Petrogenesis and tectonic implications of peralkaline-metaluminous magmatism in Kila and Shira ring complexes, north-central Nigeria

Peralkaline-metaluminous felsic-intermediate rocks are differentiates of A-type magmatism. However, their origin, sometimes obscured by hydrothermal alteration, remains a longstanding puzzle. We present new whole-rock Nd-Hf isotope, zircon U-Pb, mineral chemistry, and whole-rock data of peralkaline aegirine ± arfvedsonite granites and metaluminous aegirine-arfvedsonite quartz syenite from the Shira and Kila ring complexes in north-central Nigeria that were less affected by hydrothermal alteration. The granites are geochemically alkalic and more enriched in Zr, Hf, Rb, Y, and Nb than the quartz syenite. The Shira peralkaline aegirine-arfvedsonite granites were emplaced at 199.5 ± 0.7 and 200.6 ± 1.0 Ma, while the Kila metaluminous aegirine-arfvedsonite quartz syenite was emplaced at 232.3 ± 0.98 Ma and aegirine granites at 202.3 ± 1.3 Ma. The Kila aegirine-granites exhibit weakly to moderately negative ɛHf(t) (−4.19 to −2.54) and ɛNd(t) (−3.92 to −0.38) values, similar to the aegirine-arfvedsonite quartz syenite (ɛHf(t) = –4.46 to −0.78, ɛNd(t) = –4.15). This suggests fractionation of intermediate melts from the enriched subcontinental lithospheric mantle (SCLM), with prior melt extraction and enrichment, exhibiting crustal contamination. The Shira aegirine-arfvedsonite granites exhibit near-zero to widely positive ɛHf(t) values (+0.47 to +15.03) and mildly positive ɛNd(t) values (+0.28 to +0.42), reflecting intermediate melt fractionation from a complexly depleted SCLM source with multiple extraction episodes and localized enrichment. The magmas evolved polybarically, fractionating high-pressure and H₂O-saturated phases, including Mg-rich olivine, calcic amphibole, calcic pyroxene, plagioclase, and Fe-Ti oxides, leading to the depletion of MgO, CaO, and TiO₂. Magma evolution occurred under low pressure (~1.6 kbar), high temperature (839–1065 °C), and reduced conditions. The transition to an extensional anorogenic regime induced lithospheric stresses, reactivating major shear zones and opening associated transcurrent faults, allowing the ascent of partially melted peralkaline-metaluminous magmas from the heterogenous SCLM, which crystallized at shallow (2.8 to 7.4 km) crustal depths.