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 staurolitemuscovite 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 quartzofeldspathic) 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.

Syn-collisional granite in the northern part of the Birnin Gwari schist belt consists dominantly of granite and lesser granodiorite and quartzolite. Petrographic and ge¬ochemical data revealed three granite groups: the biotite-hornblende granite (quartzolite - BHG); the biotite granite (BG) and the biotite-muscovite granite (BMG). The rocks generally have calc-alkaline and high-K calc-alkaline affinities, and calc-alkalic to alkali-calcic, peraluminous and ferroan and magnesian geochemistry. They are characterized by LILE enrichment, high LREE fractionation factor [(La/Yb) (6.74 to 45.14] with weak to moderate negative Eu (Eu/Eu* = 0.38 to 0.62) and strong negative Nb, P and Ti anomalies. Variation in the behavior of lithophile elements (Ba, Sr and Rb) revealed diverse granite trend such as “high and low Ba-Sr”; “normal”, “anomalous” “strongly differentiated” and “granodiorite and quartz diorite” granite. Their display of similar trace elements and REE patterns suggest they are cogenetic. Major and trace element data indicate differentiation of a mafic magma and partial melting of crustal components inherited from shale-greywacke and quartzose sedimentary protoliths in volcanic arc and post collisional settings. The field and geochemical characteristics of this granite suggest that they are similar to other granites in schist belts in other parts of Nigeria, forming the lateral continuation of the same Pan-African magmatic belt.

No abstract

The granitoids and the associated volcanic rocks of the northern part of Kushaka and Birnin Gwari schist belts were emplaced in the ca. 3.5 – 1. 0 Ga remobilized basement complex terrain composed of metasedimentary and metaigneous rocks that later underwent medium- to high-grade metamorphism during the Pan-African thermo-tectonic event. They comprise dominantly of diorite, granodiorite, granite, granite gneiss and basalt, and are product of metasomatism and injections. The diorite and granodiorite occur as paleosome and the granite as leucosome with the development of high temperature minerals, locally attaining granulite facie metamorphism. Plagioclase, biotite, hornblende, pyroxene and olivine fractionation played an important role during their genesis through fractional crystallization of basaltic magma and partial melting of older dioritic-granodioritic source rock in the deep crust which were themselves ultimately derived through fusion of mantle materials contaminated by continental crust and enriched by fluids derived from oceanic crust in an arc setting. Geochemical characteristics have revealed different chemical trends in granitoids and basalts. The granitoids are calc-alkaline, ferroan and magnesian, metaluminous and peraluminous in character. They also exhibit I- and S-type signatures with enrichment in LILE, radioelements (Th and U), depletion in Nb, Sr, P and Ti, high LREE fractionation factors (La/Yb) (1.05 to 77.20), and pronounced negative Eu anomalies (Eu/Eu* = 0.34 to 1.10). Similar patterns of spidergrams show that the rocks are genetically related and were emplaced in a volcanic arc and syn-collisional setting. The basalt is tholeiitic, metaluminous and high in Fe and Mg with relative enrichment in LILE, HFSE, low and near flat LREE and HREE, low fractionation [(La/Yb)N = 1.4] with Eu/Eu* value of 1.10. It is evidently a back arc cum mid-ocean ridge (MORB) basalt. The consistent decrease in the content of MgO, Fe2O3 MnO, CaO, Sc, Cr and V of the basalt, diorites, granodiorites, and granites indicates continuous igneous crystallization process. It seems that extrusion of basaltic magmas from the sub-circular Kushaka Complex derived from subduction of oceanic crust resulted in complete change in the genesis of the magmas at the time, in this region. The granitoids and the basalt may have formed behind subducted Pan-African plate due to effects of compressional and tensional forces caused by oceanic plate roll-back which resulted to a zone of extension, parallel to the island arc. The granitoids present similar chemical characteristics to those in the other areas underlain by the basement complex and schist belts in the north and eastern parts of the Pan-African mobile belt, while basalts are similar to ophiolites and amphibolites in other schist belts of Nigeria forming a lateral continuation of the same mobile belt.

Rocks of the northern part of Birnin Gwari schist belt is underlain predominantly by (i) banded gneiss of dioritic and granodioritic composition and granitic gneisses; (ii) biotite-staurolite quartz schist; and (iii) syn-tectonic biotite hornblende (quartzolite - BHG) granite, biotite granite (BG), and biotite-muscovite granite (BMG). Banded gneiss rocks are of hybrid sedimentary–igneous protoliths; their pelitic and mafic protoliths were derived essentially from a quartz-diorite, granodiorite and granite-quartz monzonite source. Metasediments are enriched in SiO2 (63.03 to 65.13 wt %), with moderately elevated Al2O3 (15.4 – 15.16 wt %) values and depleted in Ba, V, W, La, Nb, Nd, Rb, Th and Zr trace elements; inherited from shale-greywacke sedimentary protoliths. Cogenetic syn-tectonic granites display similar trace elements and REE patterns, with diverse trends such as “normal”, “anomalous” and “strongly differentiated” and characterized by LILE enrichment, high LREE fractionation factor (La/Yb of 6.74 to 45.14) with weak to moderate negative Eu (Eu/Eu* = 0.38 to 0.62) and strong negative Nb, P and Ti anomalies. The belt consists of rocks with alkaline affinity and evolved as back arc behind subducted Pan-African plate due effect of compressional forces and differentiation of quartz diorite, granodioritic and granite-quartz monzonite magma and partial melting of crustal components inherited from shale-greywacke sedimentary protoliths in volcanic arc and post collisional settings. The precursor of these rocks originated from basalt of depleted mantle that differentiated progressively to the granite.