拉萨地块西部亚热地区林子宗群火山岩地球化学特征及其构造意义

Geochemistry and geological significance of volcanic rocks of the Linzizong Group,Yare area,western Lhasa Block

广泛分布于拉萨地块南部的林子宗群火山岩被普遍认为是新特提斯洋北向俯冲消减结束并由同碰撞向后碰撞构造背景转化过程中的岩浆作用产物,记录了从新特提斯洋俯冲结束到印度欧亚大陆碰撞后碰撞的丰富信息。本文对采集于拉萨地块西部亚热地区的林子宗群火山岩样品进行了年代学和岩石地球化学研究,结果显示:林子宗群火山岩为一套钾玄质系列的流纹岩,该岩石具有高硅(w(SiO2)=73.90%~74.03%)、高钾(w(K2O)=6.15%~6.28%)、高钾钠比(w(K2O)/w(Na2O)=2.14~2.65),富集K、Rb、Th、U、Hf等元素,亏损Nb、Ta、P、Ti、Sr等元素,具有明显的Eu负异常;林子宗群火山岩样品LA-ICP-MS锆石U-Pb年龄为50.1 Ma±0.3 Ma—51.3 Ma±0.4 Ma之间;结合前人研究,笔者认为亚热地区林子宗群火山岩为形成于印度欧亚大陆同碰撞背景下的大陆地壳部分熔融的产物。

Volcanic rocks of the Linzizong Group widely distributed in the southern part of the Lhasa Block are considered to be the product of magmatism during ending of the northward subduction of Neo-Tethys Ocean and the transition from syn-collision to post-clision thus keep abundant information from ending of the northward subduction of Neo-Tethys Ocean to Indian-Eurasian continent-continent clision-post-clision.This paper has carried out chronological and petrochemical studies on the samples of the Linzizong Group volcanic rocks collected in the western sub-thermal region of the Lhasa block.The results show that it is a set of potassium basaltic series of rhyolites with high SiO2(73.90%~74.03%),high K2O(6.15%~6.28%),high K2O/Na2O(2.14~2.65%)enrichment of K,Rb,Th,U,Hf and other elementsand and depletion of Nb,Ta,P,Ti,Sr,Ba and other elements and a significant Eu negative anomaly.The LA-ICP-MS zircon U-Pb age of the acid rock samples is between 50.1 Ma±0.3 Ma and 51.3 Ma±0.4 Ma.Combined with the previous data the author considers that the volcanic rock is the product of partial melting of continental crust during syn-clision ofI ndean and Eurasian continents.