Ion microprobe U-Pb zircon and baddeleyite ages for the Great Dyke and its satellite dykes, Zimbabwe

The Great Dyke and its two main satellites, the Umvimeela and East Dykes, trend in a north-northeast direction across Archaean basement rocks of the Zimbabwe Craton. SHRIMP analysis of baddeleyite from all three intrusions yields statistically identical results and confirms their essentially synchronous emplacement at 2574 +/- 2 Ma. SHRIMP U-Pb data for zircons from the Great Dyke are variably discordant and yield an upper intercept age of 258 +/- 11 Ma. Dispersion of the data, however, implies that the zircons have lost variable amounts of radiogenic Pb between about 1200 and 700 Ma. possibly during more than one event. Pb loss at about 700 Ma may be related to the onset of Pan-African tectonism. Pb loss at about 1100 Ma is consistent with chemical alteration during the craton-wide Umkondo igneous event, as demonstrated by previous palaeomagnetic data for the Umvimeela Dyke. Because the effects of multiple Pb loss events cannot be quantified, a more reliable age estimate for zircon crystallization is 2571 +/- 9 Ma, based on the mean Pb-207*/Pb-206* of the five most concordant analyses. Although an age of 2587 +/- g Ma has been reported for crystallization of the Great Dyke, recalculation of the previous U-Pb rutile and zircon data indicates that an age of 2574 Ma is more appropriate.