The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists

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Chokai volcano is the largest and highest (2230m) among the volcanoes belonging to the Chokai volcanic zone, and situated from 135°50'E to 140°07'E in Long. and from 39°15'N to 39°00'N in Lat. This volcano is composed of the west and the east volcanoes, both of which are double stratovolcanoes, built up on the Tertiary formation throughout the six stages of activities. The historical activities of this volcano was recorded in 861, 871, 884, 915, 1740, and 1802.
The rocks found in this volcano are classified into two series. One is the high alumina rock series derived from high alumina basalt magma through the differentiation, and includes augite olivine basalt, hypersthene bearing olivine augite andesite, and hypersthene olivine augite andesite. The other is the talc-alkali rock series derived from high alumina rock series through the assimilation. Cale-alkali rock series is devided into two types, hornblende-bearing type and hornblende-free type. The former includes hornblende-bearing hypersthene olivine augite andesite, hornblende-bearing hypersthene augite andesite, and hornblende hypersthene augite andesite and the latter includes olivine hypersthene augite andesite, olivine-bearing hypersthene augite andesite, and hypersthene augite andesite. It is characteristic that most of the hornblende-bearing type are found in the west Chokai volcano except Hottai lava flow which belongs to the east Chokai volcano.

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This paper deals with the heavy mineral association observed in the Nemuro group (late Cretaceous) in western part of the Kushiro coal field, eastern Hokkaido. 30 non-opaque and 4 opaque minerals are identified from the sandstones of various horizons intercalated in this group. They are originated mainly from granitic rocks, schalstein, diabase, ultrabasic rocks, intermediate and basic volcanic rocks, and reworked older sediments. From the view point of the heavy mineral assemblage, the Nemuro group is divided into the lower member of the Kawaruppu formation together with the underlying Kappei formation, which are characterized by abundance of zircon, garnet, and epidote, and presence of chromite, and the upper member of the Kawaruppu formation, characterized by abundance of hornblende and augite, and absence of chromite. It is concluded that such a difference is not ascribed to the difference of the geology in the background areas, but is principally due to environments of deposition.

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