Mixed mantle provenance: diverse garnet compositions in polymict peridotites, Kaapvaal craton, South Africa

Garnet compositions are used to understand mantle petrogenesis and to reconstruct the lithostratigraphy of the shallow mantle (<200 km). However, garnets in polymict peridotites from the Kaapvaal craton (>2500 Ma) have a centimeter-scale elemental and stable isotopic variability suggestive of a mixed mantle provenance. The chemical heterogeneity of the garnets is similar to that reported from rocks sampled over a considerable depth and temperature range within the lower lithosphere. For example garnets found in polymict peridotites are similar to garnets found in sheared and granular peridotites, ‘cold’ and ‘hot’ lherzolites, peridotitic (P-type) diamond inclusions, and garnets from polybaric (50–200 km) peridotites (i.e. spinel, garnet and diamond facies). These data indicate that the Kaapvaal cratonic root has been disturbed by complex processes possibly associated with crack propagation and entrainment that juxtaposed garnet-bearing lithologies of diverse petrogenesis, provenance and depth. This has preserved chemical disequilibrium in the high pressure minerals in what is, in effect, a mantle breccia possibly associated with kimberlite precursors.