大陆碰撞过程的火山岩响应：以西藏林周林子宗火山岩为例

A response of volcanic rocks to the India-Asia continental collision: A case study on Linzizong volcanic rocks in Linzhou, Tibet

关键词:  大陆碰撞  火山活动  林子宗火山岩  演化  林周  西藏

Key words:  continental collision  volcanic activity  Linzizong volcanic succession  evolution  Linzhou  Tibet

摘要:

印度-亚洲大陆碰撞伴生有大量火山活动，其中，林子宗火山岩发育最广，遍布碰撞带北侧的冈底斯带，形成长逾1200 km的火山岩带。林周地区作为林子宗火山岩的命名地，该套火山岩发育相对齐全，为安山岩、流纹岩及相应的火山碎屑岩夹沉积碎屑岩组合，顶部发育巨厚流纹质凝灰岩，可以划分出三个火山旋回，其生成时代介于63.89~48.73 Ma。岩石学和地球化学资料显示，林子宗火山岩自下而上SiO₂和K₂O含量以及Al₂O₃饱和度增加，其岩浆从早到晚由中性、中钾和准铝质变化到酸性、高钾和过铝质，晚期喷发巨厚的火山灰流，反映区域地壳明显的加厚，由早期的30~40 km变化到晚期的50~60 km。火山岩相对富集Cs、Rb、K、U，亏损Ta、Nb、Ti、Sr、Ba、P，早期与桑日组安山岩地球化学特征相近，而中—晚期与乌郁、扎嘎等地渐新世高钾火山岩相似，表明早期岩浆具有新特提斯洋俯冲板片印迹，而中—晚期具有后碰撞作用特点。林子宗火山岩作为印度-亚洲大陆碰撞过程的响应，记录着古新世至始新世（64~48 Ma）印度-亚洲大陆之间的碰撞向碰撞后演化过程。

Abstract:

Intensive volcanism is usually associated with India-Asia continental collision. The Linzizong volcanic succession (shortly LVS), a result of the India-Asia continental collision, spreads as an over 1200km long volcanic rock zone in whole Gangdise belt north to the Yarlung-Zangbo collision belt. The LVS, nominated in Linzhou area, consists of andesite, rhyolite and the related pyroclastic rocks with interlayers of sedimentary rocks. With vast rhyolitic ignimbriteat at the top of the succession, the LVS can be divided into three volcanic cycles formed between 63.89Ma and 48.73 Ma. The petrological and geochemical data show that the SiO₂, K₂O and Al₂O₃ contents in the LVS increase and the magma was graded from intermediate, moderate potassium and metaluminous to acid, high potassium and peraluminous upwards. Vast hot tuff flow developed in the late stage of the succession. All the data indicate that the thickness of the crust was thickened from 30-40 km in the early stage to 50-60 km in the late stage. The volcanic rocks in LVS are relatively enriched in Cs, Rb, K, U and depleted in Ta, Nb, Ti, Sr, Ba, P. The lower part of LVS is geochemically similar to the andesite of late Mesozoic Sanri Fm., and the upper part is likely same as Miocene high potassium volcanic rocks in Wuyu and Zhaga volcanic basins. It is indicated that the early derived magma is imprinted with subducted slab of the new Tethys oceanic crust and the midlle-late magma has post-collisional characteristics. Therefore, the LVS as a response to the collision process between India and Asia continents has recorded the evolutionary process from collision to post-collision between the India and Asia continents from 64 Ma to 48 Ma in the Paleocene and Eocene.