MAFIC TO ULTRAMAFIC SILLS OF THE EARLY ARCHEAN ZWARTWATER SUITE, SE TRANSVAAL

The Zwartwater Suite is intrusive into the De Kraalen Formation, a sui te of early Archaean supracrustal rocks, which consists of a sequence of predominantly meta-sedimentary rocks, comprising quartz-magnetite b anded iron-formations, calc-silicate gneiss and quartzite, interlayere d with subordinate meta-volcanic rocks. The sill-like nature of the Zw artwater Suite results in a concordant interlayering of the Zwartwater Suite and the De Kraalen Formation. Sills in the Zwartwater Suite hav e compositions which range from amphibolite, homblendite and pyroxene- bearing homblendite to olivine-bearing homblendite and serpentinite. R elic olivine crystals are preserved locally in serpentinite. The rocks are typically equigranular and medium- to coarse-grained. Relic textu res indicate that they had a cumulate origin. The Zwartwater Suite, De Kraalen Formation and surrounding granitoid rocks are disrupted by nu merous closely spaced NW trending faults which prevent correlation of individual units for more than 100 m along strike. Geochemical data in dicate that compositional variations in the Zwartwater Suite result fr om differentiation controlled by the crystallisation of olivine and cl inopyroxene, 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 rati os are similar to those of layered complexes developed in the Barberto n Sequence.