Petrochemistry And Tectonic Setting Of Metabasic Rocks Of Isanlu, Southwest Nigeria

Abstract

The geochemistry of amphibolite of the Isanlu area of southwest Nigeria was studied and its petrotectonic setting evaluated. Amphibolite occurs as small outcrops and residual hills that are conformably interbanded with a suite of meta-volcanosedimentary rocks. The rock reveals a mid-amphibolite facies assemblage of hornblende (magnesio and pargasitic compositions), plagioclase (oligoclase – labradorite) with small amounts of quartz and biotite. Locally, this assemblage has suffered retrogression into the greenschist facies and preserve the metamorphic assemblage actinolite + chlorite + epidote + quartz ± titanite. Amphibolite in the study area is characterized by a moderate SiO2 (46.50-53.74 wt. %), moderately high FeOtot (8.78- 14.41 wt. %), low Fe2O3/FeO (average 0.22), low MgO (6.18-10.56 wt. %), low to high CaO (8.60-18.20 wt. %) and low alkalis. Trace elements including Nb, Y and Rb are low in abundance while Sr and Ba show marked enrichment reflecting the abundance of modal plagioclase. Refractory elements including Ni, Co, Cr and V also show enhanced values probably reflecting the fairly primitive nature of its mafic ancestry. The rock classifies as high-Fe tholeiite with plume-type mid-oceanic ridge and volcanic arc tectonic affinity. However, the interbanded nature of the rock with the enclosing metasediment suggests that it was emplaced in a continental setting. This rock was probably emplaced in an ensialic basin developed on rift-controlled grabens, where the crust is sufficiently attenuated by rising mantle plumes resulting in ocean floor – type contamination-poor magmatism.

Keywords: Petrochemistry, Tectonic setting, Amphibolite, Ensialic basin, Egbe-Isanlu schist belt

Global Journal of Geological Sciences Vol. 6 (2) 2008: pp. 113-122