The transition between the mafic and the acid phase of the Bushveld complex in the Rustenburg-Brits area

Abstract

In the western Transvaal the Veekraal granite (the Lebowa Granite Suite) and the basic rocks (the Rustenburg Layered Suite) of the Bushveld Complex are separated by a thickness of acid rocks here called the transition zone. The transition zone consists predominantly of two rock types: the Beestkraal granophyre, and the Zwartbank pseudogranophyre. In addition to these rocks, microgranite, Veekraal granite in the form of downward offshoots from the overlying granite and xenoliths of sedimentary rocks are also found in the transition zone. The Zwartbank pseudogranophyre is a transformed rock type, the parent rocks being the uppermost portion of the Pretoria Group. Two textural varieties of the Zwartbank rocks are recognized, the pseudogranophyre and the poikilogranite. These are regarded as transformation products of coarse- and fine-grained parent rocks respectively. The transformation took place during, and as a consequence of, the emplacement of the layered sequence. On the other hand, available evidence indicates that the Beestkraal granophyre is a magmatic rock formed by cotectic crystallization from a melt. This melt formed and intruded after the emplacement of the layered sequence and the formation of the Zwartbank pseudogranophyre. No certainty exists concerning the derivation of the granophyre melt. Intrusion of the Veekraal granite appears to be the final event in the igneous history of the western Bushveld Complex. Tectonism occurred during and after the intrusion of the western Bushveld Complex and is taken to be responsible for controlling the intrusive events. Tectonism has also resulted in a structure consisting of a series of folds having northwest-southeast striking fold axes. An anticline was formed in the centre of the Veekraal granite along the crest of which rocks of the transition zone are exposed. Basic rocks of the layered sequence here underlie the transition zone at a relatively shallow depth {about 500 m or less).