Iron in plagioclase in the Bushveld and Skaergaard intrusions: implications for iron contents in evolving basic magmas

The evolved, iron-rich rocks of the tholeiitic Bushveld and Skaergaard intrusions are similar in containing cumulus magnetite, ilmenite, plagioclase, clinopyroxene, apatite and olivine, and also orthopyroxenes/pigeonite in Bushveld. Here, we evaluate their liquid evolution trends using the total iron content in plagioclase determined by electron microprobe analyses. To aid this analysis a revised mass balance model for the liquid evolution of Skaergaard is presented. For plagioclase in the Upper Zone of Skaergaard it was previously demonstrated that total FeO increases from ~0.25 to ~0.45 wt% with differentiation and correlates inversely with An% 100 × Ca/(Na + Ca)]. The reverse trend is observed in two recently published datasets for Bushveld, showing that total FeO in plagioclase decreases upward through the magnetite-bearing Upper Zone from ~0.30 to ~0.15% and from ~0.40 to ~0.25% in the western and northern limbs, respectively, and correlates positively with An%. The partition coefficient of total iron between plagioclase and magma increases with oxidation and polymerisation in the liquid. Although Bushveld formed under slightly more oxidizing conditions than Skaergaard, differences in the partition coefficients cannot explain the two observed trends. We therefore conclude that the differentiation trends of the liquids subsequent to magnetite saturation were fundamentally different. The inferred liquid composition for Bushveld contained about 15 wt% total FeO at the level of magnetite-in, which is slightly less than the total FeO content of the subsequent cumulates. In contrast, the Skaergaard liquid contained more total FeO than the ensuing cumulates. As a result, in Bushveld residual liquids total FeO decreased after magnetite saturation, whereas in Skaergaard the residual liquids continued to become enriched in iron. This conclusion is corroborated by simple mass balance calculations between modelled residual liquids and extracted cumulate rocks. Despite the mineralogical similarities of evolved iron-rich rocks of Skaergaard and Bushveld, their liquid evolution trends were very different, and generalizations about the extent of iron enrichment in tholeiitic magmas should be avoided.