Progressive mantle melting and crust–mantle–fluid hybridization in fluorite-bearing Sn–Nb ± Mo-rich alkaline granites of the Jos Plateau, central Nigeria

Alkaline granites of the Jos Plateau Tin Belt (central Nigeria) host Sn-Nb-Mo mineralization. However, their petrogenesis, metal enrichment, and tectono-geodynamic evolution remain poorly constrained. This study integrates whole-rock geochemistry and Sr-Nd isotopes, amphibole-fluorite chemistry, and zircon U-Pb geochronology from the Buji, Jos-Bukuru (JBK), and Kigom (KGM) complexes. Zircon U-Pb ages reveal a north-to-south younging trend from Buji (∼179 Ma) through JBK (∼168 Ma) to KGM (∼166 Ma). Buji barren granites exhibit the most negative ε Nd (t) (–4.4 to –4.2), least radiogenic 87 Sr/ 86 Sr (i) (0.7065–0.7066), and highest Neodymium Crustal Index (NCI = 0.31), indicating an enriched mantle source with strong crustal input. JBK Sn–Nb biotite-amphibole granites show intermediate ε Nd (t) (–3.3 to –3.0), NCI (0.25), and moderately radiogenic 87 Sr/ 86 Sr (i) (0.7066–0.7173), suggesting an enriched mantle source with moderate crustal input and selective hydrothermal overprint. KGM Mo–Nb arfvedsonite–riebeckite granites exhibit near-chondritic ε Nd (t) (–1.1 to –0.4), lowest NCI (0.25), and most radiogenic 87 Sr/ 86 Sr(i)r (0.7139–0.7206), reflecting extreme fluid-mediated metasomatism on a mantle-dominated magma. Decoupled fO 2 distinguishes oxidized JBK Ca-amphiboles from reduced KGM Na-amphiboles hosting oxidized zircons. Mass balance modeling indicates ∼40–50 vol% feldspar-dominated fractional crystallization coupled with ∼30–40 vol% upper crustal assimilation. JBK granites show Sn-Nb enrichment (4–126 ppm; 37–910 ppm), while KGM granites exhibit high Nb with extreme Zn-Pb-Cu (≤1326, 1054, 845 ppm), consistent with Mo-enriched systems. Fluorite compositions (F = 52–56 wt%) record F-rich reducing fluids that promoted HFSE-REE-Sn-Nb-Mo mobility. A four-stage model explains Sn-Nb-Mo enrichment in JBK and KGM: (I) magmatic enrichment; (II) magmatic-hydrothermal transition; (III) hydrothermal veining and stockworks formation; (IV) supergene placer formation. Transtension triggered adiabatic melting of metasomatized mantle, generating alkaline magmas that ascended along shear zones to form shallowly emplaced (2.6–8.7 km) anorogenic alkaline complexes. Fertile global analogues share this convergence of transtensional shearing, crustal assimilation, and F-rich fluid flux.