TrekGEO > Mineral Trail > Kanto region > Saitama pref. > Nishi-no-Iri

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Nishi-no-Iri, Yorii city, Osato county, Saitama pref., Japan



Blocks of blueschist facies metamorphic rocks in the serpentinite mélange at the boundary between the Late Cretaceous (80 Ma) Atokura Formation and the Late Cretaceous (100 Ma) high-P Sambagawa metamorphic rocks. The serpentinite mélange is considered as a part of the Kiroko mélange at the southern margin of the Atokura Formation. A part of the metamorphic rocks of the Kiroko mélange is Early Devonian (400 Ma). The Atokura Formation is composed of sandstone-mudstone turbidite and conglomerate formed at the continental shelf, and considered as a thrust nappe over the Sambagawa metamorphic rocks. Ages of sediments were determined by Ammonoidea, Inoceramus, radiolarians, and metamorphic rocks by K-Ar radiometric dating of hornblende.


Outcrop of blueschist facies metamorphic rocks in serpentinite in the Kiroko mélange. The grayish-white host is composed of albite, quartz, and jadeite. Quartz contacts jadeite. The transparent vein above the coin is quartz. The aggregates of fibrous blue crystals are crossite. Crossite is associated with quartz.

Reported Minerals

  • Glaucophane
  • Ferroglaucophane
  • Albite
  • Jadeite
  • Corundum

Mineral Assemblages

  • Quartz - Albite - Jadeite - Crossite
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