In situ Sr Isotope Compositions of Plagioclase from a Complete Stratigraphic Profile of the Bushveld Complex, South Africa: Evidence for Extensive Magma Mixing and Percolation

Abstract

The cumulates of the Bushveld Complex, which form the largest layered intrusion on Earth, are known to have crystallized from several isotopically distinct magma pulses. Here, we present in situ Sr isotope compositions combined with the corresponding mineral chemistry of plagioclase from all lithological zones, covering > 6 km of stratigraphy, to constrain the petrogenesis of the complex. The in situ data coupled with high-resolution elemental maps of individual plagioclase grains reveal complex zonation patterns with respect to mineral chemistry and Sr isotope composition. This suggests that interstitial plagioclase in the Bushveld Complex crystallized from multiple, isotopically distinct influxes of melt percolating through a mafic cumulate framework and displacing the resident melt. Similarly, cumulus plagioclase grains are the result of continual ingress of a distinct magma, which picked up previously formed plagioclase crystals. Sr isotope compositions across the layered sequence seem to be largely decoupled from differentiation indices, such as Mg#, anorthite content or trace element ratios. As these observations cannot be explained by bulk assimilation, we propose that the elevated Sr isotopic signature of the Bushveld cumulates may have resulted from the interaction of the parental magma with a fluid derived from the up to 2 km thick dolomitic footwall, which caused a major shift in Sr isotope composition without significantly affecting the degree of differentiation or trace element signature. The decarbonation and/or assimilation of dolomite during the intrusion of the Bushveld Complex may be of major importance, not only for the petrogenesis but also for the emplacement of the layered intrusion, because devolatilization is directly linked to space creation owing to volume loss, thus producing a lopolith.