Research On Genesis Of The Middle Permian Maokou Formation Siliceous Rocks In The Eastern Sichuan Basin

Siliceous rocks are widely developed in the Permian carbonate rocks in the eastern Sichuan Basin,especially the Maokou Formation.According to the field occurrence characteristics,the siliceous rocks of Maokou Formation can be divided into chert nodules,irregular chert lump and bedded siliceous rocks.In the wild,they have various shapes,heterogeneous distribution,and certain differences in mineral assemblages.Taking the siliceous rocks produced by the middle Permian Maokou Formation in three sections（Erya section,Huilongchang section and Wuyangba section）and drillings in eastern Sichuan as the research object,this thesis analyzes the field occurrence form,spatial distribution characteristics,material composition and geochemical characteristics of siliceous rocks by using the methods of petrology,mineralogy,in situ main and trace element analysis,and discusses the genetic mechanism of different types of siliceous rocks.The establishment of the genetic model of siliceous rocks in the study area is mainly based on the formation method and siliceous source.The bedded siliceous rock is uniform in thickness,with a single layer thickness of 3-14 cm.It is mostly interbedded with calcareous mudstone and micrite limestone at the bottom of the Member 1 and Member 3 of Maokou Formation,forming3-8 rhythmic layers.Microscopically,bedded siliceous rocks are rich in radiolarian and sponge spicules fossils,the biological fossils and argillaceous debris are arranged in a directional arrangement.These characteristics indicate that bedded siliceous rocks are formed by primary precipitation.The Al-Fe-Mn discriminant diagram shows that its siliceous source is mainly biological,but there are also a small amount of other silicon sources mixed in.The main driving force for the increase of biological silicon sources is the upwelling activities widely developed in the Permian Yangtze and the surrounding areas.Upwelling can transport the silicon in the bottom seawater to the shallow seawater,and the nutrients migrated together will also promote the prosperity of siliceous organisms in shallow seawater forms an environment conducive to siliceous enrichment,deposition and preservation.Chert nodules and chert lumps are exposed in the whole Maokou Formation,and the characteristics of field production are similar.Chert nodules and chert lumps are macroscopically sandwiched between limestone layers and are in abrupt contact with surrounding rock.Some of them in the shape of water drop or hollow ring,and the interior contains limestone cakes.Microscopically,calcite and quartz coexist at the edge of chert,and the interior of chert is rich in fossils,which is consistent with the species of calcareous fossils in surrounding rock.These characteristics indicate that chert nodules and chert lumps were formed in the eogenetic stage and formed through metasomatism.Chert nodules and lumps are also very similar in the composition of major,trace and rare earth elements.Although Al-Fe-Mn discrimination diagrams are mostly distributed at the Al rich end,there are strong traces of hydrothermal activity in the wild.For example,the interlayer chert nodules often have the characteristics of"infiltration"in the layer,and a large number of irregular chert lumps and carbonate lumps are also found in the member 3 of Maokou Formation,which indicates that chert nodules and lumps are affected by hydrothermal activities in the formation process,but the hydrothermal properties are different from the Fe,Mn-rich ocean floor hydrothermal near the mid ocean ridge,but the intraplate hydrothermal activities related to basement faults.At the end of the Middle Permian,the reactivation of several basement faults in the region provided a favorable channel for the migration of hydrothermal activities.The fluid in the layer infiltrated and heated along the fault,extracted Si O₂ from the underlying formation,and then moved upward to the shallow carbonate sediments for metasomatism,forming chert nodules and chert lumps of different shapes.