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Heguri-naka, Minami-Boso city, Chiba pref., Japan


Prehnite-Pumpellyite facies metamorphic rocks

Veins in the early Miocene (20 Ma) alkali basalt, hyaloclastite, gabbro, and serpentinite in the early Miocene (20 Ma) Mineoka Group, an accretionary complex corresponding to the Setogawa Group of the Shimanto accretionary complex. Protolith of serpentinite is harzburgite, and considered as refractory peridotite at shallow condition less than 30 km in depth. Subjected to Prehnite-Pumpellyite facies metamorphism. Ages of sediments were determined by radiolarians, alkali basalt by Ar-Ar radiometric dating.


Outcrop of tuff of the Mineoka Group 1 m apart from serpentinite block. The gray host is tuff, and the boundary between tuff and serpentinite is not clear. Most of white veins are pectolite with minor amount of analcime and tobermorite veins.

Reported Minerals

Mineral Assemblages

  • Hyaloclastite: Pectolite - Apophyllite
  • Hyaloclastite: Gyrolite - Laumontite - Thomsonite-Ca
  • Gabbro: Feldspar - Quartz - Augite - Hornblende - Magnetite - Ilmenite
  • Gabbro: Prehnite
  • Gabbro: Xonotlite
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