The Horoman peridotite complex, Hidaka belt, Hokkaido, northern Japan, is composed of three peridotite suites which have distinct petrographic characteristics. The first is the Main three peridotite suites which have distinct petrographic characteristics. The first is the Main Harzburgite-Lherzolite (MHL) suite, which occupies the most part of the Horoman peridotite complex. The MHL suite consists of harzburgite, spinel lherzolite and plagiocalse lherzolite, which form a layered complex with gradational layer boundaries. Two-pyroxene spinel symplectite, which is interpreted as a chemically modified garnet pseudomorph, occurs in spinel and plagioclase lherzolites. The MHL suite is of typical residual origin after various degree of magma extraction, which resulted in wide chemical variations. The second, the Banded Dunite-Harzburgite (BDH) sutie, consists of conspicuously layered dunite, harzburgite and olivine orthopyroxenite with sharp layer boundaries. The BDH peridoties are pobably exotic blocks within the MHL suite. Mineralogical characteristics of the BDH suite suggests that it is a cumulate from a high-Mg magma (e. g., high-magnesian andesite). The third, the Spinel-rich Dunite-Wehrlit (SDW) suite, consists of dunites containing abundant large oblate chromian spinel grains. The dunite becomes locally wehrlitic and contains clinopyroxene up to 8%. The SDW occurs in the MHL harzburgite zones as layers with maximum thickness of 15m. The layer boundaries with the harzburgite of the MHL suite are generally sharp. The petrographical and mineralogical characteristics show that the SDW is a cumulate from a magma segregated from the surrounding MHL suite. The melt segregation may be spreading away from the upwelling center as a corner flow. A series of layered residual peridotite (MHL suite) zones, of which degree of melt extraction increases inwards, formed around the SDW dikes.

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Rb-Sr chronological study has been carried out on the Miyamoto tonalitic complex to the east of Iwaki city, Fukushima prefecture, northeastern Japan. The rock facies of the complex can be grouped into three in terms of petrological understanding: they are gabbro group, tonalite group, and adamelite group in order of intrusion. The Rb and Sr abundances and Sr isotopic compositions were analyzed for gabbro group rocks (14), tonalite group rocks (8), and adamelite group rocks (5). The mineral separates from one of the gabbro group rocks were also analyzed.
The Rb-Sr age of the tonalite group is 120 Ma, and the initial ⁸⁷Sr/⁸⁶Sr ratio is 0.7052; the results are similar to those of the tonalitic plutonic complexes distributed nearby. The adamelite group yielded 119 Ma as the solidification age with teh inital ⁸⁷Sr/⁸⁶Sr ratio 0.7049. The age is identical to that of the tonalite group, but the initial ⁸⁷Sr/⁸⁶Sr ratio is different between them. Although the tonalite and adamelite group magmas intruded successively and solidified, the origin could largely differ between the two groups.
The gabbro group gave no consistent whole-rock age, and neither did the mineral separates. Excluding biotite, the remaning mineral separates, plagioclase, hornblende, and the whole rock make an isochron with an angle indicating the age of 100 Ma with an uncertainty of 13 Ma. The significance of the obtained mineral age, however, is unclear. The Rb-Sr system of the gabbro group seems considerably disturbed by alteration.

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Various mineral assemblages are observed in the Mineoka metamorphic rocks of basic and siliceous compositions from Byobu-jima, Kamogawa fishing port. Major basic schists are divided into two groups, hematite-bearing and ilmenite-pyrrhotite-bearing. These rocks often contain a rutile-sphene-quartz-calcite sub-assemblage in common, which may have been a buffer of CO₂ and thereby H₂O fugacities in these graphite-free rocks during metamorphism (estimated X_CO2 is around 0.3). The hematite-bearing rocks have a mineral assemblage defining the epidote-amphibolite facies whereas the ilmenite-pyrrhotite-bearing rocks the amphibolite facies, showing the great effect of oxygen fugacity on the silicate mineral mineral assemblages in basic schists. The aegirinaugite-bearing sillceous schists always contain a small amount of K-feldspar in addition to albite, while the epidote-bearing rocks are free of K-feldspar. Thus the occurrence of aegirinaugite is restricted to the rocks deficient in alumina. Some of the aegirinaugite-bearing rocks contain piedmontie and biotie rich in copper and manganese, indicating extremely high oxygen and very low sulfur fugacities in these rocks during metamorphism. Minor psammitic schist occasionally includes pebbles of (alkali) granite, offering a new insight into the sedimentary environment of the original rocks of the Mineoka metamorphi rocks.
Estimated metamorphic pressure and temperature are 5 kb and 500-550°C, respectively.

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