Abstract

The Limpopo Orogenic Belt is a linear zone of gneisses separating the adjacent granite-greenstone cratons of Rhodesia and the Transvaal. Previous age data suggested that the high-grade gneisses in the belt were formed 2,000 m.y. ago. In this Rb-Sr age study, three whole-rock isochrons and two mineral isochrons were obtained as well as 28 other isotope dilution analyses on minerals and whole rocks from scattered localities. A concordant isochron for the syngenetic Bulai and Singelele granites in the center of the belt yields an age of 2,690 ± 60 m.y. The age dates the formation of these rocks and, in addition to other evidence, indicates that the main folding and metamorphism dates back to 2,700 m.y. The average of 20 biotite and K-feldspar ages, however, confirms a strong thermal event at 2,000 ± 70 m.y. Equilibration of Sr isotopes at 2,000 m.y. in the Limpopo Belt can be related to intrusive and other contemporaneous events in southern Africa and may reflect the widespread “Eburnean” orogenic event in Africa. The formation of the Limpopo gneisses around 2,700 m.y. appears to be related to the widespread “Shamvaian” orogeny which strongly affected the Rhodesian and Transvaal cratons.

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