The Quaternary Zao volcano is situated on the Tohoku Backbone Ranges, Northeast Japan. It is known that activity of the volcano started about 1 Ma and continues to the present. The volcanic history of the Zao volcano can be divided into four stages as follows. Stage 1: A relatively small-scale volcano was formed. It is composed predominantly of pyroclastic materials of basalt and basaltic andesite. Stage 2: A stratovolcano was constructed by lavas and pyroclastic materials of andesite and dacite. The stage 2 is further subdivided into two substages, 2 a and 2 b. Stage 3: Lavas and pyroclastic materials from two vents situated near the summit widely distributed around the flank of the volcano. Rocks consist of basaltic andesite and andesite with a small amount of basalt. Stage 4: After formation of caldera, Goshikidake pyroclastic cone was formed in the caldera. The eruptive products comprise pyroclastic materials and lavas of basaltic andesite.
The rocks of stage 1 belong to low-K series, and all the others to medium-K series. The medium-K rocks from different stages show contrasting trends; the rocks of stage 3 have slightly higher K 2O than those of stage 2 and stage 4 at the same SiO2 content. The basalt of stage 3 is medium-K and a contrast to the low-K basalts of stage 1.

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Brucite-bearing white rock (pencatite), consisting mainly of fine-grained brucite and calcite, was found in the dolomitic part of the Upper Limestone (the Karasawa Limestone Member) of the Nabeyama Carbonate Formation, Kuzuu district, Tochigi Prefecture, Japan. The brucite-bearing white rock was formed in the process of the thermal metamorphism of dolostone by basalt dyke. Based on the content of the minor elements of MnO, P₂O₅ and TiO₂, the basalt is classified into island arc tholeiitic basalts, in contrast to the basalt of the Izuru Formation, which is classified into oceanic island basalts. A part of the basalt dyke was affected by an intense alteration, and changed its color into brown to reddish brown. The temperature aroud the dyke is estimated to have reached at least up to 800 to 850°C based on the compositions of coexisting Al- and Fe³⁺-spinels in the altered basalt. Therefore, it is considered that dolomite decomposed into calcite and periclase instead of brucite near the dyke, and then periclase hydrated to form brucite. The mineral assemblage of the brucite-bearing white rock indicates an equilibrium temperature below 480°C and Xco₂ below 0.01 if we assume a total pressure of 1 kb.

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標記地域の札幌岳および朝里岳より低カリウム系列のソレアイト質玄武岩を見いだした。K-Ar年代は,それぞれ1.6±0.6 Ma (札幌岳)および2.6±1.0 Ma (朝里岳)であり,標記地域に広く分布する“1平坦面溶岩”と呼ぱれている安山岩-デイサイト質溶岩の活動年代とほぽ同じである。これらの玄武岩の主・微量成分化学組成は,東北本州弧の海溝側の玄武岩組成と類似している。玄武岩の化学組成から東北日本弧の火山を海溝側と背弧側に2分すると,鮮新世から第四紀にかけての火山の帯状配列は東北本州から西南北海道までN-S方向である。この帯状配列は, N-S方向(日本海溝)からNE-SW方向(千島海溝)へと変化する現在の島弧-海溝系では説明できない。

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