北祁连大岔-大坂一带海相火山岩化学成分图解及地质意义

The Geological Significance for Geochemical Composition Analysis of Marine Volcanic Rock from Dacha-Daban Areain North Qilian Mountains

北祁连地区早古生代海相火山岩十分发育。笔者对大岔—大坂一带奥陶纪海相火山岩中细碧角斑岩化过程Na元素的来源与地质意义进行了研究。认为细碧-角斑岩比正常火山岩富Na_2O,是海水参与的结果;判别角斑质岩石的原始岩浆系列及构造环境类型时不宜采用涉及Na的图解作为判别依据,应选取惰性元素作为判别依据;同时提出采用Na_2O-SiO_2图解展示岩石细碧角斑岩化。细碧-角斑岩类与正常海相火山岩界线为一条微上凸的曲线,该界线Na_2O含量为3.5%-4%;大岔—大坂一带海相火山岩的成分点多落在该界线之上的细碧-角斑岩类分布区,可反映该区奥陶纪弧后和岛弧扩张脊火山喷发间歇期,海底热卤水对流循环活动强烈,成矿条件非常有利。

Early Paleozoic marine volcanic rocks were well developed in the North Qilian Mountains.The paper focuses on discussing the formation process and origin of Na element for Ordovician marine volcanic rocks during the process of spilite-keratophyre alteration.There is more Na_2O in spilite-keratophyre than normal volcanic rock,because of the basaltic magma react with seawater.Therefore,the discrimination diagram involved with Na element cannot be used as the discrimination criteria for distinguishing the original magma series of keratophyre and related geotectonic setting,but the immobile elements can be used as distinguishing basis.Meanwhile,the Na_2O-SiO_2 diagram is proposed tor reflecting the degree of spilite-keratophyre alteration.In this graphic,the boundary line for the percentage of Na element between spilite-keratophyre and normal volcanic rock is a slightly up-convex,which is stand for the content of Na element ranging from 3.5%to 4%.For the marine volcanic rocks in Dacha-Daban area,most of them fall into the distribution region of spilite-keratophyre,suggesting that the convection of seafloor hot brine was very strong at Ordovician back-arc and island-arc spreading ridge during intervolcanic eruption stage,and this area provides very favorableconditions for mineralization.