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 delta(18)O values in these silicates is narrow and the average values are: plagioclase, delta(18)O = +6.90 parts per thousand; orthopyroxen e, delta(18)O = +6.14 parts per thousand; clinopyroxene, delta(18)O = +6.91 parts per thousand; and melt, delta(18)O = +6.65 parts per thous and. An estimated equilibration isotopic temperature of 1091 degrees C was calculated for this Lower Mafic Succession of the Great Dyke. The oxygen isotope compositions of the silicate minerals are not very clo se to their magmatic values, but they are roughly similar to those fro m the Bushveld Complex, the Stillwater Complex and the Kiglapait intru sion. The Delta(clinopyroxene-orthopyroxene) values can be as high as 0.63 parts per thousand with an average value of 0.33 parts per thousa nd. Since the fractionation between these pyroxenes at magnetic temper atures should be approximately zero (Kyser et al., 1981), the high Del ta(clinopyroxene-orthopyroxene) values are likely to have been caused by lower-temperature crystallisation of both pyroxenes. The low Delta( clinopyroxene-orthopyroxene) values in the cumulate-rich samples are i nterpreted as products of high-temperature crystallisation while the h igh Delta(clinopyroxene-orthopyroxene) values are products of low-temp erature crystallisation associated with postcumulus textures and abund ant low-temperaturephases. The slightly O-18-rich silicates from the G reat Dyke acquired additional oxygen (up to 59%) from the Archaean roc ks of the Zimbabwe craton which has whole-rock delta(18)O values going up to +7.3 parts per thousand(Barker et al., 1976) which occur as roo f pendants in the Darwendale Subchamber of the Great Dyke.