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Grain size variation of sediments and sedimentary rocks generally complicates the assessment of the degree of chemical weathering as measured by the Chemical Index of Alteration (CIA) value. Mineralogical and geochemical research was carried out on Miocene and Pliocene fluvial sediments in southwest Japan in order to evaluate the effects of grain size on the degree of chemical weathering. A strong positive linear trend is usually exhibited between Al2O3/SiO2 ratios and quartz intensities obtained by X-ray powder diffraction, suggesting that the Al2O3/SiO2 ratio may be suitable as a grain size index for semi-consolidated and unconsolidated sediments. Most of the fine-grained sediments, for which Al2O3/SiO2 ratios are over 0.4, have higher CIA values of over 90. In contrast, coarse-grained sediments, for which Al2O3/SiO2 ratios are lower, especially below 0.2, display a wide range of CIA values. The CIA values for intermediate grain sized samples, for which Al2O3/SiO2 ratios range from 0.2 to 0.4, reflect distinctive differences in the degree of chemical weathering between Miocene and Pliocene sediments. Within this range of Al2O3/SiO2 ratios, the Miocene sediments have higher CIA values (86–98) than Pliocene sediments (71–92). To compare the degree of chemical weathering among some sediments or sedimentary rocks with variable grain sizes, it is desirable to evaluate with the Al2O3/SiO2 ratios limited to the intermediate grain size range.

This study presents the K-Ar geochronology for hydrothermal K-feldspar in plagioclase alteration, including methodology and application to the Toki granite, in central Japan. Borehole samples from the Toki granite were collected and mechanically and chemically processed to separate plagioclase from the rock and remove bulk impurities. The sample fraction of cleaned plagioclase powder was further processed to a smaller size fraction, allowing separation of the altered K-feldspar from the plagioclase host. The resulting K-feldspar represented the hydrothermal alteration product and was characterized crystallographically as microcline, and its K-Ar ages were measured. The results of the K-Ar dating and petrographic characterization indicated that in this setting, plagioclase alteration occurred through a combination of solid-state replacement and dissolution–precipitation processes. The K-feldspathization age enables constraint of the temporal conditions of the solid-state replacement process to 62.2 ± 1.4 Ma. The time-temperature (t-T) path of the sampling site is an effective tool for determining both the timing and thermal conditions of the hydrothermal microcline formation in plagioclase alteration. The combination of the t-T path and the microcline K-Ar age provides formation temperatures of about 307–325 °C. The timing and thermal conditions of solid-state replacement (62.2 ± 1.4 Ma and 325–307 °C) indicate an older age and a higher temperature than those of dissolution–precipitation (59.2 ± 1.4 Ma and 305–290 °C: Yuguchi et al., 2019A). The plagioclase alteration consists of serial processes from solid-state replacement to dissolution–precipitation. Addition of the thermal conditions and timing into petrography have implications for the sequential phenomenal variation in granite., K-feldspar K-Ar age; Plagioclase alteration; Solid-state replacement; Dissolution and precipitation; K-feldspathization and illitization; Toki granitic pluton

Fracture zones around faults (damage zones) in crystalline rocks such as granite can act as significant transport pathways because the permeability of damage zones around active faults will likely be increased by the fracture network system. Understanding the characteristics and long-term behavior of damage zones along smaller scale faults is important for the safety assessment of deep underground facilities in the plutons of an orogenic belt; for example, facilities for the disposal of high-level radioactive waste. Therefore, this paper describes the features of a damage zone and the long-term behavior of its hydrogeological structures associated with faulting in an underground environment based on the results of geological and hydraulic investigations 300 and 500 m below ground level at the Mizunami Underground Research Laboratory, Central Japan. Detailed borehole and gallery wall investigations show the distributions of fractures, fracture fillings, predominant fracture orientations, groundwater inflow points, and hydraulic transmissivity in and around a damage zone. The results indicate that there are three stages in the development of hydrogeological structures. The first stage is the formation of background fractures that formed after the temperature of granitic plutons decreased through the ductile–brittle transition, forming fractures under brittle deformation conditions. The second stage is the formation of a damage zone and corresponding increase in hydraulic permeability as a result of the formation of relatively small fractures associated with faulting. The third stage is the formation of fracture fillings because of the infiltration of groundwater through the damage zone, decreasing the hydraulic permeability of the zone. In the late third stage, unconsolidated clayey fillings form in the large fractures associated with faulting resulting in decreased permeability of the fractures in the damage zone around the fault. These results underline the importance of understanding the development stages to evaluate the effect of faulting in orogenic belt plutons.

Quartz from a granitic pluton is found to have formed through sequential growth events under different mechanisms and crystallization temperatures, which can provide new insights into magmatic processes of granitic magmas that were eventually consolidified into plutons. The events were identified using (1) the description of crystal shape and occurrence, (2) the study of the internal structure with cathodoluminescence (CL), and (3) derivation of the crystallization temperatures based on TitaniQ thermometry. The magmatic quartz crystals from the Toki granite, central Japan, are characterized as having the following internal structures: oscillatory zonation, no-oscillatory zonation with luminescence gradation (gradational zonation), and heterogeneous CL. The quartz crystals with oscillatory zonation were formed in the temperature range of about 800 °C to below 700 °C, which is referred to as oscillatory zoning temperature (OZT) conditions. The CL zonation pattern was controlled by the temperature conditions and titanium diffusivity in the melt (magma). The crystallization process of quartz within the Toki granite reveals the cooling processes of the granitic pluton; the lithofacies with a high frequency of oscillatory-zoned quartz underwent slower cooling under the OZT conditions than those in other lithofacies.

Bulk chemical composition and mineral composition of the Pliocene sediments in southwest Japan reveal the effects of source rock compositions and grain sizes, as well as the relationship between sedimentary environments and the degree of chemical weathering in the Pliocene warm period. A wide variation in the degree of chemical weathering, source rock compositions and grain sizes was observed with vertical and spatial changes in the sedimentary environment. Coarse-grained sediments, deposited in sandy channels and their floodplains, indicate a lower but wide range of CIA values. This result suggests that the degree of chemical weathering can be modified by changes in grain size due to hydrologic processes. In contrast, stagnant water sediments, composed of homogeneous fine-grained sediments, display a wide variety of REE compositions and Al2O3/SiO2 ratios mainly caused by variation in source rock compositions and grain sizes. However, the CIA values of the stagnant water sediments are over 90, such that these sediments display a tight cluster near the Al2O3 apex on the Al2O3-(CaO + Na2O)-K2O ternary diagram. This study demonstrates CIA values are generally above 90, which is due to the increased leaching of Ca, Na, and K with exposure to under intense chemical weathering conditions even if the source rocks and grain sizes are variable. Regardless of the wide variety of source rock compositions and grain sizes, REE and kaolinite-rich clay mineral concentrations in these sediments are indicative of the intense weathering conditions that prevailed in the Pliocene period (3–4 Ma) in southwest Japan.

Rock matrix permeability is mainly controlled by microcracks and is important for long-term solute transport, but may be highly heterogeneous. This study aims to identify the factors influencing the permeability of granite matrix by analyzing: physical properties, crack distribution, mineral composition, and the concentration of main elements in 48 core samples taken from three deep drillholes in the Mizunami area, central Japan. All drillholes intersect the Tsukiyoshi fault at depth. Permeability of core samples, measured by a gas permeameter, largely increases in the fault and fracture zones, which may be caused by the development of cracks as the total length of cracks extracted from thin-sections is correlated with measured permeability. Although a significant correlation is identified between permeability and P-wave velocity, a representative elastic property of rock, this correlation is enhanced by classifying the samples into two groups by the Mn/Fe concentration ratio. This ratio is identified as a key factor to separate lithofacies differentiated by the source magma. Thus, lithofacies is another control factor for permeability due to the difference in mineral composition. Moreover, permeability shows significant negative and positive correlations with Si and Ca concentrations, respectively. These concentrations are probably affected by dissolution of silicate minerals and calcite generation in the hydrothermal alteration process, which can develop both pore space and crack network and consequently, increases rock permeability in addition to crack generation and development. Therefore, a combination of hydrothermal alteration before the emplacement of granite and strong faulting are the predominant processes for controlling permeability.

Holocene environmental changes were reconstructed by analyzing diatom assemblages from sediment cores YM, KZN, NK, and MC from the Nobi Plain, central Japan. Five diatom assemblage zones were identified: (1) at the beginning of the Holocene, freshwater species were dominant; (2) then, marine and brackish-marine species increased, indicating transgression; (3) in the middle Holocene, proportions of marine and brackish-marine species became almost constant, with marine species dominant; (4) marine species began to be replaced by freshwater species, indicating marine regression as a result of delta progradation; and (5) freshwater species again became dominant. These diatom assemblages correlate with previously defined lithological units: zones 1 and 2 with unit B (fluvial to coastal plain), zone 3 with unit C (inner bay or prodelta), zone 4 mainly with unit D1 (delta front slope), and zone 5 with units D2 (delta front platform) and E (delta plain and flood plain). Although the shoreline migrated landward (transgression) faster than it migrated seaward (regression), transgressive diatom assemblage changes (decrease in marine-brackish water species) took up to 1000 years, whereas regressive changes required only a few hundred years. Diatom analysis is useful for reconstructing not only Holocene sea-level changes and sedimentary environments but also local geographic effects.

The usefulness of electrical conductivity (EC) of stirred Holocene deltaic sediments from three cores from the Nobi Plain, Japan, was assessed as a proxy for marine transgression and regression. Lack of correlation of EC with mud content for the sediments with >20% mud suggested that permeability was a negligible factor. Marine deposits showed high EC (>0.9 mS/cm), terrestrial deposits low EC (