张乐, 董永胜, 张修政, 邓明荣, 许王. 藏北羌塘中西部红脊山地区早二叠世埃达克质岩石的发现及其地质意义[J]. 地质通报, 2014, 33(11): 1728-1739.
    引用本文: 张乐, 董永胜, 张修政, 邓明荣, 许王. 藏北羌塘中西部红脊山地区早二叠世埃达克质岩石的发现及其地质意义[J]. 地质通报, 2014, 33(11): 1728-1739.
    ZHANG Le, DONG Yong-sheng, ZHANG Xiu-zheng, DENG Ming-rong, XU Wang. The discovery of the Early Permian adakitic rock in the Hongji Mountain area within central and western Qiangtang, Tibet Plateau, and its geological implications[J]. Geological Bulletin of China, 2014, 33(11): 1728-1739.
    Citation: ZHANG Le, DONG Yong-sheng, ZHANG Xiu-zheng, DENG Ming-rong, XU Wang. The discovery of the Early Permian adakitic rock in the Hongji Mountain area within central and western Qiangtang, Tibet Plateau, and its geological implications[J]. Geological Bulletin of China, 2014, 33(11): 1728-1739.

    藏北羌塘中西部红脊山地区早二叠世埃达克质岩石的发现及其地质意义

    The discovery of the Early Permian adakitic rock in the Hongji Mountain area within central and western Qiangtang, Tibet Plateau, and its geological implications

    • 摘要: 近期在羌塘中西部红脊山地区一套二叠纪蛇绿岩中首次发现了埃达克质岩石,呈脉状侵入到蛇绿岩中。对该套岩石进行了详细的岩石学、锆石U-Pb年龄和地球化学研究,重点讨论岩石成因及地质意义。埃达克质岩石岩性为白云母花岗岩,岩石中锆石晶形较完整,具典型的岩浆生长环带,结合Th/U值(0.22~0.63),表明其为典型的岩浆锆石。锆石LA-ICP-MS 定年结果为271.7Ma±2.3Ma,表明其形成时代为早二叠世。岩石具有较高的SiO2、Al2O3含量和较低的K2O/Na2O值,为弱过铝质岩石。稀土元素配分模式呈右倾的曲线,重稀土元素分馏中等,Eu异常变化较小;微量元素Y、Yb含量较低,Sr含量中等,在微量元素蛛网图上表现出Rb、U、K正异常和Nb、Ta、P、Ti负异常。这与俯冲洋壳熔融形成的埃达克质岩石特征相似。红脊山埃达克质岩石是俯冲的古特提斯洋在角闪岩相—榴辉岩相条件下部分熔融形成的,残留相主要为石榴子石+角闪石。较早产生的熔体与地幔橄榄岩充分发生交代作用,后产生的熔体与地幔橄榄岩交代作用较弱,向上运移过程中斜长石发生分离结晶。红脊山埃达克质岩石的发现,表明古特提斯洋在早二叠世开始俯冲消减,大洋进入消退阶段。

       

      Abstract: The adakitic rock was recently found in Permian ophiolite complex of the Hongji Mountain within central and western Qiangtang Block. It intruded into the Permian ophiolite in the form of veinlets. The authors studied petrology, zircon LA-ICP-MS U-Pb age and whole-rock geochemistry of the adakitic rock, with the emphasis placed on its petrogenesis and geological implications. The adakitic rock is muscovite granite. Its high Th/U (0.22~0.63) and clear zoned zircons indicate magamatic origin. Zircon LA-ICP-MS age of muscovite granite is 271.7Ma±2.3Ma, indicating that the crystallization of adakitic rock took place in Early Permian. The adakitic rock is characterized by high SiO2 and Al2O3 and low K2O/Na2O, thus belonging to aluminous-peraluminous granite. The REE model is characterized by enrichment of light REE, moderate fractionations in HREE and small Eu anomalies. The values of Y, Yb are low and the content of Sr is moderate. The primitive mantle normalized trace element spider patterns show that they are enriched in Rb, U, K and have negative anomalies of Nb, Ta, Ti. Geochemical characteristics are similar to features of the adakite formed by partial melting of the subducted oceanic crust. The Hongji Mountain adakitic rock was most probably generated by partial melting of the subducted Paleo-Tethys oceanic crust under the amphibolite-eclogite facies condition, whereas the residual facies were hornblende and garnet. The earlier adakitic melt sufficiently replaced the mantle peridotite, whereas the later melt weakly replaced mantle peridotite. The plagioclase experienced fractional crystallization during the upward migration of the melt. The discovery of Hongji Mountain adakitic rock indicates that the Paleo-Tethys oceanic crust subducted in Early Permian and entered into the regression stage.

       

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