Evolution Of Paleo-coastal Mountains Along The Southeast Coast:Sedimentation And Paleoclimate Response In Yong’an Basin Of The Late Mesozoic

The geomorphology and drainage system of East Asia have experienced a complex evolution process since the late Mesozoic,forming the current geomorphic pattern of high in the West and low in the East and large rivers flowing in the East.However,many scholars believe that in the late Mesozoic,due to the subduction of the Paleo-Pacific plate,huge Paleo-coastal mountains were formed along the southeast coast of China,which not only created a geomorphic pattern of high in the East and low in the West,which is completely opposite to today,but also led to the climate change in the interior of South China.The coastal area of South China experienced intense tectonic and magmatic activities during the Mesozoic,characterized by numerous Meso-Cenozoic residual basins and residual basins and large-scale outcropping of granites,forming a complex basin-and-ridge geomorphic pattern.Predecessors have carried out a lot of research work and made many achievements and understanding on the evolution and dynamic mechanism of tectono-magmatic since the Mesozoic along the coast of South China.However,the research on the geomorphic evolution along the coast of South China is still very weak.At present,it is not clear whether the Paleo-coastal mountains exist,the uplift process of the Paleo-coastal mountains and the coupling relationship with the deposition of the surrounding basins.In this study,the Cretaceous sedimentary rocks of Yong’an basin in Western Fujian Province are taken as the main research object.Through petrology,detrital zircon U-Pb chronology,geochemistry and neodymium isotope analysis,combined with detrital zircon U-Pb chronology of Mesozoic basins in South China,sedimentary geochemistry and neodymium isotope data of Cenozoic strata in northern South China Sea,the provenance evolution of Yong’an basin and late Mesozoic-Cenozoic basins along the coast of South China are traced,restore the Cretaceous paleoclimate evolution of Yong’an basin and the southeastern margin of South China.Combined with the research results of regional thermochronology,the geomorphic evolution process of the coastal area of South China since the late Mesozoic is reconstructed.Field investigation and microscopic identification show that the maturity of samples from Middle Jurassic to early Cretaceous is higher than that of samples from Late Cretaceous.The identification of clastic components shows that the Zhangping formation in the Middle Jurassic and Bantou formation in the early Cretaceous mainly fall into the area of self cyclization orogenic belt;The Shaxian formation in the late Early Cretaceous and the Chong’an formation in the late Cretaceous mainly fall into the magmatic arc provenance area,and are closer to the area with increased deep components and the arc orogenic belt.Element geochemical data show that the source rocks of the samples come from the medium acid felsic source area,and the characteristics of element ratio of the samples are similar to those of granite.The¹⁴³Nd/¹⁴⁴Nd ratios of the Middle Jurassic-Late Cretaceous strata are between 0.511772and 0.512253,with the correspondingεNd values ranging from-16.9 to-7.5,theεNd value generally shows a gradually increasing trend.The relative probability and distribution statistics ofεNd value indicate the transformation of provenance from Wuyishan area to the coast of South China.The study shows that CIA（69.91～85.29）,Rb/Sr value（1.13～2.21）,Mg/Ca value（0.57～3.57）,Sr/Cu value（5.44～11.66）and corrected paleosalinity index Sr/Ba value（1.18～1.55）of Bantou formation samples in the early Early Cretaceous indicate that the climate in this area changed from warm and humid to dry and hot,and the discrimination index of redox conditionsδCe value（0.89～1.03）,V/Cr（1.03～1.57）and Ni/Co（2.34～10.58）indicate the weak oxidation environment,which indirectly indicates the climate change from warm and humid to dry heat in the early Early Cretaceous.CIA（69.91～75.68）,Rb/Sr value（0.99～9.05）,Mg/Ca value（0.37～16.22）,Sr/Cu value（1.20～15.46）and corrected paleosalinity index Sr/Ba value（0.50～1.77）of Shaxian formation and Chong’an formation samples from late Early Cretaceous to Late Cretaceous indicate that the climate of late Early Cretaceous to Late Cretaceous in this area changed from hot and dry to warm and humid and then dry and cold,and the discrimination index of redox conditionsδCe value（0.89～1.03）,V/Cr（0.51～1.72）and Ni/Co（1.45～6.45）indicate the alternating environment of weak oxidation and weak reduction,which indirectly indicates that the climate in this area changed from dry and hot to warm and humid and then to dry and cold for many times from late Early Cretaceous to late Cretaceous.To sum up,the paleoclimate from Early Cretaceous to Late Cretaceous in Yong’an basin,Western Fujian was transformed from warm and humid to dry and hot to warm and humid to dry and cold（with extreme climate）.Comparing the paleoclimate of the study area with that of the surrounding areas,it is found that the paleoclimate in the interior of South China is mainly a warm and humid to dry and hot climate transition.This difference in paleoclimate change not only reflects the difference in paleoclimate change between the interior of coastal mountains and the interior of South China,but also the response to the geomorphic changes along the coast of South China in the late Mesozoic.TheεNd value of zhangping Formation in middle Jurassic is the lowerest,with an average value of-16.6.Affected by the transformation of tectonic system,the contents of zircon grains from 2000 to 1800Ma in various basins in southeastern south China are significantly increased,and the Wuyi mountain is uplifted and denuded,becoming the most important geomorphic boundary in the southeastern margin of south China.The coastal landform of South China is low,and there is no plateau with a wide range.During the Late Jurassic and the early Cretaceous,under the influence of the subduction and compression of the Pacific plate,theεNd value of the Bantou Formation increased to-14.8～-12.4.The number of young Yanshanian zircons in samples from the Early Cretaceous in most basins of the South China increased to varying degrees,indicating that geomorphic uplift occurred along the coast of South China,and the Mesozoic magmatic rocks were exposed and denuded in large areas,suggesting that the Paleo-coastal mountains in the southeast coast began to develop.In the late Early Cretaceous,the Paleo-Pacific plate experienced instantaneous compression,and the Shaxian Formation in the late Early Cretaceous and the Chong’an Formation in the Late Cretaceous have higherεNd values,ranging from-9.3 to-7.5,with an average of-8.7.The ratios of trace elements such as La/Sc and Th/Sc are synchronized with sudden changes in theεNd value.A large number of late Yanshanian（120～80 Ma）zircons appear in the late Early Cretaceous and late Cretaceous strata in the coastal basins of South China,reflecting the rapid exposure and denudation of granites in large areas along the coast of South China,and the formation and denudation of the southeast coastal mountain.By defining the sedimentary ages of the strata from different provenances,it is believed that the Paleo-coastal mountains were formed at about 110 Ma.According to the geological records of mountain uplift,such as zircon age characteristics,unconformity,molasse structure,and paleoclimate changes,the range of the Paleo-coastal mountains is divided.Compared with the late Cretaceous strata in Yong’an basin,theεNd value of Taiwan’s Cenozoic strata is significantly lower than that of the Late Cretaceous strata in the Yong’an Basin,and shows a slow decreasing trend,whereas the zircon content of 2000～1800 Ma increased,which corresponds to the breakup of the coastal mountains and the expansion of the South China Sea.The Minjiang and other water systems were captured inland,and the ancient basement in South China became the main material source of the Cenozoic strata in the northern margin of the South China Sea and Taiwan.