Chemical stratigraphy paradox of the Bushveld Igneous Complex

Abstract

The largest basaltic magmatic body in Earth’s crust – the Bushveld Complex in South Africa – reveals an intriguing paradox. All available field, chemical and isotopic data indicate that this complex has been operating as an open-system chamber that grew incrementally via several major magma replenishment events. Yet – surprisingly enough – the complex shows that the crystallisation sequence is broadly consistent with the internal differentiation of a single parental magma under closed-system conditions. Here we propose that this chemical stratigraphy paradox of the Bushveld Complex is due to its evolution being driven by magma replenishment from a much larger, deep-seated staging chamber located at the crust–mantle boundary. This deep staging chamber was formed from mantle-derived magmas and internally differentiated producing resident melts of progressively evolved composition. The influx of the increasingly evolved melts from the staging chamber into the shallow crust, followed by repeated replenishments, initiated and sustained the growth of the shallow Bushveld chamber. From the initial major influx onward, the deep and shallow magma chambers differentiated roughly synchronously, such that their resident melts were of broadly similar composition at any given time. As a result, magma influxes from the deep staging chamber have fuelled the evolution of the shallow chamber, increasing its volume without disrupting its overall evolutionary trend. We thus attribute the chemical stratigraphy paradox of the Bushveld Complex to the linked magmatic evolution and deep-to-shallow replenishment between two superlarge magma chambers of the Bushveld magmatic system.