班公湖中特提斯洋向南俯冲的时限:来自SSZ型辉长岩的制约

班公湖-怒江中特提斯洋的俯冲极性和俯冲时间一直存在争议。作者通过野外地质调查、岩相学、锆石U-Pb年代学及岩石地球化学研究,从西藏班公湖蛇绿混杂岩带中识别出一套早白垩世SSZ型蛇绿岩,岩石组合上主要由辉长岩和玄武岩组成,还有少量的硅质岩和超基性岩。本文对辉长岩进行了全岩主、微量元素地球化学及LA-ICP-MS锆石U-Pb年代学研究。地球化学组成特征显示,辉长岩富集轻稀土元素,重稀土元素平坦,相对富集大离子亲石元素,高场强元素存在一定亏损;Th/Ta比值与岛弧玄武岩相似(Th/Ta1.6),Ta/Hf比值较高(0.1),显示其既保留了俯冲环境的地球化学特征,也提供了伸展构造环境的信息。辉长岩中锆石U-Pb加权平均年龄为129.2±0.4 Ma(MSWD=0.36),该年龄是班公湖-怒江缝合带中迄今报道的最年轻蛇绿岩年龄。结合区域地质背景,认为这套蛇绿岩形成于班公湖-怒江古洋盆西段向南俯冲形成的弧前盆地,而班公湖-怒江古洋盆北向俯冲可能始于早侏罗世,晚侏罗世形成双向俯冲格局,直到早白垩世洋盆关闭,晚白垩世进入陆内构造环境。

Timing of Southward Subduction of Meso-tethys in Bangong Lake:Constraints from Supra-Subduction Zone (SSZ)-type Gabbro

The polarity and time of subduction of the Bangong-Nujiang Meso-Tethys have been controversial.By geological surveys,petrographic,geochemical studies and zircon U-Pb chronology,a set of Early Cretaceous SSZ-type ophiolite rocks from the Tibet Bangong ophiolitic mé1ange belt was identified.The ophiolite is mainly composed of gabbro and basalt as well as a small amount of siliceous rock and ultrabasic rocks.This paper systematically investigated the gabbros by conducting LA-ICP-MS zircon U-Pb dating,major and trace element geochemistry.Geochemically,the gabbro is enriched in LREE,and relatively depleted in high field strength elements.The Th/Ta ratios of the gabbro are similar to that of island arc basalts (Th/Ta＞1.6),while the Ta/Hf ratios are slightly higher (＞0.1).The geochemical characteristics showed that the ophiolite not only retained the geochemical characteristics of subduction environment,but also provided information of extensional tectonic environment.The zircon U-Pb age of gabbro is 129.2±0.4 Ma (MSWD=0.36),which is the youngest age ever reported for ophiolite in the Bangong-Nujiang suture zone.Combined with regional tectonic setting,the ophiolite was formed in a forearc basin resulted from the southward subduction of the west part of the Bangong-Nujiang ancient ocean basin.Moreover,the northward subduction of the Bangong-Nujiang ancient oceanic basin may start at the early Jurassic,and a bi-dive pattern was formed in Late Jurassic.After the closure of the ocean basin in the Early Cretaceous,the regional tectonic regime turned into the continental tectonic environment in the Late Cretaceous.