Presentation Information
[T4-P-12]Formation of a metasomatic matrix and its effect on deformation in serpentinite mélanges
*Yasushi Mori1, Miki Shigeno1, Takayoshi Nagaya2, Kohtaro Ujiie3, Nobuo Gouchi4, Tadao Nishiyama5 (1. Kitakyushu Museum of Natural History and Human History, 2. Waseda University, 3. University of Tsukuba, 4. Shiretoko Museum, 5. Kumamoto University)
Keywords:
Serpentinite mélange,Metasomatism,Deformation,Kamuikotan Metamorphic Belt,Nagasaki Metamorphic Complex
Serpentinite mélanges in high-pressure and low-temperature metamorphic belts represent the products of mechanical mixing and metasomatic reactions in subduction zones (e.g., Bebout & Penniston-Dorland, 2016; Spandler & Pirard, 2013). The mélange typically consists of a serpentinite matrix with tectonic blocks of various lithologies. The matrix is locally or mostly replaced by metasomatic rocks such as talc schist and chlorite-actinolite schist. Here we describe the formation of a metasomatic matrix and its effect on deformation in serpentinite mélanges exposed on the Pankehoronai unit of Kamuikotan Metamorphic Belt and the Nishisonogi unit of Nagasaki Metamorphic Complex.
The Pankehoronai unit of Kamuikotan Metamorphic Belt mainly consists of pelitic schist with minor amounts of mafic, siliceous and calcareous schists (e.g., Sakakibara et al., 2007; Takeshita et al., 2018). The peak metamorphic condition is estimated as 290–350°C at ~0.5 GPa. The serpentinite mélange is tectonically intercalated to the coherent schists and consists of a scaly to schistose serpentinite matrix and tectonic blocks including metabasites, metapelites and jadeite-quartz rock. The serpentinite matrix is replaced by talc schist along the contact with coherent pelitic schist to form a shear concentration zone. The petrography and whole-rock chemistry suggest Si-metasomatism and dehydration during the formation of the talc schist.
The Nishisonogi unit of Nagasaki Metamorphic Complex mainly consists of pelitic and psammitic schists with minor amounts of mafic schists and serpentinite (e.g., Nishiyama et al., 2017, 2024). The peak metamorphic condition is estimated as 370–530°C at 0.9–2.3 GPa. The serpentinite mélange is tectonically intercalated to the coherent schists and consists of scaly to schistose serpentinite and chlorite-actinolite schist matrices including metabasites, albitite, metapelites and quartz-bearing jadeitite. The chlorite-actinolite schist matrix forms a shear concentration zone and contains the lenses of serpentinite, talc schist and actinolite rock. The petrography and whole-rock chemistry suggest mechanical mixing of serpentinite and metabasites, Ca-metasomatism and dehydration during the formation of the chlorite-actinolite schist.
These findings suggest that the metasomatic matrix of serpentinite mélanges is formed by metasomatism and lithological mixing and results in weakening of rocks and concentration of shear deformation. The metasomatic matrices in the Kamuikotan Metamorphic Belt and the Nagasaki Metamorphic Complex provide examples of reaction-enhanced ductility that probably caused by the formation of mechanically weak talc, the formation of fine-grained actinolite leading to solution-precipitation creep (Yamasaki et al., 2026), and the dehydration of serpentinite (Nishiyama, 1989).
Bebout & Penniston-Dorland, 2016. Lithos, 240–243, 228–258.
Nishiyama, 1989. Mem. Geol. Soc. Japan, 33, 237–257.
Nishiyama et al., 2017. JMPS, 112, 197–216.
Nishiyama et al., 2024. Island Arc, 33, e12527.
Sakakibaraet al., 2007. Jour. Geol. Soc. Japan, 113, 103–118.
Spandler & Pirard, 2013. Lithos, 170-171, 208–223.
Takeshita et al., 2018. Jour. Geol. Soc. Japan, 124, 491–515.
Yamasaki et al., 2026. JSG, 209, 105735.
The Pankehoronai unit of Kamuikotan Metamorphic Belt mainly consists of pelitic schist with minor amounts of mafic, siliceous and calcareous schists (e.g., Sakakibara et al., 2007; Takeshita et al., 2018). The peak metamorphic condition is estimated as 290–350°C at ~0.5 GPa. The serpentinite mélange is tectonically intercalated to the coherent schists and consists of a scaly to schistose serpentinite matrix and tectonic blocks including metabasites, metapelites and jadeite-quartz rock. The serpentinite matrix is replaced by talc schist along the contact with coherent pelitic schist to form a shear concentration zone. The petrography and whole-rock chemistry suggest Si-metasomatism and dehydration during the formation of the talc schist.
The Nishisonogi unit of Nagasaki Metamorphic Complex mainly consists of pelitic and psammitic schists with minor amounts of mafic schists and serpentinite (e.g., Nishiyama et al., 2017, 2024). The peak metamorphic condition is estimated as 370–530°C at 0.9–2.3 GPa. The serpentinite mélange is tectonically intercalated to the coherent schists and consists of scaly to schistose serpentinite and chlorite-actinolite schist matrices including metabasites, albitite, metapelites and quartz-bearing jadeitite. The chlorite-actinolite schist matrix forms a shear concentration zone and contains the lenses of serpentinite, talc schist and actinolite rock. The petrography and whole-rock chemistry suggest mechanical mixing of serpentinite and metabasites, Ca-metasomatism and dehydration during the formation of the chlorite-actinolite schist.
These findings suggest that the metasomatic matrix of serpentinite mélanges is formed by metasomatism and lithological mixing and results in weakening of rocks and concentration of shear deformation. The metasomatic matrices in the Kamuikotan Metamorphic Belt and the Nagasaki Metamorphic Complex provide examples of reaction-enhanced ductility that probably caused by the formation of mechanically weak talc, the formation of fine-grained actinolite leading to solution-precipitation creep (Yamasaki et al., 2026), and the dehydration of serpentinite (Nishiyama, 1989).
Bebout & Penniston-Dorland, 2016. Lithos, 240–243, 228–258.
Nishiyama, 1989. Mem. Geol. Soc. Japan, 33, 237–257.
Nishiyama et al., 2017. JMPS, 112, 197–216.
Nishiyama et al., 2024. Island Arc, 33, e12527.
Sakakibaraet al., 2007. Jour. Geol. Soc. Japan, 113, 103–118.
Spandler & Pirard, 2013. Lithos, 170-171, 208–223.
Takeshita et al., 2018. Jour. Geol. Soc. Japan, 124, 491–515.
Yamasaki et al., 2026. JSG, 209, 105735.
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