11:30 AM - 11:45 AM
[SMP28-10] Difference in thermal evolution of the Sanbagawa Belt between Kanto Mountains and Shikoku areas
Keywords:Sanbagawa Belt (Sambagawa Belt), Raman carbonaceous material geothermometry, Pseudosection analysis, biotite , oligoclase
The Sanbagawa Belt is a intermediate-high P/T type metamorphic Belt extending for about 800 km from the Kanto Mountains in the east to Kyushu in the west. The Sanbagawa Belt in the Kanto Mountains covers an area of about 60 km, divided into the western area where the Ayukawa River and Sanbagawa River flow, and the eastern area along the Arakawa River, represented by Nagatoro. In this study, the relationship between thermal structure and metamorphic zonation was examined by using Raman carbonaceous material (CM) geothermometry and pseudosection analysis in the Ayukawa unit in the western area, which is a type locality of the Sanbagawa Belt, and the chronological data in Shikoku were compared to discuss the metamorphic history of the Sanbagawa Belt.
In previous studies, the metamorphic zonation in the Kanto Mountains were divided into the chlorite, garnet and biotite zones, with the metamorphism grades roughly increase from south to north. In the Ayukawa unit, the occurrence of oligoclase is reported throughout the biotite zone, which is not divided into an albite-biotite zone and an oligoclase-biotite zone as in the Besshi unit in Shikoku. The Ayukawa unit is divided into two units according to the age of detrital zircon grains and the K-Ar and Ar-Ar ages. The chlorite zone and part of the garnet zone on the south side are the upper unit, with exhumation age of about 60–80 Ma in the Late Cretaceous. The part of garnet zone and biotite zone on the northern side are the lower units, with a relatively young Paleogene age of about 50–60 Ma.
The results of Raman CM geothermometry indicate the estimated temperature range from 357°C to 538°C, showing a rough trend of increasing temperature from south to north. The relationship between the metamorphic zonations and the estimated CM temperatures are around 360°C for the chlorite zone, about 390–470°C for the garnet zone, and about 470–540°C for the biotite zone. Compared with the relationship between CM temperature and metamorphic zonations of other areas of the Sanbagawa Belt, the biotite zone in the Kanto Mountains appears at about 470°C, whereas the albite-biotite zone in Shikoku appears at about 500°C. This difference in temperature is probably caused by the difference of the pressure conditions. Based on the pseudosection analysis, the pressure condition required for biotite to appear at about 470°C in the Kanto Mountains is about 0.8 GPa, whereas that required for biotite to appear at about 500°C in the Shikoku is about 0.9 GPa. In addition, pseudosection analysis shows that the volume of oligoclase is higher at lower pressures, which also supports that metamorphic pressure is lower in the Kanto Mountains than in Shikoku.
The exhumation age determined from the K-Ar and Ar-Ar ages of the white mica of the Besshi unit in Shikoku is older than that of the Ayukawa unit in the Kanto Mountains, at around 80 Ma. Based on this age data, the Kanto Mountains were still undergoing subduction or peak metamorphism when the Besshi unit in Shikoku was uplifting. In previous studies, a ridge approaching model has been proposed for the Sanbagawa Belt, the younger the age, the more likely the shallower and warmer metamorphism occurred. The Kanto Mountains are younger than Shikoku, especially the biotite zone, which is reported to have an exhumation age of about 50–60 Ma, close to the subduction phase of the ridge, suggesting that warm metamorphism occurred under low-pressure conditions that facilitated the occurrence of biotite and oligoclase.
In previous studies, the metamorphic zonation in the Kanto Mountains were divided into the chlorite, garnet and biotite zones, with the metamorphism grades roughly increase from south to north. In the Ayukawa unit, the occurrence of oligoclase is reported throughout the biotite zone, which is not divided into an albite-biotite zone and an oligoclase-biotite zone as in the Besshi unit in Shikoku. The Ayukawa unit is divided into two units according to the age of detrital zircon grains and the K-Ar and Ar-Ar ages. The chlorite zone and part of the garnet zone on the south side are the upper unit, with exhumation age of about 60–80 Ma in the Late Cretaceous. The part of garnet zone and biotite zone on the northern side are the lower units, with a relatively young Paleogene age of about 50–60 Ma.
The results of Raman CM geothermometry indicate the estimated temperature range from 357°C to 538°C, showing a rough trend of increasing temperature from south to north. The relationship between the metamorphic zonations and the estimated CM temperatures are around 360°C for the chlorite zone, about 390–470°C for the garnet zone, and about 470–540°C for the biotite zone. Compared with the relationship between CM temperature and metamorphic zonations of other areas of the Sanbagawa Belt, the biotite zone in the Kanto Mountains appears at about 470°C, whereas the albite-biotite zone in Shikoku appears at about 500°C. This difference in temperature is probably caused by the difference of the pressure conditions. Based on the pseudosection analysis, the pressure condition required for biotite to appear at about 470°C in the Kanto Mountains is about 0.8 GPa, whereas that required for biotite to appear at about 500°C in the Shikoku is about 0.9 GPa. In addition, pseudosection analysis shows that the volume of oligoclase is higher at lower pressures, which also supports that metamorphic pressure is lower in the Kanto Mountains than in Shikoku.
The exhumation age determined from the K-Ar and Ar-Ar ages of the white mica of the Besshi unit in Shikoku is older than that of the Ayukawa unit in the Kanto Mountains, at around 80 Ma. Based on this age data, the Kanto Mountains were still undergoing subduction or peak metamorphism when the Besshi unit in Shikoku was uplifting. In previous studies, a ridge approaching model has been proposed for the Sanbagawa Belt, the younger the age, the more likely the shallower and warmer metamorphism occurred. The Kanto Mountains are younger than Shikoku, especially the biotite zone, which is reported to have an exhumation age of about 50–60 Ma, close to the subduction phase of the ridge, suggesting that warm metamorphism occurred under low-pressure conditions that facilitated the occurrence of biotite and oligoclase.