太行山麻棚地区高Ba-Sr花岗质侵入岩的地球化学、锆石U-Pb年龄和Hf同位素特征及其地质意义

华北克拉通经历了长期而复杂的地质过程,中生代时期在太行山地区构造岩浆活动强烈,形成了一系列中酸性岩体。麻棚岩体位于太行山北段,侵位于新太古代片麻岩和表壳岩中,由中心向边缘分别出露似斑状二长花岗岩、花岗闪长岩和石英闪长岩。前人已经对麻棚岩体进行过岩石学、岩石化学、岩体成因类型和年代学等多方面的研究,其花岗岩类型仍存有争议。本文对麻棚岩体进行了系统的岩石地球化学、锆石SHRIMP U-Pb同位素年代学分析和Lu-Hf同位素组成研究,进一步探讨了麻棚岩体的地球化学类型、形成时代和岩浆源区性质。岩石地球化学特征表明麻棚岩体具有较高的SiO₂、Na₂O+K₂O、Ba、Sr、LREE含量和K₂O/Na₂O值,较低的Al₂O₃、MgO、Rb、Nb、Ta、Th、U、Y、HREE含量和Mg^#值,无明显的Eu异常,明显亏损Nb、Ta、P、Ti等高场强元素,属于高Ba-Sr花岗岩。新获得的3个不同相带岩石的锆石SHRIMP U-Pb同位素²⁰⁶Pb/²³⁸U加权平均年龄为129.4±1.4、131.2±2.2和129.5±1.3 Ma,且具有相似主量元素组成及稀土和微量元素分布模式,反映了麻棚岩体为同一期岩浆活动产物。Lu-Hf同位素测试分析数据计算获得麻棚岩体岩浆锆石的εHf(t)值为-27.00～-15.44,二阶段模式年龄t_DM2为2 358～1 765 Ma。麻棚岩体是前寒武纪变质基底部分熔融形成的岩浆与幔源岩浆混合作用的结果。

Geochemistry, zircon U-Pb age and Hf isotope characteristics of high-Ba-Sr granites in Mapeng area, Taihang Mountains and their geological significance

The North China Craton (NCC) has undergone a long and complicated geological process. During the Mesozoic, a series of intermediate-acid granitoids were formed in the Taihang Mountains. Mapeng granitic pluton is located in the northern part of Taihang Mountains, emplaced in Fuping complex, with porphyritic monzonitic granite, granodiorite and quartz diorite exposed from the center to the edge. The petrology, petrochemistry, genetic type of Mapeng granitic pluton have been studied by predecessors, but the type and source of granitic pluton are still controversial. In this paper, the petrochemistry, zircon SHRIMP U-Pb isotopic chronology and Lu-Hf isotopic composition of porphyritic monzonitic granite, granodiorite and quartz diorite in Mapeng pluton are systematically studied, and the geochemical type, formation age and magma source nature of Mapeng granite are further discussed. Geochemical magmas show that the Mapeng granite has high SiO₂、Na₂O+K₂O、Ba、Sr、LREE and the ratio of K₂O/Na₂O, lower Al₂O₃、MgO、Rb、Nb、Ta、Th、U、Y、HREE and Mg^#value, no obvious Eu anomaly and obvious depletion of high field strength elements such as Nb, Ta, P and Ti. The SHRIMP U-Pb isotopic ages of the three newly obtained granites with different facies belts are 129.4±1.4 Ma, 131.2±2.2 Ma, 129.5±1.3 Ma, and they have similar major element compositions and REE and trace element distribution patterns. It also reflects that the Mapeng pluton is a magmatic activity of the same period, and has undergone rapid emplacement and cooling crystallization. Lu-Hf isotopic analysis shows that the εHf(t) value of magmatic zircon from the Mapeng granite is -27.00~-15.44, and the two-stage model age t_DM2 is 2 358~1 765 Ma. The magma source of the Mapeng granite is the result of mixing of mantle magma and magma formed by partial melting of Precambrian metamorphic basement.