5:15 PM - 6:30 PM
[SMP25-P14] Progressive metamorphism of the Ryoke metamorphic rocks from No-jima Island, South of Yamaguchi Prefecture
Keywords:Ryoke metamorphism, Low-pressure type, Garnet isograd
No-jima Island is located in the Seto Inland Sea, Yamaguchi Prefecture. It is thought to be the region that has undergone Cretaceous low-pressure type regional metamorphism (Ryoke metamorphism). No-jima Island is underlain by crystalline schists and metabasalts. Metabasalts show the geochemical character similar to the within plate basalts, for instance the green stones in the Mino-Tamba belt. Moreover, the metamorphic grade of the schists corresponds to the biotite zone of the Yanai region of the Ryoke belt, but the petrological character of the rocks from No-jima Island has been unclear. In addition, the geological correlation with the neighboring area, Kasado-jima Island, the Oshima peninsula, and Ohtsu-jima Island has not been understood yet. Therefore, the aim of this study, based on the mineral paragenesis of the pelitic schists, is to discuss the metamorphism of the rocks from No-jima Island, and also to unravel the thermal structure of the low-pressure type metamorphism, the Ryoke belt, in this region.
The metabasalts are mainly exposed on the northeastern part of the island but occur rarely in the west coast of the island. They are included in the schists of sedimentary origin as blocks. The metamorphic rocks are composed mainly of the pelitic schist and metamorphosed chert with bedded structure (siliceous schist hereafter) with trace amounts of calc-silicate schist as thin layers or lenses. The pelitic and siliceous schists are intermittently distributed on the western and southeastern coast of the island. Fold structures such as close folds often develop in the southern part of the island. Granite dikes with 1–2 m width intrude the schists on the western coast.
The metabasalt is porous and has a prominent pillow structure and often shows the amygdaloidal texture. Phenocryst minerals of igneous origin are of clinopyoroxene and plagioclase with minor amounts of amphibole. It contains recrystallized fibrous amphiboles partially coexisting with biotite. Calcite locally fill the amygdules and occurs as veins. The pelitic and siliceous schists show the repidoblastic texture. The calc-silicate schist is composed of Bt+Cal+Qtz. The mineral assemblage of the pelitic schists changes from north to south of the island; Bt+Ms+Chl+Pl±Grt in the north and Bt+Ms+Grt+Pl+Kfs in the south. On the other hand, the mineral assemblage of siliceous schist is mainly Ms+Bt+Grt+Pl.
The garnet of pelitic schist and siliceous schist shows a normal zoning with reduced spessartine toward the rim, probably formed during prograde metamorphism. The chemical composition of biotite depends on the mineral combination. Biotite that coexists with garnet and K-feldspar, has a high XFe in pelitie schist. Plagioclase is fine-grained and the zoning is not prominent. The composition differs for each rock such as politic schist and siliceous schist.
In addition to the change in mineral combination, the grain size becomes coarser in the south. A reaction is expected from the north to the south due to the symbiotic relationship with minerals. Therefore, the island can be divided into the biotite zone in north and the garnet zone in south in terms of the mineral paragenesis of the pelitic schists.
The geothermobarometers for garnet-biotite (Ferry and Spear, 1978) and garnet-pagioclase-biotite-quartz (Hoish, 1990) are adapted to the pelitic schist. As a result, the peak metamorphic temperature and pressure conditions are estimated to be 480–530℃ and 180–300 MPa, respectively. According to the result, the thermal structures around the Seto Inland Sea region are discussed.
The metamorphic garnet zone of No-jima Island and its cordierite zone of Kasado-jima Island, Oshima peninsula and Ohtsu-jima Island show the same metamorphic conditions. Therefore, it is considered that the cause of garnet depends on the chemical composition of pelitic rocks. As a result of comparing the chemical composition of bulk and minerals, it is considered that the crystallization of garnet is affected not only by high XFe but also by XMn values.
The metabasalts are mainly exposed on the northeastern part of the island but occur rarely in the west coast of the island. They are included in the schists of sedimentary origin as blocks. The metamorphic rocks are composed mainly of the pelitic schist and metamorphosed chert with bedded structure (siliceous schist hereafter) with trace amounts of calc-silicate schist as thin layers or lenses. The pelitic and siliceous schists are intermittently distributed on the western and southeastern coast of the island. Fold structures such as close folds often develop in the southern part of the island. Granite dikes with 1–2 m width intrude the schists on the western coast.
The metabasalt is porous and has a prominent pillow structure and often shows the amygdaloidal texture. Phenocryst minerals of igneous origin are of clinopyoroxene and plagioclase with minor amounts of amphibole. It contains recrystallized fibrous amphiboles partially coexisting with biotite. Calcite locally fill the amygdules and occurs as veins. The pelitic and siliceous schists show the repidoblastic texture. The calc-silicate schist is composed of Bt+Cal+Qtz. The mineral assemblage of the pelitic schists changes from north to south of the island; Bt+Ms+Chl+Pl±Grt in the north and Bt+Ms+Grt+Pl+Kfs in the south. On the other hand, the mineral assemblage of siliceous schist is mainly Ms+Bt+Grt+Pl.
The garnet of pelitic schist and siliceous schist shows a normal zoning with reduced spessartine toward the rim, probably formed during prograde metamorphism. The chemical composition of biotite depends on the mineral combination. Biotite that coexists with garnet and K-feldspar, has a high XFe in pelitie schist. Plagioclase is fine-grained and the zoning is not prominent. The composition differs for each rock such as politic schist and siliceous schist.
In addition to the change in mineral combination, the grain size becomes coarser in the south. A reaction is expected from the north to the south due to the symbiotic relationship with minerals. Therefore, the island can be divided into the biotite zone in north and the garnet zone in south in terms of the mineral paragenesis of the pelitic schists.
The geothermobarometers for garnet-biotite (Ferry and Spear, 1978) and garnet-pagioclase-biotite-quartz (Hoish, 1990) are adapted to the pelitic schist. As a result, the peak metamorphic temperature and pressure conditions are estimated to be 480–530℃ and 180–300 MPa, respectively. According to the result, the thermal structures around the Seto Inland Sea region are discussed.
The metamorphic garnet zone of No-jima Island and its cordierite zone of Kasado-jima Island, Oshima peninsula and Ohtsu-jima Island show the same metamorphic conditions. Therefore, it is considered that the cause of garnet depends on the chemical composition of pelitic rocks. As a result of comparing the chemical composition of bulk and minerals, it is considered that the crystallization of garnet is affected not only by high XFe but also by XMn values.