Timing Of Formation And Its Metamorphism And Deformation Of The Dunhuang Complex,Gansu Province

The study of Precambrian geology is of great significance for understanding the material composition,structure,and evolution of the early Earth.In China,the outcrops of the Precambrian rocks are concentrated in the North China Craton,the Yangtze Craton and the Tarim Craton.However,researches on Precambrian geology of the Tarim Craton are relatively insufficient.The Tarim Craton is located in northwestern China with an area of more than 600,000 km2 and is bounded by the Central Asian Orogenic Belt to the north and the Western Kunlun Orogenic Belt and the Altyn Orogenic Belt to the south.The Precambrian rocks of the Tarim Craton are mainly exposed in the four regions at the periphery,namely,the Korla-Kuruktag region on the northeastern margin,the Aksu-Wushi region on the northwestern margin,the Tiekelike-Western Kunlun region on the southwestern margin and the Northern Altyn Tagh-Dunhuang region on the southeastern margin.While,the focus of this study is the Dunhuang complex in the Dunhuang area.TTG gneisses and metamorphic supracrustal rocks make up the Dunhuang complex mainly,and the dominant rocks are composed of tonalitic gneiss,trondhjemitic gneiss,granodioritic gneiss,garnet amphibolite,garnet biotite schist and marble.We collected TTG gneisses from the Dunhuang complex systematically.All of the samples are typical and distribute throughout the area.Through the LA-ICP-MS zircon U-Pb dating,we obtain the protolith age and metamorphic age of these TTG gneisses.The metamorphic supracrustal rocks,mainly garnet biotite schists,were analyzed by electron probe.On the base of determinating the cause of composition zonings of garnets,we recognize stages of metamorphism and calculate the temperature and pressure conditions of the peak stage.This paper also explores the characteristics of structural deformation of the Dunhuang complex.Through detailed field surveys and microstructural analysis,two episodes of structural deformation are identified and their kinematic mechanisms are determined.The main opinions and conclusions in this paper are as follows:1.Diverse crystallization ages of the rock units in the Dunhuang complex are obtained,which enriches the chronological framework of the magmatic events in this area.A crystallization age of 2376±68 Ma is obtained in the granodioritic gneiss in the Hongliuxia area.A crystallization age of 2057±75 Ma is obtained in the granodioritic gneiss in the Gangou area.The crystallization ages of 1842±39 Ma and 1846±100 Ma are obtained in the Dashuixia and Sanweishan areas,respectively.Moreover,some crystallization ages of?500-400 Ma are reported in the Dashuixia,Guazhou and Diaoshigou areas.The above magmatic zircon ages reveal that there are rock units in the Dunhuang complex dating to?2.3 Ga,?2.0 Ga,?1.85 Ga and-500-400 Ma,and all indicate the magmatic events during the corresponding periods in the Dunhuang area.2.Two periods of metamorphism are identified in the Dunhuang complex and their tectonic implications are indicated.At the same time,the temperature and pressure conditions of the metamorphic peak of the last period of metamorphism are obtained.The metamorphic ages of 1884±130 Ma and 1888±75 Ma are obtained in the Gangou and Hongliuxia areas,respectively.Also,some metamorphic ages of-400 Ma are reported in the Gangou,Dashuixia and Diaoshigou areas.The two periods of tectonothermal events dating to?1.9-1.8 Ga and?400 Ma are related to the assembly of the Columbia supercontinent and the Pangea supercontinent,respectively.In addition,the metamorphic event of?400 Ma recorded a temperature-pressure condition of 667?690 ?/0.88-0.89 GPa in the Gangou area and a temperature-pressure condition of 627?667 ?/1.51?1.59 GPa in the Diaoshigou area.3.Reveal the structural deformation characteristic,deformation time and kinematic mechanism of the Dunhuang complex.By measuring the three cross-sections which are in the Gangou,Dashuixia and Guazhou areas,respectively.We reveal that the Dunhuang complex experiences two episodes of structural deformation.The first is the right-lateral ductile shearing(S1)and the second is the S-N-trending folding(S2),and the right-lateral ductile shearing(S1)with the-400 Ma metamorphism are matched at the same period.