三波川変成作用から見た沈みこみ帯の進化

Abstract

Evolution of pressure-temperature paths and fluid compositions during the Sanbagawa prograde metamorphism was reviewed on the basis of chemical characteristics and zonal structures of sodic pyroxene, amphibole, garnet and phengite. Zonal structures of sodic pyroxene, garnet and phengite imply that (1) dP/dT of prograde P-T paths of individual rocks decreased with increasing metamorphic grade and (2) dP/dT just before metamorphic climax might have been negative in some high-grade rocks. Changes of amphibole composition indicate that the prograde P-T path recorded in the higher-grade rocks was situated on the higher-temperature side of that of the lower-grade rocks and on the lower-pressure side of the metamorphic field gradient (1.4-1.8kbar/100°C). The systematic change of prograde P-T path implies an increasing dP/dT during continuous subduction by continuous cooling of the surrounding mantle. These features can be interpreted as documenting prograde metamorphism within a young subduction zone that has a non-steady-state geotherm. Xco2 [=CO2/(CO2+H2O)] value of metamorphic fluid seems to increase with increasing metamorphic grade, and reaches>0.2 in some higher-grade rocks. The increasing Xco2 might have been promoted by oxidation of carbonaceous material caused by mechanical mixing of metapelites and Fe2O3-rich metabasites. Differences in petrological and age data between the Sanbagawa high-grade rocks and pebbles of high-grade in the Eocene Kuma Group in central Shikoku were also briefly reviewed to discuss evolution of subduction zone in the southwest Japan of Jurassic-Cretaceous age.