Abstract
Geological survey of the Kume-jima Island, 90 km west of the Okinawa-jima Island, has been carried out, and petrographical description on the middle to late Miocene igneous rocks of the Ara-dake Formation has been made to clarify the nature of the rocks. The analytical results of the major elements for 62 samples were reported as well.
The rocks are distributed in the eastern half of the island, and most of the activities were subareal ones. The rocks consist of lavas, pyroclastics, and a minor amount of hyaroclastites. Andesitic lavas are dominant and dacitic lavas are minor. The lavas are divided into 7 units: nearly aphyric andesitic rocks are found only in the units 1 and 5, while the others consist of porphyritic andesites and dacites with abundant plagioclase phenocrysts. Some dacitic rocks include hornblende with/without biotite as phenocrysts.
The rocks of the units 1 and 5 are rich in TiO2 and P2O5, and plotted in the tholeiitic field in the FeO*-FeO*/MgO diagram. In contrast the others are calc-alkalic. The rocks of the units 2-7 are characterized by the highest K2O contents among the other Cenozoic volcanic rocks from the central Ryukyu Islands and Okinawa Trough region.
