252 IGNEOUS ROCKS/Kimberlite Kimberlite magmatism has occurred at various times in the Earth’s history, one major phase being in the Cretaceous South and West Africa have seen three or four periods of kimberlite magmatism, and there have been several epochs of such magmatism in the central USA and Yakutia Certain geographical areas coinciding with old cratons are ‘kimberlite prone’ Xenoliths Within kimberlites a wide variety of rock types occur as fragments Minerals derived from these rocks also become incorporated within the host kimberlite Thereby, the mineralogy becomes extremely complex There are five sources of xenoliths: Figure The relation of kimberlite occurrences in Africa to ancient cratons and their boundaries (Reproduced from Dawson JB (1980) Kimberlites and Their Xenoliths Berlin, Heidelberg, New York: Springer Verlag.) fragments of the immediate wall rocks at the level of preservation; fragments derived from earlier formations that existed at the time of intrusion but have since been removed by erosion; blocks of still-buried formations through which the intrusion passed; Figure Kimberlite and ‘orangeite’ occurrences in southern Africa, with their radiometrically derived ages Note the Precambrian age (1200 Ma) at the site of the Premier Mine and the single Permian (200 Ma) age in Swaziland (Reproduced from Mitchell RH (1995) Kimberlites, Orangeites and Related Rocks New York: Plenum Press.)