渤海湾盆地南堡凹陷新生代火山岩地球化学特征及成因

Petrogenesis and Geochemical Characteristics of Volcanic Rocks from Nanpu Sag of Bohai Bay Basin,East China

南堡凹陷位于渤海湾盆地北部,广泛发育新生代火山岩,主要为碱性玄武岩。这套火山岩分别属于3个裂陷旋回,即沙三期、沙一至东二期、东一至馆陶期。岩石的SiO2含量40.24%～51.55%,TiO2较高（1.39%～3.81%）,Mg^#变化较大（0.38～0.69）。研究表明,这套岩石经历了一定程度的单斜辉石的分离结晶作用,岩石富集轻稀土和不相容元素;（^87Sr/^86Sr）i值和εNd（t）分别为0.7036～0.7082与0.657～6.830;微量元素及Sr、Nd、Pb同位素特征排除了幔源岩浆在喷发到地表过程中受到地壳物质显著混染的可能性。^87Sr/^86Sr、^143Nd/^144Nd、^206Pb/^204Pb的相关性表明,本区玄武岩浆可能主要是DMM和EMⅠ地幔端员组分的混合,但也受EMⅡ端员的影响。它们属于典型的板内碱性玄武岩,其产出的构造环境为板内大陆裂谷环境。

The Nanpu Oilfield is located in the NW part of the Bohai Bay Basin,in east China.Petrological and geochemical studies of drill core samples indicate that the basalts belong to the alkaline series.Their SiO2 contents are 40.24%～51.55%,high in TiO2（1.39%～3.81%） and variable Mg^#（0.38～0.69）.Some basaltic rocks experienced fractional crystallization,dominated by clinopyroxene.All samples examined are enriched in high field strength elements and have trace elements patterns similar to oceanic island basalts（OIB）.High ratios of La/Yb suggest that significant fractionation of rare earth element occur.The Sr and Nd isotopic system shows that（^87Sr/^86Sr）i range from 0.703 6 to 0.708 2 and εNd（t）varies from 0.657 to 6.830.These geochemcal and isotopic characteristics indicate that they are similar to oceanic island basalts and from a within-plate setting.These basalts are formed by low degrees of partial melting from a hybrid mantle of EMⅠand DMM types,with no significant crustal assimilation.However,the contribution from the EMⅡcan not be precluded.