The LA-ICPMS U-Pb Zircon Geochronology And Provenance Study Of Sedimentary Core In The Zhoulao Town, Jianghan Plain, China

The study of the formation time of the modern Yangtze River has been a difficult problem for about a century.Because the sediments carried by the Yangtze River does not deposit in the Sichuan basin,the Jianghan Plain is one of the largest sedimentary sink in the catchment of the Yangtze River.The sediment in the Jianghan Plain records the evolutional history of the Yangtze River.But there is still no one have studied the provenance of the sediments.The author is using LAICPMS to study the U-Pb and trace elements of the detrital zircons which recovered from the Zhoulao core,and to predict the provenance of the sediments in the Jianghan Plain.This study will must be give the reference for the further study of the evolution of the Yangtze River.We should know the tectonic-thermal events of the upper reaches of the Yangtze River.The Yangtze River flows through the Qiangtang-Changdu block,Songpan-Ganze block,and the Yangtze Craton.Because of the small area,the sediments eroded from the Qiangtang-Changdu block must be very little,and then they will be drowned in the sediments from the lower reaches. Songpan-Ganze flysh zone is the largest Triassic flysh sedimentary area in the world.The felsic igneous intrusions in the Songpan-Ganze area are in the Late Triassic(～214-211Ma),some ages are Middle-Late Triassic(220Ma).The age of the Yidun Island Arc is appears Middle to Late Triassic,from 225-215Ma.This age is earlier than the time of the mass deformation in the area. There are four age populations in the detrital zircon ages of the Songpan-Ganze sediments: 2.4-2.5Ga and 1.85-1.95Ga,which is thought from the North China Craton;the third age peak (400-450Ma) is very popular in the North Qinling area.The fourth age peak(ca.250-280Ma) is different from the age peaks of the Qinling-Dabie zone,North China Craton,and the South China Craton.The provenance area of them maybe is the Guadalupian and Lopingian formations in the Longmenshan area,and is linked to the breakup of the Songpan-Ganze basin.The tectonic-thermal history of the Yangtze Craton is very complicated.U-Pb ages of detrital zircons from three Nanhua Formations of the Yangtze craton reveal four major age groups of 720-910 Ma,1.90-2.05 Ga,2.40-2.55Ga and 2.60-2.70 Ga with few＞3.2Ga grains.Three groups of igneous rocks occur in the Yangtze craton in the Mesozoic:(1) The Yangtze Igneous Group.It consists of diorite;quartz diorite and granodiorite stocks distributed along the middle and lower reach of the Yangtze River in the northeastern margin of the craton.These rocks intruded into Palaeozoic sedimentary rocks during Early Cretaceous times (127 to 135 Ma),and are closely associated with the important copper,iron,sulphur and gold ore deposits in eastern China.Some A-type granite intrusions also occur in this region.(2) The Jiangnan Igneous Group.It is distributed near the southeastern margin of the craton and includes Mesozoic granites and granodiorites,occurring as batholiths,stocks as well as felsic volcanic rocks(120 to 140Ma).(3) The intrusions in the western part of the craton,including granitic batholiths and stocks emplaced in late Palaeozoic to early Mesozoic times.There are many sedimentary rocks in the upper reaches of the Yangtze River catchment especially in the Three Gorges area.Using the U-Pb ages of the detrital zircon in the provenance study is very complicated problem.We should think about the erosion rate of the Yangtze River, the areas of the different rock types in the catchment,the velocity of the tectonic uplift.The conclusions conjectured from this study are:(1) The morphology and CL images of the detrital zircons show that majority of the detrital zircons are igneous zircons.There are obviously oscillations in the zircons.The LREE is rare and the HREE is enriched.And the content of the element Ce is positive and the Eu is negative.Most of the ratio of the Th/U is less than 0.1.Because most of them are the same,these characteristics cannot be used as a tracer in the provenance study.(2) The age peaks of the 6 detrital zircon samples are quite similar.They are concentrated in the Fuping period(＞3.0Ga),Lvliang period(1600-2500Ma),Yangtze period(720-850Ma),and Indo-china period(212Ma).Himalayan period is very rare;there are only two zircons in the Himalayan period.They all in the Gsh6,and the depositional time is ca 800ka BP,which indicates that the Yangtze River reached the Tibet Plateau at that time.(3) The U-Pb ages of the six samples do not change significantly,suggesting that the Yangtze River catchment did not change greatly fro 2.2MaBP.The older ages of the detrital zircon are becoming less and less,and the younger ages of zircons are becoming more and more,indicating the area of the Yangtze River are becoming larger and larger.Because the younger ages rocks are all in the Songpan-Ganze area,this shows that the Songpan Ganze block is one of the most important source of the sediments in the Jianghan Plain.And the formation time of the Three Gorges maybe very older than we thought before.We should search the evidence in the Pre-Quaternary sediments.The appearance of 212Ma and＜17Ma age peaks suggest that the detrital sediments of the upper reaches of the Yangtze River have been transported into the Jianghan Plain.(4) The appearance of the＞3.0Ga,2490～2386Ma,2006～1784Ma,850～720Ma age peaks show that most of the sediments are from the Kongling terrain.