A PALEOMAGNETIC STUDY OF THE UMVIMEELA-DYKE, ZIMBABWE - EVIDENCE FOR A MESOPROTEROZOIC OVERPRINT

Palaeomagnetic and magnetic susceptibility results are reported from s ixteen new sites (complementing the previous seven) on the Umvimeela D yke, one of the two major satellite dykes parallelling the Great Dyke of Zimbabwe. Four of the new sites are in the deformed northernmost se ction of the dyke with a further four just to their south. The magneti sation of the long cratonic section of the Umvimeela Dyke is character ised by a component P, carried principally by magnetite and considered primary, with a mean direction D=219.8 degrees, I=-58.7 degrees with k=116.8 and alpha(95)=3.3 degrees. This direction is almost identical to those of the Great Dyke and two other associated intrusions. The an gular dispersion of site mean directions for the Umvimeela Dyke is onl y 6.5 degrees, suggesting a short time of emplacement and cooling. A s econd component S, carried by magnetite, titanomagnetite and ilmenohae matite, is present at most sites but in bigger proportions as the Limp opo and Zambezi Belts are approached. Considerations such as geographi cal distribution, magnetic susceptibility and dyke geology suggest S i s an overprint. Its direction (D approximate to 195 degrees, I approxi mate to 10 degrees) is similar to that of the similar to 1100 Ma Umkon do 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 comp onent, but the other three have been completely remagnetized. At two o f these a component Z was detected which may have been introduced simi lar to 650 Ma ago. This suggests that this section, and the accompanyi ng Musengezi Complex of the Great Dyke, were deformed in the late Pan- African orogenic phase of the Zambezi Belt.