川西义敦岛弧中部图姆沟组流纹岩年代学、地球化学特征及其对岩浆成因的限定

Geochronology,geochemical characteristics of the rhyolites of Tumugou Formation in the central part of Yidun arc,western Sichuan:Implications for their petrogenesis

【研究目的】义敦岛弧是三江特提斯复合造山带的重要组成部分。区域现有的研究较多集中在弧花岗质侵入岩及其成矿作用上,而对区内流纹岩的研究则相对匮乏。【研究方法】本文在详细野外填图的基础上,对图姆沟组流纹岩进行岩石学、地球化学和同位素年代学研究。【研究结果】流纹岩锆石U-Pb成岩年龄216.5 Ma,为晚三叠世。岩石SiO_(2)含量为73.24%~74.72%,全碱含量为5.26%~6.27%,为钙碱性系列,富集大离子亲石元素Rb、Th、U、K和轻稀土元素,亏损高场强元素Nb、Ta、Ti、P和重稀土元素,具典型的岛弧火山岩特征。【结论】图姆沟组流纹岩是印支期甘孜—理塘洋壳向西俯冲环境下地壳部分熔融的产物。

This paper is the result of the geological survey engineering[Objetive]The Yidun island arc,as a key part of Sanjiang Tethys orogenic belt,has been hottly debated on granitic intrusions and the related mineralization in recent years.But few studies are concerned with the rhyolites such as the Tumugou Formation.[Methods]Based on detailed field mapping,this paper has conducted research on petrology,geochemistry and chronology of the Tumugou Formation’s rhyolite.[Results]LA-ICP-MS zircon U-Pb dating on one sample yield an age of 216.5 Ma,and proved that the rhyolites were formed in the Late Triassic.The rhyolites have contents of SiO2 from 73.24%to 74.72%,alkali(K2O+Na2O)of 5.26%to 6.27%as calc-alkaline rock series.All the samples are enriched in large ion lithophile elements such as Rb,Th,U,K and light rare earth elements,but depleted in the high strength filed elements such as Nb,Ta,Ti,P and heavy rare earth elements,indicative of the characteristics of island-arc volcanic rocks.[Conclusions]According to the comprehensive study,it is concluded that the rhyolites were formed by partial melting of the lower continental crust during the westward subduction of the Ganzi-Litang oceanic crust in the Indo-Chinese epoch.