内蒙古沙章土矿区闪长玢岩成岩时代及岩浆源区探讨:锆石U-Pb年代学、岩石地球化学及Hf同位素约束

闪长玢岩因其成因及含矿性特点而备受地学界关注,内蒙古阿巴嘎旗北部沙章土矿区发育闪长玢岩体,伴生着高背景化探异常。本研究通过岩石学、锆石U-Pb年代学、岩石地球化学及其Hf 同位素研究,确定沙章土矿区闪长玢岩的成岩年龄及岩浆源区。闪长玢岩的侵位年龄为(311.1±1.7) Ma,属晚石炭世海西中期构造-岩浆活动的产物。岩石中SiO₂含量为56.19%~60.45%,Na₂O + K₂O为4.81%~5.87%,铝饱和指数A/CNK为1.00~1.43,属高钾钙碱性、过铝质岩石;明显富集轻稀土和大离子亲石元素K、Rb及高场强元素Zr、Th、U,显著亏损重稀土及高场强元素Nb、Ta、P、Hf、Ti,显示了俯冲带岛弧岩浆岩的地球化学特征;稀土元素总量(REE)为139.62×10^-6~150.67×10^-6,具有轻稀土元素富集、重稀土元素亏损的右倾配分模式,显示弱负Eu异常。Hf同位素单阶段模式年龄(T_DM)为637~905 Ma,二阶段模式年龄(T_DM^C)变化范围为827~1 259 Ma,锆石ε_Hf(t)值在+1.10~+7.80之间,显示其具有幔源岩浆物质来源特征,其岩浆源区可能是由俯冲板片脱水交代上覆地幔楔,熔融体上升发生底侵作用,加热并诱发新生基性下地壳部分熔融,上升演化而形成的壳幔混源岩浆。

Formation Age and Magma Source of the Shazhangtu Diorite Porphyry,Inner Mongolia: Constraints of Petrogeochemistry,Zircon U-Pb Geochronology and Hf Isotopes

Diorite porphyry has been widely studied due to its genesis and ore-bearing characteristics. Many diorite porphyry intrusions were found in the north of Abagaqi (Inner Mongolia), accompanied by high-background geochemical anomalies. Studies on petrology, zircon U-Pb geochronology, geochemistry and Lu-Hf isotopes have been conducted for the formation age and magma source of diorite porphyry samples from the Shazhangtu mining area in Inner Mongolia. The diorite porphyry was emplaced at (311.1±1.7) Ma, and is the product of the middle Hercynian (Late Carboniferous) tectonomagmatic activities. The rocks have SiO₂=56.19%-60.45%, Na₂O+K₂O=4.81%-5.87%, and aluminum saturation index (A/CNK)=1.00-1.43. It belongs to high potassium calc-alkaline and peraluminous rocks. The LREE LILE (K, Rb) and HFSE (Zr,Th and U) are enriched, and the HFSE (Nb, Ta, P, Hf and Ti) are depleted, indicating that the geochemical characteristics of island are magmatic rocks in subduction zone. The total REE contents ranges from 139.62×10^-6 to 150.67 ×10^-6, showing a right-inclined pattern of light rare earth enrichment and heavy rare earth depletion with weak negtive Eu anomaly. The Hf isotopic data of zircons show that the single-stage and two-stage model ages range from 637 to 905 Ma and from 827 to 1,259 Ma, respectively, and the ε_Hf(t) value of zircons varies from 1.10 to 7.80, indicating the characteristics of mantle-derived magma. It is probably the partial melting magma in the mantle wedge due to the dewatering of the subduction plate that underplated under the bottom of the lithosphere and the molten body rises to cause under-invasion, heated and induced the partial melting of the new formed juvenile basic lower crust, forming the crust-mantle sources mixed magma.