北京房山岩体形成过程中的岩浆混合作用证据  

Mixing and mingling in petrogenesis of the Fangshan intrusion,Beijing

作　　者：覃锋[1] 徐晓霞[1] 罗照华[1] 

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

出　　处：《岩石学报》2006年第12期2957-2970,共14页Acta Petrologica Sinica

基　　金：国家自然科学基金重点项目(40434010);科技部基础性工作项目"国家岩矿化石标本描述标准和规范的研究制定"(2003DEA3N025);中国地质大学"创新杯"项目

摘　　要：北京房山侵入岩体主要由石英二长闪长岩单元和三种不同结构的花岗闪长岩单元组成,岩体中广泛分布的镁铁质微粒包体、富集暗色矿物的线状条带以及各岩相单元的非渐变接触关系等宏观特征,指示岩体的形成与岩浆混合作用有密切联系。122个斜长石成分剖面电子探针分析表明,它们众数成分为 An=29的更长石。但是,其成分变化范围极宽(An=14～56),晶体内不同部位的成分有很大差别,出现正常环带的 An 值较低(14～42),内环带的 An 值较高(24～56)的特殊情况,指示斜长石结晶环境的改变,暗示具有富钙基性岩浆的加入。从岩体边部到中心,斜长石 An 牌号逐渐降低,但相邻岩相单元之间有较大的重叠,表明不同岩石单元中的斜长石成分主要受控于岩浆成分的变化,与分离结晶作用无关。斜长石的韵律性环带、特别是内环带的发育则是岩浆混合作用机制的有力证据。基性岩浆的反复注入不仅改变寄主岩浆的成分,而且也控制了它的温度、定位机制和岩石的结构特征。因此,房山岩体的形成过程还包含了镁铁质岩浆多次注入的历史,并且从外到里基性岩浆的作用逐渐减弱。The Fangshan intrusion is mainly composed of a quartz monzodiorite unit and three granodiorite units with different structures. Macroscopic characteristics such as ubiquitous mafic microgranular enclaves, linear bands rich in mafic minerals, and the contact relationships between adjacent petrographic units indicate that the origin of the pluton is closely related to magma mixing. The electron microprobe profiles consisting of 122 plagioclase compositional points illustrate the range of An with a population mean of An29 （oligeclase）. However, the plagioclases show an extremely wide range of compositional variation （An = 14 -56）, and different parts of a plagieclase crystal exhibit rather different composition. The normal zone of the plagioclase has a lower An content （ 14 -42） , while the An content of the spike is much higher （24 - 56）, indicating the participation of Ca-rich basic magma. The An amounts of the plagieclase crystals decrease gradually from the early to the late stages. However, the compositional ranges of plagioclases in the adjacent overlap one on another, implying the compositional control of plagioclases in different units by magma compositions instead of fractional crystallization. Plagioclaseg oscillatory zone, especially the growth of spike, is powerful evidence of the magma mixing. The repeating injections of mafic magma not only modified the chemistry of host magma, but controlled its temperature, emplacement and structure futures of the rocks. Therefore, instead of fractional crystallization, the processes to form Fangshan intrusion include a history of multiple magma injections, and the effect of basic magma on the units granularly decreased from the outer to the inner.

关 键 词：房山岩体 花岗岩 内环带 斜长石 镁铁质微粒包体 电子探针分析 

分 类 号：P588.121[天文地球—岩石学]