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 (ε_Hf(t) = −8.08 to −4.38; ¹⁷⁶Hf/¹⁷⁷Hf = 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 (logfO₂ = −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.

中文翻译：

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尼日利亚中北部 Ropp 复合体碱性铁花岗岩的岩石成因和构造意义：来自锆石化学、U-Pb 测年和 Lu-Hf 同位素的线索

锆石拥有破译花岗岩起源的重要信息，能够对其岩浆演化进行强有力的审查。在此，我们报告了尼日利亚中北部 Ropp 碱性花岗岩的完整岩石和锆石 U-Pb 和 Lu-Hf 同位素化学。岩石单元包括里贝克石-黑云母花岗斑岩 (161.1 ± 1.7 Ma)、埃吉林-黑云母花岗岩 (162.4 ± 1.6 Ma)、黑云母花岗岩 (162.3 ± 5.1 Ma) 和钠长石里贝克石花岗岩。它们显示出高 FeOt/(FeOt + MgO)、低 Y/Nb (<1.2) 和高 Nb/Y (>1.0) 等亚铁碱性花岗岩的诊断特征。它们的锆石 Hf 同位素组成 ( ε _Hf ( t ) = −8.08 to −4.38; ¹⁷⁶ Hf/ ¹⁷⁷Hf = 0.282492–0.282543)，主要和微量元素组成，例如高 SiO ₂ >70% 和不相容的微量元素（例如，Rb、Ba 和 La），全岩 Th/U、Hf/Ta 的低比率。Ni/Co 和 Th/Ta = 2.82，以及锆石 U/Yb、Gd/Yb 和 Th/Yb 比率以及氧逸度（log f O ₂  = −26.09 至 −12.32）反映了高度还原的长时间分步结晶来自 EM2 的地幔源大洋岛玄武岩 (OIB) 岩浆具有相当大的地壳参与。钙质辉石、钙质角闪石、镁橄榄石、长石、钛石和磷灰石的分馏促进了岩浆演化，CaO、MgO、TiO ₂和 P ₂ O _{5的显着耗竭证实了这一点}. 强烈的地壳同化作用，特别是在黑云母花岗岩中，掩盖了它们丰富的地幔 (EM) 祖先。继承的泛非（约 582 Ma）锆石、升高的类地壳锆石 U/Yb、低锆石 Gd/Yb 和 Th/Yb 比率以及高全岩 Rb/Nb 比率证明了这一点和相对较低的估计温度，模拟地壳衍生的花岗岩。在周期性板块运动之后，预先存在的大剪切薄弱带的重新激活和相关横流断层的打开产生了强烈的应力场，这引起了压力和热交代流体的释放，部分熔化了较低的 EM，从而产生了罗普 OIB 岩浆。