Two stage fractional crystallization of the porphyritic biotite granite from the Iwafune granitoids in the Budo Mountains, Niigata Prefecture, Japan.

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  • 新潟県北部，蒲萄山塊に分布する岩船花こう岩類中の斑状黒雲母花こう岩の２段階分別結晶作用
  • ニイガタケン ホクブ ブドウサンカイ ニ ブンプ スル イワフネ カコウガンルイ チュウ ノ ハンジョウ クロウンモ カコウガン ノ 2 ダンカイ フンベツ ケッショウ サヨウ

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The Late Cretaceous to Paleogene Iwafune granitoids of are distributed around the border between Niigata and Yamagata Prefectures, Japan. The Budo Mountains is located in northern part of Niigata Prefecture and is exposed in an area covering more than 250 square kilometers. It consists of Iwafune granitoids, which are divided into six lithological types ; porphyritic biotite granite (PBG), biotite granite (BG), garnet two-mica granite (GTMG), two-mica granite (TMG), quartz diorite (QD) and granodiorite (GD). The six rock types can be assigned to three rock groups on the basis of petrological observations as follows : biotite granite group (BG-group ; PBG and BG), and tow-mica granite group (TMG-group ; GTMG and TMG) and diorite group (D-group ; QD and GD).Most of the granitoids in this area are attributed to BG-group. This group is composed of quartz, plagioclase, potassium feldspar, biotite and minor amounts of accessory minerals. Especially, the PBG contains potassium feldspar megacrysts and sometimes shows schlieren layering owing to the concentration of biotite and plagioclase. BG-group has medium SiO2 content ranging from 69 to 77 weight percent and is peraluminous.Modal and bulk chemical compositions show that BG-group has curved trend. Mass balance calculation indicates that the BG-series magma can be derived from common parental magma through two-stage fractional crystallization of potassium feldspar, biotite and plagioclase. The first stage is characterized by plagioclase- and biotite-rich fractionation, and the second stage is characterized by fractionation of biotite and potassium feldspar more than plagioclase.

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