Geochemistry and Petrogenetic Features of Metasediments in Northern Part of Kushaka and Birnin Gwari Schist Belts NW Nigeria

Abstract

The Kushaka and Birnin Gwari metasediments and associated banded iron formations constitute important lithological units within the Precambrian Basement Complex. They were studied to evaluate their compositional characteristics and petrogenesis in order to contribute further to the understanding of the geodynamic evolution of Nigeria’s Schist belts. The Kushaka metasediments comprise quartzite, graphite and sulphur bearing staurolite-muscovite quartz schist interbedded with Banded Iron Formations (BIFs) while the Birnin Gwari schist comprise staurolite-biotite quartz schists with lithic (angular to rounded clastic quartz, schistose, volcanic and quartzo-feldspathic) sandstones. These schists are associated with fissile and ferruginous quartzite, banded and granitic gneisses, basalts and amphibolites. Petrographic work revealed varying proportions of quartz, staurolite, biotite and muscovite with subordinate iron-oxide minerals.  Geochemically the metasediments in the Kushaka are enriched in SiO₂ (61.23 to 65.99 wt %) with elevated values of Al₂O₃ (16.53 – 20.93 wt %), Ba, V, W, La, Nb, Nd, Rb, Th and Zr; while  the Birnin Gwari schists, even though enriched in SiO₂ (63.03 to 65.13 wt %), has moderately elevated Al₂O₃ (15.4 – 15.16 wt %) values  but is depleted these trace elements. Field and geochemical characterization of the Kushaka metasediments suggests peraluminous, tholeiite and calc-alkaline character; arkosic and shale-greywacke sedimentary protoliths derived from quartzose sedimentary and granite-quartz monzonite provenance. Calculated ICV values of 0.52 - 0.99 and occurrences of graphite and sulphur in the Kushaka metasediment suggests shallow stable shelf-type sediment of carbonate and iron formations in a reducing environment with matured sedimentary protolith. The Birnin Gwari metasediments on the other hand have a peraluminous and calc-alkaline character, inherited from shale-greywacke and quartzose sedimentary protoliths derived from granodioritic and granite-quartz monzonite provenance. ICV values of 1.12 – 1.18 and angular and volcanic clasts suggest rapid subsidence of basin during genesis and / or tectonic instability in the surrounding environment with immature sedimentary protolith. This is an indication of two contrasting environment in an arc setting with contribution from basaltic and andesitic detritus. Available geochronological data on granite and granitic gneisses have ascribed the Kushaka schist belt to Kibaran and the Birnin Gwari schist belt a Pan-African age.

Keywords: Metasediments, protolith, Kushaka, Birnin Gwari, quartzite, provenance, shale, greywacke.

DOI: 10.7176/JEES/11-16-05

Publication date:June 30^th 2021