領家帯柳井地域新期花崗岩(木部花崗岩)のマグマ過程

Abstract

Cretaceous granitic rocks in the Chugoku district, Southwest Japan arc, are divided into the following three groups; Older Ryoke granitoids, Younger Ryoke granitoids and San'yo granites. The Kibe Granite, a representative Younger Ryoke granitoids in the Yanai area, Yamaguchi Prefecture, occurs as an elliptic outcrop associated with a coeval quartz diorite, and intruded the high-grade Ryoke metamorphic rocks. The Kibe Granite is dominated by homogeneous biotite-granite that contain euhedral alkali-feldspar phenocrysts. These rocks also contain local micro-magmatic enclaves and have migmatitic structures in contact zones with the pelitic gneisses in the marginal parts of the granite. The mixing and mingling structures are developed at the contact between the granite and quartz diorite. The major and trace element variations of the Kibe Granite can be explained by a subtraction of plagioclase together with minor biotite and alkali-feldspar from a parental magma. The Kibe Granite was formed through the following processes: assimilation of the pelitic gneisses, magma mixing with the quartz diorite and fractional crystallization of a parental magma. These processes took place at the exhumation stage of the Ryoke metamorphism following its isothermal decompression path.