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Mount Osa

Osa-Kominami, Niimi city, Okayama pref., Japan



Blocks of metamorphic rocks in the Osayama Serpentinite Mélange, a part of the Late Ordovisian (450 Ma) Oeyama Ophiolite in the Hida Marginal Belt in the broad sense. The Oeyama Ophiolite Thrust overlies the high-P Early Jurassic (180 Ma) Suo Metamorphic Belt. The blocks of metamorphic rocks have been subjected to the Suo metamorphism. The Osayama Serpentinite Mélange has also been subjected to Hornblend-hornfels facies contact metamorphism by the Late Cretaceous (70 Ma) granite. Approximately 2 km apart from granite, the heat source. The age of zircon in the jadeite block in the Osayama Serpentinite Mélange is the Cambrian (500 Ma). The age of ophiolite was determined by K-Ar radiometric dating of hornblende, zircon by U-Pb and Pb-Pb SHRIMP dating.

Mount Osa

Outcrop of jadeite block in serpentinite of the Osayama Serpentinite Mélange. The grayish-green part is fragile serpentinite. The white block above the scale coin is the massive white jadeite block. There are zircon crystals and white grossular veins in the jadeite block.

Reported Minerals

  • Zircon
  • Jadeite
  • Stronalsite
  • Grossular
  • Nigrine(Fe-rich rutile)

Mineral Assemblages

  • Jadeite - Zirzon - Grossular - Stronalsite
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