Numerous magmatic events between 3870 and 3625 Ma?
tic fragments were derived from metasomatically altered basaltic pillow lavas and tectonically dis-
rupted intrusive sheets of tonalite. In other parts of the felsic formation A6 of Nutman et al.
1984, Myers 2000 found large scale, along- strike
gradations between
quartzo-feldspathic schist and mylonite, and tonalitic gneiss.
Frei et al. 1999 discovered heterogeneity in late Archean metamorphic recrystallization of the
Isua greenstone belt by Pb stepwise leaching of magnetite from banded iron formations. They
recognized that the amphibolite facies metamor- phic mineral assemblages of banded iron forma-
tions in the northeastern part of the belt developed at 3.69 Ga. These rocks were cut by
magnetite and sulphide-bearing veins associated with metasomatic alteration at 3.63 Ga. In
contrast, in the western part of the Isua green- stone belt, step leaching of magnetite from simi-
lar banded iron formations yielded a PbPb isochron age of 2.84 Ma, interpreted as the age
of peak amphibolite facies metamorphism in this region.
Carbon isotopic ratios, considered to be of biogenic origin, were determined by Rosing
1999 from graphite in schistose rocks inter- preted as metagraywackes in the Isua greenstone
belt. These metasedimentary rocks were recog- nized in a lenticular zone of relatively low strain
within a thick sequence of deformed amphibolite schist, mainly derived from basaltic volcanic
rocks. This interpretation, and the \ 3700 Ma age of the graphite, remains unchallenged.
4. Numerous magmatic events between 3870 and 3625 Ma?
Nutman et al. 1993 proposed on the basis of SHRIMP UPb zircon data that ‘the Amıˆtsoq
gneisses comprise 3870, 3820 – 3810, 3760, 3730, 3700, and 3625 Ma TTG and 3660 – 3650 and
3625 Ma granites, and their inclusions belong to several supracrustal sequences with a similar
spread of ages’ ib. cit., p. 415. They concluded that ‘these results show that the complex grew by
episodic addition of new TTG and welding to- gether of rocks of different ages’ p. 415 and
proposed a plate tectonic scenario. This interpretation has been challenged in a
number of papers by Kamber, Moorbath and Whitehouse. Kamber and Moorbath 1998 dis-
cussed new and published Pb isotope data for Amıˆtsoq gneiss feldspars and whole rocks and
concluded ‘that the magmatic precursors of the Amıˆtsoq orthogneisses separated from a depleted
mantle-like source at ca. 3.65 Ga’ p. 19. They proposed that zircons described by Nutman et al.
1993, 1996 with ages significantly older than 3.65 Ga were inherited from older rocks. This
conclusion was supported by cathodolumines- cence imaging and ion-microprobe UThPb dat-
ing
of zircons
from Amıˆtsoq
gneisses by
Whitehouse et al. 1999, who found 3.8 Ga cores mantled by overgrowths at 3.65 Ga.
They interpreted the cores as inherited grains and the overgrowths as indicating the magmatic em-
placement age, in contrast to Nutman et al. 1993, who interpreted similar overgrowths as
metamorphic, resulting from granulite facies metamorphism first described from the same re-
gion south of Nuuk by Griffin et al. 1980. Subsequently, from a region unaffected by early
Archean granulite facies metamorphism, Nutman et al. 1999 provided a detailed description of the
3800 Ma age of little deformed metatonalites and metaquartz-diorites immediately south of the
Isua greenstone belt. Controversy also surrounds the interpretation
of SmNd isotopic data from the Amıˆtsoq gneisses. Bennett et al. 1993 interpreted the wide
range in initial o
Nd
values of Amıˆtsoq gneisses as indicating transient, highly depleted mantle reser-
voirs, and proposed a major change in processes between early Archean and later styles of de-
pleted-mantle evolution. These hypotheses were challenged by Vervoort et al. 1996, Gruau et al.
1996, Moorbath et al. 1997 and Frei et al. 1999, who argued that the SmNd systems, as-
sumed to have been closed systems by Bennett et al. 1993, were modified by subsequent tec-
tonothermal events. This prompted substantial debate by Bennett and Nutman 1998 and Kam-
ber et al. 1998.