Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XVIII, No.3


Petrochemistry of the Quaternary Volcanic Rocks of Hokkaido, North Japan

Katsui, Y.;Ōba, Y.;Ando, S.;Nishimura, S.;Masuda, Y.;Kurasawa, H.;Fujimaki, H.

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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.