Tectonostratigraphic terranes within Archaean gneiss complexes: examples from Western Australia 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 Yilgarn Craton, 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 zircon 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 gneisses. In both the Godthabsfjord region and the Narryer Gneiss Complex, late Archaean juxtaposition of terranes was accompanied by intrusion of crustally­ derived granites, deformation, and amphibolite facies metamorphism. 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 mafic 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 rock 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 granite Undivided rnid to late Archaean supracrustal rnetamorphisrn rocks and gabbro-anorthosite cornplexes Akia terrane r Kangimut sammisoq Mid-Archaeangneisses, including 3050-2980 Ma A TinissAq Nfik gneisses. Granulite 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 pegmatites, 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 lithologies 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 quartz-nch rocks, mica- dente to support the tectonostratigraphic terrane and amphibole-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