Thermobaric structure and metamorphic evolution of the Iratsu eclogite body in the Sanbagawa belt, central Shikoku, Japan

Abstract

The Iratsu body is the largest eclogitic body in the Sanbagawa belt, central Shikoku, composed of schists with pumpellyite–actinolite, through blueschist/greenschist transition and epidote-amphibolite, up to eclogite-facies grades. The Iratsu body and the underlying Higashi-Akaishi peridotite body are situated in the intermediate structural level of the Sanbagawa belt which occurs as a subhorizontal thin slab. In the Iratsu body, eclogites and eclogitic metagabbros (garnet-clinopyroxenites) contain garnet+omphacite+epidote+quartz+rutile±phengite±glaucophane±paragonite±barroisite±kyanite, and garnet+sodic augite+rutile±epidote±zoisite±quartz, respectively, as mineral assemblages at the peak P– T conditions. Textural relations between these minerals and secondary minerals indicate that the eclogites were retrograded by hydration under eclogite through epidote-amphibolite to greenschist-facies conditions. Geothermobarometry for the peak metamorphism of the eclogites yielded conditions of 1.4–2.5 GPa and 500–800 °C in the Iratsu body; the estimated P– T conditions gradually change across the body. Eclogites with higher P– T estimates are predominant in the southern part of the body, and the eclogites near a contact with the Higashi-Akaishi body show the highest P– T conditions. Integration of these results with previously published P– T estimates for various rock types suggests a sandwiched thermobaric structure. The highest P– T area is situated in an intermediate structural level, i.e., in the upper part of the Higashi-Akaishi body near the boundary with the Iratsu body, and the metamorphic conditions systematically decrease towards the upper and lower levels not only within the Iratsu body but also in the surrounding lower grade schists. Our thermobaric structure implies that the Iratsu and Higashi-Akaishi composite mass is not a tectonic block but is the integral part of the Sanbagawa belt, which has been formed together with the surrounding schists under a specific thermal gradient at depths in the subduction zone. The Sanbagawa metamorphic facies series, newly proposed, draws a convex curve towards the lower P and higher T side in a P– T diagram, and is similar to a steady-state P– T path in a subduction zone, calculated by numerical models.