Mirco-fabrics And Their Geological Implications Of Silica Sediments From Southern Tibet (China)

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 methods 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 cherts), 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 found 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 the 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 crystallinity 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 texture, 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 quartz granular texture. And through the surface scanning microprobe analysis, the crystal surfaces 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 contain 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 different 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 in 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 simpler than the cherts. Except a wider band in 3440cm-1, all of other peaks belong to the Si-O group. Moreover, 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 reveals 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 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 granular texture and contain some impurities that come from the earth. Luobusha cherts experience the later crystallization, they display that the larger quartz crystals pack in the smaller crystals, and they have the distinct change. Some crystal surfaces of them have the holes and gaps which indicate the boil of the hot liquid. The crystallinity of Luobusha cherts is similar to Zhongzhuo cherts. The sinters are the typical hydrothermal origin. It is composed of opal, which behave the granular and ellipsoid aggregations whose single diameter change from several microns to hundreds of micron and the surfaces of them are covered with the later hot liquid.