摘要
采用主元素、微量元素和Sr-Nd-Pb同位素地球化学研究手段和分析方法,阐明了阳信地区古近纪玄武岩地球化学和源区特征,揭示了渤海湾盆地形成的地球动力学背景。研究认为,阳信地区古近纪玄武岩为钾质碱性橄榄玄武岩;呈轻稀土富集型分布模式,无Eu异常,稀土总量较低,始新世玄武岩∑REE=97.8～115μg/g,渐新世玄武岩∑REE=143～147μg/g,并且渐新世比始新世玄武岩的轻稀土(LREE)更为富集;大离子亲石元素富集,蛛网图显示负斜率较大的右倾曲线,始新世玄武岩具有Ba、K、Sr元素富集和U-Th、Ta、Hf亏损的特征,并显示出不完全类似于OIB型的分布模式,但渐新世玄武岩则具有不同程度Ba、Nb-Ta、Sr元素富集和U-Th、Hf元素亏损的特征,蛛网图具有典型的OIB型分布模式;Sr-Nd-Pb同位素组成较为均一,表明阳信地区古近纪玄武岩源区主要来自对流软流圈地幔,但始新世玄武岩还遭受了一定程度的EMⅠ和EMⅡ富集地幔的混染。综合分析区域地质和地球物理研究成果认为,阳信地区古近纪玄武岩产出于板内构造环境,属于板内碱性玄武岩。始新世太平洋板块以俯冲作用为主,诱发了区域近南北向伸展构造事件,并形成具有EMⅠ富集地幔混染特征的玄武岩;渐新世太平洋板块以俯冲后撤作用为主,致使NW-SE向陆内扩张而形成一系列大型陆内伸展盆地,以及来源于软流圈地幔的玄武岩。
Whole rock analyses including major elements, trace and REE elements and Sr-Nd-Pb isotopic compositions were conducted to illustrate geochemical characteristics and magma source signature of Eogene basalts in Yangxin area and reveal the original geodynamic background of Bohai bay basin. The results show that Eogene basalts in Yangxin area are potassic alkaline olivine basalt and show a LREE-enriehed pattern, no Eu anomaly and low ∑REE with an range of 97.8- 115μg/g and 143 -147 μg/g for Eocene basalts and Oligocene basalts, respectively, and Oligocene basalts are more highly enriched in LREE than Eocene basalts. Eogene basalts in Yangxin area are enriched in large ion lithophile elements （LILE）, but display different patterns from Oligocene basalts. The spider diagram of Eocene basalts shows a pattern a little different from OIB, and shows an enrichment in Ba, K and Sr and a depletion in U-Th, Ta and Hf; but the spider diagram of Oligocene basalts shows a typical OIB pattern and is more or less enriched in Ba, Nb-Ta and Sr and depleted in U-Th and Hr. The relatively homogeneous Sr-Nd-Pb isotopic compositions of the two kinds of basalts indicate that Eogene basalts in Yangxin area were derived from convective asthenosphere mantle, but Eocene basalts experienced, in some extent, the contamination by EM Ⅰ and EMⅡ enriched mantle. Having comprehensively analysed achievements in regional geological and geophysical studies, it is concluded that Eogene basalts in Yangxin area were derived from intraplate tectonic setting and can be attributed to intraplate alkali-basalt. The subduction of Pacific Plate during Eocene induced the S-N extension event and basalts with EM Ⅰ and EMⅡ enriehed mantle eontamination were formed.Controlled by the roll-back subduction belt of Pacific Plate during Oligocene, the NW-SE direction intracontinental spreading happened and a series of large intracontinental extensional basins and basalts derived from asthenosphere mantle were developed in this area.
出处
《地球化学》
CAS
CSCD
北大核心
2006年第3期227-239,共13页
Geochimica
基金
中国科学院知识创新工程重要方向项目(KZCX2-SW-117)
