关于南海中央次海盆海山火山岩形成背景与构造约束的再认识

A Further Discussion on Formation Background and Tectonic Constraints of Igneous Rocks in Central Sub-Basin of the South China Sea

文章拟通过分析南海中央次海盆海山火山岩样品的岩石学及地球化学资料来审视研究区的岩浆过程及其构造背景.综合位于南海中央次海盆的中南、珍贝-黄岩、涨中、宪北、玳瑁、尖峰这6条海山链的火山岩资料及最新分析结果,将来自各条海山链的洋岛型碱性玄武岩、具有过渡性质的拉斑玄武岩,以及粗面岩、粗面安山岩、流纹英安岩等纳入统一的岩浆分异体系进行地球化学特征检验.相关参数表明,中央次海盆海山火山岩岩浆具有连续的演化关系但源区性质并不均一,属于EMⅠ和DMM双端员混合.根据分析样品的Mg#值、分异指数DI以及主、微量元素的分布特征,初步判定原始岩浆在上地幔经历了橄榄石、辉石分离结晶作用后继续演化,产生包括拉斑玄武岩、碱性玄武岩、粗面岩等在内的岩石组合.其中,富集型OIB的多项地化参数特征表明,岩浆在演化过程中似有陆壳成分的加入,这可能是被动大陆边缘破裂留给南海海盆的最重要的遗产之一.

Based on analysis of petrological and geochemical information of igneous rocks sampled from central sub-basin of the South China Sea,this article attempts to address the magmatic processes and tectonic background of the research area.By renewing the available datas and combining them with latest test results of igneous rock samples collected from 6 seamount chains in the central sub-basin of South China Sea（SCS）,we analyze the alkali basalt,transitional tholeiite,trachyte,trachyandesite and rhyodacite under a unified magmatic differentiation system in this paper.The geochemical indices show that the evolutionary relationship of these igneous rocks was relatively consistent and the regional mantle source was heterogeneous,consisting of two end-members： EMⅠ and DMM.According to the Mg#,differentiation index（DI）,attributes of major and minor elements,we conclude that subsequent to the fractional crystallization of olivine and pyroxene in upper mantle,the primitive magma continues its evolution to bring about this rock association.Furthermore,geological datas also indicate that during the process of magma evolving,different degrees of contamination occurred in different stages of sea-floor expansion,which may be one of the most important heritage left by the rupture of passive continental margin