黑龙江松嫩地块北缘晚三叠世-早侏罗世火山岩的发现及地质意义

摘要

黑龙江省松嫩地块北缘大面积分布的玄武安山岩的时代厘定对讨论区域构造演化具有重要意义。对该区原划为早白垩世板子房组的玄武安山岩进行了LA-ICP-MS 锆石U-Pb年代学、岩石学和地球化学等研究，获得两个样品的锆石U-Pb年龄分别为（200.0±1.4）Ma和（200.0±1.3）Ma，表明其形成时代为晚三叠世—早侏罗世，并非早白垩世。岩石地球化学研究表明，玄武安山岩属准铝质，以高钾钙碱性系列为主，富集Ba、Rb、K、Pb等大离子亲石元素和轻稀土元素，亏损Nb、Ce、Nd、Ti等高场强元素和重稀土元素，为典型的大陆弧火山岩，表明松嫩地块小兴安岭—张广才岭存在一条近南北向展布的晚三叠世—早侏罗世火山岩带，该火山岩带与同时代略晚的花岗岩一起构成近南北向带状展布的中生代岩浆岩带。%Basaltic andesites are widely distributed in the northern Songnen Block,Heilongjiang Province and their ages are critical to understand the regional tectonic evolution in this area.This study presents LA-ICP-MS zircon U-Pb age data and geochemical data for the basaltic andesite from the Banzifang Formation previously considered to be Early Cretaceous.Two samples yield U-Pb ages of (200.0 ±1.4)Ma and (200.0 ±1.3)Ma respectively,demonstrating that they were formed in Late Triassic-Early Jurassic rather than Early Cretaceous. The basaltic andesites are metaluminous,mainly high-K calc-alkaline.They are characterized by enrichment of large ion lithophile elements (LILE)such as Ba,Rb,K,Pb and LREE,depletion of high field strength ele-ments (HFSE)such as Nb,Ce,Nd,Ti and HREE,indicating that the basaltic andesites belong to continental arc volcanic rocks.The results demonstrate that there has been Late Triassic-Early Jurassic volcanic belts in Xiaoxinganling-Zhangguangcailing,these occurrence,together with synchronous granites,comprise a Mesozoic N-S-striking magmatic belt.