Cretaceous Subduction And Accretion Of Neo-Tethyan Ocean In Gongga-Sangri Area Of The Yarlung-Zangbo Suture Zone

Gangdese is located in the hinterland of Qinghai-Tibet Plateau,which is an important part of it.The Yarlung-Zangbo suture zone（YZSZ）on its south side represents the remnants of the oceanic lithosphere after the closure of the Neo-Tethys.The study previous show that the ages of magmatic rocks among Gangdese arc are mainly about～240-50 Ma,and they are closely related to the northward subduction of the Neo-Tethyan oceanic plate under the Lhasa terrane.The Late Cretaceous arc magmatic rock accounts for a large part of the Gangdese batholith,while the reports of the Early Cretaceous magmatic activity are relatively rare.Therefore,the Neo-Tethyan evolution patterns during Early and Late Cretaceous,and the tectonic transformation of the two stages have always been controversial.In addition,the studies on the sedimentray basin and accretionary complex related to the northward subduction of the Neo-Tethys in Cretaceous are mainly focused on the Xigaze-Laze-Sangsang-Zhongba area int the west.However,no contemporaneous sedimentary basin or accretionary complex has been reported in the eastern part of this belt.According to problems above,this paper focuses on the Early and Late Cretaceous magmatic rocks and contemporary sedimentary units in the Gongga-Sangri area,southeastern Gangdese.Sedimentology,petrology,zircon U-Pb chronology,zircon Hf isotope and whole rock geochemical research were carried out for exploring the tectonic models of the Neo-Tethys during Cretaceous.The spatial and temporal evolution sequence of Tangjia-Sumdo Paleo-Tethys and Yarlung-Zangbo Neo-Tethys since the Late Paleozoic has been constructed.We gained the following main understandings:（1）Zircon U-Pb geochronological analysis results of the Liuqiong quartz diorites in Gongga area,southeastern margin of the Gangdise Belt are 141-137 Ma（the Early Cretaceous）.Whole-rock geochemical analysis shows that the Liuqiong quartz diorites were related to plate subduction and have typical properties of Adakite.Combined with whole rock Sr-Nd isotope,zircon Hf isotopic characteristics and major,trace elements,it is concluded that the source magma of the Liuqiong Adakite originated from the partial melting of the Neo-Tethyan oceanic slab,and slightly metasomatism occurred with the peridotite in the mantle wedge during magma ascent.Based on regional data,we hold that the Neo-Tethyan oceanic lithosphere was flat-lying beneath the Lhasa terrane during the Early Cretaceous.（2）Petrographic analysis shows that the intrusive rocks of Gongga-Sangri area on the southeastern margin of Gangdese belt are mainly granite,granodiorite and quartz diorite.Zircon U-Pb dating shows that the magmatic rocks are mainly formed between 93.4 and 84.7 Ma.Theε_Hf（t）values of zircons are all high and positive（+9.67 to+14.21）,indicating that the zircons have the characteristics of the source of depleted mantle.Mantle model ages and crustal model ages of zircon Hf isotope are generally young,suggesting a close relationship with juvenile crust.The geochemical composition shows that the samples in the study area are I-type granites with volcanic arc properties,and most of the samples have affinity with adakite.Petrogenesis and tectonic setting analysis indicate that the magma originated from the partial melting of the juvenile mafic lower crust and a certain degree of magmatic mixing.Integrated regional information and Early Cretaceous results above,we suggest that the Late Cretaceous magmatism is the result of the partial melting of juvenile lower crust and subsequent magma mixing caused by the upwelling of large-scale asthenosphere mantle during the transition from low angle to normal angle（roll-back）of subducted Neo-Tethys oceanic plate.（3）The Gaxue Group in Zedong area occurs in the form of"matrix+block",and the matrix is mainly composed of mudstone,siltstone and siliceous rock,with occasional interbedded sandstone.The blocks vary in size and composition,mainly including volcanic rocks,siliceous rocks,limestone and sandstones.The matrix of siliceous argillaceous rock is characterized by strong structural deformation,plastic flow structure around those blocks,showing the characteristics of"local order,overall disorder".At the same time,faults,fold structures and metamorphism are widely developed,which is the accretionary complex formed by the northward subduction of the Neo-Tethyan oceanic lithosphere under Gangdese.Geochronological and sedimentological studies show that the Gaxue complex represents the trench basin deposited in the Early Cretaceous（～113 Ma）to Late Cretaceous（～80 Ma）,mainly receiving the siliceous sediment and weathering products of the magmatic arc in the southern margin of Gangdese and a small number of ancient components with Lhasa terrane affinity.During the process of plate subduction,the Gaxue trench deposits were scraped and accreted to the southern margin of Gangdese,representing the Cretaceous accretionary complex owing to the northward subduction of the Neo-Tethys within the Yarlung-Zangbo suture zone,southeastern Gangdese.（4）The area between Tangjia-Sumdo（Ultra-）high-pressure metamorphic belt and Yarlung-Zangbo suture zone in the southeastern part of Gangdese is a vast accretion wedge formed by the continuous northward subduction and southward recession-type accretion of the Tangjia-Sumdo Paleo-Tethys since the Late Paleozoic.The whole region is divided into Tangjia-Sumdo accretionary complex（P-T2）,Qushui-Lilong accretionary complex（T3-J1）and Gaxue accretionary complex（K1-2）from north to south.The sedimentary sequences in Mailonggang-Xionglai Basin（T3-J1）and Lhasa-Riduo Basin（J3-K2）covered the foregoing accretionary complex.These different units together constitute the compound accretive basin-mountain system.The Tangjia-Sumdo Paleo-Tethys and the Yarlung-Zangbo Neo-Tethys represent different evolutionary stages of the same ocean basin.