Chronological Characteristics Of Detrital Zircon And Basalt In Liuhe Area,nanjing And Its Relationship With The Ancient Yangtze River

The Neogene basaltic volcanic cone and the paleo Yangtze River delta sand gravel layer are widely distributed in Liuhe area,Nanjing,Jiangsu.And it is located in the lower Yangtze plate which is on the east side of the Tanlu fault zone.Some basalts directly overlay above the gravel layer of the ancient Yangtze River Delta,while some are cut and covered by the latter.The relationship between the two and the age of formation is of great significance to the understanding of the formation and evolution of the ancient Yangtze River Delta.In this paper,We Sampled the gravel layer distributed in Liuhe area of Nanjing,and selecting detrital zircon,and SHRIMP and LA-ICP-MS were used to measure the year.The zircon U-Pb ages in Liuhe can be divided into 5 groups:120-300Ma,400-600Ma,750-900Ma,1700-2000Ma,2300-2800Ma.The zircon U-Pb ages in Anhui can be divided into two groups:120-140M and 700-900Ma.From zircon U-Pb age distribution map,we can see that the zircon age map of Liuhe area sampled from the field is highly similar,and it's quite different with the samples collected from Anhui area.The gravel layer in Liuhe area is a product of a strong River,rather than a tributary.The U-Pb dating of the detrital zircon provides direct evidence for the paleo Yangtze river sedimentation.The sand and gravel layers in the Liuhe area,including the sand and gravel layers of the modern Yangtze River deposits,are caused by the sediments of the Yangtze River,which are deposited in the wobble process all the time.the ⁴⁰Ar-³⁹Ar method was used to determine the age of the basalts in the Nai mountain,the Liuhe Fangshan and the Lingyan mountain,which is located in the ancient Yangtze River alluvial plain in southeast of Liuhe.In the landscape,Fangshan and Lingyan mountain are frusto conical,and the basalts overlay above the gravel layer of the ancient Yangtze River.Nai mountain is a combination of two cone hills which leaned on each other,and it's basalt is cut by the ancient sand gravel layer of the Yangtze River.The samples were selected from fresh appearance from the three mountains,hard texture,and massive structure.The samples were processed and tested at the Argon Chronology Laboratory of the Institute of Geology,Oceanology and Atmospheric Sciences,Oregon State University,United States.The Ar-Ar result shows that,the age of the basalt in Nai mountain is 16.90±0.37 Ma,which is obtained for the first time in the history.Liuhe Fangshan and Lingyan muntain got a flat age spectrum,which is 9.55±0.03 Ma and 10.43±0.03 Ma,which is approximately close to the results of previous k-Ar dating.The new age result indicates that,there are at least two volcano eruptions in Miocene in Liuhe area,Nai mountain basalt is earlier formed than which in Liuhe Fangshan and Lingyan mountain.Combined with previous research results,the Neogene volcanic activity era in Liuhe area has been more precise,and the two periods of volcanic activity are limited to about 16-17Ma and 9-10Ma.There is still need for further research on whether there are third periods of volcanic activity during Pliocene.During the period of the two phases of basalt,a set of paleo Yangtze River delta deposits were widely developed in this area.The Ar-Ar geochronology is helpful for the study of volcanic activity and the stages of gravel formation and the evolution of the ancient Yangtze river.Since the Late Neogene to the Quaternary period,the Yangtze River flows from south to North East into the ancient Gaoyou lake depression basin,and gradually transition to that because of the continuation of the tilting movement from north to South and the differential lifting movement,the ancient Yangtze River move southward and gradually transform into the modern Yangtze River.