Geochemistry and petrogenesis of tholeiitic intrusions of possible Bushveld-age in the Vredefort Dome, South Africa

Abstract

The tholeiitic group of intrusions that occurs in the core and collar rocks of the Vredefort dome comprises three related intrusions, namely the Wittekopjes norite, the Parsons Rust dolerite-norite and the Reebokkop dolerite. The texture of these intrusions varies from hypidiomorphic-granular in the norite to the distinctive poikilitic character of the dolerite-norite to hypidiomorphic to subophitic texture of the dolerite. The major constituent minerals, pyroxene and plagioclase, are normally zoned. Chemical variations in the normally zoned minerals indicate a differentiating magma. The tholeiitic group comprises siliceous high-MgO tholeiitic rocks with a strong noritic character. The characteristic geochemistry of the group includes high MgO, SiO 2, Cr and Ni contents; low enrichment in light and heavy rare earth elements (REE) with regard to average chondrite composition; absence of significant fractionation of light REE with regard to heavy REE; and low TiO 2, P 2O 5, K 2O, Sr, Zr, Y, and Nb contents. Geochemical variation within the sills conforms to normal differentiation trends. To explain the relatively high SiO 2 content in these mafic intrusions two mechanisms can be proposed: incongruent melting of enstatite in a depleted mantle source will generate forsteritic olivine and progressively silica enriched melts; or alternatively, crystallisation of a high percentage of olivine could result in silica enrichment of an ultramafic magma. Two possible scenarios for the generation of a noritic magma with composition similar to that of the Wittekopjes norite are proposed. Firstly, derivation of a noritic magma by means of fractional crystallisation of approximately 50% olivine from an ultramafic Bushveld-type magma. Secondly, derivation of a primary noritic magma from depleted mantle material with near chondritic REE composition, by approximately 42% equilibrium partial melting. The mantle material is similar to the depleted peridotite nodules in South African kimberlite. It is illustrated that the Parsons Rust dolerite-norite and the Reebokkop dolerite can be derived from such a noritic magma by means of fractional crystallisation of olivine, pyroxene and plagioclase. The tholeiitic group is compared to other mafic intrusions in the Vredefort dome. It differs from the tholeiitic Annas Rust dolerite and associated rocks, and can, just like the epidiorite, be classified as calc-alkaline. Considering field, petrographical and geochemical evidence it is postulated that the tholeiitic group is of Bushveld age.