K-Ar ages of the Tertiary volcanic rocks from Okushiri Island and petrological characters of the Oligocene to Early Miocene volcanic rocks from the Northeast Japan arc and the surrounding areas.

K-Ar dating was carried out on the Tertiary volcanic rocks from Okushiri Island in Japan Sea. We obtained K-Ar whole rock ages of 4.4±0.2, 20.4±0.5, 19.7±0.5 and 34.4±0.8 Ma for a basalt in the Kamuiyama Formation, an andesite in the Tsurikake Formation, an andesite in the Aonaegawa Formation and a basalt in the Matsue basalt Formation, respectively. The volcanic activity during Tertiary time in Okushiri Island are divided into main three stages, Oligocene (34-29 Ma), Early Miocene (23-20 Ma) and Pliocene (-4 Ma). The chemical compositions of representative 32 samples from the three stages were determined by XRF techniques, The Oligocene Matsue basalts (lower) are characterized by high contents of MgO, Ni and Cr whereas the upper Matsue basalts have high contents of HFS elements such as Ti, P, Zr and Nb, suggesting that the latter are different in chemical compositions from the typical island arc basalts. The Early Miocene volcanics composed mainly of calc-alkaline andesite and dacite, whose incompatible element contents are similar to those of the Quaternary volcanics from the back-arc side of the NE Japan arc. The Pliocene volcanics composed mainly of rock suites ranging from andesite to dacite of calc-alkaline series with subordinate amounts of basalt which have incompatible element contents comparable to those of the Pliocene alkali basalt from Kyuroku-shima Island, northeast Japan Sea. The NE Japan arc and the surrounding areas during the Oligocene to Early Miocene are characterized by occurrence of andesite and dacite of calc-alkaline series and tholeiitic and alkali basalts with high contents of HFS elements, and the calc-alkaline rocks were generally formed before basaltic volcanism took place.