Geochemical Characteristics Of Basalts From The Daimao Seamount In The South China Sea (SCS) And From The SCS’s Neighboring Lands:Implications For The Regional Tectonic Evolution

Daimao seamount is located in the northern region of the South China Sea (SCS) basin. The eruption of the Daimao seamount (16.7-14.4Ma) and the ending of the SCS spreading (-15.5 Ma) were contemporary. In addition, one of basaltic samples from Daimao Seamount analyzed in this study is the MORB-like basalt, which is the only MORB-like sample among all the SCS seamount basalts collected so far and therefore is of the great importance in terms of its petrological and geochemical characteristics.Based on the records of volcanic eruptions, two important tectonic-magmatic events have occurred in the area of the SCS basin and its neighboring lands:one was in Sanshui basin at the end of the Paleocene to the late Eocene (60-～40 Ma) and the other was in Leiqiong area and Kontum-Khorat plateau, Vietnam after the middle Miocene (<15 Ma). Based on the previously studies, those two tectonic-magmatic events are both show characteristics of hot spot or mantle plume. In order to understand the general features of each phase during the SCS evolution, in this study, the basalts (38-43 Ma) from Sanshui Basin on the north continental margin of the SCS have been collected to represent the magmatism of the pre-spreading eruption, the basalts (16.7-14.4 Ma) from Daimao seamount in the SCS basin have been collected to represent the magmatism of the ending of the syn-spreading eruption, and the volcanic rocks (<8Ma) from Leiqiong area on the north continental margin of the SCS and from kontum plateau on the western continental margin of the SCS have been collected to represent the magmatism of the post-spreading eruption. In addition, a high-precision analytical method based on the LA-ICP-MS technique has been applied in this study to obtain high-precision data of the abundances of 70 elements in the samples, including elements with the concentrations of ppb level, such as Mo, Cd, Sn, Tl, In, Sb and W. The motivation of this study is to understand the tectonic evolution processes of the SCS based on the magmatism and mantle deep processes constrained by the geochemical characteristics of the volcanic rock samples.Besides one Daimao MORB-like samples, all the other basaltic samples analyzed in this study are OIB-like that are derived from a DM and EM2 mixing mantle source. The DM endmember is similar to the depleted MORB mantle while the EM2 endmember was formed due to the recycling of the subducted MORB slab. Pressure and potential temperature for mantle partial melting beneath the SCS basin and its surroundings are both lower than them of Hawaiian or Iceland mantle regions that are related to the deep-rooted mantle plume. More importantly, the pressure and potential temperature (1.5 Gpa,1450℃-1500℃) of mantle partial melting depth of Daimao seamount are even lower than them (3Gpa,1450℃-1500℃) of volcanic rocks from the neighboring terrestrial areas of the SCS. In addition, the siderophile and chalcophile elements in samples are depleted relative to them in typical deep-rooted OIBs, confirming that the EM2 mantle endmember of the studied area was not derived from the deep mantle or the core-mantle boundary, but derived from a shallower mantle thermal boundary, such as a sub-lithospheric mantle source.This study shows that the siderophile and chalcophile elements are powerful tools in the research of the tectonic evolution of marginal seas. Based on the deep processes constrained by the geochemistry of volcanic samples analyzed in this study, anew model of the tectonic evolution of the SCS has be proposed as follows:between Mesozoic and Cenozoic, the asthenosphere of some studied area has been uplifted extensively due to the large-scale thinning and horizontal displacement of South China continental lithosphere, which has led to the widespread volcanic eruption in Sanshui basin. The volcanic eruption in Sanshui basin was similar to volcanism of continental rift and was derived from a shallow mantle source, which could have related to the formation of the SCS. However, this asthenospheric mantle upwelling has then been suppressed by later intense movement of continental blocks induced by the collision of Indo-Asian plates and thus the magmatism related to this tectonic extension should be responsible to the opening of the SCS and should have contributed to the formation of Daimao seamount in the SCS basin. Ever since the SCS spreading stopped, the asthenospheric mantle upwelling in Sanshui basin has recovered in Leiqiong area and Kontum plateau, probably due to the change of reginal boundary conditions.