The Tagawa metamorphic rocks in eastern North Kyushu consist of the Main and Honjo bodies, which are mainly composed of pelitic-psammitic schist and amphibolite. The pelitic-psammitic schist exhibits preferred orientation of biotite, graphite and fibrous sillimanite, and cordierite and garnet include those minerals. This suggests that the Tagawa metamorphic rocks underwent low-P/high-T regional metamorphism at amphibolite facies condition. The metamorphic conditions of the Main and Honjo bodies are estimated to be 530-600°C, 0.3-0.4 GPa, and 600-650°C, 0.2-0.3 GPa, respectively. Andalusite porphyroblasts that include biotite and graphite might have been produced from contact metamorphism by the Ochiai Granodiorite and Masaki Granite. Detrital zircons in pelitic-psammitic schist display two major U-Pb age peaks at 1850 Ma and 250 Ma. These age peaks are similar to those of detrital zircons from the Suo metamorphic complex from the Kurume and Omuta districts, suggesting that the Tagawa metamorphic rocks also belong to the Suo metamorphic complex. This metamorphic complex sustained low-P/high-T regional metamorphism in the interval between the high-P/low-T Suo metamorphism and contact metamorphism by the Cretaceous granitic rocks.

The Daito granodiorite is a large plutonic mass in the San'in Batholith, southwest Japan. In this study, the igneous activity that formed the Daito granodiorite was investigated based on field and petrographic descriptions, magnetic susceptibility measurements, and whole-rock chemical analyses. The granodiorite is characterized by: 1) a ubiquitous distribution of mafic magmatic enclaves; 2) euhedral hornblende and biotite crystals; 3) abundant magnetite associated with mafic minerals; and 4) high magnetic susceptibility (>5 × 10^-3). The lithology varies from medium- to coarse-grained hornblende-biotite granodiorite to medium-grained biotite granite. These variations resulted mainly from the fractional crystallization of plagioclase, hornblende, and biotite. Significant revision of the geological map of the Daito granodiorite suggests that the intrusion occurred at ca. 57 Ma. This and previous studies show that the Daito granodiorite is an important product of Paleocene activity in the Younger Inbi intrusive group (68-53 Ma) of the San'in batholith.

Paleogene formations locally distributed in the southern Chugoku region and the eastern coastal areas of the Seto Inland Sea, southwest Japan, were analyzed in this study. The Paleogene strata were roughly divided based on their major constituents into a formation comprising non-marine and marine sandstones and mudstones and a second formation composed of fluvial conglomerates that are lithologically similar to the previously named “mountain gravels”. However, detailed age information is unavailable for some of these strata. The first reliable zircon U-Pb age was obtained using laser ablation-inductively coupled plasma-mass spectrometry on zircon grains from a fine-grained sandy tuff intercalated with an unnamed conglomerate bed that, based on lithology, can be correlated with the “mountain gravels” distributed in the western part of Mihara city, Hiroshima Prefecture. The zircon grains yield a U-Pb age of 40.8±0.1 Ma (weighted mean age of the youngest cluster), which is interpreted as the maximum depositional age of the sandy tuff, thus establishing a middle Eocene age for this formation in the southern areas of the Chugoku region.

This study analyses the joint system of the Middle Miocene Osumi granodiorite batholith in southern Kyushu, Southwest Japan. The analysis included 916 joints at 96 localities covering the northern, middle, and southern domains of the exposed area of the Osumi granodiorite batholith. Two major sets of steep joints striking NNW and ENE and a minor set of gentle joints are recognized in the northern and the middle domains. Two further sets of steep joints striking NS and EW with a minor set of gentle joints are recognized in the southern domain. Previous paleomagnetic studies indicate counterclockwise rotation of microplates in southeast Kyushu after 2 Ma. When the orientations of the joints are restored according to the rotation angles of the microplates (45° for the northern-middle domains and 30° for the southern domain), the strikes of the steep joints are largely oriented NNE and WNW.

Major and trace element compositions and a K-Ar age are reported for the Sakiura welded tuff of the Miocene Komegawaki Formation in the Mikuni coastal area, Fukui prefecture, central Japan. The rock samples are densely welded, generally show eutaxitic texture, and contain 9-17 vol.% plagioclase, minor (<1 vol.%) hornblende, and 1-14 vol.% lithic fragments. Plagioclase in the Sakiura welded tuff yields a K-Ar age of 13 Ma, which is within the 12-14 Ma age range of Komegawaki volcanic rocks. Sakiura welded tuff samples are rhyolitic (SiO₂ = 73.3-74.5 wt.%) and are near the SiO₂-rich end of the compositional trend of Komegawaki volcanic rocks on major element versus SiO₂ cross-plots. The MORB-normalized trace element patterns of Sakiura welded tuff samples are similar to those of the Komegawaki volcanic rocks. These observations indicate that the origin of the rhyolite magma that produced the Sakiura welded tuff is associated with the Komegawaki volcanic rocks. Evidently, multiple rhyolitic eruptions occurred concurrently with the formation of the Miocene volcanic rocks after the opening of the Japan Sea.