On the gabbroic rocks of the Ikomayama Mountains in the Kinki District

Abstract

The gabbroic rocks and granites are exposed in the Ikoma Mountains on the eastern border of the Osaka Basin. The gabbroic rocks are divided into the following five rock-types; 1) fine-grained pyroxene-hornblende gabbronorite, 2) olivine-pyroxene-hornblenda gabbronorite, 3) hornblende gabbro, 4) plagioclase-por-phyritic hornblende gabbro, 5) leucocratic hornblende gabbro. Among them, hornblende gabbro is the main type. Fine-grained pyroxene-hornblende gabbronorite is included in hornblende gabbro. Olivine-pyroxene-hornblende gabbronorite and hornblende gabbro are intruded by plagioclase-porphyritic hornblende gabbro and leucocratic hornblende gabbro. These gabbroic rocks are intruded by granitic rocks. The granitic rocks are classified into tonalite, K-feldspar-porphyritic biotite granite, weakly schistose coarse-grained biotite granite, massive medium-grained granodiorite and fine-grained granite. Weakly schistose coarse-grained biotite granite and K-feldspar-porphyritic biotite granite are the main types of the granites. The intrusive succession of the granites is assumed that tonalite is oldest and fine-grained granite is youngest (Fig. 3). The major elements of the gabbroic rocks vary systematically with the SiO_2 content. The study of ^<87>Sr/^<86>Sr ratios suggests that the gabbroic rocks are the products of magmatic differention from the tholeiitic magma. Plagioclase has high anorthite contents (An 85-95), but forsterite contents of olivine are rather low ranging from 0.6 to 0.7. These facts suggest the crystallization under the high water vapor pressure.