Petrological study on the Konoyama gabbroic body in the Ryoke belt, Kinki district, Japan

Abstract

Gabbroic rocks and granites are exposed in the Konoyama hill, Kinki district, Japan. The gabbroic body is composed of the following five rock-types: 1. hornblende gabbro, 2. pyroxene-olivine-hornblende gabbronorite, 3. leucocratic hornblende-gabbro, 4. pyroxene-hornblende gabbronorite and 5. cortlandite. Hornbende gabbro is the most dominant rock. Pyroxene-olivine-hornblende gabbronorite and pyroxene-hornblende gabbronorite sporadically occur in hornblende gabbro. Cortlandite is found as local facies of hornblende gabbro. These rock-types have gradational relations one another. The gabbroic and metamorphic rocks have no sign of intrusive relations. The granitic rocks intruding into the gabbros are tonalite, gneissose granodiorite, weakly schistose granite and fine-grained granite. Chemistry of the gabbroic rocks is characterized by low K2O and Rb contents. Plagioclases are highly calcic. On the other hand, Mg/Mg+Fe ratios of ferromagnesian minerals are rather low in a range from 0.70 to 0.73. These features suggest that the gabbroic rocks were formed under high vapour pressures.