A clinopyroxenite intrusion from the Pilanesberg Alkaline Province, South Africa

A number of circular negative magnetic anomalies (up to 8 km across) exist within the area encompassed by the western Bushveld Complex (150 km by 100 km) on the Kaapvaal craton in South Africa. They are covered by up to 700 m of sedimentary rocks of the Karoo Supergroup, which could not produce these anomalies. Exploration boreholes into one of these magnetic anomalies revealed a hidden volcanic complex, called the Elandskraal Volcano. One of these boreholes intersected an olivine–magnetite–apatite clinopyroxenite body, which we studied. Ages on apatite–clinopyroxene pairs using Sm–Nd dating techniques yield a poorly constrained age of 1207 ± 200 Ma, because there is little variation in Sm–Nd between all the samples. This age correlates with the Pilanesberg Alkaline Province that spans the time period from 1430 to 1200 Ma, but almost all age determinations from this suite give very large errors.Three other clinopyroxenite bodies closely related to the Pilanesberg Alkaline Province have been reported, but no geochemical data have been presented. Our mineral and whole-rock geochemical data permit an interpretation of the genesis of these clinopyroxenite bodies. The basaltic lavas of the Elandskraal Volcano are extremely unusual in having very high TiO2 (over 7 wt%), high Fe2O3 (16–21 wt%) and high incompatible element contents. Modelling the crystallization sequence using MELTS of the more magnesian lava compositions yields olivine, clinopyroxene, magnetite and apatite as liquidus phases within a temperature interval of less than 30 °C, which matches that observed in the clinopyroxenite body. In our samples the Mg/(Mg + Fe) value for the mafic minerals and incompatible trace elements abundances in clinopyroxene are consistent with crystallization from these unusual basaltic compositions. Contrasts with other clinopyroxenite bodies in the Phalaborwa Complex and the Bushveld Complex are documented in terms of rock associations and mineral compositions (both major and trace elements).

► A clinopyroxenite (with olivine, magnetite and apatite) intrusion has been found by drilling.
► We have determined its age to be part of the Pilanesberg Alkaline Province (1207 ± 200 Ma).
► No analytical data are available for the few similar bodies, so our geochemical data are new.
► A suite of associated lavas has unusual compositions, being very high Ti and Fe basalts. Modelling using MELTS shows that these lava compositions could have produced these cumulate clinopyroxenites with clinopyroxene, olivine, magnetite and apatite being liquidus minerals.