青藏高原羌塘中部榴辉岩地球化学特征及其大地构造意义

    Geochemistry and tectonic significance of eclogites in central Qiangtang, Tibetan Plateau

    • 摘要: 羌塘中部榴辉岩主要呈透镜状、块状产于石榴子石白云母片岩中,均为低温型榴辉岩。榴辉岩SiO2为45.83%~48.94%, Na2O+K2O为1.88%~3.91%, Al2O3为10.34%~15.02%,TiO2含量变化较大,大致可分为中Ti和高Ti两种类型,变化范围分别为1.62%~1.87%和3.96%~4.82%。榴辉岩的原岩主要为拉斑玄武岩,部分样品具有碱性玄武岩的特征。原岩为拉斑玄武岩的样品大多具有中等TiO2含量和不明显的Eu异常、较缓的右倾稀土元素配分模式特征,在大量判别图解上均投在MORB和E-MORB区域,稀土元素标准化配分模式和微量元素蛛网图特征与典型的E-MORB相似而与N-MORB差别很大,其原岩很可能形成于E-MORB环境;少量原岩为碱性玄武岩的样品TiO2含量高、具有相对其他样品更高的∑REE和∑LREE含量,在大量判别图解上均投在OIB区域,其原岩可能形成于OIB环境。羌塘中部榴辉岩和区域内二叠纪蛇绿岩具有相似的地球化学特征,可能是其俯冲消减的产物;样品同时具备E-MORB和OIB的特征,说明古特提斯洋在漫长的演化过程中可能存在地幔柱岩浆和正常洋中脊亏损地幔岩浆的相互作用。

       

      Abstract: The central Qiangtang eclogites occur mainly as lenses within garnet-muscovite schist. Based on equilibrium texture and mineral chemistry, thermobarometric estimates indicate that the central Qiangtang eclogites formed in low temperature condition. The geochemical characteristics show that their protolith are tholeiitic basalt and alkaline basalt with 45.83~48.94wt% SiO2, 10.34~15.02wt% Al2O3 and 1.88~3.91wt% total alkalis (Na2O+K2O). Most samples which protolith are tholeiitic basalt are characterized by moderate TiO2 content (1.62~1.87wt%), slight enrichment LREE distribution patterns, suggesting a typical E-MORB source. Immobile elements covariance diagrams show that most of them formed in an E-MORB and MORB setting and their REE distribution patterns and primitive mantle normalized trace elements patterns are similar to that of the typical E-MORB. Some samples, which are characterized by high TiO2 content(3.96~4.82wt%), relatively high ΣREE and ∑LREE contents, are derived from a typical OIB source. Immobile elements covariance diagrams also show that they formed in an OIB setting. The geochemical characteristics of the central Qiangtang eclogites are similar to that of Permian ophiolite in the area, implying that the eclogites are the product of the subduction of permian ocean crust. All the eclogite samples with the feature of both E-MORB and OIB indicate a plume-ridge interaction or hotspot-ridge interaction during the evolution process of Paleo-Tethys.

       

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