The discovery of orbicular acid hypabyssal rocks in Wulagaisumu area of Inner Mongolia and their petrographic characteristics
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摘要:
在大兴安岭乌拉盖地区发现酸性浅成球状岩石组合。球状岩石由主岩、球状体和球间基质组成。依据球状体球壳及球核的差异，可分为无壳层球粒、单壳层球状体和多壳层球状体3 种类型。无壳层球粒分布于碱性流纹斑岩和微细粒花岗岩中，直径0.5~2mm，无核，由放射状长英质和铁质纤维及针柱状碱性闪石相间分布构成。单壳层球状体分布于碱性流纹斑岩中，直径0.5~2cm，内部具放射状、扇形结构，由多个球粒相互连接构成，外壳较薄，表现为针柱状碱性闪石沿球状体切向方向环绕。多壳层球状体分布于碱性流纹斑岩中，直径2~8cm，球状体由内、中和外部层构成：内部层具圈层结构，由长英质和暗色矿物隐晶集合体及角闪石组成；中部层由大量叠瓦式、雨滴状气孔杏仁体及其间的长英质隐晶集合体组成；外部层为云朵状、棉絮状、放射状长英质隐晶集合体组成。初步认为本套球状岩石为张性环境下岩浆结晶作用形成，晚期气水热液活动强烈，球状体中矿物结晶顺序由内向外。
Abstract:The orbicular acid hypabyssal rocks outcropped in Ulgai area of Da Hinggan Mountains were discovered for the first time. They are composed of host rock (micro fine-grained graphic granite), orbicular rocks and matrix among orbicular rocks (alkaline rhyolite porphyry and micro fine-grained granite). Based on the differences between orbicular rocks'concentric shells and nuclear bulges, the authors divided the orbicular rocks into three types, i.e., spherules with non-shell layer, orbicular rocks with single shell layer, and orbicular rocks with multiple shell layers. Spherules with non-shell layer and non nuclear, whose diameters are between 0.5mm and 2mm, are distributed in alkaline rhyolite porphyry and micro fine-grained granite and are composed of radial felsic fiber, iron fiber and needle columnar alkali amphibole in alternate distribution. Orbicular rocks with single shell layer, whose diameters are between 0.5cm and 2cm, are distributed in alkaline rhyolite porphyry. With thin single shell layer presented in the form of long and needle columnar alkali amphibole around orbicular rocks'tangential direction, the orbicular rocks, composed of multiple spherules, exhibit radial and sector structure. However, orbicular rocks with multiple shell layers, whose diameters are between 2cm and 8cm, are distributed in alkaline rhyolite porphyry and can be divided into inner, middle and outer layer. The inner layer, composed of felsic and dark mineral cryptocrystalline aggregates and amphibole, has obvious layering structure. The middle layer is composed of a large quantity of stomatal amygdales in imbricate and raindrop forms, with felsic cryptocrystalline aggregates between them. The outer layer is composed of felsic cryptocrystalline aggregates in the cloud-like, cotton wool and radial forms. According to these petrographic characteristics, the authors preliminarily hold that the orbicular acid hypabyssal rocks were formed by magmatic crystallization in an extensional environment, the gas-rich hydrothermal fluid activity was strong at the late stage, and the minerals in orbicular rocks might have crystallized from the inner part to the outer part.
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图 图版Ⅰ a.灰紫色单壳层球粒流纹斑岩；b.肉红色单壳层球粒流纹斑岩；c.浅灰绿色单壳层球状流纹斑岩；d.深灰色多壳层球状流纹斑岩；e.灰绿色多壳层球状流纹斑岩；f.多壳层球状流纹斑岩中扁豆状球状体；g.单壳层球状流纹斑岩过渡为球粒流纹斑岩；h.肉红色无明显壳层球粒文像花岗岩；i.灰紫色无明显壳层球粒钠闪石花岗岩；j.灰紫色、灰白色球粒流纹岩(球粒稀疏偏小)
图 图版Ⅱ a.无壳层或隐约壳层球粒及球粒间基质(+)；b.基质中的显微嵌晶结构(+)；c:无壳层球粒聚集特征(+)；d.被微裂隙分割成雁行式暗色的矿物(-)；e.球内多球粒相连特征及隐晶质球核(-)；f.球体的单、薄壳层及球间基质特征(+)
图 图版Ⅲ a.球状体内部层呈明显圈层构造及3 个亚层特征；b.内部层亚1 层气孔中充填的粒状石英及针柱状、放射状角闪石； c.球状体中部层叠瓦式、雨滴状等气孔杏仁体；d.球状体外部层云朵状长英质隐晶集合体上点缀放射状杏仁体；e.球状体外部层长英质成分不均匀分布呈现的圈层；f.球状体外部层中碱性流纹斑岩捕虏体
表 1 部分国内外典型球状岩成因对比
Table 1. Genetic comparison between typical orbicular rocks in China and abroad
岩体 岩性 结构 成因 乌拉盖苏木球状岩 球状流纹斑岩、球状微细粒
（文象）花岗岩单壳层、多壳层、无壳层 岩浆结晶作用，交代作用，原生结晶岩塑性固结，岩
浆运移过程中快速结晶黄陵球状花岗闪长岩[7] 球状花岗闪长岩 单壳层、多壳层、无壳层 岩浆结晶作用，交代作用 石角-璜山复式岩体[3] 球状超镁铁质岩 多壳层 中深地壳环境岩体接触带结晶作用 滦平球状闪长岩[4] 球状闪长岩 单壳层、多壳层 岩浆结晶作用，交代作用，岩浆同化作用+结晶作用 Baskil岛弧岩体[8] 球状辉长岩 多壳层 原生结晶岩塑性固结，岩浆运移过程中快速结晶 Loreto岩体[15] 球状辉长岩 无壳层 绝热过冷的环境中结晶 Huaco花岗岩[9] 球状花岗岩 多壳层 过冷的熔体中异相成核结晶 Ploumanach复式岩体[16] 球状花岗岩 无壳层 绝热空间存在温度接近液相线岩浆 
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