We report occurrence and petrological characteristics of the Daisen-Kurayoshi tephra (DKP; ≥55 ka), a late Pleistocene widespread tephra in Japan, identified from borehole cores at Horinji, Nanto City, Toyama Prefecture, drilled for defining shallow subsurface geometry of the Horinji fault. DKP found from the cores contains abundant volcanic glasses compared with the heavy minerals such as orthopyroxene and hornblende, whose occurrence is very rare because the glasses are commonly altered into clay minerals. Range of refractive index of the volcanic glass, reported for the first time in and around Hokuriku region including Toyama, as well as that of orthopyroxene and hornblende, is similar to that of the source area. Result of pollen analysis of organic silt just below DKP suggests that the silt was accumulated under cool climatic condition correlated with the period of marine oxygen isotope stage (MIS) 3. Identification of intact DKP from downthrown side of the Horinji fault implies that the precise correlation of the strata or terrace surfaces of MIS 3 separated by faulting might be possible through detailed examination of the strata with tephra and pollen analyses, which provides basis for determining more reliable long-term slip rate of active faults in the Hokuriku region.
In order to construct accurate geological maps of Japan, K-Ar ages of five plutonic rocks have been determined during fiscal 2012 by the geochronological laboratory of Geological Survey of Japan, AIST. The crystals of biotite in five plutonic rocks were isolated by using an isodynamic separator and tapping processes. Each data of rock samples determined is associated with rock name, locality, collector, geological setting, K-Ar age, analytical data and geological interpretation.
Rock-forming zircon was separated from so-called Triassic granitoids at Nui Phao in the northern Viet Nam, and the contained zircons were dated by LA-ICPMS U-Pb method, as the earliest Triassic of ca. 250 Ma. These new results are suggestive to identify the formation ages of Sn-W and REE ore deposits hosted in the granitoids.