A chill sequence to the bushveld complex: Insight into the first stage of emplacement and implications for the parental magmas

Abstract

A rare occurrence of a chill sequence in drill core from the eastern Bushveld Complex has been discovered at the base of a thick succession of ultramafic rocks that forms part of the Lower Zone. The lowest 10 m of the section preserves a variety of rock types including a true chill against quartzite floor rock, crystalline quench-textured and spinifex-textured rocks as well as high-temperature olivine and orthopyroxene cumulates. It represents the first stage of magma emplacement into the Bushveld chamber and gives an insight into the nature of the parental magmas to the Bushveld, the processes that took place at that early stage, and how rocks with a high original interstitial liquid content give way to more normal cumulates higher in the section. The chill sequence also provides insight into the variety of rock types that are encountered in marginal sills that are regarded as representing early stage magmas intruded as the chamber developed but that are incompletely understood because of inadequate field exposures. Olivine compositions (up to Mg# 0·912) in a pyroxene dunite in this section are the highest recorded for the Bushveld Complex and cores of associated orthopyroxene have Mg# 0·93. Zoned orthopyroxenes in the quench- and spinifex-textured units range from Mg# 0·91 to 0·72 and preserve core compositions close to the original liquidus. Small single chromite crystals have Cr/(Cr+Al) of 0·85 and Cr/Fe(Total) of three; these are also the most primitive compositions found to date in the Bushveld Complex. The chill and quench zones represent the earliest magmas to be emplaced in the Bushveld Complex; however, these are relatively evolved and similar to the B1 liquid, long assumed to be the most primitive magma that gave rise to the Lower Zone. Major and trace element geochemistry and mineral compositions show that the true parental magmas to the Lower Zone were of komatiite composition, with the most primitive containing 19% MgO. The liquid compositions have a strong crustal signature but are also enriched in K, Rb, Pb and especially Cs and Cl. The olivines in this section are highly Ni enriched, consistent with a component of mantle pyroxenite derived from recycled ocean crust in the source of the parental magmas. It is suggested that this source, combined with PGE-fertile subcontinental lithospheric mantle, was melted within a rising mantle plume and the resultant melts then interacted with the basement rocks of the Kapvaal Craton. © The Author 2012. Published by Oxford University Press. All rights reserved.