10:00 〜 10:15
[SCG51-05] チタン石を用いた温度・圧力・年代同時推定：四国中央部三波川変成帯東平岩体が記録する約2億年前の先三波川変成作用
キーワード：チタン石U-Pb年代、LA-ICP-MS、先三波川変成作用
The pressure and temperature history of metamorphic rocks are generally deduced from the chemical composition of specific mineral assemblages and thermodynamics. Well-known conventional geothermobarometers employ cation exchange and net-transfer reactions. Recent progress in petrology provides two new approaches for geothermobarometry: trace element geothermometer and elastic geobarometer, which enables us to dissolve the petro-chronological dilemma that datable minerals themselves are not available for pressure-temperature (P-T) estimation.
Titanite is one of the potential minerals available for simultaneous P-T-t estimation. Its stable P-T conditions depend on the whole-rock compositions, where the calc-silicate system with low XCO2 makes titanite stable under a relatively wide range of up to >1000 °C and > 1.5 GPa. The closure temperature of the U-Pb system in titanites has been considered to be low; however, recent petrochrolonogic studies on natural samples indicate a much lower Pb diffusivity, suggesting robust applicability of titanite geochronology for high-temperature rock (>850 °C).
This study performed petrographic observation on an impure marble collected from the Tonaru body of the Sanbagawa metamorphic belt, central Shikoku. The Tonaru body is one of the eclogitic bodies in central Shikoku, where the peak P-T is estimated to be 700-730 °C and >1.5 GPa [1]. Geochemical data of zircon from mafic rocks indicate that the Tonaru body originated from the oceanic-arc with an igneous age of 200-180 Ma [2].
The studied impure marble mainly comprises of epidote, quartz, calcite, amphibole, clinopyroxene, and titanite, with a minor amount of muscovite (Si=3.35), zircon, apatite, and Fe-sulfide. Clinopyroxene in the sample is diopside with jadeite content of up to 18 mol% and is not omphacite. No indicative minerals have been recognized.
Titanite in the studied sample sometimes has a core and rim that are recognized under the back-scattered electron image. The differences between the core and rim are characterized by the concentration of zirconium (Zr), where the core contains Zr of up to 3500 ppm, and the Zr content in the rim is much lower (<500 ppm). The grains without such zonation have the same composition as the rim. The titanite core contains quartz showing a peak shift of 0.4-1.0 cm-1 for the 464 cm-1 band. Zr-in-titanite geothermometry and quartz-in-titanite elastic geobarometry indicate pressure-temperature conditions of 950-1100 °C and 2-2.5 GPa.
The in-situ U-Pb dating was conducted for titanite and zircon in the studied sample. The core of titanite yielded a slightly discordant age of 190-170 Ma (concordance = 0.8) with Th/U ratios of ~1.4, and the rim contained almost no U. U-Pb ages of zircon were relatively scattered among 250-180 Ma where older age grains show high Th/U ratio of ~1. In contrast, the younger age grains of 206-180 Ma, comparable to the titanite core, have a relatively low Th/U ratio of ~0.1 and are considered to be the metamorphic origin.
The above P-T-t data recovered solely from titanite are consistent with those of the pre-Sanbagawa metamorphism recovered from the impure marble of the Eastern Iratsu body [3]. In the case of the Eastern Iratsu body, the P-T-t history, including the eclogite-facies stage, was well recorded in titanite and garnet [3,4]; however, the present case of the Tonaru body lacks evidence of the eclogite-facies stage, which probably because of the whole-rock composition and retrogression. Our finding would infer the areal extent of the 200 Ma pre-Sanbagawa metamorphism in central Shikoku and the tectono-metamorphic history of southwestern Japan in the Jurassic age.
[1] Miyagi & Takasu (2005) 10.1111/j.1440-1738.2005.00468.x, [2] Aoki et al. (2020) 10.1111/iar.12332, [3] Yoshida et al. (2021) 10.1016/j.lithos.2021.106349, [4] Niki et al. (2022) 10.2465/jmps.210814
Titanite is one of the potential minerals available for simultaneous P-T-t estimation. Its stable P-T conditions depend on the whole-rock compositions, where the calc-silicate system with low XCO2 makes titanite stable under a relatively wide range of up to >1000 °C and > 1.5 GPa. The closure temperature of the U-Pb system in titanites has been considered to be low; however, recent petrochrolonogic studies on natural samples indicate a much lower Pb diffusivity, suggesting robust applicability of titanite geochronology for high-temperature rock (>850 °C).
This study performed petrographic observation on an impure marble collected from the Tonaru body of the Sanbagawa metamorphic belt, central Shikoku. The Tonaru body is one of the eclogitic bodies in central Shikoku, where the peak P-T is estimated to be 700-730 °C and >1.5 GPa [1]. Geochemical data of zircon from mafic rocks indicate that the Tonaru body originated from the oceanic-arc with an igneous age of 200-180 Ma [2].
The studied impure marble mainly comprises of epidote, quartz, calcite, amphibole, clinopyroxene, and titanite, with a minor amount of muscovite (Si=3.35), zircon, apatite, and Fe-sulfide. Clinopyroxene in the sample is diopside with jadeite content of up to 18 mol% and is not omphacite. No indicative minerals have been recognized.
Titanite in the studied sample sometimes has a core and rim that are recognized under the back-scattered electron image. The differences between the core and rim are characterized by the concentration of zirconium (Zr), where the core contains Zr of up to 3500 ppm, and the Zr content in the rim is much lower (<500 ppm). The grains without such zonation have the same composition as the rim. The titanite core contains quartz showing a peak shift of 0.4-1.0 cm-1 for the 464 cm-1 band. Zr-in-titanite geothermometry and quartz-in-titanite elastic geobarometry indicate pressure-temperature conditions of 950-1100 °C and 2-2.5 GPa.
The in-situ U-Pb dating was conducted for titanite and zircon in the studied sample. The core of titanite yielded a slightly discordant age of 190-170 Ma (concordance = 0.8) with Th/U ratios of ~1.4, and the rim contained almost no U. U-Pb ages of zircon were relatively scattered among 250-180 Ma where older age grains show high Th/U ratio of ~1. In contrast, the younger age grains of 206-180 Ma, comparable to the titanite core, have a relatively low Th/U ratio of ~0.1 and are considered to be the metamorphic origin.
The above P-T-t data recovered solely from titanite are consistent with those of the pre-Sanbagawa metamorphism recovered from the impure marble of the Eastern Iratsu body [3]. In the case of the Eastern Iratsu body, the P-T-t history, including the eclogite-facies stage, was well recorded in titanite and garnet [3,4]; however, the present case of the Tonaru body lacks evidence of the eclogite-facies stage, which probably because of the whole-rock composition and retrogression. Our finding would infer the areal extent of the 200 Ma pre-Sanbagawa metamorphism in central Shikoku and the tectono-metamorphic history of southwestern Japan in the Jurassic age.
[1] Miyagi & Takasu (2005) 10.1111/j.1440-1738.2005.00468.x, [2] Aoki et al. (2020) 10.1111/iar.12332, [3] Yoshida et al. (2021) 10.1016/j.lithos.2021.106349, [4] Niki et al. (2022) 10.2465/jmps.210814