中国地方中部の後期中生代の火成活動

The Late Mesozoic Igneous Activities in the Middle Chûgoku Province

YOSHIDA Hironao

廣島大學地學研究報告

Geological report of the Hiroshima University

Most parts of Chûgoku are occupied by various types of igneous rocks, from plutonic to effusive, which have been brought about by the Late Mesozoic igneous activities.
Plutonic members of them are divided into the following three groups or plutonic complexes: 1) the San'in granitic complex, 2) the Hiroshima granitic complex, and 3) the central plutonic group. The former two form large, continuous masses of batholithic dimension, while the latter one is re-presented by groups of plutonic stocks, the composition of which varies from diorite to typical granite.
The effusive rocks are divided into the following two series: 1) the Kisa-Takada volcanic series, and 2) the Sakugi volcanic series. The former series is composed of andesitic and dacitic rocks (Kisa member) in its lower half, and of the rhyolitic rocks (Takada member) in its upper half. The volcanic activities of the latter series start with andesitic rocks and end with rhyolitic rocks.
The age of the Kisa-Takada volcanic series seems to range mainly from the Aritan (Barremian) to the Miyakoan (Cenomanian) on the basis of the following data. 1) The lower part of the Kisa-Takada volcanic series is correlated to the upper Inakura formation, ranging from the upper Aritan to the lower Miyakoan. 2) The materials of coarser deposits in the Mifune group and the Onogawa group, which belong mainly to the Gyliakian, have been considered to be supplied from andesites, porphyrites, quartz-porphyries, etc.. 3) In the Late Mesozoic sedimentary rocks in the Outer Zone of the Southwest Japan pebbles of andesites and acidic volcanic rocks begin to appear in the Miyakoan. 4) The Izumi group, ranging from the uppermost Urakawan (Campanian) to the upper Hetonaian (Maestrichtian), covers uncomformably the rhyolitic rocks, which are supposed to correspond to the Takada member.
The age of the Sakugi volcanic series may be from the upper Urakawan (Santonian or Campanian) to the Hetonaian (Maestrichtian) from the following data. 1) From the study of the fossil plants which were collected from the Sakugi series, S. Imamura and S. Endo were inclined to regard the flora as of the upper Cretaceous, for it has much dicotylendones. 2) There is a distinct gap in time between the Kisa-Takada series and the Sakugi series, for the Sakugi volcanic rocks cover uncomformably the Kisa-Takada series and the granitic rocks which were intruded into the Kisa-Takada series. 3) The Izumi group covers unconformably rhyolitic rocks, which are supposed to be the same as the Takada member, and contains tuffaceous rocks, andesitic to rhyolitic, at various horizons. These tuffaceous rocks may be correlated to the Sakugi volcanic rocks.
From geological relations between the volcanic rocks and the plutonic rocks, the ages of the plutonic activities are considered as follows.
1) The central plutonic group. As the central plutonic group is considered to be closely related with the volcanic activities, it is divided into two series. One is the Ibaraichi plutonic group, which is connected with the Kisa-Takada volcanic activity and covered unconformably by the Sakugi series. The other is the Kuchiwa plutonic group which is related with the Sakugi volcanic activity. The activity of the Ibaraichi group is believed to range from the Aritan (Barremian) to the Miyakoan or Gyliakian (Cenomanian) and that of the Kuchiwa group may be from the upper Urakawan (Campanian) to the upper Hetonaian (Maestrichtian).
2) The Hiroshima granitic complex. The age of its activity probably ranges from the Hetonaian (from the upper Campanian to the lower Danian?) to the early Tertiary. Because the northern portion of the Hiroshima granite cuts across the structure of the Sakugi series and the intrusion of the Hiroshima granite in the southern portion seems to be earlier than the northern portion.
3) The San'in granitic complex. As the San'in granitic complex is composed of various plutonic rocks which correspond to the central group and the Hiroshima granitic complex, the ages of which range from the Aritan (Hauterivian) to the early Tertiary.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676