17:15 〜 18:30
[SMP25-P17] ジュラ紀初期の先三波川変成作用に見られる超高温・高圧グラニュライト相ステージ
キーワード：三波川変成帯、高圧グラニュライト、U-Pb年代測定、高圧変成作用、チタン石
The Sanbagawa metamorphic terrane records at least two distinct stages of metamorphism, i.e. the early metamorphism of granulite to epidote-amphibolite facies occurred at Early Cretaceous (c. 120 Ma) and the later high-pressure metamorphism covering eclogite facies, so-called Sanbagawa metamorphism, at Late Cretaceous (c. 90 Ma) (e.g., Aoya and Endo, 2017 and references therein). The evidence of the early Sanbagawa metamorphism is preserved in some coarse-grained mafic bodies in central Shikoku, such as the Western Iratsu body (Endo et al., 2009), and their protolith ages are considered to Early Jurrasic based on the U-Pb ages of zircon (c. 200 Ma) that represent the ages of the arc magmatism (Aoki et al., 2020).
We report the first evidence of the metamorphic event preceding the above ones (pre-Sanbagawa metamorphism), whose P-T conditions correspond to ultrahigh-temperature high-pressure (UHT-HP) granulite facies with the age of Early Jurrasic.
The studied sample is a high-pressure marble collected from the Eastern Iratsu body, which is composed of calcite, garnet, diopside, quartz, and minor minerals including titanite, and zircon. The eclogite facies metamorphism has been recognized by omphacite (jadeite content up to 50%) in the matrix garnet and within garnet-diopside aggregate (GDA), suggesting that diopside had become stable before the eclogite facies stage. In-situ U-Pb dating of the garnet in the quartz-rich domain yielded 96 +/- 9 Ma which is in good agreement with the previous estimates of the eclogite-facies metamorphism (Niki, Yoshida et al., JpGU2021). The sample records a unique evidence of fluid activity during the exhumation. The occurrence of datolite [CaBSiO4(OH)] in a specific part of garnet, the P-T conditions of which were well constrained, indicates the boron-rich fluid activity during the juxtaposition of the eclogite unit to the surrounding non-eclogite rocks (Yoshida et al., 2021 JMPS).
The new evidence of the pre-Sanbagawa metamorphism was discovered using titanite in the same marble. Titanite occurs in the matrix and inter-grain minerals within GDA. Titanite grains in the matrix show spindle shape while those in GDA show irregular shape. In high-contrast back-scattered electron image (BSI), most titanite are homogeneous (Ttn1), but some grains have thin rim showing dark color in BSI (Ttn2). The formation P-T conditions of Ttn1 were well constrained by means of quartz-in-titanite Raman elastic barometer (Kohn, 2014) and Zr-in-titanite thermometer (Hayden et al., 2008), yielding 900-1100 degreeC and 1.5-1.8 GPa assuming a specific range of aTiO2.
LA-ICP-MS U-Pb dating was performed for titanite and zircon. Zircon and Ttn1 yielded concordant age of Early Jurassic (200-180 Ma) while Ttn2 show a clear mixing line in the Terra-Wasserburg Concordia diagram whose lower intercept age was 126 Ma. In addition, few Ttn1 grains show intermediate ages around 164 Ma. All titanite and zircon analyzed show low Th/U ratios and thus are metamorphic origin. The Early Cretaceous age of Ttn2 is comparable to that of the early Sanbagawa metamorphism although P-T conditions were not constrained. On the contrary, Ttn1 and zircon preserved an Early Jurassic metamorphism of UHT-HP granulite conditions that has never reported from the Sanbagawa metamorphic terrane.
The oldest metamorphic ages of the Sanbagawa metamorphic terrane in the previous studies were c. 120 Ma of the early Sanbagawa metamorphism while our new data provide an evidence of the older metamorphic event. The 138 Ma of Nd-Sm age reported from the Nikubuchi peridotite body within the Eastern Iratsu body (Senda et al., 2006) was interpreted as either granulite facies metamorphism or magmatic event, which indicates that the Eastern Iratsu body records a longer history than other members of the Sanbagawa metamorphic terrane. The newly-reported pre-Sanbagawa metamorphism may provide a key to decipher tectonic evolution of the subducting Izanagi plate.
We report the first evidence of the metamorphic event preceding the above ones (pre-Sanbagawa metamorphism), whose P-T conditions correspond to ultrahigh-temperature high-pressure (UHT-HP) granulite facies with the age of Early Jurrasic.
The studied sample is a high-pressure marble collected from the Eastern Iratsu body, which is composed of calcite, garnet, diopside, quartz, and minor minerals including titanite, and zircon. The eclogite facies metamorphism has been recognized by omphacite (jadeite content up to 50%) in the matrix garnet and within garnet-diopside aggregate (GDA), suggesting that diopside had become stable before the eclogite facies stage. In-situ U-Pb dating of the garnet in the quartz-rich domain yielded 96 +/- 9 Ma which is in good agreement with the previous estimates of the eclogite-facies metamorphism (Niki, Yoshida et al., JpGU2021). The sample records a unique evidence of fluid activity during the exhumation. The occurrence of datolite [CaBSiO4(OH)] in a specific part of garnet, the P-T conditions of which were well constrained, indicates the boron-rich fluid activity during the juxtaposition of the eclogite unit to the surrounding non-eclogite rocks (Yoshida et al., 2021 JMPS).
The new evidence of the pre-Sanbagawa metamorphism was discovered using titanite in the same marble. Titanite occurs in the matrix and inter-grain minerals within GDA. Titanite grains in the matrix show spindle shape while those in GDA show irregular shape. In high-contrast back-scattered electron image (BSI), most titanite are homogeneous (Ttn1), but some grains have thin rim showing dark color in BSI (Ttn2). The formation P-T conditions of Ttn1 were well constrained by means of quartz-in-titanite Raman elastic barometer (Kohn, 2014) and Zr-in-titanite thermometer (Hayden et al., 2008), yielding 900-1100 degreeC and 1.5-1.8 GPa assuming a specific range of aTiO2.
LA-ICP-MS U-Pb dating was performed for titanite and zircon. Zircon and Ttn1 yielded concordant age of Early Jurassic (200-180 Ma) while Ttn2 show a clear mixing line in the Terra-Wasserburg Concordia diagram whose lower intercept age was 126 Ma. In addition, few Ttn1 grains show intermediate ages around 164 Ma. All titanite and zircon analyzed show low Th/U ratios and thus are metamorphic origin. The Early Cretaceous age of Ttn2 is comparable to that of the early Sanbagawa metamorphism although P-T conditions were not constrained. On the contrary, Ttn1 and zircon preserved an Early Jurassic metamorphism of UHT-HP granulite conditions that has never reported from the Sanbagawa metamorphic terrane.
The oldest metamorphic ages of the Sanbagawa metamorphic terrane in the previous studies were c. 120 Ma of the early Sanbagawa metamorphism while our new data provide an evidence of the older metamorphic event. The 138 Ma of Nd-Sm age reported from the Nikubuchi peridotite body within the Eastern Iratsu body (Senda et al., 2006) was interpreted as either granulite facies metamorphism or magmatic event, which indicates that the Eastern Iratsu body records a longer history than other members of the Sanbagawa metamorphic terrane. The newly-reported pre-Sanbagawa metamorphism may provide a key to decipher tectonic evolution of the subducting Izanagi plate.