Petrology and evolution history from late Jurassic to Paleogene of Gyangze basin,Southern Tibet.

Gyangze basin located immediately on the south of the Indus-Yalung Zangbo Suture (IYZS) evolved along with the evolution of the Tethyan Ocean.The evidences of the evolution history of a basin generally recorded well in the sediments.Based on the published stratigraphy framework of Upper Jurassic to Paleogene in Gyangze basin,this paper studied the petrology of the strata with observation on the sedimentary rocks in the field and under the microscope.The sediments of Upper Jurassic to Lower Paleogene in Gyangze Basin is a sequence of quartz arenites (Weimei Fm),shales intercalated with sublitharenites (Rilang Fm),black siliceous and calcareous shales (black unit and white unit,Gyabula Fm),chert (chert unit,Gyabula Fm),red shales intercalated with limestones (Chuangde Fm),shales with olistoliths (Zongzhuo Fm),and sandstones interbedded with shales (Gyachala Fro).This sequence indicates that the water was deepened from Weimei Fm to Gyabula Fm,while the Gyachala Fm records the shallower water sediments than Gyabula Fm to Zongzhuo Fm.Biogenic sediments,suspended sedimentation and mass flow including sandy debris-flow, turbidity current,slump and rock slide were common in the sections.The depositional environments of the strata are of deep water, most likely slope to oceanic basin.Based on the distribution of major element,we suggested that the bottom water of Gyangze basin was mainly anoxic or dysoxic during late Jurassic to early Paleogene,but the late Cretaceous red unit was deposited under an oxic condition. According to the petrology and depositional environments of sedimentary rocks,we divided the evolution history of Gyangze basin into four stages from late Jurassic to early Paleogene:stable basin stage during late Jurassic to early Cretaceous,subsiding basin of mid- Cretaceous,residual basin of passive continental margin late Cretaceous,and remnant ocean basin of latest Cretaceous to early Eocene.