| Summary: | The "Miocene Regional Basalts", proposed here by the authors, include basalts and dolerites of the Nishikurosawa and Onnagawa stages, which are widely distributed from Aomori through Yamagata to Niigata oil and gas along the Japan Sea coast. Some of these basalts play an important role as oil and gas reservoir. In the present paper, we describe occurrence, mineral chemistry and bulk chemistry of basaltic rocks in the Niigata oil and gas field, especially in the Yahiko-Kakuda area and the Higashiyama area. Systematic lateral variations in the chemical composition of the Miocene Regional Basalts are not observed on the basis of the petrochemical characters of these basalts, including basaltic rocks of the Niigata oil and gas field and those from other localities in Yamagata, Akita and Aomori Prefectures. The Miocene Regional Basalts are composed of not only the island arc-type tholeiite but tholeiitic and alkali basalts which are rich in TiO_2 and/or P_2O_5 and similar in composition to basalts of the oceanic region. It may be possible that such tholeiitic and alkali basalts of oceanic-type were derived from an upper mantle material different from the present upper mantle material in Northeast Japan arc. The origin of these basaltic magma might be derived from a mantle diapir in the same constitution as that of Iceland volcanic. The middle Miocene volcanics of the island arc-type tholeiite and of calc-alkaline series in Northeast Japan arc may have its orig1n in relation to partial melting of upper mantle and lower crust owing to the supply of heat from the ascending mantle diapir. journal article
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