The geochemical evolution of Archaean felsic gneisses in the West African Craton in Sierra Leone

There are three geochemical features of importance in this region and which form the key to understanding the evolution of felsic rocks in this area. Firstly, the older gneisses have a granitoid composition consistent with their derivation by the partial melting of a mafic source at moderate pressures of 7.5-11.5 kb and at temperatures of 900-950 °C. Secondly, the atypical and highly potassic nature of these gneisses reflects the fact that have been reworked and the in-mixing of potassic partial melts during the reworking gives rise to their present highly potassic nature. Thus they appear to be derived from older crustal materials. Finally, the younger granitoids are also the product of the partial melting of older crustal rocks.