Petrography and Geochemistry of Syenogranites and Gabbronorites from the Niergui Massif, Guéra Province, Chad

The Niergui Massif, located at the southern margin of the Sahara Metacraton, represents a poorly documented portion of the Pan-African basement in Chad. This study integrates petrographic and geochemical analyses of syenogranites and gabbronorites to better constrain their nature, origin, and the magmatic processes involved in basement formation. The syenogranites are strongly potassic calc-alkaline, weakly peraluminous, and ferroan, whereas the gabbronorites are metaluminous and moderately differentiated. Major, trace, and rare-earth element geochemistry indicates a progressive magmatic evolution: gabbronorites reflect a primitive mantle source, while syenogranites show enrichment in incompatible elements and differentiation toward crustal magmas. Sr/Y and Rb/Sr ratios, as well as Eu anomalies, suggest that syenogranites formed from a mixture of enriched mantle and crustal sources, whereas gabbronorites are weakly differentiated and mantle-derived. Nb/Zr vs. Zr diagrams and zircon saturation temperatures (700–850°C) indicate that magma crystallization involved both fractional crystallization and partial melting. Tectonic discrimination diagrams (Ta vs. Yb, Rb vs. Yb+Ta) confirm a subduction setting followed by a syn- to post-collisional phase, consistent with the Pan-African dynamics between the Congo Craton and the Adamawa-Yadé block. These results indicate that the Niergui Massif exemplifies a bimodal magmatic system, characterized by mantle–crust interaction, partial melting, and fractional crystallization. The complexity of geochemical signatures reflects local variations and multiple stages of magmatic evolution from mantle to differentiated crustal magmas. This study provides a solid framework for understanding Pan-African geodynamics in the Guéra region and contributes to the characterization of the Neoproterozoic basement of the Sahara Metacraton.