北京房山岩体黑云母矿物化学特征及其对岩石成因的指示意义

Mineral Chemistry of Biotite and Its Petrogenesis Implications in Fangshan Granite,Beijing

房山岩体位于华北克拉通燕山构造带内,在平面上为不对称环状分布的复式侵入体.其主要由多期侵入的花岗闪长质岩石组成,被划分为中央相、过渡相和边缘相3个岩相带,岩体内部分布大量的暗色镁铁质微粒包体.黑云母作为中酸性火成岩中重要的镁铁质矿物,其矿物化学特征可以记录岩浆属性以及岩石形成时的物理化学条件.本研究对房山花岗岩体和暗色镁铁质微粒包体中的黑云母进行了系统的岩相学观察,并通过电子探针分析对黑云母的化学组成进行了详细研究,从而探讨房山花岗岩体的岩浆来源以及岩浆混合作用过程.结果表明,花岗岩体和暗色镁铁质微粒包体中的黑云母具有相似的矿物化学成分.花岗岩体中的黑云母富Mg,贫Fe,属于镁质黑云母.从外向内3个相带的花岗岩中黑云母的含铁系数[(Fe^(3+)+Fe^(2+))/(Fe^(3+)+Fe^(2+)+Mg^(2+))]分别为0.42~0.47,0.45~0.47,0.41~0.46.FeO^(T)/MgO均接近0.60.MF值[2×Mg/(Fe^(2+)+Mg+Mn)]分别为1.05~1.21,1.06~1.15,1.12~1.23,指示3个相带的花岗岩的物质来源均发生了壳幔混染.从外向内3个相带中的暗色镁铁质微粒包体中的黑云母富Mg,贫Fe,属于镁质黑云母,含铁系数[(Fe^(3+)+Fe^(2+))/(Fe^(3+)+Fe^(2+)+Mg^(2+))]分别为0.44~0.48,0.45~0.50,0.44~0.52.FeO^(T)/MgO均接近0.60.MF值[2×Mg/(Fe^(2+)+Mg+Mn)]分别为1.00~1.16,1.03~1.15,1.10~1.18,说明包体的岩浆受到了中酸性岩浆的影响,发生了岩浆混合.花岗岩体与暗色镁铁质微粒包体中的黑云母矿物化学特征相似,但不同岩相带之间存在差异,推测该区域发生了一个多阶段的岩浆相互作用过程,可能为幔源基性岩浆注入壳源酸性岩浆,在岩浆房内以不同程度进行了岩浆混合.

The Fangshan granite is located in the Yanshan tectonic belt of the North China craton,and is an asymmetric annular intrusive body.It is mainly composed of multi-stage intrusive granodiorites which are divided into three lithofacies belts including central facies,transitional facies and marginal facies.In addition,there are a lot of mafic microgranular enclaves(MMEs)distributed within the pluton.Biotite is one of the main minerals in intermediate acid igneous rocks,and its mineral geochemical characteristics record the properties of host magma and the physical and chemical conditions of petrogenesis.In this study,the biotite in the granite and MMEs was systematically observed.The chemical composition of biotite was studied in detail by electron probe microanalysis(EPMA).Finally,the magma source and mixing process of the Fangshan granite are discussed.The results show that the biotite in the granite and the MMEs have similar chemical compositions.The biotite in the granite is rich in Mg and poor in Fe,which belongs to magnesian biotite.The iron content coefficients [(Fe^(3+)+Fe^(2+))/(Fe^(3+)+ Fe2 ++Mg^(2+))] of granite in three facies zones from the outside to the inside are 0.42-0.47,0.45-0.47 and 0.41-0.46,respectively.The FeO^(T)/MgO ratio is close to 0.60.The MF values [2×Mg/(Fe^(2+)+Mg+Mn)] are 1.05-1.21,1.06-1.15 and 1.12-1.23,indicating the source material were derived from crust-mantle mixed rocks.The biotite’s MF values of MMEs in three lithofacies belts from the outside to the inside are 1.00-1.16,1.03-1.15 and 1.10-1.18.The biotite is magnesia biotite,suggesting that primitive magma was influenced by intermediate-acid magma.The above results show that biotite from both the granite and MMEs has similar chemical characteristics.However,there are some differences among different lithofacies.Therefore,it speculates that magma mixing has occurred in the Fangshan granite.The magma mixing is a multi-stage interaction process and may be caused by the mantle-derived magma which is injected into the crustal-derived acid magma during multiple stages.