| As the most basic material transportation channel in nature,rivers are the main material exchange sites between the lithosphere and the hydrosphere.Some mineral particles in river sediments record the age and elemental composition of the original rock,which can reflect the history of geological evolution to a certain extent.Therefore,provenance analysis of sediments has always been a research hotspot at home and abroad.Zircon is a common heavy mineral,widely existing in modern river sediments,and its essential source is rock mass exposed during the current or geological history.It could undertake crust uplift,weathering,erosion after a long transport distances and still be preserved in sediments.Therefore,the determination of the age composition of detrital zircon in sediments reflects the potential material sources of different geological bodies,which has become one of the most popular methods for current provenance analysis.In the past two decades,laser ablation and plasma mass spectrometry(LA-ICP-MS)technology has been rapidly developed,and its characteristics of in-situ,real-time,high sensitivity,fast analysis speed,and low cost make it possible for detrital zircon U-Pb geochronology to be perfect,which has effectively promoted the progress of detrital zircon provenance tracing method.This method relies on the similarity of age distribution characteristics of detrital zircon in the sediments and the ages of the crystalline rock masses,discriminating the potential sources of the material.However,there are few discussions about the mechanism and consideration of this tracing method in the past,especially in the river basin with big catchment area,complex geological background and scattered distribution of crystallized rocks.For modern river systems,river hydrodynamics is the main driving force of detrital zircon in the process of erosion-transportation-deposition.It directly affects the microscopic transport and deposition process of zircon,and it also makes a certain consequence to the proportion of the final sediment source.In view of the insufficiency of detrital zircon research in the river transport process,this study selected the Chongling catchment in Hunan Province as a research area to conduct relevant quantitative studies.The length of the mainstream is 270 km long while the catchment area is about 6600 km2.The lithology of the catchment consists mainly of Carboniferous limestone,Paleozoic sandstone,and a small amount of granite in the upper reaches of the river.The upstream granite is the main source of zircon particles in the catchment.Based on the LA-ICP-MS zircon U-Pb dating method,18 samples and 2100 zircon sample points were measured and the results showed that the age distributions of detrital zircon in the 18 samples along the mainstream were consistent.The age peaks are all?150 Ma,which is consistent with the crystallization age of the Jurassic granite.Under single source rock conditions,the proportion of zircon in the source area is higher than in the lower reaches.The upper reaches are affected by the distribution area of the crystallization mass,while the middle and lower reaches are mainly affected by the river hydrodynamics;the changes of the zircon contribution in the middle and lower reaches of the rivers is positively correlated with the change of the median diameter of detrital zircon to a certain extent.When the zircon grain size of the original rock is relatively large(100-150 ?m),the weakening of the river hydraulic sorting will lead to a decrease in the proportion of source zircon.River hydrodynamic conditions are influenced by topography and water conservancy facilities in some sections of the river,which will change the local hydrodynamic conditions of the river,affecting the variation of the zircon contribution rate and the local proportion of sources. |
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