西藏洞错埃达克质火山岩LA-ICP-MS锆石U-Pb年龄及其构造意义

班公湖-怒江缝合带是青藏高原的主要缝合带之一,其闭合时限还存在争议,制约了班公湖-怒江特提斯洋演化历史的研究。在班公湖-怒江缝合带洞错地区新识别出一套早白垩世晚期的粗安岩,LA-ICP-MS测得其锆石~(206)Pb/~(238)U年龄为102.0±1.9Ma,岩石高SiO_2、Al_2O_3、富Na及高的Sr/Y值(25.2~42.2),贫Nb(11.1×10~(-6)~16.6×10~(-6))、Y(11.7×10~(-6)~18.3×10~(-6))和Yb(1.06×10~(-6)~1.77×10~(-6)),轻、重稀土元素分异明显,具有与埃达克质岩相似的地球化学特征,同时具有高Mg~#的特点(Mg~#=50.0~54.1)。认为洞错高Mg~#埃达克质岩很可能是残留洋壳部分熔融上升过程中交代地幔的产物,形成于板内构造环境。由此推测,洞错早白垩世晚期埃达克质岩的形成与班公湖-怒江洋盆闭合后拉萨地块与羌塘地块碰撞有关,结合区域地质资料,认为班公湖-怒江特提斯洋闭合时限应该在102Ma之前。

LA-ICP-MS zircon U-Pb ages of adakitic rocks in Dongco area, Tibet, and their tectonic implications

The Ban''gong Co-Nujiang suture zone is one of the main suture zones in the Tibetan Plateau. Its closure time has been controversial, which causes some restrictions to the study of the evolution of the Tibetan Plateau. This paper deals with the discov-ered trachyandesite which occurs in Bangong Co-Nujiang suture zone. LA-ICP-MS zircon U-Pb analyses suggest that the trachy-andesite was formed at 102±1.9Ma, and that the volcanic rocks were formed in late Early Cretaceous. They are characterized by ada-kite-like rock:high Na content, SiO₂, Al₂O₃, Mg^# and high Sr/Y values(25.2~42.2), and low Nb(11.1×10^-6~16.6×10^-6), Y (11.7×10^-6~18.3×10^-6)and Yb(1.06×10^-6~1.77×10^-6) values. The differentiation between HREE and LREE is obvious. The authors hold that high Mg^# adakite-like rock(Mg^#=(50.0~54.1) resulted from the reaction between the rising melt of remnant oceanic crust and the mantle wedge. Therefore, the origin of Dongcuo adakitic-like rocks may be related to the collision between Lasha and Qiang-tang blocks after the closure of the Bangong Co-Nujiang Ocean. Based on these data, the authors consider that Bangong Co-Nuji-ang Ocean had been closed before 102Ma.