Metamorphic P-T-time history of the Sanbagawa belt in central Shikoku, Japan and implications for retrograde metamorphism during exhumation

Kazumasa Aoki, Kouki Kitajima, Hideki Masago, Manabu Nishizawa, Masaru Terabayashi, Soichi Omori, Takaomi Yokoyama, Naoto Takahata, Yuji Sano, Shigenori Maruyama

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    45 Citations (Scopus)

    Abstract

    The peak metamorphic conditions and processes of the Cretaceous Sanbagawa HP metamorphic belt, central Shikoku, Japan, were examined based on petrological, thermodynamic and geochronological studies in the Asemi-gawa area, where garnetite (garnet > 50 wt.%) is included in mafic schists of the oligoclase-biotite zone. The representative mineral assemblage in the garnetite is Grt + Qtz + Phe + Ep + Chl + Rt. The peak and retrograde pressure-temperature (P-T) conditions were estimated by thermodynamic calculations using THERMOCALC ver. 3.31 (Holland and Powell, 1998) and THERIAK-DOMINO ver. 15.05.08 (De Capitani, 1994). The estimated peak and retrograde P-T conditions of the garnetite are in the range of P = 15-19 kbar and T = 500-520 °C, and P = 7-11 kbar and T = 460-510 °C, respectively. The P-T data indicate that the whole package of the oligoclase-biotite zone in the Asemi-gawa area once reached eclogite facies (ca. 48-60 km depth) and underwent retrograde metamorphism under epidote-amphibolite facies conditions (ca. 22-35 km depth) during the exhumation. A zircon U-Pb age was determined using Nano-SIMS for zircons extracted from pelitic schist of the oligoclase-biotite zone. Only zircon rims yielded a concordia age of 85.6 ±3.0 Ma (weighted mean age) that is considered to be the time of the retrograde recrystallization. These results imply that the eclogite-facies rocks in the Sanbagawa belt, which have been considered to be tectonic blocks or nappes, are an integral part of the Sanbagawa belt, and formed together with the surrounding schists under a specific thermal gradient at depth in the subduction zone.

    Original languageEnglish
    Pages (from-to)393-407
    Number of pages15
    JournalLITHOS
    Volume113
    Issue number3-4
    DOIs
    Publication statusPublished - Dec 1 2009

    Keywords

    • Eclogite facies
    • Garnetite
    • Nano-SIMS
    • P-T-time path
    • Retrograde metamorphism

    ASJC Scopus subject areas

    • Geology
    • Geochemistry and Petrology

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  • Cite this

    Aoki, K., Kitajima, K., Masago, H., Nishizawa, M., Terabayashi, M., Omori, S., Yokoyama, T., Takahata, N., Sano, Y., & Maruyama, S. (2009). Metamorphic P-T-time history of the Sanbagawa belt in central Shikoku, Japan and implications for retrograde metamorphism during exhumation. LITHOS, 113(3-4), 393-407. https://doi.org/10.1016/j.lithos.2009.04.033