东昆仑夏日哈木退变质榴辉岩地球化学、年代学特征及其地质意义

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

关键词:  榴辉岩  地球化学特征  锆石U-Pb定年  夏日哈木

Key words:  retrograde eclogite  gochemical characteristics  Zircon U-Pb ages  Xiarihamu

摘要:

本文对东昆仑夏日哈木退变质榴辉岩开展岩石地球化学和年代学研究，探讨其原岩性质和构造背景。岩石SiO₂含量为46.77%~49.31%，Al₂O₃含量为13.6%~14.6%，TiO₂含量为0.93%~2.29%，全碱（K₂O+Na₂O）含量为1.61%~2.66%，原岩属亚碱性玄武岩系列中的拉斑玄武岩。（La/Yb）_N=0.98~1.63，（La/Sm）_N=0.98~1.41，δEu=1.10~1.28，稀土元素球粒陨石标准化图呈弱Eu正异常的右倾型，岩石相对富集大离子亲石元素Rb、Ba和不相容元素U，相对亏损高场强元素Zr、Hf，Hf/Th=0.59~0.97，Th/Yb=0.13~0.27，显示其原岩具E-MORB特征。退变质榴辉岩的锆石Th/U比值为0.002~0.091，为变质成因锆石，其锆石SIMS U-Pb年龄为414.2 ±7.3 Ma，代表了退变质榴辉岩的峰变质年龄，结合区域地质背景，认为该岩石形成于早泥盆世早期由碰撞作用向碰撞后伸展作用转换的构造环境。

Abstract:

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.