Geological field work of Kuju volcano group in central Kyushu has been carried out to construct the evolutional history of the volcano group. Especially, at the eastern and the central area, the evolutional history after a large pyroclastic eruption, which occurred 30, 000-35, 000 y. B. P. to generate pyroclastic flows, has been clarified using a tephrochronological method.
Eleven tephra layers are recognized around Kuju volcano group. These layers are in descend-ing order ; Komekubo crater scoria-fall, Kuju-1 ash-fall, Danbaru scoria-fall, Kuju-1 weathered ash, Kuju-2 ash-fall, Kikai-Akahoya ash-fall, Kuju-2 weathered ash, Kuju-3 weathered ash, Kuju-1 pumica-fall, Kuju-4 weathered ash, Miyakono scoria-fall. Kikai-Akahoya ash-fall were erupted from Kikai caldera, which is situated at the southern Kyushu, about 6, 000-6, 500 y. B. P. The other tephras may have originated from Kuju volcano group. Kuju-1 pumice-fall was accompanied with the 30, 000-35, 000 y. B. P. pyroclastic eruption. Danbaru scoria-fall was erupted 3, 000-5, 000 y. B. P.
The evolutional history of the central area is as follows: At 30, 000-35, 000 y. B. P., a large quantity of pyroclastic materials erupted from a presumed eruption center, which situated between the Kutsukakeyama dacitic mountain and Yuzawa andesitic tablelands. After this eruption, the greater part of the central area had been formed by extrusions of lavas and pyroclastic rocks before Kikai-Akahoya ash-fall deposition (6, 000-6, 500 y. B. P.). From 6, 000-6, 500 y. B. P. to 3, 000-5, 000 y. B. P., Mimatayama dome lava erupted. The activity of the central lava domes (e. g. Kujusan lava dome) occurred after 3, 000-5, 000 y. B. P. This activity was accompanied by Kuju-1 ash-fall. After this activity, phreatic explosion and fumarolic activity occurred, and fumarolic activity continued until now.
The evolutional history of the eastern area is as follows: The activity of eastern area began after 30, 000-35, 000 y. B. P. Before Kikai-Akahoya ash-fall deposition, andesitic lavas effused north of Hiijidake. After this period, all of Hiijidake mountain and the lower part of Taisensan mountain were formed by a large quantity of lavas and a small amount of pyroclastic falls. Kuju-2 ash-fall was deposited simultaneously. About 3, 000-5, 000 y. B. P., Strombolian eruption occurred at Taisen-san, forming Danbaru and Komekubo scoria cones accompanied by the deposition of Danbaru scoria-fall. After this activity, dacitic lava erupted at the southern edge of Komekubo crater, then Komekubo crater scoria-fall erupted from the crater. Finally, Kuroda-ke dome lave erupted at the eastern end of Kuju volcano group.
The volcanic output rates are calculated for three periods; 32, 000-6.300 y. B. P., 6, 300-4, 000 y. B. P. and after 4, 000 y. B. P. At the central area, the rate for 6, 300-4, 000 y. B. P. is the largest (0.256km3/k. y.). At the eastern area, the rate became greater from 0.017km3/k. y. through 0.452km3/k. y. to 0.635 km3/k. y.

View full abstract

Graphite associated with nickeliferous pyrrhotites occurs in the Oshirabetsu gabbroic body in southeast part of the Hidaka Belt, Hokkaido. Brecciated and spherulitic aggregates of graphite are mainly found in norite and diorite.
Stable carbon isotope ratio and degree of graphitization of the graphite and carbonaceous material in the Nakanogawa Group around the gabbroid were determined.
Crystallite thickness (L_c, (002)) of the graphite is more than 1000 Â and lattice strain along c-axis (ε_c) is very small, which means that the graphite is a perfect graphite crystal.
Carbon isotopic composition of the graphite ranges from -23.34 to -23.88‰ (δ¹³C_PDB) and that of carbonaceous material in the Nakanogawa Group is about -25.7‰.
The dδ₁₃C values of the graphite range within isotopic composition of carbon in coal and petroleum. The slight difference of δ₁₃C value between the graphite and the carbonaceous material was due to the fractionation of ₁₃C between the graphite and hydrothermal fluid.
These results suggest that the graphite in the Oshirabetsu gabbroic body is of sedimentary origin and carbonaceous sedimentary rocks were incorporated in ascending gabbroic magma.

View full abstract

Edingtonite from the bedded manganese ore deposit of the Shiromaru mine, Tokyo, is found as a subordinate constituent of a thin fissure dominantly filled with celsian. It cuts a low grade manganese ore composed of braunite, hematite, cymrite, celsian and albite. The microprobe analysis gave: Si0₂ 35.82, Al₂O₃ 20.26, BaO 30.36, and the calculated H₂O 14.35, total 100.79%. The empirical formula is: Ba_1.00Al_2.OOSi_3.00O₁₀•4H₂O (basis:O=10 in the anhydrous part). X-ray powder pattern is indexed on an orthorhombic cell with a=9.542, b=9.654, c=6.514 Â. It is the later product than celsian in the fissure, where the aggregate of compositionally varied grains (Cn₉₂Or₈ to Cn₇₉Or₂₁) occupies the wall side portions, probably reserving the original texture. The close co-existence of feldspars and zeolites characterized by the same cations such as Na, Ca and K has not been known. But in case of Ba, the co-existence of these two tectosilicates has been hereby confirmed probably as the first example. It is worth alluding to that both of them have the four-membered ring composed of (Si, Al)O₄ as the basic units of the tectosilicate frame-work.

View full abstract

K-Ar age determination based on the Peak Height Comparison Method has been made on four lavas from Tateyama Volcano, which is located in the northern margin of the Norikura Volcanic Chain in Central Japan. Two lavas that erupted in the First and Third Stages gave significant K-Ar ages as 133 ± 22 and 47 ± 9 ka, respectively. K-Ar age of the First Stage Lava in Tateyama Volcano, incorporated with some ages already reported on Norikura and Yakedake Volcanoes also belonging to the Norikura Volcanic Chain, clarified that the activity of this Volcanic Chain commenced in the order of Norikura, Tateyama and Yakedake Volcanoes. K-Ar age of the Third Stage Lava is consistent with that of the Daisen-Kurayoshi Pumice (DKP: ca. 48 ka) covering the Third Stage Lava.

View full abstract