晚中新世南海珍贝-黄岩海山岩浆活动及其演化:岩石地球化学和年代学证据
Late Miocene magmatism and evolution of Zhenbei-Huangyan Seamount in the South China Sea: evidence from petrochemistry and chronology
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摘要: 南海海盆15°N附近呈东西向展布的珍贝-黄岩海山被认为是32~17Ma前南海海盆的残留扩张中心.对采自黄岩海山的两个火山岩样品(9DG,9DG-2)进行了岩石学、地球化学和年代学研究.两个样品的SiO2含量分别为60.3%和63.6%,Al2O3含量分别为17.56%和17.55%,TiO2含量分别为0.48%和0.31%,碱度率分别为3.88和3.62.根据岩石学和岩石化学分类,样品属碱性系列的粗面岩.对稀土元素和微量元素分析表明岩石具有洋岛玄武岩(OIB)型配分型式,轻重稀土总量比(∑c(LREE)/∑c(HREE))和球粒陨石标准化镧镱比((La/Yb)N)分别高达17.22和27.23,并具有铕负异常和锶、磷、钛亏损的特点.样品9DG的锶-钕-铅同位素分析结果为锶-87的含量与锶-86的含量之比值为0.704183,钕-143的含量与钕-144的含量之比值为0.512827,铅-206的含量与铅-207的含量之比值为18.68668,铅-207的含量与铅-204的含量之值为15.67962,铅-208的含量与铅-204的含量之比值为39.00261,表明初始岩浆来自软流圈地幔,具有与珍贝海山玄武岩相似的同位素组成.经钾-氩法测年,粗面岩的年龄为(7.77±0.49)Ma,略晚于珍贝海山玄武岩的年龄[(9.1±1.29)~10.0±1.80Ma],属于南海扩张期后晚中新世火山活动的产物.对比珍贝海山玄武岩的地球化学和同位素特征,认为两者有相同的岩浆源区,但是它们经历了不同程度的结晶分异过程,在晚中新世期间珍贝-黄岩海山可能有地幔柱活动.Abstract: 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 content 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-leadisotope analysis shows that the cotentratioes 87Sr/86Sr=0.704183,206Pb/204MPb=18.68668,207Pb/204Pb=39.00261,and 143Nd/144 Nd=0.512827,respectlvely,which indicates that magm as were derived from the mixture of DM (depleted mantle) and EM Ⅱ(enriched mant leⅡ).According to potassiumarg on 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-Huang yan Seamount.
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Key words:
- trachyte /
- geochemistry /
- potassium-arg on dating /
- Zhenbei Seamount /
- Huangyan Seamount /
- South China Sea Basin
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