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

Abstract

The Archaean crystalline ‘basement’ in Sierra Leone is made up of two groups of felsic rocks. These are the older gneisses and younger granitoids. In the West African Archaean Craton there are at least three different age group of older gneisses with U-Pb zircon crystallisation ages of 3.5, 3.3–3.1 and 3.0–2.9 Ga. There are two main types of younger granite – medium grained biotite granites and porphyritic granites and the main intrusive episode appears to have been at 2.8 Ga. This craton is underlain by an extremely thick root of subcontinental lithospheric mantle and its Pb-isotopic composition suggests that it may be a high-μ Archaean craton.This study presents new whole-rock and mineral geochemical data from an extensive area in the Archaean basement of Sierra Leone and shows that the older gneisses range in composition from tonalite to granite, whereas the younger granitoids are trondhjemite and granite. Both older gneisses and younger granitoids have a very similar composition range with silica contents between 60 and 77 wt % SiO2. In part this compositional range is attributed to magmatic fractionation, particularly feldspar removal and accumulation. Average compositions of both gneisses and granitoids are granitic making this area of the West African Archaean Craton more potassic than average Archaean terrains.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.