Sheared Peridotites from Northern Lesotho: Metasomatism-Induced Deformation and Craton Destabilization.

This article focuses on the deformation and metasomatic history of sheared peridotite xenoliths from Late Cretaceous (~90 Ma) kimberlites in northern Lesotho, located on the southeast margin of the Kaapvaal craton. The study identifies three groups of sheared peridotites—low-, moderate-, and high-temperature (Groups I, II, and III)—distinguished primarily by calcium contents in olivine and orthopyroxene and their pressure-temperature (P–T) equilibration conditions. All groups show evidence of multiple metasomatic events, including ancient fluid or melt metasomatism prior to deformation, younger pre-deformation metasomatism likely related to proto-kimberlite melts, and syn-deformation metasomatism involving hydrous, volatile-rich agents that caused rheological weakening. The authors propose a model in which proto-kimberlitic melts infiltrated narrow shear zones formed at the lithosphere–asthenosphere boundary, leading to progressive metasomatism, weakening, and deformation, which contributed to the destabilization and thinning of the lower lithosphere beneath the Kaapvaal craton.