Differentiation of the late-Archaean sanukitoid series and some implications for crustal growth: Insights from geochemical modelling on the Bulai pluton, Central Limpopo Belt, South Africa

The late-Archaean (similar to 2.59 Ga) Bulai pluton from North-Eastern South Africa mostly consists in granodioritic rocks and subordinate amounts of mafic lithologies, mainly monzodiorites, both belonging to the sanukitoid series. Many lines of evidence point to a genetic link between these two components, such as similar mineralogical assemblages, parallel trace-element patterns and isotopic homogeneity. The nature of this genetic link has been investigated using semi-quantitative geochemical modelling. Several models reproduce the geochemistry of the Bulai granodiorites, including magma mixing, crystallization from a primary magma and partial melting of mafic rocks. Pure mixing between two magmas that are highly contrasted in terms of geochemistry and physical properties is of doubtful reliability in generating massive volumes of homogeneous hybrid magma. Thus, the models that best fit the data involve either equilibrium or fractional crystallization from a mafic mantle-derived magma, coupled with increasing amounts of mixing with felsic melt in course of differentiation. Alternatively, melts computed by dehydration melting of monzodiorites (incongruent melting of biotite) are also similar in composition to the most differentiated Bulai granodiorites (SiO2 > 65 wt.%). However, formation of the least differentiated granodiorites (SiO2 < 65 wt.%) by such a process requires unrealistic melting conditions. Rather, they would result from entrainment of the (mafic) residual solid after melting. The widespread differentiated rocks of the sanukitoid suite were probably derived from differentiation of mafic mantle-derived melts (sanukitoids s.s.), either as residual liquids (crystallization and mixing) or partial melts. Thus, the presence of pre-existing differentiated crust is not a requirement to the generation of these granitoids that therefore can be considered as a juvenile addition to the crust. In addition, the composition of the sanukitoid series matches the one of some post-Archaean, high-K calc-alkaline suites of late-orogenic affinity, as well as the average composition of the continental crust. As a result, growth and differentiation of the crust since the end of the Archaean were likely accommodated by similar processes that gave rise to the sanukitoid series and post-Archaean high-K calc-alkaline granitoids. (C) 2012 Elsevier B.V. All rights reserved.