- © 2007 Geological Society of South Africa
Eight Jurassic to Cretaceous transitional kimberlites located on and off the Kaapvaal craton have been analysed in order to evaluate their petrogenesis and relationship to typical Group I and Group II southern African kimberlites. The transitional kimberlites tend to show intermediate petrographic and geochemical characteristics to Group I and II kimberlites for some parameters (e.g. variable phlogopite, TiO2, La/Nb, Ba/Nb and Th/Nb) and especially in their 143Nd/144Nd and 87Sr/86Sr ratios. For other parameters (e.g. MgO, Ni), they span the ranges shown by Group I and Group II kimberlites. These characteristics are interpreted to suggest that transitional kimberlites are derived from source regions with mixed Group I/Group II kimberlite source characteristics. The off-craton transitional kimberlites show greater petrographic and geochemical affinity to Group II kimberlites, and have similar emplacement ages, whereas the on-craton transitional kimberlites are petrographically and geochemically more similar to Group I kimberlites and also have similar emplacement ages. Although derived from a mixed source having both Group I and Group II geochemical signatures, the mantle source giving rise to off-craton transitional kimberlites is interpreted to have stronger Group II kimberlite-like characteristics, whereas that giving rise to on-craton transitional kimberlites had stronger Group I kimberlite-like characteristics.