Vitrinite reflectance and optical anisotropy in metamorphosed to weakly metamorphosed accretionary complexes: An example from the Sanbagawa and Nothern Chichibu belts, east Shikoku, Japan

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  • 変成～弱変成付加体におけるビトリナイトの反射率と光学異方性：四国東部の三波川変成帯－北部秩父帯を例として
  • ヘンセイ ジャクヘンセイ フカタイ ニ オケル ビトリナイト ノ ハンシャリツ ト コウガク イホウセイ シコク トウブ ノ サンバガワ ヘンセイタイ ホクブ チチブタイ オ レイ ト シテ

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Abstract

The Sanbagawa Belt is a typical subduction-related high-P/T metamorphic terrane that is composed mainly of crystalline schists that originated as clastic and basic rocks, and it is exposed along the south side of the Median Tectonic Line in SW Japan. This belt has radiometric ages of Cretaceous time. The Northern Chichibu Belt, which occurs to the south of the Sanbagawa Belt, is a Jurassic accretionary complex, and mainly made up of chert, mudstone and sandstone. We have performed systematic measurements of vitrinite reflectance on coalified phytoclasts in pelitic rock from metamorphosed to weakly metamorphosed accretionary complexes, the Sanbagawa Metamorphic and the Northern Chichibu belts, eastern Shikoku. We have measured apparent maximum and minimum reflectances on polished particulate specimens. The measurements have revealed that the vitrinites are optically biaxial. We approximated the values of maximum (Rmax), intermediate (Rint) and minimum (Rmin) reflectances. The maximum reflectance value increases toward the lower structural level in the Northern Chichibu Belt, although, it shows no systematic variation in the Sanbagawa Belt. The average of maximum reflectance values from the Northern Chichibu Belt (5.21%) is higher than that from the Sanbagawa Belt (4.79%). The flattening for maximum and minimum reflectances ((Rmax-Rmin) /Rmax) increases generally to the north, that is, to the lower structural level. This would imply an increase of burial depth or depth of accretion.

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