北京西山地区髫髻山组和东岭台组火山岩的地球化学特征与岩浆起源

Geochemical Characteristics and Origin of the Tiaojishan and Donglingtai Formations from Western Hills, Beijing

北京西山地区分布着大量的髫髻山组火山岩,区域上髫髻山组被东岭台组角度不整合覆盖。西山地区火山岩样品的主要氧化物含量变化范围比较大,如:SiO2=51.94%～77.30%,Al2O3=12.85%～19.17%,Na2O=1.65%～5.82%,K2O=0.83%～4.52%,Fe2O3=0.95%～9.30%,CaO=0.13%～7.08%,且主要氧化物的含量与SiO2具有线性相关关系。同时,这些火山岩具有LREE富集、HREE明显亏损、Eu异常不明显、高的Sr,Ba含量和Sr/Y,La/Yb比值等特征。火山岩的钕同位素成分变化大,且相当富集(εNd=-11～-17),都分布在古老下地壳和富集地幔之间,因此推断北京西山地区髫髻山组火山岩和东岭台组火山岩的形成与富集地幔起源的基性岩浆与古老下地壳组分相互作用有关。

Voluminous volcanic rocks of the Tiaojishan Formation outcrop in the Western Hills of western Beijing, which are unconformably overlain by the volcanics of the Donglingtai Formation. These volcanic rocks vary significantly in chemical compositions, with SiO2=51.94-77.30%, Al2O3=12.85-19.17%, Na2O=1.65 -5.82%, K2O=0.83-4.52%, Fe2O3=0.95-9.30%, and CaO=0.13-7.08%. Silica contents show approximately linear covariations with other oxides. These volcanic rocks are characterized by highly enriched LREE and depleted HREE, and minor Eu anomalies in the chondride-normalized REE patterns. Meanwhile, they show high abundances of Sr and Ba and high Sr/Y and La/Yb. The volcanic rocks have varied and enriched Nd isotopic compositions （εNd = -11 --17）, which plot intermediately between the field of the ancient continental crust and the field of the enriched subcontinental lithospheric mantle. These geochemical signatures indicate that the volcanic rocks of the Tiaojishan and Donglingtai Formations were originated from a process of hybridization between the enriched mantle-derived mafic magma and the lower continental crust.