tiform at the mouth of Neria Figs. 1 and 2. The north-western boundary of the block is truncated
by the Proterozoic Vesterland dextral shear zone of intense hydrous mylonitisation Watterson,
1968; Higgins, 1990. On the basis of strongly deformed blebby textures, the block is interpreted
to comprise rocks metamorphosed to granulite facies but later totally retrogressed to amphibolite
facies and moderately to strongly deformed Mc- Gregor and Friend, 1997. The northern part of
the block is dominated by hornblende-bearing tonalitic and dioritic phases enclosing abundant
enclaves, rafts and thin layers of mafic rocks, ultramafic
rocks and,
locally, gabbroic-
anorthositic lithologies. South of Neria Fig. 1 mafic lithologies are less abundant, the gneisses
being commonly finely pegmatite-layered and bi- otite-bearing and there are areas of polyphase,
leucocratic, trondhjemitic gneisses the white gneisses of Kalsbeek, 1970; Taylor and Kalsbeek,
1986. These trondhjemitic gneisses lack textural – mineralogical features indicating earlier granulite-
facies metamorphism, but appear to have had the same complex structural history as adjacent
gneisses that do preserve relic granulite-facies textures.
Field relations near the ice cap north of Sermili- gaarsuk Fig. 1 are unknown, though Masson
1967 recognised that hornblendic gneisses in the main synform were significantly different from the
rocks underneath. As one of several possible in- terpretations, he suggested a tectonic contact with
the hornblendic gneisses thrust over the biotite gneisses, though no recognition of such a contact
was documented. Late phases of biotite-bearing granitic gneiss on the south side of Neria appear
to post-date retrogression of the granulite-facies assemblages.
3
.
4
. The Sermiligaarsuk block Considerable modification of the rocks by
Proterozoic faulting has occurred on the south side of Neria obscuring Archaean relationships.
However, in contrast to the ex-granulite facies Neria block, the volcano-sedimentary Tartoq
Group to the south and associated gneisses have only been metamorphosed to greenschist and low-
ermost amphibolite facies, and are described as having had a much simpler structural history
Berthelsen and Henriksen, 1975; Higgins, 1990. It has been considered that the Tartoq Group was
tectonically emplaced and subsequently deformed into synformal structures Berthelsen and Henrik-
sen, 1975. These lower grade rocks are here named the Sermiligaarsuk block. The position of a
boundary has now been located on the outer coast where the Neria block overlies lower grade
rocks V.R. McGregor, pers. comm.. Retro- gressed granulite facies rocks of the Neria block
have been identified to within ca. 2 km of the much lower grade rocks of the Sermiligaarsuk
block Figs. 1 and 2 and there is no indication of any prograde transition preserved.
4. SHRIMP U – Pb zircon data
4
.
1
. SHRIMP ion microprobe analytical technique
Zircons were mounted in polished epoxy discs and were all studied for internal structure by
optical microscopy before analysis. In several cases, where the zircon populations were complex,
cathodoluminescence CL imaging was used to help resolve internal grain structure and choice of
analysis site. U – Th – Pb isotopic ratios and con- centrations in unknowns were determined using
SHRIMP I and referenced to the Australian Na- tional University standard zircon SL13 age 572
Ma;
206
Pb
238
U = 0.0928. Repeated analyses of the standard during each analytical session were
used for calculating the inter-element isotopic ra- tios of the unknowns and of their uncertainty.
Further details of the analytical procedure and data assessment have been given Compston et al.,
1984; Williams and Claesson, 1987; Roddick and Van Breemen, 1994; Claoue´-Long et al., 1995. A
summary of the age determinations and the zircon populations is given in Table 1 and the SHRIMP
U – Pb analytical data are presented in Table 2. Judging from the very low
204
Pb count rates, the proportion of non-radiogenic common Pb in the
unknowns is very low and has been modelled to be Cumming and Richards 1975 Model Pb at
C .R
.L .
Friend ,
A .P
. Nutman
Precambrian
Research
105 2001
143 –
164
Table 1 Summary of the relationships, with ages for protoliths and metamorphic events, where appropriate, for the Tasiusarsuaq terrane in the north, Sioraq, Paamiut, Neria and Sermiligaarsuk blocks Fig. 1
a
Sermiligaarsuk block Tasiusarsuaq terrane
Sioraq block Paamiut block
Neria block G9432
Granite sheets post 2717 9 35 ?Metamorphism
2720
b
G9417 2700 Granite sheets
extensive assembly with
extensive deformation deformation and
Akulleq terrane amphibolite facies
retrogression amphibolite facies
retrogression ? deformation and amphibolite
facies retrogression No 2820 Ma events recorded in
? granulite facies 2812 9 10
2820
c
G9432 granulite facies
this block 2820 9 10
c
? granulite facies G947
2816 9 12 Metamorphism Ilivertalik granite
G9426 2823 9 38 ?granulite facies
2827 9 11
f
?granulite facies 2820
g
amphibolite facies G9429 2826 9 10
metamorphism G9424 2835 9 70 ? Metamorphism
2863 9 10 TTG gneiss
G948 2852 9 6
TTG gneiss TTG gneiss
G9432 VM8934
2860 9 6 ? Inheritance
G947 2862 9 9
TTG gneiss Fiskenæsset
2860 9 50
d
G9429 2864 9 36 complex
G9421 2872 9 10 TTG gneiss
2872 9 14 TTG gneiss G9426
2878 9 10 G9428 2873 9 16
TTG gneiss TTG gneiss
VM904 ]
2880 Ravns Storø belt
? Supracrustal
syoracrustal rocks Anorthositic rocks
? rocks
Supracrustal rocks ?
G9414 2898 9 8 TTG gneiss
VM8934 2905 I`nheritance
G9428 2920 ? Inheritance
2922 9 4
e
Trondhjemite 2920
d
Component of old G949
2927 9 9 TTG gneiss
TTG gneiss G9424 2932 9 13 Component
2944 9 7 Grey gneiss
] 2945
Tartoq group G9424 2978 9 8
Component ?
Supracrustal rocks
a
All new data are indicated by sample numbers see Fig. 1. Previously published whole-rock and U–Pb zircon data are indicated by superscripts.
b
Friend et al., 1996.
c
Pidgeon and Kalsbeek, 1978.
d
Ashwal et al., 1989.
e
Kinny, 1987; Schiøtte et al., 1989.
f
Kalsbeek and Nutman pers. comm.
g
Nutman and Kalsbeek, 1994. Rocks equated with the Fiskenæsset anorthosite complex are cut by VM8934.
Table 2 SHRIMP U-Th-Pb data for zircons analysed from Paamiuit gneiss samples
a
Labels U ppm
Grain type Th ppm
ThU Comm.
238U206Pb 207Pb206Pb
Age disc
207Pb206Pb 206Pb
VM8934 1.1
154 p,tw
76 0.50
0.24 1.789 9 0.032
0.2035 9 0.0014 2855 9 11
238 108
0.45 0.16
p 1.757 9 0.031
2.1 0.2051 9 0.0011
2867 9 9 1
p,fr 3.1
197 108
0.55 0.08
1.773 9 0.031 0.2049 9 0.0013
2866 9 10 1
84 4.1
28 p
0.33 0.35
1.761 9 0.034 0.2031 9 0.0022
2851 9 18 2
198 81
0.41 0.18
p,fr 1.712 9 0.032
5.1 0.2044 9 0.0013
2862 9 11 4
c?,p 6.1
113 60
0.53 0.49
1.712 9 0.032 0.2099 9 0.0021
2905 9 17 2
113 35
0.31 0.30
1.783 9 0.035 0.2020 9 0.0019
7.1 2842 9 15
p 1
VM9004 431
321 0.75
0.36 1.850 9 0.028
0.1998 9 0.0011 2825 9 9
− 1
1.1 p
147 60
0.41 0.41
p 1.751 9 0.037
2.1 0.1955 9 0.0024
2789 9 20 4
3.1 364
p 142
0.39 0.18
1.924 9 0.029 0.1980 9 0.0006
2810 9 5 −
4 342
121 0.35
0.61 p
1.934 9 0.067 3.2
0.2042 9 0.0017 2860 9 14
− 6
p 4.1
276 162
0.59 0.31
1.812 9 0.028 0.2062 9 0.0010
2876 9 8 −
1 458
317 0.69
0.40 1.891 9 0.029
0.2038 9 0.0007 5.1
2857 9 5 p
− 4
140 108
0.77 0.73
p 1.886 9 0.040
6.1 0.2058 9 0.0016
2872 9 12 −
4 7.1
347 p
171 0.49
0.41 1.885 9 0.034
0.2076 9 0.0009 2887 9 7
− 5
G9432 244
22 0.09
1.1 0.01
p,fr 1.853 9 0.044
0.1981 9 0.0006 2810 9 5
− 1
172 6
0.03 0.13
p,fr 1.866 9 0.077
1.2 0.1997 9 0.0037
2824 9 31 −
2 p,fr
2.1 136
62 0.46
0.13 1.806 9 0.044
0.2043 9 0.0010 2861 9 8
− 1
115 61
0.53 0.11
3.1 1.851 9 0.056
p 0.2067 9 0.0042
2880 9 33 −
3 669
143 0.21
0.03 t,p,fr
1.830 9 0.050 4.1
0.2039 9 0.0023 2858 9 18
− 2
p 5.1
274 51
0.19 0.07
1.899 9 0.047 0.1871 9 0.0026
2717 9 23 176
10 0.06
0.09 1.847 9 0.055
0.1918 9 0.0014 2757 9 12
6.1 1
ov-p 224
10 0.04
0.08 p,fr
1.854 9 0.040 7.1
0.2051 9 0.0006 2867 9 5
− 3
139 17
0.12 0.01
8.1 1.863 9 0.048
ov 0.1971 9 0.0010
2802 9 8 −
1 137
24 0.18
0.13 c,t in p
1.942 9 0.053 9.1
0.1994 9 0.0020 2821 9 17
− 5
r on p 9.2
185 6
0.03 0.12
1.853 9 0.043 0.1986 9 0.0038
2815 9 32 −
1 82
15 0.19
0.04 p
1.781 9 0.043 10.1
0.2033 9 0.0013 2853 9 11
1 p
11.1 307
138 0.45
0.03 1.919 9 0.045
0.1870 9 0.0031 2716 9 27
− 1
p 12.1
181 47
0.26 0.00
1.914 9 0.071 0.1953 9 0.0009
2787 9 8 −
3 382
55 0.14
0.02 t,ov
1.906 9 0.061 13.1
0.2035 9 0.0038 2855 9 31
− 5
p 14.1
143 44
0.31 0.04
1.861 9 0.055 0.2004 9 0.0019
2829 9 15 −
2 113
25 0.22
0.07 15.1
1.870 9 0.137 p
0.2048 9 0.0022 2865 9 17
− 4
152 36
0.24 0.02
p 1.805 9 0.116
15.2 0.2105 9 0.0076
2909 9 60 −
2 p
16.1 31
13 0.42
0.80 1.830 9 0.089
0.1949 9 0.0074 2784 9 64
1 150
46 0.30
0.08 16.2
1.785 9 0.048 p
0.2041 9 0.0035 2859 9 28
145 36
0.25 0.04
p,fr 1.805 9 0.046
17.1 0.2041 9 0.0011
2859 9 9 −
1 ov-p
18.1 185
48 0.26
0.10 1.890 9 0.058
0.1932 9 0.0018 2770 9 16
− 1
232 85
0.36 0.06
1.901 9 0.047 0.1949 9 0.0025
18.2 2784 9 21
ov-p −
2 172
4 0.02
0.10 p
1.824 9 0.274 19.1
0.2016 9 0.0035 2839 9 29
− 1
20.1 249
p 105
0.42 0.04
1.888 9 0.044 0.1956 9 0.0006
2790 9 5 −
2 G9426
1547 42.20
0.03 0.022
1.856 9 0.050 0.1997 9 0.0024
21.3 2823 9 19
r on p −
2 188
69.17 0.37
0.074 c,p
1.891 9 0.065 21.4
0.2052 9 0.0017 2868 9 14
− 5
c,p 21.5
139 43.70
0.31 0.059
1.936 9 0.055 0.2075 9 0.0027
2886 9 21 −
7 444
199.11 0.45
0.021 1.945 9 0.049
0.2050 9 0.0030 2866 9 24
− 7
21.6 c,p
118 36.09
0.31 0.238
p 1.756 9 0.056
13.2 0.2064 9 0.0016
2877 9 12 1
130 38.29
13.3 0.29
p 0.162
1.849 9 0.043 0.2043 9 0.0029
2861 9 23 −
3
Table 2 Continued Th ppm
Labels ThU
Grain type Comm.
U ppm 238U206Pb
207Pb206Pb Age
disc 206Pb
207Pb206Pb G9429
1.1 21
ov 0.27
0.15 1.864 9 0.067
0.2040 9 0.0016 2859 9 13
− 3
79 75
0.68 0.12
1.804 9 0.045 110
0.1997 9 0.0019 p
2823 9 15 1
2.1 111
p,fr 40
0.36 0.17
1.789 9 0.058 0.2028 9 0.0017
2849 9 14 1
3.1 3.2
19 p,fr
0.27 0.09
1.875 9 0.050 0.2011 9 0.0019
2835 9 16 −
3 69
28 0.30
0.15 1.825 9 0.051
95 0.2008 9 0.0011
4.1 2833 9 9
ov 111
ov 27
0.24 0.12
1.830 9 0.047 0.1963 9 0.0027
2796 9 23 1
5.1 26
0.26 0.21
1.813 9 0.050 6.1
0.2006 9 0.0024 ov-p
2831 9 20 98
44 0.40
0.13 1.866 9 0.065
111 0.1951 9 0.0026
ov-p 2786 9 22
− 1
7.1 111
irreg 65
0.59 0.32
1.890 9 0.046 0.2012 9 0.0020
2836 9 17 −
4 8.1
27 0.64
0.13 1.785 9 0.062
0.2002 9 0.0032 2828 9 27
1 9.1
ov 42
35 0.09
0.14 1.906 9 0.043
378 0.1920 9 0.0030
p 2759 9 26
− 1
10.1 68
0.53 0.41
1.943 9 0.044 11.1
0.2015 9 0.0026 p
2838 9 21 −
6 128
88 0.61
0.07 1.858 9 0.100
144 0.1993 9 0.0035
12.1 2821 9 29
− 2
p 113
c? in ov-p 46
0.41 0.38
1.860 9 0.044 0.2046 9 0.0024
2864 9 19 −
3 13.1
28 0.26
0.06 1.817 9 0.046
108 0.2004 9 0.0015
ov,fr 2829 9 12
14.1 177
ov-p 42
0.24 0.02
1.897 9 0.074 0.1959 9 0.0027
2792 9 22 −
2 15.1
16.1 26
ov-p 0.24
0.10 1.868 9 0.046
0.1945 9 0.0018 2780 9 15
− 1
107 25
0.28 0.24
1.833 9 0.046 0.2015 9 0.0020
2838 9 16 89
− 1
17.1 p
G948 p
964 356
0.37 0.03
1.816 9 0.063 0.2050 9 0.0010
2866 9 8 −
1 1.1
328 0.59
0.00 2.1
1.865 9 0.054 p
0.2022 9 0.0017 2844 9 14
− 3
556 298
0.86 0.07
1.875 9 0.111 348
0.1946 9 0.0023 p
2782 9 19 −
1 3.1
412 p
342 0.83
0.06 1.832 9 0.068
0.1990 9 0.0055 2818 9 46
4.1 383
p 232
0.61 0.08
1.740 9 0.060 0.2023 9 0.0008
2845 9 6 3
5.1 342
0.76 0.00
1.788 9 0.073 451
0.2052 9 0.0032 6.1
2868 9 25 ov-p
289 p
190 0.66
0.03 1.770 9 0.080
0.2055 9 0.0032 2870 9 26
1 7.1
420 0.48
0.02 1.819 9 0.052
8.1 0.2028 9 0.0011
ov-p 2849 9 9
− 1
876 322
0.88 0.02
1.801 9 0.054 367
0.2038 9 0.0016 p
2857 9 13 9.1
514 p
322 0.63
0.02 1.824 9 0.056
0.1994 9 0.0015 2821 9 12
10.1 394
0.77 0.07
1.840 9 0.060 0.2015 9 0.0021
2839 9 17 11.1
− 1
p 511
32 0.09
0.03 1.987 9 0.058
0.2031 9 0.0010 2851 9 8
366 −
8 12.1
p G9421
p,fr 51
46 0.90
0.44 1.693 9 0.054
0.2057 9 0.0045 2872 9 36
4 1.1
217 0.50
0.13 2.1
1.810 9 0.040 c,p
0.2058 9 0.0019 2873 9 15
− 1
438 27
0.36 1.52
1.724 9 0.047 76
0.2051 9 0.0033 p
2867 9 26 3
3.1 161
ov 60
0.37 0.29
1.818 9 0.046 0.2036 9 0.0025
2855 9 20 −
1 4.1
56 0.57
0.00 1.786 9 0.048
0.2018 9 0.0013 2841 9 10
1 5.1
ov 98
85 0.34
0.08 1.849 9 0.041
250 0.2062 9 0.0008
ov 2876 9 6
− 3
6.1 386
rex,ov 70
0.18 0.15
1.954 9 0.048 0.2019 9 0.0025
2842 9 20 −
6 7.1
27 0.10
0.12 1.751 9 0.039
0.2074 9 0.0035 2885 9 28
8.1 1
ov 282
163 0.68
0.19 1.642 9 0.057
241 0.1886 9 0.0031
p 2730 9 27
12 10.1
11.1 34
ov 0.12
0.07 2.016 9 0.045
0.1852 9 0.0029 2700 9 26
− 4
277 G9428
72 0.68
0.08 1.811 9 0.093
0.2071 9 0.0069 2883 9 55
1.1 −
2 p
106 68
0.30 0.09
1.741 9 0.062 227
0.2072 9 0.0044 2.1
2884 9 35 2
p 101
c in ov 52
0.52 0.09
2.088 9 0.101 0.2122 9 0.0025
2923 9 19 −
14 3.1
93 0.63
0.25 1.721 9 0.045
0.2047 9 0.0018 2864 9 14
3 5.1
c in ov 148
85 0.51
0.25 1.794 9 0.059
165 0.2072 9 0.0022
p 2884 9 17
− 1
6.1 136
0.89 7.1
0.32 ov
1.777 9 0.049 0.2056 9 0.0017
2871 9 14 154
Table 2 Continued U ppm
Labels Th ppm
Grain type ThU
Comm. 238U206Pb
207Pb206Pb Age
disc 207Pb206Pb
206Pb G9424
1.1 25
ov 9
0.38 0.46
1.716 9 0.065 0.2128 9 0.0034
2927 9 26 1
840 610
0.73 0.00
t,p 1.708 9 0.039
2.1 0.2071 9 0.0013
2883 9 10 3
t,p 3.1
41 24
0.58 0.49
1.724 9 0.052 0.2205 9 0.0055
2985 9 41 −
1 77
53 0.69
0.29 1.761 9 0.053
0.2130 9 0.0032 4.1
2928 9 25 p
− 1
86 59
0.69 0.32
p 1.731 9 0.045
5.1 0.2116 9 0.0018
2918 9 14 1
63 43
0.68 0.00
3.2 1.622 9 0.059
t,p 0.2227 9 0.0019
3000 9 14 3
61 41
0.68 0.00
p 1.703 9 0.058
6.1 0.2116 9 0.0049
2918 9 38 2
t,p 7.1
83 54
0.65 0.10
1.765 9 0.049 0.2169 9 0.0017
2958 9 12 −
2 326
97 0.30
0.00 rex,p
1.976 9 0.050 8.1
0.1822 9 0.0016 2673 9 14
− 1
p 8.2
135 81
0.60 0.03
1.842 9 0.064 0.2141 9 0.0036
2937 9 27 −
5 1003
650 9.1
0.65 t,ov
0.14 1.795 9 0.046
0.2121 9 0.0017 2922 9 13
− 2
151 69
0.46 0.28
p 1.771 9 0.064
10.1 0.2064 9 0.0014
2877 9 11 81
32 0.40
0.53 11.1
1.817 9 0.049 p
0.2010 9 0.0034 2835 9 28
40 29
0.73 0.45
ov 1.728 9 0.053
12.1 0.2198 9 0.0030
2979 9 22 −
1 t,p
13.1 264
122 0.46
0.15 1.783 9 0.045
0.2097 9 0.0017 2903 9 13
− 1
29 13
0.47 0.45
t,p 1.783 9 0.092
14.1 0.2109 9 0.0040
2912 9 31 −
2 p
15.1 48
30 0.63
0.00 1.700 9 0.057
0.2168 9 0.0025 2957 9 19
1 16.1
99 p
65 0.66
0.35 1.749 9 0.057
0.2141 9 0.0032 2937 9 24
− 1
93 65
0.70 0.00
p 1.712 9 0.050
17.1 0.2140 9 0.0019
2936 9 4 1
18.1 45
p 31
0.69 0.40
1.703 9 0.067 0.2109 9 0.0029
2912 9 22 2
G9414 123
46 0.37
1.1 0.77
p 1.784 9 0.058
0.2083 9 0.0062 2892 9 49
− 1
182 53
0.29 0.22
c in p 1.786 9 0.032
2.1 0.2079 9 0.0010
2889 9 8 −
1 fr
3.1 188
24 0.13
0.14 1.759 9 0.040
0.2101 9 0.0020 2906 9 16
4.1 p
268 33
0.12 0.10
1.688 9 0.033 0.1985 9 0.0018
2814 9 15 7
302 46
0.15 0.10
p 1.856 9 0.033
4.2 0.1897 9 0.0033
2740 9 29 1
26 7
0.28 0.61
5.1 1.738 9 0.064
p 0.2069 9 0.0045
2881 9 36 2
242 38
0.16 0.19
p 1.740 9 0.039
6.1 0.2113 9 0.0013
2916 9 10 p,fr
7.1 326
63 0.19
0.23 1.779 9 0.041
0.2016 9 0.0008 2839 9 6
1 152
46 0.30
0.30 p
1.751 9 0.038 9.1
0.2090 9 0.0010 2898 9 8
1 p
10.1 273
113 0.42
0.37 1.684 9 0.032
0.2081 9 0.0013 2891 9 10
4 11.1
247 ov
27 0.11
0.09 1.778 9 0.035
0.2081 9 0.0040 2891 9 31
115 35
0.30 0.45
1.760 9 0.049 p
0.2096 9 0.0016 12.1
2903 9 13 G949
57 23
0.42 0.12
p 1.836 9 0.062
1.1 0.2127 9 0.0015
2926 9 12 −
4 86
25 0.29
1.2 0.23
p 1.830 9 0.117
0.2090 9 0.0015 2898 9 12
− 3
81 72
0.89 0.01
p 1.802 9 0.175
2.1 0.2131 9 0.0063
2929 9 48 −
3 ov
3.1 38
19 0.50
0.18 1.778 9 0.079
0.2156 9 0.0027 2948 9 20
− 2
161 4.1
33 ov
0.21 0.05
1.766 9 0.058 0.2087 9 0.0023
2896 9 18 72
35 0.49
0.05 p
1.707 9 0.131 5.1
0.2166 9 0.0076 2955 9 58
1 p
5.2 73
52 0.71
0.13 1.770 9 0.148
0.2113 9 0.0029 2916 9 23
− 1
53 32
0.60 0.01
6.1 1.728 9 0.056
p 0.2129 9 0.0030
29280 9 23 1
103 17
0.16 0.11
p 1.813 9 0.070
7.1 0.2058 9 0.0016
2873 9 12 −
1 p
7.2 165
28 0.17
0.06 1.841 9 0.059
0.2011 9 0.0021 2835 9 17
− 1
62 37
0.60 0.04
1.803 9 0.062 0.2103 9 0.0032
2908 9 25 −
2 7.3
p 31
14 0.44
0.25 p
1.741 9 0.082 7.4
0.2110 9 0.0038 2913 9 29
1 p
8.1 50
18 0.37
0.25 1.718 9 0.050
0.2073 9 0.0048 2885 9 38
3 40
24 0.60
0.27 1.720 9 0.072
0.2157 9 0.0023 2949 9 17
9.1 p
128 54
0.42 0.11
p 1.701 9 0.063
10.1 0.2114 9 0.0037
2916 9 29 2
62 18
11.1 0.30
ov 0.12
1.726 9 0.046 0.2097 9 0.0021
2903 9 16 2
Table 2 Continued Labels
U ppm Grain type
Th ppm ThU
Comm. 238U206Pb
207Pb206Pb Age
disc 207Pb206Pb
206Pb 141
126 12.1
0.89 p
0.07 1.701 9 0.038
0.2143 9 0.0011 2938 9 8
1 78
63 0.80
0.14 ov
1.620 9 0.167 13.1
0.2148 9 0.0096 2942 9 74
5 p
14.1 59
37 0.63
0.01 1.698 9 0.066
0.2131 9 0.0018 2930 9 14
2 15.1
47 24
0.53 0.18
1.684 9 0.143 0.2044 9 0.0138
2861 9 114 5
G947 194
46 0.24
1.1 0.54
r on p,fr 1.868 9 0.062
0.1960 9 0.0014 2793 9 12
− 1
47 27
0.58 0.03
1.882 9 0.055 0.2042 9 0.0036
2860 9 29 −
4 1.2
c in p,fr 101
81 0.80
0.19 ov-irreg
1.843 9 0.052 2.1
0.1995 9 0.0022 2822 9 18
− 1
ov-irreg 2.2
16 106
6.77 0.93
1.825 9 0.071 0.1983 9 0.0076
2812 9 64 27
234 8.59
0.01 1.753 9 0.064
0.1988 9 0.0027 3.1
2816 9 23 ov-p
3 286
81 0.28
0.03 p
1.848 9 0.046 4.1
0.2053 9 0.0014 2868 9 11
− 3
p 5.1
374 54
0.14 0.19
1.890 9 0.042 0.2011 9 0.0012
2835 9 10 −
3 197
56 0.28
0.06 6.1
1.792 9 0.046 p
0.2062 9 0.0007 2876 9 6
− 1
151 99
0.66 0.00
p 1.843 9 0.051
7.1 0.2064 9 0.0030
2878 9 24 −
3 r? on ov-p
8.1 117
35 0.30
0.46 1.862 9 0.042
0.2015 9 0.0019 2838 9 16
− 2
197 72
0.37 0.07
8.2 1.788 9 0.039
c? in ov-p 0.2042 9 0.0016
2860 9 13 89
42 0.47
0.26 p
1.947 9 0.076 9.1
0.1981 9 0.0034 2811 9 28
− 5
p 9.2
195 54
0.28 0.78
1.837 9 0.072 0.1978 9 0.0056
2808 9 47 64
116 1.81
0.42 10.1
1.880 9 0.047 ov
0.1984 9 0.0020 2813 9 17
− 2
51 30
0.59 0.42
ov-p 1.815 9 0.052
11.1 0.2006 9 0.0027
2831 9 22 p
12.1 149
65 0.44
0.20 1.823 9 0.054
0.2011 9 0.0017 2835 9 14
− 1
160 69
0.43 0.15
1.856 9 0.046 0.2000 9 0.0024
12.2 2826 9 20
p −
2 123
42 0.34
0.10 p
1.824 9 0.048 13.1
0.2023 9 0.0014 2845 9 12
− 1
x,ov-p 14.1
53 25
0.47 0.06
1.848 9 0.068 0.2022 9 0.0026
2844 9 21 −
2 41
24 0.57
0.37 14.2
1.865 9 0.049 r on ov-p
0.1998 9 0.0022 2824 9 18
− 2
121 49
0.41 0.20
p 1.866 9 0.303
15.1 0.1942 9 0.0044
2778 9 38 c in p
16.1 213
41 0.19
0.02 2.036 9 0.082
0.1993 9 0.0032 2820 9 26
− 9
199 39
0.20 0.14
1.981 9 0.054 0.2000 9 0.0023
17.1 2826 9 19
x, p −
7 346
53 0.15
0.43 1.841 9 0.044
c in p 0.2017 9 0.0020
18.1 2840 9 16
− 1
G9417 19
39 2.08
1.92 ov-p
1.924 9 0.131 1.1
0.1736 9 0.0194 2592 9 199
4 893
279 0.312
2.1 .40
t,p 2.405 9 0.046
0.1699 9 0.0014 2557 9 14
− 12
333 178
0.54 1.81
t,p 1.812 9 0.039
3.1 0.1899 9 0.0043
2741 9 37 3
t,p 4.1
156 114
0.73 2.03
2.027 9 0.053 0.1790 9 0.0026
2644 9 24 −
2 1489
290 4.2
0.19 t,p
2.21 2.208 9 0.044
0.1712 9 0.0025 2569 9 25
− 6
555 25
0.04 1.89
t,p 1.894 9 0.036
5.1 0.1817 9 0.0017
2669 9 16 2
t,p 5.2
1158 188
0.16 3.03
3.026 9 0.055 0.1398 9 0.0073
2225 9 93 −
17 285
43 0.15
1.82 6.1
1.823 9 0.042 p
0.1769 9 0.0036 2624 9 34
8 1054
145 0.14
2.61 t,p
2.611 9 0.057 7.1
0.1515 9 0.0014 2363 9 16
− 11
p 8.1
79 7
0.09 1.93
1.934 9 0.067 0.1793 9 0.0062
2646 9 58 2
964 75
0.08 9.1
3.23 t,p
3.232 9 0.058 0.1597 9 0.0049
2453 9 52 −
29 744
77 0.10
1.92 t,p
1.915 9 0.040 10.1
0.1718 9 0.0078 2576 9 78
5 t,p
11.1 226
105 0.46
1.81 1.810 9 0.041
0.2043 9 0.0030 2861 9 24
− 1
368 256
0.70 12.1
1.95 t,p
1.947 9 0.041 0.1681 9 0.0013
2539 9 13 5
708 14
0.02 2.55
t,p 2.549 9 0.051
13.1 0.1695 9 0.0021
2553 9 21 −
16
a
c, core; fr, fragment; irreg, irregular; p, prismatic; ov, ovoid; r, rim; rex, recrystallised; t, turbid; tw, twinned; x, xenoblastic. Corrected with 2820 Ma model Pb of Cumming and Richards, 1975
2700 Ma. However, because the proportion of non-radiogenic Pb detected in these grains is so
low, the dates quoted here are not sensitive to the choice of its composition.
Quoted dates are weighted means 2s, inverse variance weighting derived from
207
Pb
206
Pb ra- tios of analyses selected as being of the least
isotopically disturbed sites close to concordant PbU dates, with very small non-radiogenic com-
ponent to Pb in grains belonging to the same populationtype e.g. for dating of magmatic
events, sites within prismatic zircon commonly displaying micron-scale oscillatory zoning concor-
dant with the grain margins. Inititally in most cases, the spread in individual site dates meant
that the weighted mean dates calculated on such groupings of analyses had mean square weighted
deviate MSWD values \ 1.0; indicating a low probability that such dates reflect the timing of
real geological events. From multiple analyses of different sites on some grains and CL imagery on
the populations, this is best interpreted to be due to partial loss of radiogenic Pb during later mod-
ificationdamage of the ziron structure. These ob- servations suggest that the ‘oldest’ dates obtained
from any petrographic grouping are likely to give the most reliable determination of a real geologi-
cal event e.g. growth of oscillatory-zoned zircon out of a magma. Hence individual ‘youngest’
dates in any grouping were culled until those remaining had a MSWD of
1.0 for the weighted mean date. These low MSWD dates,
when combined with petrographic observations of the dated sites in the zircons, will give confident
dates on individual geological events. For more detail on the calculation of weighted mean
207
Pb
206
Pb dates from SHRIMP data see Compston et al. 1986, Nutman et al. 1997. The Steiger and
Ja¨ger, 1977 values for decay constants and present-day
238
U
235
U were used to calculate the dates.
4
.
2
. Tasiusarsuaq terrane Age determinations are reported on two amphi-
bolite facies tonalite gneisses provided by V.R. McGregor from the southern part of the Ta-
siusarsuaq terrane. Sample VM8934 represents a tonalitic sheet cross-cutting gabbro anorthosite
near the mouth of Bjørnesund 62°50N, 50°23W; Fig. 1. This yielded euhedral to slightly rounded
100 – 300 mm long colourless to light brown pris- matic zircons. They are homogeneous or show a
faint micron-scale oscillatory zoning parallel to grain exteriors. One grain 6 might contain a
homogeneous core with a mantle of zoned zircon. Analysis of the centre of grain 6 yielded the oldest
207
Pb
206
Pb date 2905 9 34 Ma. With this age and precision it could be a real core or a statistical
outlier of the main population Table 2. The remainder of the analysed grains yielded close to
concordant dates with a slight spread of
207
Pb
206
Pb dates Fig. 5a. These yielded a weighted mean
207
Pb
206
Pb date of 2863 9 10 Ma 2s, n = 6, interpreted as the intrusive age of sheet
VM8934. Of note is the slight reverse discordance of these
data. The analyses were undertaken in 1990, when it was the practice to insert an epoxy plug with a
large chip of standard SL13 into a hole in the mount containing the unknowns. This may give
rise to a small bias between PbU as measured on the standards and the unknowns on the rest of the
same mount see Friend and Nutman, 1992 for further discussion of this problem. This practice
has since been abandoned and small chips of the standard are now cast with the unknowns. This
problem does not affect the age reported which is based on the directly measured
207
Pb
206
Pb ratio. Meta-tonalitic
gneiss VM904
62°38N, 50°18W; Fig. 1 came from a unit which intruded
the south-eastern margin of the Ravns Storø belt. It yielded a population of zircons very similar to
those in VM8934 with faint CL images Fig. 4. Most of the analyses give close to concordant
dates, but with a spread in
207
Pb
206
Pb date of ca. 100 Ma Table 2, Fig. 5b. Due to the homogene-
ity of the population the grains are interpreted to be magmatic in origin, but to have suffered some
ancient Pb-loss during late Archaean tectonother- mal events 2850 – 2550 Ma known to have af-
fected the Tasiusarsuaq terrane e.g. Schiøtte et al., 1989; Friend et al., 1996. Accepting this
interpretation, filtering the analyses until those remaining
are indistinguishable
from their
weighted mean Compston et al., 1986, yielded a
207
Pb
206
Pb date of 2878 9 10 Ma 2s, n = 4.
Fig. 4. Cathodoluminescence images of selected zircon grains from the analysed samples.
206
Pb dates between ca. 2900 and 2750 Ma Table 2, Fig. 5c. If it is accepted that these grains form
a single population that underwent some later disturbance with loss of radiogenic Pb then, by
rejecting analyses with the lowest
207
Pb
206
Pb, a weighted mean date of 2860 9 6 Ma 2s, n =
13; Fig. 5c is derived and interpreted as the magmatic protolith age. Also present are stubby
prismatic to ovoid clear grains e.g. grains 1, 19 and clear overgrowths e.g. analysis 9.2, with low
ThU B 0.1. A weighted mean
207
Pb
206
Pb date of these morphologically and chemically distinct
zircons is 2812 9 10 Ma 2s, n = 4. These are interpreted to reflect new zircon growth during
high-grade metamorphism 9 anatexis, proba- bly during the granulite facies event this rock has
experienced. Two analyses of distinctive brown prismatic zircon grains 5, 11 with concordant
dates have an imprecise average
207
Pb
206
Pb date of 2717 9 35 Ma 2s, n = 2. It is uncertain if
these represent regrowth of zircon at that time or they are witness to Pb-loss from grains in an event
after peak metamorphism 2812 Ma.
Sample G9426 62°14N, 49°51W; Fig. 1 yielded predominantly clear to pale brown 100 –
300 mm long prismatic grains showing both ho- mogeneous and oscillatory-zoned domains. Some
grains are corroded, and a very few display partial overgrowths of brown zircon. Data from a first
analytical session were rejected and another ses- sion was undertaken in which only grains 13 and
21 Fig. 4 were examined with multiple analyses. Analysis was undertaken of a rim analysis 21.3
which infills a corroded area on the side of a prismatic grain. The rim is very high U, low ThU
with a
207
Pb
206
Pb age of 2823 9 38 Ma 2s. Although imprecise, this age agrees with other
2820 Ma age determinations for the high-grade ?granulite event. Multiple analyses on the pris-
matic zircons 12 and 21 yielded a weighted mean of 2872 9 14 Ma 2s, n = 5, interpreted as
giving a minimum magmatic protolith age.
The retrogressed sample, G9429 62°13.4N, 49°33.4W; Fig. 1, yielded a diverse zircon popu-
lation. Dominant are small generally B 150 mm long oscillatory-zoned to homogeneous prismatic
grains, showing little sign of corrosion. Under CL these grains show little luminescence and only
occasional zones are evident e.g. grain 2; Fig. 4.
4
.
3
. Sioraq block Two samples of granulite facies and one retro-
gressed granulite facies sample of tonalitic-dioritic gneisses from the Sioraq block were analysed.
Sample G9432 62°22N, 50°08W; Fig. 1 pro- vided a heterogeneous zircon population. Domi-
nant are brown, prismatic 100 – 300 mm long grains characterised by 1 – 2 mm scale oscillatory
zoning, embayment of some facets and rare over- growths. The grains generally show little lumines-
cence in CL, though some show faint oscillatory zonation. They generally have ThU of \ 0.10
and scatter on or close to concordia with
207
Pb
Fig. 5. Tera-Wasserburg U – Pb diagrams for the analysed zircons in samples from the southern Tasiusarssuaq terrane and the Sioraq, Paamiut and Neria blocks. See Fig. 1 for locations except GGU261014, which is from the same location as G94-24 at
Kuumiut.
Also present are equant to ovoid grains ranging from 50 to \ 300 mm, that usually show a
broad but weak zonation under CL e.g. grain 2; Fig. 4. Still larger grains did not survive sample
crushing intact and occur as fragments. Most analyses plot on or close to concordia Fig. 5e,
with
207
Pb
206
Pb dates from 2865 to 2760 Ma. The oldest analysis is of a possible core in pris-
matic grain 13 Table 2 and another old analysis 1.1 is also of a possible core Table 2. Exclud-
ing these, the remainder give a weighted mean
207
Pb
206
Pb date of 2826 9 10 Ma 2s, n = 12. Given the magmatic appearance of many of the
zircons in this population the sample must have at least partially melted at this time. Analysis 13.1 of
a core with a
207
Pb
206
Pb date of 2864 9 36 Ma 2s is, within its large error, agrees with the
protolith ages of G9426 and G9432, indicating that older components are present in this sample.
4
.
4
. Paamiut block Ages have been determined on three samples
G948, G9428, G9421, Fig. 1 considered to represent the main homogeneous biotite tonalitic
granodioritic gneisses
of this
block. G948
61°47.1N, 49°20.7W is a sample from a more homogeneous than average area of gneisses de-
void of supracrustal units. It yielded a uniform population of generally small 50 – 200 mm, clear
to pale yellow, thin, prismatic zircons showing oscillatory zoning. The grains are euhedral to
slightly corroded. In CL the show oscillatory zones mimicking the shape of the grain Fig. 4.
Most analyses are concordant within error Table 2, Fig. 5f. Interpreting the dispersion in
207
Pb
206
Pb dates as due to slight ancient radiogenic Pb-loss, then rejecting 2 analyses yielded a
207
Pb
206
Pb date of 2852 9 6 Ma 2s, n = 10. G942162°52.3N, 49°24.3W is from a sheet
of homogeneous grey gneiss that intrudes a supracrustal unit. It yielded prismatic, bipyrami-
dal, oscillatory-zoned zircons that are typically B
150 mm long e.g. grain 1. Apart from being slightly smaller than zircons in G9408, their char-
acters are similar. Rejecting a few analyses with the lowest
207
Pb
206
Pb, a weighted mean
207
Pb
206
Pb date of 2872 9 10 Ma 2s, n = 7 was determined and interpreted as giving the igneous
protolith age Fig. 5g. Sample G9428 62°14.6N, 49°27.1W is also a
homogenous tonalitic gneiss taken from a sheet intruding a supracrustal unit. It yielded prismatic,
euhedral, oscillatory-zoned zircons typically up to 300 mm long which are very similar to those in
G9408 and G9421. However, from CL imaging very rare cores might be present in a small minor-
ity of grains e.g. grain 3. After rejecting two analyses of possible cores, the remaining close to
concordant analyses give 2873 9 16 Ma 2s, n = 5, interpreted as the protolith age Fig. 5h.
4
.
5
. Neria block Four new samples have been dated from the
Neria block.
One, dioritic
gneiss G9424
62°32.2N, 49°11.1W; Fig. 1 is cut by the 2922 9 4 Ma white trondhjemitic gneiss GGU
261014 of Nutman and Kalsbeek 1994, repro- duced in Fig. 5i. Sample G9424 yielded large
\ 500 mm zircons, some of which are deep brown to opaque, rounded grains and corroded
prisms that are commonly cracked, turbid and in places metamict. Other grains are colourless to
pale pink, unflawed and only slightly corroded. Most analyses yielded close to concordant ages
Fig. 5j. The brown grains have
207
Pb
206
Pb dates from \ 2700 to 3000 Ma. Given their flawed
state, these are interpreted as a single population that has, in ancient times lost variable amounts of
radiogenic Pb. The four oldest analyses yielded a weighted mean
207
Pb
206
Pb date of 2978 9 8 Ma 2s, n = 4. With the clear, unflawed grains, two
ages might be apparent. Dominant are those which yield a weighted mean
207
Pb
206
Pb date of 2932 9 13 2s, n = 7. A single analysis of a
completely clear and euhedral grain gave an im- precise
207
Pb
206
Pb date of 2835 9 70 Ma 2s. Despite this large uncertainty, it is possible a third
generation of zircons is present. In keeping with the heterogeneous appearance of this gneiss in the
field, the zircon chronology suggests it is com- posite, containing more than one igneous phase
Table 2.
A homogenous sample, G9414 from the mouth of Neria 61°37.90N, 49°00.15W; Fig. 1 yielded
deep yellow to brown, typically 200 – 400 mm long, euhedral to slightly rounded, prismatic zircons.
Oscillatory zoning is commonly displayed, but some grains are partly metamict and mostly
showed low luminescence in CL Fig. 4. No structural cores or overgrowths were observed.
Overall the population appears simple, albeit some grains show signs of being flawed. Apart
from reverse discordant analysis 4.1, all analyses gave close to concordant ages Table 1, Fig. 5k.
Rejecting a few analyses with low
207
Pb
206
Pb, interpreted as having undergone ancient Pb-loss,
the remainder yielded a weighted mean date of 2898 9 8 Ma 2s, n = 9, interpreted as the ig-
neous age of the protolith Table 2.
Neither of the samples G9414 and 24 had obvious overgrowths, even when viewed by CL.
However, sample G9424 had young prismatic grains on which a single analysis gave a
207
Pb
206
Pb date of 2835 Ma and trondhjemitic gneiss GGU261014 from the same locality had over-
growths dated at 2827 9 11 Ma Nutman and Kalsbeek 1994. This possibly reflects the gran-
ulite facies metamorphism described by McGre- gor and Friend 1997.
Two other samples of gneisses G947 and G94 9, Fig. 1 thought lithologically representative of
the block, were dated. Gneiss G949 61°42.5N, 49°08.3W from the northern side of the block
yielded a diverse population of zircons; clear prisms with faint oscillatory zoning and buff
equant or rounded grains Table 2, Fig. 5l. No internal grain structure was visible by optical
microscopy. However, CL imagery revealed that some grains are composite, with some of the
prismatic grains having thin overgrowths on their pyramidal terminations – too thin for analysis
even using a 20 mm spot the smallest available on SHRIMP I. If the analyses are treated as belong-
ing to one population that suffered some ancient loss of radiogenic Pb, a weighted mean
207
Pb
206
Pb date 2927 9 9 Ma is interpreted as giving the igneous age of the protolith. As a justification
of this , four analysed of the large homogeneous prismatic grain grain 7, Table 2 yielded
207
Pb
206
Pb dates from 2835 to 2913, testifying to het- erogeneous loss of radiogenic Pb from single
crystals. Sample G947 61°43.9N, 49°18.7W comes
from within 2 km of a major Proterozoic fault which cuts the Neria and Paamiut blocks Fig. 1.
Dominant in the zircon population are brownish prismatic grains which are, on average, more frac-
tured and metamict that zircons from other gneiss samples discussed in this paper. Most of the pris-
matic grains have low luminescent centres and may display irregular, narrow overgrowths on
their pyramidal terminations that are bright in CL. Also present are some homogeneous ovoid to
irregular grains, up to 300 mm long that show irregular to blotchy CL luminescence grains 2
and 14; Fig. 4. Analyses of these grains and overgrowths yielded a weighted mean
207
Pb
206
Pb date of 2816 9 12 Ma 2s, n = 10, Fig. 5m.
Analyses of the prismatic grains yielded a weighted mean
207
Pb
206
Pb date of 2862 9 9 Ma 2s, n = 6. The ages are interpreted to respec-
tively represent the age of high-grade metamor- phism and a minimum for the igneous protolith.
A sample of a weakly deformed, late granite sheet G9417, 61°35.7N, 49°02.4W; Fig. 1, was
also dated. This cross-cuts retrogressed and subse- quently deformed ex-granulite facies gneisses near
the mouth of Neria. An age of this sample, to- gether with the protolith ages on the gneisses
would limit timing of the prograde and retrograde metamorphic events. However, G9417 yielded
only dark brown, widely metamict zircons with generally discordant ages Table 2, Fig. 5n. From
these results an imprecise
207
Pb
206
Pb date of 2600 Ma can be suggested. However, this does
show that by this time the major tectonothermal evolution of the Neria block had been completed.
5. Discussion and conclusions
