Termination of anorogenic alkaline magmatism in Nigerian Younger Granite province: insights from Afu A-type granite complex

The Nigerian Younger Granite (NaYG) province is renowned for its world-class anorogenic alkaline ring granite complexes and has been recognized as a potential host for abundant Nb–Sn–W–Mo mineralization. In this study, a comprehensive investigation was conducted on the southernmost complex of the province, known as the Afu granite complex, encompassing petrography, zircon U–Pb dating and Lu–Hf isotopes, and whole-rock geochemical analyses. The dominant rocks within the Afu complex consist of biotite granite and albite biotite granite. Both rock units exhibit typical characteristics of A₁-type granite, including elevated levels of alkali oxides (Na₂O + K₂O = 8.01–8.52 wt%) and high field strength elements (Zr + Nb + Ce + Y = 495–950 ppm), as well as high FeO^T/(FeO^T + MgO) and Ga/Al ratios, similar to other younger granites found in Nigeria. Bulk rock Sm–Nd and zircon Lu–Hf isotopic data (εNd(t) = −2.6 to 0.9; εHf(t) = −7.2 to −0.5) indicate a predominantly crustal material-dominated magma source, with some contribution from mantle-derived magma. The linear correlations observed between Rb/Sr and Nb/Ta ratios as well as Sr contents from biotite granite to albite biotite granite suggest the significant role played by magma differentiation. The elevated bulk Nb contents observed in all rocks, along with the presence of Sn–W-bearing quartz veins, contribute to the rare metal mineralization of the Afu complex and indicate a combination of magmatic and hydrothermal processes. Additionally, zircon U–Pb dating results from the Afu granites reveal their relatively young ages (149–148 Ma) compared to other NaYGs (263–151 Ma), suggesting the cessation of anorogenic alkaline magmatism within the NaYG province.