Abstract:
Recent studies of forearcs in the circum-Pacific regions have revealed that the widespread serpentinization of mantle wedge peridotite occurs along the subducting slab at depths of 15-30 km due to water supplied from the slab. A huge zone of diapiric serpentinite seamounts along the trench axis in the Izu-Bonin and Mariana forearcs suggests that voluminous and gravitationally unstable low-density serpentinites generated just above the subducting slab have risen to the seafloor to form the seamounts. During ODP Leg 125, metamorphic rock clasts recovered from Holes 778A and 779B at Conical seamount, one of the serpentinite seamounts, have provided essential information on the interaction between forearc material and water. A geochemical study of the 778 A metabasalts indicates that the rocks have a chemical affinity with mid-ocean ridge basalts, some of which have zigzag REE patterns due to intense interaction with seawater. There are two possible origins that are worth considering. One is the trapped oceanic crust in the area between the trench and the volcanic front when subduction of the Pacific plate started, and the other is the accreted oceanic crust supplied directly from descending oceanic slab during subduction. The Hole 778A metabasalts commonly contain quartz veins, which have been produced prior to or during blueschist facies metamorphism, because high-pressure minerals, lawsonite, pumpellyite, and aragonite, were often crystallized in the vein. When the trapped or accreted oceanic crust had been squeezed deep down by the subducting slab, it encountered the pelagic sediments on top of the subducting slab. The SiO (sub 2) -rich fluids having permeated the Hole 778A rocks were probably derived from these pelagic sediments. A phengite-rich clast, the only clast recovered from Hole 779B, is ultrabasic in composition, but is rich with incompatible elements, such as Zr, Ti and Th, and is relatively poor in compatible elements, such as Cr, Ni, and Co. Rocks with similar geochemical characteristics are found in the metasomatic reaction zone developed at the boundary between serpentinite and pelitic schist in the high-pressure Sanbagawa metamorphic belt, Japan. The clast may have been formed at the boundary between mantle wedge peridotite and subducting slab, where the hydrothermal metasomatic reactions have pervasively occurred between mantle wedge and pelagic sediments.