敦煌三危山地区白垩纪OIB型基性岩墙的特征及地质意义  

Geochemistry and geological significance of the Cretaceous OIB-type mafic dykes in Sanweishan,Dunhuang,Gansu Province

作　　者：冯志硕[1] 张志诚[1] 李建锋[1] 郭召杰[1] 

机构地区：[1]北京大学造山带与地壳演化教育部重点实验室北京大学地球与空间科学学院,北京100871

出　　处：《岩石学报》2010年第2期607-616,共10页Acta Petrologica Sinica

基　　金：国家重点基础研究发展计划项目(2007CB411305);国家自然科学基金(40672130);北京大学大型仪器开放测试基金联合资助

摘　　要：本文首次报道甘肃敦煌三危山地区早白垩世玄武质岩浆活动的记录。在三危山附近,基性岩墙侵入于敦煌群TTG和表壳岩大理岩和片岩中。全岩的K-Ar年龄为136.00±11.56Ma到99.11±6.35Ma,形成时代属于早白垩世。基性岩墙SiO_2含量变化范围较小,集中在47.95%～50.65%之间,以富TiO_2(2.07%～2.35%,平均为2.21%)、MgO(6.03%～6.51%,平均为6.32%)、贫K_2O(<1.29%),Na_2O>K_2O,Mg~#值中等且比较稳定(48.9～53.1,平均为51.7)为特征。基性岩相容元素含量相对较低,Ni含量变化相对较小,分布在112.7×10^(-6)～182.7×10^(-6)之间。而V含量变化较大,介于184×10^(-6)～267×10^(-6)之间,表明岩浆早期可能发生了一定的以橄榄石和单斜辉石为主的分离结晶作用。基性岩富集LREE((La/Yb)_N=3.97～4.66)和LILE,无Eu负异常,较高的Fe/Mn比值等,具有与洋岛玄武岩(OIB)相似的特征。微量元素比值等特征表明其来源与富集地幔关系密切,玄武质岩浆的形成可能与岩石圈的减薄和软流圈的上涌有关。This study is the first report of the Early Cretaceous mafic dykes from the Sanweishan,Dunhuang,Gansu Province.The mafic dykes,which is near Sanweishan,intruded into the TTG suite and the marble and schist of the supracrustal rock of Dunhuang Group.The K-Ar dating results of whole rocks indicate that these mafic dykes were emplaced at 136.00±11.56～99.11±6.35 Ma (Early Cretaceous).The dykes are characterized by narrow range of SiO_2(mostly 47.95%～50.65%),with high TiO_2(average 2.21%and range of 2.07%～2.35%) and MgO(average 6.32%and range of 6.03%～6.51%) and low K_2O(<1.29%) contents,together with Na_2O>K_2O and slightly varied moderate magnesium numbers(Mg~# =48.93～53.07).The mafic dyke rocks have relative low compatible elements,with narrow range of Ni(112.7×10^(-6)～182.7×10^(-6)) and wide range of V(184×10^(-6)～267×10^(-6)),showing that the magma of dyke rocks has occurred a certain degree of fractional crystallization of olivine and clinopyroxene in the early stage.The rocks have enrichment of LREE((La/Yb)_N = 3.97～4.66) and LILE,no negative Eu anomalies,relatively high Fe/Mn ratios,and it shows the similar characteristics with oceanic island basalts(OIB).The characteristics such as the ratios of trace elements show that there is an intimate connection between the magma source and the enriched lithosphere mantle,indicating that the basaltic magma possibly results from the lithospheric thinning and the upwelling asthenosphere mantle.

关 键 词：白垩纪 基性岩墙 三危山 敦煌 

分 类 号：P588.145[天文地球—岩石学]