Abstract

Thirteen biotites and eleven hornblendes in granitic rocks from the central part of the northern Abukuma mountains were chemically analysed. The biotite compositions indicate that the Po2 conditions of the Abukuma granitic rocks were intermediate between the high Po2 of the Kitakami mountains and the San'in zone, and the low Po2 of the Ryoke zone. AlVI and AlIV in hornblendes from the Abukuma granitic rocks are moderate amounts in compared with those from other granitic provinces of Japan. High AlIV in hornblendes seems to be related to low Po2 in granitic magmas. A slightly declining Po2 condition during crystallization of the Abukuma granitic rocks caused an increasing AlIV trend in hornblende towards more acidic rocks.

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