摘要
北京西山东狼沟组火山岩为钾质系列岩石(K2O=3.40%～4.70%,K2O/Na2O>1),由玄武质粗安岩和粗安岩组成。它们表现出LREE富集(La/Yb)N=31.4～44.6和HREE弱分馏(Dy/Yb)N=1.64～2.01的REE分布模式,Eu表现出弱的负异常(Eu/Eu=0.81～0.86),富集Ba、K、LREE和相对亏损Nb-Ta、Th-U。较高的Sr(87Sr/86Sr)i=0.7057～0.7059和低NdεNd(t)=-14.7～-12.4,类似于辽西义县组和华北陆内同时代的EMⅠ型基性火成岩。其Th-U相对Ba、La的强烈亏损,暗示早期可能有华北古老的下地壳再循环进入源区。结合区域构造研究,东狼沟组火山岩可能为陆内伸展拉张环境下早期交代富集的岩石圈地幔部分熔融作用的产物,并在岩浆演化过程中经历了橄榄石+辉石+斜长石的分离结晶作用。
Donglanggou Formation (DLGF) potassic volcanic rocks in Xishan, Be i jing are mainly consist of basaltic trachyandesites and trachyandesites. They ar e characterized by slight Eu negative anomaly (Eu/Eu=0.81～0.86) and signifi cant enrichment in LREE [La/Yb)N =31.4～44.6] and LILE (Ba, K and Sr) and deple tion in Nb-Ta and Th-U. These volcanics have relatively enriched Sr [87Sr/86 Sr)i=0.705 7～0.705 9] and lower Nd [?Nd(t)=-14.7～-12.4] isotopic signa ture, similar to Yixian Formation volcanics from Liaoxi and mafic igneous rocks within North China Block. All volcanics of DLGF have distinctive characteristics of significant depletion in Th and U relative to Ba and La, suggesting the earl y ancient lower crust of North China Craton recycling into the mantle source. Co mbined with regional tectonic studies, DLGF volcanics were derived from partial melting of early enriched continental lithospheric mantle under extension regime , and had undergone fractional crystallization of olivine+pyroxene+plagiocla se during magma evolution.
出处
《地球化学》
CAS
CSCD
北大核心
2004年第4期353-360,共8页
Geochimica
基金
中国科学院知识创新工程项目(KZCX1-107和KZCX2-104)
国家自然科学基金(40073011)
关键词
钾质火山岩
富集岩石圈地幔
伸展作用
东狼沟组
岩石成因论
北京
potassic volcanics
enriched lithospheric mantle
extension
Dongl anggou Formation
petrogenesis
Beijing
