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  

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

作　　者：梁桑 周涛[2,3] 李德威[2,3,4] 陈超[2,3] 郝海健[2,3] 胡维云[2,3] 李华亮[2,3] LIANG Sang ZHOU Tao LI Dewei CHEN Chao HAO Haijian HU Weiyun LI Hualiang(China Southern Petroleum Exploration ＆ Development Corporation, Haikou 570102, Hainan, China Institute of Geological Survey, China University of Geosciences, Wuhan 430074, Hubei, China Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, Hubei, China Research Center for Tibetan Plateau, China University of Geosciences, Wuhan 430074, Hubei, China)

机构地区：[1]南方石油勘探开发责任有限公司,海南海口570102 [2]中国地质大学(武汉)地质调查研究院,湖北武汉430074 [3]中国地质大学(武汉)地球科学学院,湖北武汉430074 [4]中国地质大学(武汉)青藏高原研究中心,湖北武汉430074

出　　处：《大地构造与成矿学》2017年第5期989-1000,共12页Geotectonica et Metallogenia

基　　金：中国地质调查局西藏1:50000聂拉木等4幅地质图区调(1212011121242);1:50000日土县卡易错地区4幅地质图区调(1212011121246);油气资源与探测国家重点实验室开放基金(KFKT2010-5)联合资助

摘　　要：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/Ta>1.6),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。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 melange 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〉l.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.

关 键 词：辉长岩 锆石U-Pb年代学 弧前盆地 向南俯冲 班公湖蛇绿构造混杂岩 

分 类 号：P597[天文地球—地球化学] P595[天文地球—地质学]