Mafic to ultramafic sills of the early Archaean Zwartwater Suite, SE transvaal

The Zwartwater Suite is intrusive into the De Kraalen Formation, a suite of early Archaean supracrustal rocks, which consists of a sequence of predominantly meta-sedimentary rocks, comprising quartz-magnetite banded iron-formations, calc-silicate gneiss and quartzite, interlayered with subordinate meta-volcanic rocks. The sill-like nature of the Zwartwater Suite results in a concordant interlayering of the Zwartwater Suite and the De Kraalen Formation.

Sills in the Zwartwater Suite have compositions which range from amphibolite, hornblendite and pyroxenebearing hornblendite to olivine-bearing hornblendite and serpentinite. Relic olivine crystals are presenved locally in serpentinite. The rocks are typically equigranular and medium- to coarse-grained. Relic textures indicate that they has a cumulate origin. The Zwartwater Suite, De Kraalen Formation and sorrounding granitoid rocks are disrupted by numerous closely spaced NW trending faults which prevent correlation of individual units for more than 100 m along strike.

Geochemical data indicate that compositional variations in the Zwartwater Suite result from differentiation controlled by the crystallisation of olivine and clinopyroxene, with minor orthopyroxene and plagioclase crystallisation. Reversals in chemical trends result from either a repeated influx of primitive magma and/or convective overturns. Incompatible element ratios are similar to those of layered complexes developed in the Barberton Sequence.