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 Figure 2:The Isua supracrustal belt is located in the southern West Greenland. We
move to there by a helicopter because it is far from a town, and located
at the edge of inland ice. The Isua supracrustal belt was formed at 3.8
Ga, and has the oldest evidence for presence of ocean, operation of plate
tectonics and life.
The Isua supracrustal belt is located in West Greenland (Fig. 2). Archean
geological terrane in the West Greenland is a part of North Atlantic Craton
from Labrador through southern part of Greenland to Scotland, and is composed
three terranes with different ages and lithologies, respectively (Fig.
5 inset). The Isua supracrustal belt is in the northeastern end of the
older terrane, the Akulleq terrane, among them. This terrane is composed
of 3880-3660-Ma Amitsoq gneiss and 2820-Ma Ikkattoq gneiss. The former
contains enclaves of Early Archean supracrustal rocks (Akilia association),
whereas the latter contains other enclaves of late Archean supracrustal
rocks (Malene supracrustals) and an anorthosite complex. The Akulleq terrane
was intruded by the K-rich Qorqut post-orogenic granite at about 2550 Ma
and stabilized at that time. The northern 3230-2980-Ma Akia and the southern
2920-2830-Ma Tasiusarsuaq, with the Akulleq terrane between, collided at
2820-2712 Ma. Most areas in southern West Greenland are underlain by polydeformed
amphibolite to granulite facies rocks. The northeastern part of the Akulleq
terrane is an exception. Here, a relatively less metamorphosed segment,
the Isua supracrustal belt, is exposed (Fig. 5, inset) as a fault-bounded
tectonic slice and is in tectonic contact with the tonalitic Amitsoq orthogneiss
on both sides.
The Isua supracrustal belt was affected by Barrovian-type amphibolite facies
metamorphism during Archean times (Boak & Dymek, 1982). In spite of
metamorphism, metasomatism (Rosing et al. 1996) and polyphase ductile deformation,
the original structures of the protoliths are well preserved in the northeastern
part of the belt where the metamorphic grade is the lowest. The Isua belt
is an exception displaying well-preserved primary structures that yield
information helpful in deciphering the history of pre-3.8 Ga events on
the Earth. Therefore, considerable detailed interdisciplinary works had
been conducted in this district, including tectonics, magmatism, metamorphism,
geomagnetism, evolution of surface environment, and ancient life (e.g.
Nutman 1986; Schidlowski 1988; Nutman et al. 1993, 1996, 1997; Mojzsis
et al. 1996).
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