Precambrian Research 105 (2001) 165 – 181
Geochemical comparison between Archaean and Proterozoic
orthogneisses from the Nagssugtoqidian orogen, West
Greenland
Feiko Kalsbeek
a,b,*
aGeological Sur
6ey of Denmark and Greenland,Thora6ej 8,DK-2400,Copenhagen NV,Denmark
bDanish 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 Al2O3and 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
www.elsevier.com/locate/precamres
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 melt-ing of hot oceanic crust (hydrated tholeiitic
basalts) during subduction, in contrast to most younger granitoid rocks which form from mag-mas 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-alka-line) 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 * Tel.: +45-38142253; fax:+45-38142050.
E-mail address:fk@geus.dk (F. Kalsbeek).
F.Kalsbeek/Precambrian Research105 (2001) 165 – 181 166
F.Kalsbeek/Precambrian Research105 (2001) 165 – 181 167
of lithological similarity and strong deformation it was not realised until the late 1980’s that they belong to two age groups; previously all or-thogneisses 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 chemi-cally 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 Litho-sphere 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 oro-gen, were analysed in a geochronological recon-naissance 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 geochem-istry 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 Green-land (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-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, al-though in much smaller proportions than re-worked 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 bet-ter 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 pro-vided 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 com-prise both metasedimentary and meta-igneous rocks. Detrital zircons in two samples of metased-iments 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 metavol-canic rocks (Kalsbeek and Manatschal, 1999).
F.Kalsbeek/Precambrian Research105 (2001) 165 – 181 168
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 Kals-beek and Nutman (1996).
Palaeoproterozoic meta-igneous lithologies comprise quartz-dioritic and tonalitic rocks, grad-ing 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 layer-ing are locally preserved.
After initial imbrication Archaean and Protero-zoic 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
F.Kalsbeek/Precambrian Research105 (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 sam-ples represent Archaean rocks; 10 were analysed for this study (one Archaean sample contained pegmatic veins and is not considered). Age esti-mates for these samples (Table 1) are unprecise because of the disturbance of the U-Pb zircon systems during high-grade Proterozoic metamor-phism, together with the reconnaissance nature of the age determinations. However, there is no doubt that the analysed samples represent Ar-chaean rocks. The remaining 11 samples are Palaeoproterozoic; all of these were analysed. Ages for these samples (Table 2) are better con-strained and fall between 1900 and 1950 Ma.
While this approach using dated samples only limits the number of analyses in this study, mak-ing 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, horn-blende-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 sam-ples; 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 Protero-zoic samples are very mafic, with large propor-tions 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.
trond-F
Chemical analyses of Archaean orthogneisses, Nagssugtoqidian orogen, West Greenlanda
413870 413777 413879 413883 413887 413899 415603
413752
GGU no. 413738 413766
\2700 2800 2825 2775 Arch. 2800 2850
2700
Age (Ma) \2800 2800
65.90 71.67 75.80 68.85
68.90 70.81 57.49
70.83 70.03
70.84 SiO2
0.45 0.31 0.57 0.46 0.19 0.52 0.67
TiO2 0.41 0.38 0.38
15.78 13.25 12.59 15.60 16.97
15.18 15.07
Al2O3 14.97 15.20 14.95
4.82 3.14 1.64 3.17 6.12
FeO* 2.46 2.42 2.55 3.69 2.10
0.07 0.03 0.02 0.05 0.10
0.03
MnO 0.03 0.03 0.03 0.04
1.80 1.39 0.42
MgO 1.18 1.24 1.20 1.33 0.91 1.42 4.46
4.12 3.30 1.65 2.96 7.57
2.96 2.95
CaO 2.18 2.95 2.14
4.33
4.44 4.49 4.45 4.68 3.95 3.44 3.63 4.54 3.91
Na2O
2.29
1.61 2.87 1.59 2.51 1.48 1.67 3.14 1.80 1.05
K2O
0.20 0.35 0.08 0.16 0.12
0.12
P2O5 0.12 0.13 0.10 0.09
Volat. 0.51 0.49 0.49 0.60 0.33 0.70 0.49 0.20 0.56 0.97
99.38 99.20 99.35 99.64 99.43
99.48
385 450 675 285 104
588
Ba 210 425 164 233
9 8 7 8 6 7 8 7 3
Pb 12
310 184 265 235 253
434
aAges from SHRIMP zircon U-Pb reconnaissance data (Kalsbeek and Nutman, 1996); precisions9c. 50 Ma. GGU 413887 contains Archaean zircons, but its age
F
Chemical analyses of Palaeoproterozoic orthogneisses, Nagssugtoqidian Orogen, West Greenlanda
413772 413774 413778 413850 413869 413882 413884 413898 413749
GGU no. 413740 413762
1940 1920 1820 1950 1920 1890 1910 1940
1950
Age (Ma) 1940 1910
49.94 49.56 59.37 67.88 65.62
57.59 69.21 66.99
63.13 61.78
51.02 SiO2
0.89 0.34 1.04 1.18 0.51 0.45 1.00 0.54
TiO2 0.86 0.69 0.68
17.92 15.03 16.93 15.78 14.30
15.59 15.57
17.81 Al2O3 18.70 16.61 16.85
9.29 10.47 6.08 2.62 5.85
FeO* 9.10 4.65 4.21 5.99 2.27 4.00
0.21 0.18 0.13 0.04 0.07
0.05 0.07
MnO 0.17 0.08 0.05 0.09
2.81 5.46 3.15 1.52
MgO 3.76 3.12 2.16 3.33 1.11 2.67 2.06
7.52 10.31 5.89 3.29 4.07
2.97 5.01
7.94
CaO 4.77 3.96 6.48
4.90
4.11 3.93 4.86 4.41 3.05 3.01 4.01 4.28 2.77 3.65
Na2O
2.05
2.00 2.17 1.99 1.62 4.63 2.03 2.06 2.62 2.06 0.97
K2O
0.92 0.33 0.21 0.25 0.14
0.17 0.16
P2O5 0.52 0.43 0.42 0.30
Volat. 1.11 0.96 0.78 0.92 0.39 2.06 1.92 0.97 0.49 0.98 0.54
99.39 99.48 99.31 99.21 99.52
99.42 99.55
2220 1180 1000 1720 1700
2100 566
Ba 2290 906 927 1050
8 9 8 8 34 8 11 10 9 8
Pb 8
952 382 836 888 353
890 598
aAges from SHRIMP zircon U-Pb reconnaissance data (Kalsbeek and Nutman, 1996); precisions 9c. 30 Ma. The zircons in GGU 413778 are probably of
F.Kalsbeek/Precambrian Research105 (2001) 165 – 181 172
hjemite – granodiorite (TTG) suites. Most Protero-zoic samples fall in the fields of diorite, quartz-diorite 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 gra-nodiorites. 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 sam-ples) 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 Geolog-ical Survey of Denmark and Greenland.
Major elements. Palaeoproterozoic samples are
on average less silicic (SiO250 – 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%, espe-cially 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 ox-ides. 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 signifi-cantly 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 layer-ing 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 sam-ples 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 ele-ments 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
Ar-chaean and Proterozoic samples (Table 3) are shown in Fig. 5A and B; they are very similar. LaNvaries from 47 to 134 for Archaean and from
F.Kalsbeek/Precambrian Research105 (2001) 165 – 181 173
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-lated with SiO2 (Fig. 5A, B). Therefore, more
siliceous samples often have steeper REE spectra than more mafic samples. REE in the range be-tween Tb and Yb show a somewhat concave
F
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Table 3
REE analyses for Archaean and Proterozoic orthogneisses, Nagssugtoqidian orogen, West Greenlanda
Archaean samples Proterozoic samples
413899 415603 413740 413749 413772 413774 413882 413884 413766
GGU no. 413870 413879
31.7
La 17.4 49.0 48.4 32.5 10.4 61.9 25.7 32.0 45.6 24.1
70 120 57 56 82
27 46
33
Ce 95 89 67
3
3 10 9 6 8 12 7 5 8 4
Pr
23 14 42 52 33 23 35 17
Nd 13 40 34
6.95 7.78 5.78 3.15 4.48
3.02 2.59
4.83 3.55
Sm 2.35 6.89
1.0
0.6 0.7 1.1 0.8 1.9 1.9 1.7 1.0 1.3 1.6
Eu
2.83
1.47 4.23 3.45 2.59 4.88 4.35 4.07 1.67 2.30 1.96
Gd
0.7 0.6 0.7 0.3 0.3
0.5 0.3
Tb 0.2 0.5 0.6 0.4
2.17 2.70 4.02 2.72 3.36 1.15 1.46 1.78
Dy 1.05 2.19 2.91
0.76 0.44 0.61 0.19 0.24
0.53 0.39
0.18
Ho 0.31 0.52 0.39
1.52
0.51 0.70 1.52 1.10 2.21 1.28 1.80 0.55 0.68 1.19
Er
0.32 0.16 0.26 0.07 0.08
Tm 0.07 0.07 0.22 0.16 0.24 0.20
2.49 1.19 1.94 0.58 0.63
1.67 1.67
0.48
Yb 0.59 1.36 1.20
0.25
0.06 0.09 0.20 0.18 0.36 0.16 0.29 0.09 0.09 0.24
Lu
8.6 35.1 9.0 37.3 48.9
4.2 9.8
24.1 18.3 LaN/YbN 24.4 56.1
1.04
0.99 0.40 0.82 0.81 1.00 1.00 1.07 1.33 1.24 2.17
Eu/Eu*
68.9 57.5 51.0 61.8 57.6 69.2 67.9 65.6
SiO2 70.8 68.9 65.9
aAnalysed 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*=Eu N/
(SmN×GdN) 1 2
F.Kalsbeek/Precambrian Research105 (2001) 165 – 181 175
Fig. 5. REE spectra for Archaean and Proterozoic or-thogneisses, Nagssugtoqidian orogen, West Greenland. Chon-dritic 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.
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
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 B2%) 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
sam-ples fall in this latter range, and an origin by partial melting of oceanic crust during subduc-tion, 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 sam-ples of the Proterozoic Arfersiorfik association have \2% MgO (Fig. 4), and their precursor magmas are therefore more likely to have origi-nated by melting involving mantle lithologies. An alternative possibility might be that the precursor magmas of the Arfersiorfik association were formed by melting involving komatiitic litholo-gies, 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 forma-tion, and bulk composition of the source; see Rapp, 1997), it is not possible with confidence to conclude from which source the Arfersiorfik mag-mas were formed. However, more important for the present study is the conclusion that the Ar-chaean and Proterozoic rocks in the area of study pattern for both Archaean and Proterozoic
sam-ples. 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/YbNvs. YbN(Fig. 5C), there is
F.Kalsbeek/Precambrian Research105 (2001) 165 – 181 176
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 petro-genesis, 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 Arfer-siorfik association are related to the participation of similar enriched mantle material in their petro-genesis. 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 Nor-dre Strømfjord can plausibly be interpreted in terms of an origin of their precursor magmas by melting of ocean floor and mantle wedge litholo-gies 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-Al2O3and
low-Al2O3 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 pro-portions 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 hornblende9
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 com-pared 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/YbN18 – 56 for samples with SiO2 \65%),
and although most samples have Eu*/EuB 1 (Table 3; Fig. 5A), only GGU 413870 has a pronounced negative Eu anomaly.
One of the most characteristic geochemical fea-tures of Archaean TTG, compared to later calc-alkaline 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 (norma-tive) Q-Ab-Or plots (Fig. 6). Both diagrams, how-ever, 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. Mar-tin, 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 plot-ted. 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.
F.Kalsbeek/Precambrian Research105 (2001) 165 – 181 177
batholith plotted in terms of (1) weight propor-tions K, Na, and Ca, (2) weight proporpropor-tions 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 propor-tions of K, Na, Ca is significantly displaced to-wards 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 Califor-nia 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
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 in-stead 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 (SiO2B65%) of their
gab-bro – diorite – tonalite – trondhjemite suite, how-ever, plot to the right of the lower part of the calc-alkaline reference line, along the line marked
F.Kalsbeek/Precambrian Research105 (2001) 165 – 181 178
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 ‘com-mon calc-alkaline trend’ is shown (and repro-duced by later authors) for comparison. The source of the reference line is not reported. How-ever, the rocks from the Southern California batholith, which are used as reference in the Na-K-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 sam-ples 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 Ar-chaean 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 en-richment 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 in-vestigated 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; Ta-bles 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 Al2O3and Sr
contents in the Archaean samples suggest magma formation at moderate pressures, with some pla-gioclase remaining in the residue. This is in con-trast with high-Al tonalite – trondhjemite suites, for which magma generation took place at higher pressures, with hornblende9garnet present in the residue. The relatively steep REE spectra ob-served for some of the Archaean samples (Fig. 5) would then not be the result of significant propor-tions of residual garnet during magma generation as has been suggested for high-Al TTG (e.g. Mar-tin, 1987), but, more likely, of the involvement of hornblende in the source and/or during fractiona-tion of the parent magma from which the Ar-chaean rocks were formed. The distinct negative correlation of Yb (Fig. 5) with SiO2 would seem
to support the importance of hornblende fraction-ation, because partition coefficients between horn-blende 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. Be-cause hornblende has the highest partition coeffi-cients for the middle REE, the slightly upward-concave REE patterns (Fig. 5) are consis-tent 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 ef-fects of upto 40% Rayleigh fractionation of (A) 70% plagioclase, 30% hornblende, and (B) 50% plagioclase, 50% hornblende, from a liquid with Yb and LaN/YbNas in GGU 413884 (SiO265.6%,
YbN=7, LaN/YbN=10), using the partition
F.Kalsbeek/Precambrian Research105 (2001) 165 – 181 179
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 plau-sible that in the present case hornblende fraction-ation 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 mag-matic 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-ig-neous suites in the Nagssugtoqidian orogen com-prise 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 de-scribed 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 east-ern Nordre Strømfjord area. They have been in-terpreted 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 amphi-bolitic 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
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
slab-derived 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 lithologi-cally and geochemilithologi-cally. 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 Protero-zoic 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 Cen-tre (DLC) in the Nagssugtoqidian orogen were funded by the Danish National Research Founda-tion. Analytical support from the geochemical laboratories at the Geological Survey of Denmark and Greenland (GEUS) and at the Geological Institute, Copenhagen University, is gratefully ac-knowledged. 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 Sur-vey of Denmark and Greenland.
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