An oxygen isotope study of the Lower Mafic Succession of the Darwendale Subchamber of the Great Dyke, Zimbabwe

Oxygen isotope values for plagioclase, clinopyroxene and orthopyroxene from the upper part of the Great Dyke are presented. The variation in δ18O values in these silicates is narrow and the average values are: plagioclase, δ18O = +6.90‰; orthopyroxene, δ18O = +6.‰; clinopyroxene, δ18O = +6.91‰; and melt, δ18O = +6.65‰. An estimated equilibration isotopic temperature of 1091°C was calculated for this Lower Mafic Succession of the Great Dyke. ygen isotope compositions of the silicate minerals are not very close to their magmatic values, but they are roughly similar to those from the Bushveld Complex, the Stillwater Complex and the Kiglapait intrusion. The Δclinopyroxene-orthopyroxene values can be as high as 0.63‰ with an average value of 0.33‰. Since the fractionation between these pyroxenes at magmatic temperatures should be approximately zero (Kyser et al., 1981), the high Δclinopyroxene-orthopyroxene values are likely to have been caused by lower-temperature crystallisation of both pyroxenes. The low Δclinopyroxene-orthopyroxene values in the cumulate-rich samples are interpreted as products of high-temperature crystallisation while the high Δclinopyroxene-orthopyroxene values are products of low-temperature crystallisation associated with postcumulus textures and abundant low-temperature phases. The slightly 18O-rich silicates from the Great Dyke acquired additional oxygen (up to 59%) from the Archaean rocks of the Zimbabwe craton which has whole-rock δ18O values going up to +7.3‰ (Barker et al., 1976) which occur as roof pendants in the Darwendale Subchamber of the Great Dyke.