Godtha˚bsfjord region north of Paamiut shows that the North Atlantic Craton in West Greenland from Ivittuut in the south to Maniitsoq in the north 550 km consists of a mosaic of ductile fault-bounded packages that attained
their present relative positions in the late Archaean. © 2001 Elsevier Science B.V. All rights reserved.
Keywords
:
Archaean; Zircon, U – Pb geochronology; Gneiss complexes; Terrane tectonics
1. Introduction
Understanding how high-grade gneiss terrains were constructed is hampered by polyphase, het-
erogeneous high strain and polyphase metamor- phism which normally obliterates most primary
textures and structures. This makes the identifica- tion of cogenetic rock suites very difficult. Fur-
thermore, during these processes, whole-rock and mineral isotopic systems are commonly disturbed.
Geochronological studies of rockes so affected is fraught with difficulty, and may lead to geologi-
cally meaningless results e.g. Hamilton et al. 1979 cf. Kinny and Friend, 1997, Burton et al.
1994 cf. Whitehouse et al. 1996. Ion-mi- croprobe U – Pb single zircon dating allows indi-
vidual samples to be more reliably dated so that when dealing with complicated field relationships,
analytical results can be directly compared rather than attempting to correlate using lithological
similarities. Application of this technique to the Godtha˚bsfjord region of southern West Green-
land Fig. 1 e.g. McGregor et al., 1991; Friend et al., 1996, confirmed the concept that different
terranes sensu Coney et al., 1980, had been amalgamated in the late Archaean, an hypothesis
first suggested on a structural and metamorphic basis Friend et al., 1987, 1988; Nutman et al.,
1989. Because the Godtha˚bsfjord region com- prises only a part of the West Greenland Ar-
chaean,
it is
important for
the overall
understanding of the evolution of the North At- lantic craton to know whether terrane assembly
can be recognised over a wider area. The Paamiut region Fig. 1 was mapped in the
late 1960s the region being interpreted as a single block, with a more or less uniform history
throughout e.g. Berthelsen and Henriksen, 1975; Higgins, 1990; Kalsbeek et al., 1990. A new field
study of this region McGregor and Friend, 1997, had two main aims. First, to test whether
the Paamiut region was indeed a continuous block of similar history, as indicated on existing maps
e.g. Allaart,
1975, 1982
or, like
the Godtha˚bsfjord region to the north, was con-
structed from individual blocks with different tec- tono-thermal histories e.g. Friend et al., 1988.
Second, to search for outcrops of early Archaean gneisses that, from whole-rock Pb – Pb isotopic
work on Proterozoic mafic dykes which traverse the region, were postulated to occur at depth
Kalsbeek
and Taylor,
1985. In
the Godtha˚bsfjord region such rocks, the Itsaq Gneiss
Complex, have distinct field characteristics e.g. Nutman et al., 1996. In a global context, it would
be important to identify another area where early Archaean rocks occur. However, no definitive
field or isotopic evidence was found to suggest that early Archaean rocks occur in the Paamiut
region. This paper presents the zircon geochronol- ogy carried out to establish the ages of some of
the crustal components and metamorphic events identified by McGregor and Friend 1997.
2. Pre-1992 investigations of the Paamiut region
