Abstract
Omine Acid Rocks are distributed in the mountainous part of Kii peninsula and made up of several hypabyssal plutons which are scattered in the north-south direction for the stretch of about 40km. The main constituent minerals of the acid rocks are quartz, potassium-feldspar, plagioclase and biotite. Potassium-feldspar changes toward Or-rich and An-poor composition during fractional crystallization, while plagioclase proceeds toward Ab-rich composition. Between phenocryst and groundmass in both feldspars, there is a compositional gap, which may be caused by the change of pressure and temperature conditions during cooling. Two feldspar equilibrium relation is as follows: phenocrystic plagioclase begins to crystallize first, followed by phenocrystic potassium-feldspar which coexists with sodic plaiogclase. Then, in the groundmass stage, the two feldspars crystallize at once. Other constitutent minerals are hornblende, ferro-hypersthene, augite and fayalite. These chemical compositions which are observed by an electron probe mocro-analyzer, are presented in this paper.
