Geochemical and Geochronology characteristics of Retrograde Eclogite in Xiarihamu area, East Kunlun Mountains, and its geological implications

This paper studies the geochemistry and geochronology of the retrograde eclogite and discusses the protolith properties and tectonic setting. The contents of SiO₂, Al₂O₃, TiO₂ and total alkali (K₂O + Na₂O) are 46.77%~49.31%, 13.6%~14.6%, 0.93%~2.29%, and 1.61%~2.66%, suggesting that the rocks belong to sub-alkaline tholeiite series. (La/Yb)_N value is 0.98~1.63,(La/Sm)_N value is 0.98~1.41 and δEu value is 1.10~1.28. The chondirite-normalized REE patterns of rocks show the right deviation type with weakly positive Eu anomaly. The rocks are relatively enriched in large ion lithophile elements of Rb, Ba and incompatible elements of U, relatively depleted in high field strength elements of Zr and Hf, The Hf/Th and Th/Yb ratios vary in the range of 0.59~0.97, 0.13~0.27, suggesting the protolith of the retrograde eclogite is E-MORB. The zircon Th/U values of the retrograde eclogite is 0.002~0.091, suggesting the metamorphic origin. Its zircon SIMS U-Pb age is 414.2 ±7.3 Ma, which represents the peak metamorphic age of the retrograde eclogite, in combination with regional geological setting, the authors hold that the rock was formed in an tectonic transtition environment from collisional orogeny to post-collisional extension at Early Devonian.