Constraints on the Nd-isotopic composition and nature of the last major influx of magma into the Bushveld Complex

Abstract

The Pyroxenite Marker, a thin, orthopyroxene-dominated marker horizon, is observed towards the top of the Main Zone of the Bushveld Complex, where the last voluminous influx of magma into the Bushveld Complex is thought to have occurred. In an attempt to constrain the Nd-isotopic composition of the magma added at the level of the Pyroxenite Marker, a total of 21 whole-rock samples from a borehole (BH7771) drilled on the Central Sector of the Eastern Limb of the Bushveld Complex were analysed for their Sr–Nd isotopic ratios. Modelling suggests that the added magma had a unique Sr (87Sr/86Sri = 0.7063–0.7067) and Nd (ƐNdi on the order of − 5.9) isotopic composition, distinct from any of the rocks constituting the layered sequence below the Pyroxenite Marker. Dispersion of data points around the modelled isotopic (melt–melt) mixing curves is interpreted to reflect the incorporation of minerals derived from either the incoming or resident magmas into individual rock layers occurring across the Pyroxenite Marker interval, either in response to the mixing of minerals settling through a stratified magma column, or potentially through the intrusion and mixing of crystal-laden magmas with unique isotopic compositions from a sub-Bushveld staging chamber.