Petrogenesis and tectonic implication of the alkaline ferroan granites from Ropp complex, north-central Nigeria: Clues from zircon chemistry, U-Pb dating, and Lu-Hf isotope

Zircon holds crucial information for deciphering the origin of granitic rocks, enabling robust scrutiny of their magmatic evolution. Herein, we report an integrated whole-rock and zircon U-Pb and Lu-Hf isotope chemistry of Ropp alkaline granites in north-central Nigeria. The rock units include riebeckite-biotite granite porphyries (161.1 +/- 1.7 Ma), aegirine-biotite granites (162.4 +/- 1.6 Ma), biotite granites (162.3 +/- 5.1 Ma), and albite riebeckite granites. They show diagnostic characteristics of ferroan alkaline granites such as high FeOt/(FeOt + MgO), low Y/Nb (<1.2), and high Nb/Y (>1.0). Their zircon Hf isotopic compositions (epsilon(Hf)(t) = -8.08 to -4.38; Hf-176/Hf-177 = 0.282492-0.282543), major and trace element compositions such as high SiO2 >70% and incompatible trace elements (e.g., Rb, Ba, and La), low ratios of whole rock Th/U, Hf/Ta. Ni/Co and Th/Ta = 2.82, and zircon U/Yb, Gd/Yb, and Th/Yb ratios as well as oxygen fugacity (logfO(2) = -26.09 to -12.32) reflect protracted fractional crystallization of a highly reduced mantle-derived oceanic island basalt (OIB) magma from EM2 with considerable crustal involvement. Magma evolution was fuelled by fractionation of calcic-pyroxene, calcic-amphibole, Mg-olivines, feldspars, titanite, and apatite, as corroborated by the marked depletions of CaO, MgO, TiO2, and P2O5. Intense crustal assimilation, particularly in the biotite granite masked their enriched mantle (EM) ancestry. This is attested by the presence of inherited Pan-African (ca. 582 Ma) zircon, elevated crustal-like zircon U/Yb, low zircon Gd/Yb, and Th/Yb ratios as well as high whole-rock Rb/Nb ratios and relatively low estimated temperatures, which mimic crustal-derived granites. Reactivation of pre-existing megashear zones of weaknesses and opening of related transcurrent faults following periodic plate movements generated intense stress field which invoked the release of pressure and hot metasomatic fluids that partially melted the lower EM, giving rise to the Ropp OIB magmas.