The Tugela Terrane, Natal belt: Kibaran magmatism and tectonism along the southeast margin of the Kaapvaal Craton

Abstract

This paper is a preliminary report on the geology of the Mandleni tectonostratigraphic package (MTP) part of the Tugela terrane, the northernmost element of the Natal belt in southeastern Africa. The Dondwana gneiss unit, a metamorphosed bimodal magmatic sequence comprising feldspathic gneiss, with lesser amounts of amphibolite, underlies much of the MTP. All primary structures in these rocks have been obliterated during post-depositional metamorphism and deformation. Fabrics attributable to the oldest recognised tectonic event (D1) have largely been overprinted during subsequent deformation. D2 was characterised by syn-kinematic high-pressure (P>9 kbar) granulite facies metamorphism. The dominant S2 planar fabric is a penetrative gneissic foliation that parallels lithological contacts. It is defined by colour and compositional banding, and by parallelism of coarsely crystalline mineral grains. D3 tectonism produced asymmetric north-verging folds, faults and shear zones, which accommodated overthrusting of the MTP onto the Kaapvaal craton. Plagioclase ± biotite mantles on garnet developed at this time and are attributable to uplift of the MTP from the mid- to deep-crustal levels established during D2. Ultramafic rocks (the Evuleka ultramafics) tectonically interleaved with the Dondwana gneiss unit during D3, consist of talc schist, and massive serpentinite with disseminated chromite and magnetite. These ultramafic rocks are interpreted as altered and sheared mantle or cumulate from the lower portion of an ophiolite sequence. Granitoid magmatism associated with D3 is related to subduction beneath the MTP. This subduction led to basin-closure and emplacement of the MTP onto the Kaapvaal craton. The final tectonic event (D4) produced broad open south-plunging folds and sinistral shear zones.