Through analyzing the zircon U-Pb LAICPMS dating and lithogeochemistry of the arc magmatic batholiths in the Yanhu complex massif on the south side of the west segment of the Bangonghu-Nujiang suture belt in the Nawucuo area of Tibet,this paper presents the new lithology and chronology evidences of the southward subduction of the Tethys ocean basin in the west segment of the Bangonghu-Nujiang suture belt.The lithogeochemical analyses indicate that the lithology of the intrusive rocks is acid to basic,belonging to the middle to high K calc-alkaline series,and the lithology of the volcanic rocks are intermediate andesite and basic basalt which are located in the Middle-K calc-alkaline series of the mild alkaline area.Consequently,all the above rocks are typical arc magmatite.The trace element distribution spider diagram shows that the arc intrusive rocks possess common signature of high enrichment of large-ion incompatible element,Rb,K,Pb,Th,Sr and strong depletion of high field strength elements,Nb,Ta,Zr,P.This indicates that the magmatite have the basic trace element feature of the arc granite and are resulted from the magmatism of the southward subduction stage.The combined characteristics of the rare earth elements indicate that the arc intrusive rocks are generated by partial melting of the lower crust.A further study of lithogenesis shows that the volcanic rocks are derived from the volcanic arc circumstance and are formed by the magmatic upward emplacement and eruption of the magma created by partial melting of the upper oceanic crust,and that the intrusive rocks are generated by the melting of the basaltic magma from upper mantle and the lower crust.The results of the zircon U-Pb LA-ICPMS dating show that the weighted average age of the arc granites batholiths is Late Cretaceous (about 93.0 Ma B.P.)to early Late Cretaceous (about 114.7 Ma B.P.).This illustrates that the duration of the southward subduction at least lasts about 21.7 Ma,and shows the evolutionary tendency of the arc magmatites from middle to high-K calc-alkaline and the lithology from intermediate to acid with the process of subduction from the early to the late.Based on the discussion,the authors suggest that there probably exist multistage subductions and multiple oceanic subduction zones in the Bangonghu-Nujiang suture belt and that the Tethys ocean basin may be composed of a series of limited basins.%对西藏班公湖-怒江缝合带西段南侧纳屋错地区盐湖复式岩体的岛弧型岩浆岩岩基开展岩石地球化学和锆石 U-Pb 测龄分析研究，为班公湖-怒江洋盆的南向俯冲消减提供岩石学和年代学证据。岩石地球化学分析表明，侵入岩的岩性为酸性到基性，属于中-高钾钙碱性岩；火山岩的岩性主要为中性和基性的安山岩和玄武岩，属亚碱性范围的中钾钙碱性岩，说明侵入岩和火山岩都归类于典型的岛弧型岩浆岩。岛弧型侵入岩富集大离子不相容元素 Rb、K、Pb、Th、Sr，亏损高场强元素 Nb、Ta、Zr、P，说明其具备岛弧型岩浆岩的痕量元素特征，为缝合带南向俯冲阶段岩浆作用的产物；稀土元素组合特征说明岛弧型侵入岩源于下地壳部分熔融。进一步的岩石成因研究发现火山岩产生于火山弧环境，由俯冲洋壳上部局部熔融形成的岩浆向上侵位并喷发形成，侵入岩为上地幔玄武岩浆与下地壳熔融而形成的产物。锆石 U-Pb LA-ICPMS 测龄结果显示岛弧型岩浆岩体的加权平均年龄在晚白垩世（约93．0 Ma B．P．）到早白垩世（约114．7 Ma B．P．），说明俯冲消减时间至少持续了21．7 Ma，体现出随时代从早到晚，岩浆岩从中钾钙碱性岩向高钾钙碱性岩、岩性从中性向酸性演化的趋势。通过分析认为班公湖-怒江缝合带很可能存在多期次俯冲和多条洋内俯冲带，特提斯洋盆的属性为一系列局限性小洋盆。
展开▼
