Petrology Of Intrusive Rocks Of Oban Massif, Southeastern Nigeria

Abstract

A total of forty- nine (49) fresh rock samples of the various intrusive rocks (granitoids and dolerites)were collected from the field. These rocks intruded metamorphic rocks that include pyllites, schists, gneisses and amphibolites in the Oban Massif. Intrusive rocks pegmatitic granites and granodiorite exhibit porphyritic texture rich in feldspars, quartz, muscovite, biotite, tourmaline and beryl. The granodiorite contains xenoliths of metamorphic rocks.  The dolerites are ophitic and composed of plagioclase, olivine and pyroxene. From the geochemical analyses, the intrusive rocks show SiO₂ to be the most abundant chemical component in all the rock units with its highest value recorded in the pegmatitic granite (Av. 70.07%), while the lowest value (59.68%) recorded in the dolerite. Al₂O₃ also display high concentration in rocks of Oban Massif. The rocks have concentrations of Al₂O₃> (CaO + Na₂O + K₂O), low MgO, and TiO₂ concentrations while pegmatitic granites exhibit low concentration in FeO_Total. The alkaline and calc-alkaline, granitoid intrusives in the study area were found to be I-type crustal orogenic granitoids that represent a late stage intrusion along major shear and thrust zones.The results of trace elements analysis revealed that almost all the intrusive rocks of Oban Massif contain high Ba content, the highest values occur in the pegmatitic granite with a mean concentration of (2266ppm), while meladiorite was found to contain the least amount of Ba (44ppm).