阿尔金西段榴辉岩岩石学、地球化学和同位素年代学研究及其构造意义  

Petrology, Geochemistry and Geochronology of Eclogites from the Western Segment of the Altun Tectonic Belt, Northwestern China

作　　者：张建新[1] 许志琴[1] 杨经绥[1] 张泽明[1] 崔军文[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》2001年第2期186-197,共12页Acta Geologica Sinica

基　　金：国家自然科学基金(编号49902018;49732070);国家重点研究发展规划(编号G1998040800);国土资源部"九五"重大基础项目(编号9501106)资助的成果

摘　　要：阿尔金西段的榴辉岩呈透镜状产于角闪岩相为特征的长英质片麻岩和泥质片麻岩中,岩相学研究和反应结构显示它经历了峰期的榴辉岩相、减压过程中的高压麻粒岩相和角闪岩相变质作用。峰期榴辉岩相的变质条件为t=710～850℃,p>1.5GPa。在退变质早期,通过绿辉石分解成相对贫钠的单斜辉石及斜长石后成合晶,榴辉岩转变成高压麻粒岩,局部的平衡组合显示其t=700～800℃,p=1.1～1.4GPa。晚期的退变质作用使麻粒岩组合转变成由韭闪石、韭闪石质普通角闪石和斜长石所组成的角闪岩相平衡共生组合,其平衡变质条件为t=620～740℃,p=0.65～0.95GPa。榴辉岩的微量及稀土元素分析显示其原岩主要具有“T”型大洋玄武岩特征,少量具有堆晶岩性质。保存较好榴辉岩的全岩及Sm-Nd矿物等时线测定获得500±10Ma的年龄;其变质锆石的U-Pb年龄为503.9±5.3Ma,两者基本一致的年龄数据反映了榴辉岩的峰期变质时代,从而表明了阿尔金西段加里东期与深俯冲和陆-陆碰撞作用有关的山根的存在。通过与柴达木北缘存在的类似的榴辉岩的对比,估算出阿尔金断裂的左行位移为400km。Eclogites from the western segment of the Altun tectonic belt occur as lens within quartz-feldspathic gneisses or pelitic gneisses characterized by amphibolite-facies paragenesis. Petrographic study and reaction textures reveal three main metamorphic stages the eclogites have undergone; (1) peak eclogite facies stage (p>15 kb and t = 710-850℃), (2) granulitic facies stage (p=1. 1- 1. 4 GPa and t=700-800℃) and (3) amphibo-lite facies stage (p=0. 63-0. 95 GPa and t=620-740℃ ) during decompression. Analyses of trace elements, rare earth elements and Sm-Nd isotopic data suggest that most of the eclogites have protolith features resembling T-type (transitional type) MORB (mid-ocean ridge basalt). Well-preserved eclogites were selected for Sm-Nd and U-Pb isotopic dating. The Sm-Nd isotopic data yield a whole rock-garnet-omphacite isochron age of 500±10 Ma. The U-Pb isotopic measurements of zircons from the same eclogite gave a concordia age of 503. 9 ± 5. 3 Ma. Two methods yielded a similar age, which reveals the peak metamorphic age of eclogites and reflects the existence of erogenic root related to continental subduction and continent-continent collision during the early Palaeozoic. Based on the comparison between the similar rocks on the northern margin of the Qaidam basin, a displacement of about 400 km of the Altun strike-slip fault was estimated.

关 键 词：阿尔金构造带 榴辉岩 P-T轨迹 地质年代学 片麻岩 变质岩 地球化学 同位素年代学 

分 类 号：P588.3[天文地球—岩石学]