Characteristic Of Continental Rift Volcanic Acitviites In Sanshui Basin And Early Evolution Of South China Sea

Sanshui Basin is one of the nearest Cenozoic Basins to South China Sea(SCS) in Southeast China and massive volcanic activities occurred in Sanshui Basin. Based on former research,13eruptional cycles was divided from Baihedong formation in early Cretaceous to Huayong formation in Eocene. With the most powerful eruption and thickest sediment, volcanic rocks in early Cenozoic were widely believed to be erupted in continental rift environment and may have some connection with break up of SCS in Oligocene. In this paper, trachyte, basalt and rhyolite samples in Zoumaying, Libianshan, Zidong and Wangjiegang were collected to be analyzed by Petrology, Geochemistry and Geochronology, and analysis will be helpful to revealing Characteristic of volcanic system of Sanshui Basin in Eocene and its connection with early evolution of SCS.Like Red Sea, All kinds of volcanic rocks from Sanshui Basin in Eocene performed as intracontinental and plutonic pattern, which means breaking up of Southeast China is the result of mantle upwelling. Trachyte erupted in early Eocene played an important role in volcanic evolution system in Sanshui Basin. It performed enrichment in some HFSE(Nb, Ta, Zr, Hf et al.,) and LILE and negative anomaly in Sr, Ba, Eu, P, Ti. With higher Nd isotope ratio(0.51279～0.51293), lower Sr isotope ratio(0.70461～0.71088) and similar geochemical contents, Sanshui trachyte performed similar petrological characteristic and different proportion. Base on chronology research, WJG and ZD basalt are separated which are used to be considered formed in same erupted cycle. WJG basalt has more olivine phenocrysts and lower Si content which erupted in mid-Eocene(about53Ma), while ZD one was formed in late-Eocene(about36Ma). WJG basalt is attributed to per-alkali rock which is generally erupted in peak time while ZD one belongs to calc-alkali rock erupted in later stage.Conclusions are draw as follow:(1) ZMY trachyte, WJG basalt, ZD basalt and LBS rhyolite represent Eocene volcanic rocks in Sanshui Basin which are erupted in continental rift system. Magma of these rocks are coming from two sources of DM and EMII in lower part of asthenospheic mantle, which indicate that mantle upwelling effect in early stage of SCS breaking up.(2) Trachyte is typical volcanic rock in Sanshui Basin which formed by fractional crystallization.(3) Variation of geochemical data of basalts in Sanshui Basin corresponded with the magma evolution in continental rift system. Alkalinity disappearance in Tholeiite in last eruptional cycle represent syn-rift stage which may leads to the breaking up of SCS.(4) Tectonic background of early SCS evolution performed as Red Sea pattern, but inhibition affect of volcanic activity in later Eocene(41-36Ma) indicate suspendent of magma supply, which may be the result of tectonic squeezing action from adjacent area.