华北陆块北部燕山地区中元古代辉绿岩地球化学特征及成因意义

华北陆块北部燕山地区广泛发育侵位于下马岭组的一套辉绿岩岩床。其岩石地球化学特征表明,燕山地区辉绿岩以高TiO2(1.70%~4.29%),低Al2O3(11.33%~14.98%)和低MgO(4.17%~5.86%),以及Na2OK2O为特征,属大陆拉斑玄武岩系列。稀土元素特征显示,∑REE总量较高,LREE明显富集,轻重稀土分馏不明显;配分曲线右倾,无明显Eu异常。结合Zr-Y、Zr-Nd、Zr/Y-Zr、Ti/100-Zr-Y×3、Ta/Hf-Th/Hf及Nb/Zr-Th/Zr等图解综合分析认为:该辉绿岩岩浆源自过渡性地幔,形成于华北陆块北部中元古代晚期的板内伸展构造背景。

Geochemistry of the Mesoproterozoic diabase sills from Yanshan Mountain area in the northern part of North China Plate

The intrusion of diabase sills in the shales of the Xiamaling Formation is extensively developed in Yanshan Mountain area of the northern part of North China Plate. Geochemistry of the diabase is characterized by high content of TiO2(1.70%~4.29%), low content of Al2O3(11.33% ~14.98% ), MgO(4.17% ~5.86% )and Na2O>K2O. In the K2O- TiO2- P2O5 plot, all sample points almost fall into the field of continental tholeiite. REE study indicates that the diabase is enriched in light rare earth elements with abundant rare earth elements, and the fractionation between LREE and HREE is not pronounced. Meanwhile, the chondrite- normalized distribution curve is right-oblique without obvious depletion of Eu. All these characteristics in combination with the Zr-Y, Zr-Nd, Zr/Y- Zr, Ti/100- Zr- Y × 3, Ta/Hf- Th/Hf and Nd/Zr- Th/Zr diagrams indicate that the magma was derived from the transitional mantle, and the diabase was formed in an intra-continent extension setting of North China Plate during the late Mesoproterozoic.