydration, possibly in response to decompres-sion. Based on their field occurrence and petrog-raphy we correlate these mafic dykes with theE–W component of the PaleoproterozoicKangamiut dyke swarm (Ramberg, 1949).

3. Geochemistry

At present, geochemical characterisation of theAasivik gneisses is only at a reconnaissance stage.Major element compositions of apperantly homo-geneous lithologies from the Aasivik and Ukaleqgneiss complexes and the Umiatsiaasat granitesare given in Table 1. Sample 430476 is a trond-hjemite, while samples 430441, 430451 and 94-0000 are granites. These granites are slightlyperaluminous and alkaline, and project along the

Table 1Chemical compositions of representative lithologies from the Aasivik terrain determined by XRF at Geological Survey of Greenland(GEUS)a

430441 430451 430476Sample c 430498 940000 940012

72.71 74.12SiO2 67.59 65.67 74.11 74.250.160.18TiO2 0.15 0.93 0.17 0.3

12.6813.6614.48Al2O3 13.8714.5518.370.79Fe2O3 0.5 0.47 0.31 1.45 0.35

3.83 0.73 1.140.76FeO 0.730.840.02MnO 0.022 0.02 0.02 0.07 0.02

MgO 0.290.471.250.560.330.481.013.273.27 1.171.792.25CaO

2.87Na2O 3.83 3.14 6.34 3.21 3.953.52 4.6 1.66K2O 4.48 4.33 5.37

0.160.040.05P2O5 0.070.060.230.27 0.2Volat 0.440.29 0.440.2

Sum 99.3999.14 99.36 99.36 99.5 99.14

a 430441 = felsic gneiss cut by red veins, 430451 = charnockite, 430476 = trondhjemitic gneiss, 430498 = �2730 Ma post-kine-matic rapakivi granite, 940000 = red myrmekitic granite, 940012 = �2700 Ma post-kinematic granite.


1–2 kbar water saturated cotectic in the haplo-granite system. Petrographically they are sub-solvus granites and the excess Al is expressed asgarnet in the mode. The granites are thus formedat high pressure. Compositionally they resemblethe :2800 Ma Ikattoq gneisses of the Akulleqterrain in Godthabsfjord (McGregor et al., 1991).

4. Zircon geochronology

4.1. Analytical technique and data assessment

U–Th–Pb isotopic ratios and concentrations ofzircons were determined using SHRIMP I andwere calibrated to the Australian National Uni-versity standard zircon SL13 (572 Ma; 206Pb/238U = 0.0928). Descriptions of analyticalprocedure and data assessment were given byCompston et al. (1984), Nutman (1994) andClaoue-Long et al. (1995). Comparative isotopedilution and SHRIMP analyses of zircons fromseveral well-preserved Proterozoic and Archaeansamples (Roddick and van Breemen, 1994; Ire-land, 1995) demonstrate that SHRIMP 207Pb/206Pb ratios are accurate.

As further cross-check on the accuracy of207Pb/206Pb ratios obtained with SHRIMP, zir-cons from the Paleoproterozoic norite QGNGwere analyzed interspersed with some of the un-knowns. From isotope dilution thermal ionisationanalysis, different QGNG zircon fractions have207Pb/206Pb ages of 1850 9 2 Ma (C.M. Fanning,personal communication, 1995), and �1850 Mato as low as �1810 Ma (T. Skjold, personalcommunication, 1996). QGNG analyses run withsamples presented here yielded a 207Pb/206Pbweighted mean age of 1863 9 18 Ma.

SHRIMP zircon geochronology results on ninesamples are reported here. For some of them,many zircon analyses were undertaken, whereasfor others fewer were done. The results are suffi-cient to demonstrate that there is an importantearly Archaean component to the crust in thearea, and has determined the age of late Archaeanrocks and some metamorphic zircon growth withprecisions of �10 Ma (2s).

4.2. Aasi6ik gneiss complex

4.2.1. 94-0338 and 94-000094-0338 and 94-0000 are migmatitic gneisses.

94-0338 and 94-0000 yielded similar mixed zirconpopulations. Brown prismatic grains are domi-nant, up to 200 mm long, commonly with pro-nounced mm-scale euhedral zoning and locallymetamict. In a few cases, the centres of thesegrains consist of clearer, non-zoned zircon. Thesegrains show deep embayments on prismatic facesand rounding of pyramidal terminations. Lesscommonly, there are overgrowths of clear zirconon their terminations. Also present are a fewsmall, clear to brown oval to multifaceted grains.The heavy mineral separates also contained mon-azites, which were not analyzed.

In 94-0338, three analyses of an oval grain gaveclose to concordant ages, with a 207Pb/206Pb meanage of 2712 9 12 Ma (Fig. 4a). The dominantpoorly preserved prismatic grains of 94-0338 havemoderate to high U content (up to 750 ppm) andyielded mostly discordant ages with 207Pb/206Pbages between �3200 and 3600 Ma. Several grainshave 207Pb/206Pb ages of �3600 Ma, one ofwhich is concordant, within error. These grainsare interpreted to be early Archaean in age, andto have suffered variable loss of radiogenic Pb ina zircon-growth event at �2700 Ma, also morerecently. Three analyses with the oldest 207Pb/206Pb ages yield a mean age of 3596 9 9 Ma.However, given the disturbed nature of the zir-cons, this is interpreted by us as a minimum age,but probably close to the true value.

In 94-0000, metamorphic overgrowths and abrown oval grain yielded close to concordant ageswith 207Pb/206Pb ages between 2628 9 70 and2711 9 44 Ma (2s), with a mean value of2688 9 12 Ma, indistinguishable at the 95%confidence level from the 2712 9 12 Ma age de-termined on a metamorphic grain in 94-0338.Analyses of the dominant brown prismatic grainsyielded an array of older ages, of which two withthe oldest 207Pb/206Pb ages of \3500 Ma areclose to concordant (Fig. 4b). This indicates animportant early Archaean component in thissample.


Fig. 4. U–Pb concordia diagrams for SHRIMP analysis of zircons from a) 94-0338: Migmatitic gneiss from the Aasivik terrain. b)94-0000 red myrmekitic granite/migmatite bordering the Aasivik terrain. c) 430476 Felsic orthogneiss from the Aasivik terrain, d)430475 Felsic orthogneiss from the Aasivik terrain.

4.2.2. 430441430441 is a banded gneiss migmatised by a fine

network of red coloured granitic veins just northof the E–W trending lineament separating theAasivik and the Ukaleq gneiss terrains. It yieldedmostly pale yellow to clear prismatic grains up to300 mm long with aspects ratios of 2–5 and havewell-developed mm-scale euhedral zoning. Pyrami-dal terminations are slightly rounded, and one hasa metamorphic overgrowth, too small to be ana-lyzed with the 40-mm spot used in the analyticalsession. The grains are interpreted as a magmaticpopulation, slightly modified during metamor-phism. Amongst the �50 mounted hand-pickedgrains one was devoid of euhedral zoning, butwith a possible rind of new zircon, a few micronsthick. Analyses of two of the dominant well-zonedprismatic grains showed high U contents (\1000ppm), close to concordant ages, with a 207Pb/206Pbmean age of 2817 9 10 Ma, which is interpretedas the timing of magmatic crystallisation of thegranitic neosome (Fig. 4b). Two analyses of thenon-zoned zircon gave a low U content (B100ppm), close to concordant ages, with a 207Pb/206Pb

mean age of 3784 9 18 Ma. This probably indi-cates an early Archaean age of the banded gneissprotolith.

4.2.3. 430476 and 430475The zircons in 430476 and 430475 are similar to

the main population in 94-0338, but are slightlylarger on average, and less strongly coloured. Sixanalyses of 430476 grains yielded a discordantarray on a concordia diagram, like the 94-0338zircons (Fig. 4c). In this case, no analyses yieldedconcordant ages and the oldest 207Pb/206Pb agewas 3559 9 48 (2s). Five analyses of grains from430475 yielded discordant points (Fig. 4d), withthe least discordant result having the oldest 207Pb/206Pb age of 3602 9 8 Ma (2s). As for samples94-0000 and 94-0338 the (moderate to high U)protolith zircons suffered Pb-loss in a late Ar-chaean, generating the observed discordant arrayof data. These results are not sufficient to defineaccurate and precise ages, but they do indicate thezircons are probably a strongly disturbed 3600Ma population.


4.3. Ukaleq gneiss complex

4.3.1. 94-0006The supracrustal rock 94-0006 is a garnet–bi-

otite–feldspar–quartz gneiss interpreted as asemipelitic metasediment. It gave a low yield ofzircons. Dominant are equant to oval grains, B100 mm across, strongly zoned and brown incolour. Some of these grains form twins. Alsopresent are prismatic and rounded anhedral grainsup to 150 mm long. These are brown to yellowand also generally strongly zoned. The termina-tions of these grains are rounded. The equant,oval and twinned grains have on average higher Ucontents and lower Th/U than the rounded pris-matic grains. A reconnaissance study of sixteengrains was undertaken on this sample. All analy-ses yielded close to concordant ages (Fig. 6) with207Pb/206Pb ages from 2530 9 46–2905 9 20 Ma(2s). The oval, equant and twinned grains yielded207Pb/206Pb ages of up to 2726 9 24 Ma and ifmetamorphic in origin probably grew or wererecrystallised in several high-grade events. Therounded prismatic grains yielded the oldest ages(1-1, 8-1, 10-1, 11-1 in Table 2). If they are detritalin origin, their ages indicate that the sedimentmust have been deposited in the late Archaean.

4.3.2. 430451430451 is massive granulite facies orthogneiss

or charnockite just south of the E–W trendinglineament separating the Aasivik terrane and theUkaleq gneiss complex and within tens of metresfrom the contact to a Umiatsiaasat granite dome.It yielded a population very similar in morphol-ogy to those in 430441 (just north of the linea-ment, and within the Aasivik gneiss complex) andlikewise is interpreted as a magmatic population,slightly modified during metamorphism. However,neither inherited grains nor metamorphic over-growths were observed. Six analyses were under-taken, all of which showed moderate to high Ucontents (100–987 ppm) and yielded close to con-cordant ages (Fig. 5). Grain 3-1 (Table 2) yieldedthe oldest age of 2877 9 42 Ma (2s) and is mostdiscordant. However, from optical microscopy,there is no evidence it is an inherited core. Withno rejections, all analyses yielded a 207Pb/206Pb-

weighted mean age of 2812 9 8 Ma, with analy-sis 3-1 indistinguishable from the mean. Rejecting3-1, and 6-1 (interpreted as having undergoneslight ancient Pb-loss) yielded an indistinguishableage of 2820 9 12 Ma. These results indicate themagmatic crystallisation of 430451 shortly before2800 Ma.

4.3.3. 430455440455 is a thin diorite sheet which form an

early component in the gneisses hosting the Umi-atsiaasat granites. It yielded a population of cleancolourless to pale yellow prismatic grains, up to200 mm long, with some faint euhedral zoning.Neither inherited cores nor metamorphic over-growths were observed. The grains show onlyslight rounding from metamorphic corrosion.Five grains were analyzed, which had low Ucontent (59–154 ppm) and close to concordantages (Fig. 5d). All analyses yielded a 207Pb/206Pb-weighted mean age of 2701 9 16 Ma, interpretedas the magmatic age of the sample.

4.4. Tasersiaq gneiss complex

A granulite facies gneiss 430428 from the Taser-siaq gneiss complex yielded prismatic and morerarely equant colourless to very pale yellow zir-cons, up to 250 mm long and mostly with lowaspect ratios of 1–3. They are non-zoned or showweak mm-scale euhedral zoning, more prominenttowards, the exterior of the grains. Inclusions ofapatite and opaques are common. Pyramidal ter-minations are slightly rounded, and some pris-matic faces are slightly embayed. There are noobvious overgrowths of metamorphic zircon.Twelve grains were analyzed. They have low Ucontents (73–250 ppm), yielded concordant orclose to concordant ages (Fig. 5a) with a range in207Pb/206Pb ages from 3226 9 36 Ma–2971 9 41(2s). Four analyses (4-1, 8-1, 9-1 and 11-1)yielded a 207Pb/206Pb-weighted mean age of3212 9 11 Ma. Five analyses (1-1, 2-1, 6-1, 7-1,12-1) yielded a distinctly younger age of 3127 912 Ma, and the remaining three somewhatyounger ages. There are no obvious morphologi-cal differences between grains of different age, buton average, the oldest ones have the lowest U


Table 2SHRIMP U–Pb zircon analyses

Spot Th/U f206% 206Pb/238U ratio 207Pb/206Pb ratio 207Pb/206Pb (age) % (disc)U (ppm)

94-03381.76215 0.11 0.737 9 22 0.3202 9 42 3571 9 20 01-1

0.05 0.708 9 14 0.3297 9 141.88 3616 9 07215 −51-20.18707 0.04 0.633 9 44 0.2723 9 09 3319 9 50 −52-10.19394 0.03 0.652 9 13 0.2928 9 07 3433 9 04 −63-1

0.11 0.695 9 17 0.3220 9 140.46 3580 9 074-1 −51440.16548 0.04 0.620 9 18 0.2822 9 13 3375 9 07 −84-2

0.03 0.556 9 08 0.2439 9 055-1 3145 9 03667 −90.140.20 0.516 9 13 0.1883 9 221.76 2727 9 1962 −26-1

0.45170 0.02 0.527 9 30 0.1859 9 17 2706 9 15 +16-20.016-3 0.474 9 2189 0.1854 9 29 2702 9 25 −71.240.01 0.636 9 11 0.3000 9 080.82 3470 9 04365 −97-1

0.14638 0.06 0.608 9 09 0.2636 9 13 3269 9 08 −68-10.07 0.603 9 21 0.3158 9 229-1 3550 9 11164 −140.81

B0.01 0.559 9 10 0.2430 9 160.17 3140 9 11568 −910-10.31366 0.09 0.621 9 18 0.3073 9 68 3507 9 34 −1111-1

0.01 0.668 9 15 0.3049 9 27 3495 9 14 −612-1 479 0.230.06 0.677 9 15 0.3122 9 391.07 3532 9 2013-1 −6342

0.19435 0.02 0.626 9 11 0.2747 9 21 3333 9 12 −614-10.05 0.666 9 15 0.2892 9 33 3413 9 18 −415-1 443 0.280.03 0.629 9 13 0.2831 9 14 3380 9 08 −70.6274816-1

94-0000485 0.64 0.14 0.552 9 13 0.2027 9 36 2848 9 29 01-1

0.39 0.716 9 15 0.3134 9 15 3538 9 07 −22-1 218 1.330.20 0.692 9 15 0.3164 9 181.89 3552 9 163-1 −52271.00 0.628 9 12 0.2785 9 094-1 3354 9 05680 −60.680.27 0.598 9 10 0.2745 9 100.61 3332 9 065-1 −8522

0.22253 0.01 0.514 9 11 0.1773 9 38 2628 9 35 +26-1B0.01 0.516 9 11 0.1838 9 080.55 2687 9 07128 07-1

0.41161 0.05 0.505 9 11 0.1831 9 14 2681 9 13 −28-19-1 0.01315 0.515 9 13 0.1791 9 17 2644 9 16 +10.28

B0.01 0.504 9 24 0.1864 9 250.41 2711 9 2210-1 −338811-1 0.05822 0.548 9 11 0.2400 9 07 3120 9 05 −100.338

4304750.17 0.531 9 141-1 0.2916 9 24581 3426 9 13 −200.260.06 0.616 9 18 0.3055 9 070.25 3498 9 04289 −122-1

0.61210 0.12 0.717 9 18 0.3267 9 08 3602 9 04 −33-10.59183 0.06 0.648 9 17 0.3011 9 24 3476 9 12 −74-1

0.11 0.633 9 15 0.2746 9 12 3333 9 07 −50.312005-1

43047648 0.06 0.79 0.594 9 57 0.2712 9 13 3313 9 79 −91-1

0.15 0.655 9 213-1 0.3177 9 4988 3559 9 24 −90.290.01 0.678 9 16 0.3086 9 090.17 3514 9 05644 −54-1

0.42189 0.12 0.699 9 18 0.3170 9 20 3555 9 10 −45-10.26 0.667 9 246-1 0.2604 9 6660 3249 9 40 +10.070.18 0.550 9 14 0.2251 9 13 3018 9 09 −60.117-1 204

43042873 0.76 0.24 0.619 9 36 0.2418 9 54 3132 9 36 −11-1

2-1 0.18147 0.606 9 15 0.2417 9 15 3131 9 10 −20.630.38 0.573 9 17 0.2187 9 290.54 2971 9 21109 −23-1


Table 2 (Continued)

Spot U (ppm) Th/U f206% 206Pb/238U ratio 207Pb/206Pb ratio 207Pb/206Pb (age) % (disc)

0.51 0.454-1 0.645 9 1795 0.2566 9 13 3226 9 08 −10.82 0.25 0.606 9 17149 0.2450 9 236-1 3153 9 15 −3

1427-1 1.04 0.07 0.580 9 13 0.2386 9 14 3111 9 09 −51458-1 0.57 0.09 0.634 9 15 0.2524 9 33 3200 9 21 −1

0.60 0.06 0.613 9 13116 0.2523 9 189-1 3200 9 11 −417610-1 1.05 0.03 0.568 9 23 0.2287 9 15 3043 9 11 −5

11-1 0.6383 0.31 0.591 9 15 0.2521 9 20 3198 9 12 −60.85 0.06 0.600 9 27 0.2421 9 32 3134 9 21250 −312-1

4304410.13 0.03 0.574 9 141-1 0.1978 9 121452 2808 9 10 +40.21 0.02 0.528 9 131969 0.1998 9 112-1 2825 9 09 −3

583-1 0.67 1.00 0.814 9 27 0.3648 9 30 3770 9 13 +20.90 0.38 0.793 9 32 0.3720 9 32 3800 9 133-2 −177

4304510.91 0.04 0.557 9 22 0.1978 9 23 2808 9 19 +21-1 7470.49 0.04 0.560 9 14987 0.1993 9 082-1 2820 9 06 +2

2823-1 0.55 0.14 0.516 9 14 0.2063 9 27 2877 9 21 −70.39 0.17 0.557 9 18100 0.1999 9 324-1 2825 9 26 +1

1915-1 0.37 0.22 0.535 9 14 0.1993 9 25 2821 9 20 −26-1 0.63128 0.19 0.548 9 14 0.1957 9 10 2791 9 08 +1

4304551.20 0.071-1 0.514 9 15154 0.1842 9 20 2691 9 18 −10.51 0.05 0.492 9 13 0.1822 9 35 2673 9 31 −42-1 541.08 B0.01 0.521 9 11152 0.1860 9 123-1 2707 9 10 0

604-1 0.71 0.09 0.499 9 20 0.1853 9 48 2701 9 43 −30.63 0.48 0.488 9 16 0.1859 9 46 2706 9 415-1 −559

94-00060.44 0.25 0.561 9 25 0.1938 9 19 2775 9 16 +31-1 2130.17 0.19 0.520 9 15787 0.1798 9 122-1 2651 9 11 +20.14 0.043-1 0.513 9 15617 0.1729 9 39 2586 9 38 +30.42 0.21 0.545 9 20466 0.2100 9 134-1 2905 9 10 −3

8605-1 0.20 0.03 0.508 9 15 0.1698 9 05 2556 9 05 +40.15 0.11 0.507 9 156-1 0.1777 9 26690 2628 9 24 +10.16 0.17 0.506 9 16488 0.1672 9 237-1 2530 9 23 +4

1828-1 0.49 0.06 0.564 9 20 0.2083 9 13 2892 9 10 09-1 0.20194 0.21 0.511 9 16 0.1882 9 13 2726 9 12 −2

0.12 0.14 0.516 9 15501 0.1824 9 109-2 2675 9 09 046110-1 0.81 0.25 0.571 9 16 0.1979 9 109 2809 9 93 +4

0.31 0.13 0.555 9 1611-1 0.1981 9 28366 2811 9 23 +10.17 0.21 0.489 9 14501 0.1737 9 2312-1 2593 9 22 −1

64313-1 0.22 0.11 0.500 9 14 0.1691 9 27 2549 9 26 +20.1614-1 0.12674 0.513 9 14 0.1795 9 26 2649 9 24 +10.24 1.00 0.530 9 16324 0.1911 9 3915-1 2751 9 33 0

57416-1 0.15 0.04 0.505 9 16 0.1720 9 19 2577 9 19 +20.1416-2 0.08614 0.491 9 14 0.1793 9 19 2647 9 18 −3

content. The rock is interpreted as having twoage components of 3212 9 11 Ma and 3127 912 Ma, followed by subsequent disturbance.

From the zircon data alone, it is uncertain

whether the ages represent different phases in acomposite sample or whether the protolith is3127 9 12 Ma, but carries 3212 9 11 Ma in-herited material.


5. Discussion

U–Pb age determination has been carried out onzircons extracted from a number of units. Samples430428, 430475 and 430476 are regional bandedgneisses, 94-0006 is a paragneiss, 94-0338 and430441 are red-coloured metasomatised gneisses,430451 is an isotropic charnockite, probably adiatexite, 94-0000 is a red myrmekitic granite and430455 is an undeformed diorite dyke intruded intothe supracrustal package. As can be seen fromTable 2 and Figs. 3–6, the regional gneisses givedisturbed ages in the range �3600–3780 Ma forthe samples within the Aasivik terrain as outlinedin Fig. 1. Sample 430428 south of the boundinglineament contains :3100 and :3200 Ma com-ponents. These ages have not been represented inthe zircon populations in other samples from theregion. The ages found in this sample are within therange of ages of gneisses from the Akia terrane tothe south (Friend et al., 1988; Nutman et al., 1996)and it is tentatively suggested that the Tasersiaqgneiss complex forms a part of the Akia terrane.

The supracrustal sample from the Ukaleq gneisscomplex contains 2750–2900 Ma zircons of prob-able detrital origin, which gives the maximum ageof deposition. This along with a positive oNd of theanatectic granites (tzircon oNd = 2.3–5.4; Løfqvist,unpublished data) formed within the metasedi-ments suggests that the sources of the sedimentswere dominated by juvenile or only slightly oldermaterial at 2800–2700 Ma. The chaotic intercala-tion of metapelic rocks, mafic granulites and meta-peridotites together with the occurrence wedgedbetween a late Arechaean and an early Archaeangneiss complex suggests that the Ukaleq gneisscomplex form part of a late Archaean accretionarywedge.

The red granites and the charnockitic diatexitefound in the lineaments separating the differentgneiss complexes are all dominated by 2700–2800Ma zircons. We suggest that this age range isbracketing the time of deformation in the zones,and thus the time of tectonic juxtaposition of thegneiss complexes.

Fig. 5. U–Pb concordia diagrams for SHRIMP analysis of zircons from a) Felsic gneiss from the Tasersiaq terrain. b) 430428 Redcoloured banded gneiss bordering the Aasivik terrain. c) 430451 Diatexite near contact of Umiatsiaasat granite dome. d) 430455Diorite sheet in gneisses along contact of Umiatsiaasat granite dome.


Fig. 6. U–Pb concordia diagram and relative probabilityhistogram for SHRIMP analysis of zircons from a sample ofUkaleq paragneiss.

The nature of the contact between this gneisscomplex and the Aasivik terrane has not beeninvestigated in this study, but a pronounced topo-graphic lineament coincides with contrasts instream sediment geochemistry (Steenfelt, 1994),topographic grain, occurrence of klimberlite in-trusions (Larsen et al., 1983). This lineament sepa-rates a region where no early Archaean zirconshave been identified in the gneisses (Kalsbeek andNutman, 1996) to the north from the early Ar-chaean gneisses to the south, and is here taken asthe terrane boundary.

We suggest that the gneiss complexes were as-sembled at :2700 Ma and formed a stable cra-ton during Nagssugtoqidian orogeny whichcaused penetrative deformation and metamor-phism of all rock units only 20–50 km to thenorth.

Kimberlite intrusions which are common in theadjoining terrains have not been identified withinthe Aasivik terrane, which in turn is partly definedby the apparent absence of kimberlite intrusions.By applying the well-proven method of circularreasoning, we can deduce that the terrane con-straint on the distribution of kimberlite intrusionsindicates that the Aasivik terrane has deep litho-spheric roots. The occurrence of diamond in akimberlite south of the Aasivik terrane (NunaoilA/S press release, 1996) indicate that the kimber-lite originated at \150 km depth. This is accordwith the barometric estimates by Larsen andRønsbo, 1993) for kimberlites along the inferredwestern and northern boundaries of the Aasivikterrane. If this interpretation is correct the earlyArchaean terrane is the exposed lower crustallevel of a continent which had already existed forone billion years at the time of terrane assemblyat 2700 Ma, and not merely a tectonic panel suchas a fault block or nappe. This interpretation is inaccord with Abbott (1996), who suggested thatonly accreted material with deep lithosphericroots could have been stabilised as parts of thecontinental crust during the Archaean. In thisrespect, the Aasivik terrane differs from theAkulleq terrane of the Godthabsfjord region,which represent tectonic slices of an old gneisscomplex intercalated with younger gneiss units.The Akulleq terrane is dominated by mid-crustal

The diorite dyke 430455 is associated with thepost-kinematic granites, and indicate that mem-bers of the :2700 Ma magmatic suite intrudedthe supracrustal rocks of the Ukaleq gneiss com-plex. The age of 2701 9 16 Ma for the diorite isin agreement with U–Pb TIMS analysis of zirconfrom a granite pluton which gave 2717 9 4 Ma(J. Connelly, personal communication, 1995).These ages support the field observation that thegranites were emplaced late in the 2800–2700 Matectono-metamorphic event responsible for the as-sembly of the gneiss complexes.

This study shows that the Aasivik gneiss com-plex has a major early Archaean component, andis separated from rock complexes of different agesand origin by zones of high strain. We suggestthat the Aasivik gneiss complex is a tectonometa-morphic terrane for which we propose the nameAasivik terrane. The area immediately north ofthe Aasivik terrane is dominated by :2800 Mafelsic gneisses (Kalsbeek and Nutman, 1996),which are overprinted by the PaleoproterozoicNagssugtoqidian orogeny further to the north.


amphibolite facies rocks, while the Aasivik ter-rane is at high-pressure granulite facies. The pos-sibility that the Aasivik terrane represents theroots of the allochthonous Akulleq terrane shouldbe investigated further by detailed geochemistryand chronometry.

6. Conclusions

The region south of the Nagssugtoqidian frontin West Greenland comprises three Archaeangneiss complexes, which were assembled at 2800–2700 Ma. The early Archaean Aasivik terranegives U–Pb zircon ages of 3500–3600 Ma for theformation of gneiss protoliths, but probably com-prise gneisses formed back to �3800 Ma, judgingfrom the occurrence of zircons with an age of3780 Ma. This gneiss terrain is dominated bygranitic gneisses with a complicated deformationhistory, intruded by large mafic dykes of un-known age, and now represented by pods andlayers of mafic granulite. No large supracrustalunits have been identified in the early Archaeangneiss complex at present. The Aasivik terraneprobably underlies �1500 km2. This terrane is incontact to the Southwest with the Tasersiaq mid-Archaean gness comples, which we provisionallyinterpret as the northern extension of the Akiaterrane (eg. Nutman et al., 1996) of the Nuukregion. One sample of banded gneiss from thisgneiss complex has given ages of 3127 9 12 Maand 3212 9 11 for the formation of gneiss pro-toliths. The late Archaean Ukaleq gneiss complexto the Southeast is dominated by supracrustalrocks with similarities to modern accretionarycomplexes and intruded by post-kinematic A-typegranite plutons, which probably also stitch thetectonic boundaries between the early Archaeanand the mid-Archaean and late Archaeanterranes.

Acknowledgements

This study was carried out in collaborationwith the Danish Lithosphere Centre Nagssug-toqidian project funded by the Danish National

Research Fond. Funding by the Carlsberg Foun-dation and RSES at the Australian National Uni-versity, Canberra is gratefully acknowledged. Wethank David Bridgwater, Vic McGregor, JimConnelly, Flemming Mengel, Jeroen van Gooland Clark Friend for discussions and suggestions.Heidi Thomsen, Nick Rose, Kyle Mayborn andEsbern Hoch are thanked for company in thefield.

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