THE JAGERSFONTEIN CR-POOR MEGACRYST SUITE - TOWARDS A MODEL FOR MEGACRYST PETROGENESIS

Major- and trace-element concentrations and Sr and Nd isotopic ratios were determined for selected Cr-poor megacrysts from the Jagersfontein kimberlite, South Africa. The data are consistent with fractional crystallization of the megacrysts at high pressure near the base of the mechanical boundary layer of the lithosphere from a magma of OIB-like affinity. The host kimberlite and megacrysts are suggested to be products of the same period of plume activity in the asthenosphere, but with different evolutionary histories. Zone refining, assimilation of lithospheric melts, assimilation of lithospheric and crustal fragments and near-surface alteration all contribute towards the eventual hybrid composition of the kimberlite, although the relative roles of these processes remain difficult to determine. The host kimberlite disrupts and entrains the Cr-poor megacrysts and the megacrysts must therefore be considered xenocrysts in the host kimberlite.