Gondwana Research, V: zyxwvutsrqponmlkji 5, No. 4, zyxwvutsrqp pp. zyxwvutsrqpon 801 -81 zyxwvutsr I. 0 2002 International Association zyxwvutsrq for Gondwana Research, Japan. ISSN: 1342-937X GRiW; 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 Tran Ngoc Naml", Hironobu Hyodo2,Tetsumaru Itaya2 and Takaaki Matsuda3 Department of Geosciences,Hue University of Science, 27-Nguyen Hue, Hue City, Vietnam, E-mail: nam.hue@dng.vnn.vn Research lnstitute of Natural Sciences, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005,Japan Faculty of Science, Himeji lnstitute of Technology, 2167 Shosha, Himeji, 671-2201,Japan Torresponding author (Manuscript received April 1,2002; accepted April 19, 2002) zyxwvu Abstract zyxwvuts ~~~ - ~_.~_. 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+ + Fez+) than the core. These chemical and petrological features suggest that the homblendes 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 (2056fi 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 CJK 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. Key words: Ar-Ar age, hornblende, metamorphic rocks, early Proterozoic, N. Vietnam. Introduction The Indochina Peninsula south of the Red River fault zone in Asia represents a stable continental core of South East Asia (e.g., Hutchison, 1989). Recent geochronological studies have documented three tectonothermal events widely spread in the Peninsula: Triassic Indosinian orogeny, late Jurassic-Cretaceous Yenshanian orogeny and late Oligocene-early Miocene India-Eurasian collision, which have been inferred mainly from 40Ar/39Ar and K-Ar dating of rocks (e.g., Rangin et al., 1995; Lepvrier et al., 1997; Nam, 1998; Jolivet et al., 1999). Precambrian events have also been argued about using geological evidences with some K-Ar data of rocks for the last two decades (e.g., Tran Van Tri, 1977; GGDVN, 1986; Hutchison, 1989). However, these events have been hardly accepted because of poor geochronological data for the Precambrian. Recently, Nam et al. (1998) dated hornblende separates from gneiss and amphibolite in the pre-Mesozoic metamorphic belt south of the Day Nui Con Voi in Vietnam, which gave K-Ar ages of 1700-2000 Ma. The hornblende may have recorded an early Proterozoic tectonothermal event in the Indochina Peninsula. In hornblende K-Ar system, there are many studies on its retentivity for argon. Some studies have suggested the influence of composition on Ar closure temperature in zy Gondwana Research