A Bushveld-related high-Ti igneous suite (HITIS) derived from an alkali to transitional basaltic magma, South Africa

Abstract

A suite of high-ti mafic to felsic igneous rock (acronym:HITIS) was emplaced at -2055 Ma as km-sized bodies to the South of the Bushveld Complex, South Africa, stretching from Parys and Potchefstroom in the south-west to Marble Hall in the north and Badplaas in the east. Currently known members of the suite include the Marble Hall sills and related intrusives and breccia, the Basal Gabbro Unit of the Uitkomst Complex, the Lindeques Drift and Heidelberg intrusions, the volcanic Roodekraal Complex, the Rietfontien Complex, the Koedoesfontein Complex, the Schoogezïcht (previously Kaffirskraal) Complex, the Vredefort alkali granite, as well as the high-Ti basalt and FeTip-enriched basaltic lava of the Rooiberg Group. The volcanic rocks extruded on mildly folded and denuded metasedimentary rocks and lava of the Pretoria Group, whereas the plutonic rocks intruded at a level between the upper Witwatersrand Supergroup and the lower Pretoria Group, but largely within the dolomite of the Chuniespoort Group. The mafic rocks in this suite are dominated by clinopyroxene (salite to augite), FeTi-oxide (magnetite-ilmenite), amphibole (largely edinitic and hastingsitic), olivine (FO44 to FO80), orthopyroxene and plagioclase (An<50), forming an array of dioritic and subordinate gabbroic rocks with associated clinopyroxene±magnetite(-ilmenite)±olivine±plagioclase cumulates. Locally, hybrid calc-silicate rocks derived from reaction of the magma with dolomite of the Chuniespoort Group are developed. The felsic rocks are syenodiorite and alkali granite with K-feldspar, albite, alkali pyriboles and biotite. Trace elements systematics suggests that the HITIS rocks are derived from an alkali to transitional basaltic parent. The most primitive rocks representing liquid fractions are developed in the chill zone of the Basal Gabbro Unit of the Uitkonmst Complex, (Mg# 0.55 to 0.68) and in the Marble Hall diorite sills (Mg# = 0.53 to 0.65). These are followed by the more evolved high-Ti basalt (Mg# = 0.45 to 0.50) of the Rooiberg Group, the mugearite lavas (Mg# = 0.29 to 0.47) of the volcanic Roodekraal Complex, the Fe-Tip lava (Mg# = 0.17 to 0.30) of the Rooiberg Group, and finally the Vredefort alkali granite (Mg# = 0.26 to 0.72). This rock series resulted from deep-seated (lower crustal?) amphibole fractionation, followed by shallow level clinopyroxene±magnetite(-ilmenitel±olivine±plagioclase fractionation with the FeTi-oxide phases becoming more dominant in the later stages. The Vredefort alkali granite represents the final liquid in the liquid line of descent. The HITIS magma marks an early stage in the Bushveld magmatic event and has a close temporal relationship with the boninitic B1-type magma, regarded as parental to the Lower Zone of the Bushveld Complex. It also appears to have been derived from the same mantle source that yielded the tholeiitic B3-type magma, which is in part responsible for the Main Zone of the Bushveld Complex. Minor copper and PGE concentrations are associated with cumulate rocks of the HITIS.