Archean To Proterozoic Magmatic-Metamorphic Histories Of The Cuoke Complex And Dahongshan Group Of SW Yangtze With Implications To Proterozoic Supercontinents

The early Precambrian is an important period of crustal growth and evolution.The compositions and properties of the continental crust,as well as its growth and movement differed at different stages during this period.The following subdivisions and events have been proposed: 4.45–3.80 Ga initial felsic crust generated;3.80–3.20 Ga large-scale craton formation accompanied by a large amount of sodium tonalite–trondhjemite–granodiorite series(TTGs)and greenstone belts;3.20–2.50 Ga the cratons stabilized and the continental crust transformed to high potassium granitoids,craton dispersal;and 2.50–0.8 Ga initial continental assembly and dispersal processes commenced associated with the two supercontinents of Nuna and Rodinia.On the modern Earth,especially that at convergent plate margins,the primary mechanism for crustal growth is plate tectonics,however the crust of the early Earth may have operated in non-plate tectonic modes including stagnant lid or some form of sluggish or active lid with dispersed deformation,based on observations of the geological record and numerical simulations.Establishing the crustal growth and tectonic mode(s)that operated on the early Earth,whether stagnant lid,or some forms of sluggish/active lid with varying degrees of coupling between the mantle and lithosphere is hindered by the largely incomplete and fragmentary record of the Earth through this time frame.The Yangtze Block is one of the three ancient continental cores in China and is currently enclosed by the Cathaysia,North China,and Indo China blocks and the QinghaiTibet orogen.Evidence from the rock record within the Yangtze Block shows events extending back to at least ca.3.45 Ga.In addition,the two-stage zircon Hf model ages indicate the probability of ca.4.0 Ga crust in the Yangtze Block.It records abundant information of early crustal evolution and a number of important early Precambrian oreforming events.Therefore,a detailed study of the early evolutionary history of the block is not only potentially important in understanding the role and process of early crust formation,but also with important significance to economic geology.In recent years,the discovery of ancient basement rocks in the southwestern Yangtze has provided a new window to study the early evolutionary history of the block.This thesis is mainly focused on the Cuoke Complex and Dahongshan Group in the southwestern Yangtze Block.Through a comprehensive study involving systematic geological field work,zircon U-Pb geochronology,monazite U-Th-Pb geochronology,xenotime U-Th-Pb geochronology,zircon Hf isotope analysis,as well as metamorphic petrography,and P-T evolution,was undertaken in order to provide a detailed study on the evolutionary history of the southwestern Yangtze from Archean to Proterozoic.Research outcomes from this study are integrated with previous published materials to discuss the early evolutionary history of the entire Yangtze Block,and its role of the evolution of the Proterozoic Nuna and Rodinia supercontinents.The major findings obtained in this thesis are as follows:Firstly,based on magmatic zircon U-Pb chronology and zircon Hf isotope study,this thesis establishes a multistage magmatic and crustal growth history of the southwestern Yangtze from the Archean to early Paleoproterozoic,and then compares this with that of the northern Yangtze.The results from this study support that the Yangtze Block is divisible into the northern and southwestern domains which evolved separately in time and space before they assembled to form the unified proto-Yangtze Block.The thesis also identifies 2.0–1.85 Ga metamorphic event that reset zircons of the southwestern Yangtze Block,which had been previously interpreted as a simple magmatic event.Integrated with published data,this thesis determines the assembly between the northern and southwestern Yangtze blocks took place at 2.0–1.85 Ga.This thesis reveals a ca.845 Ga metamorphic event recorded by the Dahongshan Group in the southwestern Yangtze Block.Two overlapping substages are determined at843 ± 7 Ma and 849 ± 11 Ma based on contrasting metamorphic conditions.The petrography,mineral compositions and metamorphic phase simulation indicate a clockwise P-T evolution,and medium facies metamorphic conditions and geothermal gradient.Based on field evidence and geochemical,sedimentary and metamorphic data,this metamorphic event is considered to record the assembly between the Yangtze Block and an outboard terrane/s accompanying the closure of a back-arc basin during coupling across the convergent plate margin that extended along the western part of the block for much of the early to mid-Neoproterozoic.Comparison of the geological records of the southwestern and northern domains of the proto-Yangtze Block suggest they were each initially tied to northern Laurentia and Siberia respectively,prior to at least the early Paleoproterozoic.This likely extended into the Archean for the Siberia-northern Yangtze connection.The assembly of the two domains in the late Paleoproterozoic,after 2.0 Ga,is part of the larger coming together of Laurentia and Siberia within the Nuna supercontinent.This internal location for the protoYangtze Block within Nuna contrasts with its inferred external location within Rodinia,where it was adjacent to India,either on the margin of Rodinia or as a separate continental fragment.This requires the Yangtze Block to have separated from Laurentia and Siberia during Nuna breakup in the early Mesoproterozoic.