On Metamorphic Dikes in the Hiroshima Granitic Complex

A dike swarm, extending about 50 km E-W and 5～10 km wide, is found near Hiroshima City in the Hiroshima granitic complex. The dike swarm consists of various rock types ranging in composition from andesitic to rhyolitic, especially biotite-bearing hornblende-quartz-porphyrite and diorite-porphyrite being abundant. Although porphyrite and diorite-porphyrite have been intruded into the granitic rocks, some of them show metamorphic texture, that is, crystals of hornblende have been replaced wholly or partly by biotite, iron ore, quartz, apatite, and plagioclase.
The intrusives in the Niho and Mukainada areas, situated to the east of Hiroshima City, are divided into three groups, that is, the Ôkô metamorphic porphyrite group, the Fuchizaki por-phyrite group and the Tanna porphyrite group, based on their sequence of intrusion and petrographic features. Among these dike rocks, all of the Ôkô group and the early intrusives of the Fuchizaki group have metamorphic texture. The early porphyrite groups, that is the Ôkô group and the early Fuchizaki group, are presumed having intruded not long after the intrusion of the granitic rocks, which maintained high temperature and mobile conditions. Therefore, the granitic rocks are supposed still to have had some abilities to affect metamorphism at the time of the intrusion of the early porphyrite groups. The metamorphic texture is interpreted as the result of replacement of hornblende in the early porphyrite groups under the supply of potassium from the cooling granitic rocks.