THERMAL AND GEOCHEMICAL EVOLUTION OF THE MANTLE WEDGE IN THE NORTHEAST JAPAN ARC .2. CONTRIBUTION FROM GEOCHEMISTRY

Compositions of primary basalt magmas were backcalculated from Miocene Mg-rich basalts in the Japan Sea back arc basin and the NE Japanese v olcanic front, in order to assess the spatial variation in physicochem ical conditions for magma production during opening of the back arc ba sin. Chemical compositions of these magmas together with available liq uidus experimental data suggest that the primary basalt magmas both in the Japan Sea (BABB magma) and the NE Japanese volcanic front (VFB ma gma) were equilibrated with harzburgitic mantle materials at almost id entical pressure and temperature conditions. The observation that the BABB magma is more depleted in Fe and some incompatible trace elements and more enriched in Al than the VFB magma then suggests that these m agmas may have been derived from source mantle materials with differen t chemical compositions. An equilibrium melting model further demonstr ates that the BABB magma was produced by a smaller degree of partial m elting of a more depleted source mantle material (compositional equiva lent to abyssal harzburgites) than the VFB magma. Provided that the va rying degrees of depletion in the source mantle materials were manifes tations of mantle upwelling efficient beneath the back arc side region , the more depleted BABB magma source may have been a plausible geoche mical reservoir in the deeper portion of the mantle wedge.