40Ar/39Ar Single Grain Dating and Mineral Chemistry of Hornblendes South of the Red River Shear Zone (Vietnam): New Evidence for Early Proterozoic Tectonothermal Event
40Ar/39Ar Single Grain Dating and Mineral Chemistry of Hornblendes South of the Red River Shear Zone (Vietnam): New Evidence for Early Proterozoic Tectonothermal Event
复制标题
DOI：
10.1016/s1342-937x(05)70914-x
复制
发表时间：
2002-10
影响因子：
6.1
通讯作者：
T. N. Nam;H. Hyodo;T. Itaya;T. Matsuda
中科院分区：
文献类型：
--
作者：
T. N. Nam;H. Hyodo;T. Itaya;T. Matsuda
前中生代变质带与越南南部的Day Nui Con Voi -红河剪切带平行延伸。该带主要由角闪片麻岩、斜长角闪岩透镜体和云母片岩组成。采用电子探针和40 Ar/39 Ar方法对五个角闪石进行了化学和年代学分析。电子探针分析表明，角闪石在片麻岩和斜长角闪岩中均含有大量的浅成灰岩成分，且平均成分相近。然而，重新计算的Fe 3+含量是显着的非均匀性在一个薄的部分，而总铁几乎是相同的分析。每个晶体的边缘的Fe 3 +/（Fe 3 ++ Fe 2+）高于核心。这些化学和岩石学特征表明角闪石遭受了明显的氧化作用，特别是在片麻岩中。40 Ar/39 Ar分析表明，片麻岩具有明显的高原年龄变化角闪岩的坪年龄为2056± 14和2044±21 Ma。两个样品的所有颗粒在低温下的前几个部分都有过剩年龄，在过剩和平台光谱之间有部分损失年龄。Ca/K比值表明低温谱中存在一些扰动相，但部分损失年龄也来源于角化相。这些事实表明，片麻岩中的角闪石经历了氧化和/或热激活氩扩散过程的部分氩损失。但角闪石中的片麻岩及其伴生斜长角闪岩除边缘外，均保存了早元古代的构造变质事件，为印度支那红河剪切带以南的越南北方前中生代变质带内的早元古代构造变质事件提供了新的证据。
The pre-Mesozoic metamorphic belt runs parallel to the Day Nui Con Voi - Red River shear zone in Vietnam to the south. The belt is mainly composed of hornblende gneisses, amphibolite lenses and mica-schists. Five hornblendes from a gneiss and an amphibolite were analyzed chemically and chronologically by Electron Probe Micro Analysis (EPMA) and40Ar/39Ar methods. EPMA analyses show that hornblendes in the gneiss and the amphibolite have significant amount of edenite component and similar average composition. However, the recalculated Fe3+content is significantly heterogeneous in a thin section while total Fe is nearly the same among the analyses. The rim of each crystal is higher in Fe3+/(Fe3++ Fe2+) than the core. These chemical and petrological features suggest that the hornblendes have suffered significant oxidation, in particular, largely in the gneiss.40Ar/39Ar analyses showed that the gneiss has a significant variation of plateau ages (2089±14, 1977±19 and 1873±13 Ma) among three hornblende grains, whereas the amphibolite gives the same plateau ages (2056±14 and 2044±21 Ma) for two grains. All grains of both samples have excess ages in the first few fractions at low temperatures and partial-loss ages between the excess and plateau spectra. The Ca/K ratios indicate some disturbed phases for the lower temperature spectra but the partial-loss ages are also derived from hornblende phase. These facts suggest that hornblende in the gneiss has experienced partial argon loss by oxidation and/or thermally activated argon diffusion process. However, the gneiss and the associated amphibolite have preserved the early Proterozoic tectono-metamorphic event in the hornblende crystals except for their rims, giving new evidence for the early Proterozoic event within the pre-Mesozoic metamorphic belt (northern Vietnam) south of the Red River shear zone in Indochina.