| 文象花岗岩的成分、结构和成因机制 |
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| 引用本文： | 徐海军,张超,武云,陶明.文象花岗岩的成分、结构和成因机制[J].地球科学,2016,41(9):1511-1525. |
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| 作者姓名： | 徐海军 张超 武云 陶明 |
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| 作者单位： | 1.中国地质大学地球科学学院，湖北武汉 430074 |
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| 基金项目： | 国家自然科学基金项目41172070国家自然科学基金项目41272080 |
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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c001]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.
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| 关 键 词： | 文象花岗岩 文象结构 电子背散射衍射 道芬双晶 拓扑结构 晶体光学 矿物学 |
| 收稿时间： | 2015-11-21 |
Compositions,Texture and Formation Mechanism of Graphic Granites |
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| 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 that1123]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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