Geology and Geochemistry of Some Crystalline Basement Rocks in Ilesha Area, Southwestern Nigeria: Implication s on Provenance and Evolution

Abstract

Geological and geoch emical study of the basement complex rock s i n ll esha sch i st bel t revea l ed that amph i bol i te, hornbl end e gn e i ss and grani te gn e i ss are the major consti tuents. The gn eisses are composed of si mi lar rock form i n g si l i cates with vari ation s i n abu ndance . Th e amph i bol i te bei ng a mafic rock h as different composi tion s, contai n i n g abu ndant pyroxene , act i nol i te and tremol i t e. Monazi te i s present i n th e mi neral ogy of a l l these rock s. Chem i cal com posi tion of th ese rocks revea l ed th at they a re petrogenetica l l y re lated. Geochem i ca l diagra m s, p l otted from chemical composi ti on of these rocks, R EE fractionation trend s and presence of monazi te i  n th e i r mi neralogy revea l  th at a l l   these rock s we re deri ved from a mi xed m agm a sou rce w h ich d i d not originate from a pu re u pper mantl e, but poss i bl y from a back arc tecton i c setti n g. The pattern of the R E E, progressi vel y i n creasi n g n egat i ve Eu/ Eu * anoma l y, LaN/Y b N from th e amph i bol i te to th e gran i t e gn e i ss an d mark ed Eu depletion tend to i m pl icate evol ution th rough fractionation of a mi xed basal tic magma to form the precu r sor of these rock s. The amphi bol i te proba b l y r epresents th e sam p l e of the origi na l    basa l t i c magma.