Classification And Geochemical Characteristics Of HP-UHP Metamorphic Rocks In Southwest Tianshan, China

Southwestern Tianshan HP-UHP metamorphic belt in which eclogite was discoveried in 1997 have received extensive attentions and was researched by scholars around the world. The metamorphic belt is mainly constituted by eclogite, blue schist and mica schist, and has constituted a paired metamorphic belt with low-pressure and high-temperature belt which is dominated by gneiss and plagiogneiss rocks in the north.Based on occurrence, Southwestern Tianshan eclogite can be divided into five types:thin layer and lenticular eclogite in mica schist, blue schist and eclogite interbedded thin, pillow-like structure eclogite/blue schist, eclogite in marble and large eclogite rock. Southwestern UHP metamorphism is divided into four metamorphic stages:1.Lawsonite-blue schist stage; 2.Epidote-eclogite stage; 3.Epidote-blue schist stage and 4.Greenschist facies stage.Geochemical analysis results show that the mainly original rock of blue schist, glaucophane rock and eclogite in Southwest Tianshan is tholeiite and alkaline basalt. From standardized N-MORB rare earth distribution patterns of the the original blue schist and glaucophane rock lies between rock between E-MORB and OIB. Eclogite rock could be subdivided into two types:the first is E-MORB type, and has the characteristic of Ba, U enrichment;The second type is OIB. Blue schist, glaucophane rock and eclogite tectonic environment is ocean environment near next to island. the eclogite protolith could be subdivided into two types:E-MORB type and N-MORB type,and the original blueschist rock lies between OIB and E-MORB. According to the graphic interpretation of SiO2-Zr/TiO2 rock classification, the sample is mainly constituted by tholeiitic and alkaline basalts. The main elements and trace elements analysis show that eclogite K, Rb, Cs and Ba contents are the lowest value, which is significantly less than the corresponding value of blue schist and pillow-like blue schist. Therefore, in the process of metamorphism, the original rock basalt dehydration maybe caused the loss of water-soluble elements.Larger particle size epidote group minerals in the samples ATT-75 blue schist have a clear zone, accompanied by radial cracks. By electron microprobe analysis, The mineral is mainly composed of epidote, part for oblique zoisite mineral. Radial crack and band indicates the three periods of its formation. The first phase of the nuclear department formed in the period of deep subduction. The second phase in the curtain and the third phase boundary formed in the exhumation stage. After the exhumation, as the pressure is reduced, the core portion of the volume increase causes radial cracks. XFe3+ that in nuclear-mantle-edge department of epidote minerals varies complicatedly, and the overall trend of core-edge is retrogressive metamorphism.