Sedimentary Geochemistry And Its Geological Implications Of Early Cretaceous Gyabula Formation In Rongbu Area,Southern Tibet

Rongbu area is located in the northern Tethyan Himalayas of southern Qinghai-Tibet Plateau,and belongs to Kangmar-Lhunze stratigraphic zone.On the basis of the field regional geologic survey,route exploring in and around the studied area,field section measuring with large scale,and in combination with the rock's geochemistry data and its information from the microscopes,we analyzed the rock's characteristics,sedimentary facies,tectonic backgrounds and sedimentary environment of Gyabula Formation in Rongbu area.Hereby,the Gyabula Formation of studied area is divided into two lithologic member after a comprehensive research.The first member of Gyabula Formation(K₁j¹)is characterized by:black-gray or yellow-green-gray shale,black-gray mudstone and siltsone.Black-gray shale are the main rocks,interbeded with limestone blet and limestone concretions,in which breed calcite veins.The lithology of second member of Gyabula Formation(K₁j²)is dark-gray shale sheet and black-grey mudstone with different thickness,partly interbedded with limstones,some stratigraphy with thin silty mudstone and argillaceous siltstone.It is most common to see shale and mustone interbedding with each other.The mudstone bed is 1cm?20cm in thickness.Shales developed lamellatin and slight limonite on its surface.The age of the Gyabula Formation was dated to early Cretaceous Epoch according to the fossils picked up in this area during fieldwork,including Berriasella sp.;Himalyites giganteus;Dipoloceras dingriens;Haplophylloceras pinque;Hibolites subfusiformis;Belemnopsis dingriensis;Belemnopsis alfuricus;Belemnopsis sinensis.The sedimentary face of Gyabula Formation is marine and can be divided into two facies according to the characteristics of rock associations and fossils collected in the area.The two facies are shelf face and shelf slope face.The first member of Gyabula Formation(K₁j¹)is shelf slope face,and the second member of Gyabula Formation(K₁j²)is shelf face.We analyzed the mineral compositions and geochemical data of limestone samples in the Gyabula Formation of Rongbu area.The results show that the sedimentary environment of the Gyabula Formation is reducing conditions:rear earth element Ce_anom>-0.1,V/(V+Ni)mean value0.78±0.01>0.46.Due to the reducing conditions and sea level rising with the regional tectonic movement of plate,carbon isotope is lesser than that of ocean in the same period.Eu has strongly postive anomaly demonstrated by REE+Y patterns.It is suggested that this may be mainly caused by volcanic movements.Therefore,in the early cretaceous period,the sedimentary environment of Gyabula Formation in southern Tibet was in reducing conditions and on a passive continental margin,exsiting volcanic movements.