THERMAL AND GEOCHEMICAL EVOLUTION OF THE MANTLE WEDGE IN THE NORTHEAST JAPAN ARC .1. CONTRIBUTION FROM EXPERIMENTAL PETROLOGY

Melting phase relations at high pressures have been examined for four Mg-rich basalts from both the trench and the backarc sides of the Mioc ene NE Japan are. All these basalts crystallize olivine and orthopyrox ene on the liquidus at pressures lower and higher than the point of mu ltiple saturation with those two phases, respectively. The pressure of multiple saturation is rather constant for all samples (1.0-1.4 GPa), suggesting that (1) there was no across-are variation in the depth of magma segregation in the Miocene NE Japan are, (2) the depth of magma segregation remains constant since 20 Ma in the trench side of the vo lcanic are, whereas it has changed from 30-40 km to >60 km beneath the backarc side of the are. This temporal variation may be caused by inc reasing thickness of the lithosphere beneath the backarc region associ ated with cooling of the mantle wedge after the opening of the Japan S ea backarc basin in the early Miocene.