Keywords
:
Geochemistry; Petrology; Komatiites; Archaean; Dykes; Sills; Facies
1. Introduction
The concept that Archaean komatiite melts flowed dynamically during ascent and emplace-
ment Nisbet 1982; Huppert et al., 1984 and assimilated crustal materials has critical implica-
tions for models of Archaean mantle composi- tions
and thermal
regimes Arndt,
1986a, magmatism and volcanism Huppert and Sparks,
1985a, crustal evolution e.g. Arndt and Jenner, 1986, and nickel sulphide deposit genesis Hup-
pert et al., 1984; Lesher and Campbell, 1993. A wide variety of evidence exists that some komati-
ites thermally eroded crustal wall rocks adjacent to melt conduits and footwall rocks to lava chan-
nels during magma ascent and lava emplacement Arndt and Jenner, 1986; Compston et al., 1986;
Frost and Groves, 1989; Chauvel et al., 1993; Lesher and Arndt, 1995; Perring et al., 1995.
However, virtually all the evidence is from extru- sive rocks. Contamination of intrusive komatiitic
rocks has not been reported. Recognition of con- tamination in subvolcanic dykes and sills and in
associated lava flows would greatly benefit models for
komatiitic magmatism – volcanism.
Such recognition could also be economically important
in targeting komatiite terrains for nickel sulphide deposit potential, because of the relationship of
contamination to komatiite-associated nickel de- posit genesis Lesher, 1989.
This paper 1 documents the field relationships and textural and mineralogical characteristics of
komatiitic rocks in an Archaean komatiitic dyke – sill – lava complex in the Shaw Dome, Abitibi belt,
2 documents major, minor and trace element geochemical evidence of contamination, and 3
utilises these data, in conjunction with field data and the results of MELTS program modelling
Ghiorso and Sack, 1995, to constrain models for the geochemical evolution of the complex. The
study focuses on the Shaw Dome, because it is a key area for komatiitic intrusive rocks Muir,
1979 and the intrusive and extrusive komatiitic rocks are petrologically related and form a single
stratigraphic sequence, representing a dyke – sill – lava complex Larson, 1996. The range of incom-
patible element contents in the evolved komatiitic intrusions and related komatiitic basalts is argued
to indicate crustal contamination.
2. Geologic setting and background
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