Direct dating of authigenic K-feldspar overgrowths from the Kilombero Riftof Tanzania

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 mum for in dividual Ar/Ar analyses. Here we demonstrate separation of detrital K-felds par with a mean age of 423 +/- 14 Ma and authigenic overgrowths with a mean age of 138 +/- 5 Ma in elastic sediments from the Msolwa Group of the Kilo mbero Rift of Tanzania. The young age obtained for authigenic K-feldspar ov ergrowth demonstrates that precipitation occurred more than 160 Ma after se dimentation, during the period of deepest burial and synchronous with the i nitial 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.