Comparison On Cherts From The Eastern Part Of The Paleo-Asia Ocean And The Mongolia-Okhotsk Ocean

Cherts are fine-grained sedimentary rocks mainly composed of silica particles,which record the original geochemical characteristics at the time of formation,and have important indicative significance for the restoration of ancient sedimentary environment.However,there are relatively few specialized studies on them.This paper selects cherts from Wudaoshimen Reservoir in Kashkten Banner of Inner Mongolia,Hegen Mountain and Chaogen Mountain in East Ujimqin Banner and Ulaanbaatar area of Mongolia to carry out profile measurement,major,trace and rare earth elements test and analysis,aiming to find out their petrography,geochemistry and sedimentary environment characteristics,and explore their tectonic background,so it has important scientific significance.The cherts in Hegenshan area are exposed in ophiolite with SiO₂ content of about95 %,Fe₂O₃ content of about 1 %,and Mn O content of about 0.1 %.The sedimentary water is alkaline,low salinity,and oxidation environment.The ? REE is small,and the heavy rare earth elements are enriched.It has the characteristics of negative Ce anomaly,positive Eu anomaly,and left-leaning distribution curve.Geochemical evidence shows that these cherts are formed by hot water,and the sedimentary environment is a distant ocean basin with deep source fluid / hot water injection not far from the mid-ocean ridge.Combined with previous studies,this paper argues that in the early Early Carboniferous Hegenshan area occurred tension,producing ophiolite and cherts,forming a small ocean basin.The cherts in the Chaogen Mountain area are produced in the Tarbagote Formation,which are divided into red and green,and they have obviously different geochemical characteristics.The red cherts are produced in mudstone or basalt in the form of lens,with an average SiO₂ of 88.79 %.The sedimentary water is alkaline,low salinity,and oxidation environment.The ? REE is small,and the heavy rare earth elements are enriched.The negative Ce anomaly and positive Eu anomaly,and the distribution curve is left-leaning.These indicators and geochemical diagrams show that they are formed by hot water,and the sedimentary environment is close to the mid-ocean ridge.The green chert occurs between siltstone and tuffaceous slate in layered form(the formation age is 321 Ma).The SiO₂ content is relatively low,with an average of 70.55 %.The sedimentary water is in a weak alkaline-alkaline,medium salinity and anoxic reduction environment.The ? REE is relatively large,and the light rare earth elements are enriched.There are no obvious anomalies of Ce and Eu,and the distribution curve is flat.These geochemical characteristics show that it is non-hot water origin,and the sedimentary environment is close to the continental margin.Combined with the existing geological research results,it is speculated that the chert lens was developed in the local hot water environment under the influence of magmation in the tension stage of the basin in the late Early Carboniferous.Subsequently,the magmation weakened and the water temperature inside the basin decreased,forming the non-hydrothermal stratified cherts.Therefore,these two types of cherts record the short-term development process of the small ocean basin in the Chaogenshan area in the late Early Carboniferous.Chert lenses are exposed in the pillow basalt of the Lower Permian Dashizhai Formation in Wudaoshimen area,Keshiketeng Banner,Inner Mongolia,and layered cherts are seen on them.Chert lenticle is found in the pillow basalt,the average content of SiO₂ is96.19 %,the average content of Fe₂O₃ is 3.15 %,and the average content of Mn O is0.004 %.The sedimentary water is acidic,low salinity,oxidation conditions,small ? REE,enrichment of heavy rare earth elements,negative Ce anomaly and positive Eu anomaly,and left-leaning distribution curve.These indicators and geochemical diagrams indicate that it is hydrothermal origin.Layered cherts are exposed on the basalt layer,and the SiO₂ content is relatively low,with an average of 78.81 %.The sedimentary water is in a weak alkaline-alkaline,medium salinity and anoxic reduction environment.The ? REE is relatively large,and the light rare earth elements are enriched.Ce and Eu have no obvious abnormalities,and the distribution curve is flat.The geochemical characteristics show that it is a non-hot water origin.Combined with the results of geological research,this paper believes that the Wudaoshimen area of the Early Permian is in an intraplate tensile environment.The basic magma produces pillow basalt,and the chert lens is produced in its hot water system.Later magmatism stopped,water cooled,geochemical domain changed,and layered cherts appeared.The red cherts of the Late Silurian-Late Devonian Gorkhi Formation in the Ulaanbaatar area of Mongolia are exposed in large thickness.Together with other rocks,they form an ocean-type strata characterized by OIB-type basalt-radiolarian cherts-siltstone-mudstone-sandstone-limestone assemblage,representing the Late Silurian-Late Devonian Mongolia-Okhotsk oceanic crust.The average SiO₂ content of cherts is 94.77 %,Fe₂O₃ is about 1 %,and Mn O is about 0.3 %.The sedimentary water is alkaline,low salinity,and reduction environment.The ? REE is relatively small,and the light rare earth elements are enriched.There are positive anomalies of Ce and Eu,and the distribution curve is flat.These indicators and geochemical diagrams show that they are caused by hot water,and the sedimentary environment is a typical deep ocean basin.The cherts studied in this paper reveal the geochemical characteristics of small ocean basin and stable ocean environment under extension,and provide a new comparative basis for exploring the genesis of cherts under different tectonic settings.