俯冲的大陆岛弧——柴北缘片麻岩的地球化学和同位素证据  

Subducted Continental Arc: Geochemical and Isotopic Evidence of Gneisses in the North Qaidam

作　　者：孟繁聪[1] 张建新[1] 杨经绥[1] 

机构地区：[1]中国地质科学院地质研究所,北京100037

出　　处：《地质学报》2005年第1期46-55,共10页Acta Geologica Sinica

基　　金：国家自然基金 (编号 4973 2 0 70 );地质调查项目 (编号 2 0 0 3 13 0 0 0 5 8);国土资源部科技司项目 (编号 2 0 0 10 10 10 1)资助的成果

摘　　要：柴北缘早古生代高压—超高压变质带的片麻岩分为花岗质片麻岩和副片麻岩 ,花岗质片麻岩的原岩形成于新元古代 (90 0～ 10 0 0 Ma) ,岩石类型为英云闪长岩 -奥长花岗岩 -石英二长岩 -花岗岩 ,非活动性元素的特征类似于岛弧环境形成的花岗岩。副片麻岩的原岩可能为富铝的沉积岩 ,其稀土元素特征类似于 PAAS(后太古宙澳大利亚的平均页岩 )的稀土元素分布 ,非活动性元素的特征与大陆岛弧沉积岩的特征类似。根据花岗质片麻岩与副片麻岩的共生关系结合其地球化学特征推测它们的原岩形成于岛弧环境。鱼卡河花岗质片麻岩的εNd(t) =3.2 (t=1.0 Ga) ,TDM=1.3Ga,表明岩浆源区有大量起源于亏损地幔的初始地壳物质。而落凤坡、锡铁山、沙柳河的花岗质片麻岩与副片麻岩的 Sm - Nd同位素组成类似 ,εNd(t) =- 3.4～ - 7.3(t=1.0 Ga) ,TDM=1.8～ 2 .2 Ga,表明岩浆源区为先存的大陆地壳 ,两类片麻岩地球化学特征的相似性说明岩浆源区的成分类似于变质沉积岩的组成。这项研究说明柴北缘早古生代的深俯冲卷入了大量的大陆岛弧岩石。The gneiss of early Palaeozoic HP-UHP metamorphic belt in the north Qaidam subdivided into orthogneiss and paragneiss. Protoliths of orthogneiss formed in Neoproterozoic (900～1000 Ma), rock type of protoliths belong to tonalite-trondhjemite-quartzmonzonite-granite and their immobile trace element concentration is similar to granites formed in island Arc setting. Protoliths of paragneiss possibly were Al 2O 3-rich sedimentary rocks, whose REE distribution patterns are similar to PAAS and immobile trace elements( Zr、Th、Co) are likely to sedimentary graywacks.Accoding to associating of orthogneisses occuring with paragneisses extrapolate their protoliths are products in island arc setting. The ε Nd(t) values in orhtogneisses from Yuka river for emplacement age of 1.0 Ga are 3.2～4.2, whereas the mean crustal residence age(T DM) is 1.2～1.3 Ga. The positive ε Nd(t) indicates an origin of the geniss protolith from the depleted mantle juvenile material, but the Sm-Nd isotopic composition of orthogneisses and paragneisses from Luofengpo Xitieshan and Shaliuhe area is similar, with their ε Nd(t) values are -3.4～-7.3(t=1.0 Ga); whereas the mean crustal residence age (T DM)is 1.8～2.2 Ga, indicating original source of magma is older crust of undetermined age. Geochemical and isotopic comparability between orthogneisses and paragneisses suggest s composition of original source of magma is similar to paragneisses. We consider deep subduction involved the great amount of assemblage of continental island arc.

关 键 词：花岗质片麻岩 大陆岛 岛弧 俯冲 柴北缘 岩浆源区 原岩 地球化学特征 同位素 沉积岩 

分 类 号：P588.3[天文地球—岩石学]