K-feldspars form important detrital and authigenic components in sediments but may be difficult to separate by conventional means for isotope analysis, particularly geochronology. UV lasers are being increasing applied to in situ analysis since they are strongly absorbed by clear minerals such as K-feldspar, and can achieve a spatial resolution of better than 20 μm for individual Ar/Ar analyses. Here we demonstrate separation of detrital K-feldspar with a mean age of 423±14 Ma and authigenic overgrowths with a mean age of 138±5 Ma in clastic sediments from the Msolwa Group of the Kilombero Rift of Tanzania. The young age obtained for authigenic K-feldspar overgrowth demonstrates that precipitation occurred more than 160 Ma after sedimentation, during the period of deepest burial and synchronous with the initial extensional phases of the intracontinental Anza and Sudan rifts, as well as the initial stages in the break-up and separation of South America and Africa.