The Mushandike granite: further evidence for 3.4 Ga magmatism in the Zimbabwe craton

Abstract

The conflict between independently published ages for the Mushandike Granite, Zimbabwe (2.92 ± 0.17 Ga and 3.45 ± 0.13 Ga) has been resolved in favour of the older age by SHRIMP U–Pb analyses of zircon. Two samples yield indistinguishable estimates of 3374 ± 7 and 3368 ± 11 Ma for the crystallization age of the magma. Together with published data from elsewhere in southern Zimbabwe, the results imply a widespread magmatic event at about 3.35 Ga. A single zircon core giving 3.46 Ga, together with the granite's previously measured Nd model age, suggests that the Mushandike magma could have incorporated remobilized basement similar to the c. 3.5 Ga Tokwe gneisses which crop out 30 km to the west. The published Rb–Sr and Pb–Pb datasets show evidence of late Archaean disturbance of Sr and Pb isotope systematics. In the absence of exposed contacts between the Mushandike granite and the neighbouring Mushandike stromatolitic limestone, the new U–Pb emplacement age suggests that the limestone is unconformable on the granite.