hjemite – granodiorite TTG suites. Most Protero- zoic samples fall in the fields of diorite, quartz-
diorite and tonalitetrondhjemite; 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, tonalitestrondhjemites 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 SiO
2
50 – 70 than their Archaean counterparts SiO
2
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 TiO
2
, Al
2
O
3
, MgO Fig. 4A, FeO Fig. 4B and CaO with increasing SiO
2
. Archaean and Proterozoic rocks appear to follow very similar trends, and in
the region of overlap in SiO
2
they do not show any difference in the concentrations of these ox-
ides. Concentrations of Na
2
O and K
2
O do not show any obvious correlation with SiO
2
and are similar in Archaean and Proterozoic samples,
both groups having about twice as much Na
2
O as K
2
O Tables 1 and 2. Concentrations of P
2
O
5
scatter widely; Proterozoic samples have signifi- cantly higher P
2
O
5
than the Archaean rocks, but there is a rough negative correlation with SiO
2
, and in the region of overlapping SiO
2
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 SiO
2
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 SiO
2
. 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 SiO
2
. 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. La
N
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
spectra: La
N
Yb
N
ranging from four to 56 and nine to 49 in Archaean and Proterozoic samples,
respectively, mainly as the result of variation in Yb
N
. Concentrations in Yb are strongly corre- lated with SiO
2
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
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.
F .
Kalsbeek Precambrian
Research
105 2001
165 –
181
Table 3 REE analyses for Archaean and Proterozoic orthogneisses, Nagssugtoqidian orogen, West Greenland
a
Archaean samples Proterozoic samples
413899 415603
413740 413749
413772 413774
413882 413884
413766 GGU no.
413870 413879
31.7 La
61.9 17.4
25.7 32.0
45.6 24.1
49.0 48.4
32.5 10.4
70 120
57 56
82 27
46 33
Ce 67
89 95
3 3
8 12
7 5
8 4
10 9
6 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
1.9 1.9
1.7 1.0
1.3 1.6
0.7 1.1
0.8 Eu
2.83 1.47
4.88 4.35
4.07 1.67
2.30 1.96
4.23 3.45
2.59 Gd
0.7 0.6
0.7 0.3
0.3 0.5
0.3 Tb
0.4 0.6
0.5 0.2
2.17 2.70
4.02 2.72
3.36 1.15
1.46 1.78
Dy 2.19
1.05 2.91
0.76 0.44
0.61 0.19
0.24 0.53
0.39 0.18
Ho 0.39
0.52 0.31
1.52 0.51
2.21 1.28
1.80 0.55
0.68 1.19
0.70 1.52
1.10 Er
0.32 0.16
0.26 0.07
0.08 Tm
0.20 0.07
0.07 0.22
0.16 0.24
2.49 1.19
1.94 0.58
0.63 1.67
1.67 0.48
Yb 1.20
1.36 0.59
0.25 0.06
0.36 0.16
0.29 0.09
0.09 0.24
0.09 0.20
0.18 Lu
8.6 35.1
9.0 37.3
48.9 4.2
9.8 24.1
18.3 La
N
Yb
N
24.4 56.1
1.04 0.99
1.00 1.00
1.07 1.33
1.24 2.17
0.40 0.82
0.81 EuEu
68.9 57.5
51.0 61.8
57.6 69.2
67.9 65.6
SiO
2
70.8 68.9
65.9
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. EuEu = Eu
N
Sm
N
× Gd
N
1 2
.
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 La
N
Yb
N
vs. Yb
N
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.
Yb
N
, low Yb
N
and younger calc-alkaline rocks low La
N
Yb
N
, high Yb
N
; Martin, 1986 is not in evidence, three Proterozoic samples plotting in the
field of Archaean rocks.
5. Discussion
