Mineralogical and Geochemical Characterization of the Zarnouski Felsic Suites, Anorogenic Damagaram-Mounio Province

The Zarnouski complex is located 80 km northeast of the Zinder pluton, in the province of Damagaram-Mounio, halfway between the more recent granites of the Aïr and those of Nigeria to the south. The aim of this study is to present the results of the mineralogical and geochemical characterisation of the Zarnouski felsic suites. It is a prime target for determining: (i) the nature of the rocks and (ii) tracing variations in the major elements controlling the mechanisms of magmatic differentiation. The methodological approach used is multidisciplinary, combining petrographic analysis, radiometric analysis and geochemistry of major elements. Two alkaline and hyperalkaline lineages are highlighted. The small elliptical Zarnouski pluton, consists of quartz-syenite [330 ± 3 Ma, (⁸⁷Sr/⁸⁶Sr)₀ = 0.7100] cut by aegyrin-granite [302 ± 9 Ma, (⁸⁷Sr/⁸⁶Sr)₀ = 0.7157]. The geochemical results obtained show that these rocks have high silica and alkalis contents. Indeed, using geochemical classification diagrams on rock typology coupled with calculated alkalinity indices, two magmatic lineages were highlighted. The first lineage comprises a petrographic sequence ranging from augite-syenite (K+Na: 0.30) to quartz-syenite (K+Na: 0.28) and biotite-granite, rhyolite (K+Na: 0.25), with an alkalinity index (AI) varying from 0.94 to 0.99. Hyperalkaline rocks are microsyenite (AI: 1.13; K+Na: 0.33) and riebeckite-granites (AI: 1.06 to 1.11; K+Na: 0.25-0.26). The relationship between alumina and alkalis, coupled with the decrease in CaO, FeOt and MgO, has allowed us to propose a divergent differentiation from syenite, producing an alumina-deficient fraction (riebeckite-granite) and an aluminous fraction represented by biotite-granite. The presence of Fe²⁺-rich ferromagnesian minerals (Fe²⁺/Fe³⁺>1) at the beginning and annite, Fe³⁺[biotite-granite: (Fe²⁺/Fe³⁺)< 1] towards the end of crystallization indicates that magma influx was relatively reduced. This study suggests that the rocks studied were formed from magma that underwent significant differentiation/fractional crystallisation (DI: 81.75 to 94.76) and that alkali-rich mantle sources play a key role in their genesis. A divergent differentiation of syenite, producing an alumina-deficient fraction (riebeckite granite) and an alumina-rich fraction represented by biotite granites, is proposed for the Zarnouski rocks.