LA-ICP-MS U-Pb GEOCHRONOLOGY OF ZIRCONS IN THE QUATERNARY SEDIMENTS FROM THE YICHANG AREA OF HUBEI PROVINCE AND ITS PROVENANCE SIGNIFICANCE

The cutting through of the Three Gorges of Yangtze River recently was considered as an event occurred in Quaternary Period,but there still has some diversity in the concrete time of cutting through.Therefore,the Jianghan Basin which is close to the outlet of the Three Gorges is selected as the research target and the entire Quaternary strata and preferable outcrop are thoroughly studied for this purpose.Detailed research of the sedimentary characters of Quaternary deposits,combing with the ESR dating data,following conclusions can be drawn.Yunchi Group belongs to sediments of alluvial fan and fan delta and the lower part of Shanxiyao Group is the product of sedimentation deposited in fan delta with the upper part being the lacustrine sediments.We select the profiles of 015,024,YC02,00B,which respectively represent Yunchi Group,Shanxiyao Group,the fifth level terrace and fourth level terrace of Yangtze River,and study their mineralogy characteristics and use LA-ICP-MS to date their U-Pb ages from detrital zircons of sandy matrix in gravel layer of this four sections.According to these researches,it is found that the zircons in the profiles of 015 and 024 mainly were derived from Huanglin Dome,Wudang Block,which lie in the east of the Three Gorges,and some derived form Miaochuan area which is in the border between the Hubei Province and Shanxi Province,and the rest derived from Huarong area which is in the border between the Hunan Province and the Hubei Province.These zircons are all the east of the Three Gorges.But the zircons in sediments of the fifth and sixth level terraces of Yangtze River contain not only the material deriving from the east area of the Three Gorges mentioned above,but also materials from exposed magmatic rocks in the west of the Three Gorges,such as the Micang Mountain,Daba Mountain area,Longmen Mountain,Panzhihua area and Batang-Dege area in the west of Sichuan,as well as from the Emeishan Basalt.Thereby,it is confirmed that the sediments represented by profiles of 015,024 are not the products of the Yangtze River terraces as predecessors thought,and the cutting through time of the Yangtze Three Gorges should be after the end of sedimentation of the profile 024,and before deposition of the profile YC02.Therefore,the cutting through time of the Yangtze Three Gorges should be late than 0.75 Ma.