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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32~17 Ma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0.704 183,钕-143的含量与钕-144的含量之比值为0.512 827,铅-206的含量与铅-207的含量之比值为18.686 68,铅-207的含量与铅-204的含量之值为15.679 62,铅-208的含量与铅-204的含量之比值为39.002 61,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±1.80)Ma],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。
The Zengbei-Huangyan seamounts that are distributed east-west near the 15 ° N basin of the South China Sea are considered as the residual expansion centers of the former South China Sea basin from 32 Ma to 17 Ma. Petrological, geochemical and geochronological studies of two volcanic rocks collected from Huangyan seamounts (9DG, 9DG-2) were performed. The SiO2 contents of the two samples were 60.3% and 63.6% respectively, the contents of Al2O3 were 17.56% and 17.55%, the contents of TiO2 were 0.48% and 0.31% respectively, and the alkalinity were 3.88 and 3.62 respectively. Based on the petrology and petrochemical classification, the samples are alkaline series of facies. The analyzes of REE and trace elements show that the rocks are characterized by OIB type, LREE / Σc (HREE) and chondrite standard La / Yb) N) were as high as 17.22 and 27.23, respectively, with the negative anomalies of europium and the depletion of strontium, phosphorus and titanium. The results of the strontium-neodymium-lead isotope analysis of sample 9DG show that the ratio of strontium-87 content to strontium-86 content is 0.704183, the ratio of neodymium-143 content to neodymium-144 content is 0.512 827, The content of lead-207 is 18.686 68, the content of lead-207 and lead-204 is 15.679 62, the ratio of lead-208 to lead-204 is 39.002 61, indicating that the initial magma From the asthenospheric mantle, it has a similar isotopic composition to that of the Beibei seamount basalts. According to the K-Ar dating, the age of the trachyte was (7.77 ± 0.49) Ma, slightly later than that of the Jibei seamount basalt [(9.1 ± 1.29) ~ (10.0 ± 1.80) Ma] Late Miocene volcanic activity of the product. Comparing the geochemistry and isotopic characteristics of the Chabei seamounts basalts, it is considered that they both have the same magma source areas, but they have undergone different degrees of crystallization differentiation. During the Late Miocene, the Zabei-Huangyan seamounts may have mantle plume activity .