REEVALUATION OF MAGMA COMPOSITIONS AND PROCESSES IN THE UPPERMOST CRITICAL ZONE OF THE BUSHVELD COMPLEX

A detailed geochemical study is presented of the uppermost Critical Zo ne, especially of the footwall and hanging wall to the Merensky Reef a t Impala Platinum Mines in the Bushveld Complex, The approximately 100 m-thick sequence below the Merensky Reef consists of 13 distinct laye rs which have sharp boundaries. They are adcumulates with varying prop ortions of cumulus plagioclase, orthopyroxene and chromite. Experiment al studies on the composition of coexisting orthopyroxene liquid indic ate that the magma which produced this sequence contained between 4 an d 6% MgO. The magma from which the Merensky Reef formed was more evolv ed than the footwall magma. Significant variations exist for both the En content of orthopyroxene and mg# number of whole-rock analyses in s hort vertical sections. Pyroxenite and norite always have higher value s than anorthosite. Extremely sharp breaks in these values correlate w ith changes in modal proportions, and argue against both significant f ractionation within the studied interval, and infiltration metasomatis m. Quantitative modelling shows that the entire footwall section could have contained pyroxene with a uniform primary composition of En(82), and that all the variation now observed reflects the effect of reacti on with trapped magma.Two independent methods for determining the prop ortion of trapped liquid are presented, based on mg# number and incomp atible element abundances. Both yield a uniform proportion in all samp les of approximately 10%. Immiscible sulphide liquid from the Merensky Reef can be shown to have infiltrated downwards for <5 m, despite its high density contrast with silicate magma, very low viscosity and low crystallization temperature. Residual silicate magma would have had e ven more restricted mobility. The migration of residual liquid or flui d through pothole structures in the floor of the Merensky Reef is not supported by the present data.