西藏东部三叠纪火山岩的形成和演化

Formation and Evolution of the Triassic Volcanic Rocks in East Tibet

西藏东部昌都地区的三叠纪弧-盆体系格局很清晰,三叠纪时弧火山活动达到了最高潮。从早三叠世到晚三叠世,火山作用递次增强,弧火山岩性质由以流纹质或英安质为主到安山质为主的火山岩发生变化,火山爆发指数增大,火山—沉积旋回数量也在增多。研究表明在该区普遍发育滞后型弧火山岩与碰撞型火山岩组合,岛弧火山岩的源区成分为楔形上地幔和同化混染的上覆上部地壳(40%～60%)以及少量洋壳组分(8%～20%)三者的混合。

The framework of the Triassic arc and back-arc basin system is clear in the Changdu area of eastern Tibet. The activity of the arc tended to reach its summit in the Tiassic. From Early Triassic to Late Triassic,volcanism progresses successively and types of volcanic rocks vary from predominant rhyolitic or dacite to andesitic volcanic rocks, the indices of volcanic explosion enlarge, quantities of volcanic and sedimentary cycles also grow. It turns out that there are well-developed arcs and collided volcanic rocks and the source rocks of arc volcanic rocks are a mixture of the wedge upper mantle and overlying assimilatory and hybridized crust (40%～60%) with minor oceanic crust (8%～20%).