Cratonic peridotites and silica-rich melts: Diopside-enstatite relationships in polymict xenoliths, Kaapvaal, South Africa

Silica-rich metasomatism is invoked as a mechanism for the conversion of clinopyroxene-bearing peridotites to orthopyroxene-rich, clinopyroxene-poor peridotites. While harzburgites are a major constituent of the garnet-diamond facies "keel" of Archaean (> 2500 Ma) cratons, metasomatic conversion of Iherzolite to harzburgite is not widely documented from cratonic rocks. We report on the replacement of diopside by enstatite in polymict peridotites from Kimberley, South Africa and provide elemental and isotopic constraints on the nature of this process. Silica-rich metasomatism appears to have occured at mid-craton depths and to have involved a silica-rich derivative of incompatible element-rich melts like kimberlite. The preservation of elemental and isotopic disequilibria in these mantle rocks may arise from the nearly synchronous occurences of the kimberlite entrainment process and fluid-assisted deformation/crack propagation resulting in polymict formation. Copyright © 2001 Elsevier Science Ltd.