藏北羌塘盆地望湖岭组流纹岩地球化学特征及其地质意义

摘要

It is extensively known that the Guoganjianianshan late Triassic Wanghuling Formation is the most important sedimentologic evidence of the closure of the Lungmu Co-Shuanghu-Lancang River plate suture zone;nevertheless, so far the petrogenesis of the rhyolite remains unclear. The bulk rock geochemical analysis indicates the characteristics of high silica, rich alkali and Al, suggesting calc-alkali peraluminous rhyolite. The rhyolite samples are rich in LREE, the fractionation between LREE and HREE is strong with LREE/HREE being 6.39-7.72, and there exists obvious Eu negative anomaly. In addition, the rhyolite samples are enriched in Rb, Th, U, K, depleted in Ba, Ta, Nb, Sr, and prominently depleted in P and Ti, demonstrating the characteristics of post-collision volcanic rocks. An integrated analysis of geological survey information reveals that the Lungmu Co-Shuanghu-Lancang River plate suture zone was closed in late Triassic. The authors preliminarily hold that the rhyolite resulted probably from magmatic activity in the process of decompression after the lithosphere collision, and was formed in the stress relaxation phase of the post-collision environment.%羌塘中部果干加年山上三叠统望湖岭组作为龙木错-双湖-澜沧江板块缝合带闭合最重要的沉积学证据已被广泛认知，但是望湖岭组中流纹岩的岩石成因至今尚未得到很好的解释。全岩地球化学分析表明，该流纹岩高硅、富碱、富铝，属钙碱性过铝质流纹岩；富集轻稀土元素，轻重稀土元素分馏较强，LREE/HREE=6.39~7.72，具明显的Eu负异常，富集Rb、Th、U和K，亏损Ba、Ta、Nb、Sr，强烈亏损P和Ti，具有后碰撞火山岩的特点。综合区域地质资料，龙木错-双湖-澜沧江板块缝合带于晚三叠世碰撞拼合，初步认为该流纹岩可能是岩石圈碰撞加厚之后减压过程中岩浆活动的产物，形成于后碰撞环境的应力松弛阶段。