Study On The Relationship Between Calcite Vein Microscopic Characteristics And Fracture Activities Of Permian And Triassic Limestones In Northern Chaohu Area

Based on the results of previous studies,taking petrology and geochemistry as instruction and making full use of thin section observation,cathodoluminescence(CL)technique,fluid inclusions and Carbon and oxygen stable isotopes informations,this paper carried out the study on calcite veins in Permian and Triassic limestones of Northern Chaohu area.Combining the tectonic evolution background,coupling relationship between fracture activity and vein formation is analysed from the macroscopic and microcosmic angle.The study area has experienced a long geological history with multiple tectonic movements.There are three types of veins in Permian and Triassic limestones: extensional veins,?X? type shear veins and en echelon veins.On the basis of macroscopic description and the combination of flakes and cathodoluminescence microscopy techniques,microstructure inside the veins are analysed.The Calcite crystal morphologies are mainly blocky,elongate blocky,fibrous texture.According to the growth direction and arrangement pattern,veins can be divided into three classes: antitaxial,syntaxial and stretching vein.Different growth patterns have corresponding crystal morphology.The formation of veins includes twe aspects which are the transportation and precipitation of vein forming material.Vein forming material spread in fracture space in forms of diffusional flow and fluid flow.Most of veins form while the fractures are cracking,however this is not required,such as pressure fringe which can form fibrous calcite veins without fractures.By analysing the results ofcarbon and oxygen isotopes and fluid inclusion tests,Qixia and Nanlinghu group have no significant difference in the origin and the characteristics of the fluid,which are mainly from wall rock with some exchanges of meteoric fresh water.Homogeneous temperatures of calcite veins in Nanlinghu group are within range of 56.8 ~ 97.1?,and can be separated into two periods of fluids in the range of 65~80? and 80~100?;Homogeneous temperatures of calcite veins in Qixia group are within range of 52.3~117.3?,can be separated into three periods of fluids.The two main periods are located in 60 ~ 80? and 90~100? which agree with Nanlinghu group and reflect shallow burial environment;The third period is located in 100~120? and may related to the local volcanic activities.The salinities are mainly less than 8% after converting freezing temperature to salinity,and the fluids are characteristic of medium –low salinity.