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Ohmachi Smt.
Fig. 11 Model illustrations for the genesis of geologic structure in the Ohmachi Seamount
peridotites as indicated by chemistry of relic spinels (Cr# ~0.15: Yuasa et al. 1999 ;
Niida et al. 2001, 2003 ), peridotites with similar spinel chemistry and hydrous phases
were recently discovered from southern Mariana forearcs (Yanagida et al. 2006 ;
Michibayashi et al. 2009 ) as well as from back-arc basins in the Philippine Sea Plate
(Ohara et al. 2002, 2003 ). Occurrence of least refractory peridotites in the southern
Mariana forearc are attributed to slow and rather amagmatic spreading of a back-arc
basin (Mariana Trough) in the upper plate (Michibayashi et al. 2009 ).
Concerning the structural and petrologic characteristics of serpentinites, the
simplest interpretation for the geologic structure is given in Fig. 11 , where the
fragments of the subducted oceanic crust (blueschists and eclogites) were exhumed
along with the shear zone (schistose serpentinite) at the base of the hanging-wall
mantle wedge (massive serpentinite).
Exhumation to the Surface
Because any geochronological data have not been successfully obtained so far for
metamorphic rocks of the Ohmachi Seamount, it is difficult to discuss detailed
tectonic processes of their exhumation to the surface. Here we briefly review the
tectonic history of the Philippine Sea Plate and several proposed ideas.
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