Petrochemistry of precambrian gneisses and migmatites in the western part of nigeria

The Precambrian rocks of Nigeria can be distinguished into three major divisions. The polycyclic basement complex; the series of approximately north-south trending schist belts which predominantly comprise low-medium grade metasediments with subordinate mafic-ultramafic rocks, marbles and calc-gneisses; and the Pan-African (ca. 600 Ma) granites and related rocks which commonly intrude into the above successions. The polycyclic basement complex which was evidently reactivated during the Pan-African tectogenesis, is composed dominantly of a wide range of textural and/or mineralogical types of gneisses and migmatitos. Chemical data show that the majority of these rocks are of supracrustal composition and are characterized by relatively high contents of Si0 2 , K 2 0 and related trace elements. However, some, particularly the melanocratic varieties have lower values of these elements, but are comparatively enhanced in Fe, Ti, Cr, and Ni concentrations. The chemical variations suggest that the rocks were derived from parents of varying compositions, some of which may have been partly volcanogenic.