Sequences And Basin Characteristics Of Ordovician In Northern Great Xingan,Inner Mongolia

In the study area,Ordovician strata mainly distributes in Tongshan Group(O1t),large Dayixikanghe Group(D1-2dy),Duobaoshan group(01-2d),Luoher Group(O2-3lh).Tongshan Group is composed of quartz sandstone,feldspar quartz sandstone,tuffaceous sandstone,siltstone and silty slate,argillite and limestone.Dayixikanghe Group is formaded of fine sandstone,siltstone,sericite slate,siliceous slate,argillaceous chert and crystalline limestone.Duobaoshan Group is composed of acidic volcanic,basalts,variable siltstone.The upper Luohe Group is combinated of argillaceous limestone,bioclastic limestone and argillite;while,metamorphic sandstone,metamorphosed argillaceous siltstone,fine-grained variable quartz sandstone,silty slate,contribute the lower Luohe Group,Ordovician sedimentary facies are mainly two models:tidal flat facies model,restricted platform facies mode.Tongshan Group,Dayixikanghe Group,the lower Luohe Group belong to the tidal flat facies model.And depositional environment is upper zone or intertidal zone of tidal flat environment.The upper Luohe Group belongs to the restricted platform facies mode.Depositional environment is restrictedby platform environment.Ordovician intrusive rocks are mainly granodiorite,and quartz diorite.The 206Pb/238U zircon results is 445±2Ma,453 ± 12Ma.So,it belongs to the era of the Upper Ordovician.Speculation about magmatism activity that may relate the formation of Duobaoshan island and the result of mantle upwelling.The major element analysis of volcanic rock on Duobaoshan Group,are sub-alkaline series.Rare trace element analysis,belongs to island-arc volcanic rocks.In addition the tuffaceous material,belonging to the pyroclastic flow,indicates that the large scale of volcanic activity.It proves the existence of the island-arc.Ordovician has the type of active continental margin basins,it is similar to the western Pacific-type type.Tongshan Group sedimentary type should be the early deposition of active continental margin.And Duobaoshan Group sedimentary basin type should be the inside of island-arc basin.Luohe Group sedimentary basin type has forearc basin characteristics,While Dayixikanghe Group sedimentary basin type is close to the mainland side of the back-arc basin.