柴北缘榴辉岩的峰期和退变质年龄:来自U-Pb及Ar-Ar同位素测定的证据  

Peak and retrograde age of eclogites at the northern margin of Qaidam basin, northwestern China: Evidences from U-Ph and Ar-Ar datas

作　　者：张建新[1] 杨经绥[1] 许志琴[1] 张泽明[1] 陈文[1] 李海兵[1] 

机构地区：[1]中国地质科学院地质研究所,北京100037

出　　处：《地球化学》2000年第3期217-222,共6页Geochimica

基　　金：国家自然科学基金!(49732070);国家自然科学青年基金!(49902018);国家重点基础研究发展规划项目!(G1998

摘　　要：柴北缘大柴旦榴辉岩呈透镜状产于以角闪岩相变质条件为特征的片麻岩中。选择新鲜的保存较好的榴辉岩分别进行Ｕ－Ｐｈ和Ａｒ－Ａｒ同位素年代学测定，含石榴子石的白云母片麻岩（此榴辉岩的围岩）被选用于白云母的Ａｒ－Ａｒ测定。榴辉岩的 Ｕ－Ｐｈ测定显示^（２０６）Ｐｂ／^（２３８）Ｕ表面年龄统计权重平均值为（４９４． ６±６． ５）Ｍａ，代表榴辉岩的峰期变质作用年龄。同样榴辉岩样品的多硅白云母被分选出用于^（３９）Ａｒ－^（４０）Ａｒ年代学测定。测定结果获得的坪年龄为（４６６． ７±１． ２）Ｍａ，等时线年龄为（４６５． ９±５． ４）Ｍａ，代表榴辉岩在退变过程中的冷却年龄；围岩中白云母的^（３９）Ａｒ－^（４０）Ａｒ年代学测定，获得等时线年龄为（４７７． ６７±１７． ７２）Ｍａ。相似的 Ａｒ－Ａｒ年龄值表明，榴辉岩和围岩在抬升过程中经历了一致的冷却及退变质历史。Eclogites occur as lens in gneisses with amphibolite-factes mineral parageneses at the northern margin of Qaidam basin. A fresh eclogite sample is selected for U-Ph isotopic dating of zircon and Ar-Ar dating of phengite, and a garnet-bearing muscovite gneiss sample (country rock of eclogite) for Ar-Ar dating of muscovite. The U-Pb isotopic measurements of zircons give a weighted mean age of (494. 6±16. 5) Ma which represents the peak metamorphic age of eclogites. Phengites separated from the same eclogite produced an Ar-Ar plateau age of (466± 1.2) Ma and an isochron age of (465. 9±5. 4) Ma which represents the cooling age of the eclogite during retrograde metamorphism. In addition, muscovite from garnet-bearing muscovite gneiss yields an Ar-Ar isochron age of (477. 6±17. 7) Ma. The similar age data from both eclogite and its country rocks suggest that they had probably experienced a consistent retrograde history.

关 键 词：榴辉岩 柴达木盆地 铀-铅定年 氩-氩定年 

分 类 号：P588.3[天文地球—岩石学] P597[天文地球—地质学]