Geochemistry and petrogenesis of high-K “sanukitoids” from the Bulai pluton, Central Limpopo Belt, South Africa: Implications for geodynamic changes at the Archaean–Proterozoic boundary

The Neoarchaean Bulai pluton is a magmatic complex intrusive in the Central Zone of the Limpopo Belt (Limpopo Province, South Africa). It is made up of large volumes of porphyritic granodiorites with subordinate enclaves and dykes of monzodioritic, enderbitic and granitic compositions. New U–Pb LA-ICP-MS dating on zircon yield pluton-emplacement ages ranging between 2.58 and 2.61 Ga. The whole pluton underwent a high-grade thermal overprint at ~ 2.0 Ga, which did not affect the whole-rock compositions for most of the major and trace-elements, as suggested by a Sm–Nd isochron built up with 16 samples and yielding an age consistent with U–Pb dating.The whole-rock major- and trace-element compositions evidence that the Bulai pluton belongs to a high-K, calc-alkaline to shoshonitic suite, as well as unequivocal affinities with “high-Ti” sanukitoids. Monzodioritic enclaves and enderbites have both “juvenile” affinities and a strongly enriched signature in terms of incompatible trace elements (LREE, HFSE and LILE), pointing to an enriched mantle source. Based on trace-element compositions, we propose the metasomatic agent at their origin to be a melt deriving from terrigenous sediments. We therefore suggest a two-step petrogenetic model for the Bulai pluton: (1) a liquid produced by melting of subducted terrigenous sediments is consumed by reactions with mantle peridotite, producing a metasomatic assemblage; (2) low-degree melting of this metasomatized mantle gives rise to Bulai mafic magmas. Such a model is supported by geochemical modelling and is consistent with previous studies concluding that sanukitoids result from interactions between slab melts and the overlying mantle wedge.Before 2.5 Ga, melting of hydrous subducted metabasalts produced large volumes of TTG (Tonalite–Trondhjemite–Granodiorite) forming most of the volume of Archaean continental crust. By constrast, our geochemical study failed in demonstrating any significant role played by melting of subducted metabasalts, which points to lower thermal regimes as metasediments melt at lower temperature than metabasalts. This suggests that the geodynamic changes that took place at the Archaean–Proterozoic transition and witnessed by sanukitoid-related rocks are mainly the result of progressive and global cooling of Earth. On the other hand, melting of subducted detrital material is uncommon during the Archaean, which would also indicate that significant recycling of continental material within the mantle roughly began at the Archaean–Proterozoic transition.

Research Highlights
► The high-K calc-alkaline Bulai pluton shares affinities with "high-Ti" sanukitoids.
► LA-ICP-MS dating on zircon yielded emplacement ages ranging between 2.58 and 2.61 Ga.
► The source of mafic rocks is an enriched mantle, modified by sediment-derived melts.
► Late-Archaean geodynamic changes may result from progressive cooling of Earth.
► Involvement of detrital material also indicates changes in global mass transfers.