Age and significance of the Marble Hall breccia, Bushveld Complex, South Africa

Abstract

Diorite and associated breccia form scattered outcrops on the Marble Hall Fragment, in the vicinity of the town of Marble Hall, Mpumalanga Province, South Africa. The diorite (2055.6 ± 3.1 Ma) intruded as sub-horizontal sills in highly deformed and metamorphosed supracrustal rocks of the Transvaal Supergroup. The breccia, which ranges from polymictic to monomictic, consists of angular to sub-rounded fragments of meta-dolomite, chert and adinole, as well as intensely scapolitized Marble Hall diorite and Bushveld B1-type sill rock. Predominantly sodic pargasite constitutes the breccia-fill of the monomictic breccia and represents the fractionated liquid of transitional basalt that crystallized approximately 20% amphibole and 40% plagioclase (An40). The breccia-fill of the polymictic breccia comprises ferroactinolite-albite rock, with minor diopside, calcite, sphene, scapolite, grossular garnet, chalcopyrite and pyrrhotite. Cryptocrystalline silica or calcite is less common as the matrix of the polymicitc breccia. 40Ar/39Ar step-heating analyses on the sodic pargasite yields a plateau age of 2062 ± 8 Ma (55.4% 39Ar; MSWD = 0.28). The bulk of the evidence suggests that the emplacement of the Marble Hall diorite and breccia formation overlap with the B1 sill emplacement. The former rocks may be marginally younger than the Lower Zone of the Bushveld Complex.