The Uintjiesberg kimberlite diatreme occurs within the Proterozoic Namaqua-Natal Belt, South Africa, approximately 60 km to the southwest of the Kaapvaal craton boundary. It is a group 1, calcite kimberlite that has an emplacement age of similar to 100 Ma. Major and trace element data, in combination with petrography, are used to evaluate its petrogenesis and the nature of its source region. Macrocryst phases are predominantly olivine with lesser phlogopite, with very rare garnet and Cr-rich clinopyroxene. Geochemical variation amongst the macrocrystic samples (Mg# 0.85-0.87, SiO2 = 27.0-29.3%, MgO = 26.1-30.5%, CaO = 10.9-13.5%) is shown to result from similar to 10% to 40% entrainment and partial assimilation of peridotite xenoliths, whereas that shown by the aphanitic samples (Mg# 0.80-0.83, SiO2 = 19.1-23.0%, MgO = 17.9-23.9%, CaO = 16.5-23.7%) is consistent with similar to 7-25% crystal fractionation of olivine and minor phlogopite. Changing trajectories on chemical variation diagrams allow postulation of a primary magma composition with similar to 25% SiO2, similar to 26% MgO, similar to 2.3% Al2O3, similar to 5% H2O, similar to 8.6% CO2 and Mg# = 0.85.
828ANTimes Cited:9Cited References Count:43