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Ar/Ar Single Grain Dating and Mineral Chemistry of Hornblendes South of the Red River Shear Zone (Vietnam): New Evidence for Early Proterozoic Tectonothermal Event |
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| Authors: | Tran Ngoc Nam Hironobu Hyodo Tetsumaru Itaya Takaaki Matsuda |
| Institution: | aDepartment of Geosciences, Hue University of Science, 27-Nguyen Hue, Hue City, Vietnam bResearch Institute of Natural Sciences, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan cFaculty of Science, Himeji Institute of Technology, 2167 Shosha, Himeji, 671-2201, Japan |
| Abstract: | 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) and 40Ar/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. |
| Keywords: | Ar-Ar age hornblende metamorphic rocks early Proterozoic N Vietnam |
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