冀中坳陷火成岩岩石学及地球化学特征

Petrologic and Geochemical Characteristics of Igneous Rock in Jizhong Depression

冀中坳陷发育大量隐伏玄武质岩石,尤其是坳陷北部的廊固凹陷和霸县凹陷,岩石类型以玄武岩为主,辉绿岩次之,凝灰岩分布较少,火山角砾岩极少。通过岩心、镜下薄片及铸体薄片观察,主量和微量元素测试分析,表明该区火成岩为碱性玄武岩和拉斑玄武岩系列;REE配分曲线呈轻稀土元素富集型,Eu异常不明显;火成岩微量元素原始地幔标准化曲线呈强不相容元素富集。该区玄武岩的喷发环境为水下喷发;火成岩微量元素构造环境判别图解、岩石组合、岩石系列、稀土元素和微量元素特征表明该区火成岩形成于板内大陆裂谷构造环境。通过冀中坳陷玄武岩与邻区对比发现,冀中坳陷及邻区玄武岩岩浆活动具有随时代变化往北迁移的趋势,SiO2含量显示从南往北略为增加的趋势,北部稀土元素总量变化范围相对较大,轻稀土元素富集程度较南部低。

Buried basaltia rocks are well developed in Jizhong Depression, especially in Langgu Sag and Baxian Sag, locating in the north of Jizhong Depression. The igneous rock types of Jizhong Depression are in the amount order of basalts, diabases, tufts and volcanic breccias. This paper through observing drilled cores, casting sections and thin sections by microscope, testing and analysing major element and trace element, expounds petrology and geochemistry of the igneous rock in Jizhong Depression. This paper demonstrates that the igneous rock in this district is alkaline basalt and tholeiitic series. Chondrite-normalized REE pattern of the igneous rock displays LREE enriched pattern. Weakly Eu negative anomalies and prim- itive mantle-normalized trace element spidergram of the igneous rock present high incompatible elements enriching. It points out the eruptive environment of basalts is subaquatic eruption. Synthesizing the trace element tectonic setting discrimination diagram, rock association, rock series, REE and the trace element feature of the igneous rock, it proposes that the formation environment of igneous rock could be within plate continental rift. Through the comparison of basalts in Jizhong Depression and its neighbourhood, basaltic magmatic activity presents migrating trends from south to north with the time alternation. The content of SiO2 displays a little increase from south to north, ∑ REE varying relatively larger and LREE enric- hing level being lower in the north than south.