Early Archaean sediments from China

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

The Sino – Korean craton in eastern Hebei province, northeast China is dominated by middle to late Archaean gneiss complexes, but early Ar- chaean rocks have been found at two widely- spaced localities Liu et al., 1992; Song et al., 1996. In the Anshan area, : 500 km east north- east of Beijing, a ca. 3800 Ma rocks have been found in two places. The ca. 3800 Ma component is associated with ca. 3300 Ma granite and migmatite components. The oldest proven vol- canosedimentary rocks in the Anshan area belong to the Chentaigou supracrustals, have an age of 3362 9 5 Ma Song et al., 1996 and are devoid of ancient pre-volcanic zircons. As this paper con- centrates on \ 3500 Ma detrital sediments, these rocks are not discussed further here. Approximately 150 km to the east of Beijing in the Caozhuang area, small occurrences of fuch- site-bearing metaquartzite occur in association with some mica schists and mafic rocks, within polyphase high grade gneisses Liu et al., 1992 and references therein. Amphibolite facies supracrustal units on the slopes north of Huang- baiyu village include calc-silicate rocks interlay- ered with amphibolites of probably volcanic origin, metapelite, banded iron formation and fuchsitic metaquartzite. The percentage of sedi- mentary to volcanic rocks in this package is high \ 50? Thus there is a possibility that detrital grains, particularly in fuchsite quartzites, might be of broader provenance. Attempts have been made to find early Archaean basement rocks in the amphibolite and retrogressed granulite facies or- thogneisses adjacent to the quartzites. So far, all samples dated by SHRIMP U – Pb geochronol- ogy, appear to be strongly deformed late Ar- chaean granitoids Song and Nutman, unpublished data. Thus, presently these quartz- ites seem to be an isolated occurrence as early Archaean orthogneisses associated with them have not yet been detected nearby. SHRIMP U – Pb zircon results from fuchsitic metaquartzite sample CF8926 were reported by Liu et al. 1992 and are summarised here Table 1. The youngest detrital zircon in the metaquartz- ites is ca. 3550 Ma, interpreted to be closed in age to the deposition of the quartzite, estimated at 3500 9 80 and 3470 9 107 Ma from Sm – Nd iso- topic studies of associated amphibolites on nearby outcrops Huang et al., 1986; Jahn et al., 1987. The detrital zircons obtained from fuchsite metaquartzite mostly have 207 Pb 206 Pb ages be- tween 3600 and 3850 Ma Liu et al., 1992. The distribution of ages obtained fall into several ‘peaks’, which suggests that the geological history of the source region experienced several distinct magmaticthermal events in the 3600 – 3850 Ma period, rather than a continuum of activity.

6. Ages of zircons in early Archaean sediments