Geochemistry of crystalline basement rocks SW Ugep, Nigeria

Abstract

Geochemical data on low-grade metasedimentary phyllites and schists of SW Ugep show that they were derived from predominantly pelitic parent rocks. They form a supracrustal cover on an orthogneiss of granitic composition. The metasediments are enriched in Si02, Al203 and Zr but improverished in Ni. Intrusion of pegmatite was accompanied by boron metasomatism and tourmalinization. This, as well as, the unconformable contact of the basement rocks with calcareous sandstone of Cretaceous age, may have raised the Na20, Si02, Ca0 and Mg0 contents of some outcrops. The Ti02, Al203 values (0.033 to 0.144) confirm that the metasediments were derived from a parent rock rich in quartz and clay. A dark- coloured garnet-mica schist has a composition of metagreywacke. The first episode of calc-alkaline magmatism in the area shows the emplacement of the peraluminous orthogneiss whilst the last episode indicates the intrusion of the granodiorite. Rocks in SW Ugep area correlate well in both petrography and geochemistry with those of Uwet area in the south. This confirms earlier studies, which indicated that metasedimentary schists occur in the Oban massif of southeast Nigeria and that metamorphism in the area increased in grade from upper greenschist facies in the west to upper amphibolite and locally granulite facies in the east. Key Words: Ugep, metasediments, orthogneiss, metasomatism. [Global Jnl Geol. Sci. Vol.1(2) 2003: 143-158]