Evidences of Metasomatic Processes During the Emplacement of Pan-African Granites in the Eastern End of the West African Craton.

Abstract

Pan African Orogeny has been linked with pervasive potash metasomatism that has affected most parts of the Precambrian Basement Complex of Nigeria. The Nigerian Basement Complex is characterized by several granitoid intrusives marking the Pan-African orogeny (600 Ma). They are seen emplaced within schists and migmatites. These rocks reflect syn- to post collisional environment.The Ilesha schist belt is studied to investigate which of metasomatic or magmatic processes is the more dominant process that affected the emplacement of rocks of the Nigerian basement complex.Aluminium saturation index (ASI) shows that these granites range from metaluminous – peraluminous. Tectonic model reveals that they are calc-alkaline products of continental collision events. Petrographic studies show typical composition of quartz, alkali feldspar, biotite and hornblende as the major mineral phases. Both geochemical and petrographic data infer phase changes that may be connected with potash metasomatism.

Key words: Older granites, Potash metasomatism, Microcline, Pan African orogeny and Ostwald ripening)