Geochemistry of Schists of Northwest Obudu Plateau, Southeastern Nigeria

Abstract

Precambrian, highly foliated, fine – medium grained schists, which comprise migmatitic schist (MS), quartz mica schist (QMS), biotite mica schist (BMS) and hornblende biotite schist (HBS) constitute important lithological unit in Northwest Obudu Plateau. The schists are associated with gneisses, amphibolites and metaquartzites, and variously intruded by granites, pegmatites and quartzo-feldspathic veins. Sedimentary rocks, which are mostly Cretaceous sandstones, flank the western margin of the study area. Modal analysis shows that all the schists are enriched in quartz, plagioclase (An₃₂), biotite and garnet, except the HBS which is relatively depleted in quartz but highly enriched in biotite (25 vol. %) and hornblende (55 vol. %). Sillimanite and epidote occur in accessory amounts. The co-existence of quartz- plagioclase (An₃₂)- garnet-biotite- muscovite- sillimanite in the MS assemblage indicates that the Schists are derivatives of metamorphism of pelitic and mafic rocks, which had attained the uppermost amphibolite facies grade. Geochemically, the schists are enriched in Si0₂, which ranges from 60.11 wt % in the HBS, rising to 62.94 wt % in the BMS and 66.58 wt % in the QMS, with average of 64.56 wt %. The high SiO₂ content points to a probable effect of an intruding granitic magma on the Pan – African schists. The Al₂O₃, Fe₂O₃, MgO, CaO and alkalies are relatively moderate to high. The samples are quartz normative, have high molecular value of Al₂O₃ > {CaO + Na₂O + K₂O}. Plots in chemical discrimination diagrams confirm the peraluminous characteristics of the schist and suggest pelitic rock protolith except for the HBS.

Keywords: Petrochemistry, Obudu Plateau, Protolith, Metapelite, Precambrian, Uppermost Amphibolite Facies