Geological and Petrological Studies on the Hiroshima Granite in the Togouchi-Yuu-Takehara District, Southwest Japan
The main purpose of this paper is throwing light on the mechanism of emplacement of the Hiroshima granite and on the space problem of volcano-plutonism in the Inner Zone of Southwest Japan during the late Cretaceous. In order to discuss these problems which are classical and have so far been unsolved, the author has selected the Togouchi-Yuu-Takehara district as a granite field of the San-yo zone. In the southern end of the district, the granite is in contact with Ryoke granitic and metamorphic rocks formed in 10km depth of crust, and in the central area it intruded into pre-Cretaceous sedimentary rocks and into both the sedimentary rocks and Cretaceous surficial volcanic rocks in northern and eastern areas. The author have investigated the granite distributed in the Togouchi area and another several areas in the district, focuing mainly on contact feature between the granite and wall rocks and internal structure of the granite mass. As a result, it is clarified that throughout the district the Hiroshima granite is not a uniform mass but a mass formed by accumlation of layered bodies of granitic rocks. In the mass, rather mafic granite as granodiorite or hornblende granite occurs overlying rather felsic granite as medium to coarse-grained biotite granite. The forming order of them is closely connected with the stratigraphical sequence of the Cretaceous volcanic rocks. As a whole, it can be regarded that the Hiroshima granite is a large tabular mass gently dipping toward the south, and that the Hiroshima granite passively intruded along the fracture zone gently dipping southward, and at the head of the zone the Cretaceous volcanic activities took place. Additionaly, the stress field during and after the emplacement of granodiorite in the Togouchi area has been analysed based upon microfabric and homogenization temperature of fluid inclusion planes in quartz. Finally, the author propose flat-dike type mass for the emplacement mechanism of a large granite mass.