Preliminary Research On Carbonate Exhalative Rocks In Lucaogou Formation In Permian, North Of Bogda Mountain, Xinjiang

Carbonate explosive exhalites (CEE) in Permian Lucaogou Formation in north Bogurda mountain is a type of mantle-originated hydroclastites and hydrothermal exhalites, which is a special laminated sedimentary rocks formed by the mixed explosive carbonate volcanic, exhal ative hydrothermal and normal lacustrine sediments at the floor of lake. Base on different mai n mineral, CEE is classified into calcite-explosive exhalites and dolomite-explosive exhalites.The characteristics and forming mechanism of CEE are discussed by using mineralogy, petrology, geochemistry and fluid inclusions etc. Conclusions are summarized as follow.CEE occur as laminae to thin bed (individual layer between 0.1 cm to 2 cm), and have " porph-yritic structure". Because of the different of size and amount, the phenocrysts (between 0.1 mm to 1 mm in size) show as a sharp of layered distribution. Some layers show normal graded and directional distribution, which indicate lateral fluid motion; Some layers show scattered distribution of particles, which display a characteristic of suspended sedimentary process.They are a type of special sedimentary rocks which lack of terrigenous clastic sediments.Calcite-explosive exhalites and dolomite-explosive exhalites are all have "porphyrit ic structure". "Porphyritic" clastic particles show some highlighted morphology characteristic s as angular, subangular, irregular and seriate, or crushed single mineral grains or aggregation of it. Diff-erent from the normal sedimentary characteristics of terrigenous clastic deposits, it shows that they haven’t been transported for a long distance. Furthermore, we can find some calcite particles have the regular band with different temperatures, some microcracks can be seen on the particles, all above indicate a special formation process.The geochemical characteristics of CEE show that lithophile element like Rb、Ba、Th U and light rare earth element are enrich, high field-strength element like Nb、Ta、Zr、 Hf and heavy rare earth element are loss; the ratio of Ba/La, Rb/Sr, Th/U and Sm/Nd are similar. Those features are similar with regional volcanic rocks (Permian basalt), which show some relationship with each other. The non-uniformity of fluid inclusion homogenization temperature shows that the fluid flow are alternated by multi-period of high temperature fluid. When the ultrabasic magma in magmatic chamber channel rising near to the earth’s surface, changes of the temperature and pressure cause crystallization differentiation and become the magmatic-hydrothermal material flow coexisting three phases as crystal gruel gas state, liquid state and solid state. The flow fall-out along the fracture due to the the effect of the stretching tension. In this process, the sudden changing of temperature and pressure released great energy, which cause a strong fragmentation. The mixture of fine magmatic particles and the lake water forms carbonate explosive exhalites.