南海中部岩浆岩地球化学特征及其地质意义.docVIP

南海中部岩浆岩地球化学特征及其地质意义 陈荣华, 高水土, 范维佳, 韩喜球 （国家海洋局第二海洋研究所，310012，杭州. E-mail: chrh@mail.hz.zj.cn） 摘要：对在南海中部两个站位拖网采得的玄武岩和粗面岩样品和分别进行了全岩主量元素、痕量元素和稀土元素分析以及Sr、Nd、Pb同位素分析。分析结果显示，玄武岩属于压碱性系列，痕量元素含量平均约为原始地幔的10倍左右，其中有较明显的Ti谷和K峰，Cs、Rb、Ba含量较高；粗面岩属于碱性系列，痕量元素丰度普遍较高，可以达到原始地幔丰度的200倍以上。两块岩浆岩从同位素示踪的角度来说应当与地幔岩的部分熔融有关，但从主元素的角度来说，玄武岩是地幔岩部分熔融的产物，粗面岩则可能是下地壳部分熔融的产物，但源区有沉积物的贡献，这从痕量元素方面也可以得到证实，例如含有较高的Ba含量。二者的结合使我们得出了这样一个推测：火山岩形成于大陆边缘初始裂谷环境，裂谷的成因与大洋板片的俯冲有关，区域上中国东部岩石圈巨大减薄始于晚白垩世，而粗面岩形成时应当具有较厚的陆壳，因而岩浆活动很可能发生在晚白垩世或新生代之初，这比南海打开的时间略早，符合区域地质演化规律。 关键词：南海 玄武岩 粗面岩 同位素示踪 The geochemical features and geological significances of the igneous rocks from the central South China Sea Chen Ronghua, Gao Shuitu, Fan Weijia, Han Xiqiu The Second Institute of Oceanography, State Oceanic Administration (P.O. Box:1207, Hangzhou, 310012. E-mail: chrh@mail.hz.zj.cn) Abstract: Major elements, trace elements, REE and Sr, Nd, Pb isotopes of two kinds of rocks, basalt and trachyte, were analyzed, which were sampled by dragnet from two stations in the central South China Sea. As a result, the basalt is sub-alkaline, while the trachyte belongs to alkaline. The average content of trace elements in basalt is ten times richer than the primary magma compositions, with obvious Ti-gap, K-peak and high contents for Rb and Ba. While the trace elements content in the trachyte can be up to over 200 times richer than that. To interpret from isotope tracing, the two igneous rocks ought to be related to the pyrolite partial melting, however, but to explain from the major elements, the basalt is the product of pyrolite partial melting while the trachyte may come of the partial melting of lower crust and the sediment in the origin area, which also can be concluded from trace elements, for example, the content of Ba is high enough. Combining the two evidences, we speculate that the basalt formed in the environment of continental margin primary rift valley, which was induced by the subduction of marine plate, however, the continenta