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.

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  Twin crystals of zircon in SK030 volcanic ash layer in Uonuma Formation, which is Cenozoic strata in Niigata Prefecture, central Japan, were examined mineralogically. Zircon in the SK030 layer was along with monazite, xenotime and corundum crystals. Major twin types were contact twin and penetration twin. Based on the twin forms, the former is subdivided into Fan–type I and II, and the latter into Wedge–type I and II. Cathod luminescence (CL) images of the zircon twins showed growth layer. In zircon crystals with the penetration twin, complex growth patterns were observed. Based on high resolution transmission electron micorscopic images and reciprocal lattice analysis of the corresponding ED patterns of the twin structure, the twin plane was determined as {112} plane.

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  Rb-Sr geochronological analyses were carried out for the Soeda Granodiorite, eastren North Kyushu, Southwest Japan. We obtained two Rb-Sr isochron ages from the granodiorite samples, such as 98.1±0.8 Ma defined by felsic fraction, whole-rock, and biotite, and 97.0±0.9 Ma defined by plagioclase, K-feldspar, whole-rock, and biotite. They are cooling ages for intrusion, that is, the ages represent the time at which the rocks cooled trough about 300°C. They are in range of K-Ar biotite ages from Cretaceous granitic rocks in eastren North Kyushu. Thus, the Soeda Granodiorite cooled down about 300°C at this time.

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  「鉱物結晶図鑑」
  「教授を魅了した大地の結晶　北川隆司　鉱物コレクション200選」

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