滇西南那邦变质基性岩两期变质作用的^(40)Ar/^(39)Ar年代学研究

Dating of two metamorphic events on the basalt granulite from the Nabang area on the border of China and Burma

中缅边界那邦变质基性岩出露于东缅著名的 Mogok带的东缘。那邦基性麻粒岩经历了早期麻粒岩相、晚期角闪岩相的两期变质作用的改造。早期变质作用的矿物石榴石和单斜辉石的 40 Ar/ 39Ar激光微区等时线年龄是 74.44± 1.44 Ma;石榴石和单斜辉石的 40 Ar/ 39Ar年龄谱图和等时线年龄均显示两组年龄 :74～ 76 Ma和 2 3～ 2 4Ma。晚期变质作用矿物角闪石的K- Ar法得出 2 3Ma,角闪石和斜长石的 40 Ar/ 39Ar年龄谱图和等时线年龄均显示 2 3～ 2 4Ma的变质年龄。综合这些结果 ,可以确定那邦基性麻粒岩的早期麻粒岩相变质作用的年龄是 74～ 77Ma,可能与新特提斯洋壳俯冲事件有关 ;晚期角闪岩相变质作用的年龄是 2 3～ 2 4Ma,与区内喜山期以来陆内块体响应两大陆会聚作用发生的大型走滑事件有关。那邦基性麻粒岩是世界上最年轻的麻粒岩相岩石之一 ,而且 ,两期变质作用年龄的确定对揭示区内与变质作用相关的地质事件具有重要意义。

The Nabang metamorphic basalt, discovered on the border of China and Burma recently, experienced two metamorphic events. The early metamorphism is granulite-facies and the later one is amphibolite-facies. 40Ar/ 39Ar isochron age on a laser microscope scale of the early mineral assemblage including garnet and clinopyroxene is 74.44±1.5Ma. 40Ar/ 39Ar plateau and isochron age of garnet and cliopyroxene, is about 74~76Ma corresponding to the age of the early metamorpnism and 23~24Ma to the later one. As the age of the later metamorphism, 40Ar/ 39Ar plateauand isochron age of hornblende and plagooclase is between 23 and 24Ma, the same as the K-Ar apparent age of hornblende. Consequently, it is concluded that the early metamorphic age of the Nabang metamorphic basalt is about 74~77Ma which is consistent with the Mytikyina oceanic crustal subduction age and the later is about 23~24Ma relating to the dextral strike-slip deformation. It is concluded that the Nabang metamorphic basalt is one of the youngest granulite discovered all over the world. The ages of two metamorphic events can be used to study the geological process developping in the western edge of the Tengchong block.