13:45 〜 14:00
[SMP27-07] Progressive metamorphism of high-temperature/low-pressure type metamorphic rocks from Otsu-jima Island, South of Yamaguchi Prefecture
キーワード：変成作用、高温低圧型、U-Pb年代
Otsu-jima Island is located in the Seto Inland Sea, southern Yamaguchi Prefecture. In the southern part of Yamaguchi Prefecture, Triassic high-pressure metamorphic rocks (Suo belt), Cretaceous high-temperature metamorphic rocks (Ryoke belt) and Cretaceous igneous rocks (plutonic rocks, volcanic rocks) are distributed. Otsu-jima Island was recognized as the Suo belt, but the surrounding No-jima, Oshima Peninsula, and Kasado-jima Island were recognized as the Ryoke belt. Otsu-jima Island is the boundary between the Suo belt and the Ryoke belt, where 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 Otsu-jima Island, also to clarify the attribution and geological history of metamorphic rocks.
In this region, pelitic and psammitic schists are dominant rock types accompanied by siliceous, calcalious, and basic schists. Low-angle faults and folding structures are recognized. The schistosities strike to east-west, and dip to north-south. The Cretaceous granite intruded these metamorphic rocks.
The pelitic schist is fine-grained and has a well-developed schistosity (S1). The main metamorphic minerals are biotite (Bt), muscovite (Ms), cordierite (Crd), garnet (Grt), andalusite (And), sillimanite (Sil), and corundum (Crn). To the south, the mineral combinations are Bt±Ms, And+Bt+Ms, Grt+Bt+Ms+Kfs, Bt+And+Sil+Kfs+Crd+Crn, Bt+Grt±Ms, Bt+Crd+And, Bt+Crd+Ms±Chl. Cordierite (Crd) is piniteized and forms the S1 schistosity, and is not coexistence of garnet. From such a mineral combination of S1, the metamorphic rocks of Otsu-jima Island underwent high-temperature/low-pressure type regional metamorphism condition.
The garnet of pelitic 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. The AFM diagram of cordierite has similar feature to that of biotite.
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 metam-orphic temperature and pressure conditions are estimated to be 590-660℃ and 260-380 Mpa, respectively.
We performed zircon U–Pb dating using LA-ICP-MS to determine the sedimentary age of the psammitic schist. The youngest zircon U–Pb age in the psammitic schist is 249.2±1.3 Ma. The sedimentary age of the rock is considered to be the Triassic period. This age is similar to those of the youngest inherited ages from the Suo belt; Kurume area, 260∼250Ma (Tsutsumi et al.,2003) , Omuta area, 250Ma (Miyazaki et al., 2017), and Tagawa area, 226Ma (Yuhara et al., 2021). Therefore, it is considered that the metamorphic rocks of the Otsu-jima Island is attributed to the Suo belt, and underwent high-temperature/low-pressure metamorphism, probably during the Cretaceous.
In this region, pelitic and psammitic schists are dominant rock types accompanied by siliceous, calcalious, and basic schists. Low-angle faults and folding structures are recognized. The schistosities strike to east-west, and dip to north-south. The Cretaceous granite intruded these metamorphic rocks.
The pelitic schist is fine-grained and has a well-developed schistosity (S1). The main metamorphic minerals are biotite (Bt), muscovite (Ms), cordierite (Crd), garnet (Grt), andalusite (And), sillimanite (Sil), and corundum (Crn). To the south, the mineral combinations are Bt±Ms, And+Bt+Ms, Grt+Bt+Ms+Kfs, Bt+And+Sil+Kfs+Crd+Crn, Bt+Grt±Ms, Bt+Crd+And, Bt+Crd+Ms±Chl. Cordierite (Crd) is piniteized and forms the S1 schistosity, and is not coexistence of garnet. From such a mineral combination of S1, the metamorphic rocks of Otsu-jima Island underwent high-temperature/low-pressure type regional metamorphism condition.
The garnet of pelitic 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. The AFM diagram of cordierite has similar feature to that of biotite.
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 metam-orphic temperature and pressure conditions are estimated to be 590-660℃ and 260-380 Mpa, respectively.
We performed zircon U–Pb dating using LA-ICP-MS to determine the sedimentary age of the psammitic schist. The youngest zircon U–Pb age in the psammitic schist is 249.2±1.3 Ma. The sedimentary age of the rock is considered to be the Triassic period. This age is similar to those of the youngest inherited ages from the Suo belt; Kurume area, 260∼250Ma (Tsutsumi et al.,2003) , Omuta area, 250Ma (Miyazaki et al., 2017), and Tagawa area, 226Ma (Yuhara et al., 2021). Therefore, it is considered that the metamorphic rocks of the Otsu-jima Island is attributed to the Suo belt, and underwent high-temperature/low-pressure metamorphism, probably during the Cretaceous.