1. Introduction
In recent years there have been great advances in our understanding of the mainly Proterozoic
hypabyssal dyke swarms intruded into Precam- brian cratons. Much information has come from
studies of mainly Phanerozoic continental flood basalt CFB provinces which have similar chem-
istry. Although field evidence of such a relation- ship is often lacking see arguments expressed in
Ross, 1983; Tarney, 1992; Cadman et al., 1995, it can sometimes be deduced that flood basalts were
fed from the extensive dyke systems which are now exposed in Precambrian cratons e.g. Baragar
et al., 1996.
Despite very detailed research on both phenom- ena, the degree to which various petrogenetic
processes such as fractional crystallisation, crustal contamination and mantle metasomatism control
their chemistry is still hotly debated. An obvious difficulty in any petrogenetic analysis of dykes is
that the same process may have operated on the magma at different stages in its genesis; for exam-
ple, crystal fractionation within a basaltic magma may take place both prior to dyke injection e.g.
within a magma chamber and subsequently within the dykes themselves, the relative influence
of fractionation within each environment may be very difficult to ascertain. Hence although many
studies of intradyke petrogenetic processes have been undertaken e.g. Gibb, 1968; Komar, 1972,
1976; Ross, 1983, 1986; Platten and Watterson, 1987; Blichert-Toft et al., 1992; Ernst and Bell,
1992 the degree to which basalt petrogenesis may be controlled by hypabyssal processes within
mafic dykes is still uncertain.
Study of the Kangaˆmiut dyke swarm offers an excellent opportunity to help resolve some of
these questions. Earlier workers have noted that unlike the vast majority of continental mafic
swarms, the dykes were injected into an overall contractional environment e.g. Escher et al.,
1976 and throughout much of their extent horn- blende is the dominant primary ferromagnesian
mineral Korstga˚rd, 1979; Bridgwater et al., 1995. However, the major element chemistry of
the swarm suggests a normal tholeiitic Fe-enrich- ment trend Escher et al., 1975; Bridgwater et al.,
1995. As hornblende is associated with calc – al- kaline fractionation, it would appear that the
petrogenetic processes governing the chemistry of the dykes may be unrelated to the crystallisation
processes within the dykes themselves. The field setting and unusual petrographical characteristics
of the Kangaˆmiut dyke swarm also require that models developed to explain the petrogenesis of
other dyke swarms are applied in order to test their validity. In this paper we seek to undertake
comprehensive major and trace element modelling of the chemistry of the Kangaˆmiut dykes with a
view to understanding the processes governing their formation.
2. Field relationships and geological setting
Kategori