The Kurohanayama basalt has been referred to as a representative rock of Quaternary basalts in northeast Japan. There has been a significant amount of petrological studies on these rocks, however, the effused age of the Kurohanayama basalt has not been established yet.
     In this study, we determined the K-Ar ages of rocks from the Kurohanayama basalt, and obtained 5.03±0.50, 5.2±1.2 and 5.64±0.31 Ma in age. From these data, we have shown that basalts of Kurohanayama effused some time between Late Miocene and Early Pliocene (4-6 Ma). Thus, the Kurohanayama basalt is Tertiary in age, and must be genetically separated from the Quaternary Funagata volcano.

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Manganoan winchite (up to 4.19 wt.% MnO) occurs as cores of zoned amphibole in a quartz schist from the Saruta-gawa area in the Sanbagawa metamorphic terrain, central Shikoku, Japan. The quartz schist consists of quartz, albite, amphibole, muscovite, garnet, chlorite, allanite, calcite, apatite, stilpnomelane, tourmaline, hematite and pyrite. The amphibole is euhedral to subhedral in form (up to 700 μm long). The core of the amphibole ranges in composition from barroisite through winchite to manganoan winchite. The amphibole is rimmed by alkali amphibole (mainly magnesioriebeckite) and winchite. Garnet in the quartz schist is rich in spessartine component (59-79%).

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The Shoboji Diorite distributes sporadically in the southernmost area of the Motai Metamorphic Rocks. Constituting mafic minerals are composed of clinopyroxene, hornblende and small amount of biotite. The Diorite is massive and heterogeneous, and comprising mainly diorite with subordinate gabbro, tonalite and gabbroic cumulate. Field evidences indicate that the age of the Diorite is post-Motai Metamorphic Rocks and pre-Tobigamori Formation (Upper Devonian). The K-Ar ages of hornblende have the narrow range from 446 to 432 Ma with one exception. The ages are slightly younger than those of the banded amphibolites of the Matsugataira-Motai Metamorphic rocks and the mafic to ultramafic rocks of the Hayachine Complex. The hornblende K-Ar ages indicate that the Diorite belongs to the igneous activities of the Late Ordovician to the boundary of Ordovician-Silurian age. Tonalitic rocks exposed along the Isawa-gawa River at the foot of Mt. Yakeishi-dake in the Ou Mountains, and the Hikami granite in the Ofunato district, may also belong to the products of the same igneous activity. These igneous activities at the Late Ordovician characterize that of the subduction tectonics, and the same igneous activities had continued to Siluro-Devonian and Carboniferous. Further, the characteristics of these activities suggest that the South Kitakami Belt at that time had situated in the island arc or active continental margin along the Yangtze massif in northern border of the Gondwanaland.

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