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    藏南墨竹工卡叶巴组双峰式火山岩岩石成因、时代及构造环境

    Age, petrogenesis and tectonic setting of bimodal volcanic rocks from Yeba Formation in Maizhoukunggar, southern Tibet

      摘要
    • 摘要: 青藏高原南部早中生代岩浆岩的研究对反演区域构造演化具有重要意义，其中早侏罗世叶巴组火山岩被认为是新特提斯洋俯冲的早期岩浆记录，目前对于其形成的构造环境仍存在争议。选取西藏南部墨竹工卡县地区新发现的叶巴组火山岩为研究对象，对其进行了锆石U-Pb同位素、全岩地球化学成分测试。火山岩主要岩性为玄武岩(181.0±5.0 Ma，n=1)和流纹岩(181.4±4.4 Ma，MSWD=0.017, n=18)，具有双峰式火山岩特征。玄武岩具有较高的TiO2、P2O5和Nb含量，(La/Nb)PM＜2，具有富铌玄武岩的特征，Mg#值为54.9~57.3，Ti/V＞50，富集轻稀土元素，亏损重稀土元素。流纹岩具有较高的SiO2、Na2O含量，Mg#值为49.1~50.4，Th/Ce值为0.1~0.15，Th/La值为0.22~0.31，Nb/Ta值为14.06~14.32，Sr/Y值较低(4.15~4.82)，富集轻稀土和大离子亲石元素，亏损高场强元素。研究结果揭示，玄武岩岩浆源区为板片熔体交代的地幔楔，并在后期演化过程中混入部分地壳物质。流纹岩为幔源岩浆与大陆地壳混染成因。玄武岩具有板内玄武岩及岛弧岩浆岩特征，流纹岩具有岛弧岩浆岩特征，结合区域地质背景推测，墨竹工卡叶巴组火山岩形成于新特提斯洋北向俯冲的弧后盆地环境，区域上叶巴组火山岩的形成时代及构造环境存在差异，可能代表了俯冲不同阶段的产物。

       

      Abstract: The study of the Early Mesozoic magmatic rocks in the southern Tibetan Plateau is of great significance to the inversion of regional tectonic evolution.The volcanic rocks of the Yeba Formation in Early Jurassic are considered to be the early records of the subduction of the Neo-Tethys ocean.At present, there are still controversies about the tectonic setting of their formation.We selected the volcanic rocks of Yeba Formation in Maizhoukunggar county, southern Tibet, as the research objects.We present new zircon U-Pb isotopes and whole rock geochemical composition testing, discuss the implications for evolution of this region.The main lithology includes basalts(181.0±5.0 Ma, n=1)and rhyolites(181.4±4.4 Ma, MSWD=0.017, n=18), and they have bimodal volcanic characteristics.The basalts have high TiO2(1.62%~1.78%), P2O5(0.49%~0.51%)and Nb(10.18%~10.61%)concentrations, (La/Nb)PM < 2, which are similar to the ENB.The Mg#s are between 54.9~57.3, enrichment of LREE and depletion of HREE.Rhyolites have highly SiO2 (79.25%~80.22%), Na2O(3.94%~4.26%)contents, Mg # values are 49.1~50.4, Th/Ce ratios are 0.1~0.15, Th/La ratios are 0.22~0.31, Nb/Ta ratios are 14.06~14.32, Sr /Y ratios are 4.15~4.82, and enrichment of LREE and LILE and depletion of HFSE.It is considered that the magma source area of basalts is mantle wedge metasomatized by slab-derived melt and mixed with crustal materials during the later evolution.Rhyolites are of mantle derived magma mixed with continental crust.Basalts have the characteristics of intraplate basalts and island arc magmatic rocks, while rhyolites show the characteristics of island arc magmatic rocks. Combined with the regional geological background, it is inferred that the volcanic rocks of the Yeba Formation in Maizhoukunggar were formed in the back-arc basin of the northward subduction of the Neo-Tethys ocean, and there are differences in the formation age and tectonic settings of the volcanic rocks of the Yeba Formation in the region, It may represent different products in different stages of subduction.

       

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