講演情報
[T1-O-12]Metamorphic History of Serpentinite and Amphibolite in the Sangun-Renge Belt exposed at Sasaguri, Fukuoka Prefecture
*Swarnaa ANNADURAI MUNUSAMY1, Jun-ichi ANDO1,2, Kaushik DAS1,2, Dyuti Prakash SARKAR3, Seiichiro UEHARA4 (1. Hiroshima University, 2. HiPeR, Hiroshima, 3. Yamaguchi University , 4. The Kyushu Univ. Museum)
キーワード：
Serpentinite、Cr-spinel、Amphibolite、Sangun-Renge Belt
The Sangun-Renge Belt is a high-pressure type metamorphic belt in southwestern Japan formed during the Late Paleozoic to Early Mesozoic. Around Narubuchi Dam in Sasaguri, Fukuoka Prefecture, serpentinites are exposed alongside high-pressure metamorphic rocks, including amphibolites. This study aims to constrain the metamorphic history of these rocks. The serpentinites in the study area can be classified into the following four types based on their mineral assemblages: (1) Lizardite-Chrysotile serpentinite, (2) Lizardite-Chrysotile-Antigorite serpentinite, (3) Lizardite-Antigorite serpentinite, and (4) Antigorite serpentinite. Their distribution varies across the study area. The northern region of the study area, is dominated by Type-4 serpentinite, characterized by coarse-grained antigorite with undulose extinction. A mylonitic zone formed by shear deformation is also present, where fine-grained antigorite formed through dynamic recrystallization. In contrast, all four types of serpentinite are exposed to the south of the above area. Type-1 serpentinite shows bastite and mesh texture. Type-2 serpentinite retains mesh textures with minor antigorite. Type-3 serpentinite features ribbon texture of antigorite. Type-4 serpentinite preserves pseudomorphic textures after pyroxene, including schiller structures and pyroxene replaced by magnesite and magnesio-anthophyllite. Accessory minerals such as Cr-spinel, magnetite, Cr-magnetite, chlorite, magnesio-anthophyllite, magnesite, and talc-chlorite are variably present among the different types. The composition of the accessory mineral Cr-spinel indicates the origin of the serpentinite as forearc peridotite. The presence of ferritchromite rim in the Cr-spinel grains suggests that the peridotite was re-equilibrated at greenschist to amphibolite temperatures (400-700 ℃). A magnetite overgrowth around the ferritchromite rim indicates that the peridotite was serpentinized after ferritchromitization, which suggests that the serpentinization occurred at 250-400 ℃. The bulk rock chemical analyses show that the serpentinites have very low Al2O3 (0.22-0.84 wt.%) and CaO (0.01-0.29 wt.%), and high MgO (34.75-39.63 wt.%). The very low abundances of Al2O3 and CaO are similar to the fore-arc peridotite composition. The high contents of Ni (2011-2949 ppm), Cr (1877-3637 ppm) and Co (82-105 ppm) and depletion in the incompatible elements indicate the depleted nature of the parent rock. Amphibolite bodies in the study area occur both as lenses within serpentinite and as a larger mass in the southern region. Their lithological assemblage is dominated by amphiboles: magnesio-hornblende, tschermakite, ferropargasite, pargasite, actinolite, and tremolite, along with plagioclase. Minor constituents include ilmenite, sphene, zircon, chlorite, and carbonates. Bulk rock chemistry suggests a basaltic protolith with predominantly MORB-like geochemical signatures. The formation temperature of hornblende, interpreted from hornblende-plagioclase thermometry, ranges from 550 ℃ to 700 ℃, while hornblende-only thermometry yields a comparable range of 550 ℃ to 750 ℃. The hornblende grains in the amphibolite are deformed, forming kink bands within which hornblende has been metamorphosed to actinolite. This suggests that the amphibolite was affected by retrograde metamorphism in a greenschist facies condition (250 ℃-400 ℃). Chlorite observed in the amphibolite is classified as pycnochlorite and ripidolite, and chlorite thermometry suggests retrograde metamorphic temperatures between 230 ℃ and 430 ℃ at about 0.2 GPa. In summary, these results show a two-stage metamorphic evolution in the serpentinite: initial equilibration of forearc peridotite under amphibolite to greenschist facies, followed by serpentinization during exhumation. Amphibolites record amphibolitization of a MORB-type protolith during subduction, subsequent incorporation into the continental region, and retrograde metamorphism during exhumation. Future work will involve constructing pseudosections to better constrain the pressure-temperature evolution of the studied rocks.