R8：変成岩とテクトニクス

10:00 〜 10:15研究発表優秀賞エントリー

[R8-02] Tectono-Metamorphic Evolution of Serpentinite and Amphibolite in the Sangun-Renge Belt exposed at Sasaguri, Fukuoka Prefecture

*Swarnaa ANNADURAI MUNUSAMY¹, Jun-ichi ANDO^1,2, Kaushik DAS^1,2, 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 high-pressure metamorphic rocks of the Sangun-Renge Belt are exposed around Narubuchi Dam in Sasaguri, Fukuoka Prefecture. This study aims to constrain the metamorphic evolution and tectonic history of serpentinite and amphibolite in the Sangun-Renge Belt through detailed petrographic and geochemical analyses. Serpentinites are classified into four types based on 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. Cr-spinel composition suggests a forearc peridotite origin. Ferritchromite rim around the Cr-spinel indicates re-equilibration at 400-700 °C (greenschist-amphibolite facies), while magnetite overgrowth suggests serpentinization at 250-400 °C. Bulk rock chemistry shows very low Al₂O₃ and CaO, high MgO, and enrichment in Ni, Cr, and Co, along with depletion of incompatible elements, indicating that the serpentinites originated from a depleted fore-arc mantle source. Amphibolite in the study area occurs both as lenses within serpentinite and as a larger mass in the southern region. Bulk rock chemistry suggests a basaltic protolith with predominantly MORB-like geochemical signatures. Hornblende formation temperature interpreted from hornblende-plagioclase and hornblende-only thermometry ranges between 550 °C and 750 °C with minor variations. Hornblende grains in the amphibolite are deformed, forming kink bands where they altered to actinolite, indicating greenschist-facies retrograde metamorphism (250-400 °C). The chlorite observed in amphibolite, identified as pycnochlorite and ripidolite, yields thermometry temperatures of 230-430 °C at about 0.2 GPa. In summary, the serpentinites record equilibration of forearc peridotite under amphibolite-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.