| Summary: | Aomori Prefecture in the northern part of the Northeast Japan arc is characterized by the extensive distribution of Neogene volcanic rocks. The middle Miocene volcanics have no systematic difference in alkalis between the Kitakami (outer) and Dewa (inner) zones. On the other hand, K_2O contents of the late Miocene volcanics increase gradually across the arc from the Pacific to Japan sea side. Sr and Rb of these volcanics show essentially the same lateral variation as K_2O. Similar lateral variation in K_2O contents is also recognized in the late Miocene to Pliocene volcanic rocks of southwest Hokkaido (OKAMURA, 1986). These evidences indicate that the zonal distribution of K_2O might have occurred in the volcanic rocks produced from volcanism since the late Miocene age in the region extending from northern part of the Northeast Japan arc to southwest Hokkaido. In the Northeast Japan arc of the middle Miocene, oceanic-type volcanics including TiO_2 rich basalts, icelandite-like andesites and dacites, and rhyolites occur and the latter three have high FeO*/MgO ratio and phenocrysts of Fe-enriched olivine and pyroxenes, in association with volcanics of the island arc tholeiite series and calc－alkaline series. It is possible that such oceanic-type volcanics were derived from an upper mantle material which is different from the present one in the Northeast Japan arc. These volcanics could have been generated from a mantle diapir in thc same constitution as that of Iceland volcanics. The middle Miocene volcanics of the island arc tholeiite series and calc-alkaline series in the Northeast Japan arc may have been originated from partial melting of upper mantle and lower crust heated by the ascending mantle diapir. This model for the mechanism of magma generation can explain the lack of zonal variation in alkalis of the middle Miocene volcanics in the Northeast Japan arc. 東北日本弧北部・青森県に分布する中新世火山岩類のうち，中期中新世火山岩類では，島弧横断方向で化学組成に規則的な変化はみられない．一方，後期中新世火山岩類は，島弧横断方向に（東側から西側に），K_20， Sr， ...
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