Contact Metamorphic Effect on Basaltic Rocks by the Koyama Gabbro Complex, Susa Area, Southwest Japan

Tertiary basaltic rocks around the Koyama gabbro complex mass, which occurs in the northeastern part of Yamaguchi Prefecture, have undergone intense contact metamorphism by the mass. Radiometric age determinations indicate that the metamorphism is of Miocene age. The contact aureole is rather narrow (a maximum of about 700 m), the metamorphic grade rapidly increasing toward the mass. In the comparatively lower temperature part of the contact aureole, the metamorphic paragenesis is actinolitic hornblende ± hornblende + epidote+ biotite+ oligoclase + quartz. Meanwhile, the highest-grade rocks are represented by the stable association of orthopyroxene+ clinopyroxene ± hornblende + biotite+ labradorite + quartz. The metamorphic temperature close to the contact with the mass is estimated to be 800 to 860°C. In the most-intensely metamorphosed basalts, most of such phenocrystic minerals as clinopyroxene and plagioclase can still preserve the essential chemical characteristics in the original rocks, whereas phenocrystic orthopyroxene appears to have been chemically changed its composition during the metamorphism.