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.
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 sam- ples. Modes were estimated with the help of
chemical analyses, using katanorms with norma- tive 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-bear- ing Proterozoic rocks it is problematic. Five Ar-
chaean samples fall in the field of tonalites and trondhjemites, four in the field of granodiorites,
and one plots as quartz-diorite. These composi- tions are typical for Archaean tonalite – trond-
F .
Kalsbeek Precambrian
Research
105 2001
165 –
181
Table 1 Chemical analyses of Archaean orthogneisses, Nagssugtoqidian orogen, West Greenland
a
413870 413777
413879 413883
413887 413899
415603 413752
GGU no. 413766
413738 \
2700 2800
2825 2775
Arch. 2800
2850 2700
Age Ma 2800
\ 2800
65.90 71.67
75.80 68.85
68.90 57.49
70.81 70.83
70.03 70.84
SiO
2
0.45 0.31
0.57 0.46
0.19 0.52
0.67 TiO
2
0.38 0.41
0.38 15.78
13.25 12.59
15.60 16.97
15.18 15.07
Al
2
O
3
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 0.04
0.03 0.03
0.03 1.80
1.39 0.42
MgO 1.42
1.18 4.46
1.24 1.20
1.33 0.91
4.12 3.30
1.65 2.96
7.57 2.96
2.95 CaO
2.14 2.95
2.18 4.33
4.44 3.95
3.44 3.63
4.54 3.91
4.49 4.45
4.68 Na
2
O 2.29
1.61 1.48
1.67 3.14
1.80 1.05
2.87 1.59
2.51 K
2
O 0.20
0.35 0.08
0.16 0.12
0.12 P
2
O
5
0.09 0.10
0.13 0.12
Volat. 0.70
0.51 0.49
0.20 0.56
0.97 0.49
0.49 0.60
0.33 99.38
99.20 99.35
99.64 99.43
99.48 99.53
99.45 99.35
Total 99.53
52 64
78 120
Rb 31
71 95
70 114
56 385
450 675
285 104
588 Ba
233 164
425 210
9 8
6 7
8 7
3 8
7 Pb
12 310
184 265
235 253
434 195
134 Sr
264 145
16 24
44 38
31 25
9 21
13 31
La 32
52 84
70 57
48 26
39 29
66 Ce
32 29
21 20
15 14
Nd 26
11 17
19 2
9 17
7 4
13 16
9 6
10 Y
6 15
10 9
B 1
3 25
Th 5
7 3
142 205
73 225
102 Zr
94 152
111 223
114 3.7
5.5 2.4
Hf 5.6
3.3 5.8
Nb 4.0
5.3 2.0
9.5 3.8
7.2 4.3
14 1.3
74 53
23 59
78 46
48 Zn
50 47
41 11
7 15
10 11
5 18
8 3
5 Cu
8 12
15 10
6 11
85 10
10 10
Ni 10
4 B
1 6
18 3
Sc 6
5 3
3 34
22 70
26 19
39 129
35 V
28 34
17 15
6 10
89 9
12 Cr
14 10
12 21
Ga 17
18 14
22 20
21 19
20 20
a
Ages from SHRIMP zircon U-Pb reconnaissance data Kalsbeek and Nutman, 1996; precisions 9 c. 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.
F .
Kalsbeek Precambrian
Research
105 2001
165 –
181
171 Table 2
Chemical analyses of Palaeoproterozoic orthogneisses, Nagssugtoqidian Orogen, West Greenland
a
413772 413774
413778 413850
413869 413882
413884 413898
413749 GGU no.
413762 413740
1940 1920
1820 1950
1920 1890
1910 1940
1950 Age Ma
1910 1940
49.94 49.56
59.37 67.88
65.62 57.59
66.99 69.21
63.13 61.78
51.02 SiO
2
0.89 0.34
1.04 1.18
0.51 0.45
1.00 0.54
TiO
2
0.69 0.86
0.68 17.92
15.03 16.93
15.78 14.30
15.59 15.57
17.81 Al
2
O
3
18.70 16.61
16.85 9.29
10.47 6.08
2.62 5.85
FeO 4.00
9.10 4.65
4.21 5.99
2.27 0.21
0.18 0.13
0.04 0.07
0.05 0.07
MnO 0.09
0.05 0.08
0.17 2.81
5.46 3.15
1.52 MgO
2.67 3.76
2.06 3.12
2.16 3.33
1.11 7.52
10.31 5.89
3.29 4.07
2.97 5.01
7.94 CaO
6.48 3.96
4.77 4.90
4.11 3.05
3.01 4.01
4.28 2.77
3.65 3.93
4.86 4.41
Na
2
O 2.05
2.00 4.63
2.03 2.06
2.62 2.06
0.97 2.17
1.99 1.62
K
2
O 0.92
0.33 0.21
0.25 0.14
0.17 0.16
P
2
O
5
0.30 0.42
0.43 0.52
Volat. 2.06
1.11 1.92
0.97 0.49
0.98 0.54
0.96 0.78
0.92 0.39
99.39 99.48
99.31 99.21
99.52 99.42
99.55 99.08
99.18 99.06
Total 99.28
133 60
55 47
Rb 66
32 15
108 112
46 44
2220 1180
1000 1720
1700 2100
566 Ba
1050 927
906 2290
8 8
34 8
11 10
9 8
9 8
Pb 8
952 382
836 888
353 890
598 1120
716 Sr
1000 968
36 32
68 30
21 44
21 17
66 62
26 La
62 69
129 62
44 80
38 31
126 109
56 Ce
66 31
23 33
16 24
12 Nd
31 40
51 36
6 23
36 26
14 8
11 5
15 9
19 Y
18 4
5 4
2 6
3 2
Th B
1 8
8 132
118 72
125 354
Zr 73
156 159
118 163
104 3.6
10.1 2.5
Hf 4.1
4.7 4.9
14 Nb
11 8.8
4.6 6.0
12 2.4
9.8 11
6.4 5.6
117 118
80 45
75 42
68 97
Zn 87
63 75
4 46
42 32
11 26
36 12
26 20
16 Cu
9 13
13 44
14 14
28 10
44 21
18 Ni
14 38
18 5
16 3
5 Sc
15 6
11 23
112 31
152 287
121 48
181 61
192 V
87 80
7 177
28 26
79 14
32 53
Cr 21
72 25
20 Ga
18 19
17 19
16 18
22 21
20 16
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.
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
