青藏高原羌塘中部双湖早古生代斜长角闪岩年代学及地质意义

The amphibolite from the Shuanghu region, Qiangtang Basin, Qinghai-Xizang Plateau:Geochronological and geological implications

关键词:  龙木错-双湖结合带  昌宁-孟连结合带  双湖齐陇乌如沟  锆石U-Pb年龄  斜长角闪岩

Key words:  Lungmuco-Shuanghu sutute zone  Changning-Menglian suture zone  Qilongwuru valley in Shuanghu  zircon U-Pb dating  amphibolite

摘要:

羌塘盆地中部双湖县齐陇乌如沟出露一套斜长角闪岩,通过对该斜长角闪岩锆石SHRIMP U-Pb定年,获得²⁰⁶Pb/²³⁸U加权平均年龄为450.1 ±5.0Ma,表明该变基性岩形成于晚奥陶世。元素地球化学测试分析表明,SiO₂含量在45.67%~51.68%,Na₂O为0.98%~2.54%,总体呈现明显的高Na、低K的特点。轻、重稀土元素分馏较小,Eu具有轻度的正异常。微量元素Rb、Ba和U元素相对较为富集,Nb、Ta和Zr具有不同程度的亏损。地球化学特征显示具有E-MORB的性质,代表了古特提斯洋壳的残余。齐陇乌如沟斜长角闪岩的年龄与昌宁-孟连结合带的铜厂街蛇绿岩和湾河蛇绿岩形成年龄基本一致,地球化学性质相似,说明龙木错-双湖结合带与昌宁-孟连结合带代表了一个统一的原(古)特提斯洋。

Abstract:

A succession of amphibolite crops out along the Qilongwuru valley in the Shuanghu region, Qiangtang Basin, Qinghai-Xizang Plateau. With the aid of SHRIMP zircon U-Pb dating, the amphibolite herein gives a ²⁰⁶Pb/²³⁸U weighted average age of 450.1 ±5.0 Ma, indicating that the amphibolite was formed during the Late Ordovician. Geochemically, the amphibolite is characterized by SiO₂ contents of 45.67%-51.68%, Na₂O contents of 0.98% -2.54%, high Na contents and lower K contents, lower differentiation of LREE and HREE, positive Eu anomaly, relatively enrichment of the elements Rb, Ba and U, and relatively depletion of the elements Nb, Ta and Zr. These geochemical signatures show that the amphibolite has the features of the enriched mid-oceanic ridge basalts (E-MORB), and represents the relicts of the Palaeo-Tethys oceanic crust. The similarity in the ages and geochemical signatures of the amphibolite from the Qilongwuru valley in the Shuanghu region and those of the Tongchang ophiolites and Wanhe ophiolites in the Changning-Menglian suture zone indicates that the Lungmuco-Shuanghu sutute zone and Changning-Menglian suture zone are represented by a unified Proto-(Palaeo-) Tethys ocean.