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Hirokazu Maekawa, Osaka Prefecture University (Japan)
Keiko Murata, Kobe Women¡Çs University (Japan)
Koshi Yamamoto, Nagoya University (Japan)
Hisayoshi Yokose, Kumamoto University (Japan)
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In the Mariana forearc, horst and graven structures are well developed in the outer forearc basement, of which western limit is 60 - 90 km from the trench. The basement consists of both island-arc and oceanic crust-mantle rocks, such as serpentinized peridotites, gabbro, volcanics, and their metamorphic equivalents, and pelagic chert. A zone of cone-shaped diapiric seamounts formed on the basement where horst and graven structures are dominated in the forearc regions 50-90 km west from the Mariana Trench. Diapiric seamounts are composed mainly of serpentinized peridotites, with subordinate amounts of volcanic, hypabyssal, and plutonic rocks of both island-arc and oceanic origins and their high-P/T and low-P/T metamorphic equivalents. We investigated rocks from seven diapiric seamounts and from three fault scarps of horst mounds. A clear tendency is recognized, that is, serpentinized peridotites occupies more than 90 % of whole rocks recovered from the diapiric seamounts, whereas less than 40 % from the fault scarps. Mg# [=Mg/(Mg+Fe)] values of spinels of serpentinized peridotites from fault scarps of the horst mounds are 0.55-0.68, whereas those from diapiric seamounts have relatively wide range of 0.27-0.76. It suggests that serpentinized peridotites with various degrees of depletion have been incorporated into diapir mass on the pathway in the wedge mantle from the top of subducting slab to seafloor during diapiric rise.
iiSerpentine minerals of peridotites from both diapiric seamounts and basement are mostly chrysotile and/or lizardite. Antigorite, however, is found only from peridotites recovered from Conical and South Chamorro Seamounts, which are the farthest two seamounts from the trench axis (80-90 km from the trench axis) in diapiric seamounts in the Mariana forearc. Antigorite-bearing peridotites characteristically contain cleavable olivine. Secondary olivine (Fo86.4-88.4) occurs as the overgrowth or along the cleavage traces of primary olivine (Fo89.8-91.6). Antigorite coexists with secondary olivine, acicular diopside, and brucite. The assemblage clearly shows high temperature conditions of serpentinization in comparison with chrysotile/lizardite-bearing assemblage. Serpentinization is thought to occur along the subduction boundary by supplying water from hydrated pelagic sediments on top of the subducting Pacific Plate to wedge mantle peridotites, so we can reasonably consider that diapiric seamounts farther from the trench axis contain peridotites which were serpentinized at the deeper portion. The antigorite-bearing assemblage in Conical and South Chamorro Seamounts suggests that antigorite is stable just below these two seamounts at the depth of about 25-30 km along the subduction boundary. Chrysotile/lizardite is probably stable at eastward shallower portions.
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