Tectonic Evolution Of The Cretaceous Back-arc Basin In The Middle-east Segment Of The South Gangdese,South Tibet

The continuous northward subdution of Neo-tethys oceanic lithosphere and subsequent Indus-Aisa collision lead to the Meszoic-Cenozoic ‘Gangdese magmatic arc'.Subsequently,the crust of Tibetan Plateau(TP)has been greatly shortened and thickened since the subduction and collision.The TP,marked as the highest plateau in global,consists of abundant resources,energy,thrust-fold belts and geologic phenomena.However,the condition of the plates make up the TP and their evolutional history before the collision remain enigmatic and debatable.Our research area center upon the Cretaceous basin in the middle-east of south Gangdese belt,including Lhasa-Mozhugongka-Zedong-Woka,which consists of many ranges,basins and deposits,providing an unique opportunity to test and build geological theories.To rebuild the evolution history of this area,we systematically study the structure,geochronology and petrology of the Cretaceous back-arc basin together with the Jurassic,volcanics-bearing Tertiary Linzizong strata and granitic plutons occupy the area.The detrital zircon ages form Linbuzong formation,the Lower Cretaceous,are analyzed,revealing the youngest age crowd from 147-178 Ma.The detrital zircon ages of the Cretaceous basin in southern Lhasa shows affinity with the Lhasa terrane whereas the ages in the Cretaceous strata from northern Lhasa are similar to those from Qiangtang terrane.The Cretaceous basin in south Gangdese should form in the retroarc environment in accordance with some magmatic evidences between 137-90 Ma triggered by possible slab rollback.Coeval with the thickening crust,the Cretaceous basin in south Gangdese should under compressive environment after ~90 Ma.The on-going convergence of the Great India and Aisa results in a lot of large-scale folds and decollement structures.Two main decollement structures develop in the study area,one is between the Lower Cretaceous and the Upper Jurassic strata(Gangdese Decollement,GD1),and the other is between the Lower Jurassic and the Upper Jurassic strata(GD2).Both of the deformed and metamorphism level of GD2 is deeper than that of GD1.We suspect the GD was formed between 85-69 Ma with evidence of surcture and magmatism.As the Indus-Aisa collision initiated,massive magmatic emplacement makes upper crust uplift and denudated,forming a dome-like structure in our study area(Lhasa Dome,LD).Some geological attitude and structures of the GD was deformed or overprinted to be built as thrusts belts subsequently.These thrusts mostly involve the Yeba Formation,the Lower Cretaceous,Linzizong volcanics and granitic plutons.The activation of the dome-like structure is between 44.4-10.7 Ma according to low-temperature thermochronology(mainly by apatite fission track,AFT).A status of a faster exhumation rate of the core than that of the rim is revealed.On the basis of the AFT ages and the inverse history of the low-temperature thermochronology samples,the cooling history of the samples suggest the T1 may be activated during 18-10 Ma,coupled with the mineralization age of Jiama super-large mineral deposit(15-14 Ma).After combine the evidence of the main ore body condition with the structure of the ore,and compile previous mineralization theories,we propose that the decollement system should be a good guidance for the hydrothermal fluids with mineral to migrate and emplace,offering a favorable space for ore accumulation.