Trace element stratigraphy of the bellevue core, Northern Bushveld: Multiple magma injections obscured by diffusive processes

The 3 km long Bellevue Core was recovered from the Upper Zone (UZ) and Main Zone (MZ) of the Northern Limb of the Bushveld Complexçthe largest known layered mafic intrusion. We present abundance data for 57 trace elements hosted in plagioclase, clinopyroxene, low-Ca pyroxene and olivine, obtained by laser ablation inductively coupled plasma mass spectrometry. These data permit the first comprehensive investigation of trace element distribution in the Bushveld Complex on a mineral-by-mineral basis, and show that the Cr content of pyroxene is the only proxy that records three cryptic magma injections, coincident with two pyroxenite horizons and the UZ-MZ boundary. On the basis of Cr abundance, we infer that an 'anomalous' troctolitic horizon at the base of the Bellevue Core was related to injection of relatively primitive magma, analogous to the Lower or Critical Zone magmas in the Eastern and Western Limbs of the Bushveld Complex; this horizon defines the greatest inconsistency of magmatic stratigraphy between the Northern Limb and the Eastern and Western Limbs of the complex, and is prospective for platinumgroup element exploration. However, the most significant finding of this study is that plagioclase, pyroxene and olivine have undergone extensive diffusive modification. The major and trace element geochemistry of these minerals records sub-solidus equilibration during the prolonged cooling of the Bushveld Complex, controlled by diffusion rates and local partitioning between adjacent minerals. By analogy, texturally mature cumulates in any layered intrusion are also likely to be chemically re-equilibrated.