Sixteen trace elements, Co, Cr, Sc, Ba, Hf, Th, La, Ce, Sm, Eu, Tb, Yb, Lu, Rb, Cs and Ta, in the late Cretaceous_??_Paleogene granitic rocks were analyzed by instrumental neutron activation technique for 64 samples collected from Chugoku district, southwestern Japan. SiO₂, Rb and Sr were determined by X-ray fluorescence method. The analytical results were shown in Table 1, and the average trace element contents in the granitic rocks were calculated as shown in the first column of Table 2. The late Cretaceous_??_Paleogene granitic rocks of Chugoku district have 2 or 3 times higher Th content compared with the Quaternary calc-alkaline volcanic rocks of northeastern Japan.

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The late Cretaceous Koto Rhyolite is divided into two groups according to the succession. Each group forms an igneous cycle. The older group consists of the Kaiwara Welded Tuff and the Hatasho Quartz Porphyry, and their relationship is transitional in the field showing their co-magmatic origin. The Kaiwara Welded Tuff erupted apparently first as vesiculated magma from the top of the magma_??_reservoir followed by the intrusive phase of the Hatasho Quartz Porphyry. There are enrichment of phenocrysts such as quartz and alkali feldspar in the Kaiwara Welded Tuff, whereas plagioclase phenocrysts are more commo in the Hatasho Quartz Porphyry. The K/Rb ratios of the whole rocks are larger in the Hatasho Quartz Porphyry than in the Kaiwara Welded Tuff.
The younger group consisting of the Yatsuoyama Pyroclastic Rock and the Inugami Granite Porphyry is also considered to be of co-magmatic origin although obvious intimate relationship of the two units could not be observed in the field. The differences between them in the composition of phenocryst minerals and K/Rb ratio of the whole rocks are similar to and somewhat larger than those between the Kaiwara Welded Tuff and the Hatasho Quartz Porphyry.
The Inugami Granite Porphyry of the last igneous activity intruded along the ring faults whose center subsided stepwise and resulted in a double ring dike about 30km across. Thus the Koto Cauldron was composed.
Natural remanent magnetism through the Koto Rhyolite suggests a clock-wise movement of the area during its igneous activity.

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The fission track dating has been carried out on zircon separated from six specimens of the Hyugami Lava which outcrops in Oita Prefecture. The results are as follows:
HW-20 Two-pyroxene hornblende andesite Kameishi-yama 33°1l′30″N, 131°03′45″E 0.42±0.09 Ma
HW-3 Altered hornblende andesite (borecore) Noya 33°14′28″N, 131°16′21″E 1, 56±0.41 Ma
HW-5 Altered hornblende andesite (borecore) Noya 33°14′03″N, 131°15′42″E 1.80±0.29 Ma
HW-7 Biotite-bearing hornblende andesite Yoko-yama 33°15′28″N, 131°14′03″E 1.78±0.32 Ma
HW-12 Biotite-bearing hornblende andesite Karuto-yama 33°15′51″N, 131°17′26″E 2.2110.29 Ma
HW-19 Two-pyroxene hornblende andesite Hyugami 33°10′26″N, 131°46′59″E 2.85±0.49 Ma
Considering both these fission track ages and the results of modal analysis of the rocks, HW-20 may belong to the middle Pleistocene Bungo Volcanic Rocks, and HW-3, HW-5 and HW-7 to the early Pleistocene Hohi Volcanic Rocks. However, HW-12 and HW-19 erupted in the Pliocene and come under none of known volcanic groups in central Kyushu.

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Seven new fission-track ages of zircons from pyroclastic flow deposits in the Kagoshima City area, southern Kyushu, have been determine by the external detector method. The ages of the deposits are as follows: Omine 1, 2.2±0.4 (±2σ) m. y.; Omine 3, 0.7±0.2 m. y.; Omine 4, 0.5±0.2 m. y.; Omine 5, 0.5±0.2 m. y.; Kogashira 2, 0.7±0.1 m. y.; Yoshino, 0.5± 0.2 m. y. and Mifune, 0.9±0.2 m. y. These data were supplemented with previously determined fission-track ages for the Upper Kakuto, 0.3±0.1 m. y., Lower Kakuto, 0.3±0.1 m. y. and Giono, 2.0±0.4 m. y. to provide time markers for future correlations. All the ages, except the Mifune rhyolite, are consistent with the stratigraphy in the Kagoshima City crea.

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