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

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.