Cretaceous thermo-chemical modification of the Kaapvaal cratonic lithosphere, South Africa

Peridotitic garnet xenocrysts from 33 Group II and 29 Group I kimberlites have been used to map the lithospheric mantle along a southwest to northeast traverse across the Kaapvaal Craton in South Africa. A minimum age difference of 10 m.y. exists between the Group II and Group I kimberlites in this region, and the compositions of their respective garnet xenocryst suites can define changes in mantle composition over this time interval. Depths of origin for each garnet were derived empirically using the garnet geotherm method. The location of each grain was then projected onto a cross-section. Using a gridding algorithm, the vertical and lateral distribution of Ti, Zr and Zr/Y in the garnets and the calculated XMg of coexisting olivine are contoured for the cross-section. The resulting images show that significant changes occurred in the lithospheric mantle beneath the craton between the emplacements of the two groups of kimberlites. The nature of these changes defines three domains with different metasomatic styles along the cross-section, recognisable by different geochemical signatures in garnet. In the southwest Prieska domain, metasomatism was dominated by interaction of the lithosphere with mafic silicate melts, resulting in garnets with high Ti, Fe and Zr, and low Zr/Y. In the central Kimberley domain, metasomatic signatures consistent with the interaction of the host peridotite with a hydrous, highly alkaline fluid (phlogopite-style metasomatism) are most commonly observed, resulting in high Zr and high Zr/Y, but low Ti and moderate Fe. In the northeastern Swartruggens domain, a high-Fe signature, probably related to the intrusion of the Bushveld complex, predates further phlogopite-style metasomatism during the Group II/Group I interval. Changes in the thermal structure of the lithosphere between kimberlite groups indicate that metasomatism by mafic fluids involves heating, but phlogopite-style metasomatism does not.