西天山阿吾拉勒埃达克质岩石成因:Nd和Sr同位素组成的限制  

Origin of Awulale adakitic sodium-rich rocks in western Tianshan: Constraints for Nd and Sr isotopic compositions.

作　　者：熊小林[1] 赵振华[1] 白正华[1] 梅厚钧[1] 许继峰[1] 王强[1] 

机构地区：[1]中国科学院广州地球化学研究所

出　　处：《岩石学报》2001年第4期514-522,共9页Acta Petrologica Sinica

基　　金：国家重点基础研究项目(G1999043200);国家305攻关项目(96-915-03-02);国家自然科学基金;南京大学成矿作用国家重点实验室国家教委访问学者项目。

摘　　要：西天山阿吾拉勒二叠纪钠质英安岩和钠长斑岩具有与埃达克岩一致的高Sr,低Y、Yb和Eu正异常等独特岩石地球化学特征。系统的Nd和Sr同位素组成研究表明,其(143Nd/144Nd)i为0.512384～0.512470,εNd(t)为正值(+1.57～+3.26);(87Sr/86Sr)i为0.7051～0.7054,与本区同时代幔源玄武岩的Nd和Sr同位素组成特征相似,但与俯冲洋壳部分熔融成因埃达克岩的Nd和 Sr同位素组成有显著区别。结合这些埃达克质岩石形成于二叠纪后碰撞阶段构造背景,认为本区埃达克质岩浆最有可能由新底侵的玄武质下地壳在角闪岩相向榴辉岩相过渡或榴辉岩相的条件下部分熔融形成,是西天山晚古生代后碰撞阶段地幔玄武岩浆底侵作用和地壳垂向增生的重要岩石标志。The Permian sodium-rich dacites and albite porphyries in Awulale Mountain of western Tianshan were generated at a post-collisional setting. They have unique major and trace element signatures similar to those bor subduction-related adakites. These adakitic rocks are characterized by high Na2O. Al2O3 and Sr contents and high Sr/Y and La/Y ratios (>40 and > 20, respectively), low Y and Yb contents, strong depletion in HREE, and positive Eu anomaly. The ratio (Nd-143/Nd-144)(i) ranges from 0.512384 to 0.512470 and the epsilon (Nd) (t=260 Ma) value is positive with a range of + 1.57 similar to + 3.26; the ratio (Sr-87/Sr-84)i ranges from 0.7051 to 0.7054. These Nd and Sr isotopic compositions are similar to those of the coeval basalts in this area, but obviously different from those of subduction-related adakites. The aforesaid Nd and Sr isotopic compositions indicate that the source rocks of these adakitic rocks come probably from a weakly depleted mantle, or a depleted mantle, but was contaminated by the crustal materials. These adakitic rocks were most likely derived from the partial melting of newly underplated basaltic rocks under the conditions of amphibolite to eclogite transition in the postcollisional setting during the Permian Period. They are probably an important indication of the underplating of mantle-derived basaltic magmas and the vertical growth of continental crust in western Tianshan region during the post-collisional setting of late Paleozoic.

关 键 词：阿吾拉勒 埃达克质岩石 底侵作用 地壳生长 铷同位素 锶同位素 

分 类 号：P588.12[天文地球—岩石学] P597[天文地球—地质学]