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Late Miocene magmatism and evolution of Zhenbei-Huangyan Seamount in the South China Sea: evidence from petrochemistry and chronology

WANG Ye-jian1,2,HAN Xi-qiu1,LUO Zhao-hua3,QIU Zhong-yan1,2,DING Wei-wei1,LI Jia-biao1,GAO Shui-tu1,CHEN Rong-hua1(1.Key Laboratory of Submarine Geosciences of State Oceanic Administration,Second Institute of Oceanography,State Oceanic Administration,Hangzhou 310012,China;2.Depatment of Earth Sciences,College of Science,Zhejiang University,Hangzhou 310027,China;3.College of Earth Sciences and Resources,China University of Geosciences,Beijing 100083,China)  
E-W trending Zhenbei-Huangyan Seamount at about 15°N in the South China Sea Basin was considered to be ancient spreading axis.Two samples(9DG,9DG-2) were dredged from Huangyan Seamount in 2005 and detail petrological,geochemical and chronological analyses were conducted.The samples are characterized by high contents of silica,aluminium,potassium,sodium and the low content of titanium,with contents of 60.3% and 63.6% for SiO2,17.56% and 17.55% for Al2O3,0.48% and 0.31% for TiO2,and the alkalinity ratios and 3.88 and 3.62.Based on petrological and petrochemical classification,the two samples are trachyte and belong to the alkaline series.Their REE and trace element distribution patterns are similar to oceanic island basalt(OIB),but with higher LREE content and lower contents of europium,srtontium,phosphorus and titanium.Strontium-neodymium-lead isotope analysis shows that the content ratioes 87Sr/86Sr=0.704 183,206Pb/204Pb=18.686 68,207Pb/204Pb=39.002 61,and 143Nd/144Nd=0.512 827,respectlvely,which indicates that magmas were derived from the mixture of DM(depleted mantle) and EMⅡ(enriched mantle Ⅱ).According to potassium-argon dating,the samples were formed(7.77±0.49)Ma ago,a bit later than the basalts [(9.1±1.29)~(10.0±1.80)Ma] from Zhenbei Seamount.Compared with geochemical and isotopic features of the basalts from Zhenbei Seamount,it is concluded that trachyte from Huangyan Seamount was originated from the same mantle source of basalts from Zhenbei Seamount,but formed at the different evolution stages.It is suggested that during the late Miocene,there was mantle plume activity probably or athenosphere mantle upwelling along Zhengbei-Huangyan Seamount.
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