Petrographical and whole-rock geochemical analyses were conducted on volcanic blocks from tuff breccia of the late Miocene Kitasakamoto Formation in the eastern part of northern Kyushu Island, southwest Japan for investigation of petrogenesis and magma process. On the basis of the petrography, the volcanic blocks are divided into four rock types; two-pyroxene andesite, hornblende-two-pyroxene andesite, hornblende andesite, and biotite-hornblende dacite. The former two types are main constituent of the blocks, whereas the latter two types are minor and obtained only from the lowermost part of the formation. They have microscopic disequilibrium textures as a result of magma mixing processes, such as 1) sieve texture and dusty zoning of plagioclase, 2) corroded quartz phenocrysts, and 3) reaction rims surrounding hornblende crystals. Some samples of the volcanic blocks reach the compositional range of high Sr/Y ratios coupled with low Y concentrations, similar to products from slab melting. Therefore, the adakitic magmatism is at least required as one endmember of the mixing magma.

This paper focuses characteristics of paleogeographical and geological features observed near the unconformity surface of the basal Miocene of the Chichibu Sedimentary Basin potentially indicating an ancient rocky shore, and describes and discusses those on the basis of detailed geological surveys, with investigations of grain size and roundness of the clastic sediments.

The characteristic paleo-topographies on the unconformity surface observed in the western area of Nomaki (Sakuragaya), located in the northeastern part of the Chichibu Sedimentary Basin and their interpretations, are as follows: the two upper and lower flat surfaces are judged to be abrasion platforms, the concave on the upper flat surface is a pothole, and the overhanging slope at the boundary between the two flat surfaces is a notch. The lower flat surface is a newer platform and the upper one is in the stage of erosion and collapse. Based on these paleo-topographies, the paleo-coast restored near Nomaki in the early stage of development of the Chichibu Sedimentary Basin (early Miocene) seems to be an ancient rocky shore, almost fulfilling the criteria of a modern rocky shore.

A vitric tephra layer is present in higher terrace deposit situated in the downstream of the Yura River flowing into the Sea of Japan. Major element composition and index of volcanic glass shards suggest that this tephra can be correlated with the Kakuto tephra from Southern Kyushu in MIS9, in the Middle Pleistocene. Rock compositions of gravels of this outcrop and surrounding higher terrace deposit are different from those of the river bank of the Yura River in present. This fact suggests that the Yura River was flowing into the Seto Inland Sea in MIS9, instead of the Sea of Japan.

Kosmochlor-bearing eckermannite rock is identified in the Yamanobo outcrop of Itoigawa City, Niigata Prefecture, Japan, which is emplaced within a serpentinite body of the Renge metamorphic belt. The eckermannite rock is likely a large enclave within the serpentinite body, with an albitite zone developed between the eckermannite rock and serpentinite matrix. Kosmochlor is lens-shaped or schlieren-like domains within the eckermannite matrix, and comprises spherical aggregates of radiated kosmochloric pyroxene. Spherule cores are occasionally occupied by a corroded form of chromite. The chemical compositions of the kosmochlor exhibit a solid solution between kosmochlor (Ko₉₈Jd₀Ae₁Q₁) and jadeite (Ko₂₅Jd₆₁Ae₆Q₈). The maximum kosmochlor component is 98 mol.%, likely representing the closest known composition to the NaCrSi₂O₆ end-member, worldwide. Sodium amphiboles coexisting with kosmochloric pyroxene, and especially the cores of relatively large amphibole grains, in particular, contain high Cr₂O₃ (up to 12.2 wt.%, 1.34 apfu) and exhibit significant C-site deficiency (i.e., C-site cation < 5).