西藏班戈县冈瓦纳相冰碛岩的发现及其对北拉萨地块构造演化的启示——来自1∶50000专题地质调查的新证据

Identification of Gondwana tillite in the Bange County of Tibetan Plateau and its implications for the tectonic evolution of North Lhasa terrane-New evidence from the 1:50000 thematic geological survey

【研究目的】拉萨地块作为构成青藏高原的最主要陆块之一,其构造格局的划分对深入理解特提斯洋的时空分布格局具有重要科学意义。一般来说,拉萨地块可进一步划分为北拉萨、中拉萨和南拉萨地块,但关于各地块基底属性、漂移历史、增生过程及构造亲缘性等仍不明确。【研究方法】本文报道了笔者等在北拉萨地块北拉地区开展1∶50000专题地质调查期间,在原划定的中—上侏罗统拉贡塘组中发现了大面积出露的典型冰碛岩与震积岩。【研究结果】它们与中拉萨地块申扎地区的上石炭统—下二叠统拉嘎组可对比,是冈瓦纳大陆北缘晚古生代冰期事件的记录。冰碛岩中可见明显冰筏作用形成的冰海相砾石稀散分布在砂质、粉砂质和泥质基质中,具坠石构造。震积岩以震褶岩和震裂岩为主,发育微褶皱、砂(泥)岩脉、同震节理、内碎屑副角砾岩、阶梯状断层和火焰构造等同生或准生地震沉积记录。【结论】北拉地区拉嘎组冰碛岩的发现表明,北拉萨地块与中拉萨地块具有类似的基底,它可能是在陆壳基础上逐渐形成的增生型微陆块,其形成与班公湖—怒江洋的俯冲消减密切相关,这为深入理解北拉萨地块的构造属性提供了关键证据。

This paper is the result of geological survey engineering.[Objective]As the main terrane in the Tibetan Plateau,the division of the Lhasa terrane is significance for the spatial and temporal distribution pattern of the Tethys Ocean.Generally,it can be further divided into the North,Central,and South Lhasa terranes.However,it is still ambiguous about their basement nature,drifting,accretion,and tectonic affinity.[Methods]In this paper,based on the 1:50000 geological survey in the Beila area,North Lhasa terrane,the widely exposed and typical tillite and seismite are newly identified from the Middle-Upper Jurassic Lagongtang Formation,northern Tibetan Plateau.[Results]They can be compared with the Upper Carboniferous-Lower Permian Lagar Formation in the Xainza area,Central Lhasa terrane,indicating the Late Paleozoic glacial event in the northern margin of the Gondwana.The glacial marine gravels,formed by ice raft,are distributed in the sandy,silty and argillaceous matrix,with falling structure.Seismites are mainly seismic corrugated and shattering,with syngenetic or parasyngenetic seismic sedimentary records of microfolds,sand(mud)veins,coseismic joints,intraclastpara-breccia,steplike faults and flame structures.[Conclusions]The identification of the tillite of Lagar Formation indicates that the North Lhasa terrane holds similar basement with the Central Lhasa terrane.It is probably anaccretion-induced micro-block formed gradually on the basis of local continental crust,which is closely related to the subduction process of the Bangong-Nujiang Tethys Ocean.This provides key evidence for further studying the tectonic affinity of the North Lhasa terrane.