义敦岛弧带弧后区板内岩浆作用的时代及意义  

Age of intraplate volcanism in the back-arc area of Yidun island arc and its significance

作　　者：曲晓明[1] 侯增谦[1] 唐绍华[1] 

机构地区：[1]中国地质科学院矿产资源研究所,北京100037

出　　处：《岩石矿物学杂志》2003年第2期131-137,共7页Acta Petrologica et Mineralogica

基　　金：原国家科委"九五"科技攻关资助项目 (96-914 -0 3 -0 5 )

摘　　要：通过对义敦岛弧夏囊沟一带酸性火山岩的岩石地球化学和Rb -Sr同位素研究 ,首次在该岛弧弧后区确立了一条板内火山岩带。这些火山岩属钾玄岩系列 ,以高K2 O ,低CaO ,富Nb、Ta、Zr、Hf,贫Sr、Eu为特点 ,具标准的“V”型稀土元素配分型式 ,构造环境判别图解显示其形成于板内裂陷环境。由 4个全岩样品给出的Rb -Sr等时线年龄t=189.2± 5 .0Ma ,相关系数R =0 .99982 4 ,ISr=0 .714 5 78。认为火山岩的成分分异受斜长石在岩浆源区熔融残留和岩浆形成后黑云母的结晶分离双重控制。位于该带东侧弧后扩张盆地中的晚三叠世勉戈组双峰火山岩的形成年龄为 2 13.1Ma,说明该岛弧仅仅在 2 4Ma左右时限内就完成了从俯冲造弧到板内裂陷的重大转换 。Based on the study of the felsic volcanics in Xia'nanggou district of Yidun island arc, the authors identified for the first time an intraplate volcanics belt in the back_arc region of the arc. These volcanic rocks are shoshonitic series and are geochemically characterised by K 2O_, Nb_, Ta_, Zr_, Hf_enrichment and CaO_, Sr_, Eu_depletion. They have a common “V_type' REE chondrite_normalized pattern. Their Pearce's tectonic discrimination diagrams indicate a clear intraplate rifting formation setting. Four bulk_rock samples give a good Rb_Sr isochron with t =189.2±5 Ma, R=0.999?824, and I Sr =0.714?578. The chemical variation of the volcanics are controlled by both residual of plagioclase in the melting source area and crystal fractionation of biotite after magma formation. Considering the 213 Ma formation age of the Miange Formation bimodal volcanics in the back_arc basin east of the intraplate volcanic rock belt, it is held that the island arc completed the important transition from subduction arc_forming to intraplate rifting regime only within a period of 24 Ma, from which exact constraints on the arc evolution sequence have been obtained.

关 键 词：义敦岛弧 板内岩浆作用 Rb—Sr等时线 碰撞造山 演化时序 

分 类 号：P588.141[天文地球—岩石学] P542[天文地球—地质学]