Close association in space and time of Li-, Sn- and barren pegmatites with granites suggests a link by differentiation. However, Rb-Sr data support this hypothesis only for the barren pegmatites. The Li- and Sn-pegmatites, on the other hand, have far too high initial (87Sr/86Sr) ratios to be such differentiates. These ratios are also much higher than they should be if the pegmatites were products of equilibrium melting of Damaran meta-pelites or meta-greywackes and much lower than would be expected if they were partial melts of the pre-Damaran basement. It is proposed to explain the high (87Sr/86Sr) ratios by disequilibrium melting of high Rb/Sr minerals like biotite and/or muscovite which preferentially enter the melt and thus extract radiogenic Sr and rare elements from metasediments.