The petrogenesis of the Winddam, Koedoesfontein, and Rietfontein intrusions, Vredefort Dome, South Africa

Abstract

Three boninitic magmas (labelled B1 to B3) are deemed responsible for the formation of the majority of the Rustenburg Layered Suite (RLS) of the Bushveld Igneous Complex (BIC). A Ti-rich magma, called the Bushveld Uitkomst magma (Bu), is thought to have formed from the same mantle source as B3 but represents a lower fraction melt of the mantle. Bu led to the formation of various satellite intrusions outside the BIC, collectively referred to as the high-Ti igneous suite (HITIS). The chemical evolution of the Bu magma is divided into five stages, labelled A to F. The current view is that three igneous intrusions (consisting of ultramafic to dioritic rocks) located in the Vredefort Dome, called the Winddam wehrlite (Ww), Koedoesfontein Complex (KC), and Rietfontein Complex (RietC), form part of stage D to E of the Bu magma’s evolutionary pathway based on their mineralogy. This hypothesis could not previously be confirmed geochemically as very little information regarding the geochemistry of these intrusions is available. This study serves to accomplish the following goals with the aid of geochemical data: characterise Ww, KC and RietC in terms of their mineralogical, textural, and geochemical features, propose plausible explanations for variations observed in the mineralogical content across the intrusive bodies of Ww, KC, and RietC, determine whether or not Ww, KC, and RietC are related to one another in terms of their geochemical characteristics, determine their relation with the HITIS using a rare earth element modelling approach, and provide information regarding the mineralogical and geochemical composition of the mantle source from which the intrusions originate. Ww primarily consist of olivine-hornblende clinopyroxenite and subordinate olivine-hornblende gabbronorite. KC contains an ultramafic sill consisting of wehrlite, olivine-clinopyroxenite, and clinopyroxenite. Other rock types that form part of KC include hornblende gabbronorite, hornblende norite, pyroxene hornblendite with minor quartz, and diorite. Rock types that form part of RietC include olivine (± hornblende) clinopyroxenite, olivine diorite, and magnetite-rich troctolite and dunite. Rocks from all three intrusions range from fine to medium grained with typically anhedral to subhedral grain shapes. Ultramafic rocks are typically characterised by adcumulate or heteradcumulate textures with interstitial hornblende and plagioclase. Diorite from KC is glomeroporphyritic with augite glomerocrysts with flow textures in its matrix. Flow textures are also present in the magnetite-rich troctolite from RietC and plagioclase grains have lobate grain boundaries where in contact with the magnetite. Deformation textures include planar deformation features in olivine from Ww, pseudotachylite in some ultramafic samples from KC and RietC, granoblastic textures with polygonal mineral aggregates in Ww and troctolite in RietC, and sutured grain boundaries between augite grains. These textures likely originated during meteorite impact. Ultramafic rocks from Ww, KC, and RietC are generally rich in MgO, Ni, Cr, and Co, and poor in TiO2, alkalis, and incompatible trace elements such as Zr, Y, and Rb. Ultramafic intrusions from all three intrusions show significant overlap on Harker diagrams. They commonly display negative high field strength element (HFSE) anomalies and positive Ba and Pb anomalies on multi-element spider diagrams normalised to the primitive mantle. On chondrite normalised rare earth element (REE) diagrams, significant variation in the slopes of LREE are observed while rock samples show comparable HREE patterns. Due to these geochemical similarities, ultramafic rocks from Ww, KC, and RietC are believed to be comagmatic products. Variation in the abundance of olivine and augite across the Ww intrusion is believed to be a product of preferred nucleation of augite in certain areas of the magma, enriching the surrounding melt in olivine component. The enstatite-bearing rocks from KC and troctolite from RietC typically display contrasting geochemical characteristics compared to the ultramafic rocks, and are inferred to have formed from a different magmatic lineage. It is considered possible to produce the diorite from KC via fractional crystallisation of the parental magma of the ultramafic rocks as it possesses similar incompatible element ratios and higher incompatible element contents. Olivine diorite from RietC display intermediate REE compositions between the troctolite and ultramafic rocks and is believed to have formed through mixing of the parental magmas responsible for the formation of the latter two rock types. Regarding their geochemistry, ultramafic rocks from Ww, KC, and RietC are geochemically more primitive than any member of the HITIS, and cannot fit into stage D-E of the Bu magma’s evolutionary pathway. They do display comparable REE patterns to the most primitive member of the HITIS: the Marble Hall primitive diorite (MHPD). Fractionation of olivine, augite, and plagioclase from an estimated magma parental to KC produces a near identical REE pattern compared to the parental melt of the MHPD. This observation implies that these intrusions are related to the HITIS but represent more primitive members. Low REE contents and flat HREE patterns of estimated parental melts of Ww, KC, and RietC suggest they formed by large fraction melting (approximately 40 weight percent) of a spinel lherzolite mantle source. Negative HFSE anomalies suggest the mantle was hydrothermally enriched in the vicinity of a subducting oceanic plate before melting occurred.