LITHOSPHERE PERIDOTITES OF THE KAAPVAAL CRATON

A large number of garnet peridotite xenoliths from the lithosphere below the Kaapvaal craton are too orthopyroxene-rich and SiO2-rich to have formed as residues from a normal mantle peridotite source by the extraction of basalt or komatiite. However, they can be understood as originally harzburgite rocks (i.e., olivine + orthopyroxene) which formed as either residues or cumulates from a source material that was also enriched in SiO2 compared to normal mantle peridotite or its pyrolite analogue. These harzburgites could have crystallized as cumulates from ultrabasic magmas that were higher in SiO2 than most komatiites, magmas that would have required extensive melting (> 50%) of normal mantle peridotite to form, possibly in a gigantic plume. Alternatively, the harzburgites may have crystallized directly from a high-SiO2 terrestrial magma ocean during the birth of the Earth, or from a high-SiO2 source formed by the mixing of impacted chondritic materials with normal mantle peridotite later in time. Whatever the case, there appear to have been unusual geological circumstances involved in the formation of the continental lithosphere below the Kaapvaal craton.