Geochemical comparison between Archaean and Proterozoic orthogneisses from the Nagssugtoqidian orogen, West Greenland

Precambrian Research 105 (2001) 165– 181 www.elsevier.com/locate/precamres

Geochemical comparison between Archaean and Proterozoic orthogneisses from the Nagssugtoqidian orogen, West Greenland Feiko Kalsbeek a,b,* a

Geological Sur6ey of Denmark and Greenland, Thora6ej 8, DK-2400, Copenhagen NV, Denmark b Danish Lithosphere Centre, DK-1350, Copenhagen K, Denmark Received 29 January 1999; accepted 8 June 1999

Abstract In the Palaeoproterozoic Nagssugtoqidian orogen of West Greenland reworked Archaean and juvenile Proterozoic orthogneisses occur side by side and are difficult to differentiate in the field. Archaean gneisses have tonalitic to trondhjemitic compositions with relatively low Al2O3 and Sr, and may have been derived from magmas formed by melting of basaltic or amphibolitic rocks at moderate pressures. The Proterozoic rocks are on average more mafic, and it is likely that they crystallised from mantle-derived magmas. Felsic varieties of the Proterozoic igneous suite probably formed from the original magma by fractional crystallisation, in which hornblende played an important role, and at SiO2 \ 65% Archaean and Proterozoic rocks have very similar major and trace element compositions (including REE), illustrating that different modes of origin may lead to very similar results. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Archaean orthogneisses; Geochemistry; Greenland; Nagssugtoqidian orogen; Palaeoproterozoic orthogneisses

1. Introduction The investigation dealt with in this study was inspired by the work of H. Martin and co-workers on the petrogenesis of Archaean granitoid rocks (e.g. Martin, 1986, 1987, 1993). Martin’s main conclusions can be summarised as follows: most Archaean granitoid magmas are formed by melting of hot oceanic crust (hydrated tholeiitic * Tel.: +45-38142253; fax: +45-38142050. E-mail address: [email protected] (F. Kalsbeek).

basalts) during subduction, in contrast to most younger granitoid rocks which form from magmas generated in the mantle wedge above the subducted slab. This contrast in origin can be deduced from differences in geochemistry between Archaean (trondhjemitic) and younger (calc-alkaline) granitoids, and is the result of steeper geothermal gradients during the Archaean. In the central part of the Palaeoproterozoic Nagssugtoqidian orogen, West Greenland (Fig. 1), reworked Archaean and juvenile Proterozoic meta-granitoid rocks occur side by side. Because

0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 ( 0 0 ) 0 0 1 1 0 - 8

166

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

Fig. 1. Geological sketch map of the Nagssugtoqidian orogen, West Greenland (modified after van Gool et al., 1996), with outline of the area investigated in this paper. SNF = southern Nagssugtoqidian front. The Kangaˆmiut dykes (2040 Ma, Nutman et al., 1999) predate high-grade Nagssugtoqidian metamorphism and deformation in the southern Nagssugtoqidian orogen.

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

of lithological similarity and strong deformation it was not realised until the late 1980’s that they belong to two age groups; previously all orthogneisses were thought to represent reworked Archaean rocks (e.g. Hickman and Glassley, 1984). The presence of juvenile Palaeoproterozoic rocks was documented by Kalsbeek et al. (1984, 1987) with the help of isotope data, but even then it was not possible to distinguish Archaean and Palaeoproterozoic rocks in the field, and chemically the dated Archaean and Proterozoic rocks appeared to be very similar (Kalsbeek et al., 1987). Recently, parts of the Nagssugtoqidian orogen have been re-investigated by the Danish Lithosphere Centre (Marker et al., 1995; van Gool et al., 1996; Mengel et al., 1998). To get an insight into the regional distribution of Archaean and Palaeoproterozoic rocks, zircons from a large number of samples, scattered over the whole orogen, were analysed in a geochronological reconnaissance programme (Kalsbeek and Nutman, 1996), and in nearly all cases a distinction could be made between (reworked) Archaean and Palaeoproterozoic rocks. The objective of this work is in more detail to compare the geochemistry of (dated) Archaean and Palaeoproterozoic meta-igneous rocks from a part of the orogen where they occur in close spatial association, and to see to what extent the chemical distinctions described by Martin (1986) can be recognised for these rocks.

2. Nagssugtoqidian orogen and regional setting of the investigated rocks The Nagssugtoqidian orogen of West Greenland (Fig. 1; Ramberg, 1949; Escher et al., 1976; Korstga˚rd, 1979; Marker et al., 1995; van Gool et al., 1996; Mengel et al., 1998) is a  250 km-wide ENE-trending belt north of the ‘Archaean craton’ of southern Greenland, within which Archaean rocks were strongly reworked during the Palaeoproterozoic,  1850 – 1750 Ma ago. The orogen is believed to be part of a major orogenic belt, running from Canada (the Torngat orogen), over West and East Greenland, to northern Scot-

167

land and the northern part of the Baltic Shield (e.g. Bridgwater et al., 1990), but details of the correlation between these areas are still uncertain. The central part of the Nagssugtoqidian orogen (Fig. 1) is dominated by Archaean rocks in Palaeoproterozoic granulite facies. Juvenile Palaeoproterozoic rocks are also present, although in much smaller proportions than reworked Archaean rocks. Palaeoproterozoic units comprise two major meta-igneous suites of similar age,  1920 Ma: (1) the Sisimiut charnockite complex in the southwest; and (2) the Arfersiorfik association in the northeast (Fig. 1). Within the Sisimiut complex syenitic rocks with very high Ba, Sr, LREE and P have been found, suggesting participation of a strongly enriched mantle source in the petrogenesis of these rocks (Steenfelt, 1997). In those parts of the orogen where the rocks are in granulite facies, no lithological differences have been found to distinguish between Archaean and Proterozoic units, but in the easternmost central part of the orogen around the head of Nordre Strømfjord (Fig. 2), the rocks are in amphibolite facies, and original lithological differences are better preserved. In this area Archaean and Palaeoproterozoic rocks are in tectonic contact, commonly separated by thin slivers of strongly deformed marble or calc-silicate rocks that provided glide planes during tectonic imbrication, and the Proterozoic rocks are interpreted as an allochthon (Kalsbeek and Nutman, 1996; van Gool et al., 1999). Palaeoproterozoic units comprise both metasedimentary and meta-igneous rocks. Detrital zircons in two samples of metasediments are mainly of Palaeoproterozoic age ( 2200 –2000 Ma), and indicate that the original sediments were not derived from the Archaean complexes with which they are now in tectonic contact (Nutman et al., 1999). In the area east of inner Ussuit (Fig. 2) one metasedimentary unit contains numerous lenses of komatiitic metavolcanic rocks (Kalsbeek and Manatschal, 1999). The Archaean basement consists of polyphase gneiss complexes. Felsic grey orthogneisses, with biotite as the main mafic mineral, are dominant; they contain local rafts of more mafic tonalitic to dioritic hornblende-bearing rocks, with which they have locally intrusive relationships. Younger

168

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

granitic sheets are also present. A more detailed description of the Archaean basement, illustrated with colour photographs, is presented by Mengel et al. (1998). Connelly and Mengel (1996, 2000) have obtained precise U-Pb ages between 2810 and 2870 Ma on igneous zircons from samples of the regional felsic gneisses, in broad accordance with the reconnaissance data presented by Kalsbeek and Nutman (1996). Palaeoproterozoic meta-igneous lithologies comprise quartz-dioritic and tonalitic rocks, grading into more felsic varieties; locally they have been intruded into metasedimentary rocks. The igneous rocks are collectively referred to as the Arfersiorfik association (Kalsbeek and Nutman, 1996). Their age has been determined at 1920 –

1900 Ma, and they consist mainly of juvenile components (Kalsbeek et al., 1987; Whitehouse et al., 1998), although minor contamination with older crustal components is suggested by isotope data. The largest outcrop of these rocks is the Arfersiorfik quartz diorite (Fig. 2; Henderson, 1969; Kalsbeek et al., 1987), within which igneous textures and mineralogy as well as igneous layering are locally preserved. After initial imbrication Archaean and Proterozoic rocks were complexly folded; sheets of Palaeoproterozoic hornblende-biotite gneiss in eastern Nordre Strømfjord now occur as folded layers, often only a few hundreds of metres wide, alternating with more felsic Archaean granitoids (Fig. 2; van Gool et al., 1999). From this area

Fig. 2. Geological map of the eastern Nordre Strømfjord–Arfersiorfik area outlined in Fig. 1 with sample localities (modified after van Gool et al., 1999).

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

169

zircons from 22 samples (for location see Fig. 2) were investigated by SHRIMP to determine their age (Kalsbeek and Nutman, 1996); some of the samples were collected in pairs, on both sides of a thin marble layer or mylonite zone. Eleven samples represent Archaean rocks; 10 were analysed for this study (one Archaean sample contained pegmatic veins and is not considered). Age estimates for these samples (Table 1) are unprecise because of the disturbance of the U-Pb zircon systems during high-grade Proterozoic metamorphism, together with the reconnaissance nature of the age determinations. However, there is no doubt that the analysed samples represent Archaean rocks. The remaining 11 samples are Palaeoproterozoic; all of these were analysed. Ages for these samples (Table 2) are better constrained and fall between  1900 and 1950 Ma. While this approach using dated samples only limits the number of analyses in this study, making detailed statistical comparisons impossible, classification of the samples as Archaean vs. Proterozoic is unquestionable.

3. Sample description and classification Most of the Archaean samples used in this study are relatively felsic orthogneisses with little or no K-feldspar, and biotite as the only major mafic mineral; two of the investigated samples have significant proportions of K-feldspar. One of the analysed samples represents the darker, hornblende-bearing lithologies that occur locally within the Archaean basement complex. The Proterozoic samples are on average more mafic, and, with few exceptions, they contain hornblende as well as biotite; hornblende is lacking in one sample, biotite in another. Small proportions of hypersthene occur in the two westernmost samples; here the metamorphic grade of the rocks increases westward, and further west all rocks are at granulite grade. K-feldspar is present in trace amounts in several samples. Three of the Proterozoic samples are very mafic, with large proportions of hornblende and biotite; two of these samples contain hardly any quartz.

Fig. 3. Q-A-P (Streckeisen, 1976) and An-Ab-Or (O’Connor, 1965; Barker, 1979) diagrams for Archaean and Proterozoic orthogneisses, Nagssugtoqidian orogen, West Greenland. Most samples plot in the fields of tonalite, trondhjemite, and granodiorite. See text for further information.

The investigated samples are metamorphic rocks, and their classification in terms of igneous terminology is therefore inherently problematic. Fig. 3A is part of the Streckeisen (1976) diagram with approximate modal compositions of the samples. Modes were estimated with the help of chemical analyses, using katanorms with normative hypersthene + orthoclase recalculated into biotite + quartz (6 hy + 5 or = 8 bi + 3 qz). This procedure is appropriate for the Archaean samples, but for the more mafic hornblende-bearing Proterozoic rocks it is problematic. Five Archaean samples fall in the field of tonalites and trondhjemites, four in the field of granodiorites, and one plots as quartz-diorite. These compositions are typical for Archaean tonalite –trond-

170

Table 1 Chemical analyses of Archaean orthogneisses, Nagssugtoqidian orogen, West Greenlanda GGU no. Age (Ma)

413738 \2800 70.84 0.41 14.97 2.46 0.03 1.18 2.95 4.44 1.61 0.12 0.51

Total

99.53

Rb Ba Pb Sr La Ce Nd Y Th Zr Hf Nb Zn Cu Ni Sc V Cr Ga

71 210 9 264 24 52 19 9 5 94 5.3 48 7 12 3 35 14 18

70.03 0.38 15.20 2.42 0.03 1.24 2.18 4.49 2.87 0.13 0.49 99.45 95 425 8 145 21 39 17 9 7 152 7.2 41 8 10 3 28 10 21

413766 2800 70.83 0.38 14.95 2.55 0.03 1.20 2.95 4.45 1.59 0.10 0.49 99.53 70 164 7 195 13 29 11 6 3 111 3.3 4.3 47 3 10 5 34 12 19

413870 \2700 68.90 0.45 15.07 3.69 0.04 1.33 2.14 4.68 2.51 0.09 0.60 99.48 114 233 12 134 31 66 26 10 25 223 5.6 14 50 5 10 6 34 12 20

413777 2800

413879 2825

413883 2775

413887 Arch.

70.81 0.31 15.18 2.10 0.03 0.91 2.96 4.33 2.29 0.12 0.33

65.90 0.57 15.78 4.82 0.07 1.80 4.12 3.95 1.48 0.20 0.70

71.67 0.46 13.25 3.14 0.03 1.39 3.30 3.44 1.67 0.35 0.49

75.80 0.19 12.59 1.64 0.02 0.42 1.65 3.63 3.14 0.08 0.20

99.35

99.38

99.20

99.35

56 588 8 434 16 32 14 2 3 114 1.3 46 11 8 3 22 9 20

52 385 6 310 44 84 32 17 6 142 3.7 5.8 74 15 15 10 70 17 21

64 450 7 184 38 70 29 7 15 205 4.0 53 10 10 4 26 15 17

78 675 8 265 31 57 21 4 10 73 2.0 23 11 6 B1 19 6 14

413899 2800

415603 2850

68.85 0.52 15.60 3.17 0.05 1.42 2.96 4.54 1.80 0.16 0.56

57.49 0.67 16.97 6.12 0.10 4.46 7.57 3.91 1.05 0.12 0.97

99.64

99.43

120 285 7 235 25 48 20 13 9 225 5.5 9.5 59 5 11 6 39 10 22

31 104 3 253 9 26 15 16 B1 102 2.4 3.8 78 18 85 18 129 89 20

Ages from SHRIMP zircon U-Pb reconnaissance data (Kalsbeek and Nutman, 1996); precisions 9c. 50 Ma. GGU 413887 contains Archaean zircons, but its age could not be determined with any precision. Major elements analysed by XRF on glass discs (Na by AAS). FeO*: total iron reported as FeO. Volat.: loss on ignition corrected for uptake of O during oxidation of Fe. Trace elements by XRF on powder tablets. Hf by INAA. a

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 Volat.

413752 2700

Table 2 Chemical analyses of Palaeoproterozoic orthogneisses, Nagssugtoqidian Orogen, West Greenlanda GGU no. Age (Ma)

413740 1940

413749 1950

413762 1910

413772 1940

413774 1920

413778 1820

413850 1950

413869 1920

413882 1890

413884 1910

413898 1940

51.02 0.86 18.70 9.10 0.17 3.76 7.94 4.11 2.00 0.52 1.11

61.78 0.69 16.61 4.65 0.08 3.12 4.77 3.93 2.17 0.43 0.96

63.13 0.68 16.85 4.21 0.05 2.16 3.96 4.86 1.99 0.42 0.78

57.59 0.89 17.81 5.99 0.09 3.33 6.48 4.41 1.62 0.30 0.92

69.21 0.34 15.59 2.27 0.05 1.11 2.97 4.90 2.05 0.17 0.39

49.94 1.04 17.92 9.29 0.21 2.81 7.52 3.05 4.63 0.92 2.06

49.56 1.18 15.03 10.47 0.18 5.46 10.31 3.01 2.03 0.33 1.92

59.37 0.51 16.93 6.08 0.13 3.15 5.89 4.01 2.06 0.21 0.97

67.88 0.45 15.78 2.62 0.04 1.52 3.29 4.28 2.62 0.25 0.49

65.62 1.00 14.30 5.85 0.07 2.67 4.07 2.77 2.06 0.14 0.98

66.99 0.54 15.57 4.00 0.07 2.06 5.01 3.65 0.97 0.16 0.54

Total

99.28

99.18

99.08

99.42

99.06

99.39

99.48

99.31

99.21

99.52

99.55

Rb Ba Pb Sr La Ce Nd Y Th Zr Hf Nb Zn Cu Ni Sc V Cr Ga

32 2290 8 1000 32 69 36 23 B1 156 4.9 8.8 97 46 13 23 192 21 19

108 906 9 968 66 126 51 15 8 159 4.7 9.8 75 26 44 11 87 72 22

112 927 8 1120 62 109 40 9 8 118 11 63 20 21 6 80 25 21

46 1050 8 716 26 56 31 19 2 163 4.1 6.4 87 16 18 15 112 53 20

44 2100 8 890 36 62 24 6 6 104 2.5 5.6 42 4 9 3 31 14 16

133 2220 34 952 68 129 66 36 18 132

60 1180 8 382 30 62 31 26 4 118

14 117 42 13 14 152 7 20

11 118 32 44 38 287 177 18

55 1000 11 836 21 44 23 14 5 72 4.6 80 11 14 18 121 28 17

47 1720 10 888 44 80 33 8 4 125 3.6 6.0 45 26 14 5 48 26 19

66 1700 9 353 21 38 16 11 2 354 10.1 12 75 36 28 16 181 79 16

15 566 8 598 17 31 12 5 3 73 2.4 68 12 10 5 61 32 18

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K 2O P2O5 Volat.

a Ages from SHRIMP zircon U-Pb reconnaissance data (Kalsbeek and Nutman, 1996); precisions 9 c. 30 Ma. The zircons in GGU 413778 are probably of metamorphic origin. Major elements analysed by XRF on glass discs (Na by AAS). FeO*: total iron reported as FeO. Volat.: loss on ignition corrected for uptake of O during oxidation of Fe. Trace elements by XRF on powder tablets. Hf by INAA.

171

172

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

hjemite –granodiorite (TTG) suites. Most Proterozoic samples fall in the fields of diorite, quartzdiorite and tonalite/trondhjemite; two plot as granodiorites, and one anomalous mafic sample as monzodiorite. The fact that this diagram is consistent with petrographic observations lends credence to the classification of the rocks as (quartz)-diorites, tonalites/trondhjemites and granodiorites. Also in the Ab-An-Or diagram of O’Connor (1965); fields after Barker (1979) most samples plot in the fields of trondhjemite, tonalite and granodiorite (Fig. 3B). The (quartz)-dioritic samples, for which this diagram is not suitable, are not shown.

4. Geochemistry Major elements were analysed by XRF on glass discs (Na by AAS) at the Geological Survey of Denmark and Greenland, and trace elements by XRF on powder tablets at the Department of Geology, University of Copenhagen. REE and Hf were analysed for a selection of five Archaean and six Proterozoic samples by INAA (1.5 – 2 g samples) and ICP-MS (Pr, Gd, Dy, Ho, Er and Tm) at Activation Laboratories, Canada. Results are listed in Tables 1–3, and illustrated in Figs. 4 and 5; sample numbers refer to the files of the Geological Survey of Denmark and Greenland. Major elements. Palaeoproterozoic samples are on average less silicic (SiO2  50 – 70%) than their Archaean counterparts (SiO2  57 – 76%, but mostly \ 65%), and have significantly higher concentrations of MgO and FeO* (total iron as FeO) than the latter (Tables 1 and 2, Fig. 4). This is in accordance with the more mafic nature of the Proterozoic rocks observed in the field. However, silica contents overlap between 57 and 70%, especially between 65 and 70%. Harker diagrams for major elements show a regular decrease in TiO2, Al2O3, MgO (Fig. 4A), FeO* (Fig. 4B) and CaO with increasing SiO2. Archaean and Proterozoic rocks appear to follow very similar trends, and in the region of overlap in SiO2 they do not show any difference in the concentrations of these oxides. Concentrations of Na2O and K2O do not show any obvious correlation with SiO2 and are

similar in Archaean and Proterozoic samples, both groups having about twice as much Na2O as K2O (Tables 1 and 2). Concentrations of P2O5 scatter widely; Proterozoic samples have significantly higher P2O5 than the Archaean rocks, but there is a rough negative correlation with SiO2, and in the region of overlapping SiO2 there is no major difference (Fig. 4C). The results outlined above are similar to those obtained for other samples in an earlier study (Kalsbeek et al., 1987), and the variation in major element concentrations was interpreted by these authors as the result of fractional crystallisation of two similar magmas. Based on the occurrence of mafic layers in outcrops where magmatic layering is preserved, silica-poor samples of the Proterozoic Arfersiorfik association (SiO2 50– 55%) were believed to represent cumulates; this may also be the case for some of the most mafic samples in this study. Trace elements. Most trace elements occur in very similar proportions in Archaean and Proterozoic samples (e.g. Zr, Fig. 4D). However, Ba and Sr concentrations in the Proterozoic samples are much higher than in the Archaean rocks (Fig. 5E, F); this is also the case in the region of overlapping SiO2. Spidergrams for the means of Archaean and Proterozoic samples for which REE analyses are available (Fig. 4G) illustrate the close chemical similarity of the two groups of samples. For most elements (e.g. K, Nb, the LREE, Zr and Hf) the means show no difference. Minor differences in Ti, Y, and HREE are probably largely related to the more siliceous nature of the Archaean rocks, coupled with a negative correlation of these elements with SiO2. Very significant differences are only shown for Ba, Th, Sr and P; for Th this is caused by the presence of one high-Th sample (GGU 413870) among the Archaean rocks. The cause of high Ba, Sr and P in the Proterozoic rocks will be discussed in later sections. Rare Earth Elements. REE spectra for Archaean and Proterozoic samples (Table 3) are shown in Fig. 5A and B; they are very similar. LaN varies from 47 to 134 for Archaean and from 66 to169 for Proterozoic samples. There is a marked variation in the steepness of the REE

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

spectra: LaN/YbN ranging from four to 56 and nine to 49 in Archaean and Proterozoic samples, respectively, mainly as the result of variation in YbN. Concentrations in Yb are strongly corre-

173

lated with SiO2 (Fig. 5A, B). Therefore, more siliceous samples often have steeper REE spectra than more mafic samples. REE in the range between Tb and Yb show a somewhat concave

Fig. 4. A – F Harker diagrams for some major and trace elements in Archaean and Proterozoic orthogneisses, Nagssugtoqidian orogen, West Greenland. G Spidergram comparing trace element compositions of Archaean and Proterozoic samples. Primitive mantle composition after Taylor and McLennan (1985).

174

Table 3 REE analyses for Archaean and Proterozoic orthogneisses, Nagssugtoqidian orogen, West Greenlanda Archaean samples 413766 413870

413879

413899

415603

Proterozoic samples 413740 413749

413772

413774

413882

413884

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

17.4 33 3 13 2.35 0.6 1.47 0.2 1.05 0.18 0.51 0.07 0.48 0.06

49.0 95 10 40 6.89 0.7 4.23 0.5 2.19 0.31 0.70 0.07 0.59 0.09

48.4 89 9 34 4.83 1.1 3.45 0.6 2.91 0.52 1.52 0.22 1.36 0.20

32.5 67 6 23 3.55 0.8 2.59 0.4 2.17 0.39 1.10 0.16 1.20 0.18

10.4 27 3 14 3.02 1.0 2.83 0.5 2.70 0.53 1.52 0.24 1.67 0.25

31.7 70 8 42 6.95 1.9 4.88 0.7 4.02 0.76 2.21 0.32 2.49 0.36

61.9 120 12 52 7.78 1.9 4.35 0.6 2.72 0.44 1.28 0.16 1.19 0.16

25.7 57 7 33 5.78 1.7 4.07 0.7 3.36 0.61 1.80 0.26 1.94 0.29

32.0 56 5 23 3.15 1.0 1.67 0.3 1.15 0.19 0.55 0.07 0.58 0.09

45.6 82 8 35 4.48 1.3 2.30 0.3 1.46 0.24 0.68 0.08 0.63 0.09

24.1 46 4 17 2.59 1.6 1.96 0.3 1.78 0.39 1.19 0.20 1.67 0.24

LaN/YbN Eu/Eu* SiO2

24.4 0.99 70.8

56.1 0.40 68.9

24.1 0.82 65.9

18.3 0.81 68.9

4.2 1.04 57.5

8.6 1.00 51.0

35.1 1.00 61.8

9.0 1.07 57.6

37.3 1.33 69.2

48.9 1.24 67.9

9.8 2.17 65.6

a Analysed at Activation Laboratories Ltd, Canada. La, Ce, Nd, Sm, Eu, Tb, Yb and Lu by INAA; Pr, Gd, Dy, Ho, Er and Tm by ICP-MS. Eu/Eu* = EuN/ 1 (SmN × GdN)2.

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

GGU no.

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

175

YbN, low YbN) and younger calc-alkaline rocks (low LaN/YbN, high YbN; Martin, 1986) is not in evidence, three Proterozoic samples plotting in the field of Archaean rocks.

5. Discussion

5.1. Crust or mantle origin of Nagssugtoqidian orthogneisses

Fig. 5. REE spectra for Archaean and Proterozoic orthogneisses, Nagssugtoqidian orogen, West Greenland. Chondritic REE after Taylor and McLennan (1985). C is a LaN/YbN vs. YbN plot with fields for Archaean TTG and younger calc-alkaline rocks after Martin (1993). Fractionation paths A and B show the influence of removal of plagioclase and hornblende on melt compositions, see the text.

pattern for both Archaean and Proterozoic samples. Negative correlation in between SiO2, HREE and Y is common in Archaean TTG suites (e.g. Tarney et al., 1979). In a plot of LaN/YbN vs. YbN (Fig. 5C), there is no clear separation between Archaean and Proterozoic samples. The distinction seen between Archaean tonalite –trondhjemite suites (high LaN/

Fusion of mantle lithologies is unlikely to yield magmas more felsic than andesite (e.g. Wyllie, 1984; Drummond and Defant, 1990). Melting of (hydrated) basaltic rocks, on the other hand, yields tonalitic and trondhjemitic magmas with lower MgO (commonly B 2%) and SiO2 \  65% (e.g. Rapp et al., 1991; Winther and Newton, 1991; Wolf and Wyllie, 1994; Rapp and Watson, 1995; Rapp, 1997; Wyllie et al., 1997). With the exception of GGU 415603 (a quartz-dioritic rock with SiO2 57.5%, MgO 4.5%) all Archaean samples fall in this latter range, and an origin by partial melting of oceanic crust during subduction, as suggested for other Archaean TTG suites by, for example, Barker and Arth (1976), Martin (1986) and Drummond and Defant (1990), would appear plausible. On the other hand, most samples of the Proterozoic Arfersiorfik association have \2% MgO (Fig. 4), and their precursor magmas are therefore more likely to have originated by melting involving mantle lithologies. An alternative possibility might be that the precursor magmas of the Arfersiorfik association were formed by melting involving komatiitic lithologies, the presence of which is indicated by the occurrence of meta-komatiitic rocks within metasediments in the area east of the head of Ussuit (Fig. 2; Kalsbeek and Manatschal, 1999). Since many variables are involved in determining the composition of magmas (e.g. temperature, pressure and volatile composition during formation, and bulk composition of the source; see Rapp, 1997), it is not possible with confidence to conclude from which source the Arfersiorfik magmas were formed. However, more important for the present study is the conclusion that the Archaean and Proterozoic rocks in the area of study

176

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

must have developed from magmas derived from markedly different sources, as shown by their significantly different concentrations of MgO, FeO* and SiO2 (Fig. 4). High concentrations of Ba and Sr in most samples of the Arfersiorfik association support participation of mantle lithologies in their petrogenesis, for the following reasons. Syenitic rocks ( 1900 Ma) with very high Ba, Sr, LREE and P, apparently derived from a strongly enriched, probably veined, mantle reservoir, have been found within the Sisimiut charnockite complex further west in the Nagssugtoqidian orogen (Fig. 1; Steenfelt, 1997). It is plausible that the high Ba and Sr concentrations in samples of the Arfersiorfik association are related to the participation of similar enriched mantle material in their petrogenesis. The more felsic rocks of the Arfersiorfik association share the high concentrations of Ba and Sr with the more mafic samples and could have formed by fractionational crystallisation of the same parent magma. In summary, the geochemistry of the Archaean and Proterozoic orthogneisses from eastern Nordre Strømfjord can plausibly be interpreted in terms of an origin of their precursor magmas by melting of ocean floor and mantle wedge lithologies respectively, in accordance with the views of Martin (1986).

5.2. Comparison with typical TTG suites: Al2O3, Sr and Na2O With about 15% Al2O3 at 70% SiO2 (Table 1) the Archaean samples from Nordre Strømfjord are intermediate between the high-Al2O3 and lowAl2O3 tonalite –trondhjemite suites of Barker and Arth (1976) and Drummond and Defant (1990). Differences in Al2O3 concentrations have been interpreted as the result of variations in the proportions of plagioclase and hornblende in the residue during the partial melting of mafic source rocks to produce tonalitic magmas. Low-Al2O3 tonalites (plagioclase retaining Al in the residue) are expected to have low Sr (B200 ppm) and flat REE spectra with distinct negative Eu anomalies, whereas high-Al2O3 TTG (with hornblende 9 garnet in the residue) have high Sr (\ 300 ppm)

and steep REE spectra without significant Eu anomalies (Barker and Arth, 1976; Drummond and Defant, 1990). The Archaean samples from Nordre Strømfjord have 150 –400 ppm Sr (Table 1); this is much less than in most other Archaean amphibolite-facies TTG suites (cf. Tarney and Jones, 1994, Fig. 13), and in the spider diagram (Fig. 4G) Sr shows a distinct negative spike compared to La, Ce, Pr, Zr and Hf. This suggests that plagioclase may have been present in the residue during magma genesis, and that, therefore, the magmas may have formed at moderate pressures. REE, however, display relatively steep patterns (LaN /YbN 18–56 for samples with SiO2 \ 65%), and although most samples have Eu*/Eu B 1 (Table 3; Fig. 5A), only GGU 413870 has a pronounced negative Eu anomaly. One of the most characteristic geochemical features of Archaean TTG, compared to later calcalkaline suites, is their high Na/K ratios (e.g. Barker, 1979; Condie, 1981). This is commonly illustrated with the help of K-Na-Ca and (normative) Q-Ab-Or plots (Fig. 6). Both diagrams, however, have led to confusion. In the K-Na-Ca diagram both weight proportions of K, Na, and Ca (e.g. Collerson and Bridgwater, 1979, Fig. 19), atomic proportions of K, Na, and Ca (e.g. Martin, 1987, Fig. 5B; Rapp and Watson, 1995, Fig. 6), and weight proportions of K2O, Na2O, and CaO (e.g. Luais and Hawkesworth, 1994, Fig. 5; Pidgeon and Wilde, 1998, Fig. 3) have been plotted. There is no a priori reason to prefer any of these plots, but the same location of the reference curve for calc-alkaline suites is shown by all these authors, irrespective of which units are plotted, and this may lead to incorrect comparisons. The standard calc-alkaline reference curve (CA in Fig. 6A) was originally taken by Barker and Arth (1976) from Nockolds and Allen (1953), which was based on chemical analyses for rocks from the Southern California batholith (Larsen, 1948). Neither Barker and Arth (1976) nor Nockolds and Allen (1953) state which units they use in their diagrams, but replotting the data of Larsen (1948) leaves little doubt that (not surprisingly) the calc-alkaline reference curve represents weight proportions of K, Na and Ca. Fig. 6B shows reference lines for the Southern California

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

batholith plotted in terms of (1) weight proportions K, Na, and Ca, (2) weight proportions K2O, Na2O, and CaO, and (3) atomic proportions K, Na, Ca. While the curves for weight proportions of K, Na, Ca and K2O, Na2O, CaO are only marginally different, the curve for atomic proportions of K, Na, Ca is significantly displaced towards the Na corner of the diagram. In Fig. 6A weight proportions for K, Na and Ca for the Nordre Strømfjord samples are shown together with the fields for the Southern California batholith (plotted from chemical data of Larsen, 1948), the classic Finnish Archaean TTG suite studied by Martin (1987) (replotted), and the Proterozoic tonalites and trondhjemites (SiO2 \ 65%) from the well-known gabbro – diorite – tonalite –trondhjemite suite of southwest Finland (Arth et al., 1978). All Archaean as well as the

177

more siliceous Proterozoic samples from Nordre Strømfjord fall to the left of the calc-alkaline reference curve (CA in Fig. 6A), but when the chemical variation of the rocks from the Southern California batholith is represented by a field instead of a reference curve, there is considerable overlap. The relative proportions of K, Na and Ca in the Archaean samples are very similar to those of the Finnish TTG studied by Martin (1987); the latter also overlap with the K-Na-Ca field for the Southern California batholith. The Proterozoic tonalites and trondhjemites (SiO2 \ 65%) studied by Arth et al. (1978) fall closer to the Na corner in the Na-K-Ca diagram (Fig. 6A); more mafic members (SiO2 B 65%) of their gabbro –diorite –tonalite –trondhjemite suite, however, plot to the right of the lower part of the calc-alkaline reference line, along the line marked

Fig. 6. A Na-K-Ca and C normative Q-Ab-Or diagrams comparing Archaean and Proterozoic orthogneisses from the Nagssugtoqidian orogen, West Greenland, with fields of (1) calc-alkaline rocks from the Southern California batholith (Larsen, 1948); one outlier shown with open circle, (2) Archaean TTG from Finland (Martin, 1987), and (3) Proterozoic tonalites and trondhjemites from Finland (Arth et al., 1978). CA and Tr are reference lines for calc-alkaline and trondhjemitic igneous suites, respectively (Barker and Arth, 1976). B illustrates the different positions of the reference line for the Southern California batholith, dependent upon the units used to express the proportions of Na, K and Ca. For further explanation see the text.

178

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

Tr, and there is a large gap between these and the felsic samples, suggesting their mafic and felsic rocks might not be directly related. The normative Qz-Ab-Or diagram (CIWP norms) is also commonly used to illustrate the difference between trondhjemitic and calc-alkaline igneous suites (e.g. Collerson and Bridgwater, 1979; Martin, 1987). This diagram was employed by Barker and Arth (1976, Fig. 3b), and a ‘common calc-alkaline trend’ is shown (and reproduced by later authors) for comparison. The source of the reference line is not reported. However, the rocks from the Southern California batholith, which are used as reference in the NaK-Ca diagram, do not follow this trend, but plot at lower Or. In Fig. 6C the normative proportions of Qz, Ab and Or of the Nordre Strømfjord samples are shown together with the field for the Southern California batholith (data from Larsen, 1948), with which they overlap. Also most samples of the Finnish Archaean TTG suite (Martin, 1987), as well as the Paleoproterozoic gabbro – diorite –tonalite –trondhjemite suite of southwest Finland (Barker and Arth, 1976; Arth et al., 1978) fall within the field of the Southern California batholith. In summary, the Archaean rocks (as well as the more siliceous Proterozoic samples) from the Nagssugtoqidian orogen are similar to typical Archaean TTG suites, but somewhat less aluminous; they are relatively Na-rich in comparison to young calc-alkaline rocks (exemplified by the Southern California batholith), but relative enrichment in Na of common TTG lithologies is not nearly as strong as suggested in the literature (e.g. Martin, 1987, 1993).

5.3. The role of magma fractionation The most intriguing result of this study is that, although the Archaean and Proterozoic rocks investigated in this study were apparently derived from different sources, at SiO2 \ 65% they are (apart from large differences in Ba and Sr) almost identical in chemical composition (Figs. 4 – 6; Tables 1 and 2). Even the REE spectra, which often clearly discriminate between ‘slab-derived’ and ‘mantle-derived’ granitoid rocks (Martin, 1986,

1993), are very similar, with low YbN and high (but not extreme) LaN/YbN (Fig. 5C). As discussed above, relatively low Al2O3 and Sr contents in the Archaean samples suggest magma formation at moderate pressures, with some plagioclase remaining in the residue. This is in contrast with high-Al tonalite –trondhjemite suites, for which magma generation took place at higher pressures, with hornblende 9 garnet present in the residue. The relatively steep REE spectra observed for some of the Archaean samples (Fig. 5) would then not be the result of significant proportions of residual garnet during magma generation as has been suggested for high-Al TTG (e.g. Martin, 1987), but, more likely, of the involvement of hornblende in the source and/or during fractionation of the parent magma from which the Archaean rocks were formed. The distinct negative correlation of Yb (Fig. 5) with SiO2 would seem to support the importance of hornblende fractionation, because partition coefficients between hornblende and Yb (and Y) strongly increase with increasing SiO2 (Arth and Barker, 1976), and SiO2 would have been much higher in the tonalitic magma than in the amphibolitic (?) source. Because hornblende has the highest partition coefficients for the middle REE, the slightly upward-concave REE patterns (Fig. 5) are consistent with hornblende fractionation. Since the high-SiO2 samples of the Proterozoic Arfersiorfik association have the same high Ba and Sr concentrations as the more mafic rocks, it is likely that they were formed from the same parent magma. An origin of the more felsic rocks by fractional crystallisation of a mafic, probably mantle-derived magma is therefore plausible, and once again, the low concentrations of Yb and relatively high LaN/YbN ratios for the more siliceous samples may be plausibly explained by fractional crystallisation of hornblende. The effects of upto 40% Rayleigh fractionation of (A) 70% plagioclase, 30% hornblende, and (B) 50% plagioclase, 50% hornblende, from a liquid with Yb and LaN/YbN as in GGU 413884 (SiO2 65.6%, YbN = 7, LaN/YbN = 10), using the partition coefficients listed in Martin (1987), are shown in Fig. 5C, and illustrate this point.

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

In summary, while for most high-Al2O3 TTG suites a strong case can be made for the presence of residual garnet at the site of magma generation from mafic sources (e.g. Martin, 1987), it is plausible that in the present case hornblende fractionation has played an important role to cause the relatively steep REE spectra of the investigated samples. If this is indeed the case, it is probable that the most mafic samples represent cumulitic rocks, such as observed in the field where magmatic layering is preserved, and in accordance with the wide scatter in Al2O3 and MgO at low SiO2 (Fig. 4).

5.4. Chemical similarity of Archaean and Proterozoic gneisses in the Nagssugtoqidian orogen Both the Archaean and Proterozoic meta-igneous suites in the Nagssugtoqidian orogen comprise mafic and felsic varieties, but in the Archaean complex felsic rocks are dominant, while more mafic varieties are most common in the Proterozoic Arfersiorfik association. Despite apparent differences in origin, Archaean and felsic Proterozoic orthogneisses (SiO2 \ 65%) have very similar chemical signatures. Similar convergence between mantle-derived and slab-derived magmatic rocks has been described from other areas and settings. In the Fiskefjord region of West Greenland,  250 km south of the Nagssugtoqidian orogen, Archaean meta-diorites are more common than in the eastern Nordre Strømfjord area. They have been interpreted by Garde (1997) as the result of subduction-related magma formation with major participation of mantle lithologies. The same may be true for the mafic Archaean sample 415603 (Table 1) analysed in the present investigation. In the Fiskefjord area tonalitic and trondhjemitic orthogneisses are also common, and these were probably formed by partial melting of an amphibolitic source (Garde, 1997). Also here major and trace elements concentrations for mantle- and slab-derived rocks show considerable overlap in Harker diagrams. Another case of chemical convergence of mantle- and slab-derived volcanic rocks has been

179

reported from Panama by Defant et al. (1991). ‘Old’ (13 –7.5 Ma) mantle-derived and ‘young’ (B2.5 Ma) slab-derived lavas overlap for most elements at SiO2 \ 65%, although the slabderived volcanics have slightly higher Na2O and Al2O3. Apparently, mafic magmas are mainly formed by melting of mantle lithologies, whereas siliceous magmas can be generated by fractionation of a mafic magma as well as by melting of (hydrated) basaltic or amphibolitic sources. The resulting rocks may be almost indistinguishable lithologically and geochemically. In the Nagssugtoqidian orogen, where most rocks are strongly deformed (and in large parts of the region in granulite facies), this has the unfortunate consequence that it is hard to differentiate Archaean and Proterozoic rocks in the field, which has made mapping difficult, and complicates attempts to obtain a complete insight into the details of the origin and evolution of the orogen.

Acknowledgements Investigations by the Danish Lithosphere Centre (DLC) in the Nagssugtoqidian orogen were funded by the Danish National Research Foundation. Analytical support from the geochemical laboratories at the Geological Survey of Denmark and Greenland (GEUS) and at the Geological Institute, Copenhagen University, is gratefully acknowledged. The Geochemical Laboratory at the Geological Institute is supported by the Danish Natural Science Research Council. The manuscript was critically reviewed by Adam Garde (GEUS) and Jeroen van Gool (DLC), and publication was authorised by the Geological Survey of Denmark and Greenland.

References Arth, J.G., Barker, F., 1976. Rare-earth partitioning between hornblende and dacitic liquid and implications for the genesis of trondhjemitic– tonalitic magmas. Geology 4, 534– 536. Arth, J.G., Barker, F., Peterman, Z.E., Friedman, I., 1978. Geochemistry of the gabbro– diorite– tonalite– trond-

180

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

hjemite suite of southwest Finland and its implications for the origin of tonalitic and trondhjemitic magmas. J. Petrol. 19, 289– 316. Barker, F., 1979. Trondhjemite: definition, environment and hypotheses of origin. In: Barker, F. (Ed.), Trondhjemites, Dacites, and Related Rocks. Elsevier, Amsterdam, pp. 1–12. Barker, F., Arth, J.G., 1976. Generation of trondhjemitic– tonalitic liquids and Archaean bimodal trondhjemite– basalt suites. Geology 4, 596–600. Bridgwater, D., Austrheim, H., Hansen, B.T., Mengel, F., Pedersen, S., Winter, J., 1990. The Proterozoic Nagssugtoqidian mobile belt of southeast Greenland: a link between the eastern Canadian and Baltic shields. Geosci. Canada 17, 305– 310. Collerson, K.D., Bridgwater, D., 1979. Metamorphic development of Early Archaean tonalitic and trondhjemitic gneisses: Saglek area, Labrador. In: Barker, F. (Ed.), Trondhjemites, Dacites, and Related Rocks. Elsevier, Amsterdam, pp. 205– 273. Condie, C.K., 1981. Archaean Greenstone Belts. Elsevier, Amsterdam, 434 pp. Connelly, J.N, Mengel, F.C, 1996. The Archean backdrop to Palaeoproterozoic tectonism in the Nagssugtoqidian and Torngat orogens: new constraints from U-Pb geochronology. In: Lithoprobe Report 57. University of British Columbia, pp. 15 – 24. Connelly, J.N., Mengel, F.C., 2000. Evolution of Archaean components of the Nagssugtoqidian orogen, West Greenland. Bull. Geol. Soc. America 112, 747–763. Defant, M.J., Richerson, P.M., De Boer, J.Z., Stewart, R.H., Maury, R.C., Bellon, H., Drummond, M.S., Feigenson, M.D., Jackson, T.E., 1991. Dacite genesis via both slab melting and differentiation: petrogenesis of La Yeguada volcanic complex, Panama. J. Petrol. 32, 1101–1142. Drummond, M.S., Defant, M.J., 1990. A model for trondhjemite– tonalite– dacite genesis and crustal growth via slab melting: Archaean to modern comparisons. J. Geophys. Res. 95 (B13), 21503–21521. Escher, A., Sørensen, K., Zeck, H.P., 1976. Nagssugtoqidian mobile belt in West Greenland. In: Escher, A., Watt, W.S. (Eds.), Geology of Greenland. Geological Survey of Greenland, Copenhagen, pp. 77–95. Garde, A.A., 1997. Accretion and evolution of an Archaean high-grade gneiss– amphibolite complex: the Fiskefjord area, southern West Greenland. Geol. Greenland Surv. Bull. 177, 115 pp. Henderson, G., 1969. The Precambrian rocks of the Egedesminde– Christiansha˚b area, West Greenland. Rapp. Grønlands Geol. Unders. 23, 37 pp. Hickman, M.H., Glassley, W.E., 1984. The role of metamorphic fluid transport in the Rb-Sr resetting of shear zones: evidence from Nordre Strømfjord, West Greenland. Contrib. Mineral. Petrol. 87, 265–281. Kalsbeek, F., Manatschal, G., 1999. Geochemistry and tectonic significance of peridotitic and meta-komatiitic rocks from the Ussuit area, Nagssugtoqidian orogen, West Greenland. Precambrian Res. 94, 101–120.

Kalsbeek, F., Nutman, A.P., 1996. Anatomy of the Early Proterozoic Nagssugtoqidian orogen, West Greenland, explored by reconnaissance SHRIMP U-Pb dating. Geology 24, 515– 518. Kalsbeek, F., Taylor, P.N., Henriksen, N., 1984. Age of rocks, structures, and metamorphism in the Nagssugtoqidian mobile belt, West Greenland – field and Pb-isotopic evidence. Can. J. Earth Sci. 21, 1126– 1131. Kalsbeek, F., Pidgeon, R.T., Taylor, P.N., 1987. Nagssugtoqidian mobile belt of West Greenland: a cryptic 1850 Ma suture between two Archaean continents – chemical and isotopic evidence. Earth Planet. Sci. Lett. 85, 365– 385. Korstga˚rd, J.A. (Ed.), 1979. Nagssugtoqidian geology. Rapp. Grønlands Geol. Unders. 89, 146 pp. Larsen, E.S., 1948. Batholith and associated rocks of Corona, Elsinore, and San Louis Rey quadrangles, southern California. Geol. Soc. Amer. Memoir 29, 182 pp. Luais, B., Hawkesworth, C.J., 1994. The generation of continental crust: an integrated study of crust-forming processes in the Archaean of Zimbabwe. J. Petrol. 35, 43 – 93. Marker, M., Mengel, F., van Gool, J., et al., 1995. Evolution of the Palaeoproterozoic Nagssugtoqidian orogen: DLC investigations in West Greenland. Rapp. Grønlands Geol. Unders. 165, 100– 105. Martin, H., 1986. Effect of steeper Archaean geothermal gradient on geochemistry of subduction-zone magmas. Geology 14, 753– 756. Martin, H., 1987. Petrogenesis of Archaean trondhjemites, tonalites, and granodiorites from eastern Finland: major and trace element geochemistry. J. Petrol. 28, 921– 953. Martin, H., 1993. The mechanisms of petrogenesis of the Archaean continental crust – comparison with modern processes. Lithos 30, 373– 388. Mengel, F., van Gool, J.A.M., Krogstad, E., et al., 1998. Archaean and Palaeoproterozoic orogenic processes: Danish Lithosphere Centre studies of the Nagssugtoqidian orogen, West Greenland. Geol. Greenland Surv. Bull. 180, 100– 110. Nockolds, S.R., Allen, R., 1953. The geochemistry of some igneous rock series. Geochim. Cosmochim. Acta 4, 105– 142. Nutman, A.P., Kalsbeek, F., Marker, M., van Gool, J.A.M., Bridgwater, D., 1999. U-Pb zircon ages of Kangaˆmiut dykes and detrital zircons in metasediments in the Palaeoproterozoic Nagssugtoqidian Orogen (West Greenland): clues to the pre-collisional history of the orogen. Precambrian Res. 93, 87 – 104. O’Connor, J.T., 1965. A classification for quartz-rich igneous rocks based on feldspar ratios. U.S. Geol. Surv. Prof. Pap. 525B, 79 – 84. Pidgeon, R.T., Wilde, S.A., 1998. The interpretation of complex zircon U-Pb systems in Archaean granitoids and gneisses from the Jack Hills, Narryer gneiss terrane, Western Australia. Precambrian Res. 91, 309– 332. Ramberg, H., 1949. On the petrogenesis of the gneiss complexes between Sukkertoppen and Christianshaab, West Greenland. Meddr Dansk Geol. Foren. 11, 312– 327.

F. Kalsbeek / Precambrian Research 105 (2001) 165–181

181

van Gool, J., Marker, M., Mengel, F., et al., 1996. The Palaeoproterozoic Nagssugtoqidian orogen in West Greenland: current status of work by the Danish Lithosphere Centre. Bull. Grønlands Geol. Unders. 172, 88 – 94. van Gool, J.A.M., Kriegsman, L.M., Marker, M., Nichols, G.T., 1999. Thrust stacking in the inner Nordre Strømfjord area, West Greenland: significance for the tectonic evolution of the Palaeoproterozoic Nagssugtoqidian orogen. Precambrian Res. 93, 71 – 85. Whitehouse, M.J., Kalsbeek, F., Nutman, A.P., 1998. Crustal growth and crustal recycling in the Nagssugtoqidian orogen of West Greenland: constraints from radiogenic isotope systematics and U-Pb geochronology. Precambrian Res. 91, 365– 381. Winther, T.K., Newton, R.C., 1991. Experimental melting of hydrous low-K tholeiite: evidence on the origin of Archaean cratons. Bull. Geol. Soc. Denmark 39, 213– 228. Wolf, M.B., Wyllie, P.J., 1994. Dehydration melting of amphibolite at 10 kb: effects of temperature and time. Contrib. Mineral. Petrol. 115, 369– 383. Wyllie, P.J., 1984. Constraints imposed by experimental petrology on possible and impossible mantle sources and products. Phil. Trans. R. Soc. Lond. A 310, 439– 456. Wyllie, P.J., Wolf, M.B., van der Laan, S.R., 1997. Conditions for formation of tonalites and trondhjemites: magmatic sources and products. In: De Wit, M., Ashwal, L.D. (Eds.), Greenstone Belts. In: Oxford Monographs on Geology and Geophysics, vol. 35, pp. 256– 266.

Rapp, R.P., 1997. Heterogeneous source regions for Archaean granitoids: experimental and geochemical evidence. In: De Wit, M., Ashwal, L.D. (Eds.), Greenstone Belts. In: Oxford Monographs on Geology and Geophysics, vol. 35, pp. 267– 279. Rapp, R.P., Watson, E.B., 1995. Dehydration melting of metabasalt at 8 – 32 kbar: implications for continental growth and crust-mantle recycling. J. Petrol. 36, 891–931. Rapp, R.P., Watson, E.B., Miller, C.F., 1991. Partial melting of amphibolite/eclogite and the origin of Archaean trondhjemites and tonalites. Precambrian Res. 51, 1–25. Steenfelt, A., 1997. High Ba-Sr-LREE-P components in Early Proterozoic magmas within the Nagssugtoqidian orogen, West Greenland. Terra Nova 9 (Abs. Suppl. 1), 360. Streckeisen, A., 1976. To each plutonic rock its proper name. Earth Sci. Rev. 12, 1–33. Tarney, J., Weaver, B., Drury, S.A., 1979. Geochemistry of Archaean trondhjemitic and tonalitic gneisses from Scotland and East Greenland. In: Barker, F. (Ed.), Trondhjemites, Dacites, and Related Rocks. Elsevier, Amsterdam, pp. 275– 299. Tarney, J., Jones, C.E., 1994. Trace element geochemistry of orogenic igneous rocks and crustal growth models. J. Geol. Soc. London 151, 855–868. Taylor, S.R., McLennan, S.M., 1985. The continental crust: its composition and evolution. Blackwell, Oxford, 312 pp.

.