Coupled U-Pb And Lu-Hf Isotopic Analyses Of Detrital Zircons In The Sediments From The Nanchang Section Of Ganjiang River Downstream And Its Geological Significance

The Ganjiang River is the biggest river in Jiangxi Province,which is also one of eight major tributaries of Yangtze River.It flows from the south to the north of Jiangxi province.Geologically,Ganjiang River flows through the area which are mainly located in the hinterland of the Cathaysian Block,Jiangnan orogenic belt and the southeastern margin of the Yangtze block.River is a combined product of tectonic and climate.River sediment carries denudation of source rocks,the transfer and information of the source area of drainage basin.Detrital zircons in rivers record the information derived from crustal materials.However,these crustal materials may not be preserved,or have not been exposed.Undoubtedly,detrital zircons in the sediments of the Ganjiang River mainly originated from various rocks of the drainage areas,which can carry important information of their source rocks.In order to study the source of stream sediments from the Ganjiang River,the crustal growth and evolution history of the Ganjiang River Basin,the detrital zircons of Nanchang section of Ganjiang River downstream were determined for U-Pb ages and Hf isotopic compositions by LA-ICP-MS and LA-MC-ICP-MS analytical technology,respectively.The conclusions are addressed as follows:(1)The detrital zircons U-Pb age of Nanchang section of Ganjiang River downstream can mainly be divided into 7 major age groups: 134-197 Ma,221-290 Ma,398-484 Ma,520-625 Ma,727-895 Ma,980-1110 Ma and 2502-2861 Ma,respectively.The primary ones are as follows: 398-484 Ma(Peak value of 433 Ma,accounting for 43%)and 134-197 Ma(Peak value of 151 Ma,accounting for 15%).(2)The major age groups of Nanchang section of Ganjiang River downstream's detrital zircons can correspond to the known tectonic-magmatic thermal events.2502-2861 Ma is the stage of the global crustal growth,which led to the formation of original stable continent and the ancient land nucleus.980-1110 Ma is the featured age formed by global Rodinia supercontinent assemblaged orogenic belt of Grenville period.727-895 Ma corresponds to the break-up stage of Rodinia supercontinent.520-625 Ma corresponds to pan-African events,but without coeval geological events and rocks documented in South China;398-484 Ma represents the Caledonian Movement,which is widely documented in South China.221-290 Ma is Indosinian-Hercynian period,which is a period of action of Paleo-Tethys Ocean closing and corresponding continent collision.134-197 Ma corresponds to Yanshanian period,and the period of the magma activity is occurred on the background of the formation of South China Block.(3)The detrital zircon U-Pb age distribution of sediments from the Nanchang section of Ganjiang River downstream is consistent with those of the Cathaysia Block,indicating that dominant contribution of materials of the Ganjiang River stems from the Cathaysia Block.In this regard,in-situ detrital zircon U-Pb dating is a feasible method to constrain crustal materialsl source of modern rivers.(4)Coupled detrital zircon U-Pb ages and Hf isotopic composition of the Nanchang section of Ganjiang River downstream's sediments can reflect the formation and evolution history of the Cathaysia Block.The ?Hf(t)values of the Neoarchean detrital zircons are approximately 0,showing that the strong crust-mantle mixing occurred in this period.The ?Hf(t)values of the Mesoproterozoic and Neoproterozoic detrital zircons have both positive and negative values,indicating that the material of the source region is not only the recycling of ancient crustal materials,but also the addition of juvenile crustal materials.Most of the ?Hf(t)are negative for Phanerozoic zircons and two-stage model age values correspond to the Paleoproterozoic to Mesoproterozoic,implying that the Phanerozoic crustal materials are mostly derived from the juvenile crust formed in paleoproterozoic-Mesoproterozoic,with few of them are derived from depleted mantle.Meanwhile,2.1-1.1 Ga is the main stage of crustal growth of the Cathaysia Block.The material formed in this stage consist of more than 80% of the present Cathaysia Block.