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 cl inopyroxene-bearing peridotites. to orthopyroxene-rich, clinopyroxene-poor peridotites. While harzburgites are a major constituent of the garnet-diamo nd facies "keel" of Archaean (> 2500 Ma) cratons, metasomatic conversion of lherzolite to harzburgite is not widely documented from cratonic rocks. We report on the replacement of diopside. by enstatite in polymict peridotite s from Kimberley, South Africa and provide elemental and isotopic constrain ts on the nature of this process. Silica-rich metasomatism appears to have occurred at mid-craton depths and to have involved a silica-rich derivative of incompatible element-rich melts like kimberlite. The preservation of el emental and isotopic disequilibria in these mantle rocks may arise from the nearly synchronous occurrences of the kimberlite entrainment process and f luid-assisted deformation/crack propagation resulting in polymict formation . Copyright (C) 2001 Elsevier Science Ltd.