Asthenospheric and lithospheric sources for Mesozoic dolerites from Liberia (Africa): trace element and isotopic evidence

Abstract

Combined elemental, and Sr and Nd isotopic data are presented for Mesozoic dolerite dikes of Liberia (Africa) which are related to the initial stage of opening of the Atlantic Ocean. The large scatter of both trace element and isotopic data allows the identification of five groups of dolerites which cannot be related to each other by simple processes of mineral fractionation from a common source. On the contrary, the observed chemical and isotopic variation within some dolerites (Groups I and II) may result either from variable degrees of melting of an isotopically heterogeneous source or mixing between enriched and depleted oceanic type mantle. For the other dolerites (Groups III–V) mixing with a third mantle source with more radiogenic Sr and with element ratios characteristic of subduction environments is suggested. This third source is probably the subcontinental lithospheric mantle. Finally, no significant modification by interaction with continental crust is apparent in most of the analyzed samples.