Palaeomagnetism and aeromagnetic modelling of the Mesoproterozoic Ntimbankulu pluton, Kwazulu-Natal, South Africa: mushroom-shaped diapir?

The aeromagnetic signature of the Mesoproterozoic Ntimbankulu porphyritic granite-charnockite pluton, southern KwaZulu-Natal, is conspicuously circular. Surface geological mapping of the area broadly confirms such a geometry: the pluton is expressed topographically as an annulur ring of marginal hills, composed of porphyritic granite, surrounding a low-lying central core underlain by charnockite. The pluton forms one of the least deformed of ten major granite-charnockite intrusions known collectively as the Oribi Gorge Suite. The suite, which has been dated at 1050 Ma, intrudes the Mzumbe and Margate Terranes of the 1.1 Ga Natal Metamorphic Province. A palaeomagnetic study was performed on samples from the pluton and the surrounding country rock gneisses. The measured directions of magnetisation of the different sites were combined to calculate the virtual palaeomagnetic pole position of the pluton, which was determined at 27°N and 327°E. This is consistent with previously published poles from the Namaqua-Natal Metamorphic Belt. The magnetic parameters derived during the palaeomagnetic study were integrated into the aeromagnetic data to model three selected magnetic profiles across the pluton. The aim was to test the hypothesis that the pluton has a mushroom-shaped 3D form with remnants of Mapumulo Group gneiss situated underneath, and dipping towards, the pluton. It is concluded that these remnants of gneiss with their negative inclination are responsible for the ring dyke like signature of the aeromagnetic data.