Whole rock Th-Pb age for the Masuke and Dembe-Divula complexes,Rhodesia

A whole rock U-Th-Pb study was carried out on eight granites from a suite previously dated at 177 ± 4my(λ = 1.39 × 10^?11yr^?1) by Rb-Sr methods. A U-Pb isochron was not obtained owing to recent leaching of U in the outcrop. Seven samples gave a Th-Pb isochron with age 184 ± 16my(λ = 4.99 × 10^?11yr^?1), (^20/8Pb²⁰⁴Pb)₀ = 39.2 ± 0.2 and MSWD 0.8. The eighth sample fell off the line and was rejected on geological grounds. Dissolving 5 g aliquots of the powdered rock markedly improved the reproducibility of U analyses but analysis of the variances suggests that inhomogeneities still might exist between duplicate dissolutions. Th-Pb geochronology needs further development to bring it to the level of precision of the Rb-Sr method.