特提斯喜马拉雅-印度地体初始裂解:来自藏南地区晚白垩世OIB型玄武岩的证据

The Initial Break-Up between Tethyan-Himalaya and Indian Terrane:Evidences from Late Cretaceous OIB-Type Basalt in Southern Tibet

长期以来关于喜马拉雅地体是否从印度大陆裂解以及何时裂解的问题存在较大争议,而藏南卡达地区新发现的晚白垩世枕状玄武岩为解决该问题提供了新的证据.卡达玄武岩位于卡达乡北侧,玄武岩呈北西西向不整合于中侏罗统遮拉组之上,锆石SHRIMP U-Pb同位素定年获得的成岩年龄为92.1±1.2 Ma,代表特提斯喜马拉雅地区晚白垩世时期火山活动.卡达玄武岩主量元素、微量元素分析结果显示,玄武岩为碱性玄武岩,轻重稀土明显分馏((La/Yb)N=5.7~7.1),无Nb-Ta、Eu、ZrHf负异常,以及高Fe、P、Ti含量,低(La/Nb)PM、(Th/Ta)PM比值特征.锆石εHf(t)值介于9.02~12.97,平均为10.50,地幔模式年龄tDM1为241~399 Ma.地球化学指标以及同位素组成显示卡达玄武岩为未受地壳混染的OIB型玄武岩,岩浆起源于含石榴石、尖晶石二辉橄榄岩的部分熔融.卡达玄武岩形成于特提斯喜马拉雅被动大陆边缘环境,被动大陆边缘的火山岩浆岩通常与大陆裂解有关,结合古地磁研究结果以及新特提斯洋盆演化证据,认为卡达玄武岩与特提斯喜马拉雅和印度地体的裂解有关,代表大陆地壳初始裂解的时间.

The initial break-up time between Tethyan-Himalaya and Indian terrane has been controversial for a long time.In this study,we report newly discovered pillow basalts which provide new insights to the solution of the problem.The pillow basalts trending northwestward unconformable overlap on the Middle Jurassic Zhela Formation.The zircon SHRIMP U-Pb isotopic dating shows the formation age of 92.1±1.2 Ma,representing the Late Cretaceous volcanic activity of the Tethys Himalaya.The major and trace elements analyses show the basalt is alkaline basalt with（La/Yb）N=5.7-7.1 and without Nb-Ta,Eu,Zr-Hf negative anomalies,and it has high Fe,P,Ti contents,low（La/Nb）PM and（Th/Ta）PM ratios.ZirconεHf（t）values range from9.02 to 12.97,with an average of 10.50 and tDM1 is 241-399 Ma.Geochemical and isotopic data show that the Kada basalt isOIB-type basalt without any contamination from the crust and the magma originates from partial melting of garnet-spinel bearing lherzolite.Kada basalt is present within Tethyan Himalayapassive continental margin.Mafic volcanics spreading within passive continental margins is usually related with the processes of continental break-up and formation of a new ocean basin.Combined with the latest research result of paleomagnetic and evolution of Neo-Tethys oceanic basin,we suggest that the Kada basalt is related to the initial break-up between Tethyan-Himalaya and Indian terrane.The Kada basalt provides new clues for the study of tectonic evolution of the Tethyan Himalaya.