Compositions, Texture and Formation Mechanism of Graphic Granites
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摘要: 文象花岗岩具有特殊文象结构，研究其三维拓扑结构和形成过程有助于了解花岗质岩石的结晶作用.以北京周口店房山岩体和湖北罗田蕙兰山的文象花岗岩为研究对象，综合利用光学显微镜、扫描电镜、电子探针和电子背散射衍射等技术方法，对岩石矿物组成、结晶学取向和拓扑结构进行了系统研究.结果表明：(1) 文象花岗岩的矿物组成与其形成地质环境有关，石英和长石的含量变化范围很大，其中石英含量通常在20%~45%，但是相同地区同期形成的文象花岗岩具有相对稳定的矿物组成；(2) 长石作为寄主矿物通常呈半自形－自形粗大晶体，可以是碱性长石或斜长石，其端元组分以钾长石和钠长石为主，低温下常分解为条纹长石；(3) 石英在长石寄主矿物中规则穿插生长，在三维空间通常呈近似平行板状、长条状/柱状或非连通枝杈状，并只在特定岩石断面形似象形文字；(4) 正交偏光显微镜下，石英可以具有多种消光位，但是通常在一定范围内同时消光；(5) 石英普遍发育道芬双晶，偶见日本双晶；(6) 条纹长石中钾长石与钠长石对应(100)、(010)、(001) 面和[001]轴近似平行；(7) 多数石英颗粒与寄主长石之间具有密切结晶学取向关系，即石英[1123]轴近似平行长石c[001]轴.该研究证实文象花岗岩是石英和长石同时生长的结果，而长石作为寄主矿物影响并控制着石英的成核与生长方向.Abstract: Graphic granite, found predominantly in granitic pegmatite, is a leucocratic granitic rock consisting of an intimate intergrowth of alkali feldspar and quartz with a distinctive texture as ancient cuneiform writing when viewed in certain cross sections. Deciphering the graphic texture is important for understanding its origin and the crystallization process of granitic rocks. In this paper, we present investigations on petrology, mineral compositions, crystallographic relationship and topotaxy of quartz and alkali feldspar in graphic granites from the Fangshan pluton, North China and Huilanshan, Central China. The results indicate follows (1) The volume content of quartz usually ranges from 20% to 45%, and the composition of feldspar in graphic granites depends greatly on the formation conditions. However, the quartz-feldspar ratio and the composition of feldspar in graphic granites are relatively stable in coeval graphic granites in the same area. (2) The euhedral to subhedral coarse-grained feldspar host in graphic granites can be alkali feldspar or plagioclase. Microscopically, the feldspar host is usually a perthite, which is decomposed into irregular intergrowth of sodic and potassic feldspar. (3) The majority of the quartz grains undergrown with host feldspar are in the form of sub-parallel tabular, long rods and unconnected dendritic crystals, which only show a distinctive graphic texture in certain cross sections. (4) Under cross polarized light microscopy, multiple domains of quartz grains exhibit a nearly simultaneous extinction within a single crystal of feldspar. (5) Dauphiné twin of quartz, occasionally accompanied by Japan twin, is commonly developed in graphic granites. (6) Intergrowth of sodic and potassic feldspar, resulted from sub-solidus exsolution of precursor host feldspar solid solution, has almost the same orientations of (100), (010) and (001) planes and [001] axis. (7) A definite crystallographic orientation relationship between the majority of quartz grains and the feldspar host is identified in that[1123]Quartz//[001]Feldspar. These results suggest that the nucleation and growth of quartz are controlled by the host feldspar, which supports the simultaneous growth model of quartz and feldspar in graphic granites.
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Key words:
- graphic granite /
- graphic texture /
- electron backscatter diffraction /
- Dauphiné twin /
- topotaxy /
- crystallographic optics /
- mineralogy
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图 1 文象花岗岩不同方向切面的石英拓扑结构素描
a~c.切面与钾长石a(100) 面小角度斜交；d, e.切面近似平行钾长石b(010) 面；f.切面近似平行钾长石c(001) 面.样品编号说明：HLS.湖北罗田蕙兰山，FS.北京周口店房山岩体，08和14分别代表采样时间为2008年和2014年，破折号之后的数字代表手标本顺序号
Fig. 1. Sketch showing two dimensional morphologies of quartz in graphic granites
图 2 文象花岗岩在正交偏光显微镜下的图像
Ab.钠长石；Kfs.钾长石；Qz.石英
Fig. 2. Thin section photographs of graphic granites captured under cross-polarized light
图 3 文象花岗岩背散射电子扫描图像(a~e)和取向衬度图像(f)
a, c, d.样品FS08-2，切面近似平行钾长石b(010) 面；b, e.样品FS08-1，切面近似平行钾长石c(001) 面；f.样品FS08-5；图b改自Xu et al.(2015a)；Ab.钠长石；Kfs.钾长石；Ms.白云母；Qz.石英
Fig. 3. Backscattered electron images and orientation contrast image of graphic granites
图 4 文象花岗岩化学组成归一化三角图解
a.石英-斜长石-钾长石三角图解；b.钙长石-钠长石-奥长石三角图解
Fig. 4. Normative triangular diagram of graphic granites
图 5 文象花岗岩中钠长石与钾长石的结晶学取向关系
a.EBSD测量采用人机交互模式，测量点的位置及其编号标记在电子背散射图像上；b.钾长石电子背散射衍射花样及其标定结果；c.钠长石电子背散射衍射花样及其标定结果；d.正长石结晶学取向数据散点图，上半球等角度投影；e.钠长石结晶学取向数据散点图，上半球等角度投影
Fig. 5. Crystallographic relationships between albite and potassium feldspar in graphic granites
图 6 北京周口店房山岩体文象花岗岩中石英与钾长石的结晶学取向关系
为方便对比石英和钾长石的结晶学取向关系，所有文象花岗岩样品中的钾长石均旋转到同一取向，石英颗粒的结晶学取向作协同旋转.其中，长石晶体[001]轴和(100)、(010)、(001) 面极点分别用实心原点标识，并展示在第1张极图中.N.石英颗粒数.上半球等角度投影
Fig. 6. Crystallographic topotactic relationships between quartz and potassium feldspar in graphic granites from the Fangshan pluton
图 7 湖北蕙兰山文象花岗岩中石英与钾长石的结晶学取向关系
Fig. 7. Crystallographic topotactic relationships between quartz and potassium feldspar in graphic granites from Huilanshan
图 8 文象花岗岩中石英相邻点之间方位差角相对频率直方图
Fig. 8. Correlated misorientation angle distribution of quartz in graphic granites
图 9 费氏台测试数据显示文象花岗岩中石英与长石的结晶学取向关系
a.据Fersman(1928)及其总结的前人文献资料，作者提出石英c轴主要分布在与长石c轴成42°15′的小圆环带上，可以细分为A、±B、±C、±D、Rose几种亚型；b.据Drescher-Kaden(1948)，16组测量数据分布范围很大，石英c轴与长石c轴在42°和64°的小圆环带上较为集中.文象石英的结晶学数据均在长石坐标系中展示，其中长石晶体[001]轴和(100)、(010)、(001) 面极点分别用实心原点标识
Fig. 9. Summary diagrams showing the relative orientation of quartz grains referred to feldspar axes measured by U-stage
图 10 EBSD测量数据显示文象花岗岩中石英与长石的结晶学取向关系
a~c.石英结晶学取向散点图，上半球等角度投影；d~f.石英结晶学优选方位极点密度图，上半球等角度投影，半宽5°.本文利用EBSD自动测量了10块文象花岗岩样品，其中北京周口店房山岩体6块，湖北罗田蕙兰山4块，累计获得3 980组石英颗粒数据.绝大部分石英c轴集中分布在与长石c轴成42°的小圆环带上，并且石英 <1123>轴近似平行长石c[001]轴.为获得统计性分析数据，每个文象花岗岩样品的长石均旋转到一致取向，其中长石晶体[001]轴和(100)、(010)、(001) 面极点分别用实心原点标识.石英颗粒的结晶学数据与其寄主长石一起作协同旋转，并集成展示在长石坐标系中
Fig. 10. Diagnostic crystallographic topotactic relationships between quartz and feldspar in graphic granites measured by EBSD
图 11 文象石英与长石在熔体中结晶生长过程示意图
a.长石在熔体中优先成核并快速生长，消耗体积熔体中的铝，并造成硅和水在局部富集；b.长石快速生长过程中形成粗糙颗粒边界，熔体成分出现不平衡，靠近长石颗粒边界处的熔体具有较高的水逸度和二氧化硅活度；c.靠近长石粗糙颗粒边界处，二氧化硅首先达到过饱和，石英快速成核并附生在长石晶体之上，石英平直颗粒边界处熔体二氧化硅活度降低，并开始有利于长石生长；据Lentz and Fowler(1992)
Fig. 11. Possible formation model of graphic quartz and feldspar in felsic melt
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