Magma process of Gamano granodiorite in Ryoke belt, Yanai region, Yamaguchi, Southwest Japan

Abstract

Magma processes associated with plutonic rocks are known to involve fractional crystallization, assimilation with crustal materials, and magma mixing and/or mingling with coeval intrusive rocks. We studied such processes in the Yanai region, in the western part of Yashiro-jima Island and the Murotsu Peninsula (Yamaguchi Prefecture). In this area, the Gamano granodiorite and high-grade rocks of the Ryoke metamorphic belt are exposed. Field relations and geochronological data indicate that the emplacement of the Gamano granodiorite occurred simultaneously with peak metamorphism. We identified four lithologies in the Gamano granodiorite (biotite granodiorite, garnet granodiorite, hornblende granodiorite, and biotite granite). The main facies is the biotite granodiorite, whereas garnet granodiorite and hornblende granodiorite occur around the pelitic gneiss and coeval diorite, respectively. The garnet granodiorite contains zoned garnet crystals, characterized by an almandine-rich core, possibly derived from pelitic gneiss xenocrysts, and a spessartine-rich rim, which was crystallized from the granitic magma. Mixing and/or mingling microscopic textures are recognized in the hornblende granodiorite. The initial values of Sr-Nd isotopic ratios support the idea that the garnet granodiorite and hornblende granodiorite interacted with the pelitic gneiss and the diorite, respectively. The biotite granite, which is the fourth lithology in the Gamano granodiorite, is recognized as small stocks intruding into the biotite granodiorite. Geochemical variation diagrams show overlapping trends of the biotite granite and biotite granodiorite. In addition, these two lithologies show similar isotopic compositions. Based on petrological considerations, it is suggested that the garnet granodiorite was produced by assimilation of the biotite granodiorite magma with the host pelitic gneiss. In contrast, the hornblende granodiorite was likely produced by magma mixing and/or mingling between the biotite granodiorite and the diorite magmas. The biotite granite likely represents an evolved magma derived from the biotite granodiorite.