Anomalously young volcanoes, of approximately 6, 4 and 1 Ma in age, for the 130-135 Ma Pacific Plate were discovered form the toe of the oceanward slope of the northern Japan Trench [1] and the abyssal plain on the Northwestern Pacific Basin. The new bathymetric, geological and petrological data indicate two types of eruptive style, different in the degree of vesicularity and major element composition; one is very vesicular lava forming knolls, another is dense lava forming sheet flows. The distributions of these volcanoes clearly have the trend of WNW to ESE, which are perpendicular to the hinge line of the flexural lithosphere (outer-rise) due to plate-subduction. The lavas are trapped some xenoliths of basalts, gabbros and peridotites, which are likely oceanic crust and lithospheric mantle in origin. Two types of olivine are also present in these lavas, xenocrysts with reaction rims and magmatic. The forsterite (Fo) (#91-92) values and NiO contents (0.3-0.5 wt%) of the xenocrysts are similar in composition to those of the depleted mantle peridotite. Chromian spinel inclusions in the xenocrysts also show the depleted composition in the range of abyssal peridotite. These data indicate that the xenocrysts originate from MORB-depleted mantle [2]. The calculated primitive magma, on the other hand, follows a low degree of partial melting along the 2.5 to 3 GPa cotectic lines, which would corresponds the depth of asthenosphere. The ascending magma from asthenosphere, therefore, would trap some xenoliths and xenocrysts originating in the conduit wall, possibly when brittle fractures occurs in the upper lithosphere due to the concave flexure of the plate in front of the convex bowing of outer-rise. The eruption occurred along some fissures being bent before subduction, in parallel to the maximum horizontal compression due to the down-warping of the Pacific plate motion for sub-crustal magma injection.
