The Characteristic Of Volcanic Geology And Tectonic Attribute Of Meiletu Formation In Yakeshi Region

The north part of Great Xing'an Range belonged to volcanic active belt during Jurassic-Cretaceous.The volcanicity is very active.The volcanic rocks are widely distributed.The purpose of this paper is to discuss the volcanic rock and the tectonic setting of of the Meiletu formation in Early Cretaceous Epoch.The volcanic rock of Meiletu formation is spread not only in the north part of Great Xing'an Range,but the Yakeshi area and the underground of Hailar Basin coal stratum.And the thickness is changing between 101m-1096 m.The rock suite in the studied area is consist of andesitic basalt,olivine basaltic trachyandesite rock,latite,trachyte,volcanic breccia and agglomerate.While the livine basaltic trachyandesite rock and latite is the mainly part.The volcanic rock of Meiletu formation belongs to Alkaline-sub-alkaline transitional series.Based on the comprehensive research,it belongs to shoshonite series.The Si O₂content range is between 52.87%and 61.17%.The Na₂O+K₂O content range is between 5.7 to 7.02%.The rittman index is 2.89 to 3.72,of which mean value is 3.29.It suggest that it belonged to peraluminous rock with the contend of Al₂O₃ is high,the mean value is about 16.23%.The contend of Ti O ₂is between 0.85 and 1.91%and the Mg#of most of Meiletu formation is greater than 0.4.The REE concentration and Light REE fractionation are high(?REE=203.26-308.54×10^-6).LREE/HREE is3.17-5.91 and?Eu=0.79-0.93.REE chondrite-normalized distribute curve leans overtly right.They are enriched in the large ion lithopile and depleted in high field-strength element(HFSE)with obviously LREE to HREE fractionation and no negative Eu anomaly,which shows an extensional tectonic setting.Petrochemical and geochemical evidence shows that the magma is mainly from the bottom of lithosophere and is experienced assimilation and contamination of the crustal materials.The meiletu formation is widely spread in the study area,and mostly spread along the Hailaer rivertrending NNE.It is fissure-central and fissure volcano with general overflow.With the study of Petrotectonic Assemblages and geochemistry,Meiletu formation gradually transformed between stretching stage and the compressive orogenesis,mainly formed in extensional environment.