The Late Mesozoic granitic rocks have been studied in the Okutsu area, northern Okayama Prefecture. Main portion of the area is occupied by the Okutsu granodioritic mass. It consists of mediumgrained hornblende-biotite granodiorite, mediumgrained biotite granite, fine-grained biotite granite, and aplitic granite in the succeeding order of intrusion. Contacts between these rock-types are generally gradational and An-contents of both core and margin of the plagioclase progressively decrease with increasing acidity of the rocks. These facts indicate a cognate origin of various rock-types of this mass. The Kamisaibara granite, rather uniform coarse-grained hornblende-biotite granite, occurs in large batholith. It is inferred petrographically that the Kamisaibara granite is a member of the Ningy6,t6ge granite widely distributed on the north of this area. Field evidence suggests that the Ningyo-toge granite was intruded by the Okutsu granodioritic mass.
