U-Pb single zircon geochronology of granitoids in the Vumba granite-greenstone terrain (NE Botswana): Implications for the evolution of the Archaean Zimbabwe craton

U-Pb SHRIMP single zircon crystallization ages were determined on representative samples of various granitoids exposed in the Vumba granite -greenstone terrain in NE Botswana. Based on structural and petrographic features, these granitoids were previously divided into five chronological groups: G1 (oldest) up to G5 (youngest). This chronology is not confirmed by U-Pb zircon dating, except for G5, which is the youngest granite. Our data show that igneous crystallization ages in the Vumba granite -greenstone terrain fall in the range 2696+/-4 and 2647+/-4 Ma. This time span is similar to the range of U-Pb zircon ages of 2710+/-19-2646+/-3 Ma for the Matsitama granite -greenstone terrain in NE Botswana. The oldest zircon age (2733 +/- 5 Ma) in the Vumba granite -greenstone terrain was obtained on a mafic xenolith from G2 gneiss/migmatite. This xenolith is inferred to represent a fragment of the Lower mafic-ultramafic volcanic unit of the Vumba greenstone belt. The minimum estimate of the protolith age of this gneiss is 2690+/-4 Ma and is within error of the age of the G1 tonalitic gneiss (2686+/-6 Ma). A crystallization age of 2696+/-4 Ma is yielded by the G4 dioritic granitoid, which was previously assumed to be part of the ca. 2.56 Ga post-kinematic granitoids. Field relationships and textural features suggest that this granitoid represent a subvolcanic intrusion probably coeval to the Upper intermediate to felsic volcanic units of the Vumba greenstone belt. A comparison of crystallization ages suggests that the Neoarchaean granitoids from NE Botswana might be correlated to the Sesombi granitoids and Upper Bulawayan volcanic sequence of the Zimbabwe craton. The G5 granitoids in the Vumba granite-greenstone terrain were emplaced at the same time as the Razi suite granitoids in Zimbabwe. The discrete shear zones documented in the Vumba granite -greenstone terrain developed between ca. 2686+/-6 and 2647+/-4 Ma. They are correlated to the D1-accretion-related deformation event in the Zimbabwe craton. (C) 2002 Published by Elsevier Science B.V.