Plagioclases in some granitic rocks, Kitakami mountainland, Northern Japan

Part I. Relationship between scatter in the optic axial angles and ordering degree.

Abstract

Optical characteristics of plagioclases in the Tono, Hitokabe, Uchino, Hirota, Ichinohe, Aoganebashi and Ochiai granitic masses in the Kitakami mountainland are investigated. Plagioclases in the Tono and the Hitokabe masses which are thought to be comparatively deep intrusives show the typical low-temperature type i. e. high ordering degree (0.8-1.2) proposed by Uruno (1963), and those in the Ichinohe mass show the intermediate ordering degree (0.6-1.0). However plagioclases in the Uchino, Hirota, Aoganebashi and Ochiai masses which are thought to be shallow and small intrusives show wide range of ordering degree. Optic axial angles of plagioclases having limited ordering degree show good agreement with the 2Vz-An content curve but those having wide range ordering degree do not show distinct relation between 2Vz and An content, namely the deviation of optic axial angles from the ideal curve is also due to cooling rate of granitic magma.