西藏阿里地区冈底斯花岗岩体的热年代学特征及其构造意义

冈底斯花岗岩带是沿雅鲁藏布江北侧近东西向展布的一条长约2 500 km、宽100～300 km的巨型岩浆岩带。在阿里冈底斯山主峰岗仁波齐峰附近，冈底斯花岗岩体受到多期断裂活动的影响，特别受喜马拉雅大反向断裂和喀喇昆仑断裂活动的改造，造成断裂和岩体的关系出现很多复杂的情况，一些研究者把在北阿伊拉日居山分布的32～25 Ma的花岗岩作为喀喇昆仑断裂活动引起的同构造花岗岩，并把此年龄段归结为断层活动年龄，从而引起了极大的争论。本文的锆石U-Pb年龄指示了岗仁波齐峰地区的冈底斯花岗岩是由110 Ma、60 Ma和50 Ma的3期花岗岩组成，而韧性剪切带内的锆石年龄与附近未变形岩石内的锆石年龄一致，表明锆石的形态并未受到断裂活动的影响。韧性剪切带内云母的氩氩年龄为12 Ma左右，而周围未变形花岗岩的云母氩氩年龄在60～50 Ma左右，由此表明喀喇昆仑断裂在岗仁波齐峰地区是12 Ma开始活动的。由于研究区内韧性剪切带中的变形花岗岩并没有记录32～25 Ma这期热事件，由此排除了断裂在狮泉河-门士一线是32～25 Ma开始活动的可能性。

The chronological characteristics of the Gangdese granite in the Ali area and its tectonic significance

The Gangdese granite belt is a 2 500 km long,100～300 km wide volcanic belt almost E-W striking along the north side of the Yalu-Zangpo suture in South Tibet. The granite around Kailas area experienced periods of tectonic events during the collision of the Indian and Eurasian plates,such as the activity of the Great Counter Thrust fault and the Karakorum fault,which complicated the situation between the granites and the faults. For example,some researchers attributed the 32～25 Ma granite around the North Ayilari range to the syn-tectonic granites of the Karakorum fault,which is in high debate nowadays. In this paper,new zircon U-Pb data around Kalas area demonstrates that the Gangdese granites are consisted of 110 Ma,60 Ma and 50 Ma three ages of granite. Zircon ages are similar whether in ductile shear zone or not showing no deformation of zircons by the fault activity. However,the mica ages in ductile shear zone is round 12 Ma,differs from the 60～50 Ma mica ages of undeformed rocks nearby,proving the fault imposed on the Gangdese granites around 12 Ma. The case that no 32～25 Ma ages in ductile shear zone around Kailas area precludes the hypothesis that the initiation age of the Karakoram fault is around 32～25 Ma in Ali area.