A subduction-related metasomatically enriched mantle origin for the Luoboling and Zhongliao Cretaceous granitoids from South China: implications for magma evolution and Cu–Mo mineralization

The Luoboling granodiorite porphyry and Zhongliao porphyritic biotite-granodiorite occur within the Zijinshan ore field in Fujian Province, southeast China. LA–ICP–MS zircon U–Pb dating yields ages of 103.1 ± 1.1 Ma (2σ, MSWD = 3.1) and 95.9 ± 0.6 Ma (2σ, MSWD = 1.2) for the Luoboling and Zhongliao intrusions, respectively. All rocks show high SiO₂, K₂O, and light rare earth element levels, variable CaO and Fe₂O₃^T, but low heavy rare earth element and high field strength element (Nb, Ta, Ti) concentrations. They also exhibit uniform initial ⁸⁷Sr/⁸⁶Sr ratios (0.7064–0.7068) and ε_Nd(t) values (−4.0 to −2.6), falling within the compositional field of Cretaceous basalts and mafic dikes in the Cathaysia Block. Together with their relatively high Hf isotopic ratios (ε_Hf(t) = −5.8 to +0.7), these data suggest that the Luoboling and Zhongliao intrusions were derived from a subduction-enriched mantle source. We infer fractional crystallization processes involving early fractionation of clinopyroxene and olivine, and subsequent fractionation of garnet forming the Luoboling granodiorite porphyry, eventually followed by amphibole- and biotite-dominated crystallization, with minor accessory mineral contribution, producing the (younger) Zhongliao porphyritic biotite-granodiorite. Asthenospheric mantle sources may have also contributed to generating the melts as indicated by initial Pb isotopic compositions and Hf isotopic compositions. The generation of these intrusions was associated with the Pacific subduction in an extensional setting during the Cretaceous.罗泊岭花岗闪长斑岩与中寮斑状黑云母花岗闪长岩均产出在中国东南部福建省的紫金山矿田内。采用激光剥蚀电感耦合等离子体质谱（LA–ICP–MS）锆石U–Pb定年测试，得到罗泊岭侵入体的年龄为103.1±1.1 Ma（2σ，平均平方加权偏差MSWD=3.1），中寮侵入体的年龄为95.9±0.6 Ma（2σ，平均平方加权偏差MSWD=1.2）。所有被测岩石均表现出高SiO₂、K₂O与轻稀土元素含量，CaO及全铁（以Fe₂O₃计）含量变化较大，但重稀土元素与高场强元素（Nb、Ta、Ti）含量较低。这些岩石还具有均一的初始⁸⁷Sr/⁸⁶Sr比值（0.7064~0.7068）与ε_Nd(t)值（-4.0~-2.6），其成分落入华夏地块白垩纪玄武岩及镁铁质脉岩的组分区间内。结合其相对较高的Hf同位素比值（ε_Hf(t) = -5.8~+0.7），上述数据表明罗泊岭与中寮侵入体起源于经俯冲作用交代的富集地幔源区。我们推断其经历了多阶段分离结晶作用：早期为单斜辉石与橄榄石的分异结晶，随后石榴石分异形成罗泊岭花岗闪长斑岩；最终以角闪石与黑云母为主的结晶作用（伴随少量副矿物的贡献）形成了时代更年轻的中寮斑状黑云母花岗闪长岩。初始Pb同位素组成与Hf同位素组成的特征指示，软流圈地幔源区也可能对熔体的形成有所贡献。上述侵入体的形成与白垩纪伸展构造背景下的太平洋板块俯冲作用具有成因联系。