Magmatic erosion of high-temperature-melting cumulates in the Bushveld Complex by chemical dissolution

Abstract

• At least 10–20 m of floor cumulates are eroded away in the Bushveld complex. • These cumulates have very high melting temperatures (∼1400–1500 °C). • Low-temperature (∼1200 °C) replenishing melts cannot melt such cumulates. • The cumulates can, however, be chemically dissolved in these melts. • The replenishing melts were likely slightly superheated. The Bushveld Complex in South Africa shows spectacular examples of regional and local magmatic erosion of the floor cumulates by new melt batches that replenished the evolving magma chamber. Field observations indicate that, at some stratigraphic levels, at least, 15–20 m of pre-existing floor cumulates, often nearly monomineralic in composition (e.g., anorthosite or orthopyroxenite) were completely removed on a regional scale. What was the major agent of this erosion – (partial) melting or dissolution of the floor cumulates – remains poorly understood. Thermal melting appears to be a poor candidate because normal basaltic melts (∼1220–1260°C) cannot heat up the cumulates up to their melting temperature (∼1400–1500°C). We explored, therefore, the possibility of dissolution of these high-temperature-melting cumulates by slightly superheated (15°C above the liquidus) basaltic-andesitic melts that recharged the chamber and spread out laterally along its floor as basal flows. This was done using a freely available thermodynamic tool for phase equilibria modeling of open magmatic systems – the Magma Chamber Simulator. Our thermodynamic modeling shows that the superheated melts can digest up to 4.5–8.0 wt% of the bulk floor cumulates without inducing crystallization of the melts, despite them being much colder than the liquidus temperatures of these cumulates. This is equivalent to regional erosion of 15–24 m of the floor cumulates, given a basal melt layer of about 350 m thick. We conclude that the regional erosion of the high-temperature-melting floor cumulates in the Bushveld chamber has been mostly controlled by their chemical dissolution by replenishing superheated melts.