The age of the Umkondo Group, eastern Zimbabwe, and implications for palaeomagnetic correlations

The palaeomagnetic and geochronological evidence which led to assigning an age of approximately 1800 Ma for the Umkondo Group is reviewed. New Rb-Sr data from the Umkondo dolerites and lavas are presented involving 47 whole rock determinations and three analyses of separated biotites. The former give an errorchron yielding an age of 1078 ± 143 Ma; the scatter of points is largely attributed to country rock contamination. Biotite analyses from three of the samples yielded 1075 ± 21 Ma. Comparisons of palaeomagnetic and geochronological data from various southern African igneous formations suggest widespread igneous activity in the subcontinent some 1200-1100 Ma ago. Implications for the previously suggested palaeomagnetic correlation of the Umkondon Group with the upper parts of the Waterberg Group of South Africa are discussed. -from Authors