SHRIMP U – Pb zircon geochronology Early Archaean sediments from Greenland and Labrador

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 93 – 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 A .P . Nutman Precambrian Research 105 2001 93 – 114 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 A .P . Nutman Precambrian Research 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 Precambrian Research 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