Petrogenesis and Tectonic Setting of Igneous Rocks from Kassa Area, Zing, Northeastern Nigeria: Geological and Geochemical Constraints

Pan-African granitoids of variable mineralogical compositions are emplaced in Kassa, Zing area, which form part of the Adamawa Massif of Northeastern Nigeria Basement complex. In this study, 16 samples obtained during fieldwork were subjected to geochemical analysis using XRF techniques in order to shed more light on the origin and tectonic setting of the rocks. Studied rock suites include ijolite, syenite, quartz syenite, quartz monzonite, biotite granite and granodiorite. The silica contents of the rocks range from 47.10 wt.% to 71.00 wt.%, exhibiting alkalic, calc-alkalic and calcic affinities. All the studied granitoids plot within the peralkaline and metaluminous fields on the A/NK versus A/CNK diagram. The decrease in Fe2O3T, MgO, CaO and TiO2 with increasing SiO2 and increase in Ba and Sr with increasing SiO2 is an indication of formation through fractional crystallization. Based on the field, petrographic and geochemical data, it can be inferred that the ijolite, syenite, quartz syenite, quartz monzonite and biotite granite are formed by the fractional crystallization of basic melts. On the other hand, plots of normative albite anorthite-orthoclase and Mg# versus SiO2 indicate that the granodiorite was emplaced via partial melting of tonalitic rocks from the deeper crust. In conclusion, the crystalline basement of Kassa area represents two different settings: Syn-collision and late orogenic stage. The oldest pluton, granodiorite, intruded during collision by partial melting of crustal rocks of tonalitic composition at temperatures between 850 and 950 °C. The emplacement of the ijolite, syenite, quartz syenite, quartz monzonite and biotite granite occurred during late orogenic times, via fractional crystallization of mantle-derived mafic melts.