白头山火山渣锥的岩浆演化研究  

Scoriae magma evolution at Paekdu volcano,Democratic People's Republic of Korea

作　　者：朴成哲[1] 金东旭[1] 赵日元 金守哲[1] 金仁哲[1] 

机构地区：[1]朝鲜国家科学院地质研究所,平壤3812100

出　　处：《岩石学报》2016年第10期3214-3224,共11页Acta Petrologica Sinica

基　　金：国家自然科学基金重大国际合作项目(41210003)资助

摘　　要：火山渣锥是白头山（或长白山）火山喷发的重要产物,主要沿熔岩台地周边呈圆锥状寄生小火山锥体分布。野外特征显示,火山渣是火山渣锥的重要组成;岩石手标本显示,火山渣具有气孔构造,样品自顶部至底部,颜色从赤色、赤褐色,向褐色、灰色发生转变。地球化学特征表明,火山渣岩性包括玄武岩、粗面玄武岩、玄武岩质粗面安山岩、粗面安山岩岩,具有较高SiO_2（46.22%～55.38%）,Al_2O3含量（15.28%～22.11%）,低MgO（2.05%～4.94%）,FeOT（6.79%～14.76%）的特征;同时具有较高的碱Na_2O/K_2O（Na_2O/K_2O〉1）比值,为钠质火山岩。其轻稀土（LREE）和重稀土（HREE）分异明显,具有弱的δEu正异常,并且具有富集K、Rb、Ba、Sr等大离子亲石元素（LILE）和相对亏损Nb、Ti等高场强元素（HFSE）的特征。此外,火山渣的分异指数（DI）范围为36.93～64.48,高于造盾阶段的幔源玄武岩的分异指数;其固结指数（SI）为10.73～24.09,低于早期幔源玄武岩的固结指数（SI=25～45）,这些特征说明火山渣成分发生了较高程度的岩浆分异作用。同时,火山渣的Nb/La、Sm/Nd、La/Nb和Ba/Nb比值几乎全部介于幔源玄武岩和大陆地壳之间,说明具有明显的地壳混染的特征。因此,我们认为研究区火山渣的岩浆可能是由幔源基性玄武岩上升过程中发生分离结晶和地壳混染作用形成的。Scorias are one of the major products of Paekdu（ or Changbai） volcano,widespread distributed along numbers of scoria cones in Democratic People＇s Republic of Korea.In the field,these rocks mainly appear as the unconsolidated porous pyroclastic,and the colour of pyroclastic deposits chang from red or red-brown at the surface to brown or grey to depth.In this study,we report an integrated dataset of major and trace elements for the scoriaes,and propose that scoriaes resulted from assimilation and fractional crystallization process of the mafic primitive basalt magmas.These volcanic rocks consist of basalt,trachybasalt,basaltic trachyandesite and trachyandesite,which are characterized by high Na_2O ＋ K_2O（ 5.46 % ～ 9.30 %）, SiO_2（ 46.22% ～ 55.38%） and Al_2O_3（15.28% ～ 22.11%）,and relatively low Mg O（2.05% ～ 4.94%） and Fe OT（6.79% ～ 14.76%）.Chondrite-normalized rare earth elements（ REE） patterns display positive Eu anomalies.Primitive mantle-normalized incompatible trace elements diagrams exhibit strong enrichment in large ion lithophile elements（ LILE） relative to highfield strength elements（ HFSE） and display significantly negative Nb-Ti anomalies.Besides,differentiation index（ DI） of the studied volcanic rocks ranges from 36.93 to 64.48,higher than those formed during craton-forming period; Their consolidation index（ SI） ranges from 10.73 to 24.09,lower than those derived from early mantle basalts（ SI = 25 ～ 45）.These chemical characteristics indicate that the Paekdu volcanic rocks experienced high degree fractionation.Also,values of Nb/La,Sm/Nd,La/Nb and Ba/Nb ratios all scatter between the values of mantle basalts and continental crust,which imply obvious crustal contamination.We thus propose that the scoria of the study area might have formed through fractionation and crustal contamination during the ascending of basaltic magma from mantle.

关 键 词：火山渣锥 地壳混染 分离结晶 上地幔熔融 小雁池峰火山 

分 类 号：P588.11[天文地球—岩石学]