Zircon geochronology and geochemistry of pre-Bushveld sills in the eastern Transvaal Supergroup, South Africa

Abstract

An extensive suite of sills occurring in the eastern and western Transvaal Supergroup has been termed the Marico Diabase Suite. As a result of their overall geographic proximity to the Bushveld Complex and occurrence in the Transvaal Supergroup they have been assumed to be related to the Bushveld Complex. Previous studies have identified two different types of rocks within this suite, namely the Maruleng and Lydenburg types, based on geochemical and metamorphic characteristics. The Maruleng type has mineralogical (orthopyroxene-bearing) and geochemical affinities with the mafic rocks of the Bushveld Complex, and rocks belonging to this type are much more closely spatially related to the Bushveld Complex. The Lydenburg type, on the other hand, is much more extensive spatially and was emplaced at variable depths within the Transvaal Supergroup. The latter type of rock is variably metamorphosed, far beyond the likely thermal effects arising from emplacement of the Bushveld Complex. Using LA-SF-ICPMS and LA-MC-ICMS for U-Pb-Hf isotope ratio and REE+Ti concentration analysis, we demonstrate that zircon grains from this latter type were subjected to hydrothermal alteration, which caused the introduction of common lead. Despite compositional and isotopic overprinting, an age of 2 083 ± 18 Ma (MSWD = 12, n = 16) is obtained, suggesting a pre-Bushveld in age. We suggest that the original term Marico Diabase Suite encompasses two different events. The Maruleng Suite should be grouped as part of the Bushveld Complex, whereas the Lydenburg Suite should be given a status that is independent from the Bushveld event.