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Based on the D /H and Fe/(Fe + Mg) distribution between coexisting biotite and hornblende, Japanese granitic rocks can be divided into equilibrium and disequilibrium types (Kuroda and others, 1977a). In the former rocks, the relationship between biotite and hornblende in the σD-XFe plot follows the empirical equilibrium relationship found by Suzuoki and Epstein (1976), whereas in the latter rocks, it does not.

The origin of the two types can be interpreted as follows. The magma of the equilibrium type is oversaturated with water; i.e., it contains free hydrous fluid as well as dissolved water, and later, the free hydrous fluid interacts with the hydrous silicates as shown by the Suzuoki-Epstein experiment. In contrast, disequilibrium magma is undersaturated, and hydrous fluid does not separate but remains as dissolved water and is used up consecutively by the hydrous silicates (Kuroda et al., 1977a, 1978).

Most of the Japanese granitic rocks belong to the equilibrium type, and only in the Ryoke belt is the disequilibrium type found.

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