A 2.9-b.y. event in the Rhodesian Archaean

Abstract

Rb-Sr whole rock isochrons corresponding to ages of2.87 ± 0.07 b.y. and2.93 ± 0.16 b.y. (2σ errors) have been obtained on granitic rocks from southern Rhodesia. Tuffs from the Lower Greenstones succession at Belingwe gave a Rb-Sr whole rock age of2.51 ± 0.10 b.y. with a high initial 87Sr/ 86Sr ratio of0.7056 ± 4. Samples from two localities within the Chibi granite yield sub-parallel whole rock isochrons but they demonstrate significant differences in their initial Sr compositions 2.62 b.y. ago (0.7047 and 0.7014). Rb-Sr mineral results from six localities on the Rhodesian craton indicate that biotites last cooled through their blocking conditions 2.65–2.51 b.y. ago, although co-existing muscovites sometimes preserve ages closer to the whole rock age of the host rock. Biotites from three localities along the southern margin of the craton yield ages of 2.10–1.87 b.y. reflecting the 2.0-b.y. thermal event that appears to be restricted to the Limpopo mobile belt in this area. The available data as to the nature of the 2.9 b.y. event are reviewed with particular attention given to whether greenstone belts of that age can be recognised. Problems such as Archaean K-rich adamellites with fairly low initial Sr ratios, and the relationship between high-grade granulite and low-grade granite-greenstone terrains are re-assessed in the light of our knowledge of the 2.9-b.y. event.