Microscopic Petrogenesis Of Siliceous Rocks In Different Morphology Of Upper Permian Of Shangsi Section In Guangyuan,Sichuan Province

The upper Permian strata contains a large number of siliceous rocks in east Sichuan Province.We chose siliceous rocks from Shangsi section in Guangyuan,Sichuan Province as the research object,focused on its'characteristics of sedimentary petrology,geochemistry and micro-zone in order to figure out it's macroscopic and microscopic morphology,the genesis of silicon and the deformation behavior of main rocks,analyze its'micro-depositional environment by study the different behavior of SiO₂ colloid.According to field investigation,the striped,lump,bedded and nodular siliceous rocks are distributed widely in the Upper Permian Shangsi section.The main rock-making minerals of siliceous rocks are basically consistent in Dalong formation and Wu Jiaping formation,mainly about microcrystalline-cryptocrystalline quartz and a small amount of calcite,metallic minerals and clay.Cherts from Shangsi has hidden-micro crystal structure,the large number of fossils of siliceous rocks whose cavity were filling with quartz and calcite represent close handling can be observed by microscope.Studied and analyzed siliceous'geochemical characteristic and some related illustractions,we found that most of the samples fall in biological and non-hydrothermal deposition zone in Al-Fe-Mn discriminal diagram,the ratio of K₂O/Na₂O and Cr-Zr diagram manifest the close relationships with hydrothermal sedimentation,the?REE is relatively lower,the siliceous rocks in Dalong formation indicate a certain affinity with Emeishan basalt.Comprehensive analysis reveals that the origin of these siliceous rocks are complicated,the siliceous organism is the primary source of silicon,the hydrotherm and basalt leaching is secondary.The formation of chert is mainly based on Biochemical sedimentary and supplemented by hydrothermal sedimentary.The?La/Ce?_N??La/Yb?_N ratio of Wu Jiaping formation are higher than Dalong formation which indicates the transgressions in late Permian caused rise and fall of sea level.The ratio of MnO/TiO₂ and TFe₂O₃>FeO represent that the siliceous rocks'sedimentary area is located in the oxygen-rich environment of the continental and marginal sea.The ratio of Cao/?Fe+CaO?,?MgO/Al₂O₃?×100 indicate the change of salinity during its diagenesis stage.Above all,the formation of siliceous rocks is closely related with biological and volcanical activity,and the sedimentary environment of silicious rocks is oxygen-enriched platform-basin,which is equivalent to continental slope in depth.The microstructure of siliceous rocks contains abundant information of genesis and sedimentary environment.The diagenetic process of siliceous rocks is influenced by complex physicochemical effect.The quartz is six-square crystal form under SEM,the main mineral in siliceous rocks are corroded in varying degrees.The cracks in siliceous rocks are well developed which indicate a certain reservoir potential.The formation of siliceous colloidal minerals requires a large amount of colloidal solution a certain temperature and pressure together with electrolyte.The formation of nodular siliceous rocks requires colloidal material which deposited around the center,this kind of siliceous rocks are mainly“stalacite”under SEM.The bedded and striped cherts are under the control of pressure and temperature which changed repeatedly,besides,the SiO₂ colloidal output in the state of layered or rhythmic which condensation and diffusion intermittently,indicates the salinity of sedimentary environment changed constantly,this kind of siliceous rocks'growth has a directionlity under SEM.The lump chert seems to be ellipsoid or regiment were formed under the control of surface tension of SiO₂ Colloidal solution and deposition mechanism.