西藏班戈寒武纪辉长闪长岩体的发现及其构造意义

The Newly Discovered Cambrian Gabbro-Diorite in Bange, Tibet and TheirTectonic Implications

西藏班戈地区构造单元属北拉萨地体,到目前为止,该地体尚未有寒武纪岩浆活动的报道.本文就班戈地区首次发现的辉长闪长岩体,在野外地质调查、元素地球化学及同位素年代学研究的基础上,得出以下结论:该岩体属高钾钙碱性系列,具有富集大离子亲石元素(如Rb、Ba)和轻稀土元素(La、Ce),亏损Nb、Ta、Zr、Hf和Ti等高场强元素,呈现出安第斯型岛弧岩浆特征,显示该岩体是俯冲沉积物部分熔融形成的熔体交代上覆地幔楔的产物.锆石U-Pb年代学指示其侵入年龄为512±3 Ma,形成于寒武纪,为北拉萨地体存在早古生代岩浆活动提供了可靠的年代学证据.综合证实其属于冈瓦纳大陆北缘的一部分,该成果为进一步探讨青藏高原的构造演化提供了新的约束.

The Bange area is part of North Lhasa terrane.Cambrian magmatism has not been reported in the north Lhasa terrane.In this study,field geological survey and geochemical studies,as well as geochronology,were used to study the firstly discovered gabbro-diorite in Bange and the following conclusions are drawn.The gabbro-diorites belong to the high-K calc-alkaline series.They are rich in large ion lithophilic elements(e.g.,Rb and Ba)and light rare earth elements(e.g.,La and Ce),and are depleted in high-field strength elements(e.g.,Nb,Ta,Zr,Hf and Ti),similar to the Andean island arc magmatic rocks.The rock is interpreted as resulting from partial melting of subduction sediments metasomatized with the overlying mantle wedge.Zircon U-Pb dating demonstrates that the gabbro-diorite formed at 512±3 Ma,which indicates it formed at Cambrian and provides reliable chronological evidence for the existence of Early Paleozoic magmatic events in the North Lhasa terrane.It is confirmed that the North Lhasa terrane is part of northern margin of the Gondwana land.The results provide a new window for further study of the evolution of the tectonic of Tibetan Plateau.