THE METAMORPHOSED MAFIC-ULTRAMAFIC COMPLEX OF MOKURO, ILESHA SCHIST BELT, SOUTHWESTERN NIGERIA

In the Proterozoic Schist Belt of Nigeria, lenticular bodies of metaba sites and meta-ultramafics are frequently intercalated within stauroli te bearing metapelitic schists. Such a metamorphosed mafic-ultramafic complex is particularly well exposed in the Mokuro riverbed between th e towns of Ife and Ilesha. These outcrops display contact relationship s with the surrounding metasediments, as well as between the individua l mafic and ultramafic rock types. The most common mafic rocks are ind istinctly layered amphibolites, accompanied by apatite rich amphibolit es and massive amphibolites, in part rich in ilmenite and pyrrhotite. Among the generally massive ultramafic rocks, nearly monomineralic amp hibole rocks predominate, while chlorite-amphibole, talc-chlorite-amph ibole and talc bearing olivine-chlorite-amphibole rocks occur in subor dinate amounts. Field, textural and geochemical evidence suggest that the mafic-ultramafic complex derived from a thick, structurally differ entiated basaltic sill that contained doleritic portions in its interi or. Slow cooling Fates in these inner parts enabled crystal settling w ith the formation of ultramafic cumulates. Due to the enrichment of vo latiles during the crystallisation process, higher amounts of apatite and sulphides, as well as fate magmatic amphibole, were formed in part s of the mafic-ultramafic body. Mineral assemblages in the mafic-ultra mafic complex testify to a metamorphic overprint under amphibolite-fac ies conditions. Thermodynamic modelling in the system CMFASH leads to an estimated P-T range of 1.5-3 kbar and 550-620 degrees C for the met amorphic peak assemblage talc-olivine-chlorite-Ca amphibole-orthoamphi bole. (C) 1998 Elsevier Science Limited.