Analysis Of Material Source Of Old Red Sand In Southeast Fujian Coast

Old red sand is a Quaternary red aeolian sand deposit which widely distributed along the eastern coast of China.It mainly distributes in Fujian,Guangdong and Hainan provinces.It is one of the good proxy indicators for studying Quaternary paleoclimate and environmental changes.Before the 1990s,it was believed that the old red sand was formed during the last glacial period?68-10 ka?and its material originated from the bare continental shelf at low sea level.With the development of optically stimulated luminescence,in recent years,it has been found that the age of the formation of the old red sand is as early as the last interglacial period of 120 ka or even earlier.During the last interglacial period,when the climate was high and the sea level was high,there was no large area of bare continental shelf to provide material sources,which made the climate and sea level background of the formation period of the old red sand quite different from previous explanations.The material sources of the formation period of the old red sand became a scientific issue that need to be discussed again.In this paper,three typical sections of Old Red Sand in Qingfeng,Pingtan and Keren,Jinjiang and Dongshan,Dongshan along Fujian coast and their associated areas are selected as research areas.The research content includes two parts:1.The correlation of different types of sediment debris is analyzed by mineral assemblage,clastic morphology and cathodoluminescence characteristics of quartz.The source-sink relationship between multi-element sediment in southeastern Fujian coast,including estuary sand,dune sand,coastal beach sand,island beach sand and far-source coastal drift sand,is discussed by using geochemical characteristics of common and trace elements and rare earth element geochemical tracing.2.Choose Keren old red sand section of Jinjiang River to carry out grain size,magnetic susceptibility and OSL experiments.On the basis of reconstructing the sedimentary process of the old red sand,the source parameters of the old red sand samples in the profile are compared with those of the surrounding multi-element sediment parameters,and the source changes of the old red sand cycle under different climatic and sea-level environments are studied.The results show that:According to the characteristics of mineral debris,the mineral assemblages of Qingfeng old red sand,dune sand and Minjiang estuary sand are mainly quartz debris,including plagioclase,potassium feldspar and a small amount of magnetite,and the debris morphology shows sub-angular shape.The characteristics of Qingfeng old red sand and Minjiang estuary outflow sediment debris are very similar.According to the cathodoluminescence characteristics of quartz in Pingtan,Jinjiang and Dongshan research areas,the cathodoluminescence characteristics of quartz of coastal sediments,including river sand,coastal aeolian sand,beach sand and old red sand,are mainly blue and purple,indicate that those sediment is related to the river sand from granite region in coastal area of southeast Fujian.The second is gray and black,indicating the source of sedimentary rocks,which may be related to the transport of sediment from distant sources along the coastal current.A small amount of red and brown,indicating the source of metamorphic rocks.The cathodoluminescence characteristics of quartz in the three research areas show that the old red sand and dune sand are mainly derived from granite in the river basin,and also contain metamorphic rocks and a small amount of sedimentary rocks transported by coastal currents.Sr/Ba,Al₂O₃/TiO₂,Ti/Zr and rare earth elements?Eu,?La/Sm?_N and?Gd/Yb?_N were selected as provenance indices,and rare earth elements partition patterns were used to compare and trace various samples.Sr/Ba indices show that the three study areas are continental deposits.Al₂O₃/TiO₂ and Ti/Zr indices show that the main materials are feldspathic rocks,which may be related to granite and dacite in Fujian coastal watershed.Sr/Ba-Al₂O₃/TiO₂ correlation maps and Sr/Ba-Ti/Zr correlation maps show that the old red sand is close to the nearby river sand and dune sand.Rare earth element distribution patterns indicate that the main source of old red sand is river sand.Nearshore beach sand and dune sand may be the intermediate state during the process of river sand transporting to old red sand.Some old red sand is related to island beach sand and the sand carried by far-source coastal currents.The photoluminescence age of the typical old red sand profile in Keren is?91.6±12.1?ka^13.5±2.8ka,which is the coastal aeolian sand deposit from the last interglacial period to the glacial maximum period.During the last glacial maximum,the Sr/Ba of provenance parameters increased significantly,while the Al₂O₃/TiO₂ and Ti/Zr values showed the lowest values,indicating that the change off erromagnesian protolith materials of distant source.The values of?Eu,?La/Sm?_N and?Gd/Yb?_N changed obviously,which indicated that provenances were affected by distant sources.By comparing the change of provenance parameters with the change of climatic proxy indices,it is found that the fluctuation of climatic parameters and provenance parameters during the sedimentation process of the profile shows that when the climate becomes colder,the sea level drops,the provenance shifts to offshore direction;when the climate becomes warmer,the provenance shifts to shore direction.