Some Trace-Element Data on Pliocene Basaltic Rocks from Two Districts in Hokkaido

Trace elements Rb, Sr, Zr, Y, Cu, Ni, Zn, Co, and Pb are recorded from basaltic rocks of the Kitami-Monbetsu district of east Hokkaido and the Takikawa-Fukagawa Basin and Kabato Mountains of central-west Hokkaido. Those of the former district, although petrologically similar to some from the latter district, show a different distribution of trace elements. In particular Zr and Zn are higher, and Rb and Sr are lower in the eastern basalts. Other slight differences are suggested. The reason may be an initial abundance variation

Petrochemistry of the Quaternary Volcanic Rocks of Hokkaido, North Japan

The Kurile and north Honshu island areas, which join in Hokkaido, are divided into an outer (Pacific) arc and an inner (Okhotsk or Japan Sea) arc. Quaternary volcanism has been taking place only in the inner arcs. The Quaternary volcanic rocks are composed of a basalt ― andesite ― dacite ― rhyolite suite, of which andesite and dacite of the calc-alkali series are predominant. The rocks vary markedly from the Pacific side to the marginal sea side, and are associated with basalts which vary from tholeiitic to alkalic basalt types, as follows: Pacific side............................Marginal sea side Tholeiite series(T) High-Al basalt ser.(H) Alkali rock series(A) Calc-alkali ser.(CT) Calc-alkali series(CH) Calc-alkali series(CA) 183 major element chemical analyses of the Quaternary volcanic rocks of Hokkaido are now available. Spatial zonation of the rocks is well represented in alkali - silica relations. Marked iron enrichment in the intermediate rocks is found in the tholeiite series. Trace elements were determined on 42 selected rocks. The contents of REE, Ba, U, Th and Hf increase with silica. The rocks of the marginal sea side are particularly enriched in these elements. The abundance of Sc, Cr, V and Co decreases with increasing silica content. It is noted that Sc and V behave as iron. The three groups, T & CT, H & CH and A & CA, are well distinguished from each other in the chondrite-normalized REE pattern, i.e. T & CT has distribution patterns vary similar to the ocean ridge tholeiite, showing a tendency toward depletion or equality of light REE over heavy REE, whereas A & CA has a marked enrichment of light REE over heavy REE. There is no essential difference in the REE pattern between the calc-alkali rocks and the associated basaltic rocks in each group, suggesting a common source for them. A negative Eu anomaly is a characteristic feature of the calc-alkali rocks, though this anomaly does not always appear. 87Sr/86Sr ratios were determined on 49 samples. The ratios of the rocks of the Kurile are extremely low (0.7028 ― 0.7039), and those of the north Honshu arc are also low (0.7026 ― 0.7057). The Sr-isotope ratios of basalt to dacite in a single volcano are almost identical and exhibit a small range of variation without systematic change. This implies that the igneous magmas have been produced from a relatively homogeneous mantle material and not modified by contamination with the older continental crust

Preliminary Report of the 1977 Eruption of Usu Volcano

On August 7 (09h 12m, local time), 1977, a major eruption of dacite pumice occurred from the summit of Usu Volcano (42°32'N, 140°50'E), Hokkaido, Japan, after 32 years of dormancy since the birth of Showa-Shinzan lava dome. Four notable explosive eruptions, two moderate and ten small ones have been recorded until the midnight of August 13-14, and then marked crustal movements and local earthquake swarms continue still at present. The sequence of the present eruption, the distribution and nature of the new ejecta, and the topographic deformation of the volcano, are briefly given in this report