Structural and compositional constraints on the emplacement of the Bushveld Complex, South Africa

Abstract

Despite a plethora of petrological studies, the emplacement mechanics of the world's largest layered intrusion, the 2.05 Ga Bushveld Complex, are still poorly understood. Early models considered the intrusion to comprise separate, lopolithic intrusions or even concentric cone sheets, but recently, overwhelming support for a sill-like intrusional form has emerged. Examination of the contact aureole reveals three groups of emplacement-related structures: interfinger deformation zones and bridges, formed between intruding and dilating magma fingers, preserved in both the western and eastern parts of the Complex; sub-Bushveld Complex intrusions, that reflect finger-like emplacement into the underlying contact aureole; and diapiric domes that characterize the eastern contact aureole, formed by diapiric amplification of initial interfinger deformation zones associated with the earliest mafic–ultramafic pulse of the Bushveld Complex. Most of these structures exhibit a strong NW–SE preferred orientation, while longitudinal terminations and divergence of the sub-Bushveld intrusions away from their source horizons indicate magma emplacement towards the SE. This emplacement direction is supported by petrological data, lateral lithological facies variations and mineral chemical variations within specific horizons. The thickest and most chemically primitive accumulations of the Lower Zone of the Complex are found adjacent to, and thin towards, the NE and SE, away from the Thabazimbi–Murchison lineament (TML), a crustal-scale lineament that has undergone polyphase reactivation since at least 2.7 Ga. The Bushveld Complex was most likely fed by a feeder dyke that utilised the TML, spreading laterally from the dyke-axis to form its current sill-like geometry. The stress field at 2.05 Ga was suitably oriented to allow for dilation of the ENE-trending TML, suggesting the Kaapvaal craton was subject to a component of NW–SE extension. During this time NW–SE directed collision of the Kaapvaal and Zimbabwe cratons is recorded in the Limpopo belt, which would suggest emplacement under conditions of far field extensional stress within a back-arc setting. This is corroborated by petrological data that supports a significant component of subducted crust in the Bushveld magmas.