Onara
Sanbagawa, Fujioka city, Gunma pref., Japan
"No Trespassing" sign was erected
Occurrence
Greenschist facies serpentinite
Serpentinite 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). This serpentinite block is associated with metamorphosed mafic volcanic rocks. The Sambagawa metamorphic rocks are considered that it has been near the ground surface since early Paleocene (65 Ma). 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. This locality had been a Ni mine and is a quarry for serpentinite.
Outcrop of serpentinite in Sambagawa Metamorphic Rocks. The dark green outcrop is uniformly composed of serpentinite.
Reported Minerals
- Piemontite
- Albite
- Pyrite
- Malachite
- Fuchsite
- Magnetite
- Hydromagnesite
- Brucite
- Tremolite
- Serpentine
- Pumpellyite
- Clinothulite
- Sursassite
- Aragonite
Mineral Assemblages
- Piemontite - Albite - Quartz - Muscovite
- Epidote -Pyrite - Malachite - Quartz - Albite
- Quartz - Albite - Muscovite - Graphite - Fuchsite
- Serpentine - Magnetite - Hydromagnesite - Aragonite
- Serpentine - Brucite - Tremolite
- Pumpellyite
- Albite - Quartz - Clinozoisite - Sursassite
Localities
- Orihara (Talc, High-P)
- Matsunaga Mine (Talc, High-P)
- Shiraki (Pectolite, High-P)
- Arai (Diopside, High-P)
- Osaka (Ophicalcite, High-P)
- Miyazawa (Dolomite, High-P)
- Onara (Fe-Ni, High-P)
- Yashio mine (Fe-Cr, High-P)