CARBONIFEROUS-PERMIAN TONSTEINS FORMED BY HYDROLYTIC REFORMATION OF VOLCANIC ASH SEDIMENTS IN NORTHERN CHINA

This paper deals with the material source and process of formation of tonsteins mainly on the basis of distribution, occurrence, texture, structure, and material composition. Tonsteins of volcanic origin arc exceedingly widely distributed in our country. They were found in the Inner Mongolia, Shaanxi, Shanxi, Hcbei, Shandong in Northern China and wide areas of coal fields in Southwest China.Tonsteins are commonly developed in coal measures of Carboniferous-Permian and stable in horizon.They occur as thin beds less than 1m (some only 0.02 ± m.) in thickness but their thickness keep constant in thousands of km², so they may be used as isochronous correlative horizons. The contact between the top and bottom surfaces of tonstein bed and coal is generally distinct and even, and has no indication of current scour.The bedding is not developed, and generally has massive structure.Tonstcins arc usually drak-brown in coal bed, because they contain organic carbon, but arc greyish white in coal-free bed. The mineral compositions are very simple.They are mainly composed of kaolinite and kalolinite-dickitc. Their textures are very complex and diverse, from cryptocrystalline to macrocrystalline vermicular crystals of kaolinite, and some intact pseudocrystals of feldspar are composed of microcrystalline kaolinite. The degree of order of kaolinites differs by X-ray diffraction analysis, and is generally proportional to the size of crystal. The degree of order of macrocrystal of kaolinite is high. The degree of order of cryprocrystalline kaolinite is low. The macrocrystalline kaolinite contains usually minor dickite. There are two cndothcrmic peaks at 590 and 670 on differential thermal curve. After heat treatment(550℃)the samples have still (00l)and (002)pcaks on X-ray diffraction spectrum, so the presence of dickite may be confirmed further. The content of crystal fragment of primary mineral is low, but is the main basis to analysis the source of material. The chemical compositions of tonsteins are quite simple too. They are mostly silica (45% ± ), alumina (35%± ), and water (13% ± ), total is 95% ± . The molecular ratio SiO₂/ Al₂O₃of monomineralic tonstein is close to near 2. The contents of K₂O, Na₂O, CaO and MgO are lower.The trace elements more than 30 were determined by emission spectroscopy using an inductive coupled plasma (ICP). According to this result the contents of iron family elements (Fe, Ti, Mn, V, Cr, Ni, Co)are lower, but those of Th, U, B and Zr are higher. The ratio of Ti to Th is near to acid magmtic rock. Based on colour and textural features three typical tonsteins are common: 1. Light coloured crystallinoclast-bearing macrocrystalline tonstein. 2. Dark-bropwm, macrocrystallin tonstein 3. Brown-grey microcrystalline tonstein. The difference of tonstein from common shale and mudstone lies in that the former is widely distributed, thin, stable in horizon, and not developed in bedding. The main difference is that tonstein is exceedingly simple in material composition, the kaolinite contained absolutely predominates and contains a little dickite, the crystallinoclasts of primary mineral include hypothermal mineral assemblages, the Al₂O₃content is higher, the contents of K₂O, Na₂O, CaO and MgO are lower, the iron family elements contents are lower, and radioactive element content is higher. According to the above mentioned distribution, occurrence, texture, structure, particularly mineral and chemical compositions of tonsteins. the material originated from acid volcanic ashes. The tonsteins have important significance in event stratigraphy and greater economic value. However, at present for the tonstein there are little expoundings in sedimentary petrology, and almost no proper position, these deserving extra attention also.