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.