Syenitic Rocks of the Utsugiono District, Yamaguchi Prefecture (I)

Abstract

Syenitic and alkali-granitic rocks are dike-like masses in the granitic rocks of the Utsugiono district. They are divided into the following six types. 1. Leucocratic and garnet-syenitic rocks. 2. Pyroxene-hornblende- and hornblende-syenitic rocks. 3. Epidote-syenitic rocks. 4. Potash-syenitic rocks. 5. Porphyritic svenitic rocks. 6. Syenitic aplite. Gradational relation can be found between syenitic rocks, which are very crushed and recrystallized, marginal alkali-granitic rocks, and outer granites. When alkali-granitic rocks contact with pegmatitic veins, there can not be found anv mutual intrusive relation, but plagioclase crystals in pegmatite are albitized and in latter rocks quartz often decreases. From these facts, it is concluded that shear zones in granitic rocks have been transformed to be syenitic by the effect of metasomatic reaction with alkali-solution and metamorphic differentiation in granitic rocks. The original solution might have been albitic and followed to the arrow direction in geological map, being gradually potassic by the reaction with granitic rocks, as shown by the relation between rock types, modal compositions, minerals, and the distribution of syenitic rocks. The peculiar syenitic rocks in the Setouchi region might equally be interpreted as of similar metasomatic origin.