Abstract

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.

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