Tomuraushi Volcanic Group (TVG) and the adjacent Numanohara volcano, Central Hokkaido, Japan, are situated in the middle part of the Daisetsu-Tokachi volcanic chain. The volcanic activity of the TVG can be divided into an Older and Younger stage. The volcanic activity of the Older stage is dominated by eruptions from two main volcanic centers, Koganegahara and Goshikigahara volcanoes while the three centers, Futamata, Kaunnai, and Tomuraushi volcanoes, were active during Younger stage. K-Ar age determinations have been carried out on four lavas from TVG and one lava from the Numanohara volcano.
     The K-Ar ages of the lowermost and near uppermost lavas from the Koganegahara volcano are 1.02±0.09 and 0.72±0.09 Ma, respectively. These ages define the initiation and the termination of the volcanism of the Older stage of TVG. The Younger stage of the TVG started at about 0.3 Ma inferred from the obtained K-Ar ages of the Kaunnai volcano (0.11±0.02 Ma), Tomuraushi volcano (0.05±0.01 Ma) and previously published K-Ar ages (0.29-0.15 Ma) by NEDO (1990). We therefore infer a long hiatus of about four hundred thousand years in volcanic activity occurred between the Older and Younger stages of the TVG. During this hiatus, the Numanohara volcano (0.38±0.10 Ma) was active to the east of the TVG.

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Serpentinite melange in the western Otoineppu area, northern Kamuikotan tectonic belt, which is mostly surrounded by the Early Cretaceous Sorachi Group Greenstones, contains many metamorphosed tectonic blocks. The blocks are 1) basic meta-volcanic rocks with well preserved original texture, 2) epidote amphibolite with well-zoned epidote, and 3) very weakly metamorphosed hornblende gabbro. The chemical compositions of relic clinopyroxene in the Upper Sorachi Greenstones and in the hornblende gabbro blocks suggest an orogenic belt origin whereas the chemistry of the Lower Sorachi Greenstones is similar to that of abyssal tholeiites.
     Judging from bulk chemistry, the serpentinite has derived mostly from harzburgite. Antigorite is the only serpentine mineral recognized in the serpentinites of the study area. Three types of serpentinite are discriminated; type A with fine grained antigorite with mesh structure occures, type B with coarse grained flake-shaped antigorite, and type C with coarse grained sutured antigorite.
     Texturally type A serpentinite is transposed to type B and then to type C. Antigorite with mesh structure contains higher H₂O than serpentinite of type B and C. Type C serpentinite is rich in Al₂O₃ and poor in MgO. The three types of serpentinites and mixed-type serpentinite have been separately mapped.

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Staurolite-bearing pelitic rocks have been newly found at two locations in the western Hitachi district, southernmost Abukuma Plateau, as predicted by Uruno based on his river sand survey. One is a sheared pelitic schist occurring next to the Tamadare metamorphics near Zoso. It is composed mainly of sillimanite, cordierite, biotite, muscovite, plagioclase and quartz with metastable andalusite. Staurolite is relic, being entirely enclosed in cordierite. The other is a pelitic xenolith in the Nishidohira cortlandtite mass, consisting mainly of sillimanite, garnet, biotite, plagioclase, and quartz. Staurolite occurs together with sillimanite and spinel as inclusions in idiomorphic, normal-zoned plagioclase. Thus, staurolite in these rocks is a relict enclosed in other minerals, although the matrix mineral assemblages are distinctly different between these two. Staurolite is additionally found in the Nishidohira andalusite-kyanite-sillimanite porphyroblastic pelitic gneisses. In this case, staurolite occurs as inclusions in garnet and plagioclase, being occasionally associated with kyanite, sillimanite and muscovite.

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