一种改进的简单的估算原始岩浆的方法——以峨眉山大火成岩省丽江苦橄岩为例  

An Improved Simple Method for Estimating Primitive Magma——a Case for the Picrites in the Lijiang Area of Emeishan Large Igneous Province

作　　者：李永生[1] 张招崇[1] 聂保锋[2] 

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室,北京100083 [2]首都师范大学资源环境与旅游学院,北京100048

出　　处：《地质论评》2012年第4期653-659,共7页Geological Review

基　　金：国家自然科学基金资助项目(编号40925006);国家973项目(编号2009CB421002);111计划(编号B07011)的成果

摘　　要：利用橄榄石和熔浆的Fe—Mg分配系数,制作了MgO—FeO演化图解。根据MgO—FeO体系中Fo橄榄石的Fo值:Fo=n(Mg)÷n(Mg)+n(Fen(Mg)/n(Mg)+n(Fe))等值线与Fe—Mg演化曲线交点确定原始岩浆成分。该图解优点在于可以更简单、直观地判别原始岩浆成分,适用于橄榄石分离结晶体系。同时介绍了该图解的使用方法,并以峨眉山大火成岩省丽江地区的大具与仕满两个剖面的苦橄岩为例,说明如何应用该图解来恢复原始岩浆以及解释岩石的成因。研究结果表明,仕满地区原始岩浆MgO、FeO含量分别为23.5%和12.8%,部分熔融程度较高,SM-14和SM-15是由仕满原始岩浆经过轻度橄榄石分离结晶形成的,基本可以代表原始岩浆成分。大具地区原始岩浆可分为两类,一类部分熔融程度较低,MgO、FeO含量分别为19.8%和11.3%;另一类部分熔融程度较高,MgO、FeO含量分别为23%和13.3%,与仕满原始岩浆成分类似。大具地区的岩石样品成分均不能代表该地区的原始岩浆成分,而是经历了明显的橄榄石分离结晶作用。另外,峨眉山大火成岩省中绝大多数的玄武岩中CaO含量不符合橄榄石分离结晶关系,并且MgO含量一般都低于8%,以及岩石中普遍出现单斜辉石和斜长石矿物等都表明这些玄武岩不可能由地幔部分熔融形成的原始岩浆直接通过橄榄石分离结晶作用形成。Based on Fe-Mg distribution coefficient between olivine and melt, the MgO-FeO evolution diagram is built. The compositions of primary magmas can be estimated by the intersection point of straight line and curve which illustrates liquids in equilibrium with olivine. It is suitable for the system of olivine fractional crystallization, and its advantage is that it can be easily and visually used. It is also pointed out how the diagram is applied. To show how to recover the composition of primitive magma and to explain the petrogenesis, the samples from the Lijiang area of Emeishan large igneous province are taken for examples. Results show that composition for the MgO and FeO contents of Shiman primitive magma are 23.5% and 12.8% respectively, with larger melting fraction. Samples SM-14 and SM-15 are resulted from low olivine fractional crystallization, so their contents could approximately represent primitive magma of Shiman area. Daju primitive magma may be divided two types. One has 19.8% MgO and 11.3% FeO contents, with lower melting fraction. The other one has 23% MgO and 13.3% FeO contents similar to the Shiman, with larger melting fraction. Samples from the Daju all experienced large olivine fractional crystallization. Most basalts from the Emeishan large igneous province have MgO contents less than 8%, and include many clinopyroxene and plagioclase phenocrysts,, suggesting that these basalts can not be directly resulted from olivine fractional crystallization of Emeishan primitive magma.

关 键 词：原始岩浆 分配系数 质量平衡 峨眉山大火成岩省 

分 类 号：P588.11[天文地球—岩石学]