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Tectonostratigraphic terranes within Archaean complexes: examples from and southern West Greenland

ALLEN P. NVTMAN

Nutman, A. P.: Tectonostratigraphic terranes within Archaean gneiss complexes: examples from Western Australia and southern West Greenland. Bull. geol. Soc. Denmark, vol. 39, pp. 199-211. Copenhagen, December 20th, 1991. https://doi.org/10.37570/bgsd-1991-39-09

New field work and isotopic data show that the Godthabsfjord region of West Greenland consists of a collage of tectonostratigraphic terranes, which evolved separately prior to tectonic juxtaposition in the late Archaean. In WesternA ustralia the Narryer Gneiss Complex, which lies on the northwesternm argin of the , is, unlike the Godthabsfjord region, very poorly exposed (less than 1 % ). In consequence it is impossible to follow geological boundaries in this complex, and instead the complex has been studied by a very extensive use of within-grain U-Pb geochronology on the ion microprobe SHRIMP. The zircon geochronology suggests that the Narryer Gneiss Complex also consists of several discrete terranes of early to mid Archaean . In both the Godthabsfjord region and the Narryer Gneiss Complex, late Archaean juxtaposition of terranes was accompanied by intrusion of crustally­ derived , deformation, and facies . Thus some Archaean high grade gneiss complexes consist of terranes that underwent independent evolution until they were brought together at a later time. In this respect their anatomy resembles post-Archaean orogenic belts that formed as a consequence of plate tectonic processes.

Allen P. Nutman, Research School of Earth Sciences, Australian National University, G.P.O. Box 4, Canberra ACT 2601, Australia.June 8th, 1990.

Introduction: CADS and terranes itic and tonalitic magma into supracrustal se­ quences dominated by rocks; older conti­ The purpose of this paper is to review the lines of nental crust may or may not be present. These evidence supporting the application of the tecto­ trondhjemitic and tonalitic rocks are now pre­ nostratigraphic terrane concept (sensu Coney, served as the "grey gneisses" ubiquitous in Ar­ Jones & Monger 1980) to the Archaean evolution chaean high grade gneiss complexes. Whole of the Godthabsfjord region, and then demon­ Rb-Sr, Sm-Nd and Pb isotopic studies (see e.g. strate how this concept has been applied to the Taylor et al. 1980; Moorbath & Taylor 1986; early Archaean Narryer Gneiss Complex of Moorbath et al. 1986) demonstrated that the pro­ Western Australia. Implications of recognition of toliths of these grey gneisses are dominated by tectonostratigraphic terranes in Archaean gneiss juvenile additions to the crust. The second phase complexes for the evolution of the crust are then (differentiation) of a CADS involves ultrameta­ discussed. morphism and partial melting in the depth of the In the late 1970s and early 1980s crustal accre­ new crust to give rise to a geochemically stratified tion differentiation superevents (CADS) were rec­ crust by the extraction from the lower crust of ognised as the most important factor in the pro­ granitic melt and LIL elements (see e.g. Fyfe duction of Archaean sialic crust (see e.g. Wells 1973; Wells 1979). Following a CADS the new 1979, 1980; Moorbath & Ta ylor 1986). From un­ crust is stable and may survive for a long time. certainties on Rb-Sr and Pb-Pb whole rock dating The intensive studies in the 1970s of the God­ (e.g. Moorbath & Pankhurst 1976; Ta ylor et al. thabsfjord region, southern West Greenland, 1980), CADS took less than - 200 million years. gave rise to much of the data which was firstused The first phase (crustal accretion) of a CADS to formulate the idea of CADS. The proposed involves injection of predominantly trondhjem- mid Archaean CADS in the Godthabsfjord re- 200 Nutman: Archaean gneiss complexes

Davis Strait

25 km 1

I Tasiusarsuaq terrane 1 2800 Ma Iliveralik Undivided rnid to late Archaean supracrustal rnetamorphisrn rocks and -anorthosite cornplexes

Akia terrane r Kangimut sammisoq Mid-Archaeangneisses, including 3050-2980 Ma A TinissAq Nfik gneisses. facies rnetarnorphisrn at -3000 Ma in the west. @$# 2550 Ma Qôrqut granite cornplex Akulleq terrane ' Terrane boundary 0Færingehavn sub-terrane, dominantly >3600 Ma Arnitsoq gneisses 2' Proterozoic fault Tre Brodre sub-terrane,dorninantly Permanent ice 1 2750-2800 Ma Ikkattoq gneisses

Fig. 1. Geological sketch map of the Godthåbsfjord region, West Greenland. gion entailed (1) tectonic intercalation of the lite and amphibolite facies assemblages found in "Malene supracrustal rocks" and the early Ar- the southern half of the region as having formed chaean Amitsoq gneisses (cut by the Ameralik at different structural levels during a single - dykes), (2) intrusion of the voluminous protoliths 2800 Ma metamorphic peak. Coe (1980) inter- of the (type) Nuk gneisses of Nuuk and south- preted this purported synchronous regional meta- western Godthåbsfjord and what were presurned morphic peak as marking "cratonisation" at - to be their equivalents elsewhere in the region, 2800 Ma following a CADS. Late Archaean and (3) folding and granulite facies metamor- shearing, granite emplacement and retrogression phism (see e.g. McGregor 1973, 1979; Bndgwa- were regarded as evidence of local reworking of ter et al. 1974; Chadwick & Nutman 1979; Wells crust formed in a single mid Archaean CADS, 1979; Coe 1980; McGregor et al. 1983,1986; Dy- rather than evidence of a much more complex mek 1984; Robertson 1986; Moorbath et al. crustal evolution for the Godthåbsfjord region 1986). Wells (1976, 1979) interpreted the granu- (Bridgwater et al, 1976; McGregor et al. 1983). Bulletin of the Geological Society of Denmark 201

e AKULLEQ TERRANE TASIU- AKIA TERRANE Færingehavn Isukasia Tre Br~dre TERME 1 4500 i sub-terrane sub-terrane sub-terrane 4000 -* some Isua - early g -111111111111111 3500 sialic 111111111111111 crust * -- t 3000 *-p 11111111111111 g,a a *-t-a t*- g,a -t

(Mal -k stratigraphic position of supracrustal ' rocks not yet dated t unpublished protoliths of Archaean grey gneisses SHRIMP results granite unique to a terrane deformed granites found in several terranes Qôrqut granite complex and , mostly undeformed dated supracrustal rocks g a documented granulite (g) and amphibolite (a) facies metamorphism Fig. 2. Terrane chronology in the Godthåbsfjord region. tH. Baadsgaard, M.J. Duke and A.P. Nutman, unpublished SHRIMP within-grain zircon U-Pb data.

In the mid 1980s C.R.L. Friend, V.R. McGre- are dorninated by mid Archaean gneisses (Fig. gor and the writer carried out remapping of se- 1). The Akulleq terrane, which forms most of lected areas in order to resolve the limit of Ar- Godthåbsfjord and the coastal strip south of chaean granulite facies metamorphism and the Nuuk, contains a wide variety of rocks, ranging extent of the early Archaean Amitsoq gneisses. from early to late Archaean in age (McGregor et The data gathered during this work showed that al. 1991 - this volume). The Akulleq terrane is a several associations of rocks are present in the conglomeration of the Tre Brodre and Fzringe- Godthåbsfjord region, each with its own early havn terranes described in previous publications history (Friend et al. 1987). Furthermore, "Niik (e.g. Nutman et al. 1989), which are now reduced gneisses" dorninating different associations with to sub-terrane status. The reason for this is that contrasting structural and metamorphic history, although tectonic boundaries have been identi- are now known from U-Pb zircon geochronology fied between the Tre Brodre and F~ringehavn to be of different ages (Nutman et al. 1989; sub-terranes (Friend et al. 1987), these sub-ter- Schiotte et al. 1989; H. Baadsgaard, pers. ranes contain the same but in different comm.). This suggested that the region did not proportions. This is discussed by McGregor et al. develop during a single CADS, and that it might (1991 - this volume). instead consist of a collage of tectonostrati- graphic terranes (sensu Coney et al. 1980) that were assembled in the late Archaean with amphi- The Godthåbsfjord region bolite facies metamorphism, folding, and em- placement of crustally denved granites (Fig. 2). The evidence for tectonostratigraphic terranes in The Akia terrane and Tasiusarsuaq terrane, to the the Godthåbsfjord region is demonstrated by re- north and south of Godthåbsfjord respectively, sults from part of the F~ringehavn- Tre Brodre 202 Nutman: Archaean gneiss complexes

- paragneiss and amphibolite - metagabbro and anorthosite

Fig. 3. Detailed map of part of the Færingehavn-Tre Brbdre area, West Greenland, illustrating evidence for tectonostratigraphic terranes. area as shown in Fig. 3. The eastern terrane in than 20 m wide, which are locally discordant to this area contains the controversial "old gneisses" lithological units within the terranes (Fig 3; at Titiissiq and Kangimut sammissoq (see Friend et al. 1987). The zones of flaggy rocks Schi~tteet al. 1989 and McGregor et al. 1991 - separating the terranes consist of finely-layered this volume - for discussion). Three lines of evi- quartzo-feldspathic and -nch rocks, - dente to support the tectonostratigraphic terrane and -rich schistose rocks, and (locally model for the region are considered here: struc- discordant) seams of fuchsite-bearing quartz tural, isotopic and metamorphic. rocks. The schistose rocks commonly contain thin, disrupted pegmatitic veins. Small-scale rootless folds and sheath folds are commonly pre- Structural evidence sent within the flaggy rocks of the contact zones The contacts between the terranes are zones of between the terranes, and are similar to struc- amphibolite facies fine-grained, flaggy rocks, less tures described from other high-strain zones (e.g. Bulletin of the Geological Society of Denmark

l Tasiusarsuaq terrane Tre Brodre sub-terrane which is dominated by 2800 Ma granulite facies amphibolite facies, rather homogeneous grano- metamorphism. Associated wilh inlrusion of the dioritic ''NCik" gneisses with sparse Ilivertalik granite? banding and devoid of amphibolite dykes, which 7 Amphibolite facies retrogressive seam. curling . are now named the Ikkattoq gneisses (Friend et kbar al. 1988; Fig. 3). U-Pb conventional bulk-zircon

amphibolite facies retrogression. dating of the Ikkattoq gneisses has yielded western boundary ol terrane. Associated with slightly discordant data with 207PbpffiPbages be- F2 folding and straight bel1 formation? tween 2790 and 2760 Ma (Nutman et al. 1989). . . 500 700 900 SHRIMP (Sensitive High Resolution Ion Micro- l Akulleq terrane Probe) within-grain zircon U-Pb dating of Ik- kattoq gneisses throughout Godthåbsfjord has yielded concordant ages of - 2820 Ma (H. Baadsgaard and A.P. Nutman, unpublished 7 data). Thus the Ikkattoq gneisses are consider- Late Archaean amphibolite facies kbai hism. Following crustal ably younger than the type Niik gneisses (-3000 by assembly ol the d Tasiusarsuaq terranes Ma) of south-west Godthåbsfjord. A protolith 5 age of - 2920 Ma was obtained for granulite sociated with F2 folding facies gneisses inland at Tinissaq in the Tasiu- sarsuaq terrane (Fig. 1) by Schi0tte et al. (1989) using SHRIMP. - 2800 Ma ages obtained on Fig. 4. P,T,t paths for the Tre Brodre sub-terrane of the Akul- massive low-U overgrowths and recrystallised do- leq terrane and the Tasiusarsuaq terrane. mains were interpreted as the age of the granulite facies metamorphism. Moreover, possible indica- Cobbold & Quinquis 1980). Many of the fine- tions of an intermediate - 2850 Ma event were grained flaggy rocks have mylonitic to ultramy- seen in this study. lonitic textures (cf. Sibson 1977). From Figs 1and 3 it is evident that these strongly deformed rocks Metamorphic evidence between the terranes must be shear zones, and that they were subsequently folded. Further LS Metamorphic evidence supporting the division of fabric development during Iater folding under the Godthåbsfjord region into tectonostrati- amphibolite facies metamorphism has over- graphic terranes comes from contrasting time of printed any kinematic indicators of the move- maximum metamorphism and differing P,T,t ment in the shear zones when they originally (pressure, temperature, time) paths for the ter- formed. These shear zones are now generally ranes. Examples of these can be taken from the steeply dipping, but this is the result of rotation part of the Fzringehavn-Tre Brodre area shown associated with later deformation. in Fig. 3. Geothermobarometry studies on rocks from the Tasiusarsuaq terrane in the east shows that it underwent granulte facies metamorphism Isotopic evidence with near isobaric cooling from the metamorphic Gneisses from the part of the coastal region of peak (Wells 1979; Bohlen 1987; Nutman et al. the Fzringehavn-Tre Brodre area shown in Fig. 3 1989). From the SHRIMP zircon geochronology have been dated by conventional bulk-zircon of low-U zircon overgrowths, this event has been techniques by H. Baadsgaard (in Nutman et al. established at - 2800 Ma (P. Kinny, pers. comm. 1989). The amphibolite facies, banded, tonalitic 1987; Schiotte et al. 1989). Retrogression of the gneisses cut by amphibolite dykes forming most granulites in the Tasiusarsuaq terrane is extensive of the Færingehavn sub-terrane (Fig. 3) are early near its tectonic contact with the Tre Brodre ter- Archaean in age (Nutman-et al. 1989), and are rane (McGregor et al. 1986; Friend et al. 1988; grouped with the type Amitsoq gneisses. The Nutman et al. 1989). Geothermobarometry of Færingehavn sub-terrane containing the Amitsoq retrogressed granulite facies rocks showed that gneisses is in tectonic contact in this area with the retrogression of them occurred in two or more 204 Nutman: Archaean gneiss complexes

Eurada bore gneiss

..,y ;... ::' '.'....i.,...... '.'.,.::.'."" Jack Hills supracrustal association foliation trend I 0 "Greenstones", all -3050 Ma? HProterozoic and Mesozoic cover 23600 Ma Meeberrie gn. . 3120 Ma granite Meeberrie gn. with 3300 Ma component A 2780-2700 Ma granites 0 -3300 Ma gn. and granite V 2700-2620 Ma granites e -3380 Ma Dugel gn. with 3300 Ma cornponent 3440-3490 Ma Eurada gn. Eurada gn. with 3055 Ma cornponent 30C0-2930 Ma gn.

Fig. 5. Geochronological map o€the Nanyer Gneiss Complex and adjacent regions, Western Australia. All age determinations are by SHRIMP within-grain U-Pb geochronology, apart from conventional zircon geochronology o£ the Twin Peaks volcanic rocks (Pidgeon 1986). Bulletin of the Geological Society of Denmark

stages under amphibolite facies conditions (Nut- high grade metamorphism and heterogeneous man et al. 1989). From these results an anticlock- ductile deformation. Therefore gneisses of differ- wise P,T,t path could be constructed for the Tasi- ent ages can be hard to distinguish in the field. usarsuaq terrane in the late Archaean (Fig. 4). Because of these problems, unravelling of the On the other hand, geothermobarometry studies history of the Narryer Gneiss Complex has relied of rocks from the adjacent Tre Brcbdre sub-ter- heavily on extensive within-grain U-Pb geochro- rane of the Akulleq terrane (data in Wells 1979; nology using SHRIMP. The SHRIMP geochron- Nutman et al. 1989) showed that they had never ological data presented in this review have been experienced granulite facies metamorphism and produced by several members of the "First Bil- that these rocks followed a clockwise P,T,t path lion Years" project at Australian National Uni- in the late Archaean (Fig. 4). The chronology for versity, lead by Professor W. Compston. Alto- this region (Fig. 2) shows that the recorded peak gether, over 2000 from more than 70 metamorphism in the Akulleq terrane must have rocks have been analysed from the Narryer occurred later than the - 2800 Ma granulite fa- Gneiss Complex. More detailed discussion of cies metamorphism in the Tasiusarsuaq terrane. these data are given by Froude et al. (1983), The contrasting P,T,t path in the Tasiusarsuaq Compston & Pidgeon (1986), Kinny (1987), and AkuIleq terranes could be the consequence Kinny et al. (1988, 1990) and Nutman et af. (in of thrusting of the Tasiusarsuaq terrane over the press). The technique for U-Pb isotopic analysis Akulleq terrane, which would be in accord with of zircons using SHRIMP is given by Compston et the regional mapping of these units (Friend'et al. al. (1984) with recent modifications described by 1988). Therefore, the granulites of the Tasiusar- Kinny et al. (1990). suaq terrane could have undergone retrogression Within the Narryer Gneiss Complex, the Mee- and decompression associated with rapid erosion berrie gneiss and the Dugel gneiss were recog- after thrusting, whilst the Akulleq terrane would nised by a combination of field studies and have initially undergone compression and then Sm-Nd and Rb-Sr whole rock geochronology heating due to burial associated with the thrust- (Myers &Williams 1985; Williams & Myers 1987; ing event, which would have been followed by Myers 1988a). Meeberrie gneisses in the vicinity decompression and some cooling associated with of Mt Narryer have yielded Sm-Nd model ages subsequent erosion (Fig. 4). (TcHuR)of between 3710 and 3620 Ma (De Laeter et al. 1981, 1985). Cores of zircons from a Mee- berrie gneiss gave a weighted mean 207Pb/2ffiPb The Narryer Gneiss Complex age of 3678 f 6 Ma (Kinny et al. 1988). Samples of Duge1 gneiss have yielded Sm-Nd model ages The Narryer Gneiss Complex (Fig. 5) is the most (TcHuR)of between 3540 and 3510 Ma (De Laeter northerly unit of >2900 Ma rocks in the western et al. 1981; Fletcher et al. 1983), whilst cores of and northern margins of the Yilgarn Craton, zircons in a Dugel gneiss have yielded a Concor- named the Western Gneiss Terrane by Gee et al. dia intercept age of 3381 f 22 Ma (Kinny et al. (1981). The Narryer Gneiss Complex is bounded 1988). Myers (1988a) regarded the Meeberrie to the west and east by late to mid Archaean and Duge1 gneisses as by far the most abundant "granite-greenstone" complexes. Its boundaries rocks in the Narryer Gneiss Complex. However, are not exposed, or are obscured by late Ar- further extensive SHRIMP within-grain zircon chaean granites. To the north the Narryer Gneiss geochronological studies show that gneisses of Complex is progressively reworked in the mid other ages are just as important (Kinny 1987; Proterozoic Capricorn orogen, and is also ob- Kinny et al. 1990; Nutman et al. in press). scured by early Proterozoic metasediments and Metasedimentary rocks make up about 10 granites (Williams 1986). Within the Narryer percent of the Narryer Gneiss Complex (Myers Gneiss Complex exposure amounts to less than 1988a) and are found as concordant lenses in- 1% of the area. Due to the lack of exposure, tercalated with early Archaean gneisses. Some of most important geological boundaries are not ex- the sediments contain a smal1 subpopulation of - posed. The gneisses that make up the complex 3050 Ma detrital grains, therefore they mist be are polyphase, and have undergone repeated younger than 3050 Ma (Kinny et al. 1990). The Nutman: Archaean gneiss complexes largest metasedimentary units form Mt Narryer the eastern part of the Narryer Gneiss Complex, and Jack Hills and contain a smal1 percentage of flanking the Murchison River. In this part of the 42804100 Ma grains in their detrital zircon pop- complex Myers (1988a) recognised the Meeberrie ulation~(Froude et al. 1983; Compston & Pid- and Dugel gneisses. The regional SHRIMP geo- geon 1986). The most cornmon of the chronological studies (see e.g. Nutman et al. in sedimentary units is white to pale green fuchsitic press) confirm that rocks of the same character quartzite, in which sedimentary structures have and age (predominantly 23600 components) as been obliterated by strong deformation. In areas the type Meeberrie gneisses are quite common in of low deformation original stmctures such as this part of the region. However, it is now evident cross-bedding are locally preserved (Williams & that this part of the complex contains volumetri- Myers 1987). At both Jack Hills and Mt Narryer cally important -3300 Ma gneisses (Fig. 5). quartz pebble conglomerates occur (e.g. Wil- The Eurada Gneiss Association forms a strip liams & Myers 1987; Compston & Pidgeon 1986). within the Narryer Gneiss Complex and consists The quartzites and conglomerates are found in of a group of granitic gneisses, with protolith ages association with pelitic and quartz + mag- of between 3490 and 3440 Ma. In addition, one netite . There is little or no unit of - 3120 Ma granite has also been found mafic or felsic volcanic material associated with (Fig. 5). - 3300 Ma granite and pegmatite veins, these sediments. The sedimentary sequences very common in the Murchison Gneiss Associ- were probably deposited on , in ation, have not been found in this part of the a shallow marine or fluviatile environment (Wil- Narryer Gneiss Complex. On the other hand, liams & Myers 1987). one Eurada Gneiss Association sample (Fig. 5) The gneisses and supracmstal rocks described contains abundant - 3055 Ma pegmatitic mate- above are intruded by voluminous leucocratic rial. granites, which mostly occur as sheets and lentic- Further mapping and U-Pb zircon age geochro- ular bodies. Several generations of these granites nology of , quartzites and banded iron for- are present, with ages ranging between 2750 and mations throughout the ful1 extent of the Narryer 2600 Ma (De Laeter et al. 1985; Kinny et al. Gneiss Complex reveal that they are all litholog- 1990). Many of the 2750 to 2600 Ma granites are ically similar and show a range of detrital zircon markedly foliated, demonstrating that the ob- ages from 4280 to - 3050 Ma. Because of the served amphibolite facies parageneses through- similar character of all these units, they are out the region and a significant amount of the grouped in the Jack Hills Supracmstal Associ- observed deformation must be late Archaean or ation. This supracmstal association coincides in even younger. extent with the early Archaean gneisses of the Narryer Gneiss Complex. On both sides of this: where only mid to late Archaean gneisses occur, supracrustal units are predominantly mafic to fel- Subdivision of the Narryer Gneiss Complex and sic volcanic rocks with banded iron formation adjacent parts of the region (Fig. 5). To the west and east of the Narryer Gneiss Com- plex, granitic and tonalitic gneisses with ages of - Metamorphic history 3000 Ma or less occur (Fig. 5). As yet, early Archaean rocks have not been found in these 40Ar-39Arand zircon 207Pb/206Pbgeochronology parts of the region. The early Archean gneisses shows that the whole of the Narryer Gneiss Com- seem to form a strip ca. 70 km wide within a mid plex and the adjacent regions were affected by to late Archaean gneiss complex. The early Ar- one or more high grade metamorphic events in chaean gneisses within the Narryer Gneiss Com- the late Archaean (Kinny et al. 1990; Nutman et plex are divided into the Murchison Gneiss Asso- al. in press). Prior to high grade metamorphism ciation and the Eurada Gneiss Association . In in the late Archaean, two other metamorphic addition the Jack Hills Supracrustal Association is events are apparent in the Narryer Gneiss Com- also recognised (Nutman et al. in press). plex. First, the early Archaean gneisses of the The Murchison Gneiss Association occupies Murchison Gneiss Association show widespread Bulletin of the Geological Society of Denmark 207

Narrver Gneiss Cornu Region Region Jack Hills Murchison Eurada south and north and Supracrustal 4500- Gneiss - Gneiss - east of the - west of the - Association r Association Association Narryer Narryer Gneiss Gneiss 4000 Complex - Complex

,..,.,, ..,.,.,., .,.,... ,...... m- 3000 -1 :t P----- m3¢-mm-mrnXXXX:mmmpss#d -=Y- m-- m ooooOOOOOO 2500 deformation continued after emplacement (Mal of youngest granites at -2620 Ma

protoliths of early and middle Archaean gneisses 2700-2780 Ma granites 2620-2700 Ma granites felsic volcanic rocks (R.T. Pidgeon, conventional data) detrital zircons interval during which the Murchison River and Eurada Bore Gneiss Associations were assernbled, and during which deposition of the Jack Hills Supracrustal Association occurred m docurnented metamorphisrn Fig. 6. Terrane chronology of the Narryer Gneiss ~orn~lexand adjacent regions. growth of zircons at 3300 Ma (Fig. 5). This - Recognition of terranes in the Narryer Gneiss 3300 Ma event is interpreted as the time of mig- Complex and adjacent regions matisation and high grade metamorphism coinci- dent with the emplacement of the youngest gran- The Murchison Gneiss Association and the Eu- ites unique to this association (Myers 1988a; Nut- rada Gneiss Association would appear to have man et al. in press). Second, in the Eurada Gneiss had separate early histories but in the Middle Association to the west, one sample shows clear Archaean they had come into close proximity of evidence of zircon growth at - 3055 (Fig. 5). each other, so that upon erosion they could to- It is probable that earlier events such as em- gether give rise to the detrital component in the placement of voluminous protoliths of the Mee- Jack Hills Supracrustal Association, apart from berrie gneisses in the Murchison Gneiss Associ- the smal1 proportion of detrital component de- ation were accompanied by high grade metamor- rived from >3800 Ma rocks (Fig. 6). Detailed phism and deformation. However, because of the examination of the ages of detrital zircons within severity of subsequent tectonometamorphic the Jack Hills Supracrustal Association shows events, evidence for the occurrence and nature of that samples with a large proportion of zircons these events has been obliterated. derived from rocks of the same age as the Mee- berrie gneisses of the Murchison Gneiss Associ- ation are devoid of >3800 Ma detrital grains. This suggests that the source rocks of the >3800 Ma detntal grains are either within the Eurada Gneiss Association or in aaother gneiss unit per- Nutman: Archaean gneiss complexes haps preserved farther south in the Western evolution, whereby the crust in these two regions Gneiss Terrane for which there are only meagre consists of associations (terranes) of material geochronological data. All outcrops of the Eu- which evolved separately prior to juxtaposition rada Gneiss Association visited are devoid of ma- (assembly) late in the Archaean. terial such as distinct inclusions which could be Diorite-tonalite-trondhjemite gneisses of Ar- intact >3800 Ma rocks. Furthermore, some 200 chaean gneiss complexes have geochemical and zircons in the Eurada gneisses have now been isotopic signatures which have been interpreted analysed, and neither xenocrysts nor cores of as evidence that they were generated at depth by >3800 Ma zircon have been found. of mafic rocks (e.g. Martin 1986; Ar- Although the detrital sediments in the Jack kani-Hamed & Jolly 1989), combined with pre- Hills Supracrustal Association were laid down on dominantly fractionation during their or in close proximity to the Murchison Gneiss ascent (e.g. Martin 1987). Granodiorites and Association, these supracrustal rocks now occur granites are also present in Archaean gneiss com- as tectonic intercalations with rocks of the two plexes, but these have geochemical and isotopic gneiss associations, some of which have been signatures inconsistent with their representing ju- folded into synformal keels (Williams & Myers venile crust formed by melting of mafic rocks 1987). Tectonic disruption and initial folding of (e.g. Arndt & Goldstein 1989). the Jack Hills Supracrustal Association may have In the Godthåbsfjord region, both diorite- been coeval with the earliest known high grade tonalite-trondhjemite and granodiorite-granite metamorphic event that these rocks underwent, gneiss suites are present. In the Akia and Tasiu- dated at - 2700 Ma by the 40Ar-39Armethod sarsuaq terranes diorite-tonalite-trondhjemite (Kinny et al. 1990). gneisses are dominant, and were emplaced into From 2750 to 2620 Ma the Narryer Gneiss mafic volcanic rocks and . Thus these Complex and the adjacent regions (with mid Ar- terranes contain, at least in part, juvenile (mid chaean gneisses) have a common history (Fig. 6). Archaean) crust. The Akulleq terrane of God- Using the tectonostratigraphic terrane concept of thåbsfjord, particularly in the Tre Br~dresub- Coney et al. (1980), these different entities are terrane, is dominated by granodioritic Ikkattoq interpreted as tectonostratigraphic terranes gneisses (see McGregor et al. 1991 - this vol- which evolved separately prior to late Archaean ume). The geochemistry and isotopic composi- tectonic juxtaposition (assembly), folding, high tion of the Ikkattoq gneisses (H. Baadsgaard, grade metamorphism, and intrusion of crustally pers. comm. 1988) indicates that the protoliths of derived granites. The late Archaean granites are these gneisses were derived at least in part from commonly sheared close to the margins of the anatexis of older crustal rocks, probably includ- Narryer Gneiss Complex. This suggests that the ing the early Archaean Amitsoq gneisses unique boundaries of the associations (Le. tectonostra- to the Akulleq terrane (particularly its Fzringe- tigraphic terranes in the model proposed here) havn sub-terrane). Therefore crustal evolution in might be modified by late shear zones, unrelated the Godthåbsfjord region is very complex, and to the actual assembly of the terranes at a some- certainly does not represent a single CADS. In- what earlier time. stead the terranes are samples of several major events (CADS?), in which there is evidence for both formation of juvenile crust and reworking of older sial. Discussion In the Murchison Gneiss Association of the In the well exposed Godthåbsfjord region, crus- Narryer Gneiss Complex not only are - 3300 Ma tal evolution has been studied by integrated granites important, but also most of the 23600 structural, metamorphic, and strategic geochron- Ma Meeberrie gneisses are granitic to granodior- ological studies. On the other hand, study of the itic in composition. However, the oldest quartzo- poorly exposed Narryer Gneiss Complex has re- feldspathic component in the Murchison Gneiss lied heavily on SHRIMP within-grain zircon geo- Association is volumetncally minor - 3730 Ma chronology. In both cases the results are compat- tonalite mapped as part of the Meeberrie gneisses ible with a terrane assembly model of crustal (Nutman et al. in press). In the Eurada Gneiss Bulletin of the Geological Society of Denmark

Association all the gneisses so far investigated are sequently, during terrane assembly in the late granitic to granodioritic in composition. In the Archaean (Fig. 2) crustal melting gave rise to regions adjacent to the Narryer Gneiss Complex granites common to all terranes. By analogy with the mid Archaean gneiss remnants range from terranes in post-Archaean orogenic belts, the ter- granitic to tonalitic in composition. Thus in the ranes and associations in the Godthåbsfjord re- Narryer Gneiss Complex and adjacent regions, gion and the Narryer Gneiss Complex could have granitic gneisses are much more important rela- formed as the result of plate tectonic processes. tive to tonalitic-trondhjemitic gneisses compared with the Godthåbsfjord region. Acknowledgements - SHRIMP zircon geochronological data discussed here are contributions to the "First Billion Years" -3730 tonalites of the Murchison Gneiss Asso- project at the Australian National University. I would like to ciation have E,,,,,, values of +l to +2 (R. Maas, thank my colleagues, particularly Professor W. Compston, for pers. comm. 1989; V. Bennett, pers. comm. being able to incorporate some of these data here. The Geolog- ical Survey of Western Australia and the Geological Survey of 1990). Rare leucogabbro from this association Greenland are thanked for providing logistic help in the field. (Myers 1988b) also yields a zircon age of 3730 Ma Work in Greenland was partly supported by grants from the Royal Society of London, the Carlsberg Fund, and NSERC (Kinny et al. 1988). This would suggest that, in (Canada). I thank Professor W. Compston and Drs C.R.L. the Murchison Association at least, the oldest Friend, P. Kinny and V.R. McGregor for discussions of this recognised rocks could belong to a complex of work. tonalites intruded into basic rocks (e.g. leuco- gabbro) and may thus represent - 3730 Ma juve- nile crust. Many of the Sm-Nd and Rb-Sr isotopic Dansk sammendrag results reported from the Meeberrie and Duge1 Nyere feltobservationer og isotopiske data peger på, at Godt- gneisses (e.g. De Laeter et al. 1981, 1985; håbsfjord regionen i Vestgr0nland består af et puslespil af for- Fletcher et al. 1983) are hard to interpret because skellige tektoniske og stratigrafiske brikker, som havde en for- they contain components of two ages, as shown skellig udvikling, for de blev bragt sammen i sen archaisk tid. I det vestlige Australien findes Narryer gneiss komplekset. by subsequent zircon work on the same samples Det ligger geografisk på den nordvestlige rand af Yilgam krato- (e.g. data in Kinny et al. 1988, 1990). Sm-Nd net og er i modsætning til Godthåbsfjord området meget dårligt blottet (mindre end 1%). Som felge heraf, er det umuligt at isotopic studies and whole rock geochemical folge geologiske grænser i komplekset. Derfor er der gjort studies are in progress on gneisses known from udstrakt bmg af U-Pb dateringer af enkeltkorn af zirkon vha. within-grain zircon work to consist of only one ion-mikrosonde (SHRIMP). Zirkon geokronologien indicerer, at Narryer gneiss komplekset også består af flere selvstændige component. These will give more information on delområder, hvis alder er tidlig- til mellem-archaisk. I både the early to mid Archaean evolution of the re- Godthåbsfjord omradet og i Narryer gneiss komplekset blev gion. den sene archaiske sammenforing af delområderne ledsaget af intmsioner af skorpe granitter, deformationer og amfibolit- On the basis of the relative importance of gra- facies metamorfose. Det ser altså ud til, at nogle af de ar- nodioritic-granitic gneisses relative to dioritic- chaiske, stærkt metamorfoserede gneiss komplekser gennemgik forskellig uafhængig udvikling for de blev bragt sammen se- tonalitic-trondhjemitic gneisses, it is evident that nere. I denne henseende minder de om post-archaiske orogene the processes of crustal evolution of the Narryer bælter, som er blevet dannet i forbindelse med pladetektoniske Gneiss Complex and the Godthåbsfjord region processer. were somewhat different. The associations (or terranes) which form the Narryer Gneiss Com- plex and adjacent regions developed in settings Re ferences where blocks of older continental cmst were in- Arkani-Hamed, J. & Jolly, W. T. 1989: Generation of Archean variably present and were partially melted to tonalites. Geology 17, 307-310. form granite and granodiorite. These granite- Amdt, N. T. & Goldstein, S. L. 1989: An open boundary and granodiorite-rich associations were again re- between lower continental cmst and mantle: its role in cmst formation and cmstal recycling. Tectonophysics 161, worked and injected by more crustally-derived 201-212. granite during final assembly in the late Ar- Bohlen, S. R. 1987: Pressure-temperature-time paths and a tectonic model for the evolution of granulites. J. Geol. 95, chaean (Fig. 6). On the other hand, several of the 617-632. gneiss suites in the terranes of the Godthåbsfjord Bridgwater, D., McGregor, V. R. & Myers, J. S. 1974: A region are dominated by juvenile crustal compo- horizontal tectonic regime in the Archaean of Greenland and its implications for early cmstal thickening. Precam- nents at their time of formation (albeit granites brian Res. 1, 179-197. unique to each terrane are also present). Sub- Bridgwater, D., Keto, L., McGregor, V. R. & Myen, J. S. Nutman: Archaean gneiss complexes

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