拉萨地体中北部尕尔穷晚白垩世早期高镁闪长玢岩地球化学特征指示：加厚下地壳的拆沉？

摘要

The Gaerqiong diorite porphyry developed in the mid-northern Lhasa terrane shows some adakite affinities. The geo⁃chemical features, combined with high MgO (>3.56%), Cr (>150 × 10-6) and Ni (>70 × 10-6) concentrations, and Mg# values (>61), suggest that the Gaerqiong diorite porphyry can be classified as high Mg# adakitic rocks. Previous studies have indicated that the granodiorites and diorite porphyries in the Gaerqiong area were developed during 90~86Ma. The high Mg# adakitic rocks in this study have a close spatial relationship with these rocks, which suggests that all of them were formed by the same tectonic event. The geochemical characteristics of the Gaerqiong high-Mg# adakitic rocks are consistent with a derivation from partial melting of delami⁃nated lower continental crust. The results achieved by the authors suggest that the major crustal thickening of mid-northern Lhasa terrane occurred prior to the Indo-Asian collision.%拉萨地体中北部革吉县尕尔穷矿区出露一套具有埃达克质岩特征的高镁闪长玢岩。野外观察表明，闪长玢岩与含矿斑岩（闪长玢岩、石英闪长岩、花岗闪长岩）在岩性上呈渐变关系，且共同侵位于石炭纪—二叠纪变沉积岩地层，因此它们很可能为同一岩浆事件的产物。尕尔穷埃达克质岩高镁闪长玢岩的地球化学特征显示，它们可能是拆沉下地壳部分熔融的结果。结合区域地质背景认为，在晚白垩世早期（90~86Ma），拉萨地体中北部可能因地壳加厚而发生拆沉，拆沉的下地壳部分熔融体在上升过程中与周围地幔橄榄岩发生反应，致使岩浆中的MgO、Cr及Ni含量增加，从而形成尕尔穷高镁闪长玢岩的母岩浆。尕尔穷地区具有埃达克质岩特征的高镁闪长玢岩的岩浆作用进一步表明，在印度板块和欧亚板块碰撞之前青藏高原南部中北拉萨地体已经加厚并抬升。