R8: Metamorphic rocks and tectonics

[R8P-06] Exhumation process of serpentinite in the Sangun-Renge belt exposed at Sasaguri, Fukuoka prefecture

[Presentation award entry]

*Swarnaa ANNADURAI MUNUSAMY¹, Jun-ichi ANDO^1,2, Yuki IWASAKI³, Dyuti Prakash SARKAR^1,2, Kaushik DAS^1,2, Seiichiro UEHARA⁴ (1. Hiroshima Univ., 2. HiPeR, Hiroshima, 3. NIPPON STEEL CORP., 4. The Kyushu Univ. Museum)

Keywords:Exhumation, Serpentinization, Antigorite, Quartz, CPO

Antigorite serpentinite (serp) of the Sangun-Renge Belt is exposed at the northern part of Narubuchi Dam, Sasaguri, Fukuoka Pref. Clarifying the exhumation process of serp is important for elucidating tectonics in mantle wedges. For this purpose, we conducted a petrological study of the Sasaguri serp and the amphibolite exposed in contact with this serp. Field survey reveals a mylonitized zone and siliceous schist (ss) distributed in the serp mass. The strike and dip of both rocks are similar to the serp mass. The microstructures of the serp, ss, and amphibolite are observed by optical microscope and SEM, the composition of Cr-spinel in serp is measured by EPMA, and the CPO of antigorite (atg) and quartz in ss is determined by SEM-EBSD. The following are revealed. 1) Atg, a few mm in size, shows undulose extinction and dynamic recrystallization, but no CPO. 2) Atg in the mylonitized zone is less than 100 μm in size, with (001) and [010] oriented parallel to foliation and lineation, respectively. 3) The composition of Cr-spinel (Fig. 1) indicates that the source rock (peridotite) is of forearc origin. The ferritchromite rim suggests that the peridotite is re-equilibrated at greenschist to amphibolite temperatures. 4) Magnetite formed around the ferritchromite rim indicates that the serpentinization occurred after ferritchromization. 5) The source of ss is chert. 6) SS shows a porphyroclastic texture, with X max CPO of c-axis of porphyroclast and crossed girdle of neoclasts. Crossed girdle is known to form at 400-550°C. 7) The CPO of neoclasts indicates the top-to-north shear sense, which implies that recrystallization occurred during exhumation. 8) The hornblende thermometry shows that the amphibolite is formed at 550-700°C. 9) The kink band in the hornblende is metasomatized to actinolite, suggesting retrograde metamorphism at greenschist facies condition. Based on the above results, the exhumation process of Sasaguri serp will be discussed in our presentation.