Geological and geochemical studies of the Tokuwa batholith and its surroundings and theoretical calculations of convection in the felsic magma chamber lead to the following conclusions:

(1) The Tokuwa batholith is composed of ilmenite-series and magnetite-series gran itoids. The main part of the batholith is occupied by magnetite-series rocks. The ilmenite series granitoids are recognized in the periphery of the batholith adjacent to the Kobotoke Group rocks which consist of shale and sandstone containing carbonaceous matter.

(2) The two series of granitoids have clearly geologically, mineralogically, and geo-chemically different features, for example, oxygen isotopic composition as summarized in Table V-1.

(3) It is evident that the ilmenite-series granitoids were formed under lower fO₂ than the magnetite-series granitoids based on whole-rock and mineral chemistry.

(4) The most acceptable model obtained in the present study is that the assimilation of sedimentary rocks into the magma lowered the fO₂ due to the presence of carbonaceous matter in the sedimentary rocks and generated the ilmenite-series granitoids.

(5) The mixing ratio of xenoliths and/or sedimentary rocks is estimated to be about 20% based on oxygen isotopic data.

(6) The assimilation of the sedimentary rocks and the homogenization in the margin of the batholith could have occurred, through mechanical scattering and digestion of xeno liths by type-A convection as described by Lighthill (1953). Assimilation as well as frac tionation could have modified the magma.