TrekGEO > Mineral Trail > Kanto region > Saitama pref. > Orihara

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Orihara, Yorii city, Osato county, Saitama pref., Japan


Greenschist facies serpentinite

Actinolite-bearing talc schist in the Late Jurassic (150 Ma) to Early Cretaceous (130 Ma) accretionary complex subjected to Greenschist facies metamorphism as a part of the high-P Sambagawa metamorphic belt in the Late Cretaceous (70 Ma). The Sambagawa metamorphic rocks is considered that it has been near the ground surface since early Paleocene (65 Ma). Formation of talc can be explained by the reaction of serpentinite with quartz in surrounding sedimentary rocks. The origin of this serpentinite block is considered as sedimentary serpentinite mélange from serpentinite seamounts at the fore-arc slope. Metamorphic age was determined by K-Ar radiometric dating of muscovite.


Outcrop of talc schist and pelitic schist of Sambagawa Metamorphic Rocks. The upper black part with horizontal schistosity is spotted pelitic schist. The boundary between the upper pelitic schist and lower white talc schist is a small horizontal fault. There is another pelitc schist bed below the talc schist and it is subjected to strong deformation. The scale coin is at the lower right. Formation process of carbonate-absent talc schist can be explained by reaction of serpentinite with quartz as follows:

Mg3Si2O5(OH)4 (Serpentine) + 4SiO2 >> Mg3Si4O10(OH)2 (Talc) + H2O

Reported Minerals

  • Actinolite
  • Talc
  • Fuchsite
  • Albite

Mineral Assemblages

  • Talc - Actinolite
  • Pelitic schist(Host) - Fuchsite
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