藏东江达中石炭世弧火山岩的厘定及其构造意义

Delineation of Middle Carboniferous Arc Volcanic Rocks in Jomda Area,Eastern Tibet and Its Tectonic Implications

金沙江特提斯洋盆是三江地区重要的洋盆之一,然而目前对于金沙江洋壳俯冲时限认识不一,弧火山岩对限定洋壳的俯冲消减过程具有重要意义.在西藏东部江达地区首次识别出一套中石炭世晚期的火山岩,主要岩性为玄武岩及安山岩,安山岩锆石U-Pb年龄为332±3.3 Ma(MSWD=2,n=14),指示该火山活动为中石炭世晚期.江达火山岩SiO_2含量为45.65%~55.73%,具有高Al_2O_3含量(15.48%~16.79%),TiO_2含量为0.82%~1.12%,富Na低K;轻重稀土分异明显,无δEu异常,强不相容元素Th、U等富集,高场强元素Nb、Ta等亏损,具有弧火山岩的地球化学性质.安山岩锆石εHf(t)值为较均一的正值(+11.0^+15.6),指示岩浆源区为受到富集组分影响的岩石圈地幔.研究表明江达中石炭世晚期火山岩形成于岛弧的构造环境,其火山活动与金沙江特提斯洋壳俯冲消减过程有关;结合区域地质资料认为金沙江特提斯洋壳在332 Ma之前已经开始了俯冲消减活动,进而为金沙江特提斯洋演化过程提供了岩石学证据.

The Jinshajiang Tethys is one of the most important ocean basins in the Sanjiang region.However,it is not clear when the oceanic subduction started.The research of arc volcanic rock is significant to constrain the subduction process of the oceanic plate.This paper reports the Late Middle Carboniferous volcanic rocks in Jomda area,eastern Tibet,which are dominated by basalts and andesites.Zircon U-Pb age results show they were formed at 332±3.3 Ma.The arc volcanic rocks have SiO2 contents of 45.65%-55.73%,Al2O3 contents of 15.48%-16.79%,TiO2 contents of 0.82%-1.12%,and have high Na/K ratios.They show obvious differentiation between LREE and HREE,and without Eu anomaly.They are enriched in strong incompatible trace elements,such as Th and U,and depleted in high field strength element,such as Nb and Ta.The zircons from basalts have positiveεHf（t）values（＋11.0 to＋15.6）,which indicates that the magma source might be the enriched lithosphere mantle.It is believed that Late Middle Carboniferous volcanic rocks were formed at the island arc tectonic enviorment,which is related to subduction of Jinshajiang Tethys ocean.Combining with the regional geological data,we propose that the subduction of the Jinshajiang Tethys ocean have started before 332 Ma.This study provides a lithology evidence for the evolution of Jinshajiang Tethys ocean.