The Petrogenesis Of Siliceous Rocks In Permian Lucaogou Formation,Santanghu Basin,Xinjiang

The Permian Lucaogou Formation in Santanghu Basin of north Xinjiang province develops an important set of hydrocarbon source rock and reservoir.Lucaogou Formation is long been considered as a normal lacustrine deposits.However,our team found a complicated lithology combination recently: volcanic-hydrothermal exhalative sedimentary rocks which including primary dolostone related to mantle-originated exhalative hydrothermal activities,carbonate exhalative rocks,analcite rock related to exhalative hydrothermal activities,etc.Although lots of work has been done on these rocks,their associated rock,siliceous rock,is still in less understanding.In this paper,petrology and mineralogy study,together with multiple test methods are used to confirm the characteristics of each type of siliceous rock and discuss their source and genesis.The results and understanding are as follows:The Lucaogou Formation overlies the Kalagang Formation and is covered by Tiaohu Formation.Both of Kalagang Formation and Tiaohu Formation contain volcanic and sedimentary deposits.Fine-grained sediments which deposit in deep-semi deep lake developed dominantly in Lucaogou Formation,as well as abundant volcanic-hydrothermal exhalative rocks and sedimentary rocks related to volcanic activities which indicate complicated lithology combination during volcanic interval.Siliceous rocks in Lucaogou Formation dominantly present millimeter-scale lamellar.Siliceous explosive rock,algal laminated siliceous rock and chert are identified based on their formation mechanism.Siliceous explosive rock is formed by laminar which consists of granular quartz with angular body and bright edge.Algal laminated siliceous rock is formed by laminar of short-columnar-shaped and spindle-shaped siliconized algaes,remaining microstructures of algae.Cherts are black,present mammilated structure and distribute discon-tiguously along the layers.According to cathodoluminescence results,quartz phenocrysts of siliceous explosive rocks and the edge of siliconized algaes show bright blue light,but it has no luminescence inside.Meanwhile,the chalcedony in cherts has blue light.This significant phenomenon illustrates that the center of quartz phenocrysts,silica debris and siliconized algaes are formed by hydrothermal quartz.The edge of quartz phenocrysts,silica debris and siliconized algaes and the cherts,the value of Al3+,Sr2+,K+,Na+ are abnormally high,which means there is ion substitution between cation and Si4+.In the diagram which is used to identify hydrothermal water process,all the major elements data dropped in hydrothermal water process region.The values of CaO/(Fe+CaO)are between 0.81-0.97,representing a high salinity condition;K2O/Na2 O is 0.025-1.79,reflec-ting a substantial connection with underwater volcanic activities.The value of Mn O/Ti O2 are 0.083-0.416,showing the continental characteristics.Siliceous rocks are rich in LREE but lack of HREE.?LREE/?HREE is 9.19-10.75.?Eu=0.870 and ?Ce=2.444 representing ?Eu negative anomaly and ?Ce slight positive anomaly.The characteristics of rare earth element are similar to the primary dolostone related to mantle-originated exhalative hydrothermal activities illustrating a reducing environment.In conclusion,siliceous rocks in Lucaogou Formation is a kind of mantle-derived hydroclastites and hydrothermal exhalites.