四川新龙地区中酸性侵入岩年代学、地球化学及对甘孜-理塘洋演化过程的制约

Geochronology and geochemistry of intermediate-acid intrusive rocks in Xinlong area,Sichuan Province and its constraints on the evolution of Ganzi-Litang Ocean

基于详细的野外调查,对四川新龙地区不同类型侵入岩进行了岩石学、地球化学和锆石U-Pb同位素测年,探讨其岩石成因及构造背景。研究表明,古隆巴花岗闪长岩和日果闪长岩同属准铝质系列,具有低硅碱、高铁镁特征,中等的稀土元素总量及明显的Nb、Ta负异常,属于I型花岗岩,为俯冲期岩浆活动产物,获得日果闪长岩锆石U-Pb年龄为217.1±0.8 Ma,形成时代为晚三叠世。日里隆巴花岗闪长岩和花岗岩属于过铝质系列,具有高硅碱、低铁镁及弱的Nb、Ta负异常,均属于S型花岗岩,但后者经历了更高程度的分异,分别为碰撞期和后碰撞期岩浆活动的产物,获得日里隆巴花岗闪长岩锆石U-Pb年龄为203.6±0.5 Ma,形成时代为晚三叠世末。结合区域资料认为,晚三叠世甘孜-理塘洋盆大规模向西俯冲,少量的弧岩浆岩就位于被动大陆边缘雅江残余盆地;晚三叠世末期,由于洋板块的消亡发生弧-陆碰撞,地壳发生部分熔融,形成强过铝质花岗岩。

The Yajiang residual basin is located on the southeastern margin of Tibetan Plateau and is an important part of the Yulong-Bayankala foreland basin in the Qiangtang-Sanjiang orogenic system.Based on detailed field investigations,we carried out petrology,geochemistry and zircon U-Pb isotope dating of different types of intrusive rocks in the Xinlong area of Sichuan Province,to discuss their petrogenesis and tectonic background.The results indicate that both the Gulongba granodiorite and the Riguo diorite belong to the metaluminous granites,with low silica,low alkaline,high Fe-Mg characteristics,intermediate REE total contents,and obvious Nb and Ta negative anomalies.They belong to the I-type granites,which are products of subduction magmatic activity.The Riguo diorite was formed at the Late Triassic with zircon U-Pb age of 217.1±0.8 Ma.The Rililongba granodiorite and granite are the peraluminous rocks,with high silica,high alkaline,low Fe-Mg and weak Nb and Ta negative anomalies.Both of them belong to S-type granite,but the latter experienced higher differentiation degree.They were the products of collisional and post-collisional magmatic activity.The zircon U-Pb age of the Rilongba granodiorite is 203.6±0.5 Ma,and its formation period is the end of the Late Triassic.It is believed that the Ganzi-Litang paleo-ocean basin was subducted westward in the Late Triassic,and a small amount of arc intrusions were located in the Yajiang residual basin on the passive continental margin.At the end of the Late Triassic,the arc-land collision occurred due to the extinction of oceanic plates,which caused partial melting of the crust to form strong peraluminous granites.