新疆塔什库尔干县含稀有金属伟晶岩-碳酸岩的时代——对帕米尔构造结稀有金属成矿作用的启示

Age of the rare metal(rare earth metals)bearing pegmatite and carbonatite in Tashikorgan,Xinjiang:Implications for rare metal mineralization in the Pamir Syntax

帕米尔构造结位于青藏高原北缘,记录了自新元古代晚期以来到新生代完整的特提斯构造演化过程。在长期的构造—岩浆演化过程中,发育了不同时代、不同类型的成矿事件。野外调查表明,在帕米尔北东段,发育了含绿柱石的伟晶岩及含氟碳铈矿的碳酸岩(碳酸岩—伟晶岩组合)。通过独居石及锆石U-Pb定年,识别出该区两期伟晶岩带(包括碳酸岩),分别是新生代和中生代(19~18 Ma和200 Ma),其中早期含绿柱石伟晶岩形成于古特提斯洋关闭后的伸展阶段,而新生代含氟碳铈矿碳酸岩—伟晶岩组合,与新生代陆内走滑伸展事件相关,表明帕米尔构造结伸展走滑启动的时间可能在19 Ma。结合区域地球化学异常特征,表明在帕米尔地区有望在稀有、轻稀土找矿上获得突破。

Objectives:The Pamir syntax is located in the northern margin of the Tibetan Plateau,which records the complete evolution of Tethys from late Neoproterozoic to Cenozoic.During the long period of tectono—magmatic evolution,a series of metallogenic events developed in this area.Field investigation shows that hamartite-bearing carbonatite and davidsonite-bearing pegmatite are developed in the north-east Pamir.The aim of this study is to precisely date the emplacement ages of the hamartite-bearing carbonatite and davidsonite-bearing pegmatite.Methods:By LA-ICP-MS method,we analyzed the U-Pb ages of the monazite and the zircon from the carbonatite and pegmatite.Results:The age of monazite from carbonate is 19~18 Ma,and the age of zircon from pegmatite is 197.5±1.4 Ma.Conclusions:The late Triassic pegmatites were coeval with the rare metal bearing pegmatites in the eastern Karakorum,possibly genetically related to the post-extension after the closure of the Paleo-Tethys Ocean,and the hamartite-bearing carbonatite and pegmatite is related to the strike slip events in the Neozoic.Base on the new geochronological data,we argue that the strikeslip—extension in the Pamir began at ca.19 Ma.Combined with the characteristics of regional geochemical anomalies,we suggest great potential of rare metals mineralization in the Pamire syntax and its marginal areas.