SERPENTINITE DIAPIR AND BLUESCHIST METAMORPHISM IN THE MARIANA FOREARC, WESTERN PACIFIC

A large number of seamounts of serpentinized peridotites are widely distributed in the Mariana forearc. They are commonly dome-like in shape, and up to 30 km in diameter with up to 2 km of relief. Serpentinite blocks of varying sizes are scattered in a fractured and crushed serpentinite mud matrix on the surface of the seamounts. The degree of serpentinization of the blocks varies from 30% to 100%. Drilled samples of serpentinites from Conical Seamount during ODP Leg 125 are highly sheared, and show “block-in-matrix” fabrics, which are typical in on-land serpentinite mélanges. As hydrated pelagic sediments on the top of the subducting plate supply water to wedge mantle peridotites, the peridotites react with water to form serpentinites. Voluminous low-density serpentinites generated just above the subducting slab have uplifted along faults to form a huge chain of seamounts on the ocean floor. Blueschist-facies rocks have been found from some of serpentinite seamounts. Together with abundant serpentinized peridotites, the 2-3 cm clasts of blueschist-facies rocks were recovered from one of serpentinite seamounts, i.e., Conical Seamount, during the ODP Leg 125. Many sand-sized fragments of blueschists were found from South Chamorro Seamount at the southern part of the Mariana forearc during the ODP Leg 195. Recently, we carried out the geological expedition of Twin Peaks Seamount which is situated at the central part of the Mariana forearc by means of ROV "KAIKO 7000II" and submersible vessel "Sinkai 6500" during KR06-15 and YK08-08 cruises (JAMSTEC), respectively. Blueschist-facies blocks of more than several meters are exposed in the large fault scarp, which develops at the central part of the seamount. These blueschist-facies rocks are thought to have been entrained by uprising fluidized serpentinite materials from depth in the subduction zone, and extruded onto the seafloor. The presence of high-pressure rocks in these seamounts confirms that blueschist-facies metamorphism actually took place beneath the forearc in the Mariana subduction system. Serpentinite diapir is one of the potentially important mechanisms for uplifting blueschist-facies rocks from subduction zones to forearc surface.