[SMP40-P04] Low-pressure type metamorphic rocks from No-jima Island, Yamaguchi Prefecture
Keywords:Ryoke metamorphism, Low-pressure type, Garnet isograd
No-jima island is located in the Seto Inland Sea, Yamaguchi Prefecture. It is a region that underwent 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, and are composed of Bt+Ms+Pl+Qtz±Grt±Chl. The pelitic schist locally involves Crd. 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+Qtz in the north and Bt+Ms+Grt+Pl+Chl+Qtz±Kfs in the south. On the other hand, the mineral assemblage of siliceous schist is mainly Ms+Bt+Grt+Pl+Qtz. The island can, therefore, 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 chemical compositions of garnet from the pelitic and siliceous schists show XFe > XMn and XFe < XMn, respectively. The XMg values of the pelitic and siliceous schists are less than 0.035. The XCa values of the pelitic and siliceous schists are of less than 0.05 and more than 0.1, respectively. The garnet of pelitic schist and siliceous schist shows a normal zoning with reduced spessartine toward the rim, probably formed during prograde metamorphism. 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 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, and are composed of Bt+Ms+Pl+Qtz±Grt±Chl. The pelitic schist locally involves Crd. 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+Qtz in the north and Bt+Ms+Grt+Pl+Chl+Qtz±Kfs in the south. On the other hand, the mineral assemblage of siliceous schist is mainly Ms+Bt+Grt+Pl+Qtz. The island can, therefore, 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 chemical compositions of garnet from the pelitic and siliceous schists show XFe > XMn and XFe < XMn, respectively. The XMg values of the pelitic and siliceous schists are less than 0.035. The XCa values of the pelitic and siliceous schists are of less than 0.05 and more than 0.1, respectively. The garnet of pelitic schist and siliceous schist shows a normal zoning with reduced spessartine toward the rim, probably formed during prograde metamorphism. 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.