西藏冈底斯南缘中侏罗世辉长闪长岩锆石U-Pb定年和Lu-Hf同位素组成及其构造意义

为解释冈底斯南缘新发现的中基性岩体构造环境及物质来源等信息,以西藏自治区贡嘎县昌果乡辉长闪长岩为研究对象,通过开展锆石U-Pb定年、Hf同位素组成、全岩地球化学分析,得到岩体锆石年龄为(165±1. 1) Ma。样品富集大离子亲石元素,亏损高场强元素,具有富铝(Al2O3=18. 84%～20. 70%)、富钾的特征,在构造判别图解中样品多落在挤压岛弧区域,表明其形成构造环境应为俯冲岛弧环境。样品具较为平缓的稀土配分曲线,岩浆分异程度较低,无明显Eu异常,(La/Yb)N平均值为2. 30(<4. 3),与幔源岩浆的稀土配分型式相似。锆石Hf同位素显示εHf(t)值为很高的正值(12. 97～18. 74),二阶段模式年龄集中在150～218 Ma,略大于其侵位年龄,表明幔源岩浆在侵位过程中受到地壳物质的混染。结合区域构造演化史认为该岩体形成于新特提斯洋向冈底斯俯冲过程中的成熟岛弧期,从而也进一步证实了南冈底斯在中侏罗世处于俯冲岛弧构造环境。

Zircon U-Pb Age and Lu-Hf Isotopes for Middle Jurassic Gabbro-diorite in Southern Margin of Gangdese Magmatic Belt and Its Tectonic Implications

In order to elucidate the tectonic setting and magma source of the gabbro-diorite in the southern margin of the Gangdese magmatic belt, we conducted zircon U-Pb age and Lu-Hf isotope analyses, and whole-rock major and trace elements analyses. The zircon U-Pb dating yielded (165±1.1) Ma for the gabbro-diorite. Geochemically, the gabbro-diorite is enriched in LILEs, Al₂O₃ and K₂O, but depleted in HFSEs. The sample data points are distributed in the island arc field in tectonic discrimination diagrams, and have clear island arc geochemical affinities.The chondrite-normalized REE patterns are flat with no Eu anomalies, showing low degree of magma differentiation. The mean (La/Yb)_N value is 2.30(<4.3), and the rocks have similar REE patterns to typical mantle-derived magmas. The gabbro-diorite samples have highly depleted zircon Hf isotopic compositions, with ε_Hf(t) values of 12.97-18.74 and two-stage Hf model ages of 150-218 Ma, indicating that the mantle-derived magma was influenced by crustal assimilation. Integrated whole-rock geochemical and zircon isotopic analyses suggest that the gabbro-diorite was formed in a mature island arc formed by the Neo-Tethyan subduction, which further indicates that the Gangdese magmatic arc was in a subduction setting during the Middle Jurassic.