Formation and temporal evolution of the Kalahari sub-cratonic lithospheric mantle: Constraints from Venetia xenoliths, South Africa

Abstract

The ~533 Ma Venetia Diamond Mine is located between the Kaapvaal and Zimbabwe Cratons and the study of selected xenoliths provides the opportunity to investigate the temporal evolution of the sub-continental lithospheric mantle (SCLM) underneath southern Africa, as well as the extent and potentially the timing and nature of the Si-enrichment that characterizes the Kaapvaal SCLM. Most peridotite xenoliths contain 15–25% orthopyroxene, confirming Si-enrichment of the Venetian SCLM. Mineral major element compositions for 36 peridotitic mantle xenoliths record less melt depletion than inclusions in Venetian diamonds (e.g., olivine Mg# 88.7–93.4; mode 92.5). Olivine Mg# suggest on average ~40% melt extraction and reconstructed whole rock HREE concentrations can be modelled by 20% fractional melting in the garnet stability field followed by ~10 to 13% in the spinel stability field. Calculated Nb/Sr and Ce/Yb ratios for melts in equilibrium with garnet and clinopyroxene suggest that the xenolith suite underwent metasomatism by both hydrous fluids and kimberlite/carbonatite-type melts. Garnet Nd ( T CHUR(Nd) = 2.1 Ga) and Hf ( T CHUR(Hf) = 1.8 Ga) model ages for one sample with an exclusively hydrous metasomatic character are indistinguishable from previously reported Re–Os ages of Venetian peridotitic and eclogitic diamond inclusions. Based on a geochemical and isotopic approach, we propose that the Venetian SCLM formed by shallow melting in the Archaean followed by lateral accretion. Hydrous fluids, either associated with remobilisation due to regional heating or subduction, led to metasomatism and possibly to Si-enrichment at ~2.0 Ga before a final stage of metasomatism associated with kimberlite magmatism.