The Characteristics Of The Cretaceous In The South Hainan And Its Significance To The Tectonics Of The North Margin Of The Proto-South China Sea

South China Sea is located in the junction of the Eurasian plate, the Pacific plateand the Indian plate. It belongs to transition type between the continental crustaltectonic domain and tectonic oceanic crust domain.It is the key component of theSouth China Sea system and the southernmost tip of Hainan Island in southernChina.Since the Mesozoic geological tectonic evolution has a complex history. Thismakes it the key regions to solve the tectonic evolution of the south China continentaland the northern part of south China sea.The Mesozoic is the critical period to the tectonic evolution of Hainan. Theexistence of continental margin arc and the subduction and extrusion process of thenorthern part of south China sea has been highly controversial.The Hainan IslandCretaceous strata are widely distributed. The same time, the research of usesedimentary rock-forming elements to study the tectonic evolution of the sourceregion for discriminant rock background continue to mature. This provides usimportant means to analyze the nature of the continental margin late Mesozoic and theexistence of continental margin arc.Hainan cretaceous is traditionally divided into lower cretaceous LumuwanFormation and upper cretaceous Baowan Formation, there are also volcanic series ofearly cretaceous with Lumuwan Formation at the same time. This paper mainlythrough the geochemical study of typical clastic-mudstone sequences which exposedin the Shanrong town South Hainan to determine the tectonic environment.The paperachieved the following understanding:1） The Major and trace element geochemistry studies have shown that,A12O3/SiO2(0.15～0.22) ratio in the study area is similar to the sandstones depositingin a continental island arc setting. In the major element diagram, the sandstonesamples of Baowan Formation Late cretaceous and Lumuwan Formation Uppercretaceous are mainly concentrated in the active continental margin.As for the traceelement geochemical research, the distribution of the sandstone samples of BaowanFormation and Lumuwan Formation in the south Hainan are mainly distributed in thecontinental island arc and active continental margin area.2）As for the rare earth elements, the sandstone samples of late Cretaceous in theSouth Hainan is characterized by the higher abundance and enrichment of LREE，and higher negative Eu anomaly．The differentiation of light rare earth is more apparentwith heavy rare earth. Especially the chondrite—normalized REE patterns indicatethat they are accordant with that of the CIA and the ACM sandstones．3）The Major and trace element covariant relation diagrams of Cretaceoussandstones in South Hainan have shown that the study area is continental island arc oractive continental margin type. As for the rare earth elements, ΣREE and Ce content,δEu, LaN/YbNthese important discriminant parameter values andchondrite-normalized curves are consistent indicators of the sandstone in the studyarea is continental island arc-active continental margin sandstone characteristics.This shows that Cretaceous sandstones has the characteristics of continental back-arcdeposition which associated with the subduction zone.the South Hainan in LateCretaceous should exist a continental margin arc related to compression-subduction.