Geologic setting and background

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

Komatiitic rocks are preserved in the margins of the Shaw Dome, a major anticline centred : 20 km southeast of Timmins, in the Abitibi greenstone belt, Ontario Muir, 1979; Green and Naldrett, 1981; Fig. 1. The komatiitic rocks are present in two horizons in a 4-km thick sequence of komatiitic to calc-alkalic rocks and iron-forma- tions Fig. 1, and in dykes intruding underlying calc-alkalic rocks and iron-formations. The two horizons are referred to as the lower komatiitic horizon LKH and the upper komatiitic horizon UKH. The LKH consists of intrusive komatiitic rocks, whereas the UKH consists of extrusive komatiitic rocks. The age of the LKH is bracketed by U-Pb dates for zircons from underlying calc-alkalic felsic vol- canic rocks 2725 9 1 Ma; Corfu and Noble, 1992 and a gabbro differentiate of a thick wehrlite – gabbro dyke of the LKH 2707 9 3 Ma; Corfu et al., 1989; Corfu and Noble, 1992. The age of the UKH is bracketed by a U-Pb age for zircons from overlying calc-alkalic felsic volcanic rocks 2703 9 1.5 Ma; Corfu et al., 1989. The LKH – UKH rocks altered to serpentine- magnetite, amphibole-chlorite, or talc-carbonate mineral assemblages during greenschist facies metamorphism. In the serpentine and amphibole- chlorite altered rocks, igneous textures are pre- served. Igneous olivine, chromite and amphibole are locally preserved, but clinopyroxene is com- pletely altered to secondary amphibole. In the talc-carbonate altered rocks, igneous textures are obscured and igneous minerals obliterated. This study focused on the serpentine and amphibole- chlorite altered rocks, except where no other ma- terial was available e.g. LHK wehrlite dykes. The metamorphic alteration caused extensive geochemical mobility, as indicated by wide scat- tering of Na, K, Rb, Cs, Sr, Ba, Eu, Ca, Cu, Zn, and S data on MgO variation diagrams Larson, 1996. Such mobility is common Lahaye et al., 1995; Lahaye and Arndt, 1996 and these data are not considered further. All other elements exhibit regular trends on MgO variation plots and are therefore considered immobile. The REE are mo- bile during carbonate alteration Lahaye et al., 1995, but the REE trends described below do not appear to have been affected and these samples contain only minor to trace amounts of carbonate minerals.

3. Analytical methods