Abstract

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 2581 ±11 Ma. Dispersion ofthe data, however, implies that the zircons have lost variable amounts ofradiogenic Pb between about 1200 and 700 Ma, possibly during more than one event. Pb loss at about 700 Ma may be related to theonset 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 UmvimeelaDyke. Because the effects of multiple Pb loss events cannotbe quantified, a more reliable age estimate for zircon crystallization is 2571 ±9 Ma, based on the mean 207Pb*/206Pb* of the five most concordant analyses. Although an age of 2587 ±8 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 Mais more appropriate.

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