The Basic Characteristics Of Cretaceous Andesites In The Northern Margin Of South China Sea And Its Comparison With The Andesites Of Southeast China

The South China Sea has long been a research focus of many researchers at home and abroad because of its complex geological evolution in history. Since Mesozoic it experienced the close of proto-south china sea and the cracking of modern South China Sea. Before cracking, the SCS region is a subducted and orogenic process, rather than a stable craton. In this paper, we select the cretaceous andesites in the northern margin of SCS as the main research object, carry on the detailed analysis of the petrology, geochemistry and chronology, then compare with the southeast coastal andesites, aim to explore the background and evolution process of the SCS region. After the research, conclusions are draw as follow:Typical andesites can be found in the Danxia basin, Lianping basin, Yulin of Gaungxi province, Nha trang of Vietnam and Hainan island. The andesites are both porphyritic texture, with more pyroxene and less plagioclase in Vietnam and Hainan island, while with more big plagioclase and less pyroxene in Danxia and Lianping basin. Along with the extension to inland, the acid and K2O/TiO2 ratio increases gradually, and the crust has more influence. In Zhejiang province, the andesites are also porphyritic texture but with more potassium feldspar and have higher alkalinity.The andesite’s zircon U-Pb age of Danxia basin is 143.9Ma, and 93.37Ma in Yulin of Guangxi province; the trachyte’s zircon U-Pb age of Maoming basin is 105.2Ma and 87Ma in Vietnam. The volcanic eruptions of northern margin of the SCS began in the early cretaceous, and continued until late cretaceous, while in Zhejiang region are only focused in early cretaceous.The andesite and trachyte of northern margin of the SCS both have typical characteristics of volcanic rock in subduction zone. The samples are enriched in K、Sr、 LREE, depleted in Nd、Ta, and have high Al low Ti. The andesites prefer to erupt in Andean arc. So the northern margin of the SCS was under the tectonic setting of subduction from late Mesozoic till late cretaceous and formed a broad Andean arc.Compared with the samples of the SCS, the andesites in Zhejiang region have higher alkalinity, lower ∑REE and positive Eu anomaly. They are enriched in P、Ti, and depleted in Th, U which is opposite with the SCS. The andesites of Zhejiang region are product of magma mingling, its geochemical properties are controlled by the basic and acidic magmatic rock. The andesites in northern margin of the SCS have different petro genes is compared with Zhejiang region. Therefore, the subduction zone in northern of the SCS may be resulted by the proto-south china sea rather then the western Pacific.