Contrasting Magmatic Styles in the Two Greatest Mafic Intrusions of Southern Africa: The Bushveld Complex and the Great Dyke

Allan Wilson and Gordon Chunnett
Wits University, Johannesburg, South Africa

The Great Dyke and the Bushveld Complex are two of the most important and voluminous mafic intrusions in the world. Combined, they contain over 90% of the world’s platinum resources and 75% of the chrome resources. Yet they are strikingly different with respect to magmatic origin and evolution, structure and morphology. The Great Dyke is elongate and dyke-like, while the Bushveld is lobate and sill-like. The Great Dyke possesses a primitive mantle signature and has little evidence of crustal contamination whereas the Bushveld rocks have a strong upper crustal signature. In both intrusions, complexities in radiogenic isotopes are testament to markedly different source rocks and ACF processes. The primary magma of the Great Dyke had 15% MgO. The magmas that made up the Bushveld Complex are supposedly of two types - an enriched primitive magma of about 12% MgO and a depleted magma with 7% MgO. These compositions are derived from sill complexes outside of the Bushveld. They are at variance with chill zones recognized in many areas of the Bushveld complex and of a recently discovered high-Mg basal phase containing exceptionally Mg- rich olivine. The mechanisms of emplacement are also fundamentally different. The Great Dyke contains highly predictable cyclic units of ultramafic rock whereas the Bushveld Complex contains associations of rock types and mineral compositional trends that are not consistent with simple models of magma chamber recharge and fractionation.

Continued developments in the understanding of these two intrusions provide a valid means of testing contrasting processes in magma chambers and enriche our knowledge in the primary mechanisms that give rise ultimately to subcontinental-size mafic intrusions.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery