SHRIMP U-Pb zircon geochronology of gneisses from the Gweta borehole, northeast Botswana: implications for the Palaeoproterozoic Magondi Belt in southern Africa

This paper presents new U-Pb zircon analyses from garnet-sillimanite paragn eisses from the Gweta borehole in northeast Botswana. Concordant to near-co ncordant analyses of zircon from these rocks reveal a billion year history from 3015 +/- 21 Ma for the oldest detrital grain measured, to the age of h igh-grade metamorphism, 2027 +/-8 Ma. The maximum age of sedimentation in t he Magondi belt is constrained by the age of the youngest concordant detrit al zircon at 2125 +/-6 Ma. This contrasts with the age of sedimentation in the Central Zone of the Limpopo belt which is Archaean. The comparison of o ur results with U-Pb zircon data from the Magondi belt in Zimbabwe suggests that the granulite-facies metamorphism in this belt extended between c. 20 27-1960 Ma. Granulite-facies rocks with U-Pb zircon ages in this interval a re also known in the Ubendian belt and lend support to the correlation of t hese two segments of Palaeoproterozoic belts in southern and central-easter n Africa. The granulite facies metamorphism in the Magondi belt is coeval w ith the high-grade metamorphism and granitoids documented further south in the Central Zone of the Limpopo Belt.