PETROCHEMISTRY AND AGE OF FRANZFONTEIN GRANITIC ROCKS OF NORTHERN SOUTH-WEST AFRICA

Chemical, spectrographic and isotopic analyses are presented for alkali and calc-alkali granitic rocks from the pre-Otavi Facies crystalline basement of northern South-West Africa. On the basis of new Rb-Sr whole rock isotopic studies and a previously published zircon age, the most reliable estimate for the age of emplacement of the Franzfontein granitic rocks is 1650 ± 80 m.y. Three discordant whole rock ages" of 1100-1300 m.y. are believed to reflect the post-crystallisation migration of Sr resulting from sub-aerial weathering. The chemical compositions of this suite are compared with those of a wide variety of African pre-Silurian acidic igneous rocks and with the estimated composition of granites of the earth's crust. © 1969."