Approachto Magmatic And Tectonic Activities In Post-spreading Stage Of The South China Sea By Analyzing Relics From Its Northern And Western Margins

With the cessation of seafloor spreading of the South China Sea(SCS),the large-scale magmatic activity in the surrounding land area,especially the SCS's northwest margin,is a major regional feature.The tectonic significance and deep process it represents are worthy of attention.The Late Cenozoic basalts exposed in the Leizhou Peninsula,the North Hainan Island and the Kon Tum area in Vietnam are studied in this thesis.Based on the lithology and geochemical work,combined with the distribution characteristics and activity regularity of the regional faults,new understanding on regional geological process after the cessation of seafloor spreading of SCS in Middle Miocene(~16Ma)were proposed.Eruption age and characters of basalts from the Lei-Qiong area and the Kom Tum area were similar to each other,enriched with LILE,HFSE and LREE,undoubtedly the developedby intraplate magmatism.Parental magma compositions are calculated used samples with high Mg#,mantle potential temperature are calculated by Mg O of parental magma compositions,which is 1552.02 and 154? 6.70 in Leiqiong and Vietnam,respectively,it may reve? al both region affected by the same plume;Fo of olivine in sample HK005-7 is 90.40,indicating there is a deep source,which is used to calculate the mantle potential temperature by olivine-liquid equilibria thermometry.Compare with mantle potential temperature of MORB,the mantle excess temperature is 150~300 in Leiqiong and ?Vietnam,consistent with typical hotspots like Hawaii and Iceland.Combined with geophysical evidence,we argue that the existence of Hainan Mantle Plume is reasonable.The continental margin fault structure in the northwestern margin of the South China Sea is very developed.After the Midddle Miocene,the SCS ceased to expand,and the normal fault instead of the strike-slip fault became the main element of the regional fault system.In Vietnam,the fault is mainly N-S to the distribution,and in the Lei Qiong area was NW-SE to the spread,they are the main channel of magma after mantle uplift in both areas.The typical active continental rift system,which is composed of the magmatic-tectonic activity shown in the study area of the northwest margin of SCS,is not associated with the expansion of the SCS in the order of the times.