躲南中生代以来硅质沉积物的微组构特征及其地质意义

1、躲南中生代以来硅质沉积物的微组构特征及其地质意义        【中文摘要】硅质岩形成于特定的地质地球化学环境,包含着十分丰富的古地理、古天气和古环境信息,具有十分重要的研究价值。本研究运用微区、微束的方法,分析躲南地区中新生代硅质岩和现代硅华的微组构特征,并试图揭示它们所包含的地质信息。研究中重点选择四个典型剖面:蛇绿岩套硅质岩建造(日喀则夏鲁村剖面、日喀则彭错林剖面、泽当罗布莎剖面)、非蛇绿岩套硅质岩建造(江孜宗卓组剖面)和塔格架现代硅华。主要结论和熟悉如下:1.X射线衍射结果指示,各个样品的谱图峰位相近,显示

2、为规则的石英特征谱峰,此外还含有长石和赤铁矿矿物的峰位。计算晶胞参数a、c值发现,尽大多数样品的石英晶体与标准石英相比有所增大,表现最为明显的是宗卓组硅质岩,主要由于其中有杂质混进,即Al、Fe等杂质元素与Si4+发生了类质同象置换致使其晶胞出现缺陷,相对于标准石英晶胞有增大的现象;五指峰方法计算各个样品的结晶度指数CI值,其结晶度由大到小的顺序为:罗布莎硅质岩彭错林硅质岩夏鲁硅质岩宗卓组硅质岩,依此可判定罗布莎石英的结晶程度比其他石英要好。2.四个剖面样品的石英晶体在粒度、形态和结构上存在着较大的差异。显微镜下观察显示,罗布莎硅质岩表现为变质重结晶结构;彭错林硅质岩中有一些特殊的结构,如环带

3、孔洞结构、等粒状镶嵌结构(细晶)、网脉状结构,表现了其热水成因作用。夏鲁以微晶石英居多,包括不等粒状镶嵌结构、等粒状镶嵌结构(微晶)、生物骨架结构和角砾胶结结构;宗卓组样品的主要为泥质矿物和石英颗粒交杂结构为主。扫描电镜观察显示,罗布莎样品表现为溶蚀后的不完整的晶形,有的一层层呈定向的门路状,边界以港湾状接触为特征,局部晶面会产生溶孔和溶缝,含有较多的圆形小孔洞;彭错林硅质岩结晶较好,大多为等粒状镶嵌结构(微晶质),晶体颗粒之间排列紧密,但在局部的晶体集合体之间会出现堆积不均匀而产生的空隙;夏鲁硅质岩含有的粘土矿物,泥质成分比较多,样品内保存有大量的生物化石遗迹,石英结晶程度好,呈自形微晶,含

4、蒙脱石(或高岭土)等云雾状粘土矿物;宗卓组红色硅质岩中存在着少量的玻璃质方英石,方英石发育在生物残骸体内的空腔中,石英呈不等粒镶嵌结构,含黄铁矿等杂质矿物;硅华与硅质岩样品具有截然不同的形貌特征,它由非晶质蛋白石构成,呈圆球状、椭球状聚合体,单个球体直径大小不一,可从几个微米到数百微米,有的则形成串珠状并一层一层紧密的定向排列在一起。3.红外光谱分析指示,四个剖面样品均呈现标准的石英特征峰,其中比较特殊的是,宗卓组样品在3627cm-1处出现了高频尖峰,它的基团性质属于一种“结构水”(Si-OH)而非“分子水”,指示了宗卓组硅质岩中有方英石或蛋白石等高含水矿物的存在。硅华的谱带与硅质岩样品相比

5、,要简单的多,除在3440 cm-1处出现比石英矿物更宽深的吸收带外,其他均为Si-O基团的峰位;拉曼光谱分析发现,躲南彭错林硅质岩的纤维状石英中存在方英石矿物;夏鲁硅质岩三种不同颜色的粉末样品(玄色、白色和红色),拉曼谱图特征区别明显,同时最强石英峰位有偏移;宗卓组硅质岩内一放射虫体的拉曼结果显示,其矿物成分为蛋白石。同时基质石英的结晶程度比虫体内部的石英要好。4.微组构信息对硅质岩的成因具有重要的指示意义。热水沉积硅质岩微组构特征,以彭错林硅质岩为例,表现出典型的热水成因结构,如等粒状镶嵌结构、环带孔洞构造、网状石英脉及石英与杂质矿物形成分散的椭圆形球体等;夏鲁硅质岩微组构特征,具有两种沉

6、积作用的特征,其结构包含等粒状镶嵌和不等粒状镶嵌两种结构特征,发育大量的生物骨架结构和典型的热水沉积结构如角砾胶结结构、微晶石英脉等;正常沉积硅质岩微组构特征,以宗卓组硅质岩为例,表现出硅质岩结晶度明显小于热水成因硅质岩。石英呈不等粒镶嵌结构,具典型的生物沉积特征,同时含有很多陆源杂质;重结晶作用硅质岩微组构特征,以罗布莎硅质岩为例,表现出变质重结晶结构,在粒度较细的晶体中,有明显增大的石英呈斑状拼接在一起。部分晶面会产生溶孔和溶缝,含有较多的圆形小孔洞,表现了热水流体的沸腾作用,其结晶度指数近似于彭错林硅质岩;硅华属于典型的热水沉积产物,由非晶质蛋白石构成,呈圆球状、椭球状聚合体,有的形成串

7、珠状并一层一层紧密的定向排列在一起,它们表面覆盖着后期的热水溶液。');【Abstract】 The chert formations are formed in the specifically geological and geochemical environment. They record the abundant information about the old geography, climate and environment, so they are deserved to study fully. This study use the microcosmic me

8、thods to analyze the micro- fabrics of the Mesozoic and Neozoic chert formations and the Modern sinters in southern Tibet, try to reveal their geological information. It chooses the four representative sections: the ophiolite-chert formation (include the Xialu cherts, Pengcuolin cherts, Luobusha che

9、rts), the Zhongzhuo cherts and the Modern sinters in Tagejia.It is concluded through this study as the following:1. By X ray powder diffraction, the peak shapes of samples are similar and they display the regular quartz peaks. Beside them, it also shows the peaks of feldspar and hematite. It is foun

10、d that the cell parameters of quartz crystal change from different chert samples and most of them are bigger than the standard quartz. The values of a。and c。from the Zhongzhuo cherts are obviously larger than other samples, attributed to isomorphous substitution by Al and Fe to Si. The pattern of th

11、e five finger-shaped peaks between 67°-69°indicates that the average crystallinity of SiO2 minerals is different. Through the calculation of crystal index, the order from high index to low is Luobusha cherts, Pengcuolin cherts, Xialu cherts, Zhongzhuo cherts. It indicates that the crystall

12、inity of Luobusha quartz is higher than the other samples.2. The micro-morphology of quartz in cherts is various. Observed from the microscope, Luobusha cherts show the secondary crystallization texture; Pengcuolin cherts have some special micro-texture, just as the vuggy-zonal texture, granular tex

13、ture, cryptocrystalline-microcrystalline texture, vein-netted texture. These imply their hydrothermal origin; Xialu cherts include the equal granular texture, unequal granular texture, biological skeleton texture and pseudobreccia texture; Zhongzhuo cherts is characteristics of clastic texture and q

14、uartz granular texture. And through the su*ce scanning microprobe analysis, the crystal su*ces of Luobusha cherts are destroyed also have some holes and gaps. Pengcuolin cherts show the equal granular dense packing micro-texture, but some gaps is found within the crystal aggregations. Xialu cherts c

15、ontain some clay minerals and lots of biologic fossils, some of them display the Euhedral crystals. Zhongzhuo red cherts have some glassy cristobalite, which developed in the cave of biology body and some pyrite. The quartz shows the unequal granular texture. The micro-morphology of sinter is differ

16、ent from the cherts. It is composed of opal, which behave the granular and ellipsoid aggregations whose single diameter change from several microns to hundreds of micron.3. Infrared absorption spectra of chert reflect that they have the standard quartz peaks. Besides them, a particular peak occurs i

17、n 3627cm-1 in the Zhongzhuo chert samples, which is attributed to OH-group, and indicates the existence of hydrous mineral just as the cristobalite or opal. The pattern of spectra from sinter is *r than the cherts. Except a wider band in 3440cm-1, all of other peaks belong to the Si-O group. Moreove

18、r, the micro-Roman analysis reveal that the cristobalite is also exist in the Pengcuolin cherts. The shapes of spectra of three different color samples (black, white and red) from Xialu cherts are different and the strongest peak shift. The Roman result of a radiolarian body got from Zhongzhuo revea

19、ls it is the opal, and its crystallinity is lower than the based quartz.4. The micro- fabrics of the cherts indicates their petrogenesis and mineralization. The hydrothermal original cherts, as the Pengcuolin cherts show the equal granular texture, vuggy-zonal texture, vein-netted texture which are

20、the typical hydrothermal texture. Xialu cherts contain two sedimentary characteristic. They have the equal granular texture and unequal granular texture. And they possess biological skeleton textures and typical hydrothermal texture, just like the pseudobreccia texture and tiny quartz veins. Zhongzhuo cherts are the normal terrigeneous origin cherts, whose crystallinity is lower than the hydrothermal original cherts. They behave the unequal