Regional-scale excess Ar wave in a Barrovian type metamorphic belt, eastern Tibetan Plateau

Tetsumaru Itaya, Hironobu Hyodo, Tatsuki Tsujimori, Simon Wallis, Mutsuki Aoya, Tetsuo Kawakami, Chitaro Gouzu

    Research output: Contribution to journalArticle

    20 Citations (Scopus)

    Abstract

    Laser step heating 40Ar/39Ar analysis of biotite and muscovite single crystals from a Barrovian type metamorphic belt in the eastern Tibetan plateau yielded consistent cooling ages of ca. 40Ma in the sillimanite zone with peak metamorphic temperatures higher than 600°C and discordant ages from 46 to 197Ma in the zones with lower peak temperatures. Chemical Th-U-Total Pb Isochron Method (CHIME) monazite (65 Ma) and sensitive high mass-resolution ion microprobe (SHRIMP) apatite (67 Ma) dating give the age of peak metamorphism in the sillimanite zone. Moderate amounts of excess Ar shown by biotite grains with ages of 46 to 94 Ma at metamorphic grades up to the high-grade part of the kyanite zone probably represent incomplete degassing during metamorphism. In contrast, the high-grade part of the kyanite zone yields biotite ages of 130 to 197 Ma. The spatial distribution of these older ages in the kyanite zone along the sillimanite zone boundary suggests they reflect trapped excess argon that migrated from higher-grade regions. The most likely source is muscovite that decomposed to form sillimanite. The zone with extreme amounts of excess argon preserves trapped remnants of an 'excess argon wave'. We suggest this corresponds to the area where biotite cooled below its closure temperature in the presence of an elevated Ar wave. Extreme excess Ar is not recognized in muscovite suggesting that the entrapment of the argon wave by biotite took place when the rocks had cooled down to temperatures lower than the closure temperature of muscovite. The breakdown of phengite during ultrahigh-pressure (UHP) metamorphism may be a key factor in accounting for the very old apparent ages seen in many UHP metamorphic regions. This is the first documentation of a regional Ar-wave spatially associated with regional metamorphism. This study also implies that resetting of the Ar isotopic systems in micas can require temperatures up to 600°C; much higher than generally thought.

    Original languageEnglish
    Pages (from-to)293-305
    Number of pages13
    JournalIsland Arc
    Volume18
    Issue number2
    DOIs
    Publication statusPublished - 2009

    Fingerprint

    plateau
    sillimanite
    biotite
    argon
    muscovite
    kyanite
    closure temperature
    metamorphism
    ultrahigh pressure metamorphism
    phengite
    resetting
    ion microprobe
    regional metamorphism
    monazite
    degassing
    apatite
    laser
    temperature
    crystal
    spatial distribution

    Keywords

    • Ar/Ar analyses
    • Barrovian type metamorphism
    • Closure temperature
    • Eastern Tibet
    • Excess argon wave
    • Longmenshan orogenic belt

    ASJC Scopus subject areas

    • Geology

    Cite this

    Itaya, T., Hyodo, H., Tsujimori, T., Wallis, S., Aoya, M., Kawakami, T., & Gouzu, C. (2009). Regional-scale excess Ar wave in a Barrovian type metamorphic belt, eastern Tibetan Plateau. Island Arc, 18(2), 293-305. https://doi.org/10.1111/j.1440-1738.2009.00668.x

    Regional-scale excess Ar wave in a Barrovian type metamorphic belt, eastern Tibetan Plateau. / Itaya, Tetsumaru; Hyodo, Hironobu; Tsujimori, Tatsuki; Wallis, Simon; Aoya, Mutsuki; Kawakami, Tetsuo; Gouzu, Chitaro.

    In: Island Arc, Vol. 18, No. 2, 2009, p. 293-305.

    Research output: Contribution to journalArticle

    Itaya, T, Hyodo, H, Tsujimori, T, Wallis, S, Aoya, M, Kawakami, T & Gouzu, C 2009, 'Regional-scale excess Ar wave in a Barrovian type metamorphic belt, eastern Tibetan Plateau', Island Arc, vol. 18, no. 2, pp. 293-305. https://doi.org/10.1111/j.1440-1738.2009.00668.x
    Itaya, Tetsumaru ; Hyodo, Hironobu ; Tsujimori, Tatsuki ; Wallis, Simon ; Aoya, Mutsuki ; Kawakami, Tetsuo ; Gouzu, Chitaro. / Regional-scale excess Ar wave in a Barrovian type metamorphic belt, eastern Tibetan Plateau. In: Island Arc. 2009 ; Vol. 18, No. 2. pp. 293-305.
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    AU - Wallis, Simon

    AU - Aoya, Mutsuki

    AU - Kawakami, Tetsuo

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