Spherical quartz sandstone of Mesoarchean Yuanjiang Group in central Yunnan and its petrogenesis
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摘要:
元江群是中国首次确定的中太古代浅变质地层，其记录了地球发展演化早期的一系列重大地质事件。在元江群下部的曼林组、岔河组中发育一类与火山岩、火山碎屑沉积岩、化学沉积的块状石英岩共生的特殊砂岩——球粒状石英砂岩。宏观上，球粒状石英砂岩既可与凝灰质泥质板岩、泥质板岩组成递变粒序层理，也可单独呈中-厚层状与凝灰岩、火山岩、石英岩相伴产出，表明这类球粒状石英砂岩形成于活动性较强的大地构造背景。多数石英颗粒在镜下呈近等轴的球状，部分颗粒在成岩过程中显示明显的塑性变形；球粒状石英颗粒在阴极发光照片中不发光或发微弱的蓝光。对其中的热液包裹体进行测试，结果表明，这些石英颗粒形成于温度为60~100℃的热水沉积环境，是在弱水动力条件下，SiO2含量过饱和的水体中沉淀形成的，属盆地内的自生矿物。对石英颗粒表面进行扫描，发现石英颗粒表面光滑，无擦痕，表明这些石英颗粒并未经历过长距离搬运，应属盆地内的原地-半原地沉积物。元江群下部曼林组中含有大量的凝灰质碎屑，与曼林组同期异相的迤纳厂组中普遍可见磁铁矿板岩、凝灰岩、块状含铜白云岩夹层等，指示了相似的沉积环境，表明当时火山活动较频繁，盆地水体温度总体较高，且变化较快。这类水体环境为最早的微生物提供了生存所需的能量，为中太古代末期蓝绿藻的大爆发奠定了基础。
Abstract:The Yuanjiang Group is a new lithostratigraphic unit in the central Yunnan region. It is the first time to identify the Mesoarchean shallow metamorphic strata in China, and it records a series of important geological events in the early stage of the earth's development and evolution. It was found that a typical quartz sandstone - spherical quartz sandstone was deposited in the Manlin Formation and Chahe Formation of the Lower Yuanjiang Group, associated with massive quartzite of volcanic, volcano-clastic and chemical sedimentary origin. On the macro level, the spherical quartz sandstone could be formed by graded bedding with tuff argillaceous slate and argillaceous slate, or it might be developed separately in medium-thick bedding with tuff, volcanic rock and quartzite, indicating that it was formed in an intensely active tectonic setting. The spherical quartz grains exhibit equal crystals, likely spherical crystal, showing no or slightly blue glass in the CL images, and could be formed by plastic growth. The results of hydrothermal inclusions test show that the quartz particles were formed from the SiO2-saturated water in the hydrothermal sedimentary environment of 60~100 ℃ under weak hydrodynamic conditions. They belong to authigenic minerals in the basin. Scanning the surface of quartz particles, it is found that the surface of quartz particles is smooth and has no scratches, indicating that these quartz particles were not transported over a long distance, and should belong to the autochthonous to semi-autochthonous sediments in the basin. The Manlin Formation in the Lower Yuanjiang Group contains a large number of tuffaceous detritus, and its contemporaneous different facies Yinachang Formation commonly contains magnetite slate, tuff, and massive copper-bearing dolomite intercalation, which indicates a similar sedimentary environment. It shows that the volcanic activity was more frequent, and the water temperature in the basin was generally higher and changed rapidly. Such a water environment provided the first microorganisms with the energy they needed to survive, seting the basis for a blue-green algae bloom at the end of the Middle Archean.
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致谢: 本文是《云南省区域地质志》(第二版，修编)、云南1:5万二街、易门县等4幅区调子项目的中间性成果，工作中得到全国地质志项目办公室、中国地质科学院李廷栋院士、丁孝忠研究员、肖庆辉研究员，中国地质大学(北京)邓晋福教授的大力支持和鼓励，并提出了很好的修改意见，在此一并致谢。
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图 1 剖面岩性柱状图、采样位置、野外岩石特征及剖面描述
Figure 1. Lithology histogram of the section, sampling location, field rock characteristics and section description
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图版Ⅰ
a~i.球粒状石英砂岩中矿物显微镜下特征：a.正交偏光，石英粒度大小悬殊，主要分2个粒级(中粗砂-微细砂)，其中大部分粒度较细的颗粒(微细砂)呈不规则状；少量粒度较粗颗粒(中粗砂)磨圆度较好；b、c.单偏光，中粗粒石英砂屑磨圆度较好，呈浑圆状；d.单偏光，粒序层理构造；e.正交偏光，中粗粒石英砂屑磨圆度较好，呈浑圆状；f.单偏光，粒序层理构造；g.正交偏光，不等粒状具溶蚀边；h.单偏光，因粒度大小(中粗粒-中细粒)差异显示粒序递变层理；i.正交偏光，浑圆状石英砂屑具石英次生加大边。Qtz—石英；Kfs—钾长石；Pl—斜长石
图版Ⅰ.
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图版Ⅱ
a~h.球粒状石英砂岩中石英颗粒阴极发光特征；a、b.浑圆状石英颗粒单偏光、阴极发光，大部分石英颗粒不发光，个别石英颗粒发紫光；c、d.浑圆状石英颗粒单偏光、阴极发光，石英颗粒不发光，长石碎屑发蓝光；e、f.浑圆状石英颗粒单偏光、阴极发光，大部分石英颗粒不发光，个别石英颗粒发蓝光；g、h.浑圆状石英颗粒单偏光、阴极发光，石英颗粒不发光，右侧似为一块较大的沉积石英经破碎、劈理化后形成10余粒较小的石英颗粒，大致还可拼合
图版Ⅱ.
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图版Ⅲ
a~f.球粒状石英砂岩中石英颗粒扫描电镜特征；a、b.自生石英发育有平整的自形晶棱和晶面；c.浑圆球粒状球粒石英颗粒及锯齿状边缘沉淀表面；d.浑圆球粒状球粒石英颗粒；e.气液包裹体；f.颗粒表面常见各种溶蚀坑及细微的气热包裹体
图版Ⅲ.
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图 2 球粒状石英砂岩石英颗粒中的包裹体特征
a、b—序号2、5，单偏光，沿石英颗粒微裂隙成带分布，呈无色-灰色的富液体包裹体；c—序号8，单偏光，沿石英颗粒微裂隙面成带分布，呈无色-灰色的富液体包裹体与呈深灰色的气体包裹体；d—序号10，单偏光，沿石英颗粒微裂隙成带分布，呈无色-灰色的富液体包裹体
Figure 2. Inclusions in the quartz grains of spherical quartzite
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图 3 球粒状石英砂岩中气液包裹体显微激光拉曼图谱及测点位置
Figure 3. Microlaser Raman atlas and test plots of hydrothermal inclusion fromspherical quartzite
表 1 石英阴极发光特征分类[24]
Table 1 Classification of cathodeluminescence(CL)for quartz grains
发光颜色 温度条件 产状 以紫色为主，变化于蓝-紫、红-紫之间 ＞573℃，冷却快 火山岩 深成岩 接触变质岩 棕色 ＞373℃，冷却慢 深变质岩 变质的火成岩、变质的沉积岩 300~573℃ 浅变质岩 接触变质岩、区域变质岩、回火的沉积物 不发光 ＜300℃ 沉积物中的自生石英 
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表 2 曼林组球粒状石英砂岩中包裹体特征
Table 2 Data of temperature and salinity of spherical quartz grains from the Manlin Formation
序号 赋存矿物产状 包裹体分布形态 测温包裹体类型 包裹体形状 大小/μm 气液比/% 均一相态 T/℃ 盐度/NaCl % 1 石英颗粒微裂隙 成带分布 富液体包裹体 规则 15×9 10% 液相 146 23.18 2 成带分布 富液体包裹体 规则 20×6 10% 液相 144 23.18 3 成带分布 富液体包裹体 规则 5×5 10% 液相 138 23.18 4 成带分布 富液体包裹体 规则 11×7 5% 液相 125 23.18 5 成带分布 富液体包裹体 规则 5×3 5% 液相 120 23.18 6 成带分布 富液体包裹体 规则 5×2 10% 液相 134 23.18 7 石英颗粒微裂隙面 成带分布 富液体包裹体 规则 5×10 5% 液相 71 20.97 8 成带分布 富液体包裹体 规则 4×10 5% 液相 64 20.97 9 成带分布 富液体包裹体 规则 6×2 5% 液相 80 21.54 10 成带分布 富液体包裹体 规则 2×2 5% 液相 97 21.47 11 成带分布 富液体包裹体 规则 56×13 5% 液相 98 23.18 12 成带分布 富液体包裹体 规则 26×13 5% 液相 79 23.18 13 成带分布 富液体包裹体 规则 26×14 10% 液相 100 23.18 14 石英颗粒中 成群分布 富液体包裹体 规则 7×4 5% 液相 94 21.47 15 成群分布 富液体包裹体 规则 4×4 5% 液相 109 21.54 16 成群分布 富液体包裹体 规则 10×7 5% 液相 85 23.18 17 成群分布 富液体包裹体 规则 8×2 5% 液相 83 23.18
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