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Mount Nyukasa

Fujimi, Fujimi city, Suwa county, Nagano pref., Japan


Pumpellyite-Actinolite facies high-P metamorphic rocks

Minerals in basaltic tuff of the Mikabu Greenrocks, a member of the Northern Chichibu accretionary complex. The Mikabu Greenrocks is probably a part of the Middle Jurassic to Early Cretaceous accretionary complex cropped out between the Nothern Chichibu acretional complex and the Sambagawa metamorphic rocks. The Mikabu Greenrocks are composed of Carboniferous to Triassic Basalt lava, basaltic tuff breccia, basaltic tuff, and gabbro, and has been subjected to Pumpellyite-Actinolite facies metamorphism as a low grade part of the Sambagawa metamorphic belt in the Late Cretaceous (100 Ma). The protolith of the Mikabu Greenrocks is considered as mafic volcanic rocks and ultramafic rocks of oceanic plateaux and seamounts. Ages of sedimentary rocks were determined by conodont, and metamorphic age by K-Ar or Ar-Ar radiometric dating of muscovite.

Mount Nyukasa

Outcrop of basaltic tuff of the Mikabu Greenrocks subjected to Pumpellyite-Actinolite facies metamorphism. The massive grayish-green host is metamorphosed basaltic tuff. Dark green spots are aggregates of fine clinochlore crystals. There are also veinlets of clinochlore.

Reported Minerals

  • Clinochlore
  • Epidote

Mineral Assemblages

  • Clinochlore
  • Quartz - Epidote
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