Coarse garnet peridotites that are the principal rock type in the Kaapvaal craton, southern Africa, have bulk and modal compositions that differ from oceanic preidotites in being richer in Mg and in enstatite. This distinction is fundemental in that it implies a difference in the igneous processes through which continental and oceanic lithospheres have formed. Both continental and oceanic peridotites appear to be residues and the difference is speculated to reflect a greater depth of formation for the craton peridotites. In contrast, the high-temperature peridotites underlying the Kaapvaal craton have compositions similar to oceanic lithosphere and may have been subducted. Heterogeneities in mineral distribution in the continental peridotites on a scale of several centimeters are common and are believed to be caused by exsolution of diopside and garnet from primary enstatite or pigeonite. Study of variations in bulk composition that are igneous in origin and that exclude these subsolidus features requires analytical specimens weighing 0.5-1 kg. This sample size requirement has limited acquisition of desired data. Collection of additional large xenoliths, however, is practical. This project will extend these studies to investigations of the igneous origins of the spinel peridotites from the top of the lithosphere within the craton and of the the low-Ca garnet harzburgites that form the root. The high- temperature peridotites underlying the craton appear from preliminary data to be more fertile towards the north than are those (subducted?0 beneath the southern margin. This possibility will be investigated. Bulk, modal and mineral analyses of xenoliths weighing at least several kg will be combined with isotopic studies at the Department of Terrestrial Magnetism. Determinations of the areal distribution and depths of origin of Kaapvaal ecligites are planned.
