A palaeomagnetic study of the Umvimeela Dyke, Zimbabwe: evidence for a Mesoproterozoic overprint

Abstract

Palaeomagnetic and magnetic susceptibility results are reported from sixteen new sites (complementing the previous seven) on the Umvimeela Dyke, one of the two major satellite dykes parallelling the Great Dyke of Zimbabwe. Four of the new sites are in the deformed northernmost section of the dyke with a further four just to their south. The magnetisation of the long cratonic section of the Umvimeela Dyke is characterised by a component P, carried principally by magnetite and considered primary, with a mean direction D=219.8°, I=−58.7° with k=116.8 and α95=3.3°. This direction is almost identical to those of the Great Dyke and two other associated intrusions. The angular dispersion of site mean directions for the Umvimeela Dyke is only 6.5°, suggesting a short time of emplacement and cooling. A second component S, carried by magnetite, titanomagnetite and ilmenohaematite, is present at most sites but in bigger proportions as the Limpopo and Zambezi Belts are approached. Considerations such as geographical distribution, magnetic susceptibility and dyke geology suggest S is an overprint. Its direction (D≈195°, I≈10°) is similar to that of the ∼ 1100 Ma Umkondo dolerites and it is suggested that it was introduced chemically at or just before the onset of Pan-African activity in the region. Of the four sites in the deformed section of the dyke one carries the P component, but the other three have been completely remagnetized. At two of these a component Z was detected which may have been introduced ∼ 650 Ma ago. This suggests that this section, and the accompanying Musengezi Complex of the Great Dyke, were deformed in the late Pan-African orogenic phase of the Zambezi Belt.