Evolution of the Subcontinental Mantle Beneath the Kaapvaal Craton: A Review of Evidence for Crustal Subduction for Bellsbank Eclogites

Major- and trace-element and Sr, Nd, Os, and O isotope compositions of garnet and clinopyroxene separates from xenoliths from the Bellsbank kimberlite define three eclogite groups: Group A eclogites, considered to be mantle cumulates, are characterized by δ18O, 87Sr/86Sr, and Os values typical of mantle-derived materials (+5.1 to +5.6 ‰, 0.7042–7046, and 0.615–1.98 ppb, resp.). Group B and C eclogites, believed to be metamorphosed products of ancient subducted oceanic crust, are characterized by low δ18O (+2.9 to +4.7), extremely high εNd[120] (≈ +40 to +219), radiogenic 87Sr/86Sr ratios (0.708–0.710), low Os contents (0.027–0.065 ppb), and the presence of positive Eu anomalies. The isotopic compositions of Group B and C eclogites appear to reflect a complex sequence of depletion and enrichment events, in both crustal and mantle settings, including seawater-alteration, which lowered the δ18O and raised the 87Sr/86Sr ratios, combined with partial melting and recrystallization of a crustal component to eclogite during subduction. Os concentrations substantiate the distinction of Groups B and C from Group A, however, preclude subducted MORB. The protolith may have been more primitive crust, cumulate sections of oceanic lithosphere, or cumulates from melts of such precursors. Nd model ages suggest these eclogites were formed 2.3–2.4 Ga. Lastly, Groups A, B, and C cannot be related by simple igneous fractionation.