been carried out. The high strain zones are hosts of a diffuse network of red granitic veins. In some
places, these veins consist of mylonite or fine grained annealed mylonite. In other places, the
veins are distinctly magmatic and consist of isotropic
garnet bearing
myrmekitic granite.
Gneisses in the vicinity of the veins are affected by metasomatic infiltration, seen as a red colouration
of the gneisses, which only partially overprint the compositional banding. We interpret the red
granitic rocks as syn- to post-kinematic relative to the high strain zones. They could have been
formed by flux melting within these zones as a response to fluid introduction during deforma-
tion, or be preferentially intruded into these zones from an external source.
2
.
6
. Metadolerite dykes The gneiss complexes were intruded by rare
E – W trending metadolerite dykes. Such dykes have not been observed in the post-tectonic gran-
ites. However, the dykes show no signs of dis- placement along lineaments within the Aasivik
gneiss complex. We interpret the dykes as em- placed later than the juxtaposition of the two
complexes, but preferentially intruded into the gneissic roks for structural and rheological rea-
sons. The dolerite dykes show no signs of penetra- tive
deformation, and
the igneous
mineral assemblage is widely preserved, although amphi-
bolite facies metadoleritic domains are common. In places, the metamorphic hornblende – plagio-
clase assemblage is replaced by garnet and clinopyroxene along fractures. We interpret the
two metamorphic assemblages as formed during one event of static metamorphism in which am-
phibolite facies metamorphism was followed by dehydration, possibly in response to decompres-
sion. Based on their field occurrence and petrog- raphy we correlate these mafic dykes with the
E – W
component of
the Paleoproterozoic
Kangaˆmiut dyke swarm Ramberg, 1949.
3. Geochemistry
At present, geochemical characterisation of the Aasivik gneisses is only at a reconnaissance stage.
Major element compositions of apperantly homo- geneous lithologies from the Aasivik and Ukaleq
gneiss complexes and the Umiatsiaasat granites are given in Table 1. Sample 430476 is a trond-
hjemite, while samples 430441, 430451 and 94- 0000 are granites. These granites are slightly
peraluminous and alkaline, and project along the
Table 1 Chemical compositions of representative lithologies from the Aasivik terrain determined by XRF at Geological Survey of Greenland
GEUS
a
430441 430451
430476 Sample c
430498 940000
940012 72.71
74.12 SiO
2
67.59 65.67
74.11 74.25
0.16 0.18
TiO
2
0.15 0.93
0.17 0.3
12.68 13.66
14.48 Al
2
O
3
13.87 14.55
18.37 0.79
Fe
2
O
3
0.5 0.47
0.31 1.45
0.35 3.83
0.73 1.14
0.76 FeO
0.73 0.84
0.02 MnO
0.022 0.02
0.02 0.07
0.02 MgO
0.29 0.47
1.25 0.56
0.33 0.48
1.01 3.27
3.27 1.17
1.79 2.25
CaO 2.87
Na
2
O 3.83
3.14 6.34
3.21 3.95
3.52 4.6
1.66 K
2
O 4.48
4.33 5.37
0.16 0.04
0.05 P
2
O
5
0.07 0.06
0.23 0.27
0.2 Volat
0.44 0.29
0.44 0.2
Sum 99.39
99.14 99.36
99.36 99.5
99.14
a
430441 = felsic gneiss cut by red veins, 430451 = charnockite, 430476 = trondhjemitic gneiss, 430498 = 2730 Ma post-kine- matic rapakivi granite, 940000 = red myrmekitic granite, 940012 = 2700 Ma post-kinematic granite.
1 – 2 kbar water saturated cotectic in the haplo- granite system. Petrographically they are sub-
solvus granites and the excess Al is expressed as garnet in the mode. The granites are thus formed
at high pressure. Compositionally they resemble the : 2800 Ma Ikattoq gneisses of the Akulleq
terrain in Godtha˚bsfjord McGregor et al., 1991.
4. Zircon geochronology
4
.
1
. Analytical technique and data assessment U – Th – Pb isotopic ratios and concentrations of
zircons were determined using SHRIMP I and were calibrated to the Australian National Uni-
versity standard zircon SL13 572 Ma;
206
Pb
238
U = 0.0928. Descriptions
of analytical
procedure and data assessment were given by Compston et al. 1984, Nutman 1994 and
Claoue´-Long et al. 1995. Comparative isotope dilution and SHRIMP analyses of zircons from
several well-preserved Proterozoic and Archaean samples Roddick and van Breemen, 1994; Ire-
land, 1995 demonstrate that SHRIMP
207
Pb
206
Pb ratios are accurate. As further cross-check on the accuracy of
207
Pb
206
Pb ratios obtained with SHRIMP, zir- cons from the Paleoproterozoic norite QGNG
were analyzed interspersed with some of the un- knowns. From isotope dilution thermal ionisation
analysis, different QGNG zircon fractions have
207
Pb
206
Pb ages of 1850 9 2 Ma C.M. Fanning, personal communication, 1995, and 1850 Ma
to as low as 1810 Ma T. Skjo¨ld, personal communication, 1996. QGNG analyses run with
samples presented here yielded a
207
Pb
206
Pb weighted mean age of 1863 9 18 Ma.
SHRIMP zircon geochronology results on nine samples are reported here. For some of them,
many zircon analyses were undertaken, whereas for others fewer were done. The results are suffi-
cient to demonstrate that there is an important early Archaean component to the crust in the
area, and has determined the age of late Archaean rocks and some metamorphic zircon growth with
precisions of 10 Ma 2s.
4
.
2
. Aasi6ik gneiss complex
4
.
2
.
1
.
94
-
0338
and
94
-
0000
94-0338 and 94-0000 are migmatitic gneisses. 94-0338 and 94-0000 yielded similar mixed zircon
populations. Brown prismatic grains are domi- nant, up to 200 mm long, commonly with pro-
nounced mm-scale euhedral zoning and locally metamict. In a few cases, the centres of these
grains consist of clearer, non-zoned zircon. These grains show deep embayments on prismatic faces
and rounding of pyramidal terminations. Less commonly, there are overgrowths of clear zircon
on their terminations. Also present are a few small, clear to brown oval to multifaceted grains.
The heavy mineral separates also contained mon- azites, which were not analyzed.
In 94-0338, three analyses of an oval grain gave close to concordant ages, with a
207
Pb
206
Pb mean age of 2712 9 12 Ma Fig. 4a. The dominant
poorly preserved prismatic grains of 94-0338 have moderate to high U content up to 750 ppm and
yielded mostly discordant ages with
207
Pb
206
Pb ages between 3200 and 3600 Ma. Several grains
have
207
Pb
206
Pb ages of 3600 Ma, one of which is concordant, within error. These grains
are interpreted to be early Archaean in age, and to have suffered variable loss of radiogenic Pb in
a zircon-growth event at 2700 Ma, also more recently. Three analyses with the oldest
207
Pb
206
Pb ages yield a mean age of 3596 9 9 Ma. However, given the disturbed nature of the zir-
cons, this is interpreted by us as a minimum age, but probably close to the true value.
In 94-0000, metamorphic overgrowths and a brown oval grain yielded close to concordant ages
with
207
Pb
206
Pb ages between 2628 9 70 and 2711 9 44 Ma 2s, with a mean value of
2688 9 12 Ma, indistinguishable at the 95 confidence level from the 2712 9 12 Ma age de-
termined on a metamorphic grain in 94-0338. Analyses of the dominant brown prismatic grains
yielded an array of older ages, of which two with the oldest
207
Pb
206
Pb ages of \ 3500 Ma are close to concordant Fig. 4b. This indicates an
important early Archaean component in this sample.
Fig. 4. U – Pb concordia diagrams for SHRIMP analysis of zircons from a 94-0338: Migmatitic gneiss from the Aasivik terrain. b 94-0000 red myrmekitic granitemigmatite bordering the Aasivik terrain. c 430476 Felsic orthogneiss from the Aasivik terrain, d
430475 Felsic orthogneiss from the Aasivik terrain.
4
.
2
.
2
.
430441
430441 is a banded gneiss migmatised by a fine network of red coloured granitic veins just north
of the E – W trending lineament separating the Aasivik and the Ukaleq gneiss terrains. It yielded
mostly pale yellow to clear prismatic grains up to 300 mm long with aspects ratios of 2 – 5 and have
well-developed mm-scale euhedral zoning. Pyrami- dal terminations are slightly rounded, and one has
a metamorphic overgrowth, too small to be ana- lyzed with the 40-mm spot used in the analytical
session. The grains are interpreted as a magmatic population, slightly modified during metamor-
phism. Amongst the 50 mounted hand-picked grains one was devoid of euhedral zoning, but
with a possible rind of new zircon, a few microns thick. Analyses of two of the dominant well-zoned
prismatic grains showed high U contents \ 1000 ppm, close to concordant ages, with a
207
Pb
206
Pb mean age of 2817 9 10 Ma, which is interpreted
as the timing of magmatic crystallisation of the granitic neosome Fig. 4b. Two analyses of the
non-zoned zircon gave a low U content B 100 ppm, close to concordant ages, with a
207
Pb
206
Pb mean age of 3784 9 18 Ma. This probably indi-
cates an early Archaean age of the banded gneiss protolith.
4
.
2
.
3
.
430476
and
430475
The zircons in 430476 and 430475 are similar to the main population in 94-0338, but are slightly
larger on average, and less strongly coloured. Six analyses of 430476 grains yielded a discordant
array on a concordia diagram, like the 94-0338 zircons Fig. 4c. In this case, no analyses yielded
concordant ages and the oldest
207
Pb
206
Pb age was 3559 9 48 2s. Five analyses of grains from
430475 yielded discordant points Fig. 4d, with the least discordant result having the oldest
207
Pb
206
Pb age of 3602 9 8 Ma 2s. As for samples 94-0000 and 94-0338 the moderate to high U
protolith zircons suffered Pb-loss in a late Ar- chaean, generating the observed discordant array
of data. These results are not sufficient to define accurate and precise ages, but they do indicate the
zircons are probably a strongly disturbed 3600 Ma population.
4
.
3
. Ukaleq gneiss complex
4
.
3
.
1
.
94
-
0006
The supracrustal rock 94-0006 is a garnet – bi- otite – feldspar – quartz gneiss interpreted as a
semipelitic metasediment. It gave a low yield of zircons. Dominant are equant to oval grains, B
100 mm across, strongly zoned and brown in colour. Some of these grains form twins. Also
present are prismatic and rounded anhedral grains up to 150 mm long. These are brown to yellow
and also generally strongly zoned. The termina- tions of these grains are rounded. The equant,
oval and twinned grains have on average higher U contents and lower ThU than the rounded pris-
matic grains. A reconnaissance study of sixteen grains was undertaken on this sample. All analy-
ses yielded close to concordant ages Fig. 6 with
207
Pb
206
Pb ages from 2530 9 46 – 2905 9 20 Ma 2s. The oval, equant and twinned grains yielded
207
Pb
206
Pb ages of up to 2726 9 24 Ma and if metamorphic in origin probably grew or were
recrystallised in several high-grade events. The rounded prismatic grains yielded the oldest ages
1-1, 8-1, 10-1, 11-1 in Table 2. If they are detrital in origin, their ages indicate that the sediment
must have been deposited in the late Archaean.
4
.
3
.
2
.
430451
430451 is massive granulite facies orthogneiss or charnockite just south of the E – W trending
lineament separating the Aasivik terrane and the Ukaleq gneiss complex and within tens of metres
from the contact to a Umiatsiaasat granite dome. It yielded a population very similar in morphol-
ogy to those in 430441 just north of the linea- ment, and within the Aasivik gneiss complex and
likewise is interpreted as a magmatic population, slightly modified during metamorphism. However,
neither inherited grains nor metamorphic over- growths were observed. Six analyses were under-
taken, all of which showed moderate to high U contents 100 – 987 ppm and yielded close to con-
cordant ages Fig. 5. Grain 3-1 Table 2 yielded the oldest age of 2877 9 42 Ma 2s and is most
discordant. However, from optical microscopy, there is no evidence it is an inherited core. With
no rejections, all analyses yielded a
207
Pb
206
Pb- weighted mean age of 2812 9 8 Ma, with analy-
sis 3-1 indistinguishable from the mean. Rejecting 3-1, and 6-1 interpreted as having undergone
slight ancient Pb-loss yielded an indistinguishable age of 2820 9 12 Ma. These results indicate the
magmatic crystallisation of 430451 shortly before 2800 Ma.
4
.
3
.
3
.
430455
440455 is a thin diorite sheet which form an early component in the gneisses hosting the Umi-
atsiaasat granites. It yielded a population of clean colourless to pale yellow prismatic grains, up to
200 mm long, with some faint euhedral zoning. Neither inherited cores nor metamorphic over-
growths were observed. The grains show only slight rounding from metamorphic corrosion.
Five grains were analyzed, which had low U content 59 – 154 ppm and close to concordant
ages Fig. 5d. All analyses yielded a
207
Pb
206
Pb- weighted mean age of 2701 9 16 Ma, interpreted
as the magmatic age of the sample.
4
.
4
. Tasersiaq gneiss complex A granulite facies gneiss 430428 from the Taser-
siaq gneiss complex yielded prismatic and more rarely equant colourless to very pale yellow zir-
cons, up to 250 mm long and mostly with low aspect ratios of 1 – 3. They are non-zoned or show
weak mm-scale euhedral zoning, more prominent towards, the exterior of the grains. Inclusions of
apatite and opaques are common. Pyramidal ter- minations are slightly rounded, and some pris-
matic faces are slightly embayed. There are no obvious overgrowths of metamorphic zircon.
Twelve grains were analyzed. They have low U contents 73 – 250 ppm, yielded concordant or
close to concordant ages Fig. 5a with a range in
207
Pb
206
Pb ages from 3226 9 36 Ma – 2971 9 41 2s. Four analyses 4-1, 8-1, 9-1 and 11-1
yielded a
207
Pb
206
Pb-weighted mean age of 3212 9 11 Ma. Five analyses 1-1, 2-1, 6-1, 7-1,
12-1 yielded a distinctly younger age of 3127 9 12 Ma, and the remaining three somewhat
younger ages. There are no obvious morphologi- cal differences between grains of different age, but
on average, the oldest ones have the lowest U
Table 2 SHRIMP U–Pb zircon analyses
Spot ThU
f206
206
Pb
238
U ratio
207
Pb
206
Pb ratio
207
Pb
206
Pb age disc
U ppm
94-0338
1.76 215
0.11 0.737 9 22
0.3202 9 42 3571 9 20
1-1 0.05
0.708 9 14 0.3297 9 14
1.88 3616 9 07
215 −
5 1-2
0.18 707
0.04 0.633 9 44
0.2723 9 09 3319 9 50
− 5
2-1 0.19
394 0.03
0.652 9 13 0.2928 9 07
3433 9 04 −
6 3-1
0.11 0.695 9 17
0.3220 9 14 0.46
3580 9 07 4-1
− 5
144 0.16
548 0.04
0.620 9 18 0.2822 9 13
3375 9 07 −
8 4-2
0.03 0.556 9 08
0.2439 9 05 5-1
3145 9 03 667
− 9
0.14 0.20
0.516 9 13 0.1883 9 22
1.76 2727 9 19
62 −
2 6-1
0.45 170
0.02 0.527 9 30
0.1859 9 17 2706 9 15
+ 1
6-2 0.01
6-3 0.474 9 21
89 0.1854 9 29
2702 9 25 −
7 1.24
0.01 0.636 9 11
0.3000 9 08 0.82
3470 9 04 365
− 9
7-1 0.14
638 0.06
0.608 9 09 0.2636 9 13
3269 9 08 −
6 8-1
0.07 0.603 9 21
0.3158 9 22 9-1
3550 9 11 164
− 14
0.81 B
0.01 0.559 9 10
0.2430 9 16 0.17
3140 9 11 568
− 9
10-1 0.31
366 0.09
0.621 9 18 0.3073 9 68
3507 9 34 −
11 11-1
0.01 0.668 9 15
0.3049 9 27 3495 9 14
− 6
12-1 479
0.23 0.06
0.677 9 15 0.3122 9 39
1.07 3532 9 20
13-1 −
6 342
0.19 435
0.02 0.626 9 11
0.2747 9 21 3333 9 12
− 6
14-1 0.05
0.666 9 15 0.2892 9 33
3413 9 18 −
4 15-1
443 0.28
0.03 0.629 9 13
0.2831 9 14 3380 9 08
− 7
0.62 748
16-1 94-0000
485 0.64
0.14 0.552 9 13
0.2027 9 36 2848 9 29
1-1 0.39
0.716 9 15 0.3134 9 15
3538 9 07 −
2 2-1
218 1.33
0.20 0.692 9 15
0.3164 9 18 1.89
3552 9 16 3-1
− 5
227 1.00
0.628 9 12 0.2785 9 09
4-1 3354 9 05
680 −
6 0.68
0.27 0.598 9 10
0.2745 9 10 0.61
3332 9 06 5-1
− 8
522 0.22
253 0.01
0.514 9 11 0.1773 9 38
2628 9 35 +
2 6-1
B 0.01
0.516 9 11 0.1838 9 08
0.55 2687 9 07
128 7-1
0.41 161
0.05 0.505 9 11
0.1831 9 14 2681 9 13
− 2
8-1 9-1
0.01 315
0.515 9 13 0.1791 9 17
2644 9 16 +
1 0.28
B 0.01
0.504 9 24 0.1864 9 25
0.41 2711 9 22
10-1 −
3 388
11-1 0.05
822 0.548 9 11
0.2400 9 07 3120 9 05
− 10
0.338
430475
0.17 0.531 9 14
1-1 0.2916 9 24
581 3426 9 13
− 20
0.26 0.06
0.616 9 18 0.3055 9 07
0.25 3498 9 04
289 −
12 2-1
0.61 210
0.12 0.717 9 18
0.3267 9 08 3602 9 04
− 3
3-1 0.59
183 0.06
0.648 9 17 0.3011 9 24
3476 9 12 −
7 4-1
0.11 0.633 9 15
0.2746 9 12 3333 9 07
− 5
0.31 200
5-1
430476
48 0.06
0.79 0.594 9 57
0.2712 9 13 3313 9 79
− 9
1-1 0.15
0.655 9 21 3-1
0.3177 9 49 88
3559 9 24 −
9 0.29
0.01 0.678 9 16
0.3086 9 09 0.17
3514 9 05 644
− 5
4-1 0.42
189 0.12
0.699 9 18 0.3170 9 20
3555 9 10 −
4 5-1
0.26 0.667 9 24
6-1 0.2604 9 66
60 3249 9 40
+ 1
0.07 0.18
0.550 9 14 0.2251 9 13
3018 9 09 −
6 0.11
7-1 204
430428
73 0.76
0.24 0.619 9 36
0.2418 9 54 3132 9 36
− 1
1-1 2-1
0.18 147
0.606 9 15 0.2417 9 15
3131 9 10 −
2 0.63
0.38 0.573 9 17
0.2187 9 29 0.54
2971 9 21 109
− 2
3-1
Table 2 Continued Spot
U ppm ThU
f206
206
Pb
238
U ratio
207
Pb
206
Pb ratio
207
Pb
206
Pb age disc
0.51 0.45
4-1 0.645 9 17
95 0.2566 9 13
3226 9 08 −
1 0.82
0.25 0.606 9 17
149 0.2450 9 23
6-1 3153 9 15
− 3
142 7-1
1.04 0.07
0.580 9 13 0.2386 9 14
3111 9 09 −
5 145
8-1 0.57
0.09 0.634 9 15
0.2524 9 33 3200 9 21
− 1
0.60 0.06
0.613 9 13 116
0.2523 9 18 9-1
3200 9 11 −
4 176
10-1 1.05
0.03 0.568 9 23
0.2287 9 15 3043 9 11
− 5
11-1 0.63
83 0.31
0.591 9 15 0.2521 9 20
3198 9 12 −
6 0.85
0.06 0.600 9 27
0.2421 9 32 3134 9 21
250 −
3 12-1
430441
0.13 0.03
0.574 9 14 1-1
0.1978 9 12 1452
2808 9 10 +
4 0.21
0.02 0.528 9 13
1969 0.1998 9 11
2-1 2825 9 09
− 3
58 3-1
0.67 1.00
0.814 9 27 0.3648 9 30
3770 9 13 +
2 0.90
0.38 0.793 9 32
0.3720 9 32 3800 9 13
3-2 −
1 77
430451
0.91 0.04
0.557 9 22 0.1978 9 23
2808 9 19 +
2 1-1
747 0.49
0.04 0.560 9 14
987 0.1993 9 08
2-1 2820 9 06
+ 2
282 3-1
0.55 0.14
0.516 9 14 0.2063 9 27
2877 9 21 −
7 0.39
0.17 0.557 9 18
100 0.1999 9 32
4-1 2825 9 26
+ 1
191 5-1
0.37 0.22
0.535 9 14 0.1993 9 25
2821 9 20 −
2 6-1
0.63 128
0.19 0.548 9 14
0.1957 9 10 2791 9 08
+ 1
430455 1.20
0.07 1-1
0.514 9 15 154
0.1842 9 20 2691 9 18
− 1
0.51 0.05
0.492 9 13 0.1822 9 35
2673 9 31 −
4 2-1
54 1.08
B 0.01
0.521 9 11 152
0.1860 9 12 3-1
2707 9 10 60
4-1 0.71
0.09 0.499 9 20
0.1853 9 48 2701 9 43
− 3
0.63 0.48
0.488 9 16 0.1859 9 46
2706 9 41 5-1
− 5
59
94-0006
0.44 0.25
0.561 9 25 0.1938 9 19
2775 9 16 +
3 1-1
213 0.17
0.19 0.520 9 15
787 0.1798 9 12
2-1 2651 9 11
+ 2
0.14 0.04
3-1 0.513 9 15
617 0.1729 9 39
2586 9 38 +
3 0.42
0.21 0.545 9 20
466 0.2100 9 13
4-1 2905 9 10
− 3
860 5-1
0.20 0.03
0.508 9 15 0.1698 9 05
2556 9 05 +
4 0.15
0.11 0.507 9 15
6-1 0.1777 9 26
690 2628 9 24
+ 1
0.16 0.17
0.506 9 16 488
0.1672 9 23 7-1
2530 9 23 +
4 182
8-1 0.49
0.06 0.564 9 20
0.2083 9 13 2892 9 10
9-1 0.20
194 0.21
0.511 9 16 0.1882 9 13
2726 9 12 −
2 0.12
0.14 0.516 9 15
501 0.1824 9 10
9-2 2675 9 09
461 10-1
0.81 0.25
0.571 9 16 0.1979 9 109
2809 9 93 +
4 0.31
0.13 0.555 9 16
11-1 0.1981 9 28
366 2811 9 23
+ 1
0.17 0.21
0.489 9 14 501
0.1737 9 23 12-1
2593 9 22 −
1 643
13-1 0.22
0.11 0.500 9 14
0.1691 9 27 2549 9 26
+ 2
0.16 14-1
0.12 674
0.513 9 14 0.1795 9 26
2649 9 24 +
1 0.24
1.00 0.530 9 16
324 0.1911 9 39
15-1 2751 9 33
574 16-1
0.15 0.04
0.505 9 16 0.1720 9 19
2577 9 19 +
2 0.14
16-2 0.08
614 0.491 9 14
0.1793 9 19 2647 9 18
− 3
content. The rock is interpreted as having two age components of 3212 9 11 Ma and 3127 9
12 Ma, followed by subsequent disturbance. From the zircon data alone, it is uncertain
whether the ages represent different phases in a composite sample or whether the protolith is
3127 9 12 Ma, but carries 3212 9 11 Ma in- herited material.
5. Discussion
