Magma process of the Kibe Granite (One of the Younger granitoids) in the Yanai area of the Ryoke Belt, southwest Japan 赤o 英里(Eri AKASAKI) 亀井 淳志(Atsushi KAMEI) 大和田 正明(Masaaki OWADA) 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.
The Cretaceous Ushikiri-yama granodiorite is exposed in the northern part of Tagawa city (Fukuoka Prefecture, SW Japan) and is part of the North Kyushu batholith. Three types of dikes with varying compositions are associated with the Ushikiri-yama granodiorite: porphyritic fine-grained diorite (Pr Fine Di), porphyritic finegrained tonalite (Pr Fine To), and porphyritic fine-grained granite (Pr Fine Gr). The granodiorite intrudes the pre-Cretaceous basement rocks as an isolated stock, which can be divided into two bodies (North and South bodies), separated by a fine-grained granodiorite unit with a hypidiomorphic granular texture. The South body, which contains magmatic epidote, was intruded by the North body. Higher initial Sr isotope ratios of the South body, relative to the North body, indicate that sedimentary rocks were assimilated in the magma of the South body during its emplacement in the middle/upper crust. Geochemical data indicate that the Pr Fine To resulted from mixing of the magmas that produced the South body and the Pr Fine Di, with minor crustal contamination by host sedimentary rocks. Fractional crystallization of plagioclase, biotite, and hornblende in the South body resulted in the differentiation of the evolved Pr Fine Gr. In addition, Pr Fine Di has a sanukitic high-Mg andesite composition that is similar to Cretaceous high-Mg diorite bodies from the North Kyushu batholith. Overall, the results indicate that a variety of igneous processes including magma mixing, fractional crystallization, and assimilation with the host rocks, were responsible for the formation of the Ushikiri-yama granodiorite.
Special Issue``Japanese granites and tectonics'' PrefaceGranitic continental crust is a unique feature of the Earth and, as such, the origin and growth of granitic continental crust is a major focus of research in the earth sciences. This special issue focuses on petrochemistry and isotope petrology, geology petrography of granites, U Pb zircon geochronology, K Ar geochronology, crust mantle structure beneath Northeast Japan, and evolution of the continental crust. These topics provide usuful information on petrogenesis and tectonic evolution of the arc crust of the Japanese Islands.
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