Roodeplaat

The Roodeplaat volcano occupies an oval area of 16x12 km of low relief and consists of an outer zone of trachyte lavas and minor trachytic tuffs, pyroclastic breccias, pebbly quartzites, monchiquite lavas and felsite with, on the northern and western sides, plugs and sheets of foyaite (Frick and Malherbe, 1986). The inner zone comprises poorly exposed, inward-dipping tuffaceous shales with quartzite layers. Arcuate diabase sheets cut both zones. Frick and Malherbe (1986) describe three small carbonatite dykes on the banks of the Pienaars River along the southern margin of the caldera but Verwoerd (1993) says they cannot be substantiated at present and that Roodeplaat should not be regarded as a carbonatite volcano, a view he had held earlier (Verwoerd, 1967). The trachytes, which are the most abundant rocks of the volcano are not peralkaline but the monchiquitic lavas contain barkevikite phenocrysts and an alkali amphibole in the groundmass. The foyaites comprise coarse, porphyritic and fine-grained varieties. The plug on the western side of the area, at Wallmannsthal, has been described by Shand (1922) and consists of perthite plates up to 2 cm in diameter, euhedral nepheline, sodalite, fibrous zeolite, stout prisms and needles of aegirine, titanite, apatite and an opaque phase. Two smaller plugs nearby are also of foyaite. A similar rock but with strongly zoned aegirine is described by Lombaard (1930) from Haakdoornfontein in the central northern part of the complex. A fluorite-apatite rock occurs at Wallmannsthal. It extends over nearly 150x50 metres and consists of about 20% apatite and 60% fluorite (Lombaard, 1930). Petrographic details of these rocks are also given by Frick and Malherbe (1986), who also present analyses of minerals and many rocks.