Geological and Petrological studies of the Middle Mocene volcanic rocks distributed around the volcanic front of about 15 Ma in the Northeast Japan arc

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The results obtained from this research project are summarized as follows. 1) The K-Ar dating was carried out on the Middle Miocene volcanic rocks from the trench side in the Northeast Japan arc. We obtained K-Ar ages of 15.1-15.4, 12.6-16.0 and 13-15 Ma for the Ryozen basalts, Inase volcanic rocks and Tomari volcanic rocks, respectively. 2) If we combine the data of previous studies with the present ones, we can conclude that the volcanic front of 13-16 Ma was estimated to lie 30-50 km east of the Quaternary volcanic front. 3) Mechanism for the shifting of volcanic front into the back-arc side from the Middle Miocene (forming 13-16 Ma volcanic front) to the Late Miocene (forming the 0-8 Ma volcanic front was discussed. 4) The Middle Miocene volcanic zone in the trench side can not be divided into subzones like the Quaternary Aoso-Osore and Sekiryo zones, based on differences in mafic phenocryst assemblages and incompatible element abundances of these volcanic rocks. 5) In the Middle Miocene volcanic rocks, TiO_2 rich basalts, icelanditelike andesites and dacites, and olivine basalts with primitive nature in chemical compositions are found although these volcanic rocks can not be observed in the Quaternary volcanic rocks. 6) The Middle Miocene basalts in the back-arc side have the K_2O and Rb contents lower than those in the trench side, indicating that the across-arc variations of large lithophile elements in the Middle Miocene volcanic rocks have striking contrast to those for the Quaternary volcanic rocks. These differences in petrologic characteristics of the Middle Miocene and Quaternary volcanic rocks may be ascribed to differences in tectonic environments in relation to the magma generation in the upper mantle between Middle Miocene and Quaternary ages.