EVIDENCE FOR ECLOGITE-FACIES METAMORPHISM FROM THE SANBAGAWA METAPELITES, INTERPRETED USING RAMAN GEOTHERMOBAROMETERS

An estimate of the pressure–temperature (P–T) history of metamorphic rocks and their tectonic interpretation can contribute to a clear appraisal of the evolution of subduction zones. The highest-grade rocks of the Sanbagawa belt are exposed in the Besshi and Asemi-gawa regions, central Shikoku, Japan. In the Besshi region, the eclogitic assemblage of garnet + omphacite + quartz occurs sporadically in metabasic complexes within the high-grade zone. By contrast, evidence of an eclogite facies has not been reported in the Asemi-gawa region, which is about 15 km ESE of the Besshi region. However, the coexistence of jadeite + quartz has recently been discovered as an inclusion in garnet from metapelites of the albite–biotite (Ab–Bt) zone in the Asemi-gawa region (Taguchi and Enami, 2014). This direct evidence implied that part of this region has experienced higher-P metamorphism than was previously expected. However, the exact metamorphic conditions had not been determined. In this study, we attempt to deduce the metamorphic P–T history of metapelites of the Asemi-gawa region using Raman geothermobarometers.

In the Ab–Bt zone metapelites, garnets commonly show “composite zoning” defined by a slight increase in spessartine in the grain mantle and an increase in grossular towards the rim. Metamorphic peak-T conditions were calculated from measured thermometry using the Raman spectra of carbonaceous material (RSCM). CM inclusions in garnets were estimated to indicate peak-T of 475–540 ˚C. A number of quartz inclusions in garnets indicate high residual-P conditions, corresponding to those of eclogite samples. Numerical analysis of residual-P revealed a metamorphic-P of 1.6–2.0 GPa, as indicated by the Qz-Raman barometer. Additionally, the equilibrium condition in garnet was estimated at 1.7 GPa and 470 °C using conventional geothermobarometers. The metamorphic conditions estimated from Raman geothermobarometers agreed well with those determined from established thermodynamic models.

These data suggest that a greater amount of sedimentary lithologies than previously indicated have been subducted in this region, resulting in metamorphism to up to eclogite facies. The petrological analysis of metapelites using Raman geothermobarometers has provided more precise information on the progressive metamorphism.