Geological and petrological study of the middle Miocene regional basalts from the Japan Sea side in the NE Japan arc

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Abundances of major elements and seven trace elements, Rb, Sr, Nb, Y, Zr, Ni and Cr were determined by X-ray fluorescence analysis for 111 samples from twelve Pliocene volcanic centers in the NE Japan are. The petrographical and analytical results in this study and the published ones on other Pliocene volcanic rocks indicate that systematic zonal variation of petrological features, i.e., mafic phenocryst assemblages and rock series, as well as gradual increase in K_2O, Rb and Sr contents are recognized in the Pliocene volcanic rocks from the Pacific Ocean side to the Japan Sea side across the NE Japan arc. Furthermore, systematic across-arc variation of chemical compositions and petrological features, similar to those in the Pliocene to Quaternary volcanic rocks, are also observed in the late Miocene (8-6 Ma) volcanic rocks throughout the NE Japan arc.Similarity of across-arc variation. with regard to both the petrological and geochemical features among the volcanic rocks from the late … More Miocene to Quaternary through Pliocene time suggests that igneous activity which is related to a mechanism for magma generation determined by plate subduction bas been continuous in the NE Japan arc at least during 8 Ma ago to the present.On the other hand, the NE Japan arc of the middle Miocene (16-14 Ma) are characterized by occurrence of rocks of non island arc-type volcanic rocks such as E-type MORB like basalt which is enriched in Ti and other HFS elements, basaltic rocks hailing intermediate nature between the island arc tholeiite and the back arc basin basalt in bulk rock chemistry and icelandite like andesite and dacite, in association with volcanics of the island arc tholeiite series and calc-alkaline series. These volcanic rocks increase gradually in K_2O from the Japan Sea side toward the Pacific Ocean side across the NE Japan arc, indicating the lateral variations dissimilar to the case of volcanic rocks produced during 8 Ma ago to the present.It is possible that such non island arc-type volcanics were derived from a upper mantle material which is different from the present one in the NE Japan arc. These volcanics could have been generated from a mantle diapir ascending from asthenosphere during the spreading of the Japan Sea. The middle Miocene volcanics of island arc tholeiite series and calc-alkaline series in the NE Japan arc may have been originated from partial melting of upper mantle and lower crust heated by the ascending mantle diapir. Less