In situ determination of U–Pb ages and Sr–Nd–Hf isotopic constraints on the petrogenesis of the Phalaborwa carbonatite Complex, South Africa

Abstract

The Phalaborwa carbonatite Complex, situated in the northeastern part of South Africa, is characterized by copper and zirconium mineralization, and is composed principally of pyroxenites, phoscorite and carbonatite (banded and transgressive). The complex is transected by mafic dykes, and is geographically associated with a satellite syenite and minor granite intrusions. Zircon and baddeleyite U–Pb isotopic age determinations using CAMECA 1280 secondary ion mass spectrometry have shown that the outer pegmatitic pyroxenite at the Loolekop pipe was emplaced at 2060±4Ma, and the main phoscorite at 2062±2Ma. Both ages are identical to those of 2060±2 and 2060±1Ma for the banded and transgressive carbonatites, respectively. The satellite syenite, which forms plug-like bodies outside of the border of the main complex, and the later mafic dyke have “similar” emplacement ages of 2068±17 and 2062±53Ma, indicating that these intrusions were apparently near-synchronously emplaced. In contrast to other carbonatites, the Phalaborwa Complex is characterized by high initial Sr and low initial Nd and Hf isotopic compositions. In situ isotopic analyses of apatite, calcite, zircon and baddeleyite indicate that the primary magma was derived from an enriched mantle. As the complex was emplaced slightly earlier at ~2060Ma than the nearby mafic phase of the Bushveld Complex (~2055Ma), it is proposed that the Phalaborwa carbonatite magmatism was triggered by the same mantle plume activity, which partially melted the overlying lithospheric mantle. This contribution also highlights that isotopic studies used to constrain the genesis of ancient igneous complexes should concentrate on minerals with low parent/daughter elemental ratios, such as apatite and calcite for Sr isotopes, and zircon and baddeleyite for Hf isotopes.