dykes Mengel et al., 1996. The NE – SW set constitute the main set of intrusions and include
the two dykes dated by Nutman and Kalsbeek 1996 at ca. 2.04 Ga. North of Itilleq Fjord,
Kangaˆmiut dykes were emplaced into Archaean E – W trending amphibolite facies zones.
3. Mineralogy and petrology
The mineralogical and petrographic variations within the Kangaˆmiut dyke swarm have been well
documented from
previous studies,
Windley 1970 showed that the dykes near Sukkertoppen
Fig. 1 were often composites of hornblende quartz dolerite, amphibolite and garnet amphibo-
lite, with
microporphyritic hornblende-quartz
margins. Cross-cutting
relationships between
Kangaˆmiut dykes show that the development of internal mineral foliations within the earlier intru-
sions are syn-intrusive. Fahrig and Bridgwater 1976
also noted
the existence
of zoned
Kangaˆmiut dykes.
To the
north, between
Kangaˆmiut village and Søndre Strømfjord Fig. 1 an additional texture is observed in some of the
most highly fractionated intrusions, which contain garnet – albite – quartz – diorite pods. It is impor-
tant to realise that these pods are sometimes situated within unsheared portions of the dyke,
suggesting their origin to be igneous rather than metamorphic Fahrig and Bridgwater, 1976, and
that at least some of the dykes crystallised from wet magmas Bridgwater et al., 1995.
Recent work has shown petrographic differ- ences associated with relative age and dyke trend
Mengel et al., 1996; the older E – W dykes con- tain olivine, clinopyroxene orthopyroxene and
plagioclase. The NNE-trending dykes contain ig- neous clinopyroxene, plagioclase, rare orthopy-
roxene
and primary
hornblende in
chilled margins. The NE-trending dykes contain igneous
clinopyroxene, plagioclase, igneous hornblende phenocrysts in chilled margins and felsic patches
of dioritic composition, within the centres of some wider intrusions.
Petrographically, the Kangaˆmiut dykes range from those with fresh igneous textures including
those with primary hornblende phenocrysts to those with recrystallised static or dynamic meta-
morphic textures essentially amphibolites Men- gel et al., 1996. The grade of metamorphic
overprinting generally increases northwards. Ini- tial metamorphic reactions resulted in the forma-
tion
of amphibole,
biotite and
garnet. Metamorphic pyroxene is recorded in the north-
ernmost areas of the swarm around and to the south of Itertoˆq Fjord, representing upper amphi-
bolite – granulite metamorphism Mengel et al., 1996.
Structurally the Kangaˆmiut dykes show strong evidence for syn-shear emplacement. Escher et al.
1976 recorded consistent stepping directions, oblique offsets of country rock bands across
dykes and oblique internal foliations. They con- cluded that the E – W and NNE – SSW trending
dykes were injected in conjugate shears, although more recent work suggests they were intruded
under a sinistral transpressive regime Mengel et al., 1996; Hanmer et al., 1997. Similarly Windley
1970 noted that some Kangaˆmiut dykes con- tained unfoliated chilled margins with strongly
deformed dyke centres, suggesting that the dykes had acted as loci of shear whilst still hot and
rheologically weak. Very similar field relationships were observed during this study.
Structural and mineralogical work therefore strongly suggests that Kangaˆmiut dyke emplace-
ment, shear zone formation and aqueous fluid transport were coeval or near coeval processes
Korstga˚rd, 1979; Bridgwater et al., 1995, at least within
the later
ca 2.04
Ga NE-trending
Kangaˆmiut intrusions. Mengel et al. 1996 fur- ther noted that that the two earlier dyke sets
could not be positively be classed as being of Kangaˆmiut age at the present time due to a lack
of precise age data.
4. Geochemistry and petrogenesis
