U – Pb zircon study of tectonically bounded blocks of
2940 – 2840 Ma crust with different metamorphic histories,
Paamiut region, South-West Greenland: implications for the
tectonic assembly of the North Atlantic craton
C.R.L. Friend
a,*, A.P. Nutman
baDepartment of Geology,Oxford Brookes Uni6ersity,Gipsy Lane,Headington,Oxford OX3 0BP,UK bResearch School of Earth Sciences,Australian National Uni6ersity Canberra ACT0200,Australia
Received 15 June 1999; accepted 7 December 1999
Abstract
New fieldwork, map interpretation, petrography and single zircon U – Pb geochronology has allowed the identifica-tion of different crustal blocks in the Paamiut region, in the southern poridentifica-tion of the West Greenland Archaean Craton. Changes of metamorphic grade from only amphibolite facies to granulite facies (some subsequently retrogressed) corresponds with zones of Archaean high strain ductile deformation 9 mylonites. U – Pb zircon dates are presented for the TTG (tonalite, trondhjemite, granodiorite) protoliths from each block in the Paamiut region, and the southern portion of the previously identified Tasiusarsuaq terrane lying to the north. The southern part of the Tasiusarsuaq terrane contains 2880 – 2860 Ma TTG rocks and underwent amphibolite facies metamorphism. Structurally underneath the Tasiusarsuaq terrane to the south is the Sioraq blockcontaining 2870 – 2830 Ma TTG rocks partly retrogressed from granulite facies. Structurally underneath and to the south is the Paamiut block, dominated by 2850 – 2770 Ma granodioritic rocks that have only undergone amphibolite facies metamorphism. Also structurally overlying the Paamiut block, but cropping out separately from the Sioraq block, is theNeria block. This appears to be dominated by 2940 – 2920 Ma gneisses that have been totally retrogressed from granulite facies and strongly deformed. In the southernmost part of the region the Neria block overlies the greenschist to lowermost amphibolite faciesSermiligaarsuk blockthat contains theE2945 Ma Tartoq Group. Rocks from all the blocks record ancient loss of Pb from zircons and some new zircon growth at 2820 Ma, interpreted to indicate a high grade metamorphic event at that time, including granulite facies metamorphism in the Sioraq and Neria blocks. The blocks of different metamorphic grade are interpreted to have moved to their current positions after the 2820 Ma metamorphism, explaining the change in metamorphic history across some mylonites and ductile shear zones which deform and retrogress granulite facies textures. The juxtaposed blocks and their contacts were subsequently folded under amphibolite facies conditions. The contacts are cut by undeformed Palaeoproterozoic dolerite dykes which post-date amphibolite facies metamorphism. These results, together with previously published data from the
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* Corresponding author. Tel.: +44-865-819610; fax: +44-865-819926.
E-mail address:crlf@brookes.ac.uk (C.R.L. Friend).
C.R.L.Friend,A.P.Nutman/Precambrian Research105 (2001) 143 – 164 144
Godtha˚bsfjord region (north of Paamiut) shows that the North Atlantic Craton in West Greenland from Ivittuut in the south to Maniitsoq in the north (550 km) consists of a mosaic of ductile fault-bounded packages that attained their present relative positions in the late Archaean. © 2001 Elsevier Science B.V. All rights reserved.
Keywords:Archaean; Zircon, U – Pb geochronology; Gneiss complexes; Terrane tectonics
1. Introduction
Understanding how high-grade gneiss terrains were constructed is hampered by polyphase, het-erogeneous high strain and polyphase metamor-phism which normally obliterates most primary textures and structures. This makes the identifica-tion of cogenetic rock suites very difficult. Fur-thermore, during these processes, whole-rock and mineral isotopic systems are commonly disturbed. Geochronological studies of rockes so affected is fraught with difficulty, and may lead to geologi-cally meaningless results e.g. Hamilton et al. (1979) cf. Kinny and Friend, (1997), Burton et al. (1994) cf. Whitehouse et al. (1996). Ion-mi-croprobe U – Pb single zircon dating allows indi-vidual samples to be more reliably dated so that when dealing with complicated field relationships, analytical results can be directly compared rather than attempting to correlate using lithological similarities. Application of this technique to the Godtha˚bsfjord region of southern West Green-land (Fig. 1 e.g. McGregor et al., 1991; Friend et al., 1996), confirmed the concept that different terranes sensu Coney et al., (1980), had been amalgamated in the late Archaean, an hypothesis first suggested on a structural and metamorphic basis (Friend et al., 1987, 1988; Nutman et al., 1989). Because the Godtha˚bsfjord region com-prises only a part of the West Greenland
Ar-chaean, it is important for the overall
understanding of the evolution of the North At-lantic craton to know whether terrane assembly can be recognised over a wider area.
The Paamiut region (Fig. 1) was mapped in the late 1960s the region being interpreted as a single block, with a more or less uniform history throughout (e.g. Berthelsen and Henriksen, 1975; Higgins, 1990; Kalsbeek et al., 1990). A new field study of this region (McGregor and Friend,
1997), had two main aims. First, to test whether the Paamiut region was indeed a continuous block of similar history, as indicated on existing maps
(e.g. Allaart, 1975, 1982) or, like the
Godtha˚bsfjord region to the north, was con-structed from individual blocks with different tec-tono-thermal histories (e.g. Friend et al., 1988). Second, to search for outcrops of early Archaean gneisses that, from whole-rock Pb – Pb isotopic work on Proterozoic mafic dykes which traverse the region, were postulated to occur at depth
(Kalsbeek and Taylor, 1985). In the
Godtha˚bsfjord region such rocks, the Itsaq Gneiss Complex, have distinct field characteristics (e.g. Nutman et al., 1996). In a global context, it would be important to identify another area where early Archaean rocks occur. However, no definitive field or isotopic evidence was found to suggest that early Archaean rocks occur in the Paamiut region. This paper presents the zircon geochronol-ogy carried out to establish the ages of some of the crustal components and metamorphic events identified by McGregor and Friend (1997).
2. Pre-1992 investigations of the Paamiut region
2.1. Field work
Geological mapping of the Archaean gneiss complex in the Paamiut region was completed
(Allaart, 1975) when Archaean
C.R.L.Friend,A.P.Nutman/Precambrian Research105 (2001) 143 – 164 146
gins, 1990; Kalsbeek et al., 1990) containing undi-vided, typical Archaean tonalite, trondhjemite,
granodiorite (TTG) suite gneisses from the
Ketilidian boundary north to Frederiksha˚b Is-blink (Fig. 1). Kalsbeek et al., (1990) considered that the rocks north of Paamiut were similar to those north of Frederiksha˚b Isblink, and conse-quently concentrated on describing the rocks be-tween Paamiut and Ivittuut. The gneiss protoliths were thought to have formed between 3000 – 2850 Ma and to have been metamorphosed under am-phibolite facies conditions in the late Archaean. No evidence for granulite facies metamorphism in the region was reported (e.g. Higgins, 1990; Kals-beek et al., 1990 and Refs. therein). Within the
TTG gneisses, amphibolite and
gabbro-anorthosite units occur as semi-continuous
marker units or as trains of enclaves. Locally, units of sillimanite-bearing rocks, interpreted to represent pelitic to semi-pelitic compositions have been recognised (e.g. Rivalenti and Rossi, 1972). The Tartoq Group (Higgins and Bondesen, 1966; Higgins, 1968) is a separate, thick sequence of greenschist and low amphibolite facies rocks derived from pillow lavas, other volcanic and associated sedimentary rocks. On the basis of its lower metamorphic grade and structural evidence the group was thought to be younger than the main gneisses (Berthelsen and Henriksen, 1975).
2.2. Isotopic data
Few isotopic data exist for the Paamiut region. In a whole-rock Pb – Pb isochron study, Taylor and Kalsbeek (1986) examined three units. Am-phibolite facies, grey, biotite-bearing gneisses from the south-west Vesterland (Fig. 1) yielded a
date of 2784953 Ma, interpreted as a minimum
protolith age. At Kuummiut (Fig. 1) grey, banded
gneisses gave 29859 115 Ma, and a younger,
intrusive trondhjemite component was dated at
27699110 Ma. The same trondhjemite was
dated using SHRIMP U – Pb single zircons, with
dates of 292294 Ma (2s) for prismatic grains
with later zircon overgrowth at 2827911 Ma.
These were interpreted to date the protolith and a high-grade metamorphic event respectively (Nut-man and Kalsbeek, 1994). Igneous zircons from a
tonalitic sheet cross-cutting the Tartoq Group
were dated at 294497 Ma (Nutman and
Kals-beek, 1994). This demonstrates that at least parts of the Tartoq Group is older than previously supposed (c.f. Berthelsen and Henriksen, 1975) and appears to be different from the supracrustal units found further north as suggested by Higgins, (1990).
3. New field evidence
The new field evidence for the subdivision of the Paamiut region was presented by McGregor and Friend (1997); only a summary is given here. The region between Frederiksha˚b Isblink and Ser-miligaarsuk (Fig. 1) was divided into four blocks based on: (i) the recognition of blocks where rocks have been partially to totally retrogressed from granulite facies mineral assemblages, com-pared with blocks containing amphibolite facies rocks which have never been to granulite facies conditions; and (ii) the recognition of Archaean amphibolite facies ductile shear zones with my-lonite between blocks of different metamorphic history. This division was possible because
char-acteristic metamorphic ‘blebby textures’ and
structures such as ‘spotty pegmatites’ (partial melts) produced by the growth of orthopyroxene during granulite facies metamorphism (e.g. Mc-Gregor and Friend, 1992, 1997, see below) are retained either during static retrogression or when retrogression is accompanied by only limited de-formation. Rocks which have never been to gran-ulite facies do not show these textures and structures (e.g. McGregor and Friend, 1992,
1997). Gneisses with a blebby texture developed
from pseudomorphed orthopyroxene porphyrob-lasts (McGregor et al., 1986) have been illustrated from other parts of the West Greenland Archaean (e.g. Garde, 1990, 1997; McGregor and Friend, 1992). They are also recognised elsewhere in the North Atlantic craton in Scotland (e.g. Beach, 1974; Crane, 1978) and Labrador (e.g. Collerson et al., 1982).
Fig. 2. Sketch cross section along the outer coast of the Paamiut region from Bjørnesund in the Tasiusarsuaq terrane to Tartoq in the Sermiligaarsuk block showing the relative structural positions of each of the identified blocks. For locations and legend see Fig. 1. Abbreviations used: p, protolith age; i, inheritance; af, amphibolite facies metamorphism; gf, granulite facies metamorphism.
zones. In these zones blebby textures and spotty pegmatites produced under granulite facies condi-tions were deformed, indicating tectonic
juxtapo-sition occurred after granulite facies
metamorphism. The trace of these tectonic
boundaries (Figs. 1 – 3) show they developed prior to late Archaean folding under amphibolite facies conditions. From north to south these blocks are:
3.1. The Sioraq block
This block occurs west and south of Frederik-sha˚b Isblink (Fig. 1) and is dominated by dioritic to tonalitic gneisses. Primary igneous features, such as sharp intrusive contacts between different phases and relic coarse-grained igneous textures, indicating derivation from plutonic rocks are rarely preserved. In some areas, more granodi-oritic components appear to have undergone par-tial melting, before or during granulite facies metamorphism, as they have copious leucosome veining. Supracrustal units are dominated by mafic rocks, probably derived from volcanic pro-toliths, including homogeneous and layered am-phibolite, meta-gabbroic amphibolites and some ultramafic pods. Occasional biotite-garnet-silli-manite gneisses are derived from sediments. Or-thopyroxene-bearing rocks are preserved in the north-west, but to the east and south, where post-granulite facies deformation under amphibo-lite facies conditions becomes increasingly intense, partial to total retrogression has occurred (Mc-Gregor and Friend, 1997). Garnet is widespread in rocks that have been partly retrogressed from
granulite facies, but is generally absent in rocks that have been totally recrystallised.
The position of the northern boundary with the Tasiusarssuaq terrane is uncertain and was ex-trapolated from its location on the coast by Mc-Gregor and Friend (1997), using information from Dawes (1970), Steenfelt (1994) and struc-tural trends on maps 62 V.1N Bjørnesund and 62 V.1S Nerutussoq (Fig. 1). Northwards, along the coast of Frederiksha˚b Isblink granulite facies rocks pass northwards into totally retrogressed, blebby textured rocks which, with increasing strain, become more and more mylonitic with a
C.R.L.Friend,A.P.Nutman/Precambrian Research105 (2001) 143 – 164 148
strong SSW-plunging linear fabric. About 2 km across strike to the west, an abrupt change in metamorphic grade with the amphibolite facies gneisses and rocks of the Ravns Storø belt that were never metamorphosed above amphibolite fa-cies conditions occurs. These observations suggest that the contact zone between the amphibolite facies rocks and retrogressed granulite facies rocks is tectonic and does not resemble the
pro-grade amphibolite-granulite facies transition 25
km to the north in Bjørnesund within the Ta-siusarsuaq terrane (McGregor and Friend, 1992). This northern boundary of the Sioraq block lies on the northerly dipping limb of a large antiform (Andersen and Friend 1973) and implies that the amphibolite facies rocks of the Tasiusarsuaq ter-rane are structurally above the Sioraq block (Fig. 2). Gneisses similar to those in totally retrogressed parts of the Sioraq block occur on Kangaarsup Nunaa but, on the northernmost of the Dalagers Nunatakker (Fig. 1), amphibolites with relic pil-low-lava structures occur which are equated with pillow-structured amphibolites in the Ravns Storø belt (Dawes 1970), and are thus interpreted to be part of the Tasiusarsuaq terrane.
The southern boundary of the Sioraq block (Fig. 1) is in places a 10 – 150 m wide zone of folded, sub-mylonitic to ultramylonitic rocks with amphibolite facies assemblages (Figs. 2 and 3). Rocks recognised as retrogressed from granulite facies were followed into a high strain zone to within 10 – 20 m from rocks on the other side that from textural evidenece never reached granulite facies conditions. There is no indication of a prograde amphibolite- to granulite-facies transi-tion and the contact is best interpreted as tectonic. The contact has been traced intermittently for ca. 30 km. Where observed there is an increasing strain gradient in the margin of the Sioraq block towards the contact. Structurally, the (ex-gran-ulite facies) Sioraq block overlies the amphibolite facies Paamiut block to the south which, from interpretation of published maps, occupies a northeasterly-plunging domal structure with a large parasitic synform to the south (Fig. 1). Evidence supporting this interpretation comes from two independent sources. Rivalenti and Rossi (1972) described the same textures and
structures that are now understood to be blebby textures characteristic of retrogressed granulites (e.g. McGregor and Friend, 1997) and the rocks inside the parasitic synformal structure have low K, Rb, and U, geochemical characteristics of granulite facies rocks, as elsewhere in the Sioraq block (Steenfelt, 1994; Steenfelt et al., 1994).
3.2. The Paamiut block
On the coast this block occupies the area be-tween Nordre Storø and Vesterland, (Fig. 1), whilst inland it envelops a major synform (con-taining the structurally higher Neria block) and is then continuous with similar rocks to the north of Neria (Figs. 1 and 2). In the southern inland part of the region a contact is extrapolated using struc-ture trends from published maps. The Paamiut block structurally underlies both the Sioraq and Neria blocks but differs significantly in that it has never been metamorphosed above amphibolite fa-cies, bearing none of the coarse-grained features found as a result of orthopyroxene growth and retrogression (McGregor and Friend, 1997). It is dominated by rather homogeneous tonalitic to granodioritic biotite gneisses which, at any one locality comprise several different sheets with sharp contacts and may preserve textures inter-preted to be secondary after plutonic igneous textures. Minor trondhjemitic sheets and rare hornblende-bearing mafic tonalitic and dioritic phases occur. The gneisses enclose thick units of likely supracrustal rocks dominated by amphibo-lite, with minor ultramafic and biotite schists that are continuous over tens of kilometres outlining major fold structures (Fig. 1; e.g. Andrews, 1973; Higgins, 1990). Leucogabbroic and anorthositic rocks have not been found. In some domal struc-tures the gneisses have undergone partial melting to yield metatexites and diatexites.
3.3. The Neria block
an-tiform at the mouth of Neria (Figs. 1 and 2). The north-western boundary of the block is truncated by the Proterozoic Vesterland dextral shear zone of intense hydrous mylonitisation (Watterson, 1968; Higgins, 1990). On the basis of strongly deformed blebby textures, the block is interpreted to comprise rocks metamorphosed to granulite facies but later totally retrogressed to amphibolite facies and moderately to strongly deformed (Mc-Gregor and Friend, 1997). The northern part of the block is dominated by hornblende-bearing tonalitic and dioritic phases enclosing abundant enclaves, rafts and thin layers of mafic rocks,
ultramafic rocks and, locally,
gabbroic-anorthositic lithologies. South of Neria (Fig. 1) mafic lithologies are less abundant, the gneisses being commonly finely pegmatite-layered and bi-otite-bearing and there are areas of polyphase, leucocratic, trondhjemitic gneisses (the white gneisses of Kalsbeek, 1970; Taylor and Kalsbeek, 1986). These trondhjemitic gneisses lack textural – mineralogical features indicating earlier granulite-facies metamorphism, but appear to have had the same complex structural history as adjacent gneisses that do preserve relic granulite-facies textures.
Field relations near the ice cap north of Sermili-gaarsuk (Fig. 1) are unknown, though Masson (1967) recognised that hornblendic gneisses in the main synform were significantly different from the rocks underneath. As one of several possible in-terpretations, he suggested a tectonic contact with the hornblendic gneisses thrust over the biotite gneisses, though no recognition of such a contact was documented. Late phases of biotite-bearing granitic gneiss on the south side of Neria appear to post-date retrogression of the granulite-facies assemblages.
3.4. The Sermiligaarsuk block
Considerable modification of the rocks by Proterozoic faulting has occurred on the south side of Neria obscuring Archaean relationships. However, in contrast to the ex-granulite facies Neria block, the volcano-sedimentary Tartoq Group to the south and associated gneisses have only been metamorphosed to greenschist and
low-ermost amphibolite facies, and are described as having had a much simpler structural history (Berthelsen and Henriksen, 1975; Higgins, 1990). It has been considered that the Tartoq Group was tectonically emplaced and subsequently deformed into synformal structures (Berthelsen and Henrik-sen, 1975). These lower grade rocks are here
named theSermiligaarsuk block. The position of a
boundary has now been located on the outer coast where the Neria block overlies lower grade rocks (V.R. McGregor, pers. comm.). Retro-gressed granulite facies rocks of the Neria block have been identified to within ca. 2 km of the much lower grade rocks of the Sermiligaarsuk block (Figs. 1 and 2) and there is no indication of any prograde transition preserved.
4. SHRIMP U – Pb zircon data
4.1. SHRIMP ion microprobe analytical
technique
Zircons were mounted in polished epoxy discs and were all studied for internal structure by optical microscopy before analysis. In several cases, where the zircon populations were complex, cathodoluminescence (CL) imaging was used to help resolve internal grain structure and choice of analysis site. U – Th – Pb isotopic ratios and con-centrations in unknowns were determined using SHRIMP I and referenced to the Australian Na-tional University standard zircon SL13 (age 572
Ma; 206Pb/238U =0.0928). Repeated analyses of
the standard during each analytical session were used for calculating the inter-element isotopic ra-tios of the unknowns and of their uncertainty. Further details of the analytical procedure and data assessment have been given (Compston et al., 1984; Williams and Claesson, 1987; Roddick and Van Breemen, 1994; Claoue´-Long et al., 1995). A summary of the age determinations and the zircon populations is given in Table 1 and the SHRIMP U – Pb analytical data are presented in Table 2.
Judging from the very low 204Pb count rates, the
C
Summary of the relationships, with ages for protoliths and metamorphic events, where appropriate, for the Tasiusarsuaq terrane (in the north), Sioraq, Paamiut, Neria and Sermiligaarsuk blocks (Fig. 1)a
Sermiligaarsuk block
Tasiusarsuaq terrane Sioraq block Paamiut block Neria block
G94/32
Granite sheets post 2717935 ?Metamorphism
2720b G94/17 2700 Granite sheets
extensive
assembly with extensive deformation
deformation and
Akulleq terrane amphibolite facies
retrogression amphibolite facies
retrogression ? deformation and amphibolite
facies retrogression
No2820 Ma events recorded in ? granulite facies
2812910
2820c granulite facies G94/32
this block
2820910c Ilivertalik granite G94/26 2823938? granulite facies G94/7 2816912 Metamorphism
?granulite facies 2827911f ?granulite facies 2820gamphibolite facies
G94/29 2826910
metamorphism G94/24 2835970 ? Metamorphism
2863910* TTG gneiss G94/32 TTG gneiss G94/8 285296 TTG gneiss
VM89/34 286096
? Inheritance G94/7 286299 TTG gneiss
Fiskenæsset
2860950d G94/29 2864936
complex
G94/21 2872910 TTG gneiss
2872914 TTG gneiss G94/26
2878910 TTG gneiss G94/28 2873916 TTG gneiss
VM90/4
]2880 Ravns Storø belt
? Supracrustal
(syoracrustal rocks) ? Anorthositic rocks
rocks Supracrustal rocks
?
G94/14 289898 TTG gneiss
VM89/34 2905 I`nheritance
G94/28 2920 ? Inheritance 292294e Trondhjemite
2920d Component of old
G94/9 292799 TTG gneiss TTG gneiss
G94/24 2932913 Component
294497
aAll new data are indicated by sample numbers (see Fig. 1). Previously published whole-rock and U–Pb zircon data are indicated by superscripts. bFriend et al., 1996.
cPidgeon and Kalsbeek, 1978. dAshwal et al., 1989.
eKinny, 1987; Schiøtte et al., 1989. fKalsbeek and Nutman (pers. comm). gNutman and Kalsbeek, 1994.
Table 2
SHRIMP U-Th-Pb data for zircons analysed from Paamiuit gneiss samplesa
Labels Grain type U ppm Th ppm Th/U Comm. 238U/206Pb 207Pb/206Pb Age %disc 207Pb/206Pb 206Pb%
VM89/34
1.1 p,tw 154 76 0.50 0.24 1.789 90.032 0.203590.0014 2855 911 0 238 108 0.45 0.16
p 1.757 90.031
p,fr 1.71290.032
5.1 0.204490.0013 2862911 4
c?,p
6.1 113 60 0.53 0.49 1.71290.032 0.209990.0021 2905917 2 113 35 0.31 0.30 1.78390.035 0.202090.0019
7.1 p 2842915 1 342 121 0.35 0.61
p 1.93490.067
3.2 0.204290.0017 2860914 −6
p
4.1 276 162 0.59 0.31 1.81290.028 0.206290.0010 287698 −1 458 317 0.69 0.40 1.89190.029 0.203890.0007
5.1 p 285795 −4
140 108 0.77 0.73
p 1.88690.040
p,fr 1.86690.077
1.2 0.199790.0037 2824931 −2
p,fr
2.1 136 62 0.46 0.13 1.80690.044 0.204390.0010 286198 −1
115 61 0.53 0.11
3.1 p 1.85190.056 0.206790.0042 2880933 −3
669 143 0.21 0.03
t,p,fr 1.83090.050
4.1 0.203990.0023 2858918 −2
p,fr 1.85490.040
7.1 0.205190.0006 286795 −3
t,ov 1.90690.061
13.1 0.203590.0038 2855931 −5
p,fr 1.80590.046
17.1 0.204190.0011 285999 −1
ov-p
18.1 185 48 0.26 0.10 1.89090.058 0.193290.0018 2770916 −1 232 85 0.36 0.06 1.90190.047 0.194990.0025
18.2 ov-p 2784921 −2
1547 42.20 0.03 0.022 1.85690.050 0.199790.0024
21.3 r on p 2823919 −2
188 69.17 0.37 0.074
c,p 1.89190.065
21.4 0.205290.0017 2868914 −5
c,p
21.5 139 43.70 0.31 0.059 1.93690.055 0.207590.0027 2886921 −7 444 199.11 0.45 0.021 1.94590.049 0.205090.0030 2866924 −7 21.6 c,p
118 36.09 0.31 0.238
p 1.75690.056
13.2 0.206490.0016 2877912 1
130 38.29
C.R.L.Friend,A.P.Nutman/Precambrian Research105 (2001) 143 – 164 152
Table 2 (Continued)
Th ppm
Labels Grain type U ppm Th/U Comm. 238U/206Pb 207Pb/206Pb Age %disc
206Pb% 207Pb/206Pb
G94/29
1.1 ov 79 21 0.27 0.15 1.864 90.067 0.204090.0016 2859913 −3 75 0.68 0.12 1.80490.045
110 0.199790.0019
p 2823915 1
95 0.200890.0011
4.1 ov 283399 0
111 0.195190.0026
ov-p 2786922 −1
378 0.192090.0030
p 2759926 −1
10.1
68 0.53 0.41 1.94390.044
11.1 p 128 0.201590.0026 2838921 −6
88 0.61 0.07 1.85890.100
144 0.199390.0035
12.1 p 2821929 −2
113
c? in ov-p 46 0.41 0.38 1.86090.044 0.204690.0024 2864919 −3 13.1
28 0.26 0.06 1.81790.046
108 0.200490.0015
ov,fr 2829912 0
298 0.86 0.07 1.87590.111
348 0.194690.0023
p 2782919 −1
342 0.76 0.00 1.78890.073
451 0.205290.0032
6.1 ov-p 2868925 0
289
p 190 0.66 0.03 1.77090.080 0.205590.0032 2870926 1
7.1
420 0.48 0.02 1.81990.052
8.1 ov-p 876 0.202890.0011 284999 −1
322 0.88 0.02 1.80190.054
367 0.203890.0016
p 2857913 0
9.1
514
p 322 0.63 0.02 1.82490.056 0.199490.0015 2821912 0
10.1
394 0.77 0.07 1.84090.060 0.201590.0021 2839917
11.1 p 511 −1
76 0.205190.0033
p 2867926 3
250 0.206290.0008
ov 287696 −3
163 0.68 0.19 1.64290.057
241 0.188690.0031
p 2730927 12
227 0.207290.0044
2.1 p 2884935 2
165 0.207290.0022
p 2884917 −1
6.1
136 0.89
Table 2 (Continued)
U ppm
Labels Grain type Th ppm Th/U Comm. 238U/206Pb 207Pb/206Pb Age %disc 207Pb/206Pb 206Pb%
G94/24
1.1 ov 25 9 0.38 0.46 1.716 90.065 0.212890.0034 2927926 1 840 610 0.73 0.00
t,p 1.70890.039
2.1 0.207190.0013 2883910 3
t,p
3.1 41 24 0.58 0.49 1.72490.052 0.220590.0055 2985941 −1 77 53 0.69 0.29 1.76190.053 0.213090.0032
4.1 p 2928925 −1
rex,p 1.97690.050
8.1 0.182290.0016 2673914 −1
p 0.209690.0016
C.R.L.Friend,A.P.Nutman/Precambrian Research105 (2001) 143 – 164 154
Table 2 (Continued)
Labels Grain type U ppm Th ppm Th/U Comm. 238U/206Pb 207Pb/206Pb Age %disc 207Pb/206Pb
ov-irreg 1.84390.052
2.1 0.199590.0022 2822918 −1
ov-irreg
2.2 16 106 6.77 0.93 1.825 90.071 0.198390.0076 2812964 0 27 234 8.59 0.01 1.75390.064 0.198890.0027
3.1 ov-p 2816923 3
ov-p 1.81590.052
11.1 0.200690.0027 2831922 0
p
12.1 149 65 0.44 0.20 1.82390.054 0.201190.0017 2835914 −1 160 69 0.43 0.15 1.85690.046 0.200090.0024
12.2 p 2826920 −2 199 39 0.20 0.14 1.98190.054 0.200090.0023
17.1 x, p 2826919 −7
ov-p 1.924 90.131
1.1 0.173690.0194 25929199 4
893 279 0.312
2.1 t,p .40 2.40590.046 0.169990.0014 2557914 −12
333 178 0.54 1.81
t,p 1.81290.039
1054 145 0.14 2.61
t,p 2.61190.057
ac, core; fr, fragment; irreg, irregular; p, prismatic; ov, ovoid; r, rim; rex, recrystallised; t, turbid; tw, twinned; x, xenoblastic.
2700 Ma. However, because the proportion of non-radiogenic Pb detected in these grains is so low, the dates quoted here are not sensitive to the choice of its composition.
Quoted dates are weighted means (2s, inverse
variance weighting) derived from 207Pb/206Pb
ra-tios of analyses selected as being of the least isotopically disturbed sites (close to concordant
Pb/U dates), with very small non-radiogenic
com-ponent to Pb in grains belonging to the same
population/type (e.g. for dating of magmatic
events, sites within prismatic zircon commonly displaying micron-scale oscillatory zoning concor-dant with the grain margins). Inititally in most cases, the spread in individual site dates meant that the weighted mean dates calculated on such groupings of analyses had mean square weighted
deviate (MSWD) values \1.0; indicating a low
probability that such dates reflect the timing of real geological events. From multiple analyses of different sites on some grains and CL imagery on the populations, this is best interpreted to be due to partial loss of radiogenic Pb during later
mod-ification/damage of the ziron structure. These
ob-servations suggest that the ‘oldest’ dates obtained from any petrographic grouping are likely to give the most reliable determination of a real geologi-cal event (e.g. growth of oscillatory-zoned zircon out of a magma). Hence individual ‘youngest’ dates in any grouping were culled until those
remaining had a MSWD of 01.0 for the
weighted mean date. These low MSWD dates, when combined with petrographic observations of the dated sites in the zircons, will give confident dates on individual geological events. For more
detail on the calculation of weighted mean 207Pb/
206Pb dates from SHRIMP data see Compston et
al. (1986), Nutman et al. (1997). The Steiger and Ja¨ger, (1977) values for decay constants and
present-day 238
U/235
U were used to calculate the dates.
4.2. Tasiusarsuaq terrane
Age determinations are reported on two amphi-bolite facies tonalite gneisses (provided by V.R. McGregor) from the southern part of the
Ta-siusarsuaq terrane. Sample VM89/34 represents a
tonalitic sheet cross-cutting gabbro anorthosite
near the mouth of Bjørnesund (62°50%N, 50°23%W;
Fig. 1). This yielded euhedral to slightly rounded
100 – 300 mm long colourless to light brown
pris-matic zircons. They are homogeneous or show a faint micron-scale oscillatory zoning parallel to grain exteriors. One grain (6) might contain a homogeneous core with a mantle of zoned zircon. Analysis of the centre of grain 6 yielded the oldest
207Pb
/206Pb date (29059 34 Ma). With this age
and precision it could be a real core or a statistical outlier of the main population (Table 2). The remainder of the analysed grains yielded close to
concordant dates with a slight spread of 207
Pb/
206Pb dates (Fig. 5a). These yielded a weighted
mean 207Pb/206Pb date of 2863 910 Ma (2s,
n =6), interpreted as the intrusive age of sheet
VM89/34.
Of note is the slight reverse discordance of these data. The analyses were undertaken in 1990, when it was the practice to insert an epoxy plug with a large chip of standard SL13 into a hole in the mount containing the unknowns. This may give
rise to a small bias between Pb/U as measured on
the standards and the unknowns on the rest of the same mount (see Friend and Nutman, 1992 for further discussion of this problem). This practice has since been abandoned and small chips of the standard are now cast with the unknowns. This problem does not affect the age reported which is
based on the directly measured 207Pb/206Pb ratio.
Meta-tonalitic gneiss VM90/4 (62°38%N,
50°18%W; Fig. 1) came from a unit which intruded
the south-eastern margin of the Ravns Storø belt. It yielded a population of zircons very similar to
those in VM89/34 with faint CL images (Fig. 4).
Most of the analyses give close to concordant
dates, but with a spread in 207Pb/206Pb date of ca.
100 Ma (Table 2, Fig. 5b). Due to the homogene-ity of the population the grains are interpreted to be magmatic in origin, but to have suffered some ancient Pb-loss during late Archaean tectonother-mal events (2850 – 2550 Ma) known to have af-fected the Tasiusarsuaq terrane (e.g. Schiøtte et al., 1989; Friend et al., 1996). Accepting this interpretation, filtering the analyses until those
remaining are indistinguishable from their
weighted mean (Compston et al., 1986), yielded a
C.R.L.Friend,A.P.Nutman/Precambrian Research105 (2001) 143 – 164 156
Fig. 4. Cathodoluminescence images of selected zircon grains from the analysed samples.
206Pb dates between ca. 2900 and 2750 Ma (Table
2, Fig. 5c). If it is accepted that these grains form a single population that underwent some later disturbance with loss of radiogenic Pb then, by
rejecting analyses with the lowest 207
Pb/206
Pb, a
weighted mean date of 2860 96 Ma (2s, n =
13; Fig. 5c) is derived and interpreted as the magmatic protolith age. Also present are stubby prismatic to ovoid clear grains (e.g. grains 1, 19) and clear overgrowths (e.g. analysis 9.2), with low
Th/U (B0.1). A weighted mean207Pb/206Pb date
of these morphologically and chemically distinct
zircons is 2812 910 Ma (2s, n =4). These are
interpreted to reflect new zircon growth during
high-grade metamorphism (9 anatexis),
proba-bly during the granulite facies event this rock has experienced. Two analyses of distinctive brown prismatic zircon (grains 5, 11) with concordant
dates have an imprecise average 207Pb
/206Pb date
of 27179 35 Ma (2s, n= 2). It is uncertain if
these represent regrowth of zircon at that time or they are witness to Pb-loss from grains in an event
after peak metamorphism (2812 Ma).
Sample G94/26 (62°14%N, 49°51%W; Fig. 1)
yielded predominantly clear to pale brown 100 –
300 mm long prismatic grains showing both
ho-mogeneous and oscillatory-zoned domains. Some grains are corroded, and a very few display partial overgrowths of brown zircon. Data from a first analytical session were rejected and another ses-sion was undertaken in which only grains 13 and 21 (Fig. 4) were examined with multiple analyses. Analysis was undertaken of a rim (analysis 21.3) which infills a corroded area on the side of a
prismatic grain. The rim is very high U, low Th/U
with a 207Pb/206Pb age of 2823938 Ma (2s).
Although imprecise, this age agrees with other
2820 Ma age determinations for the high-grade
(?granulite) event. Multiple analyses on the pris-matic zircons 12 and 21 yielded a weighted mean
of 2872 914 Ma (2s, n =5), interpreted as
giving a minimum magmatic protolith age.
The retrogressed sample, G94/29 (62°13.4%N,
49°33.4%W; Fig. 1), yielded a diverse zircon
popu-lation. Dominant are small (generally B150 mm
long) oscillatory-zoned to homogeneous prismatic grains, showing little sign of corrosion. Under CL these grains show little luminescence and only occasional zones are evident (e.g. grain 2; Fig. 4).
4.3. Sioraq block
Two samples of granulite facies and one retro-gressed granulite facies sample of tonalitic-dioritic gneisses from the Sioraq block were analysed.
Sample G94/32 (62°22%N, 50°08%W; Fig. 1)
pro-vided a heterogeneous zircon population.
Domi-nant are brown, prismatic 100 – 300 mm long
grains characterised by 1 – 2 mm scale oscillatory
zoning, embayment of some facets and rare over-growths. The grains generally show little lumines-cence in CL, though some show faint oscillatory
zonation. They generally have Th/U of \0.10
C.R.L.Friend,A.P.Nutman/Precambrian Research105 (2001) 143 – 164 158
Also present are equant to ovoid grains ranging
from 50 to \300 mm, that usually show a
broad but weak zonation under CL (e.g. grain 2; Fig. 4). Still larger grains did not survive sample crushing intact and occur as fragments. Most analyses plot on or close to concordia (Fig. 5e),
with 207Pb/206Pb dates from 2865 to 2760 Ma.
The oldest analysis is of a possible core in pris-matic grain 13 (Table 2) and another old analysis (1.1) is also of a possible core (Table 2). Exclud-ing these, the remainder give a weighted mean
207
Pb/206
Pb date of 2826 910 Ma (2s, n= 12).
Given the magmatic appearance of many of the zircons in this population the sample must have at least partially melted at this time. Analysis 13.1 of
a core with a 207Pb
/206Pb date of 2864936 Ma
(2s) is, within its large error, agrees with the
protolith ages of G94/26 and G94/32, indicating
that older components are present in this sample.
4.4. Paamiut block
Ages have been determined on three samples
(G94/8, G94/28, G94/21, Fig. 1) considered to
represent the main homogeneous biotite tonalitic/
granodioritic gneisses of this block. G94/8
(61°47.1%N, 49°20.7%W) is a sample from a more
homogeneous than average area of gneisses de-void of supracrustal units. It yielded a uniform
population of generally small (50 – 200 mm), clear
to pale yellow, thin, prismatic zircons showing oscillatory zoning. The grains are euhedral to slightly corroded. In CL the show oscillatory zones mimicking the shape of the grain (Fig. 4). Most analyses are concordant within error (Table
2, Fig. 5f). Interpreting the dispersion in 207Pb/
206Pb dates as due to slight ancient radiogenic
Pb-loss, then rejecting 2 analyses yielded a 207Pb/
206
Pb date of 285296 Ma (2s, n=10).
G94/21(62°52.3%N, 49°24.3%W) is from a sheet
of homogeneous grey gneiss that intrudes a supracrustal unit. It yielded prismatic, bipyrami-dal, oscillatory-zoned zircons that are typically
B150 mm long (e.g. grain 1). Apart from being
slightly smaller than zircons in G94/08, their
char-acters are similar. Rejecting a few analyses with
the lowest 207Pb/206Pb, a weighted mean 207Pb/
206Pb date of 2872910 Ma (2s, n =7) was
determined and interpreted as giving the igneous protolith age (Fig. 5g).
Sample G94/28 (62°14.6%N, 49°27.1%W) is also a
homogenous tonalitic gneiss taken from a sheet intruding a supracrustal unit. It yielded prismatic, euhedral, oscillatory-zoned zircons typically up to
300 mm long which are very similar to those in
G94/08 and G94/21. However, from CL imaging
very rare cores might be present in a small minor-ity of grains (e.g. grain 3). After rejecting two analyses of possible cores, the remaining close to
concordant analyses give 2873 916 Ma (2s,
n =5), interpreted as the protolith age (Fig. 5h).
4.5. Neria block
Four new samples have been dated from the
Neria block. One, dioritic gneiss G94/24
(62°32.2%N, 49°11.1%W; Fig. 1) is cut by the
2922 94 Ma white trondhjemitic gneiss (GGU
261014) of Nutman and Kalsbeek (1994),
repro-duced in Fig. 5i. Sample G94/24 yielded large
(\500 mm) zircons, some of which are deep
brown to opaque, rounded grains and corroded prisms that are commonly cracked, turbid and in places metamict. Other grains are colourless to pale pink, unflawed and only slightly corroded. Most analyses yielded close to concordant ages
(Fig. 5j). The brown grains have207Pb
/206Pb dates
from \2700 to 3000 Ma. Given their flawed
state, these are interpreted as a single population that has, in ancient times lost variable amounts of radiogenic Pb. The four oldest analyses yielded a
weighted mean 207Pb/206Pb date of 29789 8 Ma
(2s,n =4). With the clear, unflawed grains, two
ages might be apparent. Dominant are those
which yield a weighted mean 207Pb/206Pb date of
2932 913 (2s, n= 7). A single analysis of a
completely clear and euhedral grain gave an
im-precise 207
Pb/206
Pb date of 2835970 Ma (2s).
Despite this large uncertainty, it is possible a third generation of zircons is present. In keeping with the heterogeneous appearance of this gneiss in the field, the zircon chronology suggests it is com-posite, containing more than one igneous phase (Table 2).
A homogenous sample, G94/14 from the mouth
deep yellow to brown, typically 200 – 400mm long,
euhedral to slightly rounded, prismatic zircons. Oscillatory zoning is commonly displayed, but some grains are partly metamict and mostly showed low luminescence in CL (Fig. 4). No structural cores or overgrowths were observed. Overall the population appears simple, albeit some grains show signs of being flawed. Apart from reverse discordant analysis 4.1, all analyses gave close to concordant ages (Table 1, Fig. 5k).
Rejecting a few analyses with low 207
Pb/206
Pb, interpreted as having undergone ancient Pb-loss, the remainder yielded a weighted mean date of
289898 Ma (2s, n =9), interpreted as the
ig-neous age of the protolith (Table 2).
Neither of the samples G94/14 and 24 had
obvious overgrowths, even when viewed by CL.
However, sample G94/24 had young prismatic
grains on which a single analysis gave a 207Pb/
206Pb date of 2835 Ma and trondhjemitic gneiss
GGU261014 from the same locality had
over-growths dated at 28279 11 Ma (Nutman and
Kalsbeek 1994). This possibly reflects the gran-ulite facies metamorphism described by McGre-gor and Friend (1997).
Two other samples of gneisses (G94/7 and G94/
9, Fig. 1) thought lithologically representative of
the block, were dated. Gneiss G94/9 (61°42.5%N,
49°08.3%W) from the northern side of the block
yielded a diverse population of zircons; clear prisms with faint oscillatory zoning and buff equant or rounded grains (Table 2, Fig. 5l). No internal grain structure was visible by optical microscopy. However, CL imagery revealed that some grains are composite, with some of the prismatic grains having thin overgrowths on their pyramidal terminations – too thin for analysis
even using a 20mm spot (the smallest available on
SHRIMP I). If the analyses are treated as belong-ing to one population that suffered some ancient
loss of radiogenic Pb, a weighted mean 207
Pb/
206Pb date 292799 Ma is interpreted as giving
the igneous age of the protolith. As a justification of this , four analysed of the large homogeneous
prismatic grain (grain 7, Table 2) yielded 207Pb
/
206Pb dates from 2835 to 2913, testifying to
het-erogeneous loss of radiogenic Pb from single crystals.
Sample G94/7 (61°43.9%N, 49°18.7%W) comes
from within 2 km of a major Proterozoic fault which cuts the Neria and Paamiut blocks (Fig. 1). Dominant in the zircon population are brownish prismatic grains which are, on average, more frac-tured and metamict that zircons from other gneiss samples discussed in this paper. Most of the pris-matic grains have low luminescent centres and may display irregular, narrow overgrowths on their pyramidal terminations that are bright in CL. Also present are some homogeneous ovoid to
irregular grains, up to 300 mm long that show
irregular to blotchy CL luminescence (grains 2 and 14; Fig. 4). Analyses of these grains and
overgrowths yielded a weighted mean 207Pb
/206Pb
date of 2816 912 Ma (2s, n =10, Fig. 5m).
Analyses of the prismatic grains yielded a
weighted mean 207Pb/206Pb date of 28629 9 Ma
(2s, n=6). The ages are interpreted to
respec-tively represent the age of high-grade metamor-phism and a minimum for the igneous protolith.
A sample of a weakly deformed, late granite
sheet (G94/17, 61°35.7%N, 49°02.4%W; Fig. 1), was
also dated. This cross-cuts retrogressed and subse-quently deformed ex-granulite facies gneisses near the mouth of Neria. An age of this sample, to-gether with the protolith ages on the gneisses would limit timing of the prograde and retrograde
metamorphic events. However, G94/17 yielded
only dark brown, widely metamict zircons with generally discordant ages (Table 2, Fig. 5n). From
these results an imprecise 207Pb/206Pb date of
2600 Ma can be suggested. However, this does show that by this time the major tectonothermal evolution of the Neria block had been completed.
5. Discussion and conclusions
C.R.L.Friend,A.P.Nutman/Precambrian Research105 (2001) 143 – 164 160
trends and markers on published maps (e.g. Al-laart, 1975, 1982), it appears that these blocks were juxtaposed along mylonitic contacts that developed post-2820 Ma, the proposed age of the granulite facies metamorphism in the Sioraq and Neria blocks (Table 1), and were then folded (Fig. 2). The low state of deformation of rocks such as
2600 Ma granitoid sheet G94/17, other 2720 –
2500 Ma sheets in the region (Nutman and Mc-Gregor, unpublished data) and the undeformed state of Palaeoproterozoic mafic dykes, indicates that the folding and the post-granulite facies and amphibolite facies metamorphism are both late Archaean in age (i.e. older than Palaeoproterozoic tectonothermal events in the Ketilidian belt to the south). The folding and late amphibolite facies metamorphism appears to have obliterated any kinematic indicators in the mylonites that might have been used to establish original movement directions.
The new geochronological data suggest that all the blocks were initially formed within the period 2950 – 2850 Ma and were subsequently deformed and metamorphosed (Table 1). The Paamiut block, the structurally lowest (Fig. 2), only reached amphibolite facies conditions whilst the structurally higher Sioraq and Neria blocks at-tained granulite facies conditions, suggesting that they once occupied deeper levels in the crust and were exhumed to their present position (McGre-gor and Friend 1997). The Neria block suffered total hydrous retrogression and was deformed under amphibolite facies conditions, whilst the Sioraq block partly escaped hydration and defor-mation, or, was only statically retrogressed. The Sermiligaarsuk block contains greenschist and low amphibolite facies components and appears to have once occupied the highest crustal level. With the present disposition of the granulite facies rocks it is suggested that at least some of the boundaries between blocks are thrusts. The resul-tant crustal thickening appears to have given rise to only limited crustal remelting expressed at present erosion levels.
The question next to address is whether or not the blocks in the Paamiut region are terranes sensu Coney et al., (1980) and what relationship they have to the Tasiusarsuaq terrane which
ad-joins the Paamiut region (Fig. 1) with TTG pro-tolith ages faling in the range 2490 – 2850 Ma (Table 1). It is apparent that there are similarities between the recorded protolith and metamorphic ages of different parts of the Tasiusarsuaq terrane and the Sioraq, Paamiut and Neria blocks. The very northern part of the Tasiusarsuaq terrane is
known to have two components; one at 2920
Ma (e.g. Kinny, 1987; Schiøtte et al., 1989) and another at 2860 – 2840 Ma (e.g. Ashwal et al.,
1989; P.D. Kinny, unpublished data). This
younger component is interpreted to be the equiv-alent of that forming most of the amphibolite facies grey gneisses in the southern part of the terrane. The exact relationship between these two components is not known, largely because of the effects of granulite facies metamorphism and de-formation. However, given the time difference of
E60 Ma, they are unlikely to be products of the
same magmatic event. A possible explanation is that the Neria block represents the retrogressed equivalent of the granulite facies, northern part of the Tasiusarsuaq terrane, with two components at
2925 Ma and 2860 Ma. The amphibolite
facies Paamiut block with protoliths at 2865
Ma could then be equivalent to the southern portion of the Tasiusarsuaq terrane which has also only undergone amphibolite facies
metamor-phism. The Sioraq block with its 2865 Ma
rocks might equate to the lower crustal, granulite facies parts of the Paamiut block. However, be-cause of the different protolith ages and the lower grade of metamorphism, without more data the Sermiligaarsuk block does not easily equate with any of the identified blocks. As the suggested direction of transport on the tectonic contacts here might be south (Fig. 2), this block has the potential to be an unrelated piece of crust.
A common feature of the Tasiusarsuaq terrane and the Sioraq and Neria blocks is that they were subject to granulite facies metamorphism. In the
Tasiusarsuaq terrane this was dated, at 2820
metamorphism recorded could represent the gran-ulite event. In the samples studies, the amphibo-lite facies events in both the Tasiusarssuaq terrane and the Paamiut block have not produced new zircon that can be dated. Alternatively, it could be that some, or all, of the blocks are unrelated and
that given the analytical uncertainties of 10 Ma
(2s) on the ages, subtle real differences in their
magmatic and metamorphic histories cannot be demonstrated. By analogy with modern areas of TTG accretion and orogens, where major mag-matic and thermal events can last for only two or three million years (unresolvable in most age de-terminations on Archaean rocks whose zircons have generally lost some radiogenic Pb) this sce-nario is quite possible.
Without definitive kinematic indicators the original sense of movement on the mylonites can-not be deduced simply from the structural rela-tions between the blocks. The new data, therefore, do not allow the unequivocal identification of terranes sensu Coney et al., (1980) as in the Godtha˚bsfjord region to the north.
The Proterozoic dolerite dyke interpreted to be contaminated with non-radiogenic Pb derived from early Archaean crust (Kalsbeek and Taylor, 1985) cuts both the retrogressed granulite facies rocks of the Neria block with abundant 2920 – 2940 Ma rocks and the amphibolite facies Paamiut block, dominated by 2870 – 2850 Ma rocks. Kalsbeek and Taylor sampled gneisses where the dyke cuts the Paamiut block, and found no sign of an ancient crustal non-radiogenic Pb component in them. The non-radiogenic contami-nant Pb in the dyke is derived as they suggested from depth or, from as yet undetected older rocks ‘hidden’ within the ex-granulite facies 2940 – 2920 Ma rocks of the adjacent Neria block.
5.1. Significance to the rest of the North Atlantic
craton and other gneiss terrains
Combined with earlier studies of the
Godtha˚bsfjord region (e.g. Friend et al., 1988; Nutman et al., 1989) it can be demonstrated that the Archaean Craton of West Greenland from as far north as Maniitsoq to the Palaeoproterozoic
Ketilidian mobile belt in the south (550 km)
comprises different blocks separated by late Ar-chaean shear zones which have subsequently been folded and metamorphosed (Fig. 1b). In the Godtha˚bsfjord region unrelated tectono-strati-graphic terranes sensu Coney et al., (1980) have been juxtaposed. In the Paamiut region, it is possible that some of the blocks are slices derived from the southern continuation of the Tasiusar-suaq terrane, but it cannot yet be discounted with confidence that some could be unrelated segments of crust. However, whether they are all unrelated blocks with (given current analytical techniques) indistinguishable ages needs further investigation. Nonetheless, it is now certain that a substantial part of the North Atlantic Craton in West Green-land comprises techono-stratigraphic terranes and blocks that reached their current disposition by Archaean, post-2820 Ma, tectonic movements (Fig. 1, Table 1).
Similar findings have recently been reached for the eastern part of the craton, with new zircon geochronological data from the Lewisian complex of northern Scotland. These data have shown that it comprises two separate portions; one with
pro-tolith ages of 2960 – 3000 Ma which attained
granulite facies conditions, juxtaposed against a
terrane with protolith ages of 2700 – 2800 Ma
which only attained amphibolite facies conditions (Kinny and Friend, 1997). It thus appears that the construction of the North Atlantic Craton took place in distinct episodes, first the accretion of individual components over a wide time span from early Archaean to late Archaean and then the progressive amalgamation, along ductile shear zones, of these initially separated components into a large contiguous segment of continental crust. This situation also exists in other Archaean gnesis complexes, such the northern Yilgarn craton where zircon and Nd data have shown that it comprises different, structurally superimposed, components sufficient to conclude that distinct terranes are present (Nutman et al., 1993). In terms of understanding the construction of other gneiss complexes, finding that one of the most studied cratons can be demonstrated to comprise
many different terranes/blocks, strongly suggests
C.R.L.Friend,A.P.Nutman/Precambrian Research105 (2001) 143 – 164 162
Acknowledgements
CRLF was supported for field work and
SHRIMP analyses by NERC grant GR3/8879.
Thanks are due to Prof. W. Compston for provid-ing the SHRIMP facilities at the Australian Na-tional University, and to M.B. Fowler, V.R. McGregor and M.J. Whitehouse for comments on earlier versions of the manuscript.
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