Detrital chromian spinel grains in recent river beds in the Sangun zone of northern Kyushu are examined to infer primary characteristics of the peridotite, which has not been studied in detail. The use of the detrital chromian spinel is of great advantage to understand a general character of peridotite complexes. The peridotites were metamorphosed and serpentinized to various extent but their protoliths are possibly dunite and wehrlite. Their chromian spinel sometimes preserves almost intact cores, which are expected to be an indicator of their primary petrological characteristics. The primary spinel is high in Cr# (= Cr/[Cr+Al] atomic ratio), frequently around 0.7 to 0.9, low in Fe³⁺/(Cr+Al+Fe³⁺) atomic ratio, around 0.1, and is low in TiO₂, containing 0.15 wt% on average. Some detrital spinels, especially those from Sasaguri, have high Mg#s (=Mg/[Mg+Fe²⁺] atomic ratios) at given Cr#s, being derived from rocks with high Mg#s or high spinel concentrations. Spinels in selected metaperidotite samples are within the compositional range of the detrital spinels. These spinels from northern Kyushu are notably different from those in ultramafic rocks, mainly dunite and harzburgite, exposed in the Sangun zone of the central Chugoku district, of which Cr# is around 0.5. The dunite and wehrlite with high-Cr#, low-Ti spinel are possibly of supra-subduction zone, especially of fore-arc, origin. This type of dunite and wehrlite have not been known from the Sangun zone, and this study highlights a high lithological variety of peridotite distributed in the Sangun zone. The variety can be inherited from petrological heterogeneity of the upper mantle to Moho transition zone of a supra-subduction zone setting.

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Thermoluminescence (TL) age determination was carried out for 18 samples from four pyroclastic flow deposits(Mutsugi, Kureishibaru, Furue and Yugawachi) of Unzen volcano, SW Japan. The TL growth curve method was used for quartz phenocrysts to determine the ages. Major inaccuracy of TL dating is caused in annual dose evaluation, especially in determination of beta rays contributions. Therefore, the beta ray correction was done by measurement of quartz phenocryst diameter on thin sections.
     The TL ages for Mutsugi, Kureishibaru, Furue and Yugawachi pyroclastic flow deposits indicated 4.9±0.4 ka, 23±1 ka, 23±1 ka and 77±6 ka, respectively, which are average of 3 to 6 samples collected from same outcrop each. These ages consistent with ¹⁴C ages and stratigraphy identified by tephra.

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Basaltic volcano clusters of the Cenozoic are widely distributed in the central Chugoku district, Southwest Japan. These clusters can be divided into several zones and are considered to be produced by mantle upwelling. Most of these basaltic magmas have relatively undifferentiated characteristics and coprecipitate olivine and chromian spinel. Restites of these basaltic magmas are determined by compositional relationships between Fo content of olivine and Cr# (=Cr/(Cr+Al) atomic ratio) of chromian spinel. Each cluster has each characteristic refractoriness of their mantle restites: lherzolite (Cr# <0.5) in the Tsuyama, Kibi, Sera and Oki-Dogo volcano clusters; lherzolite to harzburgite (Cr#=0.2-0.6) in the Kuroiwa-Kogen, Yokota and Hiba volcano clusters; lherzolite (Cr#<0.4) (11 Ma) and harzburgite (Cr#=0.5-0.6) (1 Ma) in the Matsue volcano cluster; and harzburgite (Cr#=0.5) in the Kurayoshi volcano cluster. The mantle restites indicated by the Fo-Cr# relations are comparable to the residual phases deduced by melting experiments of Takahashi (1980) and Iwamori (1991) with one exception that restites indicated by the Fo-Cr# relation differ from those deduced from melting experiment for the basalt erupting at 11 Ma in the Matsue cluster. The regional distribution of possible mantle restites suggests that the relatively depleted restite (the Yokota cluster) are surrounded by the relatively fertile mantle restites. On the other hand, temporal variations of Cr# of spinel coexisting with Mg-rich olivine are recognized in each zone, i.e. refractoriness of possible restites sometimes trends to increase with time. The restites for the basalts of 1 Ma in age from the Matsue and Yokota clusters, located near the center of mantle upwelling, are relatively refractory. Therefore, generation of these basaltic cluster may be more largely affected by mantle upwelling. The change of possible restites through space and time may be caused by the repeated basaltic volcanism in central Southwest Japan due to upwelling of mantle plume.

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