1987; granitoids and gneisses, Kinny, 1986. Again, the amount of zircon grown this way is
small relative to that grown from granitic magmas.
Suites of tonalite – trondhjemite – diorite – gran- odiorite are the most abundant rocks in the Ar-
chaean geological record. They are most likely dominated by partial melting products of hy-
drated mafic crust, which was heated when de- pressed into the mantle at Archaean convergent
plate boundaries e.g. Martin, 1986. Sr and Nd isotopic tracers show that these rocks are domi-
nated by juvenile additions to the crust e.g. Moorbath et al., 1977; Bennett et al., 1993; Moor-
bath et al., 1997. Extensive experimental petrol- ogy studies of tonalite and diorite compositions
Rapp, 1997; Wyllie et al., 1997 for reviews, suggest that the observed whole rock composi-
tions in Archaean tonalite – trondhjemite – diorite – granodiorite suites are close to that of precursor
magmas, rather than migmatites of magma + abundant coexisting restite. Experimental studies
e.g. Rapp, 1997; Wyllie et al., 1997 also showed that these magmas are hot, with temperatures of
]
850°C. Archaean tonalitic and dioritic rocks typically have Zr abundances of ] 200 ppm e.g.
Wedepohl et al., 1991. Experimental studies of Zr solubility of granitic, sensu lato melts using a wide
range of compositions, pressures and tempera- tures have been reported Watson and Harrison,
1983. These studies show that if the Archaean tonalitic and dioritic rocks were close to melting
compositions, then at ] 850°C solubility of Zr in those melts would be ] 400 – 500 ppm. This is
considerably higher than the B 200 ppm Zr present in these rocks. Thus the precursor melts
were probably strongly undersaturated in Zr, and any zircon entrained in them would have had a
strong potential to dissolve. Thus, it can be ar- gued that zircons from these rocks are predomi-
nantly magmatic in origin. Zircons from these sources probably dominate early Archaean detri-
tal suites, and consequently their detrital age spec- tra can be interpreted as predominantly but not
exclusively controlled by the production of mag- matic zircon in granitoids that are juvenile addi-
tions to the crust, rather than remelted older crust.
3. SHRIMP U – Pb zircon geochronology
Following coarse crushing and pulverisation, standard heavy liquid and isodynamic techniques
were used to produce zircon concentrates. Min- eral separation was undertaken at the Research
School of Earth Sciences, the Australian National University. The separates were then hand-picked
using a binocular microscope, to produce a varied assortment least metamict and damaged grains for
analysis. Together with chips of standard zircon, these were cast into epoxy resin discs and pol-
ished. Assessment of grains and choice of sites for analysis was based on transmittedreflected light
imagery.
U – Th – Pb isotopic ratios and concentrations were
determined in
zircon separates
using SHRIMP I and II and were referenced to the
Australian National University standard zircon SL13 572 Ma;
206
Pb
238
U = 0.0928. Descriptions of analytical procedure and data assessment are
given by Compston et al. 1984, 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; Ireland, 1995 demonstrate that SHRIMP
207
Pb
206
Pb ratios are accurate within the stated errors. The decay con-
stants and present-day
238
U
235
U value given by Steiger and Ja¨ger 1977 were used to calculate the
ages. A SHRIMP produces a U – Pb zircon analysis
in 10 – 20 min. The rapidity of analysis combined with adequate precision, measurement of concor-
dancy and ability to correct for common Pb from measurement of
204
Pb makes SHRIMP an ideal tool to collect age data on detrital zircon popula-
tions, where large numbers of analyses are required.
4. Early Archaean sediments from Greenland and Labrador
4
.
1
. Geological setting Part of the Nain Complex of the northern coast
of Labrador contains early Archaean gneisses see
Schiøtte and Bridgwater, 1991 for summary. These rocks have been strongly modified by sev-
eral early and late Archaean tectonothermal events. The late Archaean events include wide-
spread granulite facies metamorphism and abun- dant intrusion of granitoids Collerson et al.,
1982; Schiøtte et al., 1989a. The early Archaean of the Nain complex is dominated by the Uivak
Gneisses, whose protoliths consist of dioritic to granitic rocks of different ages and origins
Collerson, 1983; Schiøtte et al., 1989a. Within the Uivak Gneisses there are inclusions and inter-
calations of the Nulliak assemblage, which is a diverse suite of \ 3600 Ma metasedimentary,
mafic and ultramafic rocks Nutman et al., 1989; Schiøtte et al., 1989b; Nutman and Collerson,
1991. A SHRIMP U – Pb zircon age of 3776 9 8 Ma 2s was obtained on a Nulliak assemblage
metavolcanic rock Schiøtte et al., 1989b. The sediment 83187 whose detrital zircon ages were
reported by Nutman and Collerson 1991 has a deposition age of ] 3600 Ma and possibly ca.
3800 Ma.
The early Archaean history of the Itsaq Gneiss Complex of southern West Greenland is as varied
and protracted as that of the neighbouring Nain Complex in Labrador Nutman et al., 1996 and
references therein. The informal name Itsaq Gneiss Complex was introduced by Nutman et al.
1996 to include all early Archaean rocks of the Godtha˚bsfjord
area Amıˆtsoq
gneisses, Isua
supracrustal belt and akilia association, see Mc- Gregor, 1973; McGregor and Mason, 1977; Mc-
Gregor et al., 1991, in order to emphasise that there are many groups of unrelated early Ar-
chaean rocks formed over a 300 million year period.
Supracrustal rocks, metagabbros and ultramafic rocks comprise B 10 of the Itsaq Gneiss Com-
plex and range in age from ] 3850 to 3600 Ma Nutman et al., 1996, 1997a,b. Bodies of mafic
rocks with layers of banded iron formation BIF and metachert associated with ultramafic rocks
are the main lithologies McGregor and Mason, 1977; Nutman et al., 1996. Units of supracrustal
rocks occur as rafts and tectonic intercalations in dioritic, tonalitic and granitic gneisses which form
\
90 of the complex Nutman et al., 1996 and references therein. Supracrustal units range in
size from the 30 km long Isua supracrustal belt in the north to bodies ] 100 m across scattered
throughout the complex. The well-known Isua supracrustal belt contains volcanic and sedimen-
tary rocks that are both ca. 3710 and 3800 Ma old Nutman et al., 1996, 1997a, and elsewhere in the
complex some chemical sediments are ] 3850 Ma old Nutman et al., 1997b. Only in the youngest
supracrustal sequences of the Itsaq Gneiss Com- plex 3600 Ma from Ameralik fjord, 150 km
south of the Isua supracrustal belt are detrital sediments derived from mixed-provenance clastic
sources an important lithology Nutman et al., 1996.
4
.
2
. ]
3790
Ma metasediments, West Greenland The \ 3790 Ma package dominated by amphi-
bolites forming the southern side of the Isua supracrustal belt contains rare detrital quartzites
Nutman et al., 1997a, on which data from two samples G9325 and MR81318 are used in this
paper Table 1. Both samples are found in associ- ation with mafic to felsic volcanic rocks, and are
not part of a thick detrital sedimentary sequence. The nature of the provenance region is unknown,
but as shown below it clearly contained pre-3800 Ma volcanism quartzofeldspathic rocks. A unit of
biotite 9 garnet bearing siliceous rock quartzite? MR81318 from the southern central part of the
belt yielded approximately 100 zircons. All the grains are very small typically 30 – 50 mm across
and are both equant and stubby-prismatic in habit, but somewhat rounded Nutman and
Collerson, 1991. These zircons have suffered vari- able loss of radiogenic Pb and also show a few
2696 9 6 Ma metamorphic overgrowths. Nonethe- less, there are clearly two ] 3800 Ma groups
present, which yielded weighted mean ages of 3808 9 5 Ma and 3847 9 10 Ma. Another mica-
quartzite unit occurs in the southwest of the belt sample G9325, Nutman et al., 1997a. This unit
is : 1 m wide, and is bounded to the south by a thick, partly carbonated, ultramafic schist unit
and to the north by garnetiferous mafic schist, metamylonite and felsic schists. G9325 gave a
rather low yield of zircons, only slightly larger in
A .P
. Nutman
Precambrian
Research
105 2001
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114
99 Table 1
207
Pb
206
Pb 9 1 s detrital zircon age data, after filtering according to methods described in text
a
Deposited 3700–3800Ma Deposited 3500–3600 Ma
G9325G Isua supra. CF89-26C Huangbaiyu
Nulliak 83187L S. of G9354G S. of Isua
VM9010G S. of PDK quartziteG N. of
ameralik quartzite st. johns hbr. Quartzite
ameralik graphite sch. paragneiss
Belt quartzite quartzite
VS VS
? V
V VS
3714 9 13 3851 9 4
3890 9 12 3845 9 25
3897 9 6 3859 9 3
3859 9 6 3843 9 15
3828 9 17 3713 9 6
3876 9 32 3879 9 9
3859 9 8 3832 9 11
3820 9 12 3707 9 58
3876 9 32 3837 9 16
3804 9 13 3822 9 9
3697 9 19 3872 9 19
3818 9 15 3859 9 6
3803 9 14 3821 9 12
3870 9 25 3804 9 13
3859 9 6 3677 9 22
3866 9 12 3820 9 6
3793 9 10 3859 9 12
3676 9 23 3794 9 10
3795 9 16 3859 9 20
3757 9 26 3673 9 10
3790 9 30 3865 9 10
3767 9 27 MR81318G Isua
3793 9 18 3745 9 42
3671 9 14 3862 9 19
Supra. Belt quartziteV 3670 9 8
3763 9 38 3736 9 23
3792 9 6 3858 9 6
3788 9 10 3704 9 14
3665 9 24 3689 9 15
3858 9 6 3684 9 8
3784 9 48 3665 9 54
3866 9 8 G9155G N. of
3857 9 12 ameralik graphite
sch.VS 3852 9 7
3778 9 8 3664 9 12
3857 9 14 3678 9 13
3675 9 11 3756 9 9
3857 9 9 3847 9 3
3664 9 18 3670 9 13
3752 9 4 3841 9 6
3856 9 13 3776 9 7
3653 9 73 3652 9 17
3745 9 15 3670 9 12
3762 9 9 3853 9 12
3831 9 6 3729 9 11
3816 9 7 3761 9 7
3649 9 8 3669 9 13
3853 9 8 3664 9 10
3715 9 13 3812 9 5
3852 9 7 3759 9 8
3643 9 16 3651 9 14
3713 9 4 3811 9 6
3851 9 12 3758 9 15
3638 9 23 3650 9 10
3711 9 6 3631 9 8
3849 9 12 3750 9 8
3810 9 7 3650 9 9
3702 9 9 3808 9 8
3848 9 7 3745 9 9
3629 9 15 3805 9 5
3686 9 10 3847 9 22
3734 9 11 3645 9 9
3625 9 12 3680 9 6
3804 9 4 3726 9 10
3616 9 8 3641 9 12
3846 9 6 3597 9 10
3678 9 18 3803 9 6
3846 9 12 3709 9 8
3613 9 19 3675 9 13
3707 9 8 3843 9 9
3603 9 36 3798 9 6
3842 9 11 3674 9 7
3791 9 7 3601 9 25
3691 9 8 3838 9 12
GGU221122G N. of 3669 9 6
3689 9 8 ameralik graphite
sch.VS 3646 9 6
3688 9 14 3837 9 9
3637 9 11 3836 9 11
3685 9 12 3622 9 7
3832 9 16 3677 9 9
3615 9 10 3814 9 13
3614 9 8 3669 9 10
3615 9 17 3604 9 9
3664 9 9 3590 9 15
3811 9 12
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. Nutman
Precambrian
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Table 1 Continued Deposited 3500–3600 Ma
Deposited 3700–3800Ma G9354G S. of Isua
VM9010G S. of Nulliak 83187L S. of
G9325G Isua supra. CF89-26C Huangbaiyu
PDK quartziteG N. of st. johns hbr. Quartzite
paragneiss ameralik quartzite
Belt quartzite ameralik graphite sch.
quartzite V
VS VS
VS V
? 3603 9 23
3649 9 10 3807 9 19
3551 9 12 3635 9 8
3622 9 9 3603 9 11
3601 9 9
a
Three lines heading each data set are, 1 sample number suffixes in parentheses are G, Greenland; L, Labrador; C, China; 2 locality and 3 lithology suffixes in parentheses give depositional environment — V, volcanic sequence, VS associated with volcanic rocks, but detrital sediments are more important and are likely
to be of broader provenance; ?, unknown.
size than those in quartzite? MR81318. A group at 3849 9 5 Ma is present, with possibly one older
grain at : 3900 Ma and a few at ca. 3810 Ma Nutman et al., 1997a.
4
.
3
. :
3710
Ma metasediments, West Greenland A : 3710 Ma package containing amphibo-
lites, felsic to intermediate schists and chemical sediments forms the northern side of the Isua
supracrustal belt Nutman et al., 1997a. Zirconif- erous rocks in this package are rare. Felsic vol-
canic rocks contain : 3710 Ma magmatic zircon and rare kyanite schists and some of the siliceous
metachert? units also contain : 3710 Ma zir- cons Nutman et al., 1996, 1997a and unpublished
data. So far, older pre-3710 Ma detrital zircons have not been detected in this package of the Isua
supracrustal belt. However, : 15 km to the south of the Isua supracrustal belt, there is a unit B
200 m thick of lithologically similar supracrustal rocks, interleaved with early Archaean tonalitic
gneisses Nutman, Bennett and Friend, unpub- lished
field observations
and U – Pb
zircon SHRIMP geochronology. Felsic schists in this
unit interpreted to be derived from intermediate volcanic rocks contain no protolith zircon. How-
ever, a thin lens of garnet + biotite 9 sillimanite paragneiss G9354 of likely volcanic or sedimen-
tary origin on the southern margin of the unit 64° 58 40N 50° 9W did yield zircons with early
Archaean ages. This sample belongs to a vol- canosedimentary sequence possibly arc-related?,
thus the provenance of the grains in it could be quite local.
The garnet + biotite 9 sillimanite
paragneiss G9354 yielded ca. 30 zircons. Dominant are
small 5 75 mm equantmultifaceted to anhedral grains, with high U and low ThU values Table
2. Also present are prismatic grains, with rare overgrowths. The multifaceted to anhedral grains
are generally structureless, whereas the prismatic to anhedral grains commonly display micron scale
oscillatory zoning partly obliterated and cut across by domains of featureless recrystallised zir-
con. The equantmultifaceted to anhedral grains yielded late to middle Archaean ages, with two or
three groups apparent Table 2, Fig. 2. The
Fig. 2.
206
Pb
238
U –
207
Pb
235
U concordia diagrams analytical errors depicted at the 1s level and unfiltered
207
Pb
206
Pb age histograms for zircons analysed from metasediments G9354
and VM9010.
youngest group, including analysis 18.2 of a rare rim, has a weighted mean
207
Pb
206
Pb age of 2682 9 4 Ma whereas the oldest group has an age
of 2961 9 11 Ma. In addition, two analyses 22.1 and 28.1 yield a mean age of 2827 9 10 Ma and
may represent another group. These grains are interpreted to have grown in situ during middle to
late Archaean thermal events. Non-zoned irregu- larglobular twinned grain 6 6.1 and 6.2, Table 2
yielded ages of ca. 3600 Ma. Due to its morphol- ogy, this grain is also likely to have grown in situ.
Many sites on the prismatic grains yielded concor- dant ages between 3500 and 3850 Ma. As these
are likely to be volcanic or sedimentary in origin, multiple analyses were undertaken on several of
these grains, in order to try and establish their true age. Multiple analytical sites on micron-scale
zoned prismatic grains such as five and grain 13
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. Nutman
Precambrian
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105 2001
93 –
114
Table 2 Summary SHRIMP U–Pb zircondata for metasediments G9354 and VM9010
a
Grain type Comm.
206
Pb
238
U
206
Pb ratio
235
U
207
Pb ratio
207
Pb
206
Pb ratio 207206 Age
Disc U ppm
Th ppm Labels
ThU G
93 54
garnet+biotite paragneiss, south of Isua supracrustal belt 0.686 9 0.021
1.1 29.496 9 0.979
tw 0.3119 9 0.0027
3530 9 13 −
5 260
79 0.31
0.26 0.579 9 0.016
19.284 9 0.560 0.2416 9 0.0012
3131 9 8 −
6 0.01
tw 1.2
0.09 38
422 0.06
eq 0.498 9 0.016
12.599 9 0.412 0.1835 9 0.0012
2685 9 11 −
3 1251
18 0.01
2.1 0.01
0.00 0.515 9 0.011
12.974 9 0.270 0.1829 9 0.0004
2679 9 4 eq
2.2 980
11 0.708 9 0.020
32.102 9 1.008 0.3291 9 0.0030
3613 9 14 −
5 0.08
343 0.72
3.1 p
475 0.01
p 0.708 9 0.015
30.501 9 0.666 0.3126 9 0.0011
3534 9 5 −
2 410
164 0.40
3.2 0.07
eq 0.573 9 0.016
17.126 9 0.491 0.2168 9 0.0011
2957 9 9 −
1 1232
10 0.01
4.1 0.555 9 0.012
16.023 9 0.362 0.2092 9 0.0011
2900 9 8 −
2 0.05
4.2 0.01
12 1247
eq 0.54
0.34 0.737 9 0.022
35.122 9 1.220 0.3456 9 0.0046
3688 9 20 −
4 p,f
5.1 214
116 0.688 9 0.015
29.966 9 0.733 0.3159 9 0.00217
3550 9 13 −
5 0.05
p,f 5.2
0.40 94
234 0.08
p,f 0.738 9 0.020
32.850 9 0.981 0.3228 9 0.0027
3583 9 13 −
1 211
71 0.34
5.3 0.31
p,f 0.744 9 0.048
31.900 9 2.208 0.3111 9 0.0052
3526 9 26 2
243 54
0.22 5.4
0.705 9 0.021 31.850 9 1.007
0.3278 9 0.0027 3607 9 13
− 5
0.21 6.1
0.67 128
193 m,anh
0.689 9 0.019 30.813 9 0.865
0.3244 9 0.0012 3591 9 6
− 6
6.2 m,anh
278 202
0.73 0.06
0.778 9 0.023 37.915 9 1.169
0.3536 9 0.0020 3722 9 9
0.31 92
0.64 7.1
m,p 145
0.08 e,p
0.785 9 0.018 38.407 9 0.923
0.3549 9 0.0019 3728 9 8
156 121
0.77 7.2
0.15 e,p
0.766 9 0.023 36.529 9 1.212
0.3459 9 0.0039 3689 9 17
− 1
278 82
0.29 8.1
0.776 9 0.018 38.393 9 0.935
0.3587 9 0.0013 3744 9 6
− 1
0.01 8.2
0.43 111
260 e,p
0.826 9 0.018 44.034 9 0.955
9.1 0.3868 9 0.0008
p 3859 9 3
1 252
142 0.56
0.03 0.776 9 0.017
40.860 9 1.066 0.3819 9 0.0042
3839 9 17 −
4 0.02
227 0.54
9.2 p
417 0.18
m,p,f 0.797 9 0.021
38.544 9 1.021 0.3508 9 0.0013
3710 9 6 2
266 318
1.19 10.1
0.887 9 0.049 42.907 9 2.798
0.3508 9 0.0097 3711 9 43
10 10.2
m,p,f 224
195 0.87
0.00 0.639 9 0.014
25.136 9 0.576 0.2853 9 0.0009
3392 9 5 −
6 0.01
eq 11.1
0.30 113
377 0.01
e,p 0.583 9 0.012
17.279 9 0.413 0.2149 9 0.0021
2943 9 16 1
851 8
0.01 12.1
0.04 c,p
0.696 9 0.016 29.505 9 0.781
0.3075 9 0.0031 3509 9 16
− 3
182 67
0.37 13.1
0.767 9 0.020 37.298 9 1.011
0.3527 9 0.0017 3719 9 7
− 1
0.01 94
0.40 13.2
c,p 236
0.01 c,p
0.762 9 0.031 37.106 9 1.523
0.3530 9 0.0015 3720 9 6
− 2
296 144
0.49 13.3
0.24 r+c,p
0.701 9 0.048 24.551 9 1.773
0.2539 9 0.0036 3209 9 23
7 1100
220 0.20
13.4 0.650 9 0.015
23.804 9 0.566 0.2656 9 0.0010
3280 9 6 −
2 0.01
14.1 0.21
108 515
p 0.01
m,anh,f 0.581 9 0.013
17.525 9 0.431 0.2189 9 0.0012
2972 9 9 −
1 898
9 0.01
15.1 0.623 9 0.016
24.169 9 0.625 0.2812 9 0.0011
3370 9 6 −
7 0.26
m,p 16.1
0.30 96
320 0.01
m,p,f 0.778 9 0.019
37.485 9 0.970 0.3493 9 0.0026
3704 9 12 229
80 0.35
17.1 0.18
c,eqan 0.722 9 0.015
31.789 9 0.702 0.3194 9 0.0013
3567 9 6 −
2 324
274 0.84
18.1 0.507 9 0.024
12.499 9 0.651 0.1787 9 0.0031
2641 9 29 0.39
18.2 0.02
20 1029
r,eqan 0.538 9 0.052
16.017 9 1.613 0.2159 9 0.0041
2950 9 31 −
6 19.1
eq 903
14 0.02
0.02 0.517 9 0.018
13.053 9 0.458 0.1833 9 0.0004
2683 9 4 0.01
13 0.01
20.1 m,eq
1206 0.01
eqan 0.725 9 0.067
27.287 9 2.551 0.2728 9 0.0017
3322 9 10 6
857 226
0.26 21.1
0.00 eqan
0.528 9 0.015 14.631 9 0.465
0.2012 9 0.0025 2836 9 20
− 4
733 9
0.01 22.1
0.766 9 0.075 33.624 9 3.375
0.3185 9 0.0036 3562 9 17
3 0.04
23.1 rexe,panh
0.57 114
200
A .P
. Nutman
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105 2001
93 –
114
103 Table 2 Continued
ThU Comm.
206
Pb
238
U
206
Pb ratio
235
U
207
Pb ratio
207
Pb
206
Pb ratio 207206 Age
Disc U ppm
Labels Th ppm
Grain type 0.534 9 0.014
13.494 9 0.353 24.1
0.1834 9 0.0007 m,eq
2684 9 7 3
1836 25
0.01 0.01
0.521 9 0.030 13.267 9 0.780
0.1848 9 0.0014 2696 9 13
0.01 m,eq
25.1 0.01
20 1574
0.23 m,p
0.612 9 0.055 25.331 9 2.319
0.3004 9 0.0033 3472 9 17
− 11
187 50
0.27 26.1
m,pan 0.02
0.598 9 0.013 21.116 9 0.457
0.2562 9 0.0009 3223 9 6
− 6
729 83
27.1 0.11
0.538 9 0.012 14.844 9 0.340
0.2001 9 0.0006 2827 9 5
− 2
0.01 28.1
18 969
e,ptw 0.02
VM
90 10
graphitic garnet quartzite, south coast of Ameralik 1.03
0.11 O.747 9 0.036
34.438 9 1.786 0.3345 9 0.0050
3638 9 23 −
1 99
1.1 103
mnpro 0.63
0.14 0.716 9 0.039
32.863 9 1.840 0.3327 9 0.0032
3629 9 15 −
4 118
2.1 74
m,p 0.723 9 0.026
32.320 9 1.205 0.3242 9 0.0017
3590 9 8 −
2 0.09
3.1 0.61
115 189
m,eq 0.55
0.12 0.789 9 0.049
36.098 9 2.305 0.3316 9 0.0026
3625 9 12 4
m,pro 4.1
145 79
0.545 9 0.039 13.958 9 1.024
0.1859 9 0.0013 2706 9 12
4 0.10
eq 5.1
0.17 38
225 0.02
m,pro 0.766 9 0.034
35.569 9 1.609 0.3369 9 0.0018
3649 9 8 1
836 566
0.68 6.1
0.08 m,pro
0.784 9 0.028 36.941 9 1.404
0.3418 9 0.0032 3671 9 14
2 482
257 0.53
7.1 0.781 9 0.026
36.949 9 1.389 0.3431 9 0.0050
3677 9 22 1
0.05 8.1
1.04 410
393 m,pro
0.773 9 0.035 36.501 9 1.697
0.3424 9 0.0023 3673 9 10
1 9.1
m,p 93
45 0.49
0.15 0.791 9 0.041
37.117 9 2.071 0.3404 9 0.0053
3665 9 24 3
0.03 242
1.01 10.1
m,p 239
0.08 m,eqtw
0.645 9 0.027 27.214 9 1.217
0.3060 9 0.0029 3501 9 15
− 8
55 19
0.35 11.1
0.05 m,pro
0.747 9 0.024 33.969 9 1.114
0.3297 9 0.0018 3616 9 8
− 1
174 114
0.66 12.1
0.757 9 0.026 35.798 9 1.399
0.3430 9 0.0051 3676 9 23
− 1
0.07 13.1
0.40 98
245 m,ro
0.51 0.18
0.737 9 0.059 32.779 9 2.747
0.3225 9 0.0052 3582 9 25
− 1
m,ro 14.1
169 86
0.774 9 0.050 37.353 9 2.939
0.3501 9 0.0131 3707 9 58
0.12 51
0.37 15.1
c,ro 140
0.18 r,ro
0.706 9 0.063 31.048 9 3.286
0.3189 9 0.0148 3564 9 73
− 3
188 27
0.14 15.2
0.12 eq
0.664 9 0.019 29.089 9 1.007
0.3177 9 0.0051 3559 9 25
− 8
384 47
0.12 16.1
0.780 9 0.022 37.832 9 1.166
0.3516 9 0.0030 3714 9 13
0.02 17.1
0.65 168
259 m,p
0.563 9 0.024 14.668 9 0.639
0.1890 9 0.0015 2734 9 13
5 18.1
eq 196
10 0.05
0.06 0.521 9 0.039
14.439 9 1.119 0.2011 9 0.0020
2835 9 16 −
5 0.04
26 0.14
19.1 r,eq
186 0.03
c,eqro 0.689 9 0.054
31.028 9 2.565 0.3265 9 0.0053
3601 9 25 −
6 193
81 0.42
19.2 0.08
m,ro 0.772 9 0.028
36.368 9 1.346 0.3416 9 0.0018
3670 9 8 1
183 186
1.02 20.1
0.784 9 0.024 37.984 9 1.203
0.3515 9 0.0014 3713 9 6
1 0.03
21.1 1.04
323 310
m,p 0.760 9 0.024
34.869 9 1.158 0.3330 9 0.0018
3631 9 8 22.1
m,pro 126
44 0.35
0.03 0.752 9 0.026
36.057 9 1.369 0.3476 9 0.0043
3697 9 19 −
2 0.08
m,pro 23.1
0.47 100
212 0.08
r,pro 0.698 9 0.031
30.886 9 1.576 0.3211 9 0.0067
3575 9 32 −
5 141
55 0.39
24.1 0.09
r,pro 0.682 9 0.015
27.830 9 0.649 0.2958 9 0.0011
3449 9 6 −
3 283
106 0.38
25.1 0.732 9 0.022
34.335 9 1.111 0.3402 9 0.0026
3664 9 12 −
3 0.04
25.2 0.64
64 101
C.D 0.510 9 0.016
12.884 9 0.454 0.1831 9 0.0023
2681 9 21 −
1 26.1
eqanh 386
135 0.35
0.10 0.497 9 0.011
12.805 9 0.286 0.1868 9 0.0008
2714 9 7 −
4 0.03
52 0.09
27.1 Eq
598 0.02
r,eqtw 0.690 9 0.016
29.977 9 0.775 0.3152 9 0.0023
3547 9 11 −
5 241
21 0.09
28.1 0.18
m,p 0.767 9 0.030
35.714 9 1.483 0.3376 9 0.0038
3652 9 17 1
126 61
0.49 29.1
0.733 9 0.024 34.389 9 1.253
0.3403 9 0.0039 3664 9 18
− 3
0.11 30.1
m,pro 0.62
59 95
A .P
. Nutman
Precambrian
Research
105 2001
93 –
114
Table 2 Continued
238
U
206
Pb ratio
235
U
207
Pb ratio
207
Pb
206
Pb ratio 207206 Age
Labels Disc
Grain type U ppm
Th ppm ThU
Comm.
206
Pb 0.740 9 0.026
34.726 9 1.837 0.3406 9 0.0119
3665 9 54 0.70
− 3
0.06 150
216 m,pro
31.1 0.680 9 0.015
28.856 9 0.734 0.3080 9 0.0028
32.1 3511 9 14
Eq −
5 465
52 0.11
0.07 0.064 9 0.019
28.015 9 0.869 0.3061 9 0.0021
3501 9 11 −
6 0.01
0.45 33.1
c?panh 223
100 0.714 9 0.016
31.786 9 0746 0.3227 9 0.0014
3583 9 7 −
3 33.2
r,panh 686
237 0.34
0.02 0.703 9 0.041
32.520 9 1.965 0.3357 9 0.0034
3643 9 16 −
6 0.08
34.1 0.43
132 305
e,p 0.688 9 0.026
28.384 9 1.097 0.2991 9 0.0022
3466 9 11 −
3 35.1
r,p 262
29 0.11
0.09 0.746 9 0.066
33.668 9 3.165 0.3273 9 0.0076
3605 9 36 0.03
c,p 35.2
0.57 53
94 0.05
r,p 0.690 9 0.017
28.724 9 0.772 0.3021 9 0.0029
3481 9 15 −
3 297
34 0.11
35.3 0.05
r,p 0.703 9 0.041
30.865 9 1.876 0.3184 9 0.0031
3562 9 15 −
4 433
210 0.49
36.1 0.568 9 0.033
19.336 9 1.287 0.2470 9 0.0061
3166 9 40 −
8 0.07
37.1 0.05
16 290
r,p 0.36
0.02 0.702 9 0.050
32.660 9 2.928 0.3376 9 0.0158
3652 9 73 −
6 c,p
37.2 241
87 0.520 9 0.012
15.044 9 0.369 0.2097 9 0.0013
2903 9 10 −
7 0.01
0.06 38.1
Eq 230
14 0.781 9 0.030
39.1 35.434 9 1.483
m,pro 0.3291 9 0.0040
3613 9 19 3
209 136
0.65 0.07
a
p, Prismatic; eq, equant; e, end; m, middle; r, overgrnwth; c, core; rex, rccrystallised; osr, euhedral finescale zoning; h, homogeneous; f, fragment; anh, anhedral; turb, turbid; ro, rounded comm.
206
Pb is percent of
206
Pb that is common, disc is discordance of ages in percent. Errors are 1s corrected with 3600 Ma model Pb of Cumming and Richards 1975.
Table 2; excluding composite core – rim analysis 13.4 yielded ages between 3700 and 3500 Ma,
whereas with other grains duplicate analyses yielded consistent ages e.g. grains 9 and 10. In
other cases, recrystallised domains within pris- matic grains yielded ages of 3500 – 3600 Ma e.g.
23.1, Table 2. In addition, some other sites yielded
207
Pb
206
Pb ages intermediate between 3000 and 3500 Ma. These are generally either
higher U than the \ 3500 Ma sites or discordant. Combined with ages of ca. 3600 Ma obtained on
globulartwinned grain 6, these results are inter- preted to indicate that the prismatic zircons have
an age of ] 3700 Ma, and that these grains were subjected to Pb-loss and local recrystallisation in
thermal events between 3600 – 3500 Ma events well-known in the region; e.g. Nutman et al.,
1996. The ten best analytical sites on 3700 – 3750 grains yielded a weighted mean
207
Pb
206
Pb age of 3722 9 5 Ma MSWD = 3.3. The high MSWD
linked with apparent age differences between grains, suggest that grains ranging in age from ca.
3700 to 3740 Ma are present. In addition, two age determinations on grain 9 yielded ages of ca. 3840
Ma. The 3700 – 3740 Ma and ca. 3840 Ma grains are interpreted to be of volcano-sedimentary
origin, giving a maximum age of deposition of ca. 3700 Ma. Best estimates on the ages of detrital
grains are summarised in Table 1.
4
.
4
.
3500
–
3600
Ma metasediments, West Greenland
Older sedimentary rocks of the 3700 – 3800 Ma Isua supracrustal belt and adjacent gneisses have
been discussed in most detail see Nutman et al., 1996 and references therein. A less studied part
of the Itsaq Gneiss Complex is on the north side of Ameralik fjord, where recent fieldwork and
SHRIMP geochronology have shown that 3650 – 3600 Ma gneisses and granites as young as 3570
Ma are the dominant lithologies Nutman et al., 1996. This unit may also crop out locally on the
south side of the fjord. In this part of the complex are extensive units of graphite-bearing metasedi-
ment, pelite, calc-silicate rocks, fuchsitic quartzite and amphibolites, some of which are intruded by
the : 3570 Ma granites Nutman et al., 1996 — data from several samples of these sediments are
used in
this paper
G9155, VM9010,
GGU221122, PDK quartzite, Table 1. The depo- sitional environment of these diverse supracrustal
rocks is not certain, due to the reconnaissance nature of studies in this part of the Itsaq Gneiss
Complex. However, the more widespread occur- rence of detrital sediments, including fuchsite
quartzites suggests a closer association with sialic crust when deposited such as an amalgam of arc
complexes formed in the period 3850 – 3600 Ma. As such, the detrital populations in these rocks
might provide a better sampling of sialic crust than the 3700 – 3800 Ma sediments discussed
above. Dating of zircons in a graphite-bearing metasediment and a fuchsite-bearing quartzite
found that the youngest concordant analyses of the detrital population have an age of : 3600 Ma
Kinny et al., 1988; Nutman et al., 1996. Hence the age of deposition of these sediments is : 3600
Ma, considerably younger than the ca. 3710 and 3800 Ma volcanic and sedimentary rocks of the
Isua supracrustal belt and adjacent areas Nut- man et al., 1996, 1997a. Age spectra for these
sediments show that grains with ages of 3600 – 3750 Ma are most abundant, with a lesser number
with ages of 3750 – 3900 Ma.
Zircon age data from an additional sample of graphite-bearing garnet quartzite VM9010 are
presented here. VM9010 is from the southern shore of outer Ameralik. It is from a tabular body
of metasediments no more than 100 m thick, broken-up by heterogeneous granitic gneisses. It
yielded a large number of zircons, mostly of rounded prismatic to ovoid shape. Most analyses
yielded close to concordant ages Table 2, Fig. 2. Many of the grains partial or complete over-
growths, interpreted to have grown during in situ metamorphism. Eight ‘old’ overgrowths yielded a
weighted mean
207
Pb
206
Pb age of 3577 9 9 Ma MSWD = 1.8 and 3 others yielded a weighted
mean
207
Pb
206
Pb age
of 3455 9 10
Ma MSWD = 2.6. The 3577 9 9 Ma age of meta-
morphic overgrowths gives the minimum deposi- tional age of the sediment, and agrees well with
the 3567 9 6 Ma age obtained on granites intrud- ing similar sediments north of Ameralik Nutman
et al., 1996. Younger overgrowths are also
present, such as a group with a has a weighted mean
207
Pb
206
Pb age
of 2713 9 11
Ma MSWD = 1.8, indicating metamorphism of the
rock in the late Archaean. The analysed n = 25 cores and whole grains of rounded prismatic and
rounded morphology are interpreted to be detrital in origin, and yield
207
Pb
206
Pb ages between ca. 3600 and 3715 Ma; no more ancient grains were
detected.
4
.
5
. ]
3600
Ma metasediments, northern Labrador
As in western Greenland, early Archaean quartzites and schists of likely detrital sedimen-
tary origin are rare, with most metasediments of the early Archaean Nulliak assemblage being of
chemical origin e.g. Nutman et al., 1989. Nulliak assemblage quartzite 83187 was collected from a
discontinous, heterogeneous supracrustal unit, : 2 km south of St. John’s harbour, southern side
of Saglek Bay. Unlike the sediments reported from western Greenland, there is less U – Pb zir-
con age constraint on the age of this supracrustal unit, beyond it is known to occur as an integral
part of an early Archaean ] 3600 Ma complex e.g. Schiøtte et al., 1989a,b and references
therein. The depositional setting of this unit is uncertain. However, the general paucity of detri-
tal sedimentary material and abundance of am- phibolites derived from volcanic rocks suggest
that it belongs to a volcanosedimentary sequence. In which case the provenance of detrital grains is
likely to be local rather than broad. Zircon results on 83187 were reported by Nutman and Coller-
son 1991, but are summarised here Table 1. The sample yielded abundant grains up to 300 mm
across, and often consists of thin rims over cor- roded cores. Most sites with
207
Pb
206
Pb ages of ]
3600 Ma and ca. 2760 Ma plot close to concor- dia. Most of the youngest analyses 8 are of
overgrowths and yield a weighted mean
207
Pb
206
Pb age of 2760 9 6 Ma MSWD = 2.5 and are interpreted to have grown in situ during late
Archaean regional metamorphisms. For the ] 3600 Ma analyses, 6 grains have
207
Pb
206
Pb ages of ca. 3800 – 3850 Ma. Interpretation of grains
with
207
Pb
206
Pb ages of 3600 – 3800 Ma is prob- lematic. The preferred interpretation of Nutman
and Collerson 1991 was that these grains are ]
3800 Ma old, and had lost some radiogenic Pb during early Archaean thermal events. Alterna-
tively, the sediment could be as young as ca. 3600 Ma, the age of the youngest early Archaean con-
cordant zircon analyses. Analyses with
207
Pb
206
Pb ages of B 3600 Ma are discordant and are inter-
preted as ] 3600 Ma zircon that underwent loss of radiogenic Pb in younger Precambrian events.
In compilation of age data later in this paper, this latter more conservative interpretation of the sedi-
ment being ca. 3600 Ma old is adopted.
5. Early Archaean sediments from China