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Geochemistry and petrogenesis of South African transitional kimberlites located on and off the Kaapvaal Craton

Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africa e-mail: megan.becker{at}uct.ac.za; anton.leroex{at}uct.ac.za

Cornelia Class

Lamont-Doherty Earth Observatory, Columbia University, New York, United States of America, e-mail: class{at}ldeo.columbia.edu

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.