The late Archaean mobile belt through Godthabsfjord, southern West Greenland: a continent-continent collision zone?

V. R. McGREGOR, C. R. L. FRIEND and A. P. NUTMAN

McGregor, V. R., Friend, C.R. L. & Nutman, A. P.: The late Archaean mobile belt through Godthabs­ fjord, southern West Greenland: a continent-continent collision zone? Bull. geol. Soc. Denmark, vol. 39, pp. 179-197. Copenhagen, December 20th, 1991. https://doi.org/10.37570/bgsd-1991-39-08

In the Godthabsfjord region of southern West Greenland a NE-SW-trending belt of rocks of very varied age and origin, here named the Akulleq terrane, is separated by major faults from more extensive blocks of typical high-grade Archaean rocks that, although they are superficially similar, have different ages and metamorphic histories.Th e continental crust that forms the block to the north-west, the Akia terrane, was accreted between ea. 3200 and 2980 Ma, and that forming the block to the south-east, the Tasiusarsuaq terrane, between 2920 and 2800Ma. It is suggested that the Godthabsfjord belt is the result of collision of the two continental blocks between 2800and 2650 Ma. The rocks of the Akulleq terrane are interpreted as fragments of different parts of the crust that originally separated the two continents. They include early Archaean continental crust, possible oceanic crust, and acid to intermediate rocks of intrusive and possibly also extrusive origin that may have been generated in a subduction-related environment.

V. R. McGregor, Atammik, DK-3912 Maniitsoq, Greenland. C. R. L. Friend, Department of Geology, Oxford Polytechnic, Oxford OX3 0BP, Oxon, U.K. A. P. Nutman, Research School of Earth Sciences, Australian National University, Canberra 2601, Australia. October 2nd, 1990.

A major breakthrough in understanding the com­ largely retrogressed to amphibolite facies. We plex, high-grade Archaean geology of the God­ assumed that before retrogression there must thabsfjord region of southern West Greenland have been a prograde amphibolite facies to gra­ was the recognition between 1985 and 1988 of nulite facies transition extending through Frerin­ three major blocks or terranes separated from gehavn, outer Buksefjorden and Ameralik one another by tectonic breaks. The blocks had (McGregor et al. 1986), and the main aim of our quite differentlithological associations and histo­ field work in 1984 and 1985 was to locate and ries before they were juxtaposed in the late Ar­ study this prograde zone. However in 1985, dur­ chaean (Friend et al. 1987, 1988b, Nutman et al. ing detailed remapping in the Freringehavn- Tre 1989). Previously it had been assumed that the Br�dre area and reconnaissance work in Buksef­ geology was essentially continuous across the jorden and Ameralik, we found a sharp, map­ whole region (McGregor et al. 1986). pable boundary between rocks to the south-east The firstrecognition of a major tectonic break that had been retrogressed from granulite facies came from our attempts to delimit the extent of and rocks to the north-west that had never reac­ late Archaean granulite facies metamorphism be­ hed granulite facies. Although this boundary had tween Godthabsfjord and Fiskemesset. During been modified by intense later deformation, we reconnaissance work over several fieldseasons in concluded that it must be a major fault. In the the first half of the 1980s we established a number central section of Ameralik rocks retrogressed of textural criteria for recognising rocks retro­ from granulite facies form the cores of synforms gressed from granulite facies (McGregor et al. and overlie rocks that never reached granulite 1986), and confirmed the interpretation of Win­ facies, suggesting that the boundary was origi­ dley (1972) that the rocks in the coastal strip from nally a thrust. During helicopter reconnaissance Fiskenresset north to around Freringehavn, as in 1987 the fault was traced across the area south well as in the inner part of Buksefjorden and the of inner Ameralik, where it is less modified by middle section of Ameralik, had been metamor­ later deformation and its thrust nature is clearer. phosed to granulite facies, but subsequently It was named the Qarliit nunaat thrust (Friend et

12 DGF 39 180 McGregor et al.: Late Archaean mobile belt al. 1988a). The block affected by late Archaean gneisses in the Færingehavn - Tre Brodre area granulite facies metamorphism south-east of the and the division of the geology into Fzringehavn thrust was named the Tasiusarsuaq terrane and Tre Brodre terranes appear to be local rather (Friend et al. 1987). than regional features. The name Akulleq terrane The detailed mapping in 1985 showed that in is introduced here for the belt of complex geology the Færingehavn - Tre Brodre area early Ar- north-west of the Qarliit nunaat thrust that in- chaean Amitsoq gneisses and enclosed supracrus- cludes all the rocks originally assigned to the tal and other lithologies are separated by zones of Fzringehavn and Tre Brodre terranes. "Akulleq" deformed blastomylonites from an association of is Greenlandic for "the one in the middle". lithologies that includes supracrustal rocks A second regional break that separates the (mainly quartz-cordierite gneisses), layered Akulleq terrane from mid-Archaean gneisses and metabasic rocks (leucogabbro, anorthosite, mela- enclosed supracrustal rocks to the north-west was gabbro) and a small component o£ Amitsoq recognised in 1987 in the area around Nuuk gneisses, all intruded by voluminous, mainly gra- (Godthåb) town and was traced north as far as nodioritic, quartzo-feldspathic gneisses. The Ilulialik in inner Godthåbsfjord in 1988. The name Ikkattoq gneisses was proposed for the in- name Ivinnguit fault is introduced here for this trusive quartzo-feldspathic gneisses (Friend et al. break, from a locality where it cuts the south-east 1988b). Conventional bulk zircon dating gave dis- coast of Bjorneoen in Godthåbsfjord. The block cordant results, with a maximum mPb-Z06Pbage of mid-Archaean rocks to the north-west was of ca. 2790 Ma for the least discordant sample named the Akia terrane (Friend et al. 1988a). (Nutman et al. 1989). Because all contacts be- Some of the stratigraphic groupings introduced tween on the one hand the major units of early in the late 1960s and early 1970s include rocks of Archaean rocks and on the other the Ikkattoq quite different ages and origins in different ter- gneisses and the lithologies they intrude are tec- ranes. The term Malene supracrustals was applied tonic in the Fzringehavn - Tre Brodre area, these to all supracrustal rocks of presumed mid-Ar- lithological associations were assigned to sepa- chaean age in the Godthåbsfjord region (McGre- rate terranes, named respectively the Fzringe- gor 1969, 1973). It is now apparent that this in- havn terrane and the Tre Brodre terrane (Friend cludes amphibolites of probable mid-Archaean et al. 1987). During 1987 they were traced over a age in the Akia terrane, amphibolites, quartz- large area in inner Ameralik and Kapisillit fjord cordierite gneisses and clastic metasediments of (Friend & Nutman 1988). However field work in late Archaean age in the Akulleq terrane, and 1988 and 1989 confirmed the obsewations of ear- mid-late Archaean amphibolites in the Tasiusar- lier field work (McGregor 1973) that gneisses of suaq terrane. We recommend that the use of the Ikkattoq type clearly intrude the type Amitsoq term Malene supracrustals be discontinued. No gneisses in outer Ameralik and Kobbefjord and new formational names are proposed for these on Stor0 in Godthåbsfjord. It now appears that rocks since it is uncertain to what extent they while relations between the early Archaean rocks originally formed continuous sequences or, more and the younger supracrustal sequences along likely, are fragments of a number of sequences with associated layered metabasic intrusions may that were juxtaposed during terrane formation be mainly or entirely tectonic, all these litholog- and accretion. The term Nuk gneisses was used in ical associations were intruded by Ikkattoq earlier papers for all quartzo-feldspathic gneisses gneisses (see below). Thus the tectonic separa- in the Godthåbsfjord region that did not contain tion of the Amitsoq gneisses and Ikkattoq Ameralik dykes and that intruded and migma-

Fig. 1 (opposite). Sketch map of the Godthåbsfjord region showing the disposition of the Akia, Akulleq and Tasiusarsuaq terranes as defined in this paper. There is still some doubt as to the true disposition of the boundanes in the area north-east of Godthåbsfjord towards Isukasia. The position of the granulite-amphibolite facies transition zone in the area north of the head of Bj~rnesundis unknown. The map is compiled from the 1:500 000 and 1:100 000 map sheets published by the Geological Survey of Greenland (Gr~nlandsGeologiske Unders~gelse)and our own published and unpublished data. Terrane boundaries are marked: If = Ivinnguit fault; Qnt = Qarliit nunaat thrust. Key toplace names: B = Bj~rnegen;Buk = Buksefjorden; F = Fiskenæsset; Fh = Færingehavn; I = Ililualik; Ka = Kapisillit; Ko = Kobberfjord; Q = Qooqqut; Qi = Qilanngaarsuit; Qk = Qussuk; S = Sadel@;TB = Tre Brodre. A list of Greenlandic place names with old and new spellings is to be found at the end of this paper, after References. Bulletin of the Geological Society of Denmark 181

Inland Ice 650

,' Proteroroic faults

Undivided supracrustal roc and thin anorthosites

nIkkattoq gneiss 2800 Ma BAmfisoq gneiss 3880-3400Ma

Tasiusarsuaq terrane mIlivertalik granite 2800 Ma Mid-late Archaean gneiss 2920-2800Ma 2800Ma granulite facies conditions only subject to amphibolite facies conditions

Akia terrane Taserssuaq tonalite 2980Ma and Finnefjeld gneiss Nuk gneiss 3200-2980Ma

12' McGregor et al.: Late Archaean mobile belt tised Amitsoq gneisses, Malene supracrustal and are a minor, but widespread, component of rocks and the associated metabasic intrusive the geology over a much wider area. Preliminary rocks (McGregor 1973, 1979). The term Nok SHRIMP dating of zircons indicates that dioritic gneisses is now restricted to the type rocks gneisses on southern Nordland are ca. 3200 Ma around the town of Nuuk and on Sadel0 and old (H. Baadsgaard, pers. comm.). Tonalitic BjØrne~en(Baadsgaard & McGregor 1981) and gneisses form continuous belts between the ma- to other gneisses in the Akia terrane that can be jor units of dioritic gneiss on northern Nordlan- correlated with them (Nutman et al. 1989). All or det and are the dominant lithology to the north most of the rocks mapped as Nilk gneisses within and north-east. Tonalitic gneisses from Bjarne- the Akulleq terrane can probably be included Øen in the south-eastern part of the terrane, within the Ikkattoq gneisses. where the metamorphic grade never exceeded The present paper provides a status report on amphibolite facies, have given conventional bulk our interpretation of the geology of the God- zircon concordia-intercept U-Pb ages of 3070- thåbsfjord region. We demonstrate that this re- 3000 Ma (Baadsgaard & McGregor 1981) and gion comprises two blocks of .continental crust SHRIMP concordant ages of 3040-3000 Ma (H. formed by similar processes in the middle and Baadsgaard, pers. comm.). Large, relatively ho- late Archaean respectively, and a complex belt mogeneous bodies of granodioritic gneiss that in- between them made up of rocks of very different trude the tonalitic gneisses in this area have given lithologies, ages and provenances, that were bulk zircon concordia-intercept U-Pb ages of ca. welded into a single entity in the late Archaean. 2980 Ma (Baadsgaard & McGregor 1981) and a A number of new, as yet unpublished, SHRIMP Rb-Sr whole rock isochron age of 2980 f 50 Ma (sensitive high-resolution ion microprobe) ages (Taylor et al. 1980). North of inner Godthåbs- determined at Australian National University, fjord the Taserssuaq tonalite, made up of rela- Canberra, within the last year are referred to tively weakly deformed tonalitic, granodioritic here. and minor more basic phases, crops out over an We follow Harland et al.'s (1982) division of area of 2200 km2. It has yielded a conventional the Archaean into early (4000-3400 Ma), middle concordia-intercept zircon age of 2982 f 7 Ma (3400-2900 Ma) and late (2900-2500 Ma). Place (Garde et al. 1986). names are spelt using the new orthography, offi- The tonalitic gneisses enclose many rafts and cial in Greenland since 1973. Since these spellings larger units of amphibolite, including both lay- are to be found on only very few published maps, ered and massive types. Relict pillow-lava and a list of place names with new and old spelling is agglomerate structures have been recognised at a provided at the end of this paper. The old orthog- few localities, but in most units primary struc- raphy is however retained for stratigraphic terms tures have been erased by strong deformation. originally defined when this was in use. On Sadel0 and BjØrneØen the supracrustal rocks include some with komatiitic composition that locally appear to have relict pillow structures, The northern block (Akia terrane) and abundant large pods of massive olivine-py- roxene-hornblende ultramafic rocks. Smal1 The,Akia terrane is dominated by mid-Archaean amounts of metasedimentary gneisses occur lo- dioritic, tonalitic, trondhjemitic and granitic cally with the amphibolites, for example white gneisses together with supracrustal rocks, mainly garnet-sillimanite-cordierite-bearing quartzo- amphibolites, and smal1 amounts of metabasic feldspathic gneisses in central Nordlandet. A intrusive rocks (leucogabbro, norite). Most of the large but highly fragmented body of leucogabbro, terrane was affected by granulite facies meta- intruded by both dioritic and tonalitic gneisses, morphism which was shown by Pb-Pb whole rock crops out in a complex synclinorium on western studies (Taylor et al. 1980) and SHRIMP dating Nordlandet. It is underlain by a unit of supracrus- of zircon overgrowths (H. Baadsgaard, pers. tal amphibolite with minor metasediments, but comm.) to have occurred at ca. 3000 Ma. there is no clear evidence of the relative ages of Dioritic gneisses are the dominant lithology on the leucogabbro and the supracrustal rocks. the Nordland peninsula west of Godthåbsfjord The south-easternmost part of the Akia ter- Bulletin of the Geological Society of Denmark

rane - a narrow belt immediately west of the there could not have been a pervasive fluid of l Ivinnguit fault that includes the Nuuk peninsula, constant composition and thus no regional influx Sadel@,Bj~rneoen and part of the peninsula be- of CO,. Low values of fHzo and fcoz estimated by tween Qussuk and inner Godthåbsfjord - was Riciputi et al. (1990) suggest that conditions were never metamorphosed above amphibolite facies. fluid-absent at the peak of metamorphism. Most of the rest of the terrane was metamor- The extent of the Akia terrane towards the phosed to granulite facies, with peak metamor- north is uncertain. The geology is continuous in phic conditions on Nordlandet estimated to have the coastal strip at least as far north as 6S0N, been 800 + 50°C and 7.9 + 1.0 kbar (Nutman et where rocks similar to those on Nordlandet are al. 1989, Riciputi et af. 1990). There was retro- cut by a large body of post-granulite facies ton- gression to amphibolite facies over large areas alitic gneiss, the Finnefjeld gneiss (Berthelsen (Garde 1989,1990), at least part of which was the 1962, Marker & Garde 1988). From the Finnef- result of influx of water associated with late de- jeld gneiss north to the boundary of the Protero- formation. This is clearest in and adjacent to the zoic Nagssugtoqidian mobile beft in the Sondre late belt of intense ductile deformation that ex- Stromfjord region all the rocks have been meta- tends through'rocks retrogressed from granulite morphosed to granulite facies. Between the Fin- facies along the east coast of Nordlandet and the nefjeld gneiss and Maniitsoq (Sukkertopppen) west side of Qussuk (see below). The combined there was extensive retrogression to amphibolite effects of retrogression and late deformation facies associated with deformation. Essentially have obscured the original prograde amphibolite unretrogressed granulite facies rocks extend con- to granulite facies transition. Textural evidence in tinuously north from Maniitsoq to the Nagssug- the retrogressed, amphibolite facies rocks indi- toqidian boundary. A major change in the geol- cates that it passed through the north-eastern ogy is the relative abundance of metasedimentary corner of Qussuk. gneisses in the supracrustal units north of the Weakly deformed, very leucocratic, coarse- Finnefjeld gneiss as far as Evighedsfjord com- grained trondhjemitic and less common granitic pared with their scarcity to the south. The fact gneisses break up the dioritic and tonalitic that gneisses from Maniitsoq lie on the same 3000 gneisses and the supracrustal rocks. They occur Ma Pb-Pb whole-rock isochron as rocks from as diffusely bounded patches and veins in the Nordlandet (Taylor et al. 1980) suggests that the dioritic gneisses, as sharply cross-cutting sheets, Akia terrane continues north of the Finnefjeld some quite undeformed, and as the cores of sev- gneiss. However F. Kalsbeek (pers. comm.) has eral diapiric domes that post-date more than one obtained a Rb-Sr whole-rock isochron age of generation of early folds. Orthopyroxene is gen- 2625 f75 Ma with Sri = 0.7063 k 0.0012 from erally absent from larger bodies of the late granulite facies supracrustal rocks from Hambor- trondhjemitic and granitic gneisses, but is com- gerland, 25 km north of Maniitsoq. mon in smaller bodies. Smal1 intrusive bodies of Iate, pIagiocIase-rich diorites occur in or adjacent to mafic lithologies. Pillar (1985) suggested that The southern block (Tasiusarsuaq they were the products of partial melting of mafic terrane) parents in the presence of hydrous fluids, pos- sibly in the early stages of granulite facies meta- Rocks that were metamorphosed to granulite fa- morphism. In rocks where there is no evidence of cies extend from the Qarliit nunaat thmst south retrogression associated with influx of water, the for 120 km to inner Bjornesund, where a well pyroxenes are commonly rimmed by dark green preserved prograde, amphibolite facies to granu- hornblende and the ore minerals by sheaves of lite facies transition has been recognised (McGre- biotite. Pillar (op. cif.) suggested that the late gor & Friend in prep.). There has been very hornblende was the result of reactions involving a extensive retrogression to amphibolite facies. smal1 amount of residual melt on grain bounda- Rocks in which orthopyroxene is preserved occur ries during cooling from peak metamorphic con- as discontinuous areas and patches on all scales, ditions. He found variable aHzoin samples col- There is no evidence of any major post-granulite lected from adjacent outcrops and concluded that facies break in the geology south of the Qarliit 184 McGregor et al.: Late Archaean mobile belt

Table 1. Archaean geological evolution of the Godthibsfjord area and adjacent regions. Il Age in Akia terrane Akulleq terrane Tasiusarsuaq terrane . Ma , I Minor granite and pegmatite sheets I 2500 - NNE-SSW ductile deformation zone Qtirqut granite complex. including Satut qneisses 2600 g r ;p" 2700 Collis~onof Akia and Akulleq terranes. F2 upright NE-trending folds æ$

-, Supracrustal rocks. rnainly n submarine basic lavas 31 z 7 supracrustal rocks 5 Intrusion of protoliths of Ameralik and Tarssartoq basic dykes 3200 2 - dioriticgneisses (p Intrusion of leucogabbro m 2 3300 7supracrustal rocks

- Pegmatitic gneisses. Isukasia area 3400 -

3500

m Granulite facies metamorphism. 3600 intrusion of Fe-rlch Amitsoq Suite In -0: south. intrusion of protoliths of Y Amitsoq granitic gneisses in Isukasih area Intrusion of protoliths of Amitsoq 3700 g tonalitic gneisses. Isukasia area 5 m (D 3800 g - Isua supracrustal belt - Protolith of tonalitic gneiss. Angisorsuaq island - Protolith 01 tonalitic gneiss. 3900 Akilia rsland Supracrustal rocks (part of Akilia association)

nunaat thrust at least as far as the mouth of pared with the Akia terrane there is less dioritic Bj0rnesund. The whoIe of this tract is therefore gneiss and apparently more granodioritic gneiss. considered to belong to the Tasiusarsuaq terrane. In an ENE-WSW-trending belt 40 km wide be- Like the Akia terrane, the Tasiusarsuaq ter- tween Fiskenzsset and BjGrnesund the gneisses rane is dominated by tonalitic gneisses that were also enclose lqyers and rafts derived from a major intruded as sheets into sequences of mainly basic layered complex of deformed and metamor- volcanic rocks that now occur as layers (generally phosed anorthosite, leucogabbro, gabbro and pe- less than 500 m thick) and rafts of amphibolite ridotite, the Fisken~ssetComplex (Myers 1981, with a very subordinate sedimentav component 1985). Similar rocks crop out locally farther north (Myers 1976, Kalsbeek & Garde 1989). Com- in the terrane, for example immediately above Bulletin of the Geological Society of Denmark

the Qarliit nunaat thrust on both sides of Amer- Several lines of evidence suggest that the Tasiu- alik. Just north of and overlapping with the belt sarsuaq terrane is a tilted crustal section from the in which the Fiskenzsset Complex occurs there time of the formation of the Qarliit nunaat thrust are very extensive outcrops of deformed porphyr- with progressively deeper levels exposed from itic granite, the Ilivertalik granite, associated with south-east (Bjornesund) t'o north-west (Qarliit metadiorite and metatonalite. The granite forms nunaat thrust) (McGregor & Friend, in prep.). three thick and numerous thin sheets. It post- The section has been strongly modified by defor- dates major recumbent isoclinal folds in the ear- mation that post-dates the thrust. Peak meta- lier rocks and was itself deformed by two phases morphic conditions within 10 km of the thrust are of major folding. estimated to have been 780 f 60°C and 8.9 f 1.0 Isotopic evidence indicates that the main com- kbar (Nutman et al. 1989, Riciputi et al. 1990). ponents of the Tasiusarsuaq terrane have primary There is little field evidence of partial melting crystallisation ages of 2920-2800 Ma. The oldest associated with granulite facies metamorphism in isotopic age obtained from the terrane is a this terrane except in the deepest level adjacent SHRIMP age of 2922 f 7 Ma on zircons with to the Qarliit nunaat thrust where melting of igneous features from a gneiss that contains intermediate to basic lithologies gave rise to mi- metamorphosed basic dykes 12 km south-east of nor diorites. These are more mafic than the syn- the Qarliit nunaat thrust (Schiotte et al. 1989a). granulite facies diorites in the Akia terrane. In Gneisses with metamorphosed basic dykes from the prograde transition zone in inner Bjornesund two other localities close to the thrust contain orthopyroxene grew at the expense of horn- zircons with cores that have given SHRIMP ages blende in a wide variety of lithologies without the of 2840 Ma (two samples) and 2826 Ma (P. Kinny formation of significant quantities of melt. There 1987 and pers. comm.). A Sm-Nd isochron age of are no syn-granulite facies veins of trondhjemite 2860 f 50 Ma has been obtained on samples from or granite, and orthopyroxene grains do not have the Fiskenzsset Complex (Ashwal et al. 1989). rims of late hornblende, as in the Akia terrane. A Zircons from a tonalitic gneiss that cuts leuco- sharp, but highly irregular, front that crosses lith- gabbro of the Fiskenæsset Complex near the ological boundaries separates orthopyroxene- mouth of Bjornesund, where the rocks did not bearing rocks from orthopyroxene-free rocks on reach granulite facies, have given a SHRIMP age outcrop scale. The field evidence suggests that in of 2863 f 8 Ma (20) (Nutman, unpublished the prograde transition zone orthopyroxene data). A granulite facies sample of the Ilivertalik formed as the result of influx of fluids with low granite from the northern margin of the belt in aHl0 and presumably high a,,,. This may not be which the Fiskenzsset Complex crops out has true of deeper levels in the Tasiusarsuaq terrane. given a bulk zircon concordia-intercept age of Riciputi et al. (1990) reported low estimates of 2795 f 11 Ma (Pidgeon et al. 1976, Pidgeon & fCOz in the northern part of the terrane. Kalsbeek 1978). Reconnaissance SHRIMP zircon In unretrogressed areas granulite facies rneta- dating of a granulite facies sample of Ilivertalik morphism outlasted deformation. In other areas granite from Sermilik has given an age of 2814 f retrogression of the granulite faces rocks was as- 14 Ma (20) (Nutman, unpublished data). A near- sociated with widespread ductile deformation. It concordant age of 2790 Ma for a large, euhedral involved influx of hydrous fluids that migrated zircon grain from a mafic pod in anorthosite upwards from below the Qarliit nunaat thrust within granulite facies rocks was considered by (Friend et al. 1988a) and along and out from Pidgeon & Kalsbeek (1978) to'date the granulite deformation zones. The retrogressing fluids car- facies metamorphism. These results imply that ned Pb and probably other elements from rocks the whole of the plutonic development of the of the Akulleq terrane underneath the thrust into Tasiusarsuaq terrane, including the intrusion of the northern part of the Tasiusarsuaq terrane the magmatic parents of the Fiskenzsset Com- (Nutman & Friend 1989). plex and the gneisses, intrusion of the Ilivertalik granite, three major phases of folding and granu- lite facies metamorphism, took place within a period of little more than 100 Ma. McGregor et al.: Late Archaean mobile belt

The Godthåbsfjord belt (Akulleq charactenstic feature of this unit is the fact that terrane) the rocks were metamorphosed to granulite fa- I cies at ca. 3630 Ma, before intrusion of the Amer- The NE-SW-trending belt between the Ivinnguit alik dykes (Griffin et al. 1980, Baadsgaard et al. fault and the Qarliit nunaat thrust is made up of 1984, Kinny 1986). Relict granulite facies mineral four main lithological associations of very differ- assemblages are preserved on smal1 islands south ent age and origin that have been intercalated by of Qilanngaarsuit and near Fzringehavn. Relict tectonic and intrusive processes. These are: granulite facies textures in totally retrogressed rocks have been observed where later deforma- (1) Early Archaean rocks tion has been weak as far north as Qooqqut, as (2) Supracrustal rocks not intruded by Amitsoq far east as Itilleq in inner Ameralik and as far gneisses south as the Tre Bradre area. Another character- (3) Layered metabasic intrusive rocks istic feature of the southern part of the unit is the (4) Ikkattoq gneisses. presence of an Fe-rich suite of granodioritic au- gen gneisses and ferrodioritic gneisses (Nutman et al. 1984). Zircons in an augen gneiss from the (1) Early Archaean rocks mouth of Ameralik have given a SHRIMP age of The early Archaean rocks were originally sep- 3659 f 11 Ma (P. Kinny, pers. comm.), showing arated out because they contain very abundant that the suite is about the same age as the granu- amphibolites derived from basic dykes, the Am- lite facies metamorphism and may be genetically eralik dykes, that are absent from the other lith- related to it. Rocks of the Fe-rich suite are wide- ological units in the Akulleq terrane (McGregor spread between Ameralik and Tre Brodre, but 1973). In the Isukasia area similar dykes locally have not been recognised north of Ameralik. retain many igneous features and are termed the Most isotopic ages from the southern unit cluster Tarssartôq dykes (Nutman et al. 1983). Extensive around 3600-3650 Ma and probably reflect reset- isotopic work has demonstrated that the rocks cut ting associated with granulite facies metamor- by Ameralik and Tarssartôq dykes range in age phism (Baadsgaard et al. 1984, Kinny 1986). from 3880 (Nutman, unpublished data) to 3400 However SHRIMP zircon dates of 3822 f 5 Ma Ma (see McGregor et al. 1986 for references). (Kinny 1986) and 3877 k 10 Ma (20) (Nutman, The early Archaean rocks are dominantly var- unpublished data) from tonalitic gneisses that in- iable, polygenetic, quartzo-feldspathic gneisses trude the type Akilia association on islands south grouped under the term Amitsoq gneisses. They of the mouth of Ameralik show that this unit enclose and intrude a variety of supracrustal and includes rocks older than anything recognised basic and ultrabasic intrusive lithologies, the from unit (b) to the north. largest unit of which forms a 35 km-long arcuate (b) Rocks characterised by abundant metamor- tract termed the Isua supracrustal belt (Nutman et phosed basic dykes and typical Akilia association al. 1983, Nutman 1986). The remaining occur- lithologies such as banded ironstones and clino- rences of pre-Amitsoq gneiss rocks are included pyroxene-hornblende rocks extend from north- under the name Akilia association (McGregor & ern Ivisaartoq along the margin of the Inland Ice Mason 1977). The early Archaean rocks occur as north to the Isukasia area, where they include the structural units of variable size separated by units Isua supracrustal belt. This unit (b) is separated of younger rocks. Most of the outcrops belong to from unit (a) by supracrustal amphibolites and two major units that are not connected. Ikkattoq gneisses, and it appears to be structur- (a) The southern and larger unit includes the ally above unit (a). There is no evidence to sug- type Amitsoq gneisses in outer Ameralik gest that the rocks in unit (b) reached granulite (McGregor 1973) and extends south through the facies and there are no rocks in the unit equiv- islands between the mouths of Ameralik and alent to the Fe-rich suite of unit (a). The best Buksefjorden to the Fzringehavn - Tre Br0dre dates from the Isua supracrustal belt are a area (Friend et al. 1987) and north and east to SHRIMP zircon date of 3807 f 1 Ma (Compston Stor0 and, through rafts and the roof zone in the et al. 1986) and a multigrain conventional zircon Qôrqut granite complex, to inner Ameralik. A concordia-intercept date of 3813 f 7 Ma (Baads- Bulletin of the Geological Society of Denmark

Table 2. Analyses o€Nuk and Ikkattoq gneisses al. 1989, Nutman & Fnend 1989, Schi~tteet al. 1989a).

SiO, 71.7 TiOZ 0.20 (2) Supracrustal rocks not intruded by Amitsoq A1203 15.4 gneisses Fez03 1.5 MnO 0.02 Supracrustal rocks in the Akulleq terrane are Mg0 0.5 more varied than in the Akia and Tasiusarsuaq CaO 2.2 NaZO 5.0 terranes. They occur as thin but very continuous KzO 2.6 units intercalated with the early Archaean rocks ''20s 0.08 and as layers and enclaves enclosed in and in- I.O.I. tmded by the Ikkattoq gneisses. Several litholog- Trace elements (ppm) ical associations can be distinguished. Rb 53 Sr 562 Amphibolite is the most abundant supracrustal Ba 1017 Y <2 lithology in the Akulleq terrane and forms se- Pb 24 quences with minor ultramafic rocks, but with Zr 107 Nb 2 other supracrustal lithologies absent or very sub- Cr 24 ordinate. The best preserved and thickest unit is on Ivisaartoq in inner Godthåbsfjord, where 1. Average af 13 amphibolite facies Niik granodioritic gneisses, south-eastern Akia terrane (McGregor 1979). many primary structures are recognisable even 2. Average af 4 Ikkattoq granodioritic gneisses, Færingehavn - though the rocks have been metamorphosed to Tre Bredre area, Akulleq terrane. amphibolite facies (Hall 1980, Hall et al. 1987, 3. Ikkattoq dioritic gneiss (85-363), Færingehavn - Tre Bredre area, Akulleq terrane. Chadwick 1990). Pillow-lava structures are pre- All chemical analyses an Ikkattoq gneisses were performed in served in rocks with a continuum of compositions the geochemical laborataries ol the Department o€ Earth Sci- ences, Memorial University af Newfoundland, Canada. Major from low-K tholeiitic to komatiitic. Diopside, element analyses were obtained by atomic absorption spectro- epidote, garnet, carbonate and locally scheelite photometry and colourimetry by G. Andrews. Trace element in veins and the cores of pillows are considered to data were obtained by X-ray fluorescence spectrometry on pressed powder pellets. reflect sea-floor and metamorphic alteration (Appel 1988, Chadwick 1990). The pillow-stmc- tured rocks are intercalated with concordant, ho- gaard et al. 1984). Dating by several methods mogeneous amphibolites interpreted as derived shows that the dominant tonalitic gneisses that from massive flows and sills. The amphibolites enclose and intrude the Isua supracrustal belt are enclose very continuous sheets of olivine-rich 3690 f 50 Ma, and the granitic gneisses that rocks derived from dunites and subordinate harz- intrude the tonalitic gneisses as anastomising burgites. Gradation of some of the ultrabasic sheets are 3590 f 40 Ma, while minor pegmatitic rocks into metagabbros led Chadwick (1990) to gneisses are 3370 + 60 Ma (Baadsgaard et al. conclude that they were differentiated sills in- 1986). truded into the submarine lavas. On the basis of Gneisses that contain fragments of basic dykes the primitive chemistry of the Ivisaartoq basic similar to the Ameralik dykes crop out adjacent rocks compared with the more evolved chemistry to the Akulleq terrane in both the Akia and of the Ameralik dykes (Gill & Bridgwater 1979, Tasiusarsuaq terranes and were originally Chadwick 1981), Hall et al. (1987) suggested that mapped as Amitsoq gneiss (McGregor 1973, the Ivisaartoq metavolcanics were not erupted 1984, McGregor et al. 1986) before the Qarliit through and onto early Archaean continental nunaat thrust and the Ivinnguit fault were recog- crust, but are a fragment of oceanic crust. Else- nised. Isotopic studies on the supposed Amitsoq where in the Akulleq terrane the basic supracms- gneisses in the northern part of the Tasiusarsuaq tal rocks have been transformed by intense defor- terrane give no indication af an early Archaean mation into layered amphibolites. In many places age (Jones et al. 1986, Kinny 1987, Collerson et they have discontinuous layers containing diop- McGregor et al.: Late Archaean mobile belt

side, epidote, scapolite and sphene, indicating silty and muddy sediments. The trace element that they were denved from altered pillow lavas. chemistry of the rocks suggests derivation from felsic volcanic or plutonic sources (McLennan et Quartz-cordierite gneisses are the next most al. 1984). SHRIMP dating of euhedral zircons in abundant type of younger supracrustal rocks in a single sample gave a semi-continuous range of the Akulleq terrane. They are quartz-rich, alumi- 207Pb-2"Pb ages between 3000 and 2750 Ma nous, ferromagnesian gneisses with the mineralo- (Schiotte et al. 1988). One arkosic and one pelitic gy quartz + cordierite + orthoamphibole, silli- sample gave T,,", model Sm-Nd ages of 2660 manite, garnet, biotite and staurolite (Beech & and 2820 + 30 Ma (Hamilton et al. 1983). Chadwick 1980, Dymek & Smith 1990). They Garnet-mica-graphite-bearing gneisses form a make up a unit more than 100 m thick that can be thick unit between amphibolites and layered traced for several tens of kilometres in the south- metabasic intrusive rocks on northern StorØ. western part of the terrane in contact, but not Their trace element chemistry suggests a mixed interlayered, with amphibolites. The unit is com- provenance of mafic volcanics with felsic volcan- positionally layered, although in places it is re- ics andlor tonalitic gneisses (McLennan et al. markably massive and homogeneous. Similar 1984). rocks, possibly belonging to the same unit, crop Minor quartzites are present in some supra- out in several other areas within the terrane. crustal units, especially adjacent to contacts with Baadsgaard (1976) reported a near-concordant early Archaean rocks. Some of these are recrys- date of 2720 Ma for zircons in a quartz-cordierite tallised vein quartz formed in fault zones (see gneiss from the south-western part of the Akulleq below). SHRIMP dating of zircons from a quart- terrane. Compared with most Archaean rocks of zite collected 5 m from a contact between early sedimentary origin, the Akulleq quartz-cordiente Archaean gneisses and supracrustal rocks on Si- gneisses have high but variable SiO,, enrichment miutaa at the mouth of Ameralik give a wide in MgO, Fe0 and AI20,, depletion in CaO, Na20 range of 2WPb-2"Pbages between 3755 and 2720 and K20, very high Zr, Hf, Ta, Nb, Th and U, Ma, with most euhedrally zoned grains in the low Sc, Cr, COand Ni, and enriched and fraction- ranges 3300-3200 and 2980-2800, and rounded ated REE with negative Eu anomalies (Dymek & unzoned grains mostly between 3200 and 2900 Smith 1990). Similar rocks have not been re- Ma (Schi~tteet al. 1988). These ages do not ported from other parts of the Archaean of support the hypothesis that the quartzite is a de- Greenland south of Disko Bugt. The very un- trital sediment derived from the adjacent Amit- usual chemistry implies a special mode of origin soq gneisses (Nutman & Bridgwater 1983), nor or alteration. Beech & Chadwick (1980) sug- do they correspond to known age ranges in the gested that the parents of these rocks were rnix- Akia or Tasiusarsuaq terranes. A possible, al- tures of detrital quartz and precipitated Mg-rich though somewhat contrived, interpretation is clay minerals, with detrital zircon as the source of that the quartzite was a sediment denved from all the high contents of trace elements. Dymek & three sources. Zircons from a quartzite unit up to Smith (1990) noted that the quartz-cordierite 50 m thick that can be traced for many kilometres gneisses have many chemical features in common between inner Ameralik and Kapisillit fjord with felsic igneous rocks and suggested that they (Fnend & Nutman 1988) have given SHRIMP were derived from water-lain acid to intermedi- ages between 3880 and 3650 Ma that correspond ate volcanic matenal, possibly mixed with a detri- closely to known ages ranges in the southern unit tal component of continental provenance, that of early Archaean rocks (Kinny et al. 1987). This was affected by intense chemical alteration. suggests a clastic sediment derived from rocks originally continuous with this unit. Terrigenous clastic metasediments form moder- ately thick and homogeneous sequences in sev- (3) Layered metabasic intrusive rocks eral places. Biotite-muscovite-sillimanite-bearhg quartzo-feldspathic gneisses in contact with am- Meta-anorthosites and meta-leucogabbros, asso- phibolites on Rype0 near Nuuk were interpreted ciated with subordinate marginal amphibolites by Dymek et al. (1983) as derived from sandy, denved from gabbros, and minor olivine-horn- Bulletin of the Geological Society of Denmark

blende layers, occur as continuous units and as where they are not strongly deformed, usually enclaves in the Ikkattoq gneisses that intrudeand have speckled textures that are secondary after migmatise them. Where they are not strongly coarse-grained plutonic textures in the protoliths. deformed, the leucogabbros commonly have cu- Similar textures are not common in the Ikkattoq mulate megacrystic textures. The association has gneisses, suggesting that their protoliths were many similarities to the Fiskenzsset Complex in finer grained. Ikkattoq gneisses characteristically the Tasiusarsuaq terrane (Myers 1985), although have widely-spaced, thin pegmatitic layers. These chromitites, a characteristic minor component of textures and structures suggest that the protoliths the latter, have not been reported in the Akulleq of many of the Ikkattoq gneisses were intruded terrane. The layered metabasic rocks are inte- and crystallised syntectonically, with deformation preted as derived from one or more large strati- during crystallisation of the magmas inhibiting form bodies intruded into the supracrustal rocks. the growth of large grains and producing primary Few intrusive features have survived migmatisa- gneissic foliation and layering, in part defined by tion and deformation, and it is not certain which thin pegmatites. By contrast, many of the proto- of the supracrustal associations the stratiform liths of the Niik gneisses appear to have crystal- bodies were intruded into. lised under static conditions that permitted coarse-grained plutonic textures to develop. The Ikkattoq gneisses differ compositionally (4) Ikkattoq gneisses from many other Archaean grey gneiss associ- Quartzo-feldspathic gneisses, the protoliths of ation~,for example the amphibolite facies Niik which intruded lithological associations (l), (2) gneisses in the south-eastern part of the Akia and (3), make up between a third and a half of terrane (Table 2). Tonalitic compositions are rare the outcrops in the Akulleq terrane. Until re- or absent in the Ikkattoq gneisses, but are the cently few of these had been dated and they were most abundant lithology in the Niik gneisses. The considered to be the same age as the type Niik Ikkattoq dioritic gneisses have SiO, contents in gneisses in the eastern part of the Akia terrane the range 554% which is not represented in the (McGregor 1973, 1984, Compton 1978, Coe & Niik gneisses. Ikkattoq granodioritic gneisses are Robertson 1982, 1984, Chadwick & Coe 1983, richer in Fe, K, Rb, Y, Zr and Nb, and consid- Robertson 1986). Conventional zircon dating of erably poorer in Na, Sr and Cr than Nfik grano- gneisses in the Fzringehavn - Tre Breidre area in dioritic gneisses with similar SiO, contents. the southern part of the Akulleq terrane sug- Ikkattoq-like gneisses from an area within the gested that the Ikattoq gneisses were intruded Akulleq terrane in inner Godthiibsfjord have giv- 2800-2750 Ma ago (Nutman et al. 1989). Further en a Rb-Sr whole-rock isochron date of 2767 f 40 SHRIMP dating of samples that span much of the Ma with Sr, = 0.7022 f 0.0006, and a Pb-Pb area underlain by Ikkattoq gneisses in the Akul- whole-rock isochron date of 2962 f 73 Ma with leq terrane has yielded more accurate, concord- an apparent single-stage p value of 6.64 f 0.05 ant ages of 2825-2810 Ma (L. Schieitte pers. (Robertson 1983, 1986). Comparison with zircon comm., H. Baadsgaard & A.P. Nutman unpub- ages of Ikkattoq gneisses elsewhere in the Akul- lished data). leq terrane suggests that the older Pb-Pb date and The Ikkattoq gneisses are dominantly grano- the low apparent single-stage p are the result of dioritic in composition with minor dioritic phases contamination with Pb from early Archaean in some areas. Many of them were intruded as crust. Samples of probable Ikkattoq gneisses sub-concordant sheets, commonly along contacts studied by Taylor et al. (1980) have Pb character- between the older lithological units. They differ istics that indicate contamination with early Ar- in appearance from the type Niik gneisses in sev- chaean crustal Pb. Sm-Nd characteristics of the eral ways. They are much more homogeneous in Ikkattoq gneisses also indicate involvement of outcrop than the Niik gneisses, which are com- early Archaean sialic material (H. Baadsgaard, monly polyphase, a feature shared with the ma- pers. comm.). jority of the early Archaean Amitsoq grey gneisses and the tonalitic-granodioritic gneisses in the Tasiusarsuaq terrane. The Niik gneisses, McGregor et al.: Late Archaean mobile belt

Relations between lithological they are slices of oceanic crust that were inter- associations in the Akulleq terrane leaved tectonically with the early Archaean rocks. There is local evidence of tectonic inter- Intense deformation and the intrusion of Ikkat- leaving within the supracrustal sequence, for ex- toq gneisses along many contacts have destroyed ample imbrication of quartz-cordierite gneisses most evidence of the original relations between and layered basic intrusive rocks on Quersuaq, the early Archaean rocks and the supracrustal south-west of Qilanngaarsuit. units that are interleaved with them. There is Tectonic contacts with blastomylonites be- good field evidence that at least some of the tween units of early Archaean rocks and of Ik- contacts are tectonic (McGregor 1973, 1975, kattoq gneisses with enclosed supracrustal and Chadwick & Nutman 1979, Hall & Friend 1979). Iayered basic intrusive rocks in the Fzringehavn - Contacts considered to be deformed unconform- Tre Brodre area led Friend et al. (1987) to sep- ities were described on Qilanngaarsuit by Chad- arate these units into the Fzringehavn and Tre wick & Nutman (1979) and on Simiutaa by Nut- BrØdre terranes respectively. However in several man & Bridgwater (1983). Re-examination of the parts of the Akulleq terrane Ikkattoq gneisses contact on the west coast of Qilanngaarsuit in- clearly intrude early Archaean rocks. In the F=- dicates that it, too, is tectonic. In a low deforma- ringehavn - Tre Bredre area itself there are smal1 tion area near the core of a fold, 5-10 m of areas of early Archaean rocks intruded by Ik- blastomylonite interlayered with very fine- kattoq gneisses within the Tre Brodre terrane. In grained greenish quartzite separate supracrustal outer Ameralik and Kobbefjord the type Amit- amphibolites from Amitsoq gneisses that adja- soq gneisses are clearly intruded by sheets of cent to the mylonite are strongly sheared and Ikkattoq gneiss of all sizes. A possible explana- contain clots of green mica. Along the same con- tion of these relations is that intrusion of the tact on nearby fold limbs, where deformation was Ikkattoq magmas overlapped in time with tec- more intense, the mylonites have been strongly tonic interleaving. Ikkattoq magmas appear com- thinned or completely cut out and the amphibo- monly to have been intruded along thrust planes lites are now separated from Amftsoq gneisses by between and within the older lithological units thin layers of micaceous rocks or by quartzites, or during or after interleaving. Locally, as in the are in sharp contact with the Amitsoq gneisses. Fzringehavn - Tre BrØdre area, there was tec- Relations along the amphibolite-Amitsoq gneiss tonic interleaving after the intrusion of the Ik- contacts on Simiutaa are more ambiguous, but kattoq magmer. there is no unequivocal evidence of an uncon- formity. Some of the quartzites adjacent to the contact appear to be silicified Amitsoq gneisses. Tectonic boundaries of the Akulleq Quartzite lenses immediately along the contact terrane are similar to the quartzites within the blastomy- lonites on western Qilanngaarsuit. The ages of The boundary of the Akulleq terrane to the zircons from the quartzite studied by Kinny et al. south-east is the Qarliit nunaat thrust, along (1988) show that this was a detrital sediment de- which rocks of the Tasiusarsuaq terrane that are rived from early Archaean continental rocks like retrogressed from granulite facies or that have those in the southern unit of the Akulleq terrane. relict granulite facies assemblages, overlie rocks The composition of the rock indicates a very ma- of the Akulleq terrane that have never been in ture sediment that is likely to have been lain granulite facies. As noted above, the Tasiusar- down on or immediately adjacent to the early suaq terrane appears to show a tilted section of Archaean continent. Thus while some of the the crust with progressively deeper levels ex- metasedimentary rocks in the Akulleq terrane posed from south-east to north-west towards the may well have been laid down on early Archaean thrust. The sinuous trace of the thrust reflects the rocks, there is no evidence that the amphibolites, fact that it was folded by NNE-trending upright the quartz-cordierite gneisses and the layered ba- folds (F2 in the terminology of Friend et al. sic intrusives formed a cover sequence on an 1988b). To the south-west the thrust is strongly early Archaean basement. It is quite likely that modified within zones of intense ductile deforma- Bulletin of the Geological Society of Denmark

tion, has been rotated to an almost vertical posi- that also fold the Qarliit nunaat thrust. The tion, and is difficult to locate precisely. It is less Ivinnguit fault does not appear to have been deformed to the east, south of inner Ameralik, folded by F2 and is parallel to F2 axial traces. The

I where an amphibolite facies fabric adjacent to amplitude of the F2 folds decreases away from l the thrust and parallel to it can still be seen the fault, and this led Friend et al. (1988b) to (Friend et al. 1988a, figs 2 and 3). suggest that the F2 folds could be the result of The boundary of the Akulleq terrane to the docking of the Akia terrane against the already- north-west, the Ivinnguit fault, is less modified juxtaposed Akulleq and Tasiusarsuaq terranes. than the Qarliit nunaat thrust and is presumably The timing of accretion of the Tasiusarsuaq, younger. The fault is nearly straight and does not Akulleq and Akia terranes is not well con- appear to be folded by F2. In the south and on strained. Both the Qarliit nunaat thrust and the northern Stor0 it lies within late zones of strong Ivinnguit fault cut Ikkattoq gneisses and this sets ductile deformation and is almost vertical or dips an older limit of ca. 2800 Ma. A younger limit is steeply to the east (Friend et al. 1988b, fig. 2). On given by late granites that cut all terranes and Sadel0 and the south-east corner of BjØrne0en that give dates between ca. 2700 and 2500 Ma, the fault dips at moderate angles to west-north- with a peak at ca. 2530 Ma when the Qôrqut west, is more or less parallel to the structure in granite cornplex formed (Baadsgaard 1976, the rocks on each side and has a well preserved Brown et al. 1981, Moorbath et al. 1981, Robert- blastomylonite zone 10-20 m thick. Much of the son 1983, McGregor et al. 1983, Friend et al. trace of the fault to the north as far as Ilulialik lies 1985). High initial strontium isotope ratios and under Godthåbsfjord or is obscured by Quater- other chemical features indicate that these gran- nary deposits. The lowest metamorphic grade ites are the products of crustal melting that in- and therefore presumably the highest crustal lev- volved an early Archaean component in the el in the Akia terrane crops out adjacent to the source region. Airborne gamma-spectrometer fault on Sadel0 and BjØrneØen, where the rocks measurements (Secher & Steenfelt 1981, see were never metamorphosed above middle amphi- McGregor et al. 1986, fig. 9) show that the Akul- bolite facies. This is the opposite of the situation leq terrane has high levels of K, U and Th, partic- in the Tasiusarsuaq terrane above the Qarliit nu- ularly the southern unit of early Archaean rocks naat thrust. and the Qôrqut granite complex. The late peak of granite generation and the fact that the Qôrqut granite complex is made up of a very large num- Late Archaean history of the ber of discrete, gently-dipping sheets of granite Godthåbsfjord region and pegmatite suggest slow radioactive heating of the Akulleq terrane following overthrusting by The rocks of the Akulleq terrane were affected the Tasiusarsuaq and Akia terranes, with segre- by several episodes of deformation. The earliest gation of small batches of magma that rose indi- that affected all major units (except for late gran- vidually to higher crustal levels. ites) produced major tight to isoclinal folds (Fl) Late within the period of generation of the that in the less deformed, eastern part of the granites a NNE-SSW-trending zone of strong terrane trend approximately east-west, are over- ductile deformation that crosses all three terranes turned towards the north, and are asymmetric, was formed under amphibolite facies conditions. with parts of the lower limbs commonly removed A straight belt of intense ductile deformation by shear zones (Friend & Nutman 1988, Nutman extends continuously from some distance north et al. 1989). The southern unit of early Archaean of the head of Fiskefjord (Garde et al. 1983) rocks forms the core of a major, north-west-fac- south along the west side of Qussuk, where it is ing nappe enclosed by Ikkattoq gneisses (Friend ca. 4 km wide, through the south-western parts of et al. 1988b, fig. 2). F1 folds do not affect the BjØrneØen and Sadel@,the Nuuk peninsula and Qarliit nunaat thrust, but both are manifestations the islands to the south. Within this belt the lith- of tectonic transport towards the north-west and ological units are streaked out, more or less par- they may be related. The F1 isoclines are re- allel, and have a regular steeply-dipping layering folded by upright, north-east-trending folds (F2) parallel to the trend of the belt. A strong linear McGregor et al.: Late Archaean mobile belt fabric element that plunges 20-30" to south- common with the early Archaean rocks of the south-west appears to be mainly the result of Saglek area than with those in the Isukasia area rotation of earlier stmctures. A similar belt of in the north-eastern part of the Akulleq terrane. intense ductile deformation ca. 5 km wide ex- For example, the Fe-rich Uivak II gneisses are tends through the outer parts of Buksefjorden virtually identical to the Fe-rich suite of augen and Færingehavn fjord. Between these two belts and ferrodioritic gneisses in the Amitsoq gneisses earlier stmctures have been tightened but not (Nutman et al. 1984). It is likely that the Uivak streaked out as in the straight belts, folds have gneisses in the Saglek area are part of the same acquired sheath forms, and there is commonly a block of early Archaean continental crust as the very strong linear fabric parallel to that in the southern unit of Amitsoq gneisses in the Akulleq straight belts (Chadwick & Nutman 1979), in- terrane. In Labrador the extent of the early Ar- dicating strong stretching with less flattening than chaean rocks is not as well established as in West in the straight belts. There are smaller zones of Greenland. Early Archaean rocks occur over a strong late ductile deformation outside the main distance of at least 200 km along the coast. They zone and lacunae of low deformation within it. strike approximately north-south, oblique to the The overall effect of the late deformation zone is trend of the Akulleq terrane in West Greenland. a narrowing and anticlockwise rotation of the No major tectonic boundaries equivalent to the Akulleq terrane in the south-western part of the Qarliit nunaat thmst or the Ivinnguit fault have region. The zone may be a deeper, ductile section been recognised. of a transcurrent fault zone across which there A major difference between the Saglek-Okak was also vertical displacement with relative up- region and the Akulleq terrane is the absence or throw to the west. scarcity in the former of rocks equivalent to the Where the line of the Qôrqut granite complex Ikkattoq gneisses. However, the proportion of intersects the late deformation zone there are Ikkattoq gneisses to other rocks decreases con- abundant deformed pegmatite-granite sheets, the siderably along and across the Akulleq terrane Sátut gneisses of Nutman et al. (1989). Recent from north-east to south-west, in the direction of SHRIMP dating of the S5tut gneisses confirms Labrador. Over much of the Saglek-Okak region the original correlation with the Qôrqut granite the early (?-mid) Archaean rocks were affected complex (Chadwick & Coe 1983) and suggests by granulite facies metamorphism accompanied that formation of the complex mainly predated by partial melting and mobilisation in a complex the late deformation zone. Scattered undeformed interplay of partly decoupled processes within the granite sheets within the western straight belt on period 2770-2710 Ma (Schiotte et al. 1989b). This the Nuuk peninsula show that granite generation may well prove to overlap with the period when outlasted deformation. the Tasiusarsuaq, Akulleq and Akia terranes were juxtaposed. Many of the differences in the geology along the Akulleq terrane may be ex- Correlation with northern Labrador plained in terms of differences in crustal level, with the shallowest level exposed in the Isukasia The Saglek-Hebron-Okak region of northern La- area to the north-east, progressively deeper lev- brador is made up of early Archaean rocks, the els exposed towards the south-west in West Uivak gneisses and Nulliak supracrustal rocks, Greenland, and an even deeper level, again be- interleaved with larger, more continuous units of coming deeper towards south and west, in north- Upernavik supracrustal rocks (see Bridgwater & ern Labrador. Comparison with the Akulleq ter- SchiØtte 1991 (this volume) for references). The rane in West Greenland suggests that the cause of Saglek-Okak region is approximately along the granulite facies metamorphism in the Saglek- line of the Akulleq terrane in a pre-drift recon- Okak region may have been a combination of struction of Greenland and North America, al- crustal thickening and magmatic heating. though apparently displaced a little to the north (Fig. 2). The early Archaean rocks in the south- western part of the Akulleq terrane, between the mouth of Ameralik and Tre BrØdre, have more in Bulletin of the Geological Society of Denmark

the mid-late Archaean continents. The amphibo- lites in the Akulleq terrane may be remnants of oceanic cmst that were not subducted because they were tectonically intercalated with buoyant early Archaean sialic rocks. The quartzo-feld- spathic metasediments could be derived from de- tntal sediments eroded from either of the mid- late Archaean continents, from the early Ar- chaean continental crust, from felsic volcanics associated with subduction, or from mixed sources. The Ikkattoq gneisses may have been subduction-related magmas that were intruded syntectonically into the pile of thrust slices above the subduction zone. The dominantly granodior- itic composition of the Ikkattoq gneisses, the field evidence that they intmded early Archaean rocks, and the isotopic evidence of involvement of early Archaean rocks in their genesis suggest a subduction zone descending beneath the early abundant early Archaean gneisses Archaean continent with mixing of subduction- early Archaean gneisses present in variable amounts derived magmas with magmas produced by melt- ing of the deeper parts of the continental crust. Elsonian intrusions The quartz-cordierite gneisses could be derived Fig. 2. Map showing the position of the Godthåbsfjord region from volcanic phases of the Ikkattoq magmas relative to the Saglek-Hebron area of Labrador prior to the that were erupted adjacent to a body of water in opening of the Labrador Sea. which they settled and were subject to intense leaching. Overlapping of the ages of the Iliverta- Significance of the Godthåbsfjord belt lik granite suite and the granulite facies meta- morphism in the Tasiusarsuaq terrane with the It has been shown in the foregoing that the Akul- ages of the Ikkattoq gneisses in the Akulleq ter- leq terrane is a belt of rocks of very varied ages rane suggest the possibility that this plutonic ac- and origins that is separated by major tectonic tivity in the Tasiusarsuaq terrane was related in breaks from more extensive blocks of middle and some way to subduction, perhaps underplating by late Archaean rocks on each side, and that it has basic magmas of the leading edge of the southern few if any lithological units in common with these continent, melting of the base of the crust, and blocks. It has also been shown that although mixing of magmas to form the Ilivertalik suite. there are many similarities between the blocks on In the model suggested here the rocks that now either side of the Akulleq terrane, they are in fact make up the Akulleq terrane were gathered up in made up of rocks with different primary ages and front of the continent of which the Tasiusarsuaq different metamorphic histories. terrane is part, and overridden by it. Subse- A plausible explanation of this relationship is quently the continent of which the Akia terrane is that the blocks on either side of the Akulleq part collided with the composite Akulleq-Tasiu- terrane were parts of two separate continental sarsuaq block. Deformation of the southern con- masses and that the Akulleq terrane is made up tinent accompanied by influx of fluids that caused of rocks from different parts of the crust origi- extensive retrogression of the granulite facies nally separating the two continents that were in- rocks extended at least 60 km into the Tasiu- terleaved and welded together in the late Ar- sarsuaq terrane. Deformation and associated ret- chaean as a result of the approach and collision of rogression were less extensive in the northern the continents (Fig. 3). In this model the early continent. Rocks in the Akulleq terrane that Archaean rocks are interpeted as fragments of were rich in potassium and other radioactive ele- early continental crust that were caught between ments were pushed down by the overriding McGregor et al.: Late Archaean mobile belt

Late Archaean.evolution of the Godthabsfjord region (2840-2860 al

l Ø 1 proto-Akia terran6: proto-~k~lleq*ierrane:rifted? early proto-Tasiusarsuaq dorninated by 3000 Archaean continental crust (tut by terrane: active volcanic and 3200 Ma gneisses rnafic dyke swarrns), quartzites, art, containing inclusions peiites and oceanic CrUSt. Quartz oceanic trust and also -

Ilivertalik granite intercalation of lithologies(above). complex in the and the ernplacernent of the syntectonic Tasiusarsuaq terrane Ikkattoq gneisses. Quartz cordierite gneisses rnay be volcanic equivalents (altered) of the Ikkattoq gneisses 1 2700-2720 al A A I

Akia terrane arrives via

predominantly strike slip ...... -.-S -. motion? Reactivation of the :::z x I I boundary between the Akulleq and Tasiusarsuaq Ivinnguit Qarlliit terranes forrning the Qarlliit fault nunaat ndnaat thrust. Intrusion thrust of granite sheets

Sporadic deformation and ernplacement of granitic bodies (e.g. 2550 Ma Qôrqut granite complex), with cooling and erosion of tectonically thickened crust. Fig. 3. Schematic representation of the late Archaean evolution of the Godthåbsfjord region as proposed in this paper. blocks on either side. Crustal melting that pro- extrusive and intrusive basic rocks. The great duced granites took place over a period of 150 volumes of tonalitic magmas produced within Ma or more, but peaked more than 100 Ma after short periods (5100 Ma), the chemistry and field the collision. relations of the basic rocks, and the lack of evi- The formation of the Godthåbsfjord belt in the dente of involvement of older continental crust late Archaean involved processes different from suggest intra-oceanic subduction environments. those that formed the older continental crust of However, the rocks are very different from those the blocks on either side and the early Archaean of island arcs produced by present-day intra- continent. Formation of the Amitsoq grey oceanic subduction. Consideration of the chem- gneisses at ca. 3700 Ma, of the tonalitic gneisses istry of the calc-alkaline intrusives (Martin 1986), in the Akia terrane at ca. 3000 Ma and of the modelling of thermal evolution (Arkani-Hamed gneisses of the Tasiusarsuaq terrane at ca. 2860 & Jolly 1989), and melting studies (Winther & Ma represent the intrusion of enormous volumes Newton 1991 - this volume), all suggest that the of dominantly tonalitic magmas into sequences of tonalitic-trondhjemitic-granodioritic rocks that Bulletin of the Geological Society of Denmark

dorninate typical Archaean continental crust like Appel, P. W. U. & Garde, A. A. 1987: Stratabound scheelite and stratiform tourmalinites in the Archaean Malene su- the Akia and Tasiusarsuaq terranes were pro- pracrustal rocks, southern West Greenland. Bull. GrØn- duced by melting of hydrous, low-K tholeiites in lands geol. Unders. 156, 26 pp. descending slabs of oceanic crust. This crust was Arkani-Hamed, J. & JolIy, W. T. 1989: Generation of Archean tonalites. Geology 17, 307-310. probably shorter-lived, hotter; and less dense Ashwal, L. D., Jacobsen, S. B., Myers, J. S., Kalsbeek, F. & than in present-day island arc.subduction zones, Goldstein, S. J. 1989: Sm-Nd age of the Fiskenæsset Anor- thosite Complex, West Greenland. Earth planet. Sci. Lett. and may have underplated the lithosphere rather 91,261-270. than have been subducted deep into the mantle Baadsgaard, H. 1976: Further U-Pb dates on zircons from the (Arkani-Hamed & Jolly op. cit.). The God- early Precambrian rocks of the Godthaabsfjord area, West Greenland. Earth planet. Sci. Lett. 33, 261-267. thåbsfjord belt shows a different tectonic style Baadsgaard, H. & McGregor, V. R. 1981: The U-Th-Pb sys- that involved older continental crust in addition tematics of zircons from the type NCk gneisses, Godthåbs- to oceanic crust. At the stage when the Ikkattoq fjord, West Greenland. Geochim. Cosmochim. Acta 45, 1099-1109. gneisses were formed it had features in common Baadsgaard, H., Nutman, A. P., Bridgwater, D., Rosing, M., with active continental margins, while during lat- McGregor, V. R. & Allaart, J. H. 1984: The zircon geo- chronology of the Akilia association and Isua supracrustal er stages when the three terranes were juxta- belt, West Greenland. Earthplanet. Sci. Lett. 68,221-228. posed it had more in common with mobile belts Baadsgaard, H., Nutman, A. P. & Bndgwater, D. 1986: Geo- resulting from continent-continent collisions. chronology and isotopic variation of the early Archaean Amitsoq gneisses of the Isukasia area, southern West Greenland. Geochim. Cosmochim. Acta 50,2173-2183. Acknowledgements. We thank Dansk Geologisk Forening (the Beech, E. M. & Chadwick, B. 1980: The Malene supracrustal Geological Society of Denmark) for providing the three of us gneisses of northwest Buksefjorden: their origin and signif- with the opportunity to meet again and discuss our developing icance in the Archaean crustal evolution of southern West ideas, Gronlands Geologiske Undersogelse for invaluable sup- Greenland. Precambrian Res. 11, 392-355. port throughout the years we have worked in Greenland, and Berthelsen, A. 1962: Stmctural studies in the pre-Cambrian of H. Baadsgaard, P. Kinny and F. Kalsbeek for permission to western Greenland; part III: Southern Sukkertoppen dis- quote unpublished results. VRM thanks Dansk Geologisk trict. Meddr Gr~nland123(2), 47 pp. Forening for the honour accorded to him in the award of the Brown, M., Friend, C. R. L., McGregor, V. R. & Perkins, W. Steno Medal, Carlsbergfondet for generous support for his field T. 1981: The late Archaean Qôrqut granite complex of work since 1985, the Danish Natural Science Research Council southern West Greenland. J. geophys. Res. 86, 10617- for the grant that allowed him to travel from Greenland to 10632. Europe, and the many people in West Greenland, especially in Chadwick, B. 1981: FieId relations, petrography and geoche- Atammik, who have helped him during the 25 years he has mistry of Archaean amphibolite dykes and Malene supra- worked there. CRLF acknowledges financial support from the cmstal amphibolites, northwest Buksefjorden, southern Royal Society and Oxford Polytechnic. West Greenland. Precambrian Res. 14, 221-259. Chadwick, B. 1990: The stratigraphy of a sheet of supracrustal rocks within high-grade orthogneisses and its bearing on Late Archaean structure in southern West Greenland. J. geol. Soc. Lond. 147, 63-52, Dansk sammendrag Chadwick, B. & Coe, K. 1983: Descriptive text to 1:lOO O00 I Godthåbsfjord-regionen i det sydlige Vestgronland findes et sheet Bukesforden 63 V.1 Nord. Copenhagen: Gronlands N@-SV-gående bælte, der består af bjergarter med varierende geol. Unders., 70 pp. alder og oprindelse. Dette bælte, som i nærværende artikel Chadwick, B. & Nutman, A. P. 1979: Archaean structural kaldes 'Akulleq terrane', er begrænset af vigtige forkastninger evolution in the northwest of the Buksefjorden region, mod to storre blokke af typiske hojmetamorfe Arkæiske bjer- southern West Greenland. Precambrian Res. 9, 19%226. garter, der på trods af mange ligheder har forskellige aldre og Coe, K. & Robertson, S. 1982: Mapping of Archaean rocks in metamorfe udviklingshistorier. Den kontinentale skorpe, som part of the Ivisirtoq sheet. Rapp. GrØnlands geol. Unders. danner blokken mod nordvest (kaldet 'Akia terrane') blev dan- 110, 63-67. net for mellem ca. 3200 og 2980 mill. år siden, hvorimod den Coe, K. & Robertson, S. 1984: Contrasting styles of Archaean , sydostlige blok ('Tasiusarsuaq terrane') dannedes for ca. 2800- cmstal evolution in parts of southern West Greenland. J. 2650 mill. år siden. Bjergarterne i Akulleq terrane tolkes som Geodynamics 1,301-31 1. forskellige skorpefragmenter, som oprindeligt adskilte de to Collerson, K. D., McCulloch, M. T. & Nutman, A. P. 1989: Sr storre blokke, og som omfatter tidlig Arkæisk kontinental and Nd isotope systematin of polymetamorphic Archean skorpe, mulig oceanskorpe og sure til intermediære intrusive og gneisses from southern West Greenland and northern La- maske også ekstrusive magmabjergarter, som kan være dannet brador. Can. J. Earth Sci. 26,446-466. under subduktionslignende omstændigheder. 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