大兴安岭北段塔河地区晚寒武世中基性火山岩的发现及其地质意义

新林-喜桂图断裂北东段塔河地区发育一套中基性火山岩,岩石类型为玄武安山岩、玄武粗安岩和粗面玄武岩,前人曾将其划归为中-晚侏罗世塔木兰沟组。本次工作区在塔河北瓦拉干林场一带,采用LA-ICP-MS同位素测定方法,在玄武安山岩和粗面玄武岩中分别取得锆石~(206) Pb/~(238) U年龄平均值为(501.1±1.4)Ma(23个数据点)和(500.50±0.95)Ma(14个数据点);首次确定了大兴安岭北段塔河以北寒武系火山地层的存在。中基性火山岩,具有亚碱性岩系列钾玄岩特征,且无任何变质变形;Eu微弱负异常,(La/Yb)_N=16.63~25.91;富集Ba、Rb、Sr、K,亏损Nb、Ta、Ti。岩浆的形成可能与岛弧背景下的拉伸作用有关,同时指出额尔古纳地块塔河以北地区寒武系的存在并非是局部现象,很可能有一定的分布范围。这一研究成果为重新认识大兴安岭北段局部地区中生代火山岩、探讨新林-喜桂图构造带演化历史及西伯利亚南缘额尔古纳地块大陆地壳演化提供了新的资料。

Discovery of the Late Cambrian Intermediate-Basic Volcanic Rocks in Tahe,Northern Da Hinggan Mountain and Its Geological Significance

There exist a set of intermediate-basic volcanic rocks in Tahe,north east part of Erguna massif,which consist of basaltic-andestite,basaltic-trachyandesite and trachybasalt.This set of volcanic rocks was previously allocated to the Middle-Late Jurassic Tamulangou Formation.However,according to the 206Pb/238U isotope of zircon determination data of (501.1±1.4) Ma (n =23) and (500.50±0.95) Ma(n =14)for basaltic-andestite and trachybasalt,it is the first reported existence of Cambrian volcanic rocks north of Tahe.Intermediate-basic volcanic shows the sub alkalne shoshonite series feature,and without any metamorphose;negative Eu anomalies,(La/Yb) N =16.63-25.91;Rich Ba,Rb,Sr,and K and depletion Nb,Ta,Ti.The magma is likely related to the extension in the island-arc background,and it is considered that there exists the Cambrian in the certain range of this area.The research results provide new materials for re-evaluating the Mesozoic volcanic rocks in local area of northern Da Hing gan,discussing the evolution history of Xinlin-Xiguitu fault,and the continental crust evolution of Erguna massif at the southwest edge of the Siberia plate.