Abstract

A Rb-Sr isotope study of gneiss units in the Limpopo orogenic belt at Pikwe, Botswana, southern Africa, confirms two previously established isotopic events in the belt at 2,700 and 2,000 m.y. B.P. The new data make it possible to relate these ages to specific events in the tectonometamorphic history of the Pikwe area and to show that some deformation took place substantially later than the emplacement of the satellite extensions of the Great Dyke (2,600 ±120 m.y. ago), which cut Limpopo belt gneiss units to the north in Rhodesia.

In the first recognizable tectonometamorphic event affecting the Pikwe area, the formation of regional gneissic foliation was accompanied by granulite facies conditions and extensive partial melting. This metamorphism has a Rb-Sr whole-rock isochron age of 2,660 ± 40 m.y. (2σ) and resulted in the complete isotopic homogenization of Sr across distances of at least 350 m in the gneissic succession.

Upright folds were superimposed on recumbent folds in a late polyphase event 2,100 to 2,000 m.y. ago. Minor structures in a late D3 shear zone show a transition from ductile to brittle deformation, and from early D2 to late D3, metamorphic conditions fell from upper amphibolite to greenschist facies. Upper amphibolite facies conditions caused near-complete isotopic rehomogenization of Sr on a scale of 7 cm; isotopic mixing was restricted to less than 25 cm.

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