Volcanic products from Tokachidake volcano were observed mineralogically and petrographically, focusing on hydrothermal alteration of ash particles to interpret the pre-eruptive interaction between magma and the subvolcanic hydrothermal fluid. The 4.7 ka Ground Crater pyroclastic flow deposit, the 3.3 ka Ground Crater pyroclastic flow deposits, and the 1926 AD lahar deposits were examined. Each sample consists of ash grains undertaken alteration in various degrees from unaltered to intensely altered. Based on mineral assemblage, the alteration of ash is classified into three types: silica type (silica mineral alone), alunite type (silica mineral-alunite±kaolin), and kaolin type (silica mineral-kaolin). Most ash grains in the 4.7 ka and 1926 AD products belong to either of silica and alunite types. The 3.3 ka products are rich in kaolin-type ash and exhibit a stratigraphic variation in the proportion of unaltered ash: unaltered ash increases with the stratigraphy. All the alteration observed in the ash imply acid hydrothermal alteration, regardless of degrees of alteration. The weakly altered ash grains consist of unaltered volcanic rock and acid alteration parts, indicating that an acid hydrothermal fluid-rock interaction at open-fluid flow system formed their altered rock texture. It shows the reaction process proceeding during discharging the reactant fluid from the inside of the rock. These features reflect an unstable process of a short-lived subvolcanic hydrothermal system accompanying with a magma intrusion to the shallow potion and also with the reaction between the rock and acidic fluid that passed through the rocks.

This study describes the brachiopod Shittakazawa fauna from the upper part of the Shittakazawa Formation in the Okuhinotsuchi area, South Kitakami Belt, northeastern Japan. The following four brachiopod species, which are assigned to three genera, are identified: Rhipidomella kusbassica, Schizophoria pinguis, S. mayesensis, and Unispirifer kozuboensis. The Shittakazawa fauna corresponds to a late Tournasian age, thus indicating a late Tournasian age for the upper part of the Shittakazawa Formation.

Zircon U-Pb and fission track (FT) single-grain double dating was conducted on two samples from the Nakatsugawa I and II volcanic ash layers (Tono area, Gifu Prefecture). The U-Pb results, with the exception of discordant ages and ages obtained from heterogeneous grains, yield a weighted average age of 3.94±0.07 Ma. The weighted average FT age from the same grains is 3.97±0.39 Ma. The similar age (within error) for the U-Pb system (higher closure temperature) and FT system (lower closure temperature) indicates that zircon crystallization occurred at high temperature and under quenching conditions. The age is therefore interpreted as an eruption age. The results are consistent with the age of the Ohta Tephra in the Tokai Group, which is one of the most widespread Pliocene tephras in Central Japan, thus contributing to regional correlations of Pliocene strata in Japan.

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 fine-grained 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.

The Tetori Group in the Shiramine area (Ishikawa Prefecture, central Japan) is subdivided into the Gomijima, Kuwajima, Akaiwa, and Kitadani formations (from bottom to top). A variety of animal and plant fossils have been reported from the Kuwajima, Akaiwa, and Kitadani formations, but the depositional ages are still poorly constrained. We obtained the first reliable zircon U-Pb age from the Akaiwa Formation of the Tetori Group in the Shiramine area using laser ablation inductively coupled plasma mass spectrometry. The dated sample was taken from the lower part of the Akaiwa Formation along the Osugidani River (Shiramine area), and its U-Pb age is 121.2±1.1 Ma (95% confidence interval).