Petrological studies on the mantle peridotites recovered from the ocean floor in the western Pacific

Geological and geophysical models for the various oceanic crusts (or lithosphere) have been proposed on the basis of the combined studies between seismic observation for the oceanic crusts and petrological models of the onland ophiolites, which have been assumed as fossil of oceanic crusts. It is very important to collect basement rocks constituting various oceanic crusts and to characterize those petrological features. Ocean floor is commonly covered by effusive volcanic rocks, however occasionally hypabyssal and plutonic rocks are observed among the unique geological environments in the Western Pacific as partly shown in the followings. VOLCANIC DIATREME(?): Very unique volcanic knolls have been recently discovered by N. Hirano at the typical oceanic crust in the Northwestern Pacific, off Tohoku of Northeastern Japan. The constituting rocks for the main volcanic edifice are porous alkaline lavas with 1-5Ma age containing abundant lithic fragments including gabbros as well as mantle peridotites. They are assumed as a volcanic diatreme induced in the Cretaceous oceanic lithosphere . Geological and petrological analyses on those volcano and volcanic rocks can make clear the geological cross (or columnar) section of the typical oceanic lithosphere including crust as well as upper mantle down to 100 km deep asthenospheric mantle. PARECE VERA BASIN: The Parece Vela Basin (PVB) is an extinct backarc basin in the Philippine Sea. The NNE extending escarpments and depressions (maximum depth 7500 m) are fossil fracture zones and extinct segmented spreading axes (first-order segments), respectively. Oceanic core complexes (OCCs), or megamullions, develop at each first-order segment. Recently discovered OCCs at slow-spreading ridges have been interpreted as exhumed footwalls of oceanic detachment faults in magma-starved ridge environments. Godzilla Mullion, one of the OCC in the PVR, is the worlds largest OCC, 10 times larger in area than the normal OCCs in the Mid-Atlantic Ridge (Ohara et al., 2001). Various plutonic rocks including mantle peridotites were recovered from the megamullion. TONGA FOREARC: The serpentine seamounts have never been observed along the forearc, on the other hand, geological cross section is recognized along the Tonga Trench inner wall including fresh mantle peridotite. IZU-OGASAWARA(BONIN)-MARIANA FOREARC: Many topographic highs are recognized along the Izu- Ogasawara-Mariana forearc. A number of igneous rocks including lavas, gabbros and serpentinized depleted peridotites; so called ophiolitic rocks were dredged from those seamounts by several investigators, who concluded that these seamounts originated from serpentinite diapirs derived from the upper parts of the mantle wedge. Remnant mantle diapir is assumed to be the depleted source peridotite of diapiric serpentinite seamount. Tonga forearc and Izu-Ogasawara-Mariana forearc may be assumed as modern analogue of the Oman ophiolite and the Trodos ophiolite, respectively.