北京延庆白河堡水库周边地区火成岩地球化学特征

Geochemical Characteristics of Igneous Rock around the Baihebu Reservoir in Yanqing,Beijing

通过野外地质调查与镜下薄片观察发现,北京延庆白河堡水库周边地区火成岩发育,岩性包括基性辉绿岩、中性粗安岩与酸性的流纹岩及火山角砾岩等。岩石地球化学分析显示该地区火成岩碱质含量(Na_(2)O+K_(2)O)介于5.97%~9.87%,K;O含量介于2.21%~5.95%,岩石δ值介于2.01~4.86,属于高钾碱性-钙碱性系列;A/CNK=0.73~1.01,A/NK=1.06~1.77,属于准铝质-弱过铝质岩石,岩石具有中高等的Mg^(#)(0.65~0.78),中高程度的TFeO/MgO比值(3.05~4.59)。微量元素表现为Ba、Pb富集,Nb、Ta亏损,Y轻度亏损的特征;稀土元素表现出较高的REE含量(208.18~318.46μg/g),分配模式表现为右倾平滑型,LREE相对富集,Eu轻度亏损,δEu在0.46~0.92之间,Eu负异常。轻稀土分馏较强,(La/Yb)_(N)=10.55~26.94,重稀土分馏较弱,(Dy/Yb)_(N)=1.1~1.63。地球化学的特征表明白河堡水库周边早期中酸性的火成岩形成伸展背景下类似于岛弧的环境,而晚期基性岩的形成则可能与西太平洋板块西北向运动有关。

Diabase,trachyandesite,dacite liparite and volcanic breccia are exposed around the Baihebu reservoir in Yanqing,Beijing.They contain 5.97%-9.87%(Na_(2)O+K_(2)O)and 2.21%-5.95%K_(2)O withδvalues of 2.01-4.86,A/CNK of 0.73-1.01 and A/NK of 1.06-1.77,belonging to quasi-aluminous-weak peraluminous rock of high potassium alkaline calc alkaline series.MgO content varies from 0.65 to 0.78.TFeO/MgO ratio ranges from 3.05 to 4.59.Trace elements are characterized by enrichment in Ba and Pb and depletion in Nb and Ta,and slight depletion in Y.The REE geochemistry is characterized byΣREE of 208.18-318.46 ppm,enrichment in LREE,slight depletion in Eu withδEu values of 0.46-0.92,(La/Yb)_(N)of 10.55-26.94 and(Dy/Yb)_(N)of 1.1-1.63.The geochemical characteristics indicate that the early intermediate-acid igneous rocks were derived from an extensional setting similar to the island arc environment,while the late basic rocks may be related to the northwest movement of the Western Pacific plate.