东昆仑造山带中灶火地区早中生代镁铁质岩墙群的成因及地质意义
Petrogenesis and geological significance of the Early Mesozoic mafic dyke swarms in Zhongzaohuo area, East Kunlun orogenic belt
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摘要: 提要：东昆仑造山带中灶火地区镁铁质岩墙群以闪长玢岩为主，含少量闪斜煌斑岩、辉绿玢岩及辉绿岩，LA-ICP-MS锆石U-Pb年代学指示该套岩墙群结晶侵位年龄为(249±1)Ma。稀土元素含量整体较高，富集轻稀土元素(∑REE=99.9×10-6~173.9×10-6 ，(La/Yb)N=3.5~9.3)；微量元素表现出富集大离子亲石元素(LILE)，亏损高场强元素(HFSE)的特征；源区分析表明，镁铁质岩浆为俯冲洋壳析出的流体交代富集地幔的结果，且在岩石成因中部分熔融起到主导作用，地壳混染和分离结晶作用对岩浆成分分异起到的作用有限。构造环境分析表明，岩石的形成与俯冲作用有关，结合区域构造演化认为镁铁质岩墙群的成因为：早三叠世，在古特提斯洋向北俯冲的环境下，俯冲板片释放的流体交代富集地幔，诱发地幔部分熔融形成镁铁质岩浆，受弧后伸展的动力学背景影响，岩浆最终上升侵位形成镁铁质岩墙群。Abstract: Abstract: The Zhongzaohuo mafic dyke swarms in East Kunlun are principally composed of dioritic porphyrite with minor lamprophyre, porphyritic diabase, and diabase. The LA-ICP-MS zircon U-Pb dating indicates that the mafic dykes were emplaced at (249±1) Ma. The mafic dykes are enriched in LILE and depleted in HFSE. The rocks have higher total REE，with slightly enriched LREE (∑REE=99.9×10-6~173.9×10-6 ，(La/Yb)N=3.5~9.3). Source region analysis shows that mafic magma resulted from the fluid from subducted ocean crust and the mantle metasomatism, and partial melting played a dominant role in magma evolution，while crustal contamination and fractional crystallization played a limited role in magmatic differentiation. Tectonic setting analysis shows that the formation of the rocks was related to subduction; in combination with the tectonic evolution of East Kunlun, the authors hold that the formation process of mafic dyke swarms in this paper is as follows: in early Triassic, as the result of northward subduction of Paleo-Tethys Ocean, fluids from the subduction slab were dehydrated and mantle peridotite experienced metasomatism, which promoted the mantle peridotite partial melting into mafic magma; the magma was affected by the dynamic background of the back-arc extension, and then it migrated upward to the upper crust along the fissures to form the mafic dykes.
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