微量元素和Pb同位素对西藏雅鲁藏布构造带日喀则蛇绿岩形成环境的制约

Trace elements and Pb isotope of the mafic rocks from the Xigaze ophiolite of Yarlung Zangbo suture zone,southern Tibet:Constraints on the tectonic setting

西藏南部雅鲁藏布构造带分布有一系列蛇绿岩体。人们对这些蛇绿岩体的形成环境仍然存在较大的争议。雅鲁藏布构造带中段日喀则蛇绿岩路曲和大竹曲岩体镁铁质岩石的微量元素和Pb同位素特征指示其母岩浆起源于亏损地幔源区。这些镁铁质岩石的La/Sm和Sm/Yb比值显示其岩浆产生于尖晶石二辉橄榄岩地幔经过大约10%部分熔融作用。综合岩相学和全岩主量元素特征暗示这些镁铁质岩石形成于无水玄武质岩浆。而且这些镁铁质岩石的微量元素和REE元素配分模式均非常相似于N-MORB,除了弱Nb-Ta负异常。这些特征表明路曲和大竹曲岩体形成于大洋中脊环境。此外,路曲和大竹曲镁铁质岩石的Pb同位素结果指示其地幔源区与印度洋MORB地幔域具有相似的地球化学特征。这些镁铁质岩石N-MORB标准化微量元素模式显示弱Nb-Ta负异常可能是由于其地幔源区交代了古老的俯冲带物质。

A series of ophiolitic massifs are distributed along the Yarlung Zangbo Suture Zone(YZSZ),southern Tibet.However,the formation settings of these ophiolites are still controversial.Trace elements and Pb isotope geochemical features indicate that the magmas that formed the mafic rocks in the Luqu and Dazhuqu massifs of the Xigaze ophiolite in YZSZ were derived from the depleted mantle source.The La/Sm and Sm/Yb ratios of the mafic rocks show that their parental magmas were produced by^10%partial melting of spinal lherzolite.Combined with the petrographic observations and major elements of the mafic rocks,the authors hold that they were formed from an anhydrous basaltic magma.Furthermore,the normalized patterns of trace elements and REE of the mafic rocks are very similar to those of N-MORB,except for weakly negative Nb-Ta anomalies.These features indicate that the Luqu and Dazhuqu massifs were formed in the MOR environment.Moreover,the Pb isotope of the mafic rocks indicates that their mantle sources have similar geochemical characteristics to Indian MORB mantle domain.N-MORB normalized trace element patterns of the mafic rocks show that weakly negative Nb-Ta anomalies might have resulted from the process that their mantle source metasomatized old subducted materials.