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.
